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HomeMy WebLinkAboutGEOtech Slopes Report - BLD Engineering / Geo-tech Reports - 7/3/2006 Bradley-Noble Geotechnical Services A Division of The Bradley Group, Inc. PO Box 12267, Olympia WA 98508-2267 Phone 360-357-7883 • FAX 360-867-9307 3 July 2006 RECEIVED Mr. Richard Moore JUL .I B 2006 South Shore Construction 230 E. Hillside Drive MASON COUNTY Belfair, Washington 98528 Subject: Geotechnical assessment of slopes on Lot J7, Highlands at Sunset, Mason County Tax Parcel 122187700070, Trails End Lake Area. Dear Mr. Moore: As requested, we visited the above property in order to observe site geology and slope conditions on this and adjacent lots. Our work consisted of a site visit, review of information you provided us on the proposed location of the new residence and utilities, review of available geologic information, and preparation of this report. The project site is located on the Vashon recessional outwash plain. In this area we find a gently undulating surface of a series of low ridges with shallow valleys between. These ridges and valleys have a north-south trend and reflect the movement of the Vashon glacial ice mass. Deeper ravines have been incised into the outwash plain by melt water during the later stages of the retreat of the ice sheet. These steeper ravines are not within 300 feet of the project area. At this project site, we find the ground to rise about five feet above Grew Bluff Court, the road that provides access to this property. In the area of proposed construction, we find an area of low relief. To the west of the building area, there is a slope down to a wetland. Measuring the slope with a Leitz Pocket Transit, we measured the slope angle from top of slope to toe. This slope averaged 17 degrees or 25% slope. Measurement of the slope was facilitated by a clearing of the small diameter Douglas firs growing on the slope in a clearing about 125 feet wide that extended from top to toe. Slope distance from top of slope to toe is estimated to be about 200 feet. We observed that at the time of our site visit there was standing water in the wetland area. We expect that during the summer, this wetland will become seasonally dry. Site soils are exposed in the road cuts of the project and in the test pits excavated on- site for the drain field design. In this area, we find a thin layer, about three feet maximum of Vashon recessional outwash (Qvr). These soils are silty sandy gravels and 06070301 Page I of 3 5 ' 06070301 Page 2 of 3 silty gravelly sands. Underlying the outwash soils is the Vashon subglacial till (Qvt). Soils exposed are consistent with the geologic mapping of the area. Plate I of Water- Supply Bulletin 29, Geology of Southeastern Mason County, Washington, by John B. Noble and Dee Molenaar maps the project and general area as Qvt, Vashon subglacial till. The surface soils of outwash were fluvially deposited during the retreat of the Vashon lobe of the Cordilleran glacier as it retreated from the Puget Sound Lowland during the Fraser glaciation in late Wisconsinan time. The thickness of these soils varies, but is typically less than six feet in the general project area. Underlying the outwash soils is Vashon subglacial till. Subglacial till is a mixture of silt, sand, and small gravels. It was deposited at the base of the advancing ice sheet. The grain size distribution and loading by the ice sheet, thought to have been between one half to one mile thick in this area, have created a soil unit that has many engineering properties similar to those of lean concrete. With the slopes being underlain at shallow depth by subglacial till, deep rotational type of failures is not considered to be a risk. With the cementing and overconsolidation of the till, these soils have high angles of internal friction, significantly greater than slope angles on this property. When slope angles are less than the angle of internal friction, slope stability is inferred. With the till being of very low porosity and permeability, for all practical purposes impermeable, flows of ground water in the till is not considered to be a destabilizing factor to slope stability. We do expect that during periods of heavy or prolonged rainfall, flows of water will develop on the slope surface of the subglacial till. This downslope flow of water feeds the seasonal wetland area. Examination of the surface of the cleared area on the slope does not indicate that active mass wasting events have developed or areas of accelerated surface erosion. Trees growing on either side of the clearing did not indicate that surface movement of the near-surface active soil zone was occurring. Placement of the new residence was indicated on the site plan you provided us . We have no objections based on geotechnical considerations to the location. This location is behind the break in slope and in the area of low relief. We do recommend that the site be graded around the structure in conformance with the requirements of Section 1803.3 of the International Building Code (IBC) to ensure positive drainage away from the structure and prevent ponding against the foundations. It is good practice to collect all roof rain leaders into a tightline collection system. This is to prevent concentrated surface flows of storm water from being directed over the slope from impervious surfaces. Concentrated flows can create areas of localized instability or accelerated surface soil erosion due to saturation of the outwash soils on the slopes. We recommend that all roof storm water be collected into a tightline system with the collected water conveyed to the toe of slope for infiltration. 06070301 Page 3 of 3 Based on our site observations and research, we have no objections to the placement of a new single-family residence at the location indicated based on geotechnical considerations. We do not consider that the project site or surround slopes are at risk of mass wasting events that would adversely affect soil foundation support. Good grading practices around the structure are required to prevent ponding. We do not consider that the preparation of a Geotechnical Report is warranted at this site due to the site being underlain at shallow depth by highly overconsolidated soils. If you have any questions on this report, or if we may be of additional service to you or your client, please contact us at our Olympia office. Cordially, BRADLEY-NOBLE GEOTECHNICAL SERVICES David C. Strong, L.E.G. 7 'cP 1129 qf" d Geo�o DAVID C. 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