Soft Clay Bed Research Articles

Shaking Table Test and Numerical Modeling of Seismic Behavior of Rectangular Tunnel Structures Embedded in Soft Clay

In this paper, a series of 1-g shaking table model tests were carried out to investigate the seismic behavior of a relatively stiff rectangular tunnel structure installed in soft clay bed, accounting for different ground motions with varying peak accelerations. Using a validated numerical analysis procedure, a suite of three-dimensional (3D) finite element (FE) analyses was performed to systematically study the factors of tunnel burial depth, seismic intensity, flexural rigidity of the middle column and tunnel wall thickness on the seismic response of rectangular tunnel structures installed in clayey ground. It was found that, with the increasing burial depth, the seismic response of rectangular tunnel structure became more intense due to the increased inertial force arising from the overlying clay; in terms of improving the seismic performance of tunnel structure, increasing tunnel wall thickness seemed more effective than increasing flexural rigidity of the middle column. Furthermore, it was found that the existing simplified approaches generally tended to overestimate the earthquake-induced racking distortions of rectangular tunnels installed in clayey ground. A new semi-empirical relationship was derived for better correlating the racking ratio with flexibility ratio for rectangular tunnels embedded in soft clays, which could provide a useful reference for the seismic design and risk assessment of similar clay-tunnel systems.

Use of Sustainable Material as Column Filler in Soft Clay Bed Reinforced with Stone Column: Numerical Study

Use of Sustainable Material as Column Filler in Soft Clay Bed Reinforced with Stone Column: Numerical Study

Ground Improvement for Transportation Infrastructure: Experimental Investigations on Cyclic Behavior of a Group of Granular Columns

This paper presents the results of experimental and numerical investigations conducted on groups of granular columns under loading conditions similar to transportation routes. Cyclic and monotonic tests were conducted on reduced-scale groups of floating and end-bearing granular columns of 30 mm diameter installed in very soft clay beds of two different undrained shear strengths of 6 and 10 kPa and arranged in plane-strain state to simulate the loading conditions present in transportation routes such as highways and railways. The measured results for floating and end-bearing groups of stone columns under monotonic loading are then validated numerically using three-dimensional finite-element analyses. The experimental results show that the settlements induced by cyclic loading are nearly 4–11 times greater than the settlements induced by monotonic loading of the same magnitude of maximum vertical stress. The end-bearing granular columns are observed to be more effective than floating granular columns in improving the bearing capacity of the soft ground and reducing the induced settlements under both cyclic loading and monotonic loading. The results of three-dimensional finite-element analysis show that the major deformation for peripheral columns in both floating and end-bearing stone column groups is lateral, whereas the central columns either undergo shortening and punching in the case of floating stone columns or bulging in the case of end-bearing stone columns.

Geosynthetic Encased Concrete Debris Stone Columns

Rapid urbanization has compelled the building industry to improve the soft soil grounds which otherwise are unsuitable for construction activities. Ground improvement helps achieve better performance by avoiding several problems like excessive settlements, large lateral flow of soft soil beneath the structures, and loss of global or local stability under operational loading conditions by modification of foundation soils. The growing quantities and types of waste materials, shortage of landfill spaces, and lack of natural earth materials highlight the urgency of finding innovative ways of recycling and reusing waste material additionally can reduce the demand for natural resources creating sustainable ecosystem. The aim of this experiment is to use crushed concrete debris (CCD) in soil stabilization. A series of experiments are to be carried out to develop an understanding on the performance of soft clay foundation beds reinforced using geosynthetic encased stone-concrete debris column. Model tests was conducted on clay reinforced with encased vertical columns filled with both CCD and aggregates. A comparative study is to be done and an assessment of ultimate bearing capacity of the composite ground is to be done. The column failure mechanism in the both the cases are to be carefully studied. Modal tests and later numerical analysis is done in PLAXIS 2D for analyzing the behavior of geosynthetic encased stone column stabilized clay bed numerically. Analyses were carried out using Mohr-Coulomb’s criterion. Lab conditions were simulated in PLAXIS for accurate comparison for same model tests.

Shear Strength Behaviour of Pervious Concrete Column Improved Soft Clay Bed: A Numerical Study

Shear Strength Behaviour of Pervious Concrete Column Improved Soft Clay Bed: A Numerical Study

Numerical Study on Behaviour of Encased Stone Columns with Partial Content of Shredded Tyre Chips in Soft Clay Bed

The utility of construction of stone columns in soft soil like clay, marine soil in coastal regions as a ground improvement technique is widely acknowledged. This study presents a numerical analysis of behaviour of ordinary and encased columns, made of various mix proportions of shredded tyre chips and stone aggregates, through Finite Element modelling (with Plaxis 3D software). It is observed that encased columns exhibited remarkable decrease in bulging with respect to ordinary columns. This was true even for columns composed of high proportion of tyre chips (50–100% by volume) owing to generation of higher hoop strain in such columns. The development of hoop strain increased the tensile strength of encasement and hence the column stiffness. The study illustrates the variation of maximum lateral deformation and the corresponding depth with the amount of tyre chips in the mix proportion forming the column. A decrease in bulging along with bulging at greater depths was observed in encased columns, the bulging was negligible for columns wherein the tyre content varied between 0 and 30% by volume. The settlement reduced remarkably with the construction of encased columns irrespective of the mix proportions composing the columns. The study concludes that partial replacement (30–70% by volume) of stone aggregates with shredded tyre chips in encased columns in soft soil is feasible with encasement of significant stiffness.

Use of recycled materials in floating stone columns

In this research, to improve the bearing capacity of a soft clay bed, the performance of floating stone columns constructed of recycled aggregates from demolition waste was investigated. For this purpose, a single stone column with three types of materials, including recycled crushed brick (CB), recycled crushed concrete (CC) and gravel as natural aggregates was modelled. Furthermore, the quality of the recycled aggregates used was evaluated by aggregate index tests such as the aggregate crushing value and the aggregate impact value. The results of the recycled aggregate index tests demonstrated a poorer performance of these materials compared to the natural aggregate. Despite this, the bearing capacity of the clay bed reinforced with the floating column, which was filled with CB or CC, was approximately five times the bearing capacity of the unreinforced clay bed. Moreover, the loading results showed that the floating stone columns constructed of a type of aggregate delivered a better performance in comparison with the columns constructed of a combination of several types of recycled aggregates together or a combination of natural and recycled aggregates.

Settlement Behaviour of Soft Clay Bed Reinforced with Stone Column under Sustained Loading

This research paper investigates the behaviour of soft clay reinforced with stone column under sustained loading. Experiments were conducted in the laboratory on stone column reinforced prepared soft soil bed of kaolin having strength of 7.5 kPa with aggregate of size 2.5 to 10 mm as column material. The stone column with four diameters of 38.1, 50.8, 63.5 and 76.2mm were constructed which correspond to low to high area replacements ratios (i.e. 6.93% - 26.49%). The plain and reinforced soft clay beds were subjected to a sustained load of 150, 200, 250 and 300 kPa where each applied load has been maintained for 24 hours and the settlement behavior of composite ground was taken into account. The test results represent the settlement of reinforced soil bed decreases with increase of column diameters. The settlement reduction ratio is a measure of ground improvement which increases with area replacement ratio. The experimental and theoretical results values were compared as per IS15284 (Part 1): 2003 with reference of stress concentration ratio ‘n’(The ratio of stress in the column to the stress of surrounding ground area). The % variation in theoretical and experimental results is in the range of 50% and therefore the theoretical procedure needs to be revised.

Effect of fill properties on load−settlement behaviour of granular columns in marine clays

This paper describes an experimental investigation carried out to study the load−settlement behaviour and bulge characteristics of granular columns in a soft marine clay bed. The effects of gradation and compacted density of the granular fill material on the load response, bulge length and bulge diameter of granular columns are presented. Granular columns were formed using five kinds of crushed stone aggregates of different gradation, compacted in loose and dense states. The ultimate capacity of the composite ground, bulge diameter and bulge length were more in the case of the densely compacted granular columns than for the loosely compacted granular columns. The relative density and the angle of internal friction of the granular fills, in both loose and dense states, increased with decreasing particle size of the aggregates. Consequently, the load-carrying capacity also increased with decreasing particle size of the aggregates. The percentage increase in the ultimate capacity ranged from 106 to 210% when different aggregates were compacted in their loose and dense states. For a given gradation, the ultimate capacity increased up to an l/d ratio of 11 and decreased thereafter.

Experimental Study on Shear Strength of Cohesive Soils Reinforced with Stone Columns

Stone columns are mostly designed to carry compressive loads. However, in some cases, the movement of soil mass can result in lateral deformations, and consequently shear stresses in soil and stone column. The study presented herein is an experimental program, aimed to investigate the shear strength of soft clay bed reinforced with stone column. Modeling of undrained, short-term behavior of clay bed reinforced with stone column was performed using a large direct shear testing device with in-plane dimensions of 305 × 305 mm. In this study, the effect of key parameters including area replacement ratio, stone column arrangement, normal pressures value, and stone column material was experimentally evaluated. For this purpose, three different replacement ratios, stone column arrangements (single, square and triangular) and normal pressures (35, 55 and 75 kPa) and two different materials including crushed gravel and fine-grained sand were used in experiments. Results showed that in the presence of stone column, the shear strength and the overall stiffness of soft clay bed increased. The stone column arrangement, area replacement ratio and stone column material were also shown to have an impact on improving the shear strength. The most and the least increase in shear strength and stiffness values were related to square arrangement of columns and single column, respectively. In this study, the variation of stress concentration ratio of stone columns under shear loads were measured using suitable instruments. Also, the equivalent shear strength and equivalent shear parameters measured from experiments were compared with those predicted by analytical relationships at stress concentration value of 1 and stress concentration value obtained from experiments.

Laboratory Investigation on Quasi-Static Penetration Testing Using SPT Sampler in Soft Clay Bed

Abstract The standard penetration test (SPT) is one of the most widely used in-situ penetration tests for subsurface exploration. Its suitability for assessing and predicting the design parameters of granular materials has been well appreciated by the geotechnical engineering community over several decades. However, in the case of soft to very soft clays, the correlation between the standard penetration number (N) and the undrained shear strength is poor. In the recent past, tests conducted at a constant rate of penetration, such as the cone penetration test (CPT), ball penetration test, and T-bar penetration test, have become popular in soft clay regions. These tests also have a distinct advantage over SPT in terms of test setup because, in the case of SPT, the entire weight of the sampler and the rod assembly is directly applied on the subsoil, whereas in the case of CPT, the weight of the assembly is supported externally prior to the penetration. This can cause significant operational advantage in case of soft clay deposits. Despite the shortcomings, SPT is still one of the most widely available site investigation tools in the field, as samples are available for inspection and index tests. The operational disadvantages in the case of soft clay deposits can be overcome by altering the penetration process of SPT. A quasi-static penetration at a constant rate rather than a dynamic penetration will help establish a relationship between the penetration resistance and the strength parameters of soft soils. In this article, a quasi-static penetration test using the standard SPT sampler is conducted on reconstituted clay samples in order to develop an empirical correlation with the laboratory vane shear strength. The particle image velocimetry technique is used for detecting the influence zone and plugging during penetration.

An experimental test study on ring footing resting on clay bed reinforced by stone column

Stone columns are used as a ground improvement technique, and they not only enhance the bearing capacity and reduce the settlement, but also serve as a primary function of reinforcement and drainage. Wrapping the stone columns with geosynthetic materials makes ordinary stone column (OSC) stronger and stiffer by enhancing its performance. Ring footings are more often provided for structures such as storage tanks and bridge piers. Stone column is generally used with square, rectangular and circular footings. The idea of using ring footing with encased stone column is very popular nowadays. By using geosynthetic-encased stone column (GESC) with combination of ring footing, more increase in bearing capacity and reduction in settlement are achieved as compared to OSC. Based on the experimental results, pressure–settlement response of the stone column-reinforced clay was studied. This paper also presents the subgrade modulus aspect of geosynthetic-encased stone column-reinforced clay bed The aim of this paper is to study the effect of different parameters such as the number of columns, length of column, diameter of column and the effect of encasement provided on OSC and GESC on bearing capacity and on subgrade modulus. The variation of bearing capacity ratio and settlement are also reported for different parameters. The experimental data were further used for regression analysis to fit the equation for bearing capacity of the improved soft clay bed. Thus, it was concluded that with the increase in the number of columns, length and diameter of column, bearing capacity and subgrade modulus of reinforced clay have increased.

Shaking table tests on geosynthetic encased columns in soft clay

This paper presents the results of an experimental research on the behavior of geosynthetic encased stone columns and ordinary stone columns embedded in soft clay under dynamic base shaking. For this purpose, a novel laminar box is designed and developed to run a total of eight sets of 1-G shaking table tests on four different model soil profiles: Soft clay bed, ordinary stone column installed clay bed, and clay beds with geosynthetic encased columns with two different reinforcement stiffnesses. The geosynthetic encased columns are heavily instrumented with strain rosettes to quantify the reinforcement strains developing under the action of dynamic loads. The responses of the columns are studied through the deformation modes of the encased columns and the magnitude and distribution of reinforcement strains under dynamic loading. The response of the granular inclusion enhanced soft subsoil and embankment soil and the identification of the dynamic soil properties of the entire soil body are also discussed in this article. Finally, to determine the effect of dynamic loading on the vertical load carrying capacity, stress-controlled column load tests are undertaken both on seismically loaded and undisturbed columns.

Experimental and intelligent techniques to estimate bearing capacity of cohesive soft soils reinforced with soil-cement columns

Deep Mixing (DM) method is a soil improvement technique, in which the soil is mixed in situ with stabiliser agents (cement and/or lime) in the form of slurry or powder. The group column is one of DM type that has been extensively used to treat subsoil. Limited attempts have been made to investigate the failure behaviour of a group of floating soil–cement columns. This paper presents a series of 1-G physical tests to identify the bearing capacity of floating soil–cement columns formed via DM method. Pre-formed soil cement columns of 24 mm diameter with different lengths (i.e. 50 mm and 100 mm) were installed beneath a rigid foundation of 80 mm width in a soft clay bed using replacing method. The study also compares the effect of different improvement area ratios i.e. 17%, 26% and 35% on the bearing capacity. Overall, the laboratory results suggest the improvement ratio in the range of 29–79% when the bearing capacity of the treated soil is compared with that of untreated soil. Additionally, using the results of the laboratory tests as well as a dataset comprising other related works (overall 55 sets of data), the paper proposes a predictive model of bearing capacity using adaptive neuro-fuzzy inference system (ANFIS) for cohesive soft soils reinforced with soil–cement columns. The prediction performance of the proposed ANFIS model (coefficients of determination equal to 0.989 and 0.960 for training and testing data, respectively) reveals the feasibility of the model.

Behaviour of soft soil improved by floating soil–cement columns

Deep mixing method is a soil modification technique, in which the soil is mixed in situ with cement stabiliser agents. Limited attempts have been made to investigate the failure behaviour of soil–cement columns constructed in a floating manner. Hence, this paper aims to examine the failure mechanisms and bearing capacity of floating soil–cement columns, in which columns of 24 mm diameter and 100 mm length were constructed by means of the replacing method below a rigid foundation of 100 mm width in a soft clay bed. Improvement area ratios of 17, 26 and 35% have been explored for parametric investigation. The close range photogrammetry and particle image velocimetry techniques were used to evaluate the associated failure mechanisms. The obtained bearing capacity and failure mechanisms were then compared with the ultimate limit state theory and analytical solution for verification. Departing from a good agreement from the verification, a non-linear relationship was established between the average shear strength of improved ground underneath the footing and the strength of the surrounding soil, cua/cus and bearing capacity factor, Nc. In general, it was found that the column failed in both shearing and bending modes.

Improvement of Bearing Ratio of Clayey Subgrade Using Compacted Flyash Layer

This paper presents a laboratory study on California Bearing Ratio (CBR) of flyash overlying a soft clay bed. Flyash sample (Class F) has been collected from Titagarh Thermal Power Plant situated near Kolkata. The type of soil collected locally is silty clay. Different configurations of soil and flyash overlying the soil have been maintained by varying the thickness ratio (ratio of thickness of flyash to that of clay) in the ratios of 1:2, 1:1 and 2:1. Both Standard Proctor and Modified Proctor compaction tests have been carried out to obtain respective maximum dry density and optimum moisture content to be used for CBR tests. The water contents of soil used in this study are 16, 22, 28, 34 and 40% where 16% is the optimum moisture content of the soil as obtained in Standard Proctor compaction test. Similarly water content of the soil has been kept at 12, 18, 24, 30, 36 and 42% for samples prepared using Modified Proctor energy where 12% is the corresponding optimum moisture content of soil. Improvement of CBR values of composite soil-flyash system has been observed when compacted flyash layer is placed above the compacted soil. Out of three thickness ratios tested, CBR value has been found to be the lowest at thickness ratio of 1:2 and the highest at 2:1. The improvement factor is found to be maximum for highest moulding water content irrespective of thickness ratio and compaction energy. It indicates the effective use of flyash soil composite matrix under worse affected condition of high water content. The paper highlights the nature of improvement of clayey soil when compacted flyash layer has been placed on it with different values of thickness ratio and placement moisture content and compaction energy.

Experiment and 3D-numerical studies on soft clay bed reinforced with different types of cellular confinement systems

Abstract The manuscript deals with the experimental and 3-dimensional numerical studies carried on the soft clay bed reinforced with three different types of 3D-cellular confinement systems. The geogrid cells, commercial geocells and the bamboo cells are the three different types of 3D-cells used in the study. First, laboratory plate load tests were performed on the soft clay beds. Subsequently, numerical simulations were carried out to compliment the experimental findings. The numerical simulations were carried out in 3-dimensional framework using FLAC3D by considering the actual honeycomb shape of the geocells. The foundation soil, infill soils and the cell materials were modeled with three different material models, namely, modified Cam-clay, Mohr Coulomb and the linear elastic models. The maximum bearing capacity was observed in the case of foundation bed reinforced with the bamboo cells due to its higher stiffness, higher tensile strength and the higher surface roughness values. Using the validated numerical model, the effect of tensile strength and the surface roughness of the geocell on the performance of the reinforced soft clay beds were quantified. The tensile strength of the geocell material found to have a more pronounced influence than the surface roughness. In addition, a hypothetical case of the prototype foundation on the soft clay has been analyzed and results of the model and prototype cases were found to be in accordance with each other.

Numerical Evaluation on the Different Shapes of Gravelly Sand Columns to Increase the Loading Capacity of Soft Clay

Improvement on soft clay by the installation of stone column is one of the most popular methods followed worldwide. Different analytical and numerical solutions have already been developed for understanding the load transfer mechanism of soft soil reinforced with stone column. This study investigated a bearing capacity of the gravelly sand column, installed in soft clay bed at 15kpa of undrained shear strength. The column variable of length and diameter ratio at 7, 8 and 9 were evaluated. On top of that, the combination of two diameters in single column was tested and the uniform diameter was used as a control. In the numerical analysis, Mohrcoulomb model was adopted in the idealization of the behaviour of the gravelly sand column and soft clay materials. The results revealed that the optimum design that gave the highest loading capacity of the combination 11=12 of column diameter was the length and diameter ratio of 8.

An experimental study on random fiber mixed granular pile

Granular pile (GP) is one of the ground improvement techniques. Randomly mixed fiber GP in place of plain GP is a new development. In this paper some parametric studies on randomly mixed fiber GP have been presented in order to evaluate the effect of fiber content, fiber length and depth of sand-random fiber mixture in GP on load carrying capacity and other characteristics of GP. Significant improvement in load carrying capacity of soft clay bed has been observed with the increase in fiber content, fiber length and depth of sand-random fiber mixture in the GP. An equation has been developed by regression analysis to estimate the load carrying capacity of randomly mixed fiber GP. The findings of this model study indicate that the random fiber-reinforced GP may open a new option in the field of ground improvement techniques.

Expanded Polystyrene Geofoam Based Cellular Reinforcement

Abstract This paper reports the effectiveness of proposed expanded polystyrene (EPS) geofoam cellular reinforcement with fly ash as filling material overlying a soft clay bed through small-scale laboratory model tests. The test beds were subjected to uniformly distributed load by means of a rigid steel plate. Loads with corresponding settlement were measured during the testing. EPS geofoam cellular reinforcement of densities 15, 20, 22, and 30 kg/m3 were prepared in the form of a mattress with three different heights of 50, 75, and 100 mm. The effects of density of EPS geofoam, height of cellular reinforcement, and the planar jute geotextile layer at the base of cellular reinforcement were tested through a series of tests to systematically study the overall performance of the system in terms of load carrying capacity and settlement behavior. Test results indicated that the provision of EPS geofoam cellular reinforcement in fly ash bed significantly improved load carrying capacity above the clay bed. The load carrying capacity decreased with increasing density of EPS geofoam and increased with increase in height of cellular reinforcement. A layer of jute geotextile placed at the base of cellular reinforcement further enhanced load carrying capacity for less height of cellular reinforcement. However, the effect of planer reinforcement was decreased with the increase in height of cellular reinforcement.