Site Development and Foundation Planning for a Wastewater Treatment Plant Expansion-Paper

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The 8-hectare expansion of a wastewater treatment plant requires raising site grades by about 5 m for flood protection. The ground comprises loose to compact, liquefiable sand over deep, compressible clay, creating risks of earthquake-induced damage and long-term settlement. The expansion could also affect existing facilities, nearby properties, and an adjacent viaduct.<br /><br />The study used three-dimensional settlement analyses to assess historic and proposed loading, soil conditions interpolated between boreholes and cone penetration tests, and the effects of ground improvement, preloading, surcharge, and foundation design. The model was calibrated against available short- and long-term settlement measurements. Vibro-replacement stone columns were proposed to mitigate liquefaction, while preloading would reduce post-construction settlement in the clay.<br /><br />Two development options were compared. Option 1 used broader preload extents and raft foundations in the Lagoon D area. It predicted generally uniform settlement of the proposed facilities, but 75–125 mm of incremental settlement at adjacent viaduct piers. Option 2 removed preload from Lagoon D, limited the preload footprint near the viaduct, and supported facilities there on piled rafts with 40 m friction piles. This reduced predicted viaduct settlement to 45–65 mm overall, with incremental differential movement between piers of up to 5 mm.<br /><br />The results indicate that restricting preload and fill near sensitive infrastructure, and using deep piles for selected structures, can reduce impacts on the viaduct while maintaining reasonable performance for the new plant. The study also identifies longer piles and reduced nearby fill as potential ways to further limit settlement if needed.

Keywords

wastewater treatment plant

settlement analysis

liquefaction mitigation

preloading

surcharge loading

stone columns

piled raft foundation

viaduct settlement

ground improvement

earthquake-induced damage