600 RIVERSIDE RD Geotech Report 2022-10-25

1 P 1 1 1 Landau Associates In contrast, it is anticipated that a significant portion of the excavations for the proposed pump stations will extend below the groundwater table, especially if they are installed following an extended period of wet weather or during the winter and spring months. Because the fill and upper sand units are relatively permeable, a substantial flow of water into the excavations will occur where excavations extend below the groundwater level. Groundwater seepage will tend to destabilize excavation slopes and increase lateral loads on shoring systems. In addition, groundwater flowing upward into the excavation can cause "quick" conditions. Under "quick" conditions, the relative density of the sandy onsite soil will be reduced, resulting in increased pipe and pump station settlement during and after construction. Because of these impacts, we recommend that dewatering be accomplished so that excavation and placement of the pump stations, pipe, pipe bedding, and backfill materials is completed in the dry. In our opinion, a combination of wells or well points and sumps will be required to effectively dewater excavations that extend below the groundwater level. Extended dewatering could result in a lowering of the water table, which could cause consolidation of the underlying alluvial soil and settlement of the ground surface. The magnitude of the settlement and its lateral extent would depend on the amount of change in the water level; the length of time the water level was lowered; and the compressibility, thickness, and permeability of the underlying soil. If wells are used to facilitate installation of the proposed improvements, LAI should review the contractor's proposed dewatering system design to evaluate potential impacts. To the extent possible, considering the potential for settlement of adjacent improvements and structures, dewatering wells should be operated continuously for as long as they are needed in a given area. Turning the wells off at night and turning them back on the next day causes rapid drawdown conditions in the soil and tends to increase caving and sloughing of excavation slopes. The contractor will be required to manage any extracted groundwater generated during construction consistent with all local, state, and federal laws, including all permitting, treatment, and disposal activities. The contractor will be required to either obtain a temporary NPDES discharge permit or waste discharge permit (as applicable) and abide by all associated pretreatment and monitoring requirements, or temporarily store all extracted groundwater onsite in holding tanks for transport, treatment, and disposal in conformance with applicable laws and regulations. Groundwater resulting from construction dewatering should be handled in accordance with the Soil and Groundwater Management Plan (LAI 2017). It should be noted that a wide range of groundwater arsenic concentrations exist at the site, some of which may be high enough to preclude discharge to the existing sanitary sewer system without pretreatment. Therefore, excavation procedures at the site should be implemented to minimize the need for construction dewatering. Use of tight sheeting and dewatering from within the shored excavation is considered a feasible alternative to limit the quantity of groundwater generated. Staging excavations to coincide with periods of seasonally low groundwater elevations is also another method that could be used to limit the quantity of extracted groundwater. Geotechnical Engineering Report 0121030.010.011 Riverside Business Park March 22, 2018 Public Access Trails and Roadway Improvements 3-6