End-bearing Columns Research Articles

Lime Treatment of Lake Texcoco Clays with Some Insights on the Application of Deep Mixing Columns

The soils across Lake Texcoco (LT) basin are notorious for the presence of highly compressible salt-affected lacustrine clays. The geological formation of the area accompanied by the unmonitored flow of water from Mexico City region has resulted in a unique sodium-rich clay that is highly plastic (plasticity index~200), and impermeable ( k~1.5 × 10−9 m/s) and possesses low compressive strength (unconfined compressive strength < 10 kPa). Over the last few decades, the entire basin has settled at an average of 13.2 cm per year and will continue further with increased loads as a result of rapid urbanization. In planning a solid foundation for the airport, as well as developing the surrounding area, trial sections were built to analyze the effect of quicklime (CaO) on the natural clay s physical, chemical, and engineering properties through laboratory testing. The lab-scale investigations were accompanied with field measurements using the variable energy dynamic cone penetrometer (DCP) and plate load tests. Additionally, a numerical analysis of the effect of deep mixed columns (DMC) to reduce consolidation settlement was performed. The numerical analysis comprised of various DMC stabilizers, loading pressures, column lengths, and area replacement ratios. DMC lengths were observed to be the most dominant parameter to reduce consolidation settlements. Settlement reduction was more sensitive to the area replacement ratio for end-bearing columns, and to the loading pressures for floating columns. Based on the stabilizers evaluated, a combination lime and Portland cement performed the best across all loading pressures and column lengths. Other stabilizer combinations also had a remarkable effect on limiting the consolidation settlement.

Relative contribution of various deformation mechanisms in the settlement of floating stone column-supported foundations

Failure mechanisms of single, isolated stone columns are known to include bulging, punching and general shear failure for long, short floating and short end-bearing columns, respectively. Block punching and lateral displacement of edge columns are additional failure mechanisms relevant to groups of floating stone columns. The term “deformation mechanism” is used here to refer to a source of settlement of a stone column supported foundation (SCSF) which, under a sufficiently large load, leads to failure due to the development of a corresponding failure mechanism. The three main mechanisms that contribute to the total settlement of floating SCSFs are considered in this study. Contributions of these mechanisms to the overall SCSF settlement are quantified using settlements at three representative points of the stone column group. A novel chart is introduced to relate values of these contributions to the overall settlement using results of 3D finite element modeling of a large number of SCSFs. Locations of points on this chart for cases with the same governing deformation mechanism are determined and regions related to the various mechanisms are identified. Knowledge of these mechanisms helps determine the design parameters that contribute the most to the settlement of SCSFs and the most efficient way to minimize this settlement.

Laboratory Investigation on Performance of Soil–Cement Columns Under Axisymmetric Condition

The inclusion of soil–cement columns in soft soil is a ground improvement technique that is used to reduce the settlement and improve the bearing capacity of the soft soil ground. In the present study, model tests have been carried out on soft soil improved with groups of end-bearing and floating columns under the axisymmetric condition to evaluate the relative improvement in the stiffness and failure stress of the soft ground due to the installation of soil–cement columns. The effect of various group foundation parameters, such as area ratio, length and diameter of columns and binder content are investigated. The failure mode of end-bearing and floating columns after exhumation are presented. For the same area ratio, of the improved soil–cement columns to the present soft clay deposit, the usage of the smaller sized soil–cement columns rather than large size soil–cement columns is found to be relatively more beneficial in case of both end-bearing and floating columns. The stiffness and failure stress of the composite ground increase with an increase in the length of the columns. However, the increase in strength is only marginal for the increase in column length beyond 10 times the column diameter. In the case of end-bearing columns, area ratio has a significant effect on failure pattern. At an area ratio of 25%, the column failed by outward displacement and bending. On the other hand, at an area ratio of 32%, the columns failed due to bending at approximately one-half to two-third of the column length from the base of the footing. In the case of floating column, the relative strength of columns to the soil $$( {c_{\text{uc}}}/{c_{\text{us}}})$$ appears to be the major governing factor. For the high value of $$( {c_{\text{uc}}}/{c_{\text{us}}})$$ used in the present study, punching failure was observed with slight outward displacement and some horizontal cracks for area ratios of 25% and 32%. For validation, the failure stresses of model ground were compared with the results obtained using numerical analysis. The results show a good agreement between the bearing capacity value obtained from the experiment and the numerical software.

EFFECT OF ENCASEMENT LENGTH ON GEOSYNTHETIC REINFORCED STONE COLUMNS

Being cost effective and environmental friendly, stone columns are used worldwide for supporting flexible structures such as embankments, oil storage tanks etc. which can tolerate some settlements. However, when the stone columns in very soft soils are loaded; stone columns undergo excessive settlement due to low lateral confinement provided by the soft soil, leading to the failure of the structure. The excessive settlements may be avoided or at least reduced to a great extent, when the columns are reinforced with geosynthetic. The technique of wrapping the individual stone column with a suitable geosynthetic (encasement) is one of the reinforcement modes which improve the performance of stone columns. In this study, model tests were performed on single floating as well as end-bearing stone columns with and without encasement. The column was reinforced by providing geosynthetic encasement over varying column length. The tests indicate that the encasement over the full column length gives higher failure stress as compared to the encasement over partial length of column for both floating and end bearing columns. Further, the performance of end-bearing columns was found to be better than the floating columns.

Model tests on single and groups of stone columns with different geosynthetic reinforcement arrangement

ABSTRACT: Stone columns have proved to be a useful technique for supporting flexible structures resting on soft soils. However, in the case of very soft soils, stone columns have encountered excessive settlements due to insufficient lateral resistance offered by the soft soil against bulging of the columns upon loading. The bulging may be reduced to a great extent by reinforcing the columns either by encasing them with geosynthetics or by placing horizontal circular discs of geosynthetics within the columns at a regular interval. In the present study, model tests have been carried out on long floating and end-bearing single and groups of columns with and without reinforcement to evaluate the relative improvement in the failure stress of the composite ground due to different types of reinforcement. The exhumed deformed column shapes were used to understand the failure mechanism for different types and configurations of reinforcement. The geogrid was the best geosynthetic type for encasement for end-bearing columns; for floating columns, geotextile and geogrid were equally good for horizontal circular discs and encasement configurations.

Influence of stone column installation on settlement reduction

The paper presents numerical simulations investigating the settlement reduction caused by stone columns in a natural soft clay. The focus is on the influence of the soft soil alteration caused by column installation. A uniform mesh of end-bearing columns under a distributed load was considered. Therefore, the columns were modelled using the “unit cell” concept, i.e. only one column and the corresponding surrounding soil in axial symmetry. The properties of the soft clay correspond to Bothkennar clay, which is modelled using S-CLAY1 and S-CLAY1S, which are Cam clay type models that account for anisotropy and destructuration. The Modified Cam clay model is also used for comparison. Column installation was modelled independently to avoid mesh distortions, and soft soil alteration was directly considered in the initial input values. The results show that the changes in the stress field, such as the increase of radial stresses and mean stresses and the loss of overconsolidation, are beneficial for high loads and closely spaced columns but, on the contrary, may be negative for low loads, widely spaced columns and overconsolidated soils. Moreover, whilst the rotation of the soil fabric reduces the settlement, in contrast the soil destructuration during column installation reduces the improvement.

Influence of geosynthetic encasement on the performance of stone columns floating in soft clay

This paper investigates the influence of geosynthetic encasement on the performance of stone columns floating in soft clay. It was found that with unencased columns the bearing capacity improvement is about 3.5 fold, but with geogrid encasement the improvement increases to 5 fold, where 60% of the column length is encased. With full-length encasement (i.e., 100%), the improvement is only about 3 fold. It is therefore evident that partially encased floating columns are superior to the fully encased ones. In contrast, with end-bearing stone columns, full-length encasement is reported to have exhibited better performance improvement than the partially encased ones. In the former case (floating columns), it is the bulge formation at a deeper depth that enhances the bearing capacity, while in the latter case (end-bearing columns), it is the stiffening effect of the encasement that enables the column to transmit the surcharge pressure onto the competent strata below.

Consolidation and deformation around stone columns: Comparison of theoretical and laboratory results

A unit cell, i.e. only one stone column and its corresponding surrounding soil, is used to study the consolidation and deformation around end-bearing columns under distributed loads. The results of laboratory measurements are compared with analytical solutions and numerical simulations. The large amount of data available from the laboratory allows for a detailed comparison. The main soil responses, such as the equivalent coefficient of consolidation, the stress concentration factor and the settlement reduction are analysed using the different approaches.The soil improvement directly depends on the stress distribution between soil and column. Column yielding, and consequently the friction and dilatancy angles of the gravel control the final improvement. Therefore, only the analytical solutions that properly consider plastic strains in the column accurately predict the soil improvement. An issue that influences the stress distribution with time and as a result the consolidation process is the high small-strain stiffness of the column, which is not considered in the analytical solutions but has a small influence for high degrees of consolidation, and hence, for practical purposes.

Model tests on geosynthetic-reinforced stone columns: a comparative study

ABSTRACT: The use of stone columns is popular as a ground-reinforcing technique for supporting flexible structures on soft to very soft soils. When a very soft soil improved by stone columns is loaded, the stone columns undergo excessive bulging, because the very low lateral confinement provided by such a soil gives rise to a very low failure stress of the composite ground. The performance of stone columns in such conditions can be improved either by encasing them with geosynthetics or by placing horizontal strips of geosynthetics within the columns at regular intervals. In the present study, model tests have been carried out on short, floating and fully penetrating single columns with and without reinforcement to evaluate the relative improvement in the failure stress of the composite ground due to different types of reinforcement. A comparative evaluation of the column shape on exhumation reveals subtle differences in failure mode due to the different configurations and types of reinforcement. Whereas for the end-bearing stone columns encasement is clearly the best method, for the floating columns there is no significant difference between the performance of the horizontal strip reinforcement and that of the encasement. For end-bearing columns a geogrid is the best type of geosynthetic to use for both types of reinforcement, but for floating columns the best types of geosynthetic are geogrid as the horizontal strip and geotextile as the encasement.

Limit Analysis of Rigid Foundations on Floating Columns

This study applies the limit analysis to estimate the bearing capacity of a rigid foundation on a soil reinforced by a group of floating columns. The lower bounds of the bearing capacity are derived in terms of a dimensionless factor that depends on the characteristics of the soil and the inclusion, the area replacement ratio, the columns length, and a uniform surcharge surrounding the foundation. The established results are valid for a variety of reinforcing configurations in terms of arbitrary foundation shape and any randomly distributed column arrangements under the foundation. The computation of the bearing capacity is extended to the kinematic approach of the limit analysis for reinforcement of purely cohesive soils. For symmetrical column arrangement the upper bound of bearing capacity factor is calculated by the Prandtl’s mechanism failure. Previous results established for the reinforcement case by end-bearing columns remain valid when considering floating columns. Nevertheless, a limitation on th...