Vertical cyclic loading response of geosynthetic-encased stone column in soft clay

https://doi.org/10.1016/j.geotexmem.2020.07.006Get rights and content

Highlights

  • Laboratory tests have been conducted to study the dynamic responses of GESCs in soft soil under cyclic loads.

  • Soil-column stress distribution, accumulated settlement and column bulging are obtained.

  • Variation of the excess pore water pressure in GESCs improved ground subjected to cyclic loads is obtained.

Abstract

Dynamic responses of the geosynthetic-encased stone column (GESC) supported embankment under traffic loads have become a hot topic. This study investigates the responses of GESC improved ground under vertical cyclic loading. A series of laboratory tests in a designed model test tank have been carried out with different loading parameters (varied loading amplitudes and frequencies), different column dimensions (varied encasement lengths and column diameters). In the tests, the soil-column stress distribution, accumulated settlement of loading plate, excess pore water pressure in the surrounding soil and lateral bulging of the stone column are monitored. Experimental results indicate that the vertical stress on the stone column increases with the increment of encasement length, and decreases with the increment of column diameter, loading amplitude and loading frequency. The increasing stress on the surrounding soil leads to a greater accumulated settlement of the loading plate and excess pore water pressure, while the increasing stress on the column leads to larger lateral bulging of the column. Excess pore water pressure dissipates effectively through vertical and horizontal drainage channels provided by the stone column and the sand bed. The geosynthetic encasement prevents the clay from obstructing the drainage channel by filtration and guarantees the drainage effect.

Introduction

A growing number of embankments have to be constructed over soft soil to satisfy the demands of the rapid development of expressways in China. Under the unsuitable geotechnical conditions, ground improvement technique has become an important issue, and the ground improvement methods can be divided into following categories: densification, replacement, drainage, dewatering, consolidation, chemical stabilization, reinforcement, thermal and biological treatment (Han, 2015). The ordinary stone column (OSC), as the subcategory of replacement methods, has been widely used to address the problematic soft soil (e.g., Basack et al., 2017; Salem et al., 2017). It can reduce the loads transferred to the soil and facilitate the rapid dissipation of excess pore water pressure. Surrounding extremely weak soil (i.e., the undrained shear strength cu of the soil is less than 15 kPa), however, may not be capable of providing sufficient circumferential confinement to prevent the column from excessive radial bulging. To solve this problem, encasing the OSC with geosynthetics is an alternate ground treatment method, namely the so-called geosynthetic-encased stone column (GESC) method.

The geosynthetic-encased stone column (GESC) was first proposed by Van Impe (1989) and extensively studied by many researchers. Its performance has been investigated by numerous laboratory tests, such as uniaxial tests (e.g., Chen et al., 2018; Gniel and Bouazza, 2010), triaxial tests (e.g., Wu and Hong et al., 2009), single-column model tests (e.g., Gu et al., 2016; Miranda et al., 2017), group column model tests (e.g., Ali et al., 2014; Debnath and Dey, 2017; Ghazavi and Afshar, 2013), and GESCs supported embankment model tests (e.g., Chen et al., 2015). Great achievements on GESCs have been made through above laboratory investigations, however, they are restricted for static loaded cases.

A sufficient understanding of cyclic behavior of the stone column is essential, as the GESC improved foundation also carries dynamic loads such as moving traffic loads in transportation corridors (Arulrajah et al., 2009; Basack et al., 2016). However, investigations on the performance of either OSCs or GESCs under vertical cyclic loading are relatively scarce. The sporadically available literature on this topic is the investigations for vertical cyclic loaded OSCs in clay bed by Raju et al. (2013), Fahmi et al. (2018), and Karkus and Jabbar (2019). Recently, the GESCs improved ground under vertical cyclic loading has been investigated. Numerical analyses of GESCs improved soil ground under vertical cyclic loading by Ardakani et al. (2018) concluded that end bearing columns provide higher resistance than floating columns. Laboratory model tests have been performed by Yoo and Abbas (2019, 2020) to study the cyclic loading response of GESCs in the soft clay and the sand, considering the effect of the geosynthetic stiffness, encasement length and cyclic loading characteristics. Although the above studies provide a basic understanding of the vertical cyclic loading response of GESC improved ground, more comprehensive investigations are required to study the mechanisms of the load transfer, the accumulation of the settlement, the development and dissipation of excess pore water pressure and lateral bulging of the stone column.

To study the complicated behaviors of GESC improved soil foundation, the investigations on GESC can be conducted from two perspectives in terms of the components. The investigations on the cyclic behavior of gravel and geosynthetics could be one of the perspectives, since the GESC is made up of granular materials and geosynthetic encasement. Subjected to vertical loading, the column will not only compress vertically but also bulge laterally. The vertical cyclic loading will cause periodic compression and resilience of granular materials, and also periodic tension and contraction of geosynthetics. Investigations on the cyclic behavior of the surrounding soil should be the other perspective, since the GESC gain their bearing capacity from circumferential confinement provided by the surrounding soil. In this respect, the reduction in soil strength induced by cyclic loading is usually the concern of existing studies as it would lead to the reduction of the bearing capacity of the foundation (e.g., Ni et al., 2015; Wang et al., 2019). For the GESC improved soil foundation, however, the GESC serving as vertical drainage can accelerate the dissipation of excess pore pressure in the surrounding soil. The dissipation of excess pore pressure increases the effective stress in the surrounding soil, thus improving the soil strength and the bearing capacity of GESCs. Therefore, in addition to the reduction in soil strength induced by cyclic loading, the improvement of soil strength caused by the dissipation of excess pore pressure should be equally emphasized in GESC improved ground. However, limited efforts have been devoted to the investigations on the development of excess pore water pressure in GESC reinforced ground, especially for the dynamic loading scenarios. Behavior of GESCs under seismic excitation has been reported by Cengiz and Guler (2018a, b), and the soil layer was considered in an undrained state due to the short loading time. Differing from the seismic load, the traffic load, however, is a long-term cyclic load. During the cyclic loading, the accumulation and the dissipation of excess pore water pressure are supposed to occur alternately. Taking this into account, the soil under traffic loads should be considered in a drained or a partially drained state. It should be noted that there is still little information on the accumulation and dissipation of pore water pressure under vertical cyclic loading. To the authors’ knowledge, only Yoo and Abbas (2019) have experimentally investigated the excess pore pressure dissipation in the clay under cyclic loading. Therefore, an improved understanding of the excess pore water pressure in GESC improved foundation under traffic loads is necessary.

This study aims to investigate the dynamic responses of the GESC improved ground under vertical cyclic loading by a series of laboratory tests in a designed test tank. Dynamic performances of GESCs are monitored by measuring soil-column stress distribution, accumulated settlement of loading plate, excess pore water pressure in the surrounding soil and radial bulging of the stone column. The influence of various factors, such as loading parameters and stone column dimensions, on vertical cyclic responses of GESCs can be studied.

Section snippets

Soft soil

Soft soil (clay adopted in the experiment) was collected from a nearby lake bed. To identify the water content of the clay corresponding to undrained shear strength (cu) of clay less than 15 kPa, a series of unconfined compressive strength (UCS) tests were conducted. The results of variation of undrained shear strength with water content are shown in Fig. 1. In this study, the clay with water content w = 38% and cu = 13.7 kPa was adopted, and the water content was kept all the same in the

Soil-column stress distribution

Soil-column stress distribution is a significant feature of GESC improved ground and a key factor in the theoretical analysis. Since the stiffness of GESC is much greater than that of soft soil, most of the vertical stress from the upper loads will be transferred onto the column, then the vertical stress acting on the soil is decreased compared with the unimproved ground. As shown in Fig. 3(c), the vertical stress was measured by miniature stress transducers. For illustration, the vertical

Conclusion

In this investigation, a series of laboratory tests have been conducted in the designed model test tank to study the dynamic responses of GESCs in soft soil under vertical cyclic loading. Dynamic performances of GESCs are analyzed by measuring the soil-column stress distribution, accumulated settlement of loading plate, excess pore water pressure in the surrounding soil and lateral bulging of the stone column. Based on the results of the experimental investigation, the following conclusions can

Acknowledgments

This research was sponsored by the National Natural Science Foundation of China (NSFC No. 51678231), and the Basal Research Fund Support by Hunan University.

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