RecDoc 2016-104272 STMMA (849 Veterans)

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SAN MATEO COUNTYWIDE WATER POLLUTION PREVENTION PROGRAM systems can be terraced to step down a steep slope, maintaining level bed bottoms separated by earthen berms. BASE LAYER • To allow for subsurface water storage, the base must be open graded, crushed stone (not pea gravel), meaning that the particles are of a limited size range, with no fines, so that small particles do not choke the voids between large particles. • When subject to vehicular traffic, all open-graded aggregates should conform to the following or to similar specifications as directed by the municipality: crushed material, minimum 90% with at least 2 fractured faces conforming to Caltrans test method CT 205; have Los Angeles Rattler no greater than 40% loss at 500 revolutions per Caltrans test method CT 211 ; and a minimum Cleanness value of 75 per Caltrans test method CT 211 . Sieve analysis should conform to Caltrans test method CT 202. • If the subbase/base layer is sized to hold at least the Municipal Stormwater Regional Permit Provision C.3 .d volume of rainfall runoff, the area of pervious paving is not considered an impervious surface and can function as a self-treating area (see Section 4. 2). • If the subbase/base layer has sufficient capacity in the void space to store the C.3.d amount of runoff for both the area of pervious paving and the area that drains to it, it is not considered an impervious surface and can function as a self-retaining area, described in Section 4.2. • Pervious paving designed to function as a self-retaining area may accept runoff from an area of impervious surface that has a surface area of up to two times the surface area of the properly-designed pervious paving area. • If an underdrain is used, position the perforated pipe a minimum of 2 inches above bottom elevation of base course. To be considered a self-treating area or self- retaining area, the underdrain shall be positioned above the portion of the base layer that is sized to meet the C.3.d sizing criteria. • Design calculations for the base shall quantify the following: ▪ Soil type/classification and soil permeability rate; if subject to vehicular traffic, k- values (psi/cubic inch) or R-values characterizing soil strength when saturated; LI Fill type if used, installation, and compaction methods plus target densities; • Lifetime expected vehicular traffic loading (in 18,000 lb. equivalent single axle loads or Caltrans Traffic Index); the maximum Traffic Index = 9. LI Drainage routing of detained flows within the open graded subbase/base as well as expected infiltration into in-situ soils, or collection in a raised underdrain if infiltration rate cannot meet design criteria. PAVEMENT MATERIALS • The pavement materials shall not crack or suffer excessive rutting under anticipated traffic loads. This is controlled by designing pervious concrete and porous asphalt surfacing materials and layer thicknesses that minimize the horizontal tensile stress at their base. All pervious pavements benefit from using 6-30 CHAPTER 6 ATTY/AGR.2016.231/849 Veterans RWC, LLC - STMMA