Compensated Raft Foundation on a Preloaded Soil Improved by Vertical Drains

Compensated raft foundation is used to support heavily loaded structures resting on soft and low-permeable soils. It also aids in the reduction in settlement due to lowering of the stress transferred to the underlying soil. Such soils often require pre-loading and pre-treating with vertical drains to arrest the majority of the final settlement under the actual construction in less time. This paper reports the attainable efficacy in adopting the above methods for the foundation of the 10^th Boy's Hostel at the Indian Institute of Technology, Guwahati. Soft, clayey, and marshy fill soil is prevalent in the hostel site. FEM modelling using PLAXIS 2D v2012 has been used to interpret the possible benefit of the adopted method. In comparison to the condition when no preload was used, it has been observed that the application of staged preloading resulted in a reduction of the total settlement in the range of 45-90%. Moreover, in comparison to the untreated soil, the application of vertical drains significantly accelerated the rate of consolidation and dissipation of pore-pressure (~30-50% and 15-25 times, respectively). The comparative results reveal that the adoption of the above methods will substantially help improve the settlement characteristics of the soft soil site in discussion.