Increasing the Undrained Shear Strength of Soft Ground Improved by Vacuum Consolidation

This paper discusses the use of vacuum consolidation to improve soft ground and increase its undrained shear strength (Su). Since soft soils, particularly peat and organic clay soils, have very low Su values, the construction of embankments and structures on these soils often cause ground failure. For the trial embankment conducted in this research project, soft ground composed of peat, organic clay and clay was improved by using vacuum consolidation, resulting in a significant increase in Su values, and an embankment 10.7 m in height could be constructed on the improved ground without causing ground failure. The initial Su values of the peat and organic clay were lower than the strength of the clay, namely, 48% (peat) and 59% (organic clay) of that of clay. However, after vacuum consolidation of these soils, their final Su values greatly increased to 199% (peat) and 154% (organic clay) of the final strength of clay. It was demonstrated that the increased Su was the result of synergy of the relatively high normalized undrained shear strength (Su/σ′v, where σ′v is the effective overburden stress) of peat and organic clay, and the unique loading mechanism of vacuum consolidation. It was also demonstrated that in case of a relatively narrow embankment (e.g., road embankment and river levee), the effect of isotropic consolidation also contributes to increase the stability of the vacuum consolidated peaty ground. It was also found that if the (Su/σ′v)NC of the original ground and the over-consolidation ratio (OCR) are known, the (Su/σ′v)OC of the given ground over-consolidated by vacuum consolidation can be predicted.