4626 HARBOR LN 2018-01-02 MF Import

Geotechnical Engineering Evaluation Fox Residence Underpinning and Retaining Wall Design Everett, Washington NGA File No.853512 August 3, 2012 Page 5 or better native soils expected to be encountered in the wall subgrades after fill removal should provide suitable support for the wall fills and new backlill. We recommend that the wall and retained fill footprint be over -excavated down to the underlying medium dense or better native soil and the wall be supported on this rn.tcriel. The total height of the wall will vary, but should be limited to 8.0 feet in total height, including a 1.5-foot base embedment into the native material. The wall recommendations are discussed further in the Wall Design and Construction Recommendations subsection of this report. We recommend that the pin piles for the garage foundation support be installed before the undocumented fill is removed and new retaining wall constructed. The soils encountered on this site are considered moisture -sensitive, and can disturb when wet. We recommend that construction take place during the drier summer months, if possible. If construction is to take place during wet weather, the soils may disturb and additional expenses and delays may be expected due to the wet conditions. Additional expenses could include the need for placing a blanket of rock spalls to protect exposed subgradcs and construction traffic areas. The non -organic on -site soils could be used as structural fill provided they could be compacted to specifications. This will depend on the moisture content of the soils at the time of construction. NGA should be retained to determine if the on -site soils can be used as structural till material during construction. Under no circumstances, should water be allowed to flow on the sloping ground surface both during construction and after construction have been completed. We recommend that all of the footing and roof drains be investigated to ensure that all of drains are functioning property. All drains including existing roof, footing, yard drains, and any new drains associated with the new wall be directed to a new main catch basin to be located downhill of the residence and directed to a level area and outletted with in a level spreader. This is discussed further in the site Drainage subsection of this report. The sloping ground below the wall area should be protected from erosion. We recommend that all disturbed areas be replanted with vegetation to re-establish vegetation cover as soon as possible. Specific recommendations for erosion control are presented in the Erosion Control and Slope Protection Measures subsection of this report. Erosion Control and Slope Protection The erosion hazard for the on -site soils is listed as moderate to high, but the actual hazard will be dependent on how the site is graded and how water is allowed to concentrate. Best Management Practices NELSON GEOTECHNICAL ASSOCIATES, 'NC.