3301 SEAWAY BLVD WHOLE SITE 2025-10-30

Mr. Greg Bertch January 18, 2017 In addition to the overall slope stability, we understand there is concern that the proposed flexible membrane liner will leak. As we addressed previously, we acknowledge that the FML can leak. However, in this case, the liner is being installed to prevent increases in natural groundwater seepage levels that may otherwise occur and potentially impact the current stability of the adjacent steep slope. In our opinion, a reasonable allowed leakage rate (ALR) for this condition would be one that matches the current amount of precipitation that enters the ground by infiltration. Even if the liner leaks, it is more than likely that the leakage rate would be less than this amount of water and; therefore, potential impacts to the global stability of the steep slope would be avoided if not improved. Localized leaks in the liner adjacent the cast in place concrete containment wall could result in excessive seepage velocities and potential for piping below and around the wall foundation. Design would include measures to prevent this occurrence. Such measures could include installation of sub -drains adjacent the wall foundation to capture and control this seepage to prevent piping or increasing the length of the seepage flow path thereby reducing the critical flow gradient below the wall foundation. This can be accomplished by increasing the depth of the proposed footing, constructing a longer vertical keyway, or increasing the length of the footing. One of these options would be included in the design of the cast -in -place retaining wall. We trust the additional information presented is sufficient for your current needs. If you have any questions or require additional ipf9mation, please call. Encl: i!9 l7 Location Plan Subsurface Explorations i Project No. e-7342 �