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ESLHA Wet Season request 9-28-09.pdffi r'Jrrr. D CIENCw. G ll tl`,dltr('lfa? �-ars rJ'r u�,�a�Vr,r;v September 28, 2009 IIWA Project No. 2.003-174-2.1-1300 Gr rr of frol Ulo`o7 of , f r0u ' �i /iYftT TO: CC: Aunt Bullis and Pat Lawler David F'ritschen Edmonds Development Services and Deborah Stewart 1215 1h Avenue North 19225 13'x' Drivc SE Edmonds, WA 98020 Bothell, WA 98012 Subject: WINTER SEASON EARTHWORK REQUEST Fritsclren/Stewart Property Development 16319 75"' 'lace West, Edmonds, W.A. City of Edmonds Plan Check leo. 20090376 Dear Ann and Pat: J� SEP 2 IIWA is providing geotechnical special inspection services for the above -subject project. We advocate that the site earthwork be permitted to proceed into the upcoming wet season, in a manner such that the subject and adjacent properties do not become unstable, and that no sediment leaves the site. To this end, we recommend the following wet season earthwork measures: 1. All vertical or near vertical excavations should be minimized in extent and duration of exposure. We recommend that the rack -faced gravity retaining walls along the south and east sides of the grading area be constructed in sections, i.e., excavation, foundation preparation, and rock placement should be completed in sections. Concurrent with site excavations down to final grade these south/east site walls should be constructed. Temporary vertical to near -vertical excavations should not be left exposed to precipitation for more than 8 hours. If it is not possible to complete a section of rock - faced retaining wall in one day's work shift, the exposed excavation face should be covered with plastic sheeting secured with sandbags on ropes to protect the excavation from exposure to wet weather. 2. All excavations for underground utilities, such as the storm water detention pipe/vault„ should be quickly completed and backfilled. Underground utility excavations more than 3 feet deep should not be left open overnight. Efforts should be made to minimize the amount of open excavations in order to minimize erasion hazard. 3. The approximately 1.,400 cubic yards of on-site soil that will be excavated is generally very silty/clayey, As such this material is extremely moisture 19730 - 6401 Avenuc W. sensitive and will become difficult or impossible to work with when a ,4c v arty L�mnwood, WA 98036,5957 1�4: 42577,1,0106 1 -ax: 425.77,1,2714 www lwvsagaao.com September 28, 2009 HWA Project No. 2003-174-211300 wet. Tire -mounted equipment should not operate on this exposed silty/clayey soil without a quarry spall construction access path. Excavated material should not be stockpiled on site, but rather loaded directly into dump trucks for offsite disposal. 4. Excavated onsite material should not be re -used as trench or structure backfill. All retaining wall and trench backfill should be comprised of imported, clean and freely - draining sand or gravel, as applicable to the structure being backfilled and/or as specified in the City -approved plans. All TESC provisions must be maintained and, if necessary, replaced or improved upon as construction progresses. In our opinion, the approved TESC plan for the project will be adequate for wet season (winter) site earthwork construction. The underlying objective of the TESC measures is to ensure that no sediment leaves the site, except that which is hauled off via dump truck to an approved soil disposal site. These measures must function effectively in October and November, just the same as they must function in August and September. If at any time any silt fence, catch basin insert, or other measure is observed to be either inadequate or failing, then immediate corrective action should be taken to correct the problem. 6. At the conclusion of each day's work,, the site should be protected and stabilized with plastic sheeting, quarry. spalls, and erosion control curtains as required. During each day's site inspection, the HWA geotechnical special inspector will systematically inspect the site for erosion control and soil stability. Any areas identified by the geotechnical inspector as deficient or requiring attention will be brought to the attention of the general contractor (David Fritschen) for corrective action. 7. The pin piles should be installed after the site retaining walls are completed and after the underground utilities are installed. A gravel working surface should be placed so that the pin pile subcontractor is not exposed to the silty/clayey subgrade. Once all of the pin piles are installed, pile caps and grade beams should be promptly formed and poured. After this wort, is completed, the exposed ground surface should be hydroseeded and/or covered with straw mulch. Provided the above measures are implemented, it is our opinion allowing continued site earthwork during the upcoming winter wet season will not reduce the stability of the subject or adjacent sites, nor increase the risk of sediment leaving the site. 2003-174-21-300 Wet Weather Work Extension 2 HWA GEOSCIENCES INC. September 28, 2009 HWA Project No. 2003-174-21-1300 We are committed to working with the City of Edmonds to ensure that the work is completed safely, and without any problems. We hope the foregoing is adequate for the City of Edmonds to provide permission to continue with site earthwork and grading. If you require additional information or we may be of further service, please contact the undersigned at (425) 774-0106. Sincerely, HWA GEOSmNms INc. Erik O. Andersen, P.E. Senior Geotechnical Engineer, Vice President 2003-174-21-300 Wet Weather Work Extension 3 HWA GEOSCIENCES INC.