HomeMy WebLinkAboutA21_Malick EmailFrom: Geoff Malick <geoff@stratumgroup.net>
Subject: Slope Stability Assessment Summary at 161 Twana Way
Date: August 15, 2023 at 4:43:59 PM PDT
To: jdimaggio@mac.com
Hi John,
The email serves as a summary of my findings and recommendations from yesterday’s site visit.
Hillside above driveway- This slope is subject to soil settlement, raveling, and creep due to the presence
of loose weathered soil as well as side cast material left over from previous road construction. This
loose, low cohesion sand mantles much more compact sandy till that is unweathered and predominantly
stable. The observed soil raveling and creep is mostly driven by weathering processes (wetting and
drying, freeze thaw) as well as slope disturbance from animal foot traffic.
Based on my assessment, it is my opinion that the rate of raveling and creep can be greatly reduced by
planting drought tolerant, native vegetation and achieving majority ground cover. Recommended
species include salal, ocean spray, sword fern, Oregon grape, Evergreen huckleberry, Scouler’s willow,
Snowberry, and Nootka Rose. Douglas fir trees can also be planted. I do not believe any retaining
structures are needed at this time. However, wooden crib walls that support and buttress the
uppermost 3 feet of the slope would temporarily arrest and delay settlement at the top edge of the
slope when used in conjunction with planting.
Eroding slopes below the home- The existing home is located on building pad partially underlain by fill
material. The fill is composed of low cohesion sand that is highly susceptible to soil raveling, creep,
settlement, and erosion. The observed soil movement and erosion is caused by a combination of
weathering processes (e.g., wetting and drying, freeze thaw cycles), slope disturbance from foot traffic,
as well as stormwater discharge near the southwest corner deck pier.
Based on my assessment, I recommend that the upper 3 feet of the slope is supported and buttressed
by wooden crib walls. The bottom of the crib walls should be located at least 10 inches below the
ground surface. The crib wall posts should be driven as deep as possible to prevent tilting (5 to 6 feet if
possible). Crib walls will work to greatly reduce soil raveling, creep, and settlement for an approximately
15-year period when combined with planting of drought tolerant native vegetation on the remainder of
the slope (species listed above). Efforts to reduce foot traffic should also be made.
I recommend that the downspout that is currently discharging on the slope near the southwest deck
pier is connected to a pipe and infiltrated into the ground or discharged in an area away from the fill
pad.
Leveling of yard area: Fill slopes in this area should be supported with wooden crib walls that are under
4 feet in height. The crib wall posts should be driven as deep as possible to prevent tilting of the wall (5
to 6 feet if possible). Well drained fill should be used and properly compacted. If a wall 4 feet or higher is
required, it will need to be engineered and have drainage. We do not recommend the use of concrete or
rockery walls due to the mass inducing a load on the sensitive soil and causing settlement.
Please let me know if you have any questions or need anything else. The company I was thinking about
to do the work is Peninsula Environmental in Port Angeles. They would be good for the planting but I’m
not sure if they install wooden crib walls. I will send an invoice in a separate email shortly. Thank you!
--
Geoff Malick
Licensed Geologist
Office:360-714-9409
Cell: 802-535-2690