Soil Consolidation and Vacuum Pressure Distribution Under Prefabricated Vertical Drains

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Geotechnical and Geological Engineering






Vacuum preloading is an effective ground improvement technique for treating soft soils. Most previous research focused on the soil’s consolidation within the treatment depth of the prefabricated vertical drains (PVDs). Researches on the soil consolidation and the vacuum pressure distribution below the PVDs are very limited. Another debatable issue is the distribution of vacuum pressure within the treatment depth. Some researchers suggested that vacuum pressure decreases rapidly with depth, so there exists an effective treatment depth. Other researchers suggested that vacuum pressure does not change significantly with depth. Based on the massive monitoring data from the Shanghai Theme Park ground improvement project, it was found that all of the final surface settlements were greater than the target settlements with an average increase of 13%, and the settlement under PVDs contributed 8–18% to the final settlement. Therefore, for a project with thick soft soils under PVDs, the substratum settlement cannot be ignored. Meanwhile, the vacuum pressure below the PVDs was significant to an influence depth of up to 10 m, and the back-calculated coefficient of consolidation was 4 times higher than that determined from laboratory tests. In addition, the measured vacuum pressures were almost constant along the depth up to 20.5 m. This result indicates that the decrease of vacuum pressure with depth may not be an issue if a good sealing system is used.


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