Vacuum-PVD improvement of reclaimed deep excavation pits filled with soft clay lumps

Improvements to settling of soft clay lumps in landfill were investigated using the Vacuum Consolidation Method (VCM) with surcharge loading applied to clay lumps deposited in deep ponds (15–20 m) for housing development. Unlike naturally deposited clays, soft clay lumps are heterogeneous and highly compressible, making settlement predictions challenging. VCM is used widely with natural soft clays but rarely with soft clay lumps in reclaimed or backfilled areas. Thus, site monitoring data were collected and used for back-analysis. The coefficient of horizontal consolidation (C_h) is a crucial parameter; however, existing values are based on naturally deposited layers. The obtained C_h values for soft clay lumps were in the range 1.39–4.13 m²/year. Furthermore, the study proposed a new empirical approach for calculating settlement in soft clay lumps improved by applying the VCM, estimating that settlement at a 90% degree of consolidation corresponds to 4%–9% of the prefabricated vertical drain installation depth. This finding should enable more accurate pre-construction predictions by improving existing knowledge, as previous equations have been appliable only to naturally deposited clays. Overall, the research should contribute to the planning and design of soft ground improvement projects involving reclaimed or backfilled soft clay lumps.