4711 GLENHAVEN DR 2016-01-01 MF Import

unit weight of 35 pounds per cvbic foot (pcfl. If the walls are to be restrained at the top from free movement, the equivalent fluid weight should be �• .ased to 55 pcf. These values are based on a horizontal backfillslope, and assume that no other surcharges due to steeper backfill slopes, hydrostatic pressures, savctural loads, trfric loads, or other surcharges loads will act on die wall. If such surchazges apply, they should be ��dded to the above design lateral pressure. Since craffic loads will lx exerted on the north retainiug walls located within the . upper parkin� area, we recommend that a vaffic load equivalent to 2 feet of additional bacicfill height be added�to the pressures recommended above. Retaining walls should be backfilled with a free-draining washed rock containing less than 5 pement fines ba,,ed on the minus No. 4 sieve fraction. The free-draining material should extend at least 18 inches behind the wall. The remainder of the badcfill may consist of compacted � natural soil. A perforated drainpipe should be placed at the base of the wall. The drainpipe . should be perforated with holes less than one-quarter inch in diameter, and surrounded by t}uee- eighths inch pea gravel. Compaction behind retaining walls should be completed with hand held equipment. Heavy equipment should not be used so that hi�li lateral wall stress does not develop and potentially damage the walls. 10 2 Drilled Piers for RetaininQ Wall Sunnort The fill soils located at the top of the soudi slope are not suitable for support of tbe proposed parking area retaining walls utilizing conventional foundations. Therefore, we recommend that the retauung wall foundations Ue extended throueh the loose fill and gain support within the underlying medium dense to dense till soils. These competeqt soils should be encountered at a � depth of about 15 to 20 feet below existing grades. Given the deep excavations required ro conswct conventional spread footings, we suggest that the retaining wall foundations ucilize drilled piers. Installadon of drilled piers consisu of placing concrete in pre-drilled, open holes. This rype of pier system requires that the sediments will not cave into the holes afrer auger �yithdrawal and during concrete placement. If the holes will not stay open, it may be necessary to case d�e l�oles. Based on observations during our field explorations, it is our opinion that die holes will generally stay open during and immediately following augering. We reconunend die use of a flexible drop clwte or concrete pump �vith its hose extended to the bottom of d�e l�ole to assure that seepage is adequately displaceu during concrete pumping. Tliis method would also tend to reduce the amount of sloughine caused by concrete liitting the side of the l�ole as it is placed. Por preliminary estimating purposes, a 16 inch diameter drilled pier penetratin� ro a depdi of 25 feet below existing erades (minimum 4 feet into bearing lodgemcnt ull soil) may be desiened using an allowaUle venical loading of 10 tons, and an allowable uplifr load'uig of 3.5 l:ips. Each pier ma�� be desiened to resist a lateral loading of 0.5 ton at a deflection of 0.35 inch. Allowable 1'