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Home My WebLink About938400524 Geotech Assessmentv ,~..., ~~ ~~ M ,,,~ ~ } - .` ~ ~ ~. s` ~ ..~ ~"` ,4 . ~ ~. ~~ ~~. flee t ,~' ~. _~~~~~~'_~~ Geologic .Conditions..: ~ , . ~.~..... ~ ., ..~.. The SCS Soil Survey of Jefferson County-maps the soil in the area of the. subject property as the Clallam gravelly sandy loam. This soil formed in glacial till. Runoff is classified as slow to medium and the hazard of water erosion is slight to moderate. The Survey classifies this soil as a Silty Sand (SM) from 0" to 23"and as a Silty G[ra~el (GM) or Silty Sand (S:M) from 23" to 36" using the Unified .Soil Clas ification System (USCS). The Survey labels the shrink-swell _potential as lows the corrosvity to uncoated steel and concrete as moderate, and the frost action potential as none. This soil is typically cemented below a depth of 20" to 40'' and water may move laterally above the cemented layer-during the wet season. The Washington State Department of Ecology in their "Geology. and Ground-vVater Resources of Eastern Jefferson :County, Washington" maps the soil in the area- of the subject property. as Lodgement till (Qvt) or as Recessional .outwash {Qvr). These u-nits are assoca#ed with the Fraser Glaciation and consist of primarily of gravel and sand with boulders, cobbles, silt and clay. Visual observations of-the two test pits revealed the soil to be compact light brouvn to tan sandy rounded gravel (GW or GP using the USCS) with a trace of cobbles (Photo 2). There was~about 10" of organic:topsoil on the surface. Granite gravel was present in the test pits and there was a very large granite boulder (glacial erratic) nearby which suggests that the material is from the Fraser Glaciation as :granite is not found. locally on thee-Peninsula but was carried south from Canada by the glaciers. The test... pits were standing; vertical with no sloughing. (Photo 3) and no groundwater was present. Conclusions and Recommendations The „hillside at the subject property appears grossly stable and the. development of the property seems feasible from a geotechnical perspective. The slope is well vegetated. - The slope angle is.below the angle of repose. No springs or seeps were noticed on the slope face': and no evidence of past slope failures were seen. Due to the slope of the property and the proposed location of the houses on the slope, deepened. foundations will be necessary. Also, retaining walls will be necessary to retain cut lopes and also to provide; protection to the house from slope drainage,. erosion and shallow sloughing from the hillside above. Using the Uniform Building Code {UBC) guidelines, we recommend that foundation elements be embedded five feet below grade and founded infirm undisturbed native soil. As evidenced by the lack of sloughing of the test pits, it appears that similar _2_ ` j ., ~ ~ rvx .. t t ~, # r ~"+ ~ ~ •.€ r ~. r "~ .. excavations for footing elements could be achieved at a cost :advantage to typical auger cast piers. Further, site .access with a typica truck mounted drill rig would be difficult. Foundation elements could consist of piers cast into square holes- dug with an excavator or backhoe The1 sidewalls would need to be approximately plumb,. consi tent in width and the bottom. would need to be level and free of loose soil. Even though the test pits did not cave in there is a risk of the: holes caving. in. All codes andlor regulations should C be followed with regard #o trench excavations, shoring and backfill in order to protect workers. Based on Part N of Section 29.6-155 of the Washington Administrative Code (WAC), the soils at the site classify as Type C soils. Based on Table 18-I-A of the Uniform Building` Code (UBC), the allowable foundation pressure of the sandy gravel found in the test pits is 2000 pounds per square foot (psf). This-value.can be increased up to 6000 psf subject to footnote 2 of the table. The lateral bearing is 2:00 Lbs. Per square foot. per foot of depth below natural~grade. This value may be increased per footnote 3 of the table.'The lateral sliding coefficient is 0.35 to ba multipkied by the dead load. For retaining structures, where freeunrestrained walls: are planned to retain Level, clean, granular, nonexpansive backfill, an equivalent fluid pressure of 35 pcf forwalls allowed to -deflect at least 0.002 times the wall height (active condition) may be used. For wal'Is not allowed to deflect at least 0.002 times the wa11 height, the above value should be increased to 67 pcf (at-rest condition). An appropriate safety factor should be a lied for . .. pp the design of retaining structures. A safety factor of at least 1.5 for tiding and overturning is recommended. Appropriate allowances should be made for anticipated surcharge loadings. Backdrainage systems are recommended for solid retaining walls. Forgabion baskets or :rock walls, than are pervious.,- backdraina_ge systems are not required, but may be ..used-to-direct water that might otherwise weep through the wall and be unsightly. Atypical `batter angle for low gabion or rock walls is 1 H:4V. The compact glacial till s-.oils found in the-test pits may temporarily stand at this angle long. enough to face the cut with the wall (as evidenced by the~test pits standing vertical). However, his steep of an ..angle should not be considered stable and-the slope should be monitored for any signs of instability and laved backto a less steep angle if necessary. Surcharge loads should, betaken into consideration. walls .over 4 feet high generally require the design services ~ofa structural engineer. Lightweight compactors such as hand-operated tampers should be utilized for compaction in .close proximity to .retaining walls in order to reduce the possibility of excess wall loading and potential movement. .The following recommendations should also be considered with regards to deve opment of the subject property: 1. It will be necessary to .maintain ground cover on the hillside to: reduce erosion .from surface runoff. Any bare areas that develop should be revegetated. 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