Soil liquefaction and its associated ground failures during an earthquake are major potential hazards. The important aspect of geotechnical earthquake engineering is to mitigate liquefaction and its associated effects for assuring the safety of foundations. There are different ground improvement techniques available for improving the liquefaction resistance of sand deposits.

Ground improvement techniques such as sand compaction piles, stone columns, encased stone columns and prefabricated vertical drains are the most commonly adopted techniques for dissipating generated pore-water pressures and for improving the strength of the soil. Though many studies are using this technique available for improving the seismic performance of liquefiable soils, the performance evaluation of reinforced ground subjected to sequential acceleration conditions was limited.

The present study conducted at CSIR-Central Building Research Institute (CBRI), Roorkee, aims to assess different ground improvement techniques for liquefaction mitigation and to evaluate the efficiency of stone columns and encased stone columns reinforced ground subjected to independent and sequential acceleration conditions.

Uni-axial shaking table tests were performed on poorly graded sand with varying density levels subjected to different acceleration conditions. For experimental investigations, a tank of size 1.4m × 1m × 1m was used. The ground having 0.6 m depth was prepared using wet sedimentation technique with 40% and 60% relative density. For soil reinforcement, stone columns and encased stone columns having area replacement ratio 2.67%, 5% and 9.6% were selected for the study (Fig. 1-3).

The prepared ground was then subjected to independent and repeated incremental sinusoidal acceleration loading of 0.1g, 0.2g, 0.3g and 0.4g simulating medium to very high earthquake magnitude. In addition to acceleration loading, shaking time also plays a major role in initiating liquefaction and reliquefactionbehaviour. Hence, the entire acceleration loading was applied for a period of 40s with a total of 200 sinusoidal cycles.

For repeated loading, the acceleration loading was applied sequentially to the model ground after complete dissipation of excess pore water pressure generated from previous acceleration loading. The variation in soil density, maximum generated excess pore water pressure, soil displacement and foundation settlement at each acceleration loading was monitored continuously and compared with and without ground improvement techniques (Fig. 4). 

Using the PLAXIS 3D Finite element program, experimental test results were validated and parametric studies were performed. From combined experimental and numerical results, the efficiency of soil reinforced ground under independent and sequential seismic loading conditions was assessed and the typical design system for field application was presented (Fig. 5-7).

Based on the results from the present investigation, the following conclusions are drawn:

1. The liquefaction and reliquefaction resistance of untreated soil deposit depends on past input, motion characteristics and its shaking duration. Also, the occurrence of reliquefaction increases with the increase in subsequent acceleration loading.  
2. Installation of ordinary and geosynthetic encased stone columns effectively reinforce the liquefiable deposits and make resistant against liquefaction compared to other ground improvement techniques. 
   1. When stone column improved ground is subjected to repeated acceleration loading, the intrusion of surrounding soil reduces drainage characteristics, induce the generation of pore water pressure and affect column performance. 
   2. In case of an encased stone column, provision of geosynthetic encasement confines the stone aggregates inside the column preventing column deformation and controlling clogging. This resulted in effective load carrying capacity and drainage characteristics even under repeated loading events.  
3. The experimental study verified that provision of proper drainage is highly required for improving liquefaction resistance. To mitigate liquefaction and reliquefaction, the ground improvement technique should have a combination of densification and effective drainage system. This will help in improving the efficiency of ground improvement technique in liquefiable soils, especially under repeated earthquake conditions.

Contributed by S. Ganesh
CSIR-Central Building Research Institute, Roorkee