|S. Siva Gowri Prasad||Department of Civil Engineering, GMR Institute of Technology, Andhra Pradesh 532127, India|
|P.V.V. Satyanarayana||Department of Civil Engineering, Andhra University, Visakha Patnam, Andhra Pradesh 532127, India|
Stone columns are the most suitable and economical ground improvement technique for soft soils. Stone columns accelerate the consolidation process, thereby increase the stiffness of the soil. This increase may not be sufficient because of the less lateral confinement, which leads to excessive bulging. The strength of the composite soil can also be increased further by encasing the column with geotextile. In this paper, model tests were conducted on end-bearing stone columns with geotextile encasement and compared with the unreinforced (plain) stone columns. The stone columns were prepared by placing the silica-manganese slag, sand and were reinforced with geotextile with different encasement lengths of D, 2D, 3D, and 4D (D is the stone column diameter; i.e., 5 cm). The tests demonstrated that the engineering behavior of the soil was improved by introducing the silica-manganese slag (when compared with conventional stone columns) and also with encasement. Bulging can also be reduced by providing encasement beyond the zone of bulging.
Bulging; Encasement; Geotextile; Marine clay; Silica-manganese slag; Stone column
Due to development of infrastructure in metropolitan cities, suitable sites for construction have been reduced and caused a rise in land prices. Because of this problem, industries are looking for cheaper land for construction. As a result, some sites which were not used earlier due to low strength are now being used for construction. When these soils are loaded, they may experience failure due to excessive settlement. Greenwood (1970) was first to propose load transfer theory, settlement prediction, and estimation of ultimate bearing capacity. Hughes and Withers (1974) found that stone columns fail under compressive loads in general shear, bulging, and sliding. The load-carrying capacity of the columns is acquired via lateral confinement from the surrounding soils (Greenwood, 1970). While the stone columns improve soft soil, sufficient load-carrying capacity may not be achieved because of the less lateral confinement. To overcome this situation, geosynthetic material can be used for encasing stone columns. This is the most popularly used method.
Many researchers have used geosynthetic material as encasement for stone columns to improve soft soils. Murugesan and Rajagopal (2009; 2010), Gniel and Bouazza (2009), Samadhiya et al. (2009), and Hasan and Samadhiya (2016) studied the behavior of geosynthetic/geogrid-encased stone columns and found that the stiffness of soft soil can be improved by increasing the encasement length. Malarvizhi and Ilamparuthi (2004) reported that settlement can be reduced by providing the encasement by increasing the stiffness of the stone column. Murugesan and Rajagopal (2009) studied geosynthetic-encased stone column performance and found that the pressure settlement response showed linear behavior.
Malarvizhi and Ilamparuthi (2004; 2007) and Ali et al. (2011) studied the effect of length to diameter ratio (L/D) and found that the load-carrying capacity was increased by increasing the L/D ratio whereas the influence is much less in floating columns (Malarvizhi & Ilamparuthi, 2004). The bearing capacity of composite soil increases with column length, but the increase is not significant when the length exceeds beyond six times the column diameter (Ali et al., 2011). Samadhiya et al. (2009), Murugesan and Rajagopal (2010), Ali et al. (2011), and Hasan and Samadhiya (2016) conducted tests on stone columns of different diameters and concluded that the stiffness of the soil increases with a decrease in the diameter of the column. This is because of the higher confining stresses mobilized on smaller diameter columns. Fattah et al. (2016) studied the behavior of stone columns in embankments and concluded that the Stress Concentration Ratio (SCR; the ratio of the stresses in the column to the surrounding soil) increases gradually with increasing L/D ratio.
Dheerendra Babu et al. (2010) conducted experiments on stone columns reinforced with vertical nails placed along the circumference and found that the circumferential nails enhanced the stone column performance. Furthermore, the behavior of composite ground was improved with the number of nails. They also found that in order to enhance the stone column performance significantly, the depth of embedment of nails required was 3D to 4D. Fattah & Majeed (2012a) studied the behavior of capped stone columns encased with geogrid by the finite element method and found that the capped stone column increased the bearing improvement ratio (q treated/q untreated) and decreased the settlement for all L/D ratios. The bearing improvement ratio also increased with the thickness of the cap, up to 0.4 times the footing diameter.
Samadhiya et al. (2009) and Hasan and Samadhiya (2016) studied the lateral reinforcement of geogrid strips by varying the vertical spacing and concluded that the load intensity was increased by decreasing the spacing. The strength of granular pile was increased by increasing the length of reinforcement to a depth of three times the diameter and no further increment was observed. Basu et al. (2016) worked with fiber-reinforced stone columns and found that the diameter of bulging can be decreased by increase the length and the fiber content. The depth of maximum bulging from the surface also decreased, but the total length of bulging was increased. Prasad and Satyanarayana (2016) studied the behavior of geotextile-reinforced stone columns by placing the reinforcement laterally at different spacings and found that the load-carrying capacity increased with the decrease in spacing.
Ambily and Gandhi (2004) carried out experimental studies by loading stone columns on their area alone and found that the failure occurred in the form of bulging of the stone column at a depth of about 0.5D to 1.0D below the surface. When the load was applied to the tank wall, the load/settlement behavior was linear and the failure did not take place. Fattah & Majeed (2012b) studied the geogrid-encased floating stone columns and found that the maximum lateral displacement occurred at an effective encasement length ratio (length of geogrid encasement along the stone column/total stone column length) of 0.6. Gniel & Bouazza (2009) carried out experiments on geogrid-encased stone columns and found that maximum bulging occurred at a depth of 2D. This could be reduced by providing encasement beyond the zone of bulging. Damoerin et al. (2015) carried out a series of tests to increase the shear strength of the soil by improving the cement column and found that this increased the shear strength of the soil. Fattah and Majeed (2009) studied the behavior of encased floating stone columns and found that the bearing improvement ratio increased by increasing the area replacement ratio for both ordinary and encased stone columns.
Fattah et al. (2010) carried out tests on stone columns by varying the SCR and found that the stiffness was increased with an increase in stiffness of the treated soil. Malarvizhi and Ilamparuthi (2007), and Murugesan and Rajagopal (2010) studied the stone column behavior and concluded that the SCR increased by inclusion of the encasing material and also with the stiffness of the encasing material.
Ambily and Gandhi (2007) carried out tests on stone columns by varying the spacing between the columns and shear strength of the soil for both single and group columns. They found that the stiffness improvement factor (the ratio of stiffness of treated soil to untreated soil) depends on the angle of internal friction between the stones and the spacing between the columns, independent of the shear strength of the soil. They also found that the settlement increased and the load-carrying capacity decreased with an increase in spacing up to an L/D ratio of 3 (beyond this, the change was negligible). For stone columns reinforced to L = 2D the improvement ratio was very high and the settlement reduction ratio was very low (Fattah et al., 2016).
From the literature review, it is clear that many researchers have studied soft soil improvement by using different types of stone aggregates. However, there has been limited research on the replacement of stone aggregates with other materials. In this study an alternate material (Silica-Manganese slag) was used as the column material and the sand was replaced within the voids between the aggregates. This column was further encased with geotextile material with different encasement lengths, and the bulging and load versus settlement behavior was studied.
Experimental studies were conducted on stone columns by replacing the column material with silica-manganese slag, and the columns were reinforced with geotextile for various encasement lengths. The following conclusions were made: (1) Silica-manganese slag is a potential alternative for improvement of soft soil as it has a better load-carrying capacities than conventional stone columns (about 9%) because of the superior properties of the slag over the stone chips; (2) The load-carrying capacity of the stone column was increased by introducing the encasement, due to mobilization of the hoop stresses which resist bulging. These hoop stresses help to transfer the load to the bottom of the stone column, and thus the bearing capacity increases. The load-carrying capacity also increased by increasing the encasement length of the column; (3) Bulging was reduced by providing reinforcement to the columns. The maximum bulging occurred at the center of the column for both the plain and fully-reinforced columns. For the other reinforcement lengths, bulging was found just below the encasement depth. This indicates that the reinforcement transfers the bulging to the greater depths; (4) The bulging can be decreased by providing reinforcement beyond the zone of bulging (i.e., beyond an encasement length of 2D) due to the increase in confinement on the stone column where the bulging is occurring.
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