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HomeMy WebLinkAbout6801 Panama Lane -Percolation Letter - Winco Panama Ashe (6) 1 Terracon Consultants, Inc. 2817 McGaw Avenue Irvine, California 92614 P [949] 261 0051 F [949] 261 6110 terracon.com May 29, 2013 WinCo Foods, LLC 650 North Armstrong Place Boise, Idaho 83704 Attn: Mr. David Van Etten P: 208.672.2017 E: david.vanetten@wincofoods.com Re: Proposed Infiltration Basin Summary Letter Proposed WinCo Foods Store - Bakersfield Bakersfield, California Terracon Project No. 60125053 Dear Mr. Van Etten: As requested by you, we are providing this summary letter regarding our understanding of the proposed stormwater infiltration pond at the subject site. Background Terracon provided a summary letter dated May 21, 2013 that summarized our work related to the proposed infiltration basin area. As described in that letter, our work consisted of borehole infiltration tests as well as hydraulic conductivity testing (required by the City). The borehole infiltration tests resulted in measured rates of 5 inches per hour for sandy silty clay soils and 12 inches per hour for silty sand over silty clay soils. CTA Architects and Engineers (CTA) designed the infiltration basin and summarized their design method in a letter dated May 22, 2013. Soil Conditions Terracon’s borings in the vicinity of the proposed basin show sandy silt clay overlying silty sand, which in turned overlaid poorly graded sand. As discussed with you, Terracon recommends extending the proposed basin to the silty sand layer. Borings Perc-3 and Perc-4 (see Terracon’s addendum report dated May 7, 2013) showed this layer was encountered at a depth of 11 ½ and 10 ½ feet, respectively. Summary of CTA’s Design Based on the CTA letter referenced above, we understand the pond was designed by taking a measured infiltration rate of 5 inches per hour, applying a factor of safety of 4.5, and then using the result to determine a volumetric infiltration rate. Their letter shows they estimate the pond will drain in a little less than 7 days using this rate. Infiltration Basin Summary Letter Proposed WinCo Foods Store - Bakersfield ■ Bakersfield, California May 29, 2013 ■ Terracon Project No. 60125053 Responsive ■ Resourceful ■ Reliable 2 Conclusions As indicated above, Terracon recommends that the basin extend to the silty sand layer to increase the drainage rate above that within the sandy silty clay layer. A simple calculation using the infiltration rate and the depth of water within the basin (as previously published by CTA) shows that the basin will drain in 7 days or less. We note that they used the slower/more conservative infiltration test result in their calculations and extending the pond to the silty sand layer should improve infiltration rates. As previously recommended in our original report, our addendum report, and our summary letter (May 21, 2013), infiltration testing should be performed after the basin has been excavated to verify the design infiltration rates. It should be noted that siltation and vegetation growth along with other factors may affect the infiltration rates of the infiltration areas. We expect that the infiltration rate of the basin will decrease over time unless adequate maintenance is performed to remove vegetation, dust, debris, grease/oil, and fine grained soils. Failure to adequately maintain the basin will result in slower infiltration times. General Comments Terracon’s scope of work related to the proposed basin was limited to field exploration, performing in-situ infiltration testing, and performing laboratory permeability testing in general conformance with the requirements of the City of Bakersfield. The design of the proposed basin was performed solely by CTA based on the results of our tests. Any conclusions in this letter about the performance of the basin are based on the validity of CTA’s design, which we have not independently verified. Terracon is not responsible for the design of the basin or any associated features. Site safety during and after construction is the responsibility of others. The analysis and recommendations presented in this report are based upon the data obtained from the borings and tests previously performed for this project. This report does not reflect variations that may occur between borings, across the site, or due to the modifying effects of construction or weather. The nature and extent of such variations may not become evident until during or after construction. If variations appear, we should be immediately notified so that further evaluation and supplemental recommendations can be provided. This report has been prepared for the exclusive use of our client for specific application to the project discussed and has been prepared in accordance with generally accepted geotechnical engineering practices. No warranties, either express or implied, are intended or made. Site safety, excavation support, and dewatering requirements are the responsibility of others. In the event that changes in the nature, design, or location of the project as outlined in this report are planned, the conclusions and recommendations contained in this report shall not be considered valid unless Terracon reviews the changes and either verifies or modifies the conclusions of this report in writing.