9900 19TH AVE SE Geotech Report 2018-01-02 MF Import

Geotechnical Engineering Report Home Base Warehouse - Everett July 31,1997 NCA File No. 213497 Page 11 1 should be placed over the sand or gravel. All slabs should be supported on medium dense or better native soils or structural fill extending to these soils. Pavements Subgrades in pavement areas should be prepared as described in the Site Preparation and Grading suls- section. Structural fill, if needed, should be placed and compacted. Prior to paving, the subgrade should be proofrolled with a heavy rubber -tired piece of equipment such as a loaded 10-yard dump truck to identify soft or yielding areas that require repair. We should be retained to observe the proofrolling and recommend repairs, prior to placement of the asphalt or hard surfaces. ADDITIONAL CONSIDERATIONS Although recommendations for the detention system were not in our scope of services, there are some geotechnical issues that we thought could help in your design. As we understand it, the 6-inch diameter perforated pipe that is to be placed in the bottom of the ravine is to recharge water into the proposed fill, ( and then drain water out of the fill. If this is the case, there should be concern about washing or "piping" fines out of the fill, resulting in loss of ground, and the possibility of loss of support for footings and/or slabs above the pipe. To lessen the potential for this to occur, there should be a filter around the perforated pipe. This could be accomplished by surrounding the pipe in free -draining material such as pea gravel or drain rock, and completely wrapping the free -draining material with filter fabric. The site geology includes relatively impermeable till over relatively more permeable outwash. Outwash was encountered in Test Pit 5, located in the proposed on -site detention pond, at an elevation estimated to be above the bottom of the pond. If the base of the pond were in outwash, the pond could be expected to leak. If this is not the intent of the pond, it may be necessary to line it. The on -site till should be a suitable soil liner material, and liner design could be based on an estimated permeability of 0.00001 centimeters per second (1'10-5 cm/sec). The necessity of a liner can be evaluated during construction after the pond has been excavated and the base of the pond is exposed. As discussed in the Subsurface Conditions sub -section, the test pits were not deep enough to extend to the bottom of the planned excavation for the detention vault and on -site detention pond. Although it is apparent that the soil beneath the bottom of the test pits is glacially consolidated, we were not able to NELSON-COUVRETTE & ASSOCIATES, INC.