Geotechnical Report – Ryzuk Geotechnical, November 16, 2021

Re: Proposed Retaining Walls & Pool, 53 Knollwood Road – View Royal, BC

As requested, we attended the referenced site most recently on September 14, 2021, to complete a visual geotechnical assessment of the property as such relates to the construction of a seawall, several small landscape walls, as well as new pool and associated structures. The site is located within the Environmental Protection & Natural Hazard Development Permit Area, in accordance with the View Royal Official Community Plan. Our associated observations, comments, and recommendations in this regard are summarized herein. Our work has been carried out in accordance with, and is subject to, the previously provided Terms of Engagement.

The property is located at the west end of Knollwood Road, and slopes down to the southwest from the road to the shoreline of Price Bay, which is part of Esquimalt Harbour. The site is bounded to the northwest and southeast by similar sized residential properties. We understand that the residence was constructed in about 1973 and was recently renovated in the last year. The sewer lateral for the property extends down the slope near the eastern property line, connecting to a sewer main located on the beach at the toe of the slope. The overall foreshore slope is about 8 – 10 m high from the edge of the upper back yard to the shoreline, over a horizontal distance of about 20 m.

Ryzuk has been involved in work at this property ongoing since 1996. A summary of our involvement to date is provided below.

Our first inspection of the property was in April 1996 after a slope failure occurred at this site. The failure occurred in November 1995, resulting in loss of a portion of the back yard and the destruction of three mortared rock retaining walls. The retaining walls south of the residence that were involved in the 1995 slope failure were constructed in about 1993 or 1994, to terrace the foreshore slope/waterfront portion of the property.

Our observations in April 1996 indicated that the slope failure appeared to involve mostly fill, which had possibly been placed on the slope at the time of construction of the residence to expand the level portion of the back yard, although it may have also involved some undisturbed surficial soils on the margins of the slide area. The failure reportedly followed some heavy rain, but is also known that the sewer lateral was ruptured, and there is a possibility that leakage of the sewer line prior to the failure could have been a factor. Information provided to us by the insurance company at the time suggested that there was some movement of the crest of the slope, extending about 2 m back of the uppermost retaining wall, a few days prior to the large-scale movement that destroyed the three walls and left a scarp about 2 m high at the crest of the slope. Although the retaining walls were cracked or severed at various locations they generally remained upright, as did a number of large trees that were transported downslope during the failure.

Following our 1996 inspection we understand the sewer lateral was repaired, however, no slope remediation or work on the slope was completed until 2014. In 2014 we were retained to complete design and construction of an Allan Block retaining wall near the crest of the current backyard. During construction of the upper wall, the existing fills were removed to expose native dense till-like soils at the base of the wall, and the wall was backfilled with geofoam and incorporated helical anchors and geogrid. In addition, adequate drainage was provided behind the wall, which was tight piped to the base of the foreshore. No disturbance was completed on the lower half of the slope as part of this work.

A comparison of our photos from 1996 with our recent observations suggests that there has been only minor change to the lower portion of the slope configuration over the past 26 years. Parts of the severed retaining walls have now toppled and some shallow surface creep is evident. In addition, at least one of the large fir trees has died, and the larger willow tree located at the current Present Natural Boundary (PNB) has leaned considerably towards the foreshore, indicating further creep of the slope. A small soil scarp, of 1 to 1.2 m height is present at the current natural boundary, which shows evidence of ongoing erosion and creep. From our review of the forshore, bedrock outcrops are exposed near the property line at both the east and the west ends of the foreshore.

From our review of the preliminary drawings, we understand that the current proposed work will involve partial removal of the upper retaining structure and construction of the new pool and associated mechanical room, however exact details and elevations have yet to be finalized. In additional several tiered landscape walls will be constructed in the lower section of the slope including a new seawall along the PNB.

Given the history of the site, creep/erosion of the lower slope is anticipated to be ongoing. While the overall environment of the bay is relatively calm, the foreshore is also subject to coastal effects such as wave impact at the toe, particular due to wave wash during storm events in conjunction with high tide and in consideration of future sea level rise. Construction of the seawall wall will help limit the rate of erosion, however some ongoing creep is likely to occur unless all existing fills can be removed.

Recommendations

For construction of the lower seawall and tiers we recommend that a flexible wall be used. As noted, the slope is anticipated to experience shallow soil creep of surficial soils, which has the potential to result in some movement over time of walls constructed on such slopes. Flexible wall types such as boulder stack, gabion basket, Flex MSE, etc. have the ability to deflect somewhat in response to minor slope movement without compromising the overall stability or aesthetic value of the wall. In contrast, rigid walls such as rock and mortar or cast in place concrete may crack in response to minor movements. From discussion with you we understand that a gabion type wall is preferred due to access restraints. Gabion walls can constructed with small equipment and provided a natural exposed face which can be readily planted with native soils to create a “green” look.

Proposed works on the lower portion of the slope should attempt to balance cuts and fills to avoid increased surcharging and potential acceleration of the rate of creep or instability. The attached Location Plan shows the approximate location of the proposed seawall and gabion walls. Base of the wall should be embedded a minimum of 0.5 m below existing grade placed on a level pad of crushed rock material. We are proposing the gabion wall consist of the Maccaferri Green Gabion, or approved similar, which can be constructed with a 60 degree batter. The lower seawall the wire mesh of the gabion baskets should be both galvanized and coated in polymer for corrosion protection (Maccaferri Polimac), while the upper terraces should be a minimum of galvanized.

Backfill behind the walls should consist of a minimum 0.3 m wide drainage layer of granular backfill. For seawalls, we typically recommend a 10kg class rip rap or approved alternate. It is critical that the gradation of the seawall backfill match the specification, in order to prevent erosion of the material from behind the wall through the face of the gabions. A medium weight non-woven geotextile is recommended against the seawall temporary bank cutslope and behind the backfill in order to prevent migration of fines from the slope soils into the granular backfill. For retaining walls not interacting with the shoreline, any free draining granular fill could be used. Well graded and thoroughly compacted fill is recommended where such will support finished surfaces atop the wall. Clear drain rock could be used if desired but should be fully wrapped with medium weight geotextile in order to prevent migration of fines into the voids within the drain rock. Attention should be paid to gradational compatibility to prevent the planting soil from hour-glassing into the voids within the free draining backfill.

Tiered walls, where proposed, should be located such that the upper tier is set back behind a minimum 1.5H to 1V plane extending from the toe of the lower wall, so as not to induce a surcharge on the lower wall. The noted walls are intended as landscaping walls only and have been not designed for support of additional structures above. Foundations for structures upslope of the retaining walls should also be located behind a 1H to 1V plane from the base of the upper wall.

We recommend that drainage be provided behind all the upper retaining walls to help limit flow of surface or groundwater through the slope. To avoid concentrated surface run-off, all collected water should be tight piped to the base of the slope were it is disposed of on a splash pad or bedrock outcrop.

We understand that the upper walls for the new pool and mechanical structures (beyond 15 m setback) will be part of a separate permit application. Based on our review of the preliminary drawings, construction of the pool and lower mechanical room will result in a net overall unloading of the crest of the slope which is anticipated to enhance overall slope stability. We understand that the final grade below the pool will not be significantly modified, however in the event the grade is to be raised significantly, the use of geofoam to avoid locally surcharging the slope may be required. The requirement for geofoam above would need to be confirmed once final design is complete.

Based on the above, and provided our recommendations are followed, we consider the construction of the noted retaining walls and structures can be completed without significant adverse impacts to slope stability or erosion on this or neighbouring properties. Our review has considered a seismic event with a 10% probability of exceedance in 50 years (1 in 475 year event). To ensure the work is completed in accordance with our recommendations, site inspection will be required during construction. In addition, all work should be completed following environmental guidelines in accordance with a qualified environmental consultant.

We trust the preceding is suitable for your purposes at present. If you have any questions, or require anything further, please do not hesitate to contact us.

Sincerely, Ryzuk Geotechnical

Matt Mueller, P. Eng. Project Manager

Attachments:

• Gabion Wall Details
• Maccaferri Green Gabion Technical Data Sheet

---

[Diagram: LOCATION PLAN]

Notes:

1. This drawing is for the intended use of the client for the specified project, and should not be used elsewhere without the express permission of the client and/or Ryzuk Geotechnical.
2. This drawing is scaled for 11x17 sheet and does not require further scaling to fit. Scales will differ if printed on different sheet size.
3. Base Plan taken from Polaris Survey drawing
4. Proposed work traced from site concept sketch

Refer to “Environmental Review for 53 Knollwood Road, Victoria, BC” prepared by Fish-KW Environmental, dated April 12, 2022 for updated plans

---

[Diagram: Section A - A]

GREEN GABION WALL DESIGN NOTES:

1. Wall construction and fill placement to be completed under supervision of Ryzuk Geotechnical.
2. Base gabion course placed directly upon approved subgrade. A 100 mm thick leveling lift of well compacted granular fill is optional.
3. Gabions to consist of high abrasion resistant polymer coated Maccaferri (or approved equivalent) green gabion baskets. Baskets to be 2 m in length by 1 m in width by 0.5 m in height. Face angle to be 60°. Gabions to be prepared and installed in accordance with the manufacturers specifications. Manufactures specification attached for reference.
4. Backfill to consist of 10 kg class rip-rap. Method of compaction to vary depending on lift thickness and to be compacted to a minimum of 95% of Standard Proctor Maximum Dry Density (SPMDD), or judged equivalent.
5. No construction machinery larger than a Bobcat S70 should encroach within 1.0 m of the back of wall during construction.
6. Wall design assumes an additional surcharge load associated with a maximum 1.5H:1V (34°) slope. No additional loads from other adjacent structures (i.e. out-buildings) should be applied near crest of wall unless confirmed by a qualified professional.
7. The design of the retaining wall structure is based on the following soil parameters:

Maximum Allowable Bearing Pressure = 145 kPa

8. Wall designed in accordance with a minimum global stability factor of safety in seismic = 1.2.
9. Seismic design horizontal acceleration coefficient = 0.15 g (50% 10% probability of ground motion exceedance in 50 years).
10. Seawall encroachment beyond the Present Natural Boundary (PNB) to be confirmed acceptable by the relevant regulatory agencies and provincial authority.

Refer to “Environmental Review for 53 Knollwood Road, Victoria, BC” prepared by Fish-KW Environmental, dated April 12, 2022 for updated plans