2.5 Property-Scale Landslides

Property-scale slope movements are limited to steep slope portions of the site. Our site review did not reveal recent shallow soil slide evidence within the area in close vicinity of the proposed home. The trees observed on the site did not exhibit tilting or trunk curvature features to suggest past or ongoing soil slope movements. The review of available geological and geotechnical information was found to be consistent with a review of historical air photographs and with the findings of the site review. Previous work by Rollerson (2005) and Barrow (2009) in coastal areas of BC determined potential for open slope soil slides increases substantially for slope gradients exceeding about 60-70%.

The localized steep slopes near the proposed building location have the approximate inclination in the range of 30° to 35° (60% to 70%), which are considered extremely steep slopes. It is considered that over a time period between 500 and 10,000 years there exists a chance of at least one natural or anthropogenic open slope slide occurring on the steep site slope areas and that results in direct building impact. The 1:500 annual probability approximately corresponds to a 10% probability of occurrence in the next 50 years, which is considered an appropriate minimum standard for a subdivision. Based on the rationale outlined above, smaller localized shallow sloughing type slide recurrence frequencies between 1/500 and 1/10,000 (Unlikely to Possible) from steep site slopes are considered applicable.This risk evaluation is generally consistent with Hazard Acceptability Criteria 4 (Cave, 1993) defined as subdivision approval, but with siting requirements to avoid the hazard registered against the title.

 2.6 Snow Avalanche

 Snow avalanche is not considered a credible hazard to the proposed home.

 2.7 Rock Fall

Rock fall is not considered a credible hazard to the proposed development due to the dense silty nature of the soils above the proposed development. The surrounding area is densely vegetated with medium to large trees. This vegetation will add to the stability of the soils. Based on this information the Property is not at risk of rock fall

 2.8 Liquefaction

Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid. Due to the low fundamental period of vibration for a typical residential building and the dense silty nature of the soils with no evidence of groundwater, the possibility of risk of liquefaction failure of the foundation subgrade material in the event of a design earthquake is very low.

3.Conclusion

Geotechnical hazards considered for the proposed home included inundation by flood waters, stream erosion and avulsion, debris flows and floods, regional-scale landslides, property scale landslides, snow avalanche, rock fall and liquefaction. It is considered small-scale localized landslip is a credible geotechnical hazard at the subject site as defined by Dr. Cave. For the development area in general, there is considered to be a probability of direct building impact from small-scale localized landslip corresponding to approval with siting requirements to avoid the hazard or requirements for protective works to mitigate the hazard with a registered covenant against the title (Approval Response #4 – Cave, 1993) for proposed subdivision. In view of the above information and in accordance with the hazard acceptability criteria, the proposed house area is considered safe for a new building, subject to the siting measures discussed below. Based on the results of our field work and analyses, we recommend a geotechnical setback for proposed home at the site of 5 m from the top of the slope for foundations. The top of the slope is defined as a line where the slope inclination exceeds 15°. Where the recommended 5 m setback is not feasible due to the area constraints, lowering the house foundations may be considered. The vegetation and geometry of the natural slope outside the building area must not be altered without competent professional advice. A formal geotechnical hazard statement that meets the requirements of Section 86 of the Land Title Act is provided in Section 6.3 below. The estimated frequency of geohazards discussed above is provided in Table 3 below.