600 RIVERSIDE RD Geotech Report 2022-10-25

Landau Associates 3.6.2 Trenching and Excavation Support We understand that excavations up to about 13.5 ft below the final grade will be required to facilitate j installation of the proposed utility structures. Based on the soil conditions observed in our explorations, we anticipate that it will be possible to excavate the onsite soil using conventional construction equipment. Actual excavation configurations and maintenance of safe working conditions, including temporary excavation stability, should be the responsibility of the contractor. All applicable local, state, and federal safety codes should be followed. Temporary excavations in excess of 4 ft should be sloped in accordance with Safety Standards for Construction Work Part N, WAC 296-155-657, or shored. In the absence of groundwater seepage, the fill and alluvial soil expected within the trench zone classify as Type C soil and may be inclined no steeper than 1%H:1V according to WAC 296-155-657. In areas where groundwater flow is present, an unstable condition may develop, requiring flatter slopes, temporary shoring, and/or dewatering. All open cuts should be monitored during and after excavation for any evidence of instability. If instability is detected, the contractor should flatten the side slopes or install temporary shoring. If groundwater or groundwater seepage is present, and the trench is not properly dewatered, the soil within the trench zone may be prone to caving, channeling, and running, thereby resulting in trench widths that may be substantially wider than a properly dewatered trench. Trench boxes should provide suitable support for shallow excavations in the fill and upper sand unit, provided the groundwater level is below the base of the excavation and settlement sensitive structures or utilities are not situated immediately adjacent to the excavation. Where a trench box is used to support excavations, one or both sides of the trench are likely to cave against the box. The caving may extend out on either side of the trench for a distance approximately equal to the depth of the trench. Additional bracing or sheeting may be required where the near edge of the trench will be closer than about 1.5 times the trench depth to settlement -sensitive structures or utilities. When the trench box is moved, precautions should be taken to minimize disturbance of the pipe, underlying bedding materials, and native soil. Excavations that will be open for an extended period of time, such as at the proposed pump stations may require shoring. Use of a temporary shoring system, such as cantilevered sheet piles, will also help reduce the amount of potentially contaminated soil and groundwater that has to be handled and disposed. If used, the temporary shoring system should be designed and constructed to support lateral loads exerted by the retained soil mass. In addition, any surcharge loads from construction equipment, construction materials, and excavated stockpiled soil should be included in the shoring design. The primary disadvantage to using sheet piles is that the sheets will probably have to be installed through the aquitard, which could potentially allow groundwater from the upper contaminated aquifer to reach the lower aquifer, particularly after the sheet piles are pulled. As a Geotechnical Engineering Report 0121030.010.011 Riverside Business Park March 22, 2018 Public Access Trails and Roadway Improvements 3-7