The impact of centered load of flexible pavement on soft clay with stone column reinforcement

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.

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.

Summary Mat-supported jackup rigs often experience mat penetrations approaching the thickness of the mat in soft soil penetrations approaching the thickness of the mat in soft soil areas. Data obtained with an electronic bottom sensor show that actual mat penetrations differ from divers' observations because of a soil mound that forms near the mat's edge. The paper compares the mat penetration data with various bearing capacity procedures and strength data to help assess which procedure gives the most accurate prediction. The paper describes geologic features and soil properties that may influence the foundation performance of these rigs. The paper concludes by performance of these rigs. The paper concludes by recommending types of geophysical and geotechnical studies to be performed to evaluate the expected foundation performance of mat-supported rigs more thoroughly. performance of mat-supported rigs more thoroughly. Introduction About 60% of the offshore oil and gas exploration is being conducted with a fleet of more than 350 mobile jackup drilling rigs. Although there are many different mobile rig designs, the rigs can be divided into two broad categories according to their foundation type:individual footings ormat-supported. Many of the mats are A-shaped. Mat-supported rigs have a much larger bearing area and develop lower bearing pressures than rigs with independent footings. The lower bearing pressures enable mat-supported rigs to operate in areas covered by very soft clay soils with only a few feet of mat penetration below the seafloor. Observed mat penetrations, however, can approach the mat thickness in active delta areas around the world, such as the Mississippi River, where the soils are very soft underconsolidated clays. In these cases, an accurate measurement of the mat penetration and an assessment of the resulting foundation penetration and an assessment of the resulting foundation stability is important since designers caution against using the rigs at locations where the mat top penetrates below the seafloor. Hirst et al. showed that foundation performance of mat-supported jackup rigs was safe and performance of mat-supported jackup rigs was safe and acceptable as indicated by a record totaling 176 rig years without loss of a rig due to wind, wave, or current activities during drilling. Their performance data do, however, indicate that vertical and lateral movements have occurred in very weak soils during four severe huricanes. The purpose of this paper is to assess the geotechnical and geological factors that may influence the foundation performance of mat-supported jackup rigs operating in areas with very soft clay soils. The paper then presents a series of field measurements made at two sites in the West Delta Area to determine soil strength characteristics and mat penetration at various stages during rig placement and later, after Hurricane Allen (July 1980). These measurements show:classical bearing capacity equations underpredict actual mat penetrations of a jackup rig,a soil mound forms adjacent to the edge of the mat, resulting in divers making misleading observations of actual mat penetration below the seafloor, andthe method of placement can greatly influence mat penetration. Later sections describe the type of soil-strength data that should be used with bearing capacity equations to allow more accurate predictions of mat penetration in very soft deltaic clay predictions of mat penetration in very soft deltaic clay soils. JPT P. 2958

Stone columns are recognized as a highly effective technique for enhancing the load-bearing capacity and reducing settlement in soft clay soils. Recently, encased stone columns have become increasingly important in improving the properties of soft soils. In a series of controlled laboratory tests on prototype models, both uniform and non-uniform stone columns were examined under various loading conditions, with and without the application of geogrid encasement. The material characteristics of the soil and the stone columns, as well as their geometry, were the main subjects of this experimental study. The results revealed that geosynthetic encasement significantly reduced bulging and settlement, especially in non-uniform stone column configurations. The study found that using a non-uniform stone column design could reduce the quantity of materials required while still enhancing the soil's bearing capacity. The findings showed a significant improvement in performance due to the encasement, with the best load-settlement behavior occurring in columns where the encasement height was optimized for the specific geometry. This research underscores that a non-uniform encased stone column section can achieve results comparable to those of a traditional stone column.

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

Nowadays, some projects have to be constructed in areas having thick layers of soft clay.These soft soils with low shear strength and high voids ratio, lead to excessive settlements even if subjected to low vertical or lateral loads.For this reason, soft clays are considered problematic soils for foundation purposes.Several improvement techniques have been done to enhance such clay.Stone columns have been used to improve soft soils by increasing its carrying capacity and reducing the settlement.In this paper, a numerical study on seismic behavior of stone column in soft clay subjected to earthquake loading has been performed.A two-dimensional plane strain program PLAXIS (dynamic version) is used for the present numerical modeling.A series of modeled stone columns were simulated with different diameters and spacing between columns.Also, the influence of geotextile encasement on the performance of stone columns, foundation systems in soft clay is investigated and compared with the behavior of ordinary stone columns without casing.The results showed that the ordinary stone column with small spacing and larger diameter has a greater bearing capacity and give a smaller settlement, compared to the column with large spacing.In addition, the geotextile encasement for stone column can be provided a significant increase in stone column capacity as well as a huge reduction in settlement is considered with increasing the encasement strength.

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.

Improvement of consolidation settlement beneath square footings using uncased and encased stone columns

The response of floating stone columns of different lengths to diameter ratio (L/D = 0, 2, 4, 6, 8, and 10) ratios exposed to earthquake excitations is well modeled in this paper. Such stone column behavior is essential in the case of lateral displacement under an earthquake through the soft clay soil. ABAQUS software was used to simulate the behavior of stone columns in soft clayey soil using an axisymmetric finite element model. The behavior of stone column material has been modeled with a Drucker-Prager model. The soft soil material was modeled by the Mohr-Coulomb failure criterion assuming an elastic-perfectly plastic behavior. The floating stone columns were subjected to the El Centro earthquake, which had a magnitude of 7.1 and a peak ground acceleration of 3.50 m/s2. The surface displacement, velocity, and acceleration in soft clayey enhanced by floating stone columns are also smaller than in natural soft clay. The findings of this research revealed that under the influence of earthquake waves, lateral displacement varies with stone columns of various lengths.

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

The increase in population in many locations and the value of land has increased significantly. This has made the use of all areas including soft problematic soils inevitable. Due to the lack of bearing capacity of these soils, different methods of soil improvement techniques such as stone columns are used. In this study, four different analytical methods published before are briefly motioned. Also, three-dimensional (3D) numerical analyses were carried out on a stone column under footing on the soft clay soil. The main point of the current paper is to make a comparison between a numerical model and different analytical methods to represent a footing on the soft clay soil reinforced by a group of stone columns.

Soft clay is consolidated upon any slight change in the effective stress. Consolidation and settlement of soft clay surrounding a pile usually drag the pile down ward. The down drag movement adds additional loads to the already loaded pile. This force is expressed as a negative skin friction. Negative skin friction starts from the pile head to a neutral depth depending on the interface properties. This depth is referred to as the neutral plan. This paper presents a study on the effect of stone columns on the behaviour of piles during consolidation of soft clay soil. For this goal the experimental tests were done to study effect of stone columns on the behaviour of single pile embedded in soft clay. The study concluded that the neutral plane is located closer to the end of the pile as the end bearing increases. Stone columns are considered one of methods used to reduce the settlement of soft clay soil, and helps to speed up the rate of consolidation by shorting the length of drainage path within the soft clay layers, and then decrease the relative displacement between the pile and the surrounding soft soils. The use of stone columns reduced the amount of negative friction developed along the pile depth.

The composite system of stone columns reinforced soil overlaid by granular mattress (GM) has been given more attention as a hybrid improvement technique. This work presents 3D finite element analyses using PLAXIS 3D software for a composite system of floating stone columns group constructed in a deep soft clay deposit in Port-Said in Egypt. The limited area of loading was investigated in conjunction with the system key parameters, i.e., length, diameter, and the GM main properties. Results validated the effectiveness of the GM in settlement reduction and system behavior enhancement. The GM thickness results in considerable settlement reduction up to value of 1.5d where d is the stone column diameter. The thickness increase can be an economical substitution instead of increasing stone column length where a new factor named “Mattress improvement factor: Ƞgm” is defined to represent the occurring improvement due to GM existence. Results also show notable reduction of settlement with increasing the length to a limiting value of L = 1.8B where B is the footing width.

Sand and stone columns are used to improve bearing capacity of soft clayey soils… because of their stiffness which is higher than the soil was replaced, the compacted columns… produce shearing resistances which provide vertical support… for overlying structures… or embankments. Also the sand… and stone columns… accelerate the settlement… in the native surrounding soil… and improve the load settlement… characteristics… of foundation. The technique… consists of excavating… holes of specific… dimensions and arrangement… in the soft soil… and backfilling… them with either… sand or… stone particles. The present work investigates the behavior of soft soil reinforced with group of stone columns, sand columns and sand columns stabilized with lime or cement. The percentage of lime and cement used in this research, were determined previously in papers of single sand column stabilized with lime and cement, 11% by weight lime and 9% by weight cement. The model tests were carried out on a soil with undrained shear strength ranging between 16-18 kPa. The models consist of eight… columns at area replacement ratio of (0.196) in square pattern, the holes 50 mm in… diameter and 300 mm… in length were excavated… in a bed… of soft soil. The holes… were backfilled… with stone, sand and sand… stabilized with lime or cement particles. Each group… of columns was loaded… gradually through… a rectangular… rigid footing, its dimensions… 400×200 mm with 50 mm thickness, up to failure… with continuous… monitoring of the settlement. The test… results are analyzed… in terms… of bearing improvement… ratio… and settlement reduction… ratio for all… columns… and in terms… of the stress… concentration… ratio and… stiffness ratio. The results show that the improvement in bearing capacity was about 70% and 62% for sand columns stabilized by lime and cement respectively, and the improvement in bearing capacity was about 42% and 34% for sand columns stabilized by lime and cement compared with stone columns respectively.