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5. REVISIONS TO THE SLOPE STABILITY
GUIDELINES FOR DEVELOPMENT APPLICATIONS IN THE CITY OF OTTAWA RÉVISION DES LIGNES DIRECTRICES SUR LA STABILITÉ DES
PENTES RELATIVEMENT AUX DEMANDES D'AMÉNAGEMENT À OTTAWA |
Committee
recommendation
That Council approve the revisions to the Slope Stability Guidelines for Development Applications in the City of Ottawa to correct the description of organic soils in Section 2.5, and to update Section 5.6 to reflect changes in the Ontario Building Code as detailed in Documents 1.
Recommandation DU Comité
Que le Conseil approuve
les révisions aux Lignes directrices sur la stabilité des pentes
relativement aux demandes d’aménagement à Ottawa, afin de corriger la
description des sols organiques contenue à la section 2.5 et de mettre à jour
la section 5.6 pour tenir compte des modifications apportées au Code du
bâtiment de l’Ontario qui est énoncée dans le document 1.
Documentation
Deputy City Manager's report, Infrastructure Services and Community Sustainability, dated 6 January 2012 (ACS2012-ICS-PGM-0030).
Report
to/Rapport au :
Comité de l'urbanisme
and Council / et au Conseil
6 January 2012 / le 6 janvier 2012
Submitted by/Soumis
par : Nancy Schepers, Deputy City
Manager, Directrice
municipale adjointe, Infrastructure
Services and Community Sustainability, Services d'infrastructure et Viabilité des
collectivités
Contact Person/Personne-ressource : Richard Kilstrom,
Manager/Gestionnaire,
Policy Development
and Urban Design/Élaboration de la politique et conception urbaine, Planning
and Growth Management/Urbanisme et Gestion de la croissance Élaboration de la
politique et conception urbaine
(613) 580-2424
x22653, Richard.Kilstrom@ottawa.ca
REPORT RECOMMENDATION
That Planning Committee recommend Council approve the revisions to the Slope Stability Guidelines for Development Applications in the City of Ottawa to correct the description of organic soils in Section 2.5, and to update Section 5.6 to reflect changes in the Ontario Building Code as detailed in Documents 1.
RECOMMANDATION DU
RAPPORT
Que le Comité de l’urbanisme recommande au Conseil d’approuver les
révisions aux Lignes directrices sur la stabilité des pentes relativement aux
demandes d’aménagement à Ottawa, afin de corriger la description des sols
organiques contenue à la section 2.5 et de mettre à jour la section 5.6 pour
tenir compte des modifications apportées au Code du bâtiment de l’Ontario qui
est énoncée dans le document 1.
BACKGROUND
On November 24, 2004, City Council approved the Slope Stability Guidelines for Development Applications in the City of Ottawa.
The Guidelines provide the development industry with directions for the preparation of slope stability assessment reports when they are required as part of the development review process. The Guidelines provide basic background information on the soils and geology of Ottawa, as well as direction on various technical analyses and calculations to be undertaken during a slope stability assessment.
During one of the Ontario Municipal Board (OMB) hearings held on the appeals to Official Plan Amendment No. 76 (OPA 76), a technical issue was raised regarding the Guidelines’ characterization of organic soils. The Guidelines were not before the Board, but City staff agreed to undertake a review of the wording in Section 2.5 (Organic Soils) and to revise this section as needed to ensure its technical accuracy.
Staff also subsequently identified the need to revise Section 5.6 of the Guidelines to reflect changes in the National Building Code of Canada and in the Ontario Building Code regarding the seismic coefficient and other factors used in assessing seismic conditions and liquefaction potential. The need for revision of Section 5.6 came as a result of discrepancies between the Guidelines and the respective Building Codes being raised as an issue during a recent Ontario Municipal Board hearing over a plan of subdivision.
DISCUSSION
The issue of organic soils was raised during the Ontario Municipal Board hearing into the City’s policies for the protection of endangered and threatened species, which was held in December 2010. The appellant’s expert witness, a soil scientist, expressed concern with the description of organic soils in the Guidelines, arguing that it was not consistent with accepted scientific definitions, and should not be used as a basis for mapping such soils in the Official Plan. In fact, the organic soils mapping in the Official Plan (Schedule K) is not based on the description in the Guidelines but is rather taken from mapping by the Ontario Institute of Pedology. There is, therefore, no issue with regards to the Official Plan’s mapping of organic soils.
However, City staff did acknowledge the witness’ comments regarding the accuracy of the Guidelines’ description of organic soils and agreed to undertake a review of Section 2.5 to address the concerns raised. This undertaking was reported to the Planning Committee and the Agriculture and Rural Affairs Committee in January 2011 in the staff report ACS2011-CMR-LEG-0003. Council received this report on February 9, 2011 and directed staff to review and report back to Planning Committee and Council on the definition of Organic Soils in the Slope Stability Guidelines in order to clarify the definition and its role in the context of the Guidelines.
Staff subsequently reviewed the content of Section 2.5 in the Guidelines. Section 2 of the Guidelines provides an overview of Ottawa’s geology, and includes subsections with additional detail on several types of geological material present in the city. Section 2.5 is one such subsection, addressing what are termed “organic soils” within this document. From a soil science perspective, “organic soils” must meet specific criteria with respect to the percentage and depth of organic matter in the soil. The Guidelines’ description of “organic soils” includes alluvium and marl, which do not meet the scientific definition of an organic soil. The Guidelines’ use of this term is therefore overly broad and not technically correct.
A review of this section of the Guidelines was conducted, and the new text proposed in Document 1 is recommended to clarify that the Guidelines refer to soils with organic content, rather than organic soils. The proposed new text is provided in Document 1 with the currently approved text for comparison.
During the course of staff’s review of the Guidelines, an additional issue was noted regarding Section 5.6 of the Guidelines (Seismic Conditions and Seismic Liquefaction). The information in the Guidelines is no longer consistent with the requirements of the current National Building Code of Canada (NBCC) and the Ontario Building Code (OBC) with respect to the seismic coefficient and other factors used in assessing seismic conditions and liquefaction potential of soils. The information in the Guidelines was taken from previous versions of the NBCC and OBC, and had not yet been updated to reflect the changes to those documents in 2005 and 2006, respectively. Although in most cases the new standards are already being applied by consultants in accordance with the requirements of the respective Building Codes, there was a recent Ontario Municipal Board hearing held over a plan of subdivision where the appellant’s consultant argued that the requirements of the Guidelines should take precedence. In order to resolve this issue, a review of Section 5.6 was conducted and changes are recommended to ensure the Guidelines reflect the most current Code standards. The proposed changes are attached as Document 2.
RURAL IMPLICATIONS
These Guidelines apply City-wide. The changes to Section 2.5 do not affect how
slope stability assessments are to be performed, and the changes to Section 5.6
reflect current legal requirements for such assessments.
CONSULTATION
The proposed changes to Section 2.5 were circulated to the appellants who raised the issue with the organic soils description during the OMB hearing on OPA 76. They have reviewed and approved the revised wording.
This change is recommended in order for the Guidelines to reflect consistency with applicable legislation
RISK MANAGEMENT IMPLICATIONS
There are no risk implications.
FINANCIAL IMPLICATIONS
There are no direct financial implications.
ACCESSIBILITY IMPACT
There are no accessibility implications.
ENVIRONMENTAL IMPLICATIONS
The changes to Section 5.6 may result in greater setback distances being applied to areas with sensitive soils. This should reduce the potential for environmental impacts and may also contribute to the protection of natural features such as significant valleylands and watercourses.
N/A
These changes make the Guidelines more accurate and consistent with federal and provincial standards, and will also reduce the risk of environmental impacts when applied, which supports the City’s goals in the fields of service excellence and environmental stewardship.
N/A
SUPPORTING DOCUMENTATION
Document 1 – Proposed changes to the Slope Stability Guidelines
DISPOSITION
The Business Support and Evaluation Unit of the Planning and Growth Management Department will revise the Slope Stability Guidelines for Development Applications in the City of Ottawa to incorporate the new text.
CURRENT TEXT OF SECTION 2.5:
2.5 Organic
Soils
Organic soils are generally formed
by the decomposition of vegetation, in relatively recent times, forming soils
that may consist partly or almost entirely of organic matter (rather than
mineral particles). Common types include topsoil, peat, and alluvium. Topsoil
is generally a mineral soil (such as sand or clay) with up to about 10 percent
organic matter; it is the darker upper-most soil (usually less than about 0.3
metres thick) found in most locations. Peat is generally found in “swampy” or
“marshy” conditions and is formed by the accumulation and decomposition of
organic matter to form a material that is almost entirely organic (i.e., no
mineral soil). Alluvium is the name of soils (generally silt and sand)
deposited as a dark silty “mud” along river and creek banks. A fourth organic
soil type is marl, which is often present beneath peat deposits, is white or
grey in colour, feels much like a soft wet clay, but is weaker and often
behaves “jello-like” when shaken.
TO
BE REPLACED WITH PROPOSED NEW TEXT:
2.5
Soils with Organic Content
The
decomposition of vegetation, in relatively recent times, forms soils that may
consist partly or almost entirely of organic matter (rather than only mineral
particles). Common types include topsoil, peat, and alluvium. Topsoil is
generally a mineral soil (such as sand or clay) with up to about 10 percent
organic matter; it is the darker upper-most soil (usually less than about 0.3
metres thick) found in most undeveloped areas. Peat is generally found in
“swampy” or “marshy” conditions and is formed by the accumulation and
decomposition of organic matter to form a material that is almost entirely
organic (i.e., no mineral soil). Alluvium is the name of soils (generally silt
and sand) deposited as a dark silty “mud” along river and creek banks. A fourth
type is marl, which, though not typically highly organic in composition itself,
is commonly overlain by peat and is typically biologic in origin (containing
numerous shells). Marl is white or grey in colour, feels much like a soft
wet clay, but is weaker and often behaves “jello-like” when shaken.
(Note: highlighting indicates new
text or other changes, such as the start of a new paragraph; strikeout
indicates deletions).
5.6 Seismic
Conditions and Seismic Liquefaction
The ground shaking that
develops during an earthquake can generate both horizontal and vertical forces
within a slope and reductions in soil strength which may lead to instability.
It should be noted that the ground vibrations travelling up to the Earth’s
surface from depth travel in waves such that, except for low slopes, the ground
motion would not all be in the same direction simultaneously. In other words,
although part of the slope would be pushed outwards (encouraging failure),
other parts would be pushed inwards (resisting failure), as shown on
Figure 21. Vertical ground motions also occur, but are generally of lesser
magnitude and impact and are often not taken into consideration unless analyses
indicate that the slopes are marginally stable when the horizontal earthquake
forces alone are taken into consideration.
It must be recognised that a
realistic assessment of the magnitude of the seismic forces and their impacts
on the slope is complex and thus not practical for most projects. Simplified
methods exist and are commonly used in North America, but vary from
jurisdiction to jurisdiction. No specific methods or minimum criteria are
mandated for use on projects in the Ottawa area at this time.
One common method is to carry
out a separate simplified “seismic” slope stability analysis (often
referred to as a pseudo-static analysis) for which a horizontal force is
included, the value of which is related to a seismic coefficient, typically
identified as kh, as shown on Figure 22. The magnitude of kh
that should to be
used in the analyses is debatable, however a typical value of half of the peak (horizontal) ground acceleration (PGA) specified in
the appropriate for the design earthquake acceleration for the Ottawa area
(as described in the 1995 National Building code)
Code of Canada (NBCC), as referenced by the current edition of the Ontario Building Code (OBC),
is typically used. For example, for a
PGA of 0.42 (as specified in the 2005 NBCC and referenced by the 2006 OBC), a kh
of 0.21 would be used. would be kh = 0.1.
[Changed to new paragraph] However, the design earthquake accelerations described in the National
Building Code NBCC are
typically considered to be the “firm ground” values, representing shaking
forces at the surface of the bedrock or dense soil. Loose or soft soils
overlying the “firm ground” may,
and often do, result in
a significant increase (possibly 10 to 30 percent) in the earthquake
accelerations within these soils, which should be considered in selecting the
design value of kh.
The analysis for seismic
conditions would be carried out separately from the “static” analyses.
A minimum factor of safety of 1.1 is generally required (for example, by
the Los Angeles County government), although other jurisdictions require 1.0
(for example, suggested by the Southern California Earthquake Centre). The use
of these seemingly low required factors of safety (relative to the 1.5 factor
of safety required for “static” conditions), is based on a number of factors,
namely the limited accuracy of the method, the relative infrequency and short
duration of earthquakes, and that seismic slopes failures often result only in
movement of the slope, rather than a more dramatic “landslide” type failure.
If the factor of safety is
determined as being less than 1.0 or 1.1, the permanent displacement (i.e.,
movement) of the slope is calculated; a maximum allowable movement of either 50
millimetres or 150 millimetres is often specified, depending on the
jurisdiction and the nature of the project. However the methods for evaluating
these displacements, though commonly used in many seismically active areas of
western North America, should only be used with caution for sites in the Ottawa
area since the methods were generally developed for soils with very different
properties and for earthquakes with different characteristics. The Champlain
Sea clay is a very “sensitive” soil meaning that its strength reduces
drastically once it’s sheared, so the displacements from a slope in the
Champlain Sea clay may be much larger than would be calculated by those
methods, unless that possible strength loss is considered in the calculation.
Based on the above, a minimum
factor of safety of 1.1 is suggested for seismic slope stability analysis.
Where the factor of safety is less than 1.1, a Limit of Hazard Lands may be
determined corresponding to a factor of safety of 1.1, in the same manner as
used for the ‘static’ loading cases. However more sophisticated analyses may
also be justified, which might yield a more accurate assessment of the
stability of the slope. Caution is urged in these circumstances. It should also
be noted that a Toe Erosion Allowance need not be included in the determination
of the Limit of Hazard Lands under seismic conditions, since erosion is not the
trigger of the slope movement. As well, as a general guideline, an Erosion
Access Allowance need also not be included since this Limit of Hazard Lands for
seismic design corresponds to a post-disaster condition. The general philosophy
for seismic design (due to their low frequency of occurrence) is to avoid
immediate collapse and loss of life; structures are not expected to remain
serviceable.
The resulting Limit of Hazard
Lands determined for seismic loading conditions is then compared to the limit
corresponding to ‘static’ loading conditions and the more conservative value is
used.
It is important to note that
the soil strength parameters that apply to the silty clay for the assessment of
the factor of safety under “seismic” loading conditions are not those that
apply to the more conventional “static” analyses. “Undrained “ soil strength
parameters need to be used, to reflect the rapid nature of the loading, whereby
the clay soil has a much higher cohesion than described previously for “static”
analyses (typically between 25 and 150 kilopascals) and a friction angle of 0
degrees. However, as indicated above, some clays, in particular sensitive
clays, may have a reduction in strength due to “softening” when subject to
earthquake shaking.
Another very important
consideration for these assessments is the potential for seismic “liquefaction”
of loose sandy soils. Seismic liquefaction is a phenomenon whereby the ground
vibrations from an earthquake generate high (pore) water pressures (i.e., the
water pressure in the pores between the soil particles is increased above its
natural level). These excess pore water pressures reduce the stresses between
the soil grains and thus reduce the frictional resistance to shearing of the
soil. This phenomenon leads to a temporary but dramatic reduction in the shear
strength of the soil (potentially changing it to a liquid-like state) and thus,
in the period immediately following an earthquake, could lead to failure of an
otherwise stable slope. Even very flat slopes may experience large, permanent
movements if the soils underlying these slopes liquefy.
The details for assessing the
potential for seismic liquefaction are beyond the scope of this document.
However, the potential should be considered wherever the slope contains “looser” sand, sand and gravel, or
silt located below the highest projected groundwater level in the slope. “Loose”
In
this context, “looser” means a Standard Penetration
Test blow count less than about
1810, as measured in a borehole. Many routine projects do not
include borehole drilling and so that information would often be unavailable.
Caution is urged on such occasions.
It should also be noted that
there are planned changes to the design earthquake for Ottawa specified in the
National Building Code. Those changes (anticipated for release in 2005, for
subsequent adoption by the Ontario Building Code) would double the magnitude of
the design earthquake for Ottawa. In that case, the horizontal acceleration for
slope stability analyses would also double and a greater range of soils would
be classified as being susceptible to seismic liquefaction.
In summary, a review of a
slope stability report should identify that:
·
the factor of safety against
slope instability under seismic conditions is at least 1.1;
·
a seismic coefficient of about
0.1 half of the PGA specified in the current version of
the OBC (and applicable referenced version of the NBCC), or greater, was
used for the analyses, and that the potential for increase in the seismic
coefficient from the “firm ground” value has been taken into consideration;
·
undrained soil strength
parameters were used for clay in the analyses, with consideration of the
potential for decreased strength due to “softening” during the earthquake;
·
where sand or silt deposits
are present, the potential for seismic liquefaction is addressed.