Effectiveness of Stone and Deep Mixing Lime Columns on Stability of Embankments Constructed on Soft Consolidating Soil

The objective of this study was to investigate the effectiveness of soil–cement (SC) columns, provided either individually or in combination with lime columns, on the stability of embankment constructed on soft consolidating soil. The stability of the embankment was evaluated in terms of both the settlement and factor of safety at various time intervals during the consolidation. The effectiveness of columns was studied numerically using the two-dimensional plane strain finite element method. Settlement, excess pore water pressure and factor of safety were obtained at various time intervals. From the study, it was concluded that SC columns were effective in improving both the settlement and factor of safety of embankment throughout the consolidation of foundation soil. Provision of SC columns with lime columns was also effective in improving the stability of embankment. In addition, the stability of the embankment was affected by the arrangement of columns in the lime and SC composite system. The arrangement most effective to improve the settlement need not be the most effective system to improve the factor of safety after the construction of the embankment.

The improvement of soft soil foundations is essential in geotechnical engineering, particularly for infrastructure projects, such as highways, railways, and embankments. The soft soil formations pose significant challenges, including excessive settlement, low shear strength, and insufficient load-bearing capacity, which often render the traditional foundation methods inadequate. Stone columns are a widely adopted ground improvement technique due to their ability to enhance the soil stability, bearing capacity, and drainage efficiency. Advanced reinforcement techniques, such as stone columns, provide an effective solution, yet an understanding of the coupled soil-fluid interactions remains limited. This study employs a numerical approach to evaluate the stone column performance using a finite element model developed in FORTRAN 90. The model examines the embankment response under three conditions: without stone columns, with stone columns in a free-field scenario, and with stone columns under a surface foundation surcharge of 0.5 kN/m². The key parameters analyzed include Excess Pore Pressure (EPP) dissipation, deformation characteristics, and site stiffening effects. The numerical simulations indicate that the stone columns significantly reduce the settlement, accelerate the pore pressure dissipation, and improve the embankment stability by redistributing the stresses within the soil matrix. The findings confirm that the stone columns are a viable solution for soft soil reinforcement. The developed numerical model provides valuable insights into the effectiveness of the stone columns in embankment stabilization. Future research should focus on optimizing the stone column configurations, integrating real-time field monitoring, and exploring hybrid reinforcement techniques to further enhance the soil stabilization strategies.

Inclusion of stone columns in the underlain soft soil is one of the most prominent methods for improving the stability of embankments. The stone columns are encased with a geosynthetic material to further enhance the stability. The influence of this partial replacement of weak foundation soil with stone columns on the performance of embankments needs to be quantified. In this study, performance of ordinary stone column (OSC) and geosynthetic-encased stone column (GESC)–supported embankments is carried out using a three-dimensional finite element programme (PLAXIS3D). A parametric study was conducted to quantify the influence of various factors viz. spacing to diameter ratio (S/D), stiffness of encasement, cohesion of soil, friction angle of stone column and friction angle of embankment on the factor of safety against deep-seated failure. The results show that encasing the stone columns enhances the stability of embankments. Decreasing the column spacing (S/D) enhances the stability, reduces the excess pore pressure development and average settlement. Increase in geotextile encasement stiffness, cohesion of underlain soft soil, friction angle of stone column and friction angle of embankment improves the performance of embankments and also reduces the average settlement of the ground under embankment. Results of the parametric study were used to develop two types of data-driven models viz. multiple linear regression (MLR) and artificial neural networks (ANN) to simplify the evaluation of the factor of safety (FOS) of embankments. Among the two approaches, ANNs were able to predict the factor of safety values more accurately.

There can be many reasons for engineers to place the footings near a slope such as leakage of suitable sites or architectural considerations. One of the approaches to increase the amount of bearing capacity, especially in soft soils, is adding stone columns to the soil. In this research, the behavior of a strip footing placed near a stone column reinforced clayey slope was investigated. For this purpose, some small-scale model tests were performed on a clayey slope reinforced with stone columns. The effects of the length of the stone column and the length of encasement on the footing were studied. Additionally, vertical encased stone columns in a group arrangement were investigated. Some numerical analyses were also performed using the Midas GTS NX finite element software, and the factor of safety was studied. Results show that the optimum length was equal to four times the diameter of stone columns. It was observed that by increasing the length of encasement, the bearing capacity of strip footing was also increased. The safety factor of slope showed an increase when stone columns were added to the slope, but the maximum influence on the factor of safety appeared when the stone column was in the upper middle of the slope.

The engineering structures constructed on thick deposits of soft soil strata have problems of low bearing capacity, excessive total and differential settlement, lateral spreading etc. To mitigate such problems, different ground improvement techniques are available namely; vertical drains, lime/cement column, stone (granular) column etc. in view of their proven performance, short time schedule, durability, constructability and low costs. Stone column technique seems to be very suitable and favourable ground improvement technique for deep soft soil improvement. Further to prevent excessive bulging, squeezing of stone into soft soil, stone column can be encased with suitable geosynthetic. Another advantage of encasement is having high load carrying capacity and lesser settlement of composite foundation. This paper presents the current state of the geosynthetic encased stone column as a ground improvement technique. A review is provided aiming to: (a) identify key considerations for the general use of encased stone columns, (b) provide insights for design and construction, (c) compile the latest research developments. Case histories of field applications and observed field performance are cited to portray different stone column applications and observed effectiveness. The paper identifies areas where more research is needed and includes recommendations for future research and development.

Stone columns have become a widely used method of increasing bearing capacity of soft soils. This study investigates floating stone columns with and without encasement in soft soils. Although the stone columns in single-layered soil have been studied extensively, stone columns constructed in a base of varying soil layers are not fully understood. In the present study, the behavior of both single-layered soft soil and layered soil consisting of loose sand overlaying the soft soil was investigated by using small-scale laboratory pilot tests. The bearing capacity of soft soil was improved in all cases of stone column application. The contribution of stone columns on the bearing capacity of soft soil was presented by the term bearing improvement ratio (BIR). With a non-encased stone column in single-layered soft soil, the BIR was about 3.3-fold and with geotextile encasement in the same soil, the improvement ratio increased to 3.4-fold. For a non-encased stone column in layered soils, the BIR was about 2.0-fold and with geotextile enhancement in the same soil, this improvement ratio increased to 4.0-fold. The inclusion of geotextiles resulted in improved bearing capacity by distributing the induced stresses over larger areas. The maximum bulging of non-encased stone column in single-layered soft soil was observed at the depth of 1.5 times the original diameter of the stone column from the top, whereas for encased stone column in single-layered soft soil, the maximum bulging was transferred to a depth of 3.0 times the original diameter of the stone column.

ABSTRACT: Construction of embankments on soft natural soil may be a challenge, owing to its low shear strength and high compressibility. Stone columns, which depend on lateral support from the soft ground, have been utilised to accelerate foundation consolidation, and to increase foundation bearing capacity. To keep the drainage function, and to improve the stone columns as reinforcing elements, geosynthetics are used for column encasement. In this research, the behaviour of full-scale unreinforced and reinforced Bremerhaven clay with conventional and geogrid-encased stone columns under embankment loads is analysed numerically. The consolidation analysis is applied to investigate the long-term behaviour of the clay. The results show that the stone columns in the Bremerhaven clay increase the bearing capacity and accelerate the reduction of excess pore water pressure of the foundation. Once the stone columns are encased, more improvement occurs in their performance in soft soil. The analyses also indicate that stress concentration generation in the stone columns contributes significantly to the acceleration of soil consolidation.

Geotechnical Engineering has developed many methods for soil improvement so far. One of these methods is the stone column method. The structure of a stone column generally refers to partial change of suitable subsurface ground through a vertical column, poor stone layers which are completely pressed. In general terms, to improve bearing capacity of problematic soft and loose soil is implemented for the resolution of many problems such as consolidation and grounding problems, to ensure filling and splitting slope stability and liquefaction that results from a dynamic load such as earthquake. In this study, stone columns method is preferred as an improvement method, and especially load transfer mechanisms and bearing capacity of floating stone column are focused. The soil model, 32 m in width and 8 m in depth, used in this study is made through Plaxis 2D finite element program. The clay having 5° internal friction angle with different cohesion coefficients (c 10, c 15, c 20 kN/m2) are used in models. In addition, stone columns used for soil improvement are modeled at different internal friction angles (ϕ 35°, ϕ 40°, ϕ 45°) and in different s/D ranges (s/D 2, s/D 3), stone column depths (B, 2B, 3B) and diameters (D 600 mm, D 800 mm, D 1000 mm). In the study, maximum acceleration (amax = 1.785 m/s2) was used in order to determine the seismic coefficient used. In these soil models, as maximum acceleration, maximum east–west directional acceleration value of Van Muradiye earthquake that took place in October 23, 2011 was used. As a result, it was determined that the stone column increased the bearing capacity of the soil. In addition, it is observed that the bearing capacity of soft clay soil which has been improved through stone column with both static and earthquake load effect increases as a result of increase in the diameter and depth of the stone column and decreases as a result of the increase in the ranges of stone column. In the conducted study, the bearing capacity of the soil models, which were improved with stone column without earthquake force effect, was calculated as 1.01–3.5 times more on the average, compared to the bearing capacity of the soil models without stone column. On the other hand, the bearing capacity of the soil models with stone columns, which are under the effect of earthquake force, was calculated as 1.02–3.7 times more compared to the bearing capacity of the soil models without stone column.

Although fully penetrating stone columns have widely been studied in the literature, the number of published studies on floating stone columns – which also have been efficiently used to improve vertically lengthy soft-clay deposits – is very limited. Basal reinforcement with geosynthetic is another technique that increases overall stability of embankments on soft soils and can be combined with stone columns. In the paper, mechanical-hydraulic coupled finite element analyses are performed to study a floating stone column-supported embankment on a thick soft soil deposit. A parametric analysis is conducted so that the influence of two governing factors is evaluated: length of stone columns; basal reinforcement with geosynthetic. An overall stability analysis method, which uses the results of finite element analyses, is also applied. Results of several key variables are analysed: overall safety factor, settlements, horizontal displacements, vertical stress at the embankment base, stress concentration ratio, excess pore pressures and geosynthetic tensile force.

Booming infrastructure activities demand suitable subsoil conditions to accomplish design requirements in terms of strength and serviceability. Soft soils encountered during such activities may pose problems such as very low strength, poor hydraulic conductivity, shrinkage and swelling with seasonal moisture variations etc. Utilization of sites with soft soils required the invention of various ground modification techniques. Stone columns is one of such techniques used in soft soils to improve its load bearing capacity, dissipate excess pore water pressure rapidly and reduce the total settlement effectively and economically. Stone columns require use of natural aggregates to be transported from a distant place to the construction site that causes an increase in construction cost, carbon footprint and diminish natural resources. This paper presents laboratory investigations of load test results on virgin clay bed and clay bed with stone columns. Two different conditions of stone columns viz. (1) un-encased stone columns with natural and lightweight aggregates (2) encased stone columns with natural and lightweight aggregates. Clay beds were prepared using slurry consolidation method by mixing dry soil with water content equal to 1.5 times liquid limit of the soil. Displacement controlled load tests were performed on clay beds with and without stone columns. Loads against applied displacements were continuously monitored for all the tests conducted. Clay bed with stone columns demonstrated higher load carrying capacity as compared to virgin clay beds. Un-encased stone columns indicated increase in bearing capacity by 31.7% for natural aggregates and 19.5% for light weight aggregates as compared to virgin clay bed. Further, provision of encasement to the stone columns increased load carrying capacity to 36.6% for natural aggregate stone columns and 31.7% for light weight aggregate stone columns as compared to virgin clay bed.

Construction on soft natural soil is considered a risk due to its low shear strength and high compressibility. Stone columns are an effective improvement method for soft soils under light structures such as rail or road embankments. Stone columns are generally used to increase the bearing capacity which depends on their lateral support. In this research full scale stone columns in Bremerhaven clay, a soft soil layer of 6.0 m thickness, were analyzed using the finite element program Plaxis. The stone columns were loaded under undrained and drained conditions of the surrounding soft soil to investigate the effect of varying parameters like spacing distance between columns, column diameter and stiffness of the geogrid encasement on the behavior of the stone column in short and long term conditions. The results showed that the ordinary stone columns with narrower spacing distances and smaller diameters have a greater bearing capacity and show smaller settlement as well as lateral bulging than wider spacings and greater diameters of stone columns. When using geogrids as encasement for stone columns, a huge increase in the bearing capacity of the stone column as well as a huge reduction in the lateral bulging and the settlement occur. The bearing capacity of the stone columns increases as well as their lateral bulging, settlement and differential settlement decrease with increasing encasement stiffness in both short and long term conditions.

The use of stone columns (otherwise called granular piles) has proved to be an economical and technically viable ground improvement technique for construction on soft clay soils. Though the stone columns are designed to carry vertical compressive loads, soil movements occurring in the field conditions may cause shear deformations in the stone columns. The stone columns, particularly installed in very soft soils, may not be able to resist these shear movements because of the low confinement offered by the surrounding soil. The shear load capacity of such stone columns can be significantly improved by encasing the individual stone columns with suitable geosynthetic. The encasement confines the aggregate and makes the stone column act like a semirigid pile; thus leading to increased shear stiffness of the column. This paper discusses some laboratory model tests performed to investigate the shear load capacity of stone columns with and without geosynthetic encasement. The laboratory tests were performed by inducing lateral soil movements in a stone column treated soft soil. The results have shown qualitative improvement in the shear stiffness of the stone column due to geosynthetic encasement.

The study represents an experimental investigation of the development of bearing capacity in soft clayey soil using stone column with bamboo sheet plate. Basically, application of bamboo sheet plate in stone column helps to decrease the lateral displacement of the soft clayey soil. Accordingly, bulging of stone column decreases and enhances the load bearing capacity of the soft clayey soil. A series of model experiments are conducted on a specially fabricated steel tank filled with soft clayey soil in the laboratory. Single stone columns are constructed in the soft soil by digging hole and then placing and compacting stones. Bamboo sheet plates are placed at different locations of the stone column in a single layer such as L/3, L/2, and 2L/3 from the top, where L stands for the depth or length of stone column. One experiment is performed only for stone column without the bamboo sheet. Another experiment is conducted by placing two bamboo sheet plates together; one at the depth of L/3 and another is at the depth of 2L/3. A plate load test is also performed to find out the capacity of soft clayey soil. Load verses settlement graphs are plotted for all the tests, and load carrying capacity and settlement are calculated. A comparative study is conducted from the results of different experiments and noticed that load bearing capacity of the soft clayey soil improves by the application of bamboo sheet plate.KeywordsBamboo sheet plateStone columnLoad bearing capacity

Use of stone column technique to improve soft foundation soils under roadway embankments has proven to increase the bearing capacity and reduce the potential settlement. The potential contribution of stone columns to the stability of roadway embankments against general (i.e. deep-seated) failure needs to be thoroughly investigated. Therefore, a two-dimensional finite difference model implemented by FLAC/SLOPE 7.0 software, was employed in this study to assess the stability of a roadway embankment fill built on a soft soil deposit improved by stone column technique. The stability factor of safety was obtained numerically under both short-term and long-term conditions with the presence of water table. Two methods were adopted to convert the three-dimensional model into plane strain condition: column wall and equivalent improved ground methods. The effect of various parameters was studied to evaluate their influence on the factor of safety against embankment instability. For instance, the column diameter, columns’ spacing, soft soil properties for short-term and long-term conditions, and the height and friction angle of the embankment fill. The results of this study are developed in several design charts.

An experimental study is carried out to evaluate the performance of Lime mortar-Well graded Soil (Lime-WS) columns for the improvement of soft soils. Tests are conducted on a column of 100 mm diameter and 600 mm length surrounded by soft soil in different area ratios. Experiments are performed either with the entire area loading to evaluate the load - settlement behavior of treated grounds and only a column area loading to find the limiting axial stress of the column. A series of tests are carried out in soaking condition to investigate the influence of moisture content on the load - settlement behavior of specimens. In order to compare the behavior of Lime-WS columns with Conventional Stone (CS) columns as well as Geogrid Encased Stone (GES) columns, the behavior of these columns have been also considered in the present study. Remarkable improvement in the behavior of soft soil is observed due to the installation of Lime-WS columns and the performance of these columns is significantly enhanced by increasing the area ratio. The results show that CS columns are not suitable as a soil improvement technique for extremely soft soils and should be enhanced by encasing the column or replaced by rigid stone columns.