Improving weak soils with reinforced stone columns

Stone columns are widely used as an effective and environmental friendly improvement method for increasing the load-carrying capacity of soft clay soils. In very soft clay soils, reinforced stone columns are used because of the lack of the lateral confinement created by the surrounding soil. To provide lateral additional confinement, geosynthetics are usually used. This study intends to evaluate the use of vertical steel bars and horizontal steel discs as an alternative way to geosynthetics to investigate the effect of reinforcement on the footing load-carrying characteristics. Therefore, some large-scale laboratory tests were conducted on stone columns with diameters of 80 and 100 mm and a length to diameter of 5. The results show that changing the arrangement of the bars to a higher stiffness leads to increase in load-carrying capacity. Reinforcing the full-length of the stone columns with the bars in comparison to half-length reinforced has significant influence in capacity. However, in the case of horizontal discs, this increase is negligible. Also by decreasing the space of the discs, load-carrying capacity increases. Moreover, the performance of the vertical reinforced stone column seemed to be better than the horizontal reinforced stone column. The increase of load-carrying capacity in reinforced stone columns with vertical bars or horizontal discs is higher than geotextile reinforcement in the same conditions.

Chapter Fifteen - Experience in using sensitivity analysis and ANN for predicting the reinforced stone columns’ bearing capacity sited in soft clays

One of the most common, effective and innovative ways to improve soft soil is to use stone columns or lime columns. An extensive study has been conducted to find the effectiveness of lime or stone columns to improve the settlement of embankments constructed over soft consolidating soil. There are very few studies to evaluate the effects of columns on embankment safety factor during foundation soil consolidation. This study examined the effectiveness of stone and lime columns on the embankment stability, both in terms of safety factor and settlement, which are determined at different intervals of time, during the foundation soil consolidation. Further, it was also investigated the effectiveness of stone and lime column composite system. In this numerical study, two-dimensional plane strain finite element method is used. This study shows that, stone columns are more effective than lime columns in reducing post-construction settlement and to accelerate consolidation process, whereas lime columns are observed to be more effective than stone columns in improving the safety factor after the embankment construction indicating that the usage of stone or lime columns is significantly affected by the purpose for which columns are used. In addition, it is also observed that the column effectiveness on safety factor is strongly influenced by the relative shear strength of embankment and foundation soils. Further, provision of stone and lime columns as a composite system performs better than providing only stone or lime columns.

The advantage of utilizing stone columns in very low bearing capacity of soil has been demonstrated as an effective strategy to improve the load bearing characteristics of soils. For stone columns, the load bearing capacity primarily relies upon the circumferential confinement given by the local delicate soils. In this research article, several tests were performed on group of 3 and 4 stone columns having diameter of 40 mm and length of stone columns of 300 mm. Stone columns were tested for both unreinforced and geotextile reinforced stone columns, i.e. encased stone columns and horizontally reinforced stone columns. Reinforced stone columns have been considered to explore the load settlement behaviour and failure mechanism. The principle target of this exploration is to compare the adequacy of encased stoned columns and horizontally reinforced stone columns. Results show that horizontal reinforced stone columns provide better ground improvement in comparison with encased stone columns. Utilizing of geotextile helps reduce failure due to the lateral bulging. Finite element modelling (FEM) has also been performed with Plaxis 2D and 3D software. The FEM results show that the load bearing capacity of reinforced stone columns is better than unreinforced stone columns both depicting failure due to bulging. The validation of experimental and FEM results are found in good agreement within permissible range of variation.

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.

One common approach for treatment of soft clay soils is the installation of stone columns. The load capacity of the stone columns depends very much on the shear strength of the surrounding soft clay soil. The stone columns help in both reduction of settlements and accelerated pre-consolidation of clay soil deposit. However, in case of extremely soft clay soils, the stone column formation may be difficult due to lateral spread of stones. The contamination of stone aggregate by the ingression of soft clay soil may inhibit the drainage function of stone columns. The geosynthetic encasement of stone columns is an ideal solution for enhancing the performance of stone columns in such conditions. The geosynthetic encasement helps in easy formation of the stone column and improves the strength and stiffness of the columns. This paper presents the results from a laboratory based studies on the performance of the encased stone columns. The laboratory studies consisted of load tests on stone columns with and without encasement in a clay bed formed in unit cell tank. The encasement was found to significantly improve the load capacity of the stone columns. Using the tension membrane theory for analysing the hoop strains in the geosynthetic encasement, design methodology has been developed for the selection of the geosynthetic material for use as encasement.

A significant challenge arises when pavements are built on soft clay soils, such as the Gedebage area of Bandung; when centered loads from heavy vehicles are applied, the soft clay soil will experience significant settlement and uneven deformation. This causes the pavement structure to fail, threatening road users' safety and increasing maintenance and repair costs. Often, conventional techniques are not effective enough or have an undesirable environmental impact, so stone columns are used as an alternative to soil reinforcement. This research aims to determine the effect of stone column parameter variation on settlement reduction, bearing capacity, and road stability improvement on soft clay soil, as well as designing flexible pavement thickness using CBR data and average daily traffic surveys using the Pavement Design Manual 2024 method. Stone column diameter variations of 0.7 m, 0.8 m, and 0.9 m and stone column spacing variations of 2 m, 2.1 m, and 2.2 m are given in the finite element numerical analysis with PLAXIS 3D software, where the stone column is soft clay stabilized with palm shell ash and asphalt emulsion. Stone column diameter of 0.8 m and spacing of 2.2 m has a safety factor of 13.65, displacement of 24.42 cm, and bearing capacity of 320.3 kN/m2; it can be concluded that the reinforcement of the stone column can significantly improve the original soil condition.

Soft clay soil is known for its high compressibility and low bearing capacity, making it one of the most challenging soil types. Sand columns and sand layers reinforced with geosynthetics are effective techniques to enhance the performance of foundations built on soft clay. Stone or sand columns improve load-bearing capacity by utilizing the natural lateral confinement of the soil. However, in very soft soil, a significant design challenge arises due to bulging in the stone columns, as the surrounding soil may not provide adequate confinement to support the required load capacity. This issue has been addressed by grouting the columns, resulting in highly stable and solid structures. Additionally, the grouting pressure enhances frictional resistance and fills any voids within the soil, contributing to increased overall stability. In the current study, soil improvement methods using ordinary sand columns and grouted sand columns were investigated and then compared with adding sand layers with geogrid reinforcement. The study demonstrated that grouted sand columns improved the bearing capacity by 90% over untreated clay. With geogrid reinforcement, sand columns achieved a 180% increase, while grouted columns with geogrid reinforcement reached a 260% improvement. Increasing the thickness of reinforced sand (H/B = 1.5) further raised capacity improvements to 300% for ungrouted and 420% for grouted columns.

Bearing capacity of geosynthetic encased stone columns

Short-term and long-term behavior of geosynthetic-reinforced stone columns

Stone columns are often used as an effective technique for improving the performance of soft ground. Stone columns derive their load-carrying capacity due to lateral confinement from the surrounding soil. Very soft soils offer very low lateral confinement, leading to large settlements and low load-carrying capacities. In this paper, an alternative method of enhancing the performance of stone columns in soft soils by reinforcing the stone columns with circumferential nails driven vertically is suggested. The method was developed in laboratory-scale model tests and a series of plate load tests were performed in unit cell tanks to investigate the performance of stone columns reinforced with circumferential nails. The investigation was carried out by varying the depth of nails below ground level, the number of nails and the diameter of nails with different diameter stone columns and area ratios (or spacing). It was found that the circumferentially reinforced stone columns have much higher load-carrying capacity with a significant reduction in settlement and less lateral bulging in comparison with plain stone columns.

The soil is generally a heterogeneous material with very variable characteristics. Soil’s main issues are reflected through low bearing capacity and significant deformations (settlement) under loads. For Geotechnical experts, such problems present a challenge in many Moroccan regions like Rabat (Bouregreg Valley) and the northern regions. To overcome these defects, stone columns are used as a soil improvement technique to reduce settlement and to increase the foundations’ bearing capacity on soft clay soil. The benefits arise from the fact that there is a partial replacement of the compressible soil by a more competent material (compacted stone aggregates). Moreover the stone columns are highly permeable and act as vertical drains facilitating the soft soil’s consolidation and improving the foundation’s performance. Numerical modeling is a simple and effective method to approach the real behavior of soils reinforced by stone columns. It allows settlement analysis, lateral deformation, vertical and horizontal stresses in order to understand the behavior of columns and soil. It also has the advantage of integrating the settlements of the underlying layers, especially those of least resistance. The aim of this paper is to study the numerical simulation’s results. The properties of the soft soil correspond to “Bouregreg valley”-soil. This paper is structured as follows: the first part provides the soil conditions and the parameters related to columns and the second part presents 3D finite element analyses that study the stone columns’ performance. These 3D analyses aim to clarify the most important parameters, the influence of the constitutive models and the column geometry. The study also shows the mechanisms of functioning and interactions of stone columns vis-a-vis the various parameters characterizing the granular material “ballast” and the surrounding soil.

Using stone columns is an efficient method to increase the bearing capacity of soft soils. This has led to an increased interest in further developing and improving the method. In addition, granular blankets are used to increase the bearing capacity of the stone columns. In this research, the bearing capacity of stone columns, granular blankets, and a combination of both methods in reinforced and unreinforced modes was examined using large-scale laboratory tests. A scale factor of 1–10 is used for the geometry of the models, and the stone columns are a floating type that are 60 mm in diameter and 350 mm in length. These columns are either reinforced with vertical encasement of a geotextile or they are unreinforced. The granular blankets are either reinforced by using a biaxial geogrid or they are unreinforced with 40 and 75 mm thicknesses. In general, 16 large experimental tests have been carried out. Results indicate that using all these variations (granular blankets, stone columns, and a combination of both) improves bearing capacity. Using geogrid as the reinforcement of granular blankets and geotextile as stone-column encasement increases the efficiency of granular blankets and stone columns significantly. The maximum bearing capacity was obtained when reinforced granular blankets and reinforced stone columns were combined. The stress-concentration ratio and bearing capacity increased as geotextile encasement was used in the stone columns.

The application of stone columns increases the stiffness of soft soils which contributes to its load carrying capacity and accelerates the process of consolidation leading to reduction in settlement. However, under external loading, squeezing of adjacent soil into the columns not only compromises the integrity of the columns but also reduces its stiffness, strength and drainage properties. The present study investigates the use of vertically and horizontally reinforced stone columns, as a remedial measure for ordinary unreinforced stone columns. The vertical reinforcement is done by encasing the stone columns in geotextile and horizontally by placing geotextile circular discs within the columns at regular interval. Model tests on group of 3 and 4 unreinforced and reinforced stone columns have been conducted in weak sandy soil. The load–settlement response and failure modes for both reinforced and unreinforced groups have been studied. It is observed that reinforced group of stone columns depict better load bearing capacity as compared to unreinforced group. Moreover, bearing capacity for vertically encased and horizontally reinforced is almost similar, with horizontally reinforced group of 4 stone columns depicting slightly higher (1–2%) bearing capacity for a settlement of 30 mm. The experimental results are also validated with theoretical results and are found to be in good agreement.

The stone column is a useful method for increasing the bearing capacity and reducing the settlements of foundations. The confinement provided by the native soft soils has an effective influence on the stone column bearing capacity. In this paper, laboratory tests were performed on unreinforced and reinforced geotextile-encased stone columns. Tests were performed on columns with diameters of 60, 80, and 100 mm and a length to diameter ratio of 5. Vertical encased stone columns (VESC) and horizontal reinforced stone columns (HRSC) were used for stone column reinforcement to investigate the effect of reinforcement type on the bearing capacity. The main objective of this research is to study the efficiency of VESC and HRSC under the same conditions for diameters of 60, 80, and 100 mm. Experimental results show that the bearing capacity of stone columns increases using vertical or horizontal reinforcing material. Moreover, the bearing capacity of reinforced stone columns increases by increasing the strength of reinforcement in both VESC and HRSC. Results show that bulging failure mechanism governed in all tests and that lateral bulging decreases using geotextiles and increasing strength of reinforcement. In addition, for both VESC and HRSC, the stress concentration ratio of the columns also increases.