RYZUK GEOTECHNICAL Geotechnical Field Review / Site Instruction (March 5, 2019)

Slope Stability Assessment – Addendum Revision 1

As requested, we have undertaken a lot specific slope stability analyses using computer modeling for each of the proposed lots. The following is intended to supplement our preliminary analysis as detailed in our Geotechnical Assessment Letter, which indicated a general setback from any slope crest of 7.5 m.

Our analysis applied a design seismic event with 2% probability of exceedance in 50 years. Slope contour information was based on the Layout Plan prepared by Wey Mayenburg Land Surveying Inc, attached. Our model was generated in accordance with Section 4.1.3, Factor of Safety and/or Slope Displacement, of the Guidelines for Legislated Landslide Assessments for Proposed Residential Developments in BC. Two-dimensional limit equilibrium analyses was utilized to identify potential failure surfaces and Newmark analyses was utilized to determine slope displacements during seismic ground motions. For Newmark analyses, a magnitude 9.0 seismic event originating from the Cascadia Subduction Zone was used and scaled to a Seismic Site Classification of 'C'.

Lot 1

A reduction to the general setback is possible provided the slope crest is unloaded. We recommend unloading the slope crest by incorporating a walk-out basement level to any proposed development. Our model considered a walk-out basement level would lower site grades near the crest by approximately 3.5 m which, in turn, provides an addition 4.5 m horizontal distance relative to the existing crest location. Based the analysis, we consider a safe setback for residential foundations from the lowered crest to be 6.0 m. Results from the Newmark analysis indicate that ground displacement would not exceed 2.6 cm beyond a 6.0 m setback, which is well below the maximum tolerable slope displacement of 15 cm in accordance with the legislative guidelines.

Lot 2

The proposed Lot 2 includes the existing residence/open foundation, as indicated on the attached Survey and Proposed Subdivision Plan. We recommend that a walk-out basement level is maintained for this lot. The existing building setback from the crest is approximately 4.5 m. Based on our analysis, a minimum setback of 6.0 m should be maintained for any new residential development. Results from the Newmark analysis indicate that ground displacement would not exceed 0.2 cm within the 6.0 m setback line.

Lot 3

We consider a safe setback for residential foundations to be 5.0 m provided the residence is constructed roughly at the existing grade of the lot. This setback could be further reduced provided the slope crest is unloaded by 3.5 m. Based on our analysis, we consider a safe setback for residential foundations from the lowered crest to be 4.0 m. Results from the Newmark analysis indicated that ground displacement would not exceed 15 cm within 5.0 m of the slope crest provided existing grade is maintained.

Closure

In general, we consider that unloading the slope crest and constructing the residential buildings behind the provided setbacks would improve slope stability on the site. The following table summarizes the minimum setback for residential foundations from the crest of the predominant slope based on our stability analysis.

Table 1: Summary of Setbacks

Proposed Lot	Minimum Setback for Residential Foundations
1	6.0 m from the lowered crest
2	6.0 m from the lowered crest
3	5.0 m from the existing crest
3	4.0 m from the lowered crest

Minimal disturbance of vegetation should occur within the area between the slope crest and the proposed residences. Areas subject to excavation/unloading along the slope crest should be excavated with care and re-vegetated as soon as possible. Landscaping features should be constructed such that surface flows are not concentrated to discharge at the slope crest. Alternatively, collected water could be conveyed downslope via secured solid pipe network and discharged onto non-erodible soils.

We trust that the above is suitable for your purposes at present. Please don't hesitate to contact our office if you have any questions.

Sincerely, Ryzuk Geotechnical

Colden Van Ommen, EIT Junior Engineer

Andrew Jackson, P.Geo., Eng.L. Geoscientist / Engineering Licensee

Attachments - Layout Plan prepared by Wey Mayenburg