Behaviour of Stone Columns Research Papers (original) (raw)

Sabkha soils are widely distributed throughout Saudi Arabia, especially along the coastal areas. Typical problems encountered in foundations and structures built over Sabkha beds. The use of stone columns as a technique of soil improvement is frequently implemented in Sabkha soils. Besides, their use in Sabkha soils has been found to provide moderate increases in load carrying capacity accompanied by significant reduction in settlement. In this paper a case study chosen from the Eastern Saudi Arabia is studied in detail to investigate the performance behavior of ground improved with stone columns. The stone columns were constructed using the wet method. Comparison between Pre-and Post-Cone Penetration Test CPT was conducted in this study to discuss different methods that are used to assess the improvement of the soil layers upon installation of stone columns.

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This study investigated the behaviour of single rammed stone columns. Prior to this assessment, the load – settlement and bulging responses of the columns were evaluated by conducting laboratory tests using a specially designed bench scale tank. The latter was a mild steel cylindrical tank which represented unit cells reinforced with single stone columns. The testing technique enabled the evaluation of the influence of variation of column diameter on stone column deformation responses and also provided data that was compared with the numerical results. In the numerical analysis, Mohr-Coulomb and modified Drucker-Prager models were used in the idealization of the behaviour of the column and soil materials respectively. Results revealed that the bigger the column diameter, the higher the loading capacity of the columns. Increasing the column diameter,D by 1.5, 2 and 3 times its initial size generally led to improved load carrying capacity by approximately 2, 4 and 10 times the initial strength respectively. The radial expansion of the columns was prominent in their upper parts with the highest value being experienced at a depth of about 0.5D from the ground surface.

Ground improvement by installation of stone columns has emerged as a powerful and effective means of ground improvement technique. The field application of the technology has developed faster than the design methodology because the mechanism of interaction between the two basic elements- the installed column and the native soil is complex, both in terms of induced stresses and resulting strains. The output behaviour of the ‘unit-cell’ comprising of the column and soil is influenced by the interdependence of the external stress level, the material properties of host soil and backfilled column and its dimensions, the construction procedure and its installation array. An effort has been made in the present paper to predict the upper and lower bounds in application of this technology for optimal cost effectiveness in terms of the area ratio of unit-cell and height-diameter ratio of column for a given set of material properties. For validation of the model, the results are compared with some field tests reported in literature, and the salient interesting findings are discussed

The stone columns improve the performance of foundations on soft and loose soil due to the ability of composite ground to sustain increased structural loads under reduced settlements. The interaction between the two basic elements: the ambient subsoil and installed column, present a complexity of behaviour, both in terms of applied stresses and resulting strains. Moreover, as the same is provided in-group in a regular array beneath foundations, the performance of an individual column is likely to be influenced by the presence of neighbouring columns. The present paper addresses this very issue. Analysis of reported field hydro-test data of large diameter oil storage tanks in soft ground in reference to some existing theories led to the development of the stone-column 'group efficiency factor'. In order that such factor is of general use, it was felt necessary to examine the applicability of the factor to different sizes of groups under varied subsoil conditions. With this in view, some model test results reported in literature of single and group of columns have been analysed and in the process an explicit relationship regarding the stone-column 'group-effect' could be established.

The inclusion of granular columns in soft clay deposits leads to improvements in bearing capacity and overall stiffness, along with a reduction in consolidation settlement. Many laboratory investigations have focused on aspects of bearing capacity, but published data on settlement performance are limited. This paper reports on some interesting findings obtained from a laboratory model study with respect to these issues. In this investigation, 300 mm diameter by 400 mm long samples of soft kaolin clay were reinforced with single or multiple granular columns of various lengths using the displacement and replacement installation methods. The experimental findings revealed that, for the same area replacement ratio, limited settlement reduction was achieved for single long floating columns and end-bearing column groups. Marginal improvements in settlement performance were also achieved for columns installed by the displacement method. No settlement reduction was achieved for short single floating columns, whereas short floating granular column groups produced increased settlements. These observations were verified using contact pressure measurements between the footing and column/surrounding clay.

The emphasis of current work is on assessing the settlement improvement ratio, which is described as the ratio between the settled soils treated with a stone column and the settlement of the non-treated soil (Sr = Streated/Suntreated). The research was conducted using a 300 mm diameter and 300 mm high stone-column container testing model. On 14 modeled stone columns made only from crush stones and using various backfill content, model tests including static axial compression tests were performed. The substance used in the stone backfill column had been changed by sand or lime or cement percentages. The shear strength prepared by the containers varied between 5.5 kPa and 13.5 kPa. Results show that the settlement ratio values, Sr achieved with crush stone, crushed stone +50% sand, crushed stone +5% dry lime, crushed stones +10% dry lime, crushed stone +2.5% cement +5.0% crushed stone +5.0% cement, respectively, was 0.23, 0.12, 0.16, 0.15 and 0.09. In other words, there is a drop in the settlement from 77% to 91%.