Effects of Sand Compaction Piling on Soft Ground

Sand compaction piling (SCP) is a cost-effective method of ground improvement which is commonly used to improve soft seabed soils prior to land reclamation works. This method involves driving closely-spaced sand columns into the soft seabed to form a grid of sand columns, which imparts higher strength and stiffness to the improved ground. The installation of these sand compaction piles often involves a large amount of cavity expansion displacement of the soft clay around the sand piles, which in turn, leads to significant changes in the strength and stiffness of the soft clay around the sand piles. In practice and research, the properties of the soft clay are taken to be those of the in-situ soil. 

Figure 1: In-flight sand pile installer on the NUS Geotechnical Centrifuge.

Between 1994 and 1999, the Centre for Soft Ground Engineering (CSGE) developed an in-flight model sand pile installer, which, to date, is still the only one of its kind in the world. This apparatus (Figure 1) was developed with a research grant from the Housing & Development Board (HDB). In the past year, a research team at the CSGE has been using this equipment to conduct a range of experiments into the method of ground improvement. 

The improvement in the strength of the softy clay is around 25% to 50%. 

The results of these experiments allow the changes in total stress and pore pressure due to the installation of sand compaction piles to be quantified. Comparison of the measured changes in lateral stress and pore pressure with conventional plane strain cavity expansion theory shows that the latter gives a reasonably good estimate at large depth for the entire installation process. However, deviation from plane strain cavity expansion theory was noted at shallow depths. To account for the effect of the ground surface, a semi-empirical plane stress cavity expansion theory was proposed for the shallow zones. The establishment of the two limits formed by the plane strain and plane stress theories allows semi-empirical relations to be fitted to the data. These findings also indicated that, in order to mobilize significant set-up of stress in the improved ground, there must be substantial further cavity expansion during the sand injection stage of SCP. The cumulative total stress and pore pressure increment at a given location due to the installation of multiple piles in a grid is also reasonably estimated by superimposing the increments due to the installation of each pile. The measured and computed undrained shear strength after the dissipation of the excess pore pressure is also higher in the clays enclosed within the SCPs. The quantum of improvement in the strength of the soft clay due to the cavity expansion displacement is typically of the order of 25% to 50%. 

Tests were also conducted to quantify the relative performance of different methods of installing sand piles. The results of this comparative study show that the effects of cavity expansion displacement in the soft clay do lead to a significant improvement in the strength and stiffness of the soft clay. The sequence of installation also affects the resulting properties of the soft clay, although this is second-order compared to the effect of installation method.

Some of the results of this study have been published in international journals and conferences. The significance of the published results was recently highlighted by a detailed citation of the teamís research by Professor Osamu Kusakabe in a keynote address in the International Conference on Physical Modelling in Geotechnics, in Newfoundland, Canada.

Contact Person: Assoc Prof FH Lee 
Tel: 68742274
Fax: 67791635 
Email: cveleefh@nus.edu.sg