5700 23RD DR W 2016-01-01 MF Import

( result from infi�cration. Such modeling could be conducted by AESI once srorm water volumes � have lxen calculated by others. DETENTION POND BERM CONSTRUCTION Prior to construction of the pond becros, a11 topsoil, organic material, and loose soils should be ezcavated from the subgrade. In general, the berm base should be excavated down iaco the dense, very der,se, or hard nadve soils, and a key cxcavated into this macerial to a depth of at least one foot. Th:,• k:r width should be one-half the berm base width. When the excavation and strip�ing of the �,m subgrade has been completed and approved, ihe upper 12 inches of exposed ground should be recompacted to at least 90 percent of the modified Proctor maximum density using ASTM:D-1557 as the standard. Afcer re��ompaction of the exposed ground is tested and approved, strucaral fill for the berms may be pl�ced to attain desired grades. Strucmral fill is defined as non-organic soil, acceptable to the geotechnical engineer, placed in ma�cimum 8 ineh loese lifcs, with each lift being compac[ed to at least 95 pement of the modified Proctor maacimum density using ASTM:D-1557 as the standard, Fill placed on slopes steeper than 4H:1V (Horizontal:Verrical) should be ded to firm, stable subsoil by benc}ung inco the medium dense to very dense or hacd naqiral soils. The benches should be cut horizoatally across the lull, following ihe contours of the slope. No specific width is required for the benches, although they are usuat?y a few feet wider than the dour being used to cut them. � The contractor should note that any proposed fill soils must be evaivated by ,A�,SI prior to their use in fills. This would require tl�at we have a saznple of the maceria( 48 hours in advance to . perform a Proctcr test and deurmine its field compaction standard. Soils in wtuch the amount of fine-grained maurial (smaller than No. 200 sieve) is greater thaa approximauly 5 percent (measured on the minus No. 4 sieve siu) shouid be considered moisture-sensitiv�, 'The use of moisture-sensitive soil as structural fil1 should be limited co favorable dry weather conditions. The on-site til] soils, pre-Vashon undiffaentiaced Soils� a� some of the Vashon advancc outwash sedimenu may be used to coacn�uct the berms, however tlu.y ge�rally contain significant amounu of silt and are considered moisnue-sensitiv�, In Pddiiion, const:vction equipment traversing the site when tfx soils are wet can cause considerable disnubance. If fil� is placed during wet weather or if proper compaction cannoc be obtai�d, a select unpoR material consisting of a clean, free-draining gra�el and/or sand should be used, pree.drai�8 fill consists of non-organic soil with the amount of fine-gcain rnaterial limited �0 5 p�� bY weight when measured on the minus No. 4 sieve fraction. For construction of betms for unlined ponds, we recommend that only fill soiLs that will provid� a nladvely impen,uabk barrier from �ltration be used, such as the on-siu till. Berm geometry should conform to Snohomish County design stat�dards. Addidonally, we recommend that interior slopes of berms for lined ponds be constr�cted no sueper than 4H:1 V. Upon completion, the disturbed areas should be re-vegetated usin�; a suitable hydroseed mixture. 3