Model Tests on Stone Columns Reinforced with Lateral Circular Discs

Transcription

1 International Journal of Civil Engineering Research. ISSN Volue 5, Nuber 2 (2014), pp Research India Publications Model Tests on Stone Coluns Reinforced with Lateral Circular Discs K. Ali 1, J.T. Shahu 2 and K.G. Shara 3 1 Civil Engineering Section, University Polytechnic, Aligarh Musli University, Civil Lines, Aligarh, INDIA. 2,3 Civil Engineering Departent, Indian Institute of Technology, Hauz Khas, New Delhi, INDIA. Abstract Stone coluns have proven to be the ost suited technique for iproving the bearing capacity of weak or soft soils and are used worldwide for supporting flexible structures such as ebankents, oil storage tanks, etc. Stone coluns installed in very soft soils undergo excessive bulging on loading due to very low lateral confineent and consequently, failure occurs. The strength of stone coluns in such conditions can be enhanced by reinforcing these with a suitable geosynthetic. In the present study, the stone coluns were reinforced by providing lateral circular discs of geotextile within the colun. The circular discs were placed at two different spacings (d and d/2) over varied colun lengths (l/4, l/2, 3l/4 and l). Sall scale laboratory odel tests have been carried out on floating and end-bearing single coluns with and without reinforceents to evaluate relative iproveent in the failure stress of the coposite ground due to different configurations of the reinforceent. The tests indicate that the lateral circular discs distributed over full colun length at d/2 spacing is the best configuration. The perforance of reinforced end-bearing coluns was uch better than the reinforced floating coluns. Both geotextile and geogrid were found to be equally effective for both floating and end-bearing stone coluns. Keywords: Geosynthetics; Soft Soil; Stone Colun; Ground Iproveent; Reinforceent.

2 98 K. Ali et al 1. Introduction Due to rise in population and an increase in infrastructure growth in etropolitan areas, there is a draatic rise in land prices and lack of suitable sites for developent. Therefore, now-a-days construction is also being carried out on arginal sites having extreely poor ground conditions like soft clays that were earlier considered unsuitable due to their poor strength and high copressibility. Such soils when loaded cause excessive settleents and early failure of structures. It is a challenge to the geotechnical engineer to provide a suitable foundation with satisfactory perforance at a reasonable cost in such soils. Stone coluns have been used for both ground iproveent and providing foundations to structures (Rao and Ranjan 1985). For lowrise buildings and structures such as liquid storage tanks, abutents, ebankents, factories, etc. that can tolerate soe settleent, stone coluns provide an econoical ethod of support in copressible and fine-grained soils (Mitchell 1981; Bergado et al. 1996). Stone colun foundation is capped with a granular at on the top. The granular at helps in transitting the load uniforly over the stone coluns and drains out the water fro the stone coluns. 2. Literature Review When the stone colun reinforced ground is being loaded, the coluns defor laterally into surrounding soft soil strata. For stone coluns having lengths greater than critical length (i.e., about four ties the diaeter of the colun), it is recognized that the bulging failure governs the load carrying capacity whether they bear on stiff layer or penetrate partially into the ediu stiff soil (Madhav et al. 1994). When the stone coluns are installed in extreely soft soils, the lateral confineent offered by the surrounding soil ay not be adequate. Consequently, the stone coluns installed in such soils will not be able to develop the required load-bearing capacity. In such situations, the bearing capacity of the stone colun can be iproved by providing circular lateral discs of a suitable geotextile as a reinforcing aterial along the length of the stone colun at a regular spacing. In this case, the bulging of the coluns is resisted by the frictional stresses obilized on the surface of the geotextile owing to lateral oveent of loose stone chips. Madhav et al. 1994; Shara et al. 2004; Ayadat et al have studied the behaviour of such reinforced stone coluns. Most of the work done so far is liited to fully penetrating coluns; therefore, in this paper experients have also been conducted on floating coluns. 3. Experiental Prograe 3.1 Materials, Instruentation and Test Prograe The odel tests were conducted on soft soil bed reinforced with stone coluns. The soft soil bed was ade up of fully saturated reolded kaolin clay. The properties of the kaolin clay are given in Table 1. The coluns were ade up of stone chips of size varying fro 2 to 4.75 copacted at a relative density D r = 60% and having an angle of internal friction ϕ = 42º as deterined by the direct shear test.

3 Model Tests on Stone Coluns Reinforced with Lateral Circular Discs 99 Table 1: Properties of the clay used in the odel tests. Paraeter Value Specific Gravity 2.64 Liquid Liit (%) 54 Plastic Liit (%) 23 Plasticity Index (%) 31 Saturated unit weight (kn/ 3 ) Dry unit weight (kn/ 3 ) 14.5 Water content (%) 40±1 Shear strength (kpa) 6 7 A 20 thick at was provided below the footing area in all odel tests. The at consisted of sub-angular Badarpur sand of predoinantly quartz particles of sizes passing through 1 sieve and retained on 600 icron sieve having an angle of internal friction ϕ = 38º. A woven geotextile of tensile odulus = 97.5 kn/ was used to reinforce the odel stone coluns. The odel tests for floating coluns were conducted in a perspex cylindrical tank of 300 diaeter and 600 depth and for end bearing coluns a tank of 300 diaeter and 360 depth was used. The diaeter and length of floating as well as end-bearing coluns were kept as 30 and 360 respectively. A suary of the odel tests is given in Table 2. Tes t No. d l Table 2: Suary of the odel tests conducted. Colu n Type Reinf. Type s x Tes t No. d l Colu n Type Reinf. Type Clay EB UR FL UR EB LCD FL LCD EB LCD FL LCD EB LCD FL LCD EB LCD FL LCD EB LCD FL LCD EB LCD FL LCD EB LCD FL LCD EB LCD FL LCD d = Diaeter of colun, l = Length of colun, s = Lateral circular discs spacing, x = Reinforceent length, FL = Floating, EB = End-bearing, UR = Unreinforced, LCD = Lateral circular discs s x

4 100 K. Ali et al 3.2 Preparation of Soft Clay Bed The soft clay bed was prepared for undrained shear strength of 6 7 kpa. The oisture content (40%) required for the desired shear strength was deterined by conducting several vane shear tests on a cylindrical specien of 76 height and 38 depth. After adding the water to the clay powder it was thoroughly ixed to a consistent paste and then left for 48 hours covered with wet gunny cloth for oisture equalization. This paste was then filled in the tank in 10 thick layers to the desired thickness by hand copaction such that no air voids are left in the soil. Before filling the soil in the tank, the inner surface of the tank wall was first coated with silicon grease and then covered with a polythene sheet to iniize the friction. The tank filled with soil was then again left for 48 hours for thixotropic gain. 3.3 Construction of Stone Coluns After finishing the top surface of the clay bed the position of colun was arked and three vane shear tests were conducted at different depths within the colun length for assessing the undrained shear strength of the clay bed. After conducting the vane shear tests, an open-ended perspex tube of external diaeter of 30 and 1 thick was pushed to 15 into the soft soil at dearcated location. The soil fro inside the casing pipe was then taken out with the help of an augur. The pipe was then again pushed into the soil by 15 and the soil was again reoved fro the pipe. The process was continued till the casing pipe to full colun length is pushed into the soil. The inner surface of the casing pipe was then properly cleaned off and then the stone colun was casted in steps by copacting the stone chips and withdrawing the casing pipe siultaneously. Stone coluns reinforced with lateral discs were constructed by placing the lateral circular discs of reinforcing aterial at desired levels inside the colun during the copaction of stone chips. The coposite soil with the colun inside was again left covered with a wet jute fabric in the controlled conditions for 24 hours to develop proper bonding between the stone chips of the colun and the soft soil. Before loading, a sand at of 20 thickness was then constructed by pouring Badarpur sand of required gradation over the footing area. 3.4 Test Procedure After construction of stone colun, sand at of 60 diaeter was copressed at a constant strain rate of 1 /in to ensure the undrained condition and the corresponding load was observed through a proving ring. The stone colun and its tributary soft soil area were loaded through a 12 thick perspex plate of diaeter double the colun diaeter, representing a 25% area replaceent ratio (A r ). The coposite soil was copressed to a axiu footing settleent of 60. A coplete test set up arrangeent and scheatic view of typical stone colun foundation for test has been shown in Fig. 3.

5 Model Tests on Stone Coluns Reinforced with Lateral Circular Discs 101 Loading plate 60 Sand at 50 Stone colun Soft soil 300 Tank Figure 1(a): Test set up ready for loading All diensions are in 300 Figure 1(b): Scheatic view of stone colun Figure 2. Defored shapes of coluns 3.5 Post Test Analysis After copletion of the test, the stone chips fro the colun were carefully picked up and a thin paste of plaster of Paris was poured into the cavity to establish the defored shape of the colun. The hardened plaster of Paris representing the defored colun shape was isolated by reoving the surrounding soft soil. Soe of the photographs of defored coluns have been shown in Figure Results and Discussion To study the relative perforance of coposite soil iproved with reinforced stone coluns, non-diensional charts were prepared with the help of noralized applied vertical stress and footing settleent. The applied vertical stress (σ) was noralized by dividing it with undrained shear strength (c u ) of soft clay bed and footing settleent (δ) by dividing it with the colun length (l). Thus in the ongoing text, the word failure stress stands for noralized failure stress. σ/c u σ/c u δ/l Plain Clay Unreinforced s = d, x = l/4 s = d, x = l/2 s = d, x = 3l/4 s = d, x = l (a) (b) Figure 3. Effect of reinforceent for the coposite ground iproved with floating coluns (d = 30, l = 360, D r = 60%, A r = 25%). Figures 3(a) and 3(b) show the effect of reinforceent on the failure stress of coposite ground iproved with floating coluns having geotextile discs at d and d/2 spacing respectively for different reinforcing lengths. The failure stress of the δ/l Plain Clay Unreinforced s = d/2, x = l/4 s = d/2, x = l/2 s = d/2, x = 3l/4 s = d/2, x = l

6 102 K. Ali et al coposite ground iproved with reinforced coluns increased by 12% for coluns having discs at d spacing for full colun length. In case of coluns reinforced with discs at d/2 spacing for full colun length, the failure stress increased by 32%. The reduced disc spacing increased the resistance against bulging and consequently bearing capacity iproved. But the iproveent was not significant because colun terinates into soft soil; the increased stiffness due to reinforceent facilitates the penetration of colun into the soft soil. (a) (b) Figure 4: Effect of encaseent for the coposite ground iproved with end bearing coluns (d = 30, l = 300, D r = 60%, A r = 25%). Figures 4(a) and 4(b) show the effect of reinforceent on the failure stress of coposite ground iproved with end-bearing coluns having geotextile discs at d and d/2 spacing respectively for different reinforcing lengths. End-bearing coluns exhibited ore increase in failure stress as copared to floating colun for all corresponding cases. The increase in bearing capacity of coposite ground iproved with end-bearing coluns with disc spacing of d and d/2 for full colun length was found to be 52% and 90% respectively. In this case also, colun reinforced at reduced spacing for full colun length is ore beneficial. 5. Conclusions 1. Whether floating or end-bearing, provision of reinforceent increases the bearing capacity of coposite ground. 2. Bearing capacity of coposite ground increases as the reinforcing length increases.. 3. Bearing capacity of coposite ground increases as the discs spacing decreases. 4. End-bearing coluns perfor better than floating coluns for each reinforceent configuration. 5. Whether floating or end-bearing, the best configuration for lateral discs of geotextile is the placeent of the discs over full colun length at d/2 spacing.

7 Model Tests on Stone Coluns Reinforced with Lateral Circular Discs 103 References [1] Ayadat, T., Hanna, A. M., and Haitouche, A. (2008). Soil iproveent by internally reinforced stone colun. Ground Iproveent, 161(2), [2] Bergado, D. T., Anderson, L. R., Miura, N., and Balasubraania, A. S. (1996). Soft ground iproveent in lowland and other environents. Aerican Society of Civil Engineers, New York. [3] Madhav, M. R., Alaghir, M., and Miura, N. (1994). Iproving granular colun capacity by geogrid reinforceent. Proceedings of 5 th International Conference on Geotextile, Geoebranes and Related Products, Singapore, pp [4] Mitchell, J. K. (1981). Soil iproveent state-of-the-art report. Proceedings of 10 th International Conference on Soil Mechanics and Foundation Engineering, Stockhol, June, 4, A. A. Balkea, Rotterda, Netherlands, [5] Rao, B. G., and Ranjan, G. (1985). Settleent analysis of skirted granular piles. Journal of Geotechnical and Geoenvironental Engineering, 111(11), [6] Shara, R., S., Kuar, B., R., P., and Ngendra, G. (2004). Copressive load response of granular piles reinforced with geogrids. Canadian Geotechnical Journal, 41,

8 104 K. Ali et al