2201 100TH ST SW AD SYSTEMS Geotech Report 2022-10-25

GeoTest Services, Inc. April 25,2017 2201 100th Street Southwest, Everett,WA Job No. 17-0186 Utilities It is important that utility trenches be properly backfilled and compacted to reduce cracking or localized loss of foundation, slab, or pavement support. It is anticipated that excavations for new shallow underground utilities will be in native, weathered and unweathered glacial till soils. Trench backfill in improved areas (beneath structures, pavements, sidewalks, etc.) should consist of structural fill as defined earlier in this report. Outside of improved areas, trench backfill may consist of re-used native deposits or clean fill provided it can be compacted to the project specifications. Trench backfill should be placed and compacted in general accordance with the recommendations presented in the Fill and Compaction section of this report and as shown on Figure 4, Typical Utility Trench Section. The native soil is dense to very dense and is not expected to drain efficiently. It should be expected that utility trench backfill is likely to be more permeable than the native soil. As such, up-gradient utility trenches have the potential to route subsurface sources of water towards new construction. GTS recommends that low-permeability trench dams and water stops be considered should utility trenches be installed up-gradient of any planned structures. Prior to implementing these mitigations, a review of the trench depth and gradients should be performed to determine if these mitigations will be included in the final design. Surcharge loads on trench support systems due to construction equipment, stockpiled material, and vehicle traffic should be included in the design of any anticipated shoring system. The contractor should implement measures to prevent surface water runoff from entering trenches and excavations. In addition, vibration as a result of construction activities and traffic may cause caving of the trench walls. Actual trench configurations are the responsibility of the contractor. All applicable local, state, and federal safety codes should be followed. All open cuts should be monitored by the contractor during 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. Trench widths may be substantially wider than under dewatered conditions. Stormwater Infiltration Potential As noted previously, the native soils underlying the subject site and in the vicinity of the subject site are glacially consolidated. In addition, the native soils contain a high fines content. Perched groundwater seepage was encountered in the borings, which were drilled in late March. During the wet weather months, perched groundwater seepage can be expected atop the low permeability, unweathered glacial till soils. Based on our experience working with projects in the nearby vicinity of the subject site, these properties have also encountered shallow, perched groundwater seepage during wet weather months. Thus, based on this information, it is our professional opinion that the underlying native soils would not be suitable for subsurface infiltration. Page 11 of 13