Effect of vertical drains and preloading on the creep behavior of soft clay

摘要

Construction on very soft clay is challenging due to its long-term settlement. Surcharge preloading is one of the most effective soil improvement techniques for such compressible soil formation. However, post-construction settlement is usually underestimated in slightly overconsolidated soil condition. This underestimation is due to neglecting the effect of the secondary consolidation after eliminating the primary consolidation settlement. Numerical analysis utilizing 3D FEM is performed for an embankment constructed on soft soil deposits improved using preloading accompanied with prefabricated vertical drains (PVDs) in the Eastern Nile Delta of Egypt. The numerical analysis is then used to estimate the secondary consolidation value for the investigated case study and was found to form about 78.5% of the total residual settlement expected during the project's service time. A parametric study is carried out to determine the effect of the preloading stress, the length of the PVDs, the spacing between the PVDs, and the degree of soil consolidation under the preloading stress, on the secondary compression index. The results show that the preloading stress is the main factor affecting the improvement of the secondary compression index. As the preloading stress increases from about 50% to 250% of the permanent loads, the secondary compression index decreases to be 50% to nearly zero of its initial value. Additionally, as the length of the PVDs increases, the secondary compression value decreases. The secondary compression value is reduced from 68 to 45% of its initial value as the PVDs' length increases from 22 to 100% of the thickness of the total compressible layer. Furthermore, the secondary compression value reduced from 62 to 42% of its initial value as the spacing between the PVDs reduced from 6.00 to 0.75 m. Finally, the increase of the degree of consolidation of the soft clay under the preloading surcharge from 65 to 95% decreases the secondary compression index from 56 to 46% of its initial value.