Report
to/Rapport au :
Planning and Environment Committee
Comité de l'urbanisme et de
l'environnement
and Council / et au Conseil
9 September 2005 / le 9 septembre 2005
Submitted
by/Soumis par : R.G. Hewitt,
Acting Deputy City Manager/Directeur municipal adjoint intérimaire,
Public Works and Services/Services et Travaux publics
Contact Person/Personne
ressource : John Manconi, Acting Director/Directeur
intérimaire
Surface Operations/Opérations de surface
(613) 580-2424 x 21110, John.Manconi@ottawa.ca
REPORT RECOMMENDATIONS
That Planning and Environment Committee recommend Council
approve:
1. That Forestry Services
staff use the revised 4-Phase assessment process, as outlined in this report,
when reviewing individual claims of foundation damage to buildings potentially
caused by City-owned trees.
2. That property owners submitting claims of city-owned trees causing damage to their foundations continue to be responsible for all associated costs of the required geotechnical report prescribed in the 4-Phase assessment process as outlined in this report.
3. That the City implement
a communications strategy that:
·
Informs area residents on tree and foundation
issues in areas where sensitive marine clay is known to exist as outlined in
Documents 1 and 2 attached to this report;
·
Ensures
the City’s website contains information on how area residents can educate
themselves on sensitive marine clay issues;
·
Issues
special watering advisories in times of drought requesting that residents
assist the City by watering trees adjacent to their properties;
·
In coordination with the City’s Water Efficiency
program, provides information on the importance of watering trees as outlined
in Document 3 of this report and;
·
Develops
an information exchange with other professionals involved in the trees and
foundation assessment process
4. That
a precommitment to the 2006 Forestry Operating Budget
of the Surface Operations Branch of Public Works and Services Department be
established for increased
funding of $873,000 for tree trimming, supplemental watering, infrastructure
improvements and for 2 FTE positions for implementation to cover the additional
costs for tree maintenance activities that mitigate the effect of trees growing
in proximity to buildings and structures.
Further that $50,000 be provided from the 2006
capital funding envelopes to purchase two vehicles.
5. When
planting on city property in areas where sensitive marine clay is known to
exist:
·
Only
low water demand trees with a lateral separation distance of 1 full mature
tree height be planted in proximity to buildings or structures;
·
In
areas where adjoining properties result in one combined greenspace
only 1 tree per front yard area will be planted;
6. That in areas of sensitive
marine clay where the revised 4-Phase assessment process determines that city
owned trees may be a contributing factor to foundation damage the trees will be
retained in recognition of the significance of the City’s urban forest and its
contribution to the quality of life in the City of Ottawa and the City will
implement arboriculture mitigation measures as outlined in this report.
RECOMMANDATIONS DU
RAPPORT
Que
le Comité de l’urbanisme et de l’environnement recommande au Conseil
d’approuver :
1. que le personnel des Services forestiers utilise
le nouveau processus d’évaluation en quatre étapes, comme l’énonce ce rapport,
pour l’examen des réclamations individuelles pour des dommages aux fondations
possiblement causés par des arbres appartenant à la Ville.
2. que les propriétaires qui font des
réclamations à la suite de dommages à leurs fondations causés par des arbres
appartenant à la Ville continuent à être responsables de tous les coûts
associés au rapport géotechnique prescrit par le processus d’évaluation en
quatre étapes, comme l’énonce ce rapport.
3. que la Ville mette en œuvre une stratégie de
communication :
· qui informera les résidants sur les
problèmes liés aux arbres et aux fondations dans les secteurs où on décèle la
présence d’argile marine, comme l’indiquent les documents 1 et 2 de ce rapport;
· qui veillera à ce que le site Web de la
Ville contienne de l’information sur la façon dont les résidants des secteurs
touchés peuvent se renseigner sur les problèmes liés à l’argile marine
sensible;
· qui prévoira l’émission d’avis spéciaux
en période de sécheresse demandant aux résidants d’aider la Ville en arrosant
les arbres avoisinant leur propriété;
· qui, en coordination avec le programme
de valorisation de l’eau de la Ville, offrira de l’information sur l’importance
d’arroser les arbres, comme l’indique le document 3 de ce rapport;
· qui permettra d’échanger de
l’information avec d’autres professionnels participant au processus
d’évaluation des arbres et des fondations.
4. qu’un engagement préalable du budget de
fonctionnement de Services et Travaux publics de 2006 pour les Services
forestiers soit établi en vue du
financement accru de 873 000 $ pour l’ébranchage, l’arrosage
supplémentaire, l’amélioration des infrastructures ainsi que la mise en place
de deux ETP afin de couvrir les coûts additionnels relatifs aux activités
d’entretien qui limitent les effets des arbres qui croissent à proximité des
bâtiments et des structures. En outre, que la somme de 50 000 $ soit
transférée des enveloppes de dépenses en capital en vue de l’achat de deux
véhicules.
5. que lorsqu’on plante des arbres sur des
terrains appartenant à la Ville dans des secteurs ou il y a présence d’argile
marine :
· Planter uniquement des arbres qui demandent peu
d’arrosage, en prévoyant un espacement latéral de la taille d’un arbre mature à
proximité des immeubles ou des structures;
· Dans les secteurs où des propriétés attenantes ne disposent que d’un
espace vert combiné, planter un seul arbre par cour avant;
· Réviser tous les plans d’aménagement paysager
pour s’assurer qu’ils sont conformes au processus de la Ville en ce qui a trait
aux arbres et aux fondations;
· Lorsqu’il faut remplacer des arbres sur des terrains de la Ville où il
n’y a pas suffisamment de place pour permettre l’espacement approprié, la Ville
procèdera sur demande à la plantation des arbres sur le terrain privé adjacent
à la condition que le propriétaire concerné signe une décharge par laquelle il accepte d’assumer la
propriété de l’arbre et de libérer la Ville de toute responsabilité.
6. que dans les secteurs où on décèle la
présence d’argile marine sensible et pour lesquels le nouveau processus
d’évaluation en quatre étapes permet de déterminer que les arbres appartenant à
la Ville contribuent possiblement aux dommages causés aux fondations, on
préserve les arbres en reconnaissance de la signification de la forêt urbaine
de la Ville et de sa contribution à la qualité de vie dans la Ville d’Ottawa,
et que la Ville mette en œuvre des mesures d’atténuation arboricoles, comme
l’indique ce rapport.
EXECUTIVE SUMMARY
The former City of Ottawa approved a policy in 1997
that authorized staff to remove city trees if it was proven that the tree(s)
was a contributing factor in damaging a building’s foundation due to the
dewatering of the underlying clay soils.
Each claim was evaluated starting with a geotechnical analysis provided
by the resident making the claim. Tree
removal was only considered as a last resort.
After amalgamation in 2001, this policy was carried
forward into the new city although 92% of the claims continued to be received
within the geographic boundaries of the former City of Ottawa. In the summer of 2002, the City initiated an
extensive review that studied all the potential causes of foundation damage in
areas where sensitive marine clay is known to exist. In 2004 a review of the Trees and Foundation
process was undertaken as a component of an overall Forest Maintenance Strategy
and revised citywide Forest policy that is currently targeted for submission to
Committee and Council in early 2006.
Additional issues arose in 2004 as a result of 3
proposed removals under the existing policy in the Centretown Conservation
District. Concerns were expressed about
the scope of the policy, particularly where the removal of larger trees may
have a reduction on the quality of the streetscape in urban
neighbourhoods. In consideration of
these concerns, a motion was passed at City Council on 23 February 2005,
placing a moratorium on all foundation related tree removals pending the review
of the Trees and Foundation Policy.
In response to this motion the development of a
harmonized policy has been accelerated and staff have undertaken a thorough
review of the existing policy as presented in this report. As part of its review staff met with tree
experts from the Faculty of Forestry at the University of Toronto, with
municipal staff of the Cities of Toronto and Montreal and with representatives
of the Ottawa Forests and Greenspace and Local Architectural Conservation
Advisory Committees. The City has also
reviewed the revised geotechnical requirements with the local chapter of the
Canadian Geotechnical Society.
This report recommends that a 4 Phase Assessment
process be followed in situations where home owners are claiming foundation
damage from city trees, that home owners continue to bear the cost of the
geo-technical assessments, that a communications strategy be adopted to inform
residents and other stakeholders on trees and foundation issues and that city
owned trees be retained in conjunction with the appropriate mitigation
measures.
The revised policy is in keeping with the City’s
Official Plan’s goal to increase the city’s forest cover from 28% to 30% and
supports the objective that identifies the preservation of urban trees and
greenspaces as a priority and a key factor in the quality of life in the City
of Ottawa.
RÉSUMÉ
L’ancienne Ville d’Ottawa a
approuvé une politique en 1997 autorisant le personnel à abattre les arbres
appartenant à la Ville s’il avait été prouvé que ceux-ci contribuaient aux
dommages causés aux fondations d’un immeuble en raison de la déshydratation des
sols argileux sous-jacents. L’analyse géotechnique fournie par le résidant touché
constituait la première étape de l’évaluation de chaque demande. Ce n’est qu’en
dernier recours qu’on envisageait d’abattre les arbres.
Après la fusion en 2001, la nouvelle ville a
conservé cette politique même si 92 % des demandes reçues se situaient dans
les limites géographiques de l’ancienne Ville d’Ottawa. À l’été 2002, la Ville a lancé un examen
complet étudiant toutes les causes potentielles des dommages occasionnés aux
fondations dans les secteurs où on décèle la présence d’argile marine sensible.
En 2004, on a entrepris d’examiner le processus d’évaluation des arbres et des
fondations dans le cadre d’une stratégie globale d’entretien des forêts et de
la révision de la politique générale d’aménagement forestier, qui devrait être
déposée devant le Comité et le Conseil au début de 2006.
D’autres problèmes sont survenus en 2004
lorsqu’on a proposé d’abattre 3 arbres conformément à la politique actuelle
dans le District de conservation du patrimoine du Centre-ville. Certains ont
émis des doutes quant à la portée de la politique, particulièrement dans les
cas où l’abattage d’arbres de grande taille pourrait piller le paysage de rue
dans les quartiers urbains. Compte tenu des préoccupations exprimées, une
motion a été approuvée au Conseil de municipal le 23 février 2005, instaurant
ainsi un moratoire sur tout abattage d’arbre lié aux dommages causés à des
fondations en attendant l’issue de l’examen de la politique sur les arbres et
les fondations.
À la
suite de cette motion, on a accéléré l’élaboration d’une politique harmonisée
et le personnel a entrepris un examen complet de la politique en place, comme
elle est présentée dans ce rapport. Dans le cadre de cet examen, le personnel a
rencontré trois spécialistes de la faculté de foresterie de l’Université de
Toronto, le personnel de la Ville de Toronto et de la Ville de Montréal, de
même que des représentants du Comité consultatif sur les forêts et les espaces
verts et du Comité consultatif local sur la conservation de l'architecture
d’Ottawa. La Ville a également révisé les exigences géotechniques de la section
locale de la Société canadienne de géotechnique.
Le
présent rapport recommande qu’on recoure au processus d’évaluation en quatre
étapes lorsque des propriétaires soutiennent que des arbres appartenant à la
Ville ont causé des dommages à leurs fondations; que les propriétaires
continuent d’assumer les frais liés aux analyses géotechniques; qu’on adopte
une stratégie de communication en vue d’informer les résidants et d’autres
intervenants sur les problèmes liés aux arbres et aux fondations, et qu’on
préserve les arbres appartenant à la Ville en appliquant les mesures
d’atténuation appropriées.
La
nouvelle politique s’accorde avec l’objectif du Plan officiel de la Ville qui
consiste à accroître la superficie boisée de la Ville de 28 % à 30 %, et appuie
l’objectif selon lequel la préservation des arbres et des espaces verts est une
priorité et un facteur clé pour la qualité de vie dans la Ville d’Ottawa.
BACKGROUND
The City of Ottawa is unique among major urban centres across Canada in that it is made up of both urban and rural communities. As such, the City has both urban and rural forests. The City-owned urban forest consists of individual trees along streets, in urban plazas, in parks and at city facilities as well as urban woodlots and ravines. Its rural forests consist of individual trees on rural roads, hedgerows and community forests.
Trees are important and distinctive features within cities. Their benefits to our daily lives and their contributions to the well being of our communities are well recognized. Trees provide social, cultural, environmental, economic and aesthetic benefits to both urban and rural communities. They offer shelter and shade from the elements and provide natural features in urban environments. Trees are well known to increase property values and moderate temperatures, thus reducing heating and cooling costs. Collectively, trees in the urban and rural forests become natural air and water filters. Though often taken for granted, trees are an integral part of our daily lives. They define the spaces that we live in, and their aesthetic beauty has a great impact on the quality of life in our City.
The urban environment differs greatly from the typical rural forest setting and there are unique challenges when caring for urban street trees. Trees in nature frequently contain dead wood, either as a result of insect attack, disease or injury and a percentage of dead wood has a minimal effect on the health of a tree. However a higher standard of care, such as the requirement to remove dead branches, is required along urban streetscapes due to safety concerns. Reconstruction projects and competition in rights-of-way for growing space also have a significant impact on the health and longevity of mature urban trees, which are becoming increasingly difficult to replace as our cities become more and more developed.
There is currently a trend towards an increased recognition of landscape features such as trees as essential components of heritage districts and this is reflected in several of the Ontario provincial government’s more recent policies. Conservation districts are now being defined in the new Provincial Policy Statement, which came into effect on 1 March 2005, as “cultural landscapes”, a notion that encompasses the natural dimension of these neighbourhoods, of which mature trees and soft landscaping are significant features. The new Ontario Heritage Act (April 2005) also goes in a similar direction by legislating the approval of not only the district, but also the management plan that defines its main characteristics. Although never formally approved by the former City of Ottawa Council, the Centretown Conservation District Study prepared in 1995 identified trees and street canopy cover as being one of the significant streetscape features in the core area.
The City of Ottawa’s forestry programs ensure that every effort is made to maintain and retain city trees for as long as possible. However, there are times when trees have to be removed and when they are this frequently has a significant impact on the adjacent community. While removal of dead trees is generally understood, the removal of an apparently healthy tree is contentious, as in the case when a tree is removed as a contributing factor to foundation damage. As a result the City has developed a detailed 4 Phase Assessment process for managing this issue.
Forestry
Services Mandate
The maintenance, preservation, protection and enhancement of over 250,000 street and park trees and 10,000 hectares of forested land falls under the responsibility of the Forestry Services Section of the Surface Operations Branch within the Public Works and Services Department.
Forestry Services is responsible for the direction of strategic planning, design, and implementation of maintenance, preservation, protection and enhancement programs and policies for city-owned trees and forests. This Section acts as a liaison with the community for tree related issues and is responsible for the promotion and public awareness of trees and forests. In addition to the provision of forestry-related services to the public, Forestry Services also provides advice and technical expertise to, and collaborates with, other City of Ottawa Branches such as Parks and Recreation, Infrastructure Services, Planning and Infrastructure Approvals, Real Property Asset Management and By-law Services.
In 2004, as part of the budget process, the City conducted an EKOS poll to evaluate area residents' satisfaction with City services. Parks and green spaces were identified as important features in our community, particularly in the city centre. In fact, 99% of respondents indicated that the existing level of services should be either maintained or enhanced.
In 2005, the City of Ottawa spent $5.91 per capita on its forestry programs. As the table below illustrates this figure compares fairly well to cities such as London and Quebec City. However Ottawa still falls short of other Canadian cities such as Winnipeg or Vancouver, which are known internationally for their proactive tree maintenance and forestry programs.
|
Municipality |
Forestry Programming per Capita (2004) |
|
London |
$5.44 |
|
Ottawa |
$5.91 |
|
Quebec City |
$6.00 |
|
Winnipeg |
$6.75 |
|
Vancouver |
$8.61 |
Ottawa is generally situated on clay soils deposited at the time of the last ice age when receding glaciers formed the Champlain Sea. As a result, many buildings have been constructed on fine-grained sensitive marine clays (commonly referred to as leda clays) that shrink according to their water content. The tiny particles that form these clays are arranged like a house of cards, which are held together when the spaces between them are filled with water. When there is a severe loss of water they collapse, leading to a reduction of soil volume and soil shrinkage. Across Canada, sensitive marine clay is primarily found in Ottawa and the St. Lawrence River lowlands, which also includes the City of Montreal. World-wide, Great Britain also has a long history of tree and foundation concerns and the National House-Building Council (NHBC) in that country has recently modified its Technical Standards to address building construction in areas where sensitive marine clay is known to exist.
Although foundation damage can be caused by many factors, it can sometimes occur as a result of soil shrinkage in close proximity to the structure through differential settlement, or shrinkage, of the surrounding soils. There have been several studies conducted in the Ottawa area since the 1950's that have investigated this phenomenon. A well-known study conducted by the National Research Council (NRC) in the 1960's affirms that the cause of the damage due to differential settlement can be attributed to trees since they remove water from the soil. This information is widely quoted in the geotechnical community today. However, it is important to note that the NRC study only looked at trees as a contributing factor and ignored all other potential causes for the damage.
More recent studies confirm that, dependent upon site conditions, trees can have a contributing role to foundation damage; however, in most cases there are a combination of factors working together that result in soil shrinkage and differential settlement of foundations. All of these factors must be considered when assessing foundation damages. It is not sufficient, nor accurate, to focus only on the tree.
Prior to 1991, the former City of Ottawa had received very few complaints related to City trees damaging foundations. At that time there was no formal evaluation process in place to assess the causes, complaints were dealt with individually and the trees were usually removed. However, from 1991 to 1995, the City received 69 complaints - all of which were associated with trees in the road allowance. The subsequent investigations resulted in the removal of 42 large diameter (> 50cm) trees. Concerns raised by citizens groups at that time about the loss of core area trees resulted in a decision in 1996 to review the City’s existing tree removal policy.
The goal of the study was to create an effective strategy and action plan for the City’s standards, policies and procedures that guided the preservation and enhancement of the mature urban tree population. This included, as a focus, a recommended policy and procedure for dealing with healthy mature trees whose roots had been blamed for foundation damage.
The five (5) objectives of the study were:
1. To review and assess the City’s existing standards, policies and procedures with respect to the maintenance and preservation of the City’s mature urban trees and the City’s policy for the removal of healthy mature trees where their roots were believed to be causing foundation damage;
2. To obtain the current knowledge and experience available with respect to the maintenance and preservation of mature urban trees and on the various methods to control and/or reduce foundation damage believed to be caused by the roots of mature trees;
3. To undertake public consultation to obtain public opinion on the City’s current policies and procedures for maintaining and preserving its mature trees;
4. To identify and evaluate various options for improving on the City’s standards, policies and procedures with respect to the maintenance and preservation of the City’s mature urban trees and to report on the financial impacts of each option; and,
5. To develop a strategy and action plan to assist the City in reducing the number of healthy mature trees removed as a result of causing damage to adjacent foundations and for enhancing and preserving its mature urban tree population.
Two principles became apparent during the review that guided the development of the City’s policy and actions on this issue.
1. Tree Preservation: The preservation and health of individual trees should be promoted, as trees were a highly valued natural resource that brings many benefits to the City of Ottawa.
2. Risk Reduction: The risk of deep soil shrinkage that may cause foundation damage should be examined by addressing all of the contributing factors, not just the tree.
Until 1997, when a claim was received the engineering studies often only confirmed that damage had occurred, that a mature tree existed in close proximity to the building, and that therefore the tree was the likely cause. Such studies were rarely multi-disciplinary in nature, and usually gave little or no attention to the other factors that contribute to deep soil shrinkage. As a result of the study a more detailed 4 Phase Assessment process was developed that is still used by Forestry Services staff today.
Upon receipt of a complaint the city conducts a site inspection and an information brochure (Document 1) is provided that outlines the process on how the city manages these claims. The city orders a legal survey that determines the ownership of the tree and should the tree be privately owned this information is provided to the resident at no cost and the file is closed. Should the tree be owned by the city the 4 Phase Assessment process is initiated as outlined below.
|
Phase
1 Confirmation of Sensitive Marine Clay |
Phase 2 First Level Screening |
Phase 3 Detailed Site Review |
Phase 4 Best Management Practices |
Decision Retain/Remove the Tree |
Since soil conditions can vary from site to site, before reviewing any aspects of a claim or a complaint it must first be determined that sensitive marine clay soils of appropriate type and composition, and in sufficient quantity and depth, underlie the foundation. This can only be accomplished by an on-site investigation using subsurface testing performed by a qualified soils expert who will determine the exact location of the sensitive marine clay layers. The clay samples should then be analysed to identify whether the clay soils are a possible problem. Upon confirmation of the presence of sensitive marine clay soils on the property, an analysis must then be done to determine if the tree could have contributed to the cause of the settlement problems. Simply identifying the presence of sensitive marine clay soils is not sufficient to confirm that the tree is the sole cause of the damage.
As a policy option, the City had to decide whether the soils information was to be provided at the cost of the complainant, or of the City. In view of the findings in the 1996 report, it was argued that there was sufficient scientific evidence to assert that even a mature tree could not affect the foundation of a building located on soils other than those composed of sensitive marine clay, whether by soil shrinkage or otherwise.
Therefore any policy requesting the complainant to provide a soil test before the City investigation could proceed would have to be based on scientific evidence specific to the site. If no soils report is provided the process is terminated. If the required report is provided staff proceed to Phase 2.
|
Phase 1 Confirmation of Sensitive Marine Clay |
Phase 2 First Level Screening |
Phase 3 Detailed Site Review |
Phase 4 Best Management Practices |
Decision Retain/Remove the Tree |
Once the presence of sensitive marine clay soils on the site has been confirmed, the City responds to the complaint by dispatching City staff to perform a site inspection. Trained Forestry staff conduct the inspections and structural or geotechnical engineers assist the team.
The Phase 2 review is designed to seek information on six key indicators, the results of which could absolve any responsibility of the City and would terminate the investigation. In other words, if any of the indicators are positive, the City street tree should not be considered as a probable cause of the claim against it. The six indicators are:
1. Has there been deep excavation in the vicinity? If so then the excavation could be the cause of the damage.
2. Are the tree(s) considered to be low water demand? Trees that were either low water demand or too small for their root systems to extend beneath the footings could not be considered as contributing factors.
3. Does the site have a large amount of vegetation? If there is significant vegetation on private property, at the choice of the property owner, it is not considered reasonable to attribute deep soil shrinkage solely to the city tree. Under this situation, the tree is not considered as a probable cause of the damage.
4. Is the front yard surface highly impermeable? One of the documented contributing factors to deep soil shrinkage is the inability of rainwater to infiltrate into the soil and subsequently recharge the ground water. When front yards in the vicinity of street trees are rendered impermeable due to paving or soil compaction, trees are denied an otherwise natural source of water. The hard landscaping of front yards is the choice of the homeowner, and the homeowner should bear the environmental consequences of this choice.
5. Is the Foundation sub-standard? A foundation that fails because it has fundamental imperfections, flaws or weaknesses is viewed as a probable cause of damage. If City staff has reason to believe that the foundation is sub-standard an independent engineering study would be required.
6. Is there evidence of a lack of proper maintenance or structural modifications to the building? Lack of maintenance and upkeep may have resulted in a detrimental effect on the overall structure. Structural modifications can lead to a redistribution of loads to the building’s footings that can lead to differential settlement. If City staff has reason to believe that the building has not been properly maintained, or has been modified, a detailed report on the status of the building from a licensed building inspector or a structural engineer may be appropriate.
In summary, if any of the above six indicators are considered positive, the investigation is terminated, and the complainant is notified of the reasons. If all answers are negative staff proceed to Phase 3.
|
Phase 1 Confirmation of Sensitive Marine Clay |
Phase 2 First Level Screening |
Phase
3 Detailed Site Review |
Phase 4 Best Management Practices |
Decision Retain/Remove the Tree |
If the investigation continues following the first-level screening, a detailed review of the site and the circumstances surrounding the claim is completed. The information required to complete this evaluation includes building and foundation construction details, the overall size of the site and the exact location, quantity and size of all vegetation on the site.
|
Phase 1 Confirmation of Sensitive Marine Clay |
Phase 2 First Level Screening |
Phase 3 Detailed Site Review |
Phase 4 Best Management Practices |
Decision Retain/Remove the Tree |
It is clear that there were several contributing factors to deep soil shrinkage, of which the City street tree is only one. In the past, the street tree had been considered to be the easiest factor to deal with as it could simply be removed. This kind of approach may well have ignored other, perhaps more significant factors, which were actually responsible for the damage. However, individual mature trees have many social and environmental values, and efforts should be taken to preserve them whenever possible and practical. Therefore, Phase 4 of the assessment process focuses on determining what options exist to mitigate a damage problem, without removing the tree.
For example, in a situation where the Phase 3 site review determines that on-site drainage could be changed easily to divert rainwater toward the tree, that excessive ground vegetation could be removed or that some impermeable surfaces could be removed to improve ground water recharge, the City could recommend that the landowner make these on-site changes rather than removing the city tree. In other cases, pruning or watering the tree or even altering the foundation itself could be considered as viable mitigation measures.
Perhaps the simplest solution was the most obvious, to water the tree. In areas where sufficient greenspace exists to allow percolation of water staff have consistently made the recommendation to residents that they ensure that their city tree receive an adequate amount of supplemental water. Since a mature tree can uptake in excess of 150 litres of water on a warm summer day the replacement of this water is a key element in ensuring not just the continued good health of the tree but also the avoidance of any dewatering concerns.
More recently the installation of watering tubes, to facilitate the provision of supplemental water to the level of the foundation footings and directly to the root zone, or root barriers that block future growth of tree roots, has also been considered. All these practices are intended to retain the tree and preserve the urban canopy while reducing the risk of future damage for the homeowner, as well as liability to the city.
|
Phase 1 Confirmation of Sensitive Marine Clay |
Phase 2 First Level Screening |
Phase 3 Detailed Site Review |
Phase 4 Best Management Practices |
Decision Retain/Remove the Tree |
Decision:
Retain/Remove the Tree
As outlined in the table below, since 1990, a total of 373 inquiries have been received through the office of the City Forester where residents have indicated that a city-owned tree is responsible for their foundation damage. Of the total enquiries 69% (256) provided the required soils reports and proceeded through the 4 Phase Assessment process. The assessments at these sites resulted in the removal of 381 trees (64%) and the retention of 219 trees (36%) due to their small size or through the implementation of mitigation measures. Forestry Services staff monitor all retained trees annually.
|
WARD |
INQUIRIES |
ASSESSMENTS |
TREES REMOVED |
TREES RETAINED |
|
1 |
2 |
2 |
2 |
0 |
|
2 |
1 |
1 |
3 |
3 |
|
3 |
1 |
0 |
0 |
1 |
|
4 |
0 |
n/a |
n/a |
n/a |
|
5 |
0 |
n/a |
n/a |
n/a |
|
6 |
0 |
n/a |
n/a |
n/a |
|
7 |
11 |
7 |
9 |
5 |
|
8 |
31 |
12 |
49 |
36 |
|
9 |
3 |
1 |
0 |
2 |
|
10 |
107 |
87 |
166 |
45 |
|
11 |
2 |
1 |
2 |
6 |
|
12 |
27 |
22 |
32 |
12 |
|
13 |
8 |
4 |
0 |
12 |
|
14 |
49 |
28 |
27 |
16 |
|
15 |
49 |
36 |
44 |
27 |
|
16 |
39 |
29 |
46 |
18 |
|
17 |
23 |
12 |
9 |
15 |
|
18 |
20 |
14 |
21 |
21 |
|
19 |
0 |
n/a |
n/a |
n/a |
|
20 |
0 |
n/a |
n/a |
n/a |
|
21 |
0 |
n/a |
n/a |
n/a |
|
TOTALS |
373 |
256 |
381 |
219 |
DISCUSSION
New City of Ottawa Trees and
Foundation Protocol
In 2001, Ottawa experienced its 6th driest and 3rd hottest year since 1960. Towards the latter part of that year, and continuing until 2003, the City received 88 new foundation inquiries. As a result the City undertook a detailed review in 2002/03 to determine why there was such a large number of foundation damage problems. A geotechnical specialist, Sarafinchin and Associates, was retained to review and analyze existing data and records and to undertake in-depth soil studies in the affected areas.
The objectives of this study were to determine the causes of damage to existing foundations and to provide geotechnical recommendations that would minimize further settlement. An extensive review of existing geo-technical information, as well as studies on the relationship of trees and foundations in areas of sensitive marine clay, was conducted. In addition, boreholes were drilled in some areas where sensitive marine clay was known to exist to start the gathering of information through long-term monitoring.
The 2003 Sarafinchin Report identified a wide range of factors that could individually, or in combination, contribute to settlement of sensitive marine clay and be the cause of differential settlement of foundations.
The report concluded that there were both explicit contributing factors (factors that caused the direct lowering of the groundwater levels and soil dehydration) and implicit contributing factors (factors triggered by the lowering of the groundwater levels and soil dehydration). It went on to report that vegetation, including tree species with varying water demand, location and maintenance could contribute to differential settlement, however, in most cases it was a combination of factors that resulted in the damages.
The factors that were identified included:
· Tree species (high vs. low water demand)
· Zone of influence of the root system (both depth and distance)
· Distance from the building or structure
· Size of the tree (Leaf area and transpiration rate)
· Degree of sensitivity of marine clay soils
· Stratification of soils and backfill adjacent to foundations
· The percentage of hard surface between the tree and the foundation
· Location of utility/sewer/water service trenches (tree roots follow the path of least resistance)
· Poor groundwater recharge due to low on-site infiltration and retention
· Water table fluctuations both seasonally and over time
· Hard surfaces draining surface water prior to recharge
· Municipal services lowering water tables (water or sewer trenches drain free water in the absence of clay dykes)
· Water movement (capillary action) of soil going from wetter to drier
· Consolidation due to Climate Change (global warming drought effects)
(2001 was the 6th driest and 3rd hottest and 2002 was the 3rd driest and 8th hottest year in Ottawa since 1960)
· Frost heaving
· Differential temperature gradients on buildings (i.e. southern exposure)
· Size and spread of the footings
· Failure to use piles or pillars
· Poor construction practices and materials
· Building renovations and changes to load distributions
· Different elevations of building footings
· Heavy backfill materials adjacent to footings
· Building designs that do not recognize Climate Change
· Reduction of tree trimming programs that reduce leaf area and water uptake
· Lack of supplemental watering of trees in times of drought
Although all these factors had been previously known, the Sarafinchin Report confirmed a comprehensive site evaluation is required that starts with identification of the degree of sensitivity of the site’s marine clay soils. The specific contribution of the tree can then be reviewed along with the site’s other elements through the detailed assessment process.
The sensitivity of the clay to moisture depletion is a key factor in determining the role of the tree. Properties of soil such as the field capacity (how tightly water is “bound” to the soil and in some cases unavailable to tree roots), the plastic limit (a measure of the point where the soil maximizes its cohesive (sticky) properties) and the liquid limit (a measure of the point where the soil minimizes its cohesiveness and becomes a liquid) must all be known. By doing so it is possible to identify, or even exclude, the tree as a contributing factor.
Another key factor that has been previously unknown is consolidation, or soil shrinkage, due to climate change. As soil dries out it shrinks and this is accelerated during hot dry periods. Although difficult to quantify, climate change has been the focus of the Adaptation and Impacts Research Group (AIRG) of the Meteorological Service of Canada at Environment Canada. Since the 1890’s global temperatures have shown an upward trend and the 1990s have been recorded as the warmest decade in recent history. The work being undertaken by Environment Canada indicates that the Ottawa area is getting warmer, that drought conditions are on the rise and that average conditions are no longer average. This coincides with the increase in foundation damage concerns during 2001 and 2002.
As the effects of climate change become better known, it may be determined that the main cause of soil shrinkage results from consolidation and this could mean that the removal of a tree is less likely to reverse the problem. In fact, the continued presence of the tree may actually assist with water retention on the site due to the cooling effect of the canopy of the tree. The challenge is the quantification of the role of the tree in comparison to all the other factors.
It is interesting to note that the Plasticity Index, as a measure of soil volume potential, is also included in Great Britain’s National House-Building Council’s Technical Standards. The Standards state that the interaction between trees, soils and buildings is dependent upon many factors and is inherently complex. The relationship becomes less predictable as factors combine to produce extreme conditions and the Standards have been recently amended to require consideration be given to the potential effects of climate change. The proposed changes in the City’s revised geotechnical requirements are reflective of these NHBC standards and have been reviewed with local geotechnical experts.
The existing 4 Phase assessment process continues to be an appropriate and effective tool to evaluate the site; however, Document 3 shows the revised geotechnical information that is required to more precisely quantify the role of the tree. These revised geotechnical requirements represent the most significant change in the revised 4 Phase Assessment process and will be used during Phase 1 of the review. They will provide staff with more information related to the properties of the soil and will facilitate the evaluation of the specific role of the tree in the dewatering process.
Current costs for a standard geo-technical report at a private residence range from between $500-$1,000. Additional costs for the wider range of soil tests are estimated to add approximately $200-$500 to the average report.
However it must be noted that a responsible homeowner should confirm the geo-technical information for their property prior to actioning any building repairs in order to avoid future problems. Since this is information they already require for their own needs, the City’s additional requirements are considered incremental.
The revised geo-technical requirements are seen as being the most appropriate mechanism to quantify the role of the tree as a contributing factor and to even determine if it can be eliminated as a contributor altogether. However both tree and geo-technical experts warn that the nature of soil, the fluctuation of underground water tables and the very nature of roots by being underground and hidden will continue to make the tree an ongoing target.
City
of Montréal Program
As mentioned earlier, the City of Montréal also has areas of sensitive marine clay and has a process in place to address claims received regarding city trees causing foundation damage. The evaluation process, with slight variations, is essentially the same as the City of Ottawa’s. However since the early 1990’s only 15 claims have been received through their Risk Management office that have alleged foundation damage attributable to city trees.
The main reason for this appears to be the existence of a joint municipal-provincial program called Renovation Québec. Following an extended period of dry weather in 1983, foundation problems were so extensive and severe in the province that the Quebec National Assembly adopted a special law allowing owners to receive partial compensation for damage to their buildings. Residents in participating municipalities who experience foundation cracking caused by the drying of clay soil are eligible under the program for financial assistance if they meet the following 4 conditions:
1. The work must cover the residential portion of the building only;
2. The work must be aimed at restoring the foundations and correcting other elements of the building damaged by foundation shifts;
3. The work must include the installation of piles to stabilize the foundation; and
4. A contractor holding the appropriate licences from the Régie du bâtiment du Québec must carry out the work.
The resident is responsible for paying one third of the eligible costs and the municipality is responsible for determining the level of assistance and the terms and conditions for calculating the grant which may cover up to two-thirds of the eligible work.
Habiter Montréal is the office that coordinates this on behalf of the City of Montréal. The applicant must be the owner of a residential building with a municipal assessment not exceeding $200,000, it must have been their principal residence for at least 6 months and the structure must be over 5 years old. The building must have at least one unstable foundation wall with a subsidence of more than 2.5cm caused by soil settlement regardless of soil type. Owners are granted a lump sum payment based on the work required and the maximum amount of financial assistance is $15,000 per building. Residents are responsible for an administration fee of $350 and they must supply the city with plans and specifications approved by a qualified engineer.
The installation of piles (vertically placed steel beams which support the foundation on the underlying bedrock) mitigates the issue of tree roots since the building is then stabilized regardless of additional soil shrinkage. City of Montréal staff are not aware of any instances where a request for tree removal has been received once the building has been repaired under this program.
For the period of 1996 to 2005 inclusive, this program has produced the following results:
|
A. Total Building Claims Processed |
3036 |
|
B. Total Value of Damages Claimed |
$28,000,000 |
|
C. Average Damage Claim |
$9,222 (B/A) |
|
D. Total Value of Grants Paid |
$9,400,000 |
|
E. City’s Portion of Grant |
$4,700,000 (50% of D) |
|
F. Average Grant Payment per Claim |
$3,096 (D/A) |
|
G. City’s Average Cost per Claim |
$1,548 (E/A) |
In total over the last 10 years, $3,096 has been paid per claim and the City incurred an average annual expense of $470,000.
The City of Montréal’s Legal staff are currently reviewing the research of Mr. Vincent Silvestri, a Professor at the École Polytechnique de Montréal. In his research Mr. Silvestri has been studying trees in clay soils since the 1980s and he contends that the city is under the influence of a rain deficit, which has resulted in decreased water in the clay soils, and over many years cracks have developed in the soil. Tree roots by nature are adventitious, following a path of least resistance and it is theorized that this may result in roots growing through these cracks in the soil and eventually extending under foundations. Montreal Legal staff are considering the use of an argument that this climate change factor is responsible for damages rather than the tree.
The City of Ottawa 4 Phase assessment process was developed as a way to ensure that any soil reports were as multi-disciplinary as possible and to evaluate all the factors that contribute to deep soil shrinkage. City staff have also met with Dr. Andy Kenney, Professor of Urban Forestry Studies; Dr. Vic Timmer, Soil Scientist; David Fayle, Registered Professional Forester (RPF) and Dr. Tony Price, a Soil Hydrologist. These tree specialists at the University of Toronto were consulted on tree physiology as it relates to the uptake of water through a tree’s root system.
Tree roots remove water from the soil by osmosis. Water is always present but at different concentrations in the soil and it surrounds the soil particles that make up the sands, silts and clays. At the molecular level there are positive and negative charges that allow water to flow either into or out of the plant. This is what is responsible for grass burning when a fertilizer spill occurs because water flows out of the plant into the surrounding soil.
These tree specialists confirmed that the City of Ottawa process was a reasonable and suitable process to follow. It was also, the opinion of the University of Toronto experts that a comparison of the potential for soil to hold water against failure rates of sensitive marine clays could become another evaluation criterion in Ottawa’s 4 Phase assessment process. This soil water classification has therefore been included in the revised geotechnical requirements.
Best
Management Practices and Mitigation Measures
To minimize the likelihood of damage to building foundations from adjacent trees in areas of sensitive marine clay and/or to mitigate the effects of tree roots at locations where foundation damages have occurred, a number of Best Management practices are recommended.
An effective tree maintenance program includes regular monitoring and the necessary follow-up care to detect problems and correct them before failure occurs. By providing annual inspections of mature trees, the City can prevent or reduce the severity of future foundation concerns as well as provide a baseline of information relative to the effect the tree is having on its immediate environment.
Trimming is one of the most common tree maintenance activities and, following standard arboricultural practice, is usually performed every 7-10 years. Trimming is often desirable or necessary to remove dead, diseased, or insect infested branches, and to improve tree structure, enhance vigor or maintain safety. Removing foliage can reduce photosynthesis and the overall growth of the tree. Trimming, if properly performed by a professional arborist, can therefore affect the overall water demand of a tree while still providing an aesthetic contribution to the urban landscape.
The City’s current Tree Maintenance Program encompasses all activities related to the care of city-owned trees within the urban and rural forests. In 2004, Forestry Services responded to over 8000 service requests for maintenance to trees (up from 5000 calls in 2001). The current Forestry budget does not provide for any planned routine pruning. Funds exist to respond to maintenance requests, to rectify sightline and health and safety concerns and to respond to emergency calls such as storm damage complaints.
This equates to approximately 3% of the city’s estimated 250,000 street trees being trimmed annually but entire streetscapes are seldom trimmed and life cycle trimming in accordance with the previously mentioned arboricultural standard on City streets known to have sensitive marine clay is beyond existing resource levels. It is estimated that to achieve this standard for trees growing in such areas 2 additional work crews at an additional annual cost of $400,000 would be required.
Root pruning is undertaken when the roots of a tree are severed prior to a disturbance such as road reconstruction. This mitigates damage that occurs due to soil compaction from heavy equipment. Since the amount of the root system that a tree can lose, and still remain healthy, is limited (approx. one third), and since this operation could destabilize a tree creating a safety hazard, it should only be undertaken by individuals with a sound understanding of tree physiology.
Another application with the potential to mitigate root growth is the use of commercial root barriers. Such a barrier diverts the roots in a downward direction into the sub soil, allowing the tree to grow naturally, while diverting potentially damaging roots away from foundations, sidewalks and curbs. These barriers have been known to extend the life of trees in urban settings by up to fifteen years.
Root barriers come in a variety of forms: pre-formed containers, plastic sheets, and molded modular panels. The installation of these products shows potential when implemented in conjunction with root pruning operations.
The potential for severing roots that could be contributing to foundation damage and installing root barriers is limited due to hard surfaces, small urban yards and underground utilities. However this treatment may be appropriate in certain circumstances. Costs for this operation are approximately $100 per linear metre.
Water is essential to the survival of all trees. In addition to its role in physiological processes, water provides the means for the transport of minerals and nutrients through the plant. Lack of water is potentially the single most important contributing factor to the decline and death of young trees.
The City issues watering bulletins in times of extended drought advising residents to water both newly planted, as well as established trees. This message is included in the City’s 2005 “Healthy Lawn, Trees & Gardens – Naturally” campaign. Messages are also included on the City’s website and in information brochures available to the public as shown in Document 4. The City will continue to issue special watering bulletins in times of drought requesting that residents assist the City by watering trees adjacent to their properties.
A more comprehensive approach to providing supplemental water is to provide artificially constructed watering pits or tubes, which also assist in containing the spread of a tree’s root system when placed when the tree is planted. By creating a substitute water source tree roots can stay at a pre-determined location rather than continuing their search through the soil where they could eventually encounter a foundation. Costs for boreholes are approximately $500 per tree and, like root barriers, their use is limited, however this treatment may be appropriate in certain circumstances. Given that the use of hydraulic excavation methods are more common, this may allow boreholes to be placed in closer proximity to existing utility services. When installed in proximity to larger trees, watering tubes allow direct access to the root system and facilitate the recharging of water to the level of the foundation footing.
An effective watering program continues to depend on the public doing its part since the city cannot action all the required watering in an affordable manner. Supplemental watering, when not provided by adjacent residents, can only be achieved by trucking water to the site. Supplemental watering is scheduled during the growing season from May to October but only for recently planted trees in parks, on arterial roadways and for trees surrounded by hard surfaces.
There are approximately 10,000 city owned trees growing in areas known to have sensitive marine clay. Of these, 3,000 are core area trees many of which are in either hard surfaces or in locations where there is limited greenspace available for the natural percolation of water. None of these trees are being watered through the city’s existing watering program and they would therefore have to be watered through an increase to the existing budget and, where possible, following the installation of water tubes.
It is estimated that to expand the watering program to provide supplemental water for trees along streets in the core area known to have sensitive marine clay and to install water tubes or root barriers as outlined in this report an additional $300,000 would be required.
The Sarafinchin Report raised concerns regarding tree planting in areas known to have sensitive marine clay and suggests that fast growing, high water demand trees should not be planted closer to a building than a distance equal to their height at physiological maturity.
Small sized trees such as flowering crabapples, serviceberry or Japanese lilacs must therefore be placed a minimum distance of 7.5m from any building when planting along road allowances. Newly planted trees must be a minimum of 2.5m from the curb and have a small sized canopy at maturity to allow sufficient space for snow and ice control purposes. Note that coniferous trees are the one exception to the height rule since they are rated as a low water demand tree species. However the spread of such trees at maturity (4m+) conflicts with the snow and ice control program and requires their placement 6.5m from the curb.
The trees that are regularly planted as part of the municipal tree-planting program includes moderate water demand species such as locust, linden and maples which all attain heights in excess of 7.5m. Such trees will not be planted in close proximity to buildings and will be replaced with tree species, aside from conifers, that only achieve heights at maturity that meet the recommendations in the Sarafinchin report.
Forestry Services has received requests from residents who have had foundations claims with the city where they have requested a replacement tree. Replacement trees will be planted in such circumstances, but only on the adjacent private property and after a waiver acknowledging ownership of the tree and absolving the city of all future liability is received from the resident. If there is no waiver then no replacement tree will be planted.
As mentioned earlier in this report trees are important and distinctive features within our City. They provide many benefits to our daily lives and contribute to the social, cultural, environmental, economic and aesthetic well being of our community. At the same time the City has a responsibility to minimize the risks associated with trees, such as when a broken limb or a dead tree threatens the safety of the public or when there is the potential for liability to the city.
Considering the overall benefits to the community at large, in instances where trees are suspected of being a contributing factor in damages to adjacent building foundations, it is important that a tree’s role in the quality of life also be considered during the site evaluation. This is of particular importance when the removal of large trees is being considered in the core area where there are fewer such trees and where their successful replacement is difficult to achieve.
These
Social Factors include:
· City responsibility to effectively manage its liability and risk based upon claims that street trees have caused damage to private property;
· The social importance of significant trees on urban streetscapes (particularly in heritage conservation areas);
· Canada’s Kyoto Protocol commitments, which identify the role of urban trees in air quality, energy conservation and reducing greenhouse emissions.
To date such arguments have received only secondary consideration, with liability issues usually taking precedence. However municipalities have been placing greater emphasis on their green spaces and the importance of living in communities where these quality of life criteria are given a high degree of importance.
As mentioned earlier in this report, the former City of Ottawa conducted a study in 1997 that designated a Centretown Heritage Conservation District. It was felt at that time that guidelines were needed to ensure the preservation of the heritage buildings and the character of the streetscape in the area. Many of the residents in the area identified the preservation of street trees as their most important priority in re-establishing the heritage character of the district.
The City of Ottawa made commitments in the 2003 Official Plan and in Ottawa 2020 to protect mature trees, increase the canopy cover in the city to 30% and create liveable communities that maintain greenspace and the high quality of life that enhances the attractiveness of the city for business development.
In 2003, for the first time, the importance of urban forests was included in Canada’s National Forest Strategy with the inclusion of a section on urban forestry. This strategy, which runs until 2008, seeks to develop a common vision for urban forestry across Canada. In addition, the Tree Canada Foundation will soon be releasing a National Urban Forest Strategy. This document will identify urban forest priorities for discussion at all political levels and identify areas where new legislation is envisioned to protect Canada’s urban forests.
These social factors suggest that serious consideration for the retention of trees, even under circumstances where there may be liability concerns, is appropriate.
Such an approach suggests that a tree should be retained even after the 4 Phase Assessment process determines that the implementation of mitigation measures would not absolutely guarantee that future damage might occur. However the justification for retention would be based upon recognition of the significance of the City’s urban forest and its contribution to the quality of life in the City of Ottawa.
In such cases the City’s position would be that any effects caused by the tree would be mitigated through the placement of the tree on a life cycle monitoring program where the tree would be trimmed, root barriers or water tubes would be installed if site conditions allow and a pro-active watering program would be initiated.
Communication
Strategy
As this report has illustrated there are highly complex interactions that take place between trees and the soils in which they grow. This report has outlined many of the geotechnical, biological, engineering and climatic factors that all have an effect on the stability of buildings. Since these issues are interdisciplinary in nature much of the information is new, or isolated within one profession, and this has resulted in mixed information to area residents and the loss of trees that may have been avoidable.
A proactive communications strategy, directed at both the City’s residents (Document 2) and the various professional disciplines involved, would resolve misconceptions and allow a proactive exchange of information. Such a communication strategy would advise residents on the issues surrounding sensitive marine clay and how it interacts with buildings and structures, how all the various factors interact and how they can take steps themselves, such as watering their tree, to avoid future problems.
In addition to written information, the local Chapter of the Canadian Geotechnical Society has already had discussions with City staff on the possibility of having this topic form the basis of an open forum with their membership. Developing such an information exchange could also include experts from other cities where sensitive marine clay is known to exist, members of Ottawa’s development community as well as interested residents. It is anticipated that such a forum would allow the development of appropriate mitigation tools that would be known to, and recommended by, all stakeholders.
There are no reported Ontario Court decisions that deal with situations of trees depleting moisture in soil thereby resulting in structural damage to nearby buildings. This is likely because the sensitive marine clay that principally causes this situation is not prevalent throughout Ontario, but is limited to a narrow area in Eastern Ontario.
Court decisions in Quebec dealing with similar situations and somewhat analogous Court decisions in Ontario seem to indicate that the owner of a tree will only be liable for the damage that occurs after the initial notification of damage if the initial damage could not have been reasonably foreseen. This appears to indicate that so long as the City takes reasonable steps once notified of damage occurring, then there will be no liability of the City for the initial damage. Such reasonable steps would include the mitigation measures discussed earlier in this report and if such measures cannot properly prevent further damage, removal of the tree.
It must also be borne in mind that these types of claims are novel and the professional evidence relating to them is in its infancy and is still evolving. It could ultimately be determined that trees will not be considered to be the “cause” of the damage or the sole cause of the damage, but some or all of the other factors discussed in this report may be considered to be the cause of the damage. In such a circumstance, (still not proven to date), the City likely would not be liable for any damage caused by its trees.
CONSULTATION
Local Architectural and Conservation Advisory Committee (LACAC)
Ottawa Forests and Greenspace Advisory Committee (OFGAC)
Ottawa Geotechnical Society
FINANCIAL IMPLICATIONS
That a precommitment in the 2006 Forestry operating budget be established for:
· $400,000 for tree trimming
· $200,000 for supplemental watering
· $100,000 for infrastructure improvements
· Two vehicles capital costs of $25,000 each with combined annual operating expense of $16,000
· 2 FTE positions totaling $156,000 to implement the additional costs for tree maintenance activities that mitigate the effect of retained trees growing in proximity to buildings and structures.
This represents a total operating impact of $873,000 and a total tax increase of .10518% not including the funding of $50,000 for two vehicles.
SUPPORTING DOCUMENTATION
Document 1 Trees and Foundation 4 Phase Assessment Information Brochure
Document 2 Landowner Bulletin - Protecting Your Building Foundation from Damage Due to Soil Settlement in Areas of Sensitive Marine Clay.
Document 3 Geotechnical Report Requirements
Document 4 Tree Watering Brochure
DISPOSITION
Following Council approval, the Public Works and Services Department will implement the revised Trees and Foundation Strategy as outlined in this report including notification of Council’s decision to the Ottawa Geotechnical Society.
Trees are often considered to be the cause of
damage to building foundations, and as a result are sometimes removed. However, trees are not always the cause of
the problem.
Damage to building foundations is often a result of the behaviour of the soils under the foundation. Uneven soil shrinkage can lead to uneven settlement and foundation damage. The Ottawa area is dominated by sensitive marine clay, which has a high water content. When water is removed from sensitive marine clay, it shrinks and does not regain its original volume.
In the Ottawa area, shrinkage of sensitive marine clay is a major contributor to building foundation damage.
Factors that contribute to sensitive marine clay shrinkage include:
Studies
confirm that dependent upon site conditions, trees can have a contributing role
to the foundation damage however in most cases there are a combination of
factors working together that result in soil shrinkage and differential
settlement of foundations. All of these
factors must be considered when assessing foundation damages. It is not sufficient, nor accurate, to focus
only on the tree.
As a responsible corporation, the City of Ottawa is committed to protecting its natural environment and resource base. In an effort to preserve mature trees and be fair to its citizens, the City has instituted a four Phase assessment process to investigate the circumstances related to mature municipal trees that are suspected of contributing to damage to building foundations.
Phase 1
Confirm the Existence of Sensitive Marine Clay
Action: The complainant is responsible for
determining the existence of sensitive marine clay in proximity to the
foundation footings. Borehole
information must extend from surface level to 1.2 meters below the foundation
footing and be within 2 metres of the physical damage
to the foundation. More than one
borehole may be required to ensure a representative cross-section of all
damaged areas.
The claimant must provide the following information
to the City of Ottawa to confirm the presence of sensitive marine clay:
Measurements at the surface, 0.5m above the
footing, at the footing, and 0.5m and 1.2m below the footing are required.
The
certified geo-technical report must also include the following information:
If no soils report is provided or there is no
evidence of sensitive marine clay, the process will be terminated. If the required report indicates the presence
of sensitive marine clay, the assessment will proceed to Phase
2.
Phase 2
Apply First Level Screening
Action:
City of Ottawa representatives will conduct a site assessment to
determine the probability that the tree is a contributing factor to the
foundation damage. The assessment is based
on six key indicators of soil shrinkage.
The tree will not be considered as the probable cause
of damage and the process will be terminated if any of the following indicators
are present:
·
Deep trenching has occurred within
10 meters of the building foundation;
·
The tree has a low demand for water;
·
The site has a large amount of other
plants, such as shrubs and trees;
·
The front yard surface is highly
impermeable;
·
The foundation is found to be
sub-standard;
·
Evidence of a lack of proper maintenance or structural modifications to
the building.
If none of these indicators are present, the
assessment will proceed to Phase 3.
Phase 3
Detailed Site Review and Evaluation
Action: A detailed inspection of the site and the
circumstances of the case will be completed by the City of Ottawa. This includes an investigation into building
and foundation construction details, site drainage patterns, quantity of
vegetation versus the area of hard surface on the site, an assessment of the
size, health and species of the municipal tree that is considered to be the
problem, and a general assessment of activity that has occurred in the area
over the last few years.
Once the damage has been assessed and the
causes of the damage confirmed, the city will conduct Phase 4 of the process.
Phase 4
Prescribe Best Management Practices
Action: Due to the recognition of
the significance of the City’s urban forest and it’s
contribution to the quality of life in the City of Ottawa the City’s position
is that any effects attributable to the city-owned tree can be mitigated
through the placement of the tree on a life cycle monitoring program where the
tree will be trimmed, root barriers or water tubes will be installed if site
conditions allow and a pro-active watering program will be initiated.
The City of Ottawa will work with you to determine the best options to
mitigate any damage that might be attributable to the city-owned tree.
Landscape design measures that help to mitigate
the tree’s contribution to foundation damage:
·
Maximize surface water permeability
by increasing the green area;
·
Maximize use of rainfall by
directing storm water for use by the trees, keep area around trees permeable,
and redirect rood runoff towards green space and away from sewers;
·
Augment surface and groundwater
content by watering trees during droughts;
·
Reduce the water consumption of new
trees by planting species that have low water demands;
·
Monitoring and pruning the tree to
reduce its water demand;
·
Installing
watering tubes, to facilitate the provision of supplemental water to the level
of the foundation footings and placing the tree on a supplemental watering
program;
·
Installing
root barriers that block future growth of tree roots.
Note: There is no evidence that
tree removal eliminates the risk of future soil shrinkage and foundation
damage.
Did you know?
Just a few facts about tree roots…
·
Tree roots are usually located
within 60 cm of the soil surface;
·
Tree roots may span a diameter equal
to or greater than the height of the tree;
·
In urban settings, trees are most
often located within 3 to 7 meters of building foundations;
·
Tree roots generally grow along the
path of least resistance;
·
The only force exerted directly by a
tree root is that caused by its growth;
·
Larger trees demand more water than
do smaller trees;
·
Trees uptake water from the soil
through their root system;
·
The rate of tree growth slows during
summer months;
·
Some tree species require more water
than others.
Water Demands of Various Tree Species
|
High Elm Oak Poplar Willow Silver Maple Manitoba Maple Hawthorn |
Medium Cherry Ash Hawthorn Honey Locust Hornbeam Linden Other Maples (Sugar, Red) Cedar |
Low Beech Birch Mulberry Honey locust Fir Pine Spruce |
For more information please contact:
City of Ottawa
Call Centre at 580-2400
e-mail to
forestry@ottawa.ca
Document 2
LANDOWNER
BULLETIN
Protecting Your Building Foundation
from Damage Due to Soil Settlement in Areas of Sensitive Marine Clay.
The Issue…
Cracked
building foundations can lead to costly repairs. As a building owner, you can take steps to
prevent certain types of cracking. Most often, cracks in foundations walls and floors develop
because they have been subject to a stress for which they were not
designed. One such stress can result
from the shrinkage of sensitive marine clay soils.
Parts
of the City of Ottawa are situated on clay soils deposited at the time of the
last ice age when receding glaciers formed the Champlain Sea. As a result, many buildings have been
constructed on fine-grained sensitive marine clays (commonly referred to as leda clays) that shrink according
to their water content. The tiny
particles that form these clays are arranged like a house of cards, which are
held together when the spaces between them are filled with water. When there is a severe loss of water they
collapse, leading to a reduction of soil volume and soil shrinkage and possibly
causing a building foundation to settle unevenly and to crack.
Although
this is a rather uncommon occurrence, you may want to consider some
precautionary steps that can be taken to protect your building from the risk of
foundation damage due to sensitive marine clay soil shrinkage.
Assessing the Risk…
First
of all, you should determine if your building is at risk. The most important factor is of course the
presence of sensitive marine clay soils under your foundation. Unfortunately, there is no readily available
source of information to determine the presence or extent of sensitive marine
clay soils on individual properties. A
soil test by a qualified geotechnical company to confirm the presence of
sensitive marine clay on your property may be appropriate.
Next,
you should consider the key factors that can combine to increase the risk of
soil shrinkage. If the any of the
following five indicators apply to your property, you may be at risk:
1. Has there been deep excavation in
the vicinity of the foundation? If so
then the excavation could have contributed to the lowering of the water table,
which could be a cause for potential damage.
2. Does the site have a large amount of
vegetation? If there is significant
vegetation on the property they could be contributing to soil shrinkage.
3. Is the front yard surface highly
impermeable? One of the documented
contributing factors to deep soil shrinkage is the inability of rainwater to
infiltrate into the soil and subsequently recharge the surface ground
water. When front yards in the vicinity
of street trees are rendered impermeable due to paving or soil compaction,
trees are denied an otherwise natural source of water.
4. Is the Foundation sub-standard? A foundation that fails because it has
fundamental imperfections, flaws or weaknesses could be the cause of
damage.
5. Has your building been properly
maintained or have you recently made modifications to the structure? Lack of maintenance and upkeep may have
resulted in a detrimental effect on the overall structure. Structural modifications can lead to a
redistribution of loads to the building’s footings that can lead to
differential settlement. If so, an inspection
on the status of the building from a licensed building inspector or a
structural engineer may be appropriate.
Another key factor that has been previously unknown is consolidation, or soil shrinkage, due to climate change. As soil dries out it shrinks and this is accelerated during hot dry periods. As the effects of climate change become better known it may be determined that the main cause of soil shrinkage results from consolidation and this could mean that the removal of a site’s vegetation is less likely to reverse the problem. In fact the continued presence of a tree may actually assist with water retention on the site due to the cooling effect of the canopy of the tree.
If you believe your foundation to be at risk, you can take some simple precautions. During very dry and hot summers, you may consider regularly watering tress and other vegetation growing in proximity to your building. Refer to the City of Ottawa’s brochure entitled Watering Your Trees. Since all vegetation takes its water through the root system watering will replenish the ground water. You may also consider modifications to your landscaping with the objective of improving water permeability. As well, if you retain a geotechnical company to review your property, it will be able to provide you with more specific recommendations.
For more information on maintaining your home, visit your local library and/or the following web sites:
www.cmhc.ca/en/burema/gesein/abhose ce45.cfm – The CMHC Guide to Trees and Foundations in Clay
GEOTECHNICAL REPORT REQUIREMENTS
A geo-technical report is usually as the first step in
determining the most cost effective method to action any building repairs
sustained as a result of differential settlement of foundations.
The
confirmation of the existence of sensitive marine clay, as well the specific
properties of the clay in proximity to the foundation footings, is
required to confirm that a tree(s) may be contributing factors to the
damage.
All
Borehole information must extend from surface level to 1.2 meters below foundation
footing and be within 2 meters of the physical damage to the
foundation. More than one borehole may be required to ensure a representative
cross-section of ALL damage locations.
lTo attempt to confirm the tree’s role as
a contributing factor the City of Ottawa requires the following information
from the claimant:
Note that measurements at the surface and at the footing, as well as at .5m above and below and 1.2 m below the footing are required.
The
certified geo-technical report must also include the following information:
Watering Your Tree
Newly Planted Trees
A
50 mm (2") diameter tree needs a minimum of 130 litres (30 gallons) of
water per week (this is approximately the amount held by six kitchen sinks).
The best way to ensure that the water applied to the tree actually penetrates
the rootball, is to maintain the earth ring or saucer that was placed when your
tree was planted. If you are able to water using a soaker hose or slow drip,
give the tree a minimum of two hour's worth of water. For just over 50¢ per month
in water, you can ensure the survival of your new tree. During a week when it
rains two or more days, watering is not necessary. Watering should continue
until the first frost, usually around the beginning of October. At that time,
one final soaking should get your tree through the winter. The most common
reason for poor survival of new trees is inconsistent watering. If you plan to
take holidays away from home, please ask a neighbor to water while you are
gone. Regular watering will greatly improve the tree's health and appearance
during this critical establishment period.
Mature Trees
A
tree that has been planted for 15 years or more may appear to be able to fend
for itself under any condition. At this point in a tree's development, its root
system has spread out away from the trunk and can reach water and nutrients
even when surface conditions appear very dry. There is, however, a critical
point during extended drought periods where the remaining soil moisture is so
tightly held to soil particles that roots cannot use it. Trees of all ages
suffer when extended drought conditions are experienced. As with humans, stress
can make an otherwise healthy tree more susceptible to pathogens looking for
weakness. Extra water to all trees during a drought, regardless of age, can
keep defenses strong against invading insects or disease. Let the garden hose
run at a trickle for several hours or if possible, overnight. Water stress
happens very quickly for trees planted in difficult areas such as sidewalks,
patios, or raised lawns where water naturally drains away. Trees in these
situations should be helped with an overnight soaking three times a week
through the hottest part of the summer. For under $2.00 per month you can give
your tree approximately 550 litres (120 gallons) of water.
Trees Near Foundations
During
very dry weather, soil particles will "lock up" water molecules at a
threshold level. This is especially true of clay soil particles, which have
very strong electrical charges and therefore, hold water very tightly. This is
the beginning of a battle between soil particles and roots for water uptake.
Clay soils will actually shrink in volume due to water loss, which reduces the
soil's capacity to support adjacent structures such as foundations. Foundation
damage from unstable clay soils may be avoided by ensuring that trees close to
foundations are always well irrigated. Again, the overnight trickle from a
garden hose will help maintain the water balance necessary to keep clay based
soils stable. This should happen on a three times per week basis for as long as
drought conditions persist. For more information about watering your tree or to
obtain the City of Ottawa's Trees and Foundation Damage brochure, please
contact the City at 580-2400.