Deep Excavation in Clayey Soils for a Sanitary Sewer Pump Station in Maple Ridge, British Columbia

This paper presents the design and construction of a new pump station and sanitary sewer overflow tank in Maple Ridge, British Columbia. The site is underlain by soft to firm silts and clays, overlying compact to very dense sand, which, in turn, are overlying stiff to very stiff marine clay. The natural groundwater level was as shallow as about 0.5 m depth below the ground surface. Excavation between 4.5 and 11 m depth below the ground surface was required. Vertical shoring walls were necessary to limit the excavation footprint and to cut off groundwater. Sheet pile walls with internal bracing were assessed as the most practical and economical temporary shoring option. The contractor’s alternative design, which was constructed, included sheet pile walls with external bracing using soil anchors. This paper presents the geotechnical design of temporary shoring, the construction details, and settlement monitoring adjacent to the excavation. The measured settlement data are compared against settlement magnitudes and patterns estimated using published correlations. In general, the measured settlement adjacent to the excavation was two to three times the settlement predictions using published correlations. The discrepancy between the predicted and measured settlements is believed to be the result of a combination of construction method and staging, including the time delay between the excavation and soil anchor installation, insufficient soil anchor capacity, the time delay in the installation of new anchors, and lateral deflection of cantilevered sheet piles.