Recent Advances in Soft Soil Consolidation

Buddhima Indraratna    Professor and Research Director, Centre for Geomechanics and Railway Engineering, Faculty of Engineering and Information Sciences, University of Wollongong, NSW, Australia

Acknowledgments

This chapter is based on my 2009 E.H. Davis Memorial Lecture, and the relevant content has been reproduced here with kind permission from the Australian Geomechanics Society (AGS). Much of the contents of this chapter are also elaborated in numerous issues of the Canadian Geotechnical Journal, ASCE Journal of Geotechnical and Geoenvironmental Engineering, Géotechnique, ASCE International Journal of Geomechanics, Ground Improvement—Case Histories (Elsevier, Indraratna and Chu (Eds.), 2005), and several keynote papers at various international conferences. Selected contents from some of these articles have been reproduced with kind permission.

I gratefully appreciate the help of Dr. Rui Zhong, Associate Prof. Cholachat Rujikiatkamjorn, and Dr. Xueyu Geng during compiling and editing the vast amount of data from 15 years of research in vertical drains and vacuum preloading conducted at University of Wollongong (UOW). At least a dozen Ph.D. students, who I have had the pleasure of supervising, have contributed to the contents, as reflected by the cited references. A number of research projects on the application of vertical drains and vacuum preloading have been supported in the past and at present by the Australian Research Council (ARC) through Discovery and industry Linkage projects. My special thanks are conveyed to Vasantha Wijeyakulasuriya, Prof. Harry Poulos, Prof. (Bala) Balasubramaniam, Geoff McIntosh, Prof. Chu Jian, Dr. Jayantha Ameratunga, Prof. Dennes Bergado, Prof. Serge Leroueil, Prof. Jian-hua Yin, Dr. Richard Kelly, Prof. Chandra Desai, Dr. Ana Heitor, Dr. Jayan Vinod, Peter Boyle, Associate Prof. (Siva) Sivakugan, Dr. Brook Ewers, Henk Buys, Prof. Dave Potts, Prof. Gholamreza Mesri, Mark Adams, Prof. Scott Sloan, Prof. John Carter, Prof. Dave Chan, Prof. Sarah Springman, Daniel Berthier, Prof. Maosong Huang, Prof. R. Robinson, and Dr. Sanjay Nimbalkar, who have all helped during some stage of these projects over the years. Collaborations with industry through numerous projects have facilitated the application of theory to practice, and, in particular, the following organizations warrant sincere acknowledgment: Port of Brisbane Corporation, RTA (now RMS), Coffey Geotechnics, Douglas Partners, Arup, ARTC, Austress Menard, and Soilwicks. The support of the University of Wollongong under the Centre for Geomechanics and Railway Engineering, and the dedication of its technical staff, has been an influential factor. Alan Grant, in particular, has been involved with me in the design and building of in-house equipment that are unique for physical modeling of PVD and vacuum preloading simulation.

1.1 Introduction

Due to the rapid development of urbanization and the population increase along coastal lines, exploitation of undeveloped low-lying areas has became a necessary strategy for many countries (Indraratna et al., 1992; Indraratna, 2010). Many coastal cities, such as Brisbane and Ballina in Australia, and Tianjin in China, have to confront the common problems with very soft alluvial deposits. In the long-term, the postconstruction settlement, especially the differential settlement, may cause malfunctions or even damage to the infrastructures. In the short-term, ground failure is a very real hazard to the construction. These problems arise because of the low bearing capacity and high compressibility of soft clays. Thus, it is vital to apply proper ground improvement in these areas before the commencement of any construction to increase the shear strength of the soil as well as to avoid excessive postconstruction settlement.

Surcharge loading with multistaged embankment is a traditional and very successful soil treatment technique in soft soil consolidation. With enough consolidation time excess pore pressure can dissipate to a required extent and thus the surcharge load will be undertaken by the effective stress of the soils. This process both enhances the bearing capacity of the ground and completes a large part of settlement before the construction. It could be dangerous to apply the surcharge loading instantaneously, as failure of the ground may occur if the soil strength cannot bear the weight of the embankment. As a result, in most cases, the embankment is raised in a multistage effort, with designed intervals between stages (Jamiolkowski et al., 1983). However, because most soft clays have very low permeability, it may take a very long time to reach the desired degree of dissipation of excess pore pressure through the only permeable boundaries at the soft clay surface (and bottom if underlain by a high permeable granular layer). This could result in a sizable economic loss due to the delay of the construction schedule.

To allow radial drainage paths to be formed for a faster consolidation process, a number of optional techniques have been adopted in past industrial practices. These include sand drains, sand compaction piles, prefabricated vertical drains (PVDs), gravel piles, and stone columns. Compared to the addition of granular materials, the disadvantage of PVDs is that they do not have the necessary stiffness to provide reinforcement to soft soils. However, their advantages are more obvious and significant, including the avoidance of large lateral ground movements and a relatively low price. Moreover, PVDs are more environmentally friendly than granular materials without a consumption of quarries, which has a negative impact on the environment. Using well-designed mandrels, PVDs can be readily installed into moderate to highly compressible soils.

A meaningful assistant technique that can be combined with PVDs and surcharge preloading is the vacuum. Vacuum pressure is propagated into the PVD from a pump within a membrane system or membraneless system. The suction along the PVD accelerates the dissipation of the excess pore pressure induced by the surcharge loading, and thus reduces the time required to reach the designed degree of consolidation. Different from surcharge loading, the effect of the vacuum is isotropic which allows the soil particles to move inwardly in a lateral direction. This effect can partially neutralize the outward lateral movement of soils induced by the surcharge loading. Consequently, PVDs combined with surcharge loading and vacuum have become more and more popular in soft soil consolidation.

This chapter describes the fundamentals of vacuum-assisted consolidation, the concepts of 3D-to-2D conversion in numerical modeling, the constitutive model for soils under cyclic loading, the application of PVDs in past case histories, and design charts of PVDs for industrial convenience.

1.2 Principles of vacuum consolidation via prefabricated vertical drains

1.2.1 Fundamentals of vacuum preloading

The radial drainage paths formed by the existence of PVDs accelerate the consolidation and thus reduce the duration of the ground improvement process. Due to tight construction schedules, a higher embankment that is heavier than the weight of the planned infrastructure is always required to expedite the consolidation to an acceptable degree (usually more than 95%). This method is costly and can even be risky as instability can occur if the soil’s effective stress has not been increased enough at the time of a further surcharge increase. The use of vacuum...