4626 HARBOR LN 2018-01-02 MF Import

Geotechnical Construction Monitoring Final Letter Fox Residence Underpinning and Retaining Wall April 24, 2013 NGA File No. 8592B13 Page 2 the structural engineer inspected the brackets. We then observed and evaluated the construction of the Ultra Block retaining wall along the west and north side of the residence. The wall within the northern section was 10.0 feet tall (four blocks high) while the western section of the wall was 7.5 feet tall (three blocks high). Before construction of the wait and placement of the wall backtill, the subgrade was evaluated. We interpreted the subgnde to be native material in a mcdium dense or better condition, and adequate to support the wall. An approximate 6-inch thick Icvcling pad consisting of 5/8-inch minus crushed rock was placed on the wall subgrade and compacted with a vibrating drum roller. The wall blocks consisted of full and half Ultra Block were placed in a staggered pattern. A tapered Ultra Block was utilized at the connection of the northern and western wall sections. The base of the wall was founded on the competent native material and was also embedded a minimum oft8-inches below the ground surface. The entire height directly behind the wall was backfilled with a one foot layer of 2-inch washed rock drainage layer. We observed that a 4-inch diameter, PVC, perforated drain pipe was placed behind the base of the walls. A solid 4-inch PVC drain pipe for the roof drains was also placed behind the base of the retaining wall. The roof drain tightline and will foundation drain were then directed to discharge into an approximate 6-foot diameter by 9-foot deep drywell to the southwest of the wall. The sides of the drywcll generally exposed light brown fine sand soils with silt soils exposed at the base of the drywell. The drywell was backfrlled with 2-inch washed rock up to three feet below the existing ground surface. The drains were directed to discharge directly onto the top of the washed rock. Filter fabric was placed on the washed rock and then covered with approximately three feet of native soils to the existing ground surface. Geogrid consisting of Mirafi 8XT and 3XT was placed perpendicular to the wall and stretched tight behind the wall on top of each layer of block. The 8XT geogrid was utilized in the lower two levels of the northern portion of the wall and within the lower level of the western portion of the wall. The 3XT gcogrid was utilized within the upper level of both the northern and western walls. The gcogrid extended hack a minimum of 9.0 feet behind the back of the wall for the -northern portion of the wall and a minimum of 7.0 feet for the western portion of the wall. The backtill behind the wall generally consisted of imported gray -brown, fine to medium sand with silt and gravel and imported gray fine to medium sand with gravel that was compacted in thin lifts with a large drum roller or jumping jack. Our density test NELSON GEOTECHNICAL ASSOCIATES, INC,