3535 RIVERFRONT BLVD Geotech Report 2023-02-22

September 24, 2018 HWA Project No. 2015-061-21 separates the upper perched groundwater table from the lower tidally influenced groundwater table. These perforations through the peat and organic silt deposits act as windows that allow the tidally influenced groundwater table to extend up into the trench backfill across the site. Prospective contractors should be aware that excavations which expose existing trench backfill are located near existing utilities and will most likely require increased dewatering effort in comparison to excavations advanced wholly in native soils. Prospective contractors should also expect that groundwater within or adjacent to existing utility trenches will vary tidally as shown on Figure 4. The ground water conditions herein are for the specific dates and locations indicated and, therefore, may not necessarily be indicative of other times and/or locations. It is anticipated that ground water conditions will vary depending on the season, local subsurface conditions, and other factors. 4. CONCLUSIONS AND RECOMMENDATIONS 4.1 GENERAL The 3-Acre Park site is underlain by a sequence of fill over compressible and potential liquefiable alluvial soils. The soils will make development of the site challenging. The compressible soils are expected to undergo consolidation settlement under the weight of the soils used to increase site grades as well as the loads from the proposed structures. The magnitude and orientation of the anticipated settlement are such that they would be expected to damage the proposed improvements. A preload fill has been constructed in the vicinity of the proposed restroom structure to reduce the magnitude of future settlements in this area. However, long term settlements are still expected to occur across the site. Proposed improvements should be designed to accommodate the anticipated long term settlement. After removal of the preload, long term settlements are still expected to occur over the design life of the proposed structures. We recommend that the restroom structure be supported on a mat foundation capable of tolerating the anticipated long term settlement in this area. We understand that the proposed picnic shelter structures are settlement sensitive and will not be able to tolerate the anticipated long term settlements. Therefore, we recommend that the picnic shelter be supported on pin pile foundations, bearing in the very dense alluvial soils encountered at depth, or the associated foundation systems be constructed with the ability to relevel the structure over time. Given the potential for damage to the proposed concrete flat work, we recommend that the pin pile foundations be installed prior to construction of the flat work, if utilized. Proposed underground utilities servicing the restroom structure will be subject to differential settlements across the site. All utilities should be routed to minimize differential settlement and use flexible connections to accommodate anticipated long term settlement. Utilities should be Final Geotechnical Report-3-Acre Park.docx 6 HWA GeoSciences Inc. I