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Sand compaction pile and stone column method have been used widely for the improvement of soft ground during last several decades as a technique to increase ultimate bearing capacity and to accelerate the consolidation settlement of the foundation ground. However, stone column method where a compaction stone column is made of crushed stone is difficult to apply to the ground with insufficient lateral confining pressure, accordingly, no bulging failure resistance. Hence, in the present study, the geogrid reinforced stone column system is developed to solve the settlement reduction problem and to widen the applicability of stone column method. To develop this system, 3- dimensional numerical analysis are carried out for the evaluation of the GRSC (geogrid reinforced stone column) system by investigating the behavior characteristics and settlement reduction effect of stone column when type and depth of the applied geogrids are changed. INTRODUCTION Even though the demand for the hard foundation is increasing more and more in the recent construction work, it is difficult to get the hard foundation due to the limited land spaces. So, it is inevitable that the construction work like roads, railways, and industrial complexes proceeds on the soft ground. In this case, the soft ground should be improved. So, the ground improvement method like the stone column system is applied. When the stone column system is applied, the bearing capacity and settlement must be considered, and recently various studies to reduce settlement have been carried out. The stone column system using stone with the high stiffness and low compressibility constitutes the compaction stone column, and used very often as the improving method for the soft ground. However, it can't be applied to the ground with the insufficient horizontal confining pressure because it doesn't have the sufficient resistance against bulging and shear failure.
Stone column is one of the ground improvement systems that is used for accelerating consolidation and increasing bearing capacity for settlement sensitive structures like load embankments, bridge abutments, oil storage tanks etc. This method enhances ground bearing capacity, reduces settlement, prevent liquefaction and lateral ground movement. Recently, the geosynthetic reinforced(encased) stone column approach, which increase the confinement effect, has been developed to improve the load carrying capacity of stone column. Although such a concept has been successfully applied in practice, the fundamentals of the method have not been fully explored. This paper investigates the failure mechanism and load carrying capacity of the geogrid encased stone column by model tests. The results of the analyses indicated an improved bearing capacity of the geogrid encased stone column method over the conventional stone column method with no encasing. Also, the length of the geogrid encasement was found to be more effective for installation of the column within a length from the surface of about 2~4 column diameters. INTRODUCTION Recently, the stone column method, which uses gravel or stone instead of sand to improve ground strength at weak ground construction sites, has received increasing interest. The stone column method is a construction method that replaces 10 ~ 30% weak ground with a column filled with crushed stone or gravel to improve ground strength. Advantageously, the stone column may function as a vertical drain material to promote consolidation, dissipate porewater pressure generated by the foundation or earthquake load, improve the ground support, reduce the foundation settlement, induce the stabilization of slope etc.(Barksdale et al, 1983) However, for disadvantages, stone column construction may induce bulging failure on the upper part of the stone column because of the foundation load and require numerous stones depending on the ground condition.
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ABSTRACT: The stone column technique of ground improvement is extensively used to improve the strength of weak soil layers. Stone columns essentially increase the bearing capacity of cohesive soils. In cohesive soils, the drainage path provided by stone columns accelerates the rate of residual settlement due to consolidation. The paper gives the design and construction methods of stone columns and highlights the errors that can affect stone columns. Two case studies of stone column failure are presented in detail. These studies reveal the errors that occurred in the design and construction methods. The case studies also highlight the probable field conditions that could create excessive settlements. Based on these observations, conclusions are derived indicating the specific care that should be taken in the design and execution of stone columns. INTRODUCTION Stone column technique for ground improvement is being extensively used to undertake constructions in weak soils. The stone columns essentially increase the bearing capacity of loose cohesionless soils. In cohesive soils, along with the increase of bearing capacity, the consolidation settlement of the ground under loading is also considerably reduced. In addition, in cohesive soils stone columns act as drainage paths to accelerate the rate of consolidation of the residual settlement. Even though stone columns are very useful for these purposes, designs made without proper concept of the behaviour of the stone column and execution of work not understanding the stone column and behaviour pattern of the non-treated ground leads to complications. This paper attempts to highlight these factors. A case study where the foundation failure of a structure constructed on a soil improved by stone columns highlights the various aspects discussed. CONCEPT OF DESIGN Stone columns are essentially designed to take load when the columns bulge under loading and the surrounding soil offers passive resistance.
Behaviors of Foundation System Improved With Stone Columns
Bae, Woo-Seok (Department of Civil Engineering, Chungbuk National University) | Shin, Bang-Woong (Department of Civil Engineering, Chungbuk National University) | An, Byung-Chul (Department of Civil Engineering, Chungbuk National University) | Kim, Joo-Sub (Ki-Hwa Construction Co.)
ABSTRACT Foundation system improved with stone columns is many difficulties in quantitative analysis of soil-column interaction due that bearing capacity and consolidation behavior of stone column is affected by various parameters. Because, behavior of composite ground develops in the soil-foundation system interface, behavior of stone column is best investigated in term of various parameters which affect the horizontal resistance in the interface. In the present study, failure mechanism and various parameters for the behavior of end-bearing stone column groups are investigated by loading tests and unit cell consolidation tests. Results of model tests are verified through the failure behavior and bearing capacity by FEM analysis. Finally, the improving characteristics of soft ground and the influence of design parameters are investigated in this study using FEM results and PR value. From the PR value calculation and test results, bearing capacity of stone column is affected by undrained strength of surrounding ground, area replacement ratio of composite ground and installation of mat. And, behaviors of foundation are affected by diameter and spacing of pile rather than embedment ratio and mat. Also, the behavior of stone column is mainly affected by initial undrained shear strength of surrounding ground and column spacing. And, analyses by FEM using Mohr-Coulomb model and PR values are applicable to the prediction of stone column behavior. INTRODUCTION Stone column is one of the soft ground improvement methods, applicable to a wide range of soil strata and a economical method of support in compressible and cohesive soils for low-rise buildings, lightly loaded foundations, earth structures and storage tanks that can be tolerate appreciable movement. However, there are many difficulties in quantitative analysis of soilcolumn interaction due that bearing capacity and consolidation behavior of stone column-mat foundation system is affected by various parameters.
ABSTRACT Laboratory model test results on the ultimate bearing capacity of ∼ surface strip foundation supported by a nearly saturated clayey sot! reinforced with layers of geogrid are presented. The optimum values. for the width of reinforcement layers, depth of reinforcement and location of the first layer of geogrid for mobilization of maximum bearing capacity were determined. Based on the model test results, an empirical procedure to estimate the ultimate bearing capacity of strip foundations on geogrid-reinforced clay was developed. INTRODUCTION During the last fifteen years or so, a number of small-scale. laboratory model test results were published which substantiate that the ultimate and allowable bearing capacities of shallow foundation can be increased by using single or multilayer(s) of geogrid as reinforcement in the soil located below the foundation. These studies were conducted primarily in sand (e.g., Guido et al., 1986; Khing et al., 1993; Yetimoglu et al., 1994). The studies were essentially carried out to evaluate the following parameters, in a nondimensional form, where the most beneficial effect will be derived from the reinforced soil with respect to the ultimate bearing capacity (Fig. 1): (Equations are shown in the paper) A review of the existing literature shows that, unlike the bearing capacity studies on reinforced sand, theoretical and!0r experimental studies relating to the ultimate and allowable bearing capacities of shallow foundations supported by geogrid-reinforced saturated clayey soil are practically nonexistent. Limited data on the topic of geosynthetic-reinforced clay can be found in the works of Ingold and Miller (1982), Milligan and Love (1984) and Dawson and Lee (1988). This paper reports the results of some recent laboratory model tests conducted to determine the ultimate bearing capacity of a surface strip foundation supported by geogrid-reinforced saturated clay.