7010 COLBY AVE 2023-06-01 - Laserfiche WebLink

Mr. Paul Korenovsky ES-6067 June 20, 2018 Page 9 Drainage Discrete zones of perched groundwater seepage should be anticipated in site excavations depending on the time of year grading operations take place, particularly within deeper excavations for utilities. Temporary measures to control surface water runoff and groundwater during construction would likely involve interceptor trenches and sumps. ESNW should be consulted during preliminary grading to identify areas of seepage and to provide recommendations to reduce the potential for instability related to seepage effects. Finish grades must be designed to direct surface water away from the new structures and/or slopes. Water must not be allowed to pond adjacent to the new structure and/or slopes. In our opinion, foundation drains should be installed along the building perimeter footings. A typical foundation drain detail is provided on Plate 4. Infiltration Evaluation As requested, we evaluated the capacity for infiltration into native soils in accordance with project development plans. Applicable requirements and procedures of the referenced 2014 SWMMVVW were incorporated into our evaluation. At this time, we understand the project will incorporate limited infiltration facilities (i.e. drywells, permeable pavement, rain gardens, etc.) into the design. ESNW conducted two small-scale pilot infiltration tests (PITs) within the proposed development area. The infiltration tests were conducted approximately four feet below the subgrade surface at TP-1 and approximately three feet below the subgrade surface at TP-2. The PITs were completed in general accordance with the 2014 SWMMWW test method described in Volume III, Chapter 3. Upon completion of the six-hour soaking period, measurements were collected at each PIT location. One test pit was advanced further to verify soil stratigraphy and to check for ponding and confining layer characteristics. The in-situ testing yielded measured infiltration rates of approximately 3.0 inches per hour (iph) at TP-1 and 1 iph at TP-3. Per the 2014 SWMMWW, correction factors must be applied to the measured infiltration rates and incorporated into the following equation: Design Infiltration Rate = Measured Infiltration Rate x CFv x CFT x CFM. • Short-term Infiltration Rate (Measured) —TP-1 3.0 iph • Short-term Infiltration Rate (Measured) —TP-3 1.0 iph • Site Variability CFv= 0.7 • Test Method (Small PIT) CFT= 0.5 • Degree of Influent Control CFnn = 0.9 Earth Solutions NW,LLC