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HomeMy WebLinkAbout1992, 08-17 Geotechnical Reconnaissance LetterMINIM Madden Construction E. 10601 Nixon Ln. Spokane, WA 99206 Attn: Frank Madden Dear Frank: 3820 E. Broadway Spokane, WEehington enol budinger & associates geotechnical & material engineers August 17, 1992 Project Number F92204 PROJECT: Single Family Residence 15920 22nd Ct. Spokane, WA SUBJECT: Geotechnical Reconnaissance As requested, we met you on site the morning of August 17, 1992 to review the soil and groundwater conditions which are currently plaguing the project. We understand that the proposed structure is to be a two-story, wood frame building with a basement which will partially daylight to the west. The site is situated at the toe of the lower foothills of Mica peak, about a mile south of Shelly Lake along the southern margin of the Spokane Valley. At the time of our visit, the site had been cleared and grubbed and the basement area was excavated to grade. The perimeter footing form work was in place although no concrete had been delivered. We understand that construction has been halted by the Spokane County Building Department pending a Geotechnical Review of the prevailing soil and groundwater conditions. Groundwater was encountered in the basement excavation with the most significant seeps occurring along the east and southeastern sides of the proposed building. We understand that the earthwork contractor experienced some difficulty along the eastern footing line which has resulted in serious disturbance of the foundation soils and has precipitated a small slope failure of the temporary excavation. Currently, groundwater is seeping from the cut at a slow trickle and is collecting along the eastern footing line. (509) 535-8841 =AY /cm riAS-Q589 F.C. Bud lnow PE J. Andrew Siemens PE $tephen D. Burchett EIT Ridgemont Subdivision Project Number F92204 Spokane, WA In our opinion, the sloughed material from the slope failure along with other soils loosened and disturbed should he carefully removed, preferably using hand methods. These soils can he replaced utilizing an open -graded, drainfield type gravel, or a rounded pea gravel, either of which should be fully enveloped by filter fabric to prevent infiltration of fines. We do not anticipate a great volume of soil to be removed and as a result, we expect this removal and replacement operation to be a fairly economical and readily accomplished solution providing reasonable support for the proposed footings. Following the stabilization of the area affected by the slope failure and the excavation, it is our opinion that construction can proceed as scheduled. However, it will be important to provide positive drainage for the basement floor and walls which will effectively be constructed below the water table. We recommend utilizing a pre -fabricated drainage composite, such as "Miradrain 6000" fitted with a perforated pipe (at least 4" in diameter, enveloped in filter fabric) around the footing perimeter. The drainage system should extend up the stemwall to within about 6" of the proposed finished grade. Further, the Miradrain should discharge into a gravel filled gallery that directs the groundwater to a safe, unobstructed outlet that will not become blocked by debris, ice, or snow, The outlet must daylight down gradient of the low point along the perimeter drainage system. Even though a perimeter drain is in place, the floor slabs will remain at risk of collecting moisture from a saturated subgrade. Consequently, we recommend that the slabs be underlain by a 6" course of open -graded gravel such as that commonly utilized to provide a radon ventilating course. This underslab vapor barrier should be connected with the perimeter drain to provide a means of discharge for free water which may collect below the slab. 2 budinger & assolcates geotechnical & material engineers Ridgemont Subdivision Project Number F92204 Spokane, WA In addition to these procedures, normal practice would include applying a waterproofing compound or material to the foundation and stem walls. In our opinion, a water stop along the joint between the foundation and the stemwall could promote a buildup of hydrostatic forces and lead to buoyant uplift, cracking and leaking of the basement floorslab. Consequently, although waterstops are commonly used for this type of situation, they are not recommended. In our opinion, adherence to these recommendations will result in a foundation constructed to standards equal to or greater than that normally practiced by contractors building similar homes in this area of Spokane with similar shallow groundwater. This does not guarantee that the basement will remain completely dry although it is our opinion that the drainage system described is adequate to dispose of the majority of the groundwater occurring in a normal year. Extraordinary wet years could promote interior moisture which could only be controlled by a much more elaborate and expensive drainage and waterproofing system. We present this report in lieu of any warranties express or implied. It has been a pleasure to be of service and if we can provide further assistance or if this report requires clarification, please call. JAS/dr Addressee - 1 City of Spokane - 1 budinger & assoicates Respectfully submitted, BUDINGER & ASSOCIATES J. Andrew Siemens, PE 3