STUDY ON IMPROVEMENT IN BEARING CAPACITY OF SOIL USING GEOGRID REINFORCEMENT

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 7, Issue 6, November-December 216, pp , Article ID: IJCIET_7_6_19 Available online at ISSN Print: and ISSN Online: IAEME Publication STUDY ON IMPROVEMENT IN BEARING CAPACITY OF SOIL USING GEOGRID REINFORCEMENT Amala Raju Arul. A Assistant Professor, Department of Civil Engineering, Sathyabama University, Chennai, India Madhumathi R.K Research Scholar, Department of Civil Engineering, Anna University, Chennai, India ABSTRACT The influence of geogrid reinforcement on bearing capacity of granular soil is investigated in model tests. The tests were performed for an isolated footing resting on a sand to establish the Load versus settlement response of Unreinforced and Reinforced Soil system. The result shows that the bearing capacity increases significantly with decrease in vertical spacing of geogrids. The observations proved that the Geogrid reinforced system creates an enhanced resistance and minimizes the differential settlement. A numerical study is also carried out using Plaxis 2D and the results are compared with the experimental results. The results thus obtained from the FEM compares reasonably with the results of 1g model test. Key words: Geogrid reinforcement, Granular Soil, Isolated Footing, Settlement, Numerical Analysis. Cite this Article: Amala Raju Arul. A and Madhumathi R.K, Study on Improvement in Bearing Capacity of Soil Using Geogrid Reinforcement. International Journal of Civil Engineering and Technology, 7(6), 216, pp INTRODUCTION Sandy soils instigate many problems for geotechnical engineers. Usually, their low shear strengths combined with the magnitude of the proposed loads require the soil to be stabilised. The granular soil deposits are very loose and undergo more settlement. Depending on the structural load acting and the depth of the sand layers, large settlement may occur in these deposits. Hence the in situ soil is treated to accommodate the proposed loads with maximum efficiency. Several techniques are available for ground stabilisation, e.g. grouting, freezing, dewatering, compacting, etc. Most, however, are site specific, often costly and time consuming. The technique of reinforcing the soil below shallow foundations with geosynthetic reinforcement is one of the fastest growing techniques in the field of geotechnical engineering. Considerable improvement in the bearing capacity was observed in reinforced soil over the unreinforced soil by introducing geogrid reinforcement. The reduction in the cost was 1.% of total cost. Construction time for geogrid reinforcement bed was less than of a stone column and is expected to be more stable with time editor@iaeme.com

2 Amala Raju Arul. A and Madhumathi R.K The placement of tensile reinforcement members significantly improves the soil stability and ultimate load bearing capacity. Various materials were used like thin steel strips, special plastic grids and geotextiles. The most common tensile reinforcement materials are uniaxial geogrid and biaxial geogrid, because of their durability and low cost. The factors like the capacity of element to withstand tensile stresses, the amount of extension exhibited by the element under tensile stress and the shearing resistance between the reinforcement and surrounding soil governs the effectiveness of reinforcing element embedded in soil. The most effective location of the geosynthetic for subgrade improvement is at the interface between the selected granular material and the subgrade 2. Since biaxial geogrids cannot provide uniform tensile strengths when subjected to tension in different directions. A new product with triangular apertures was developed which has a more stable grid structure and is expected to provide uniform tensile strengths in all directions as compared with uniaxial and 9 biaxial geogrids 3. Considerable work has been done to study the effect of geomembranes on soil experimentally and numerically (using approximate methods such as finite element and finite difference techniques) [4-1].The works reported in this area require better understanding on the behaviour of geogrid reinforcing technique for granular soil. The objective of the present study is to establish a technique to improve the load carrying capacity of the soil using Geogrids and to evaluate the efficiency of the system in term of strength characteristics of selected soil and reinforcement material. 2. EXPERIMENTAL INVESTIGATION 2.1. Loading Arrangement and Test Tank Tests were conducted using a model tank having internal dimensions of 9 mm in length, 9mm in width and 9mm in height. Figure 1 shows the schematic arrangement of test setup with model footing in position. Dimension of wooden footing used in this 1g model test is 1mm x 1mm x 2mm. The tank is made up of concrete and it has high stiffness. The portal frame is supported to the floor by means of base plate with anchor bolt arrangements. A stable beam acts as a support for the pre-calibrated proving ring and hydraulic loading jack arrangement. The sandy soil is levelled up to the top of the soil. Isolated footing is placed at the centre of the tank, and then the proving ring is placed on top of the footing. The piston in the loading frame touches the proving ring with the help of hydraulic jack. Two dial gauges are fixed on the footing to measure the settlement. Figure 1 Test setup editor@iaeme.com

3 Study on Improvement in Bearing Capacity of Soil Using Geogrid Reinforcement 2.2. Properties of Test Medium Uniformly graded river sand was chosen as the test medium. Tests were conducted on sand samples for gradation, specific gravity, maximum and minimum dry density and strength of the sand. Controlled pouring was employed in order to achieve homogeneous sand beds. For the unit weights studied in this investigation, the required quantity of sand was weighed for each layer to determine the total weight of the sand bed. The unit weights were measured to be 14.4 kn/m 3 for soil of RD(relative density) 17% and 16.6 kn/m 3 for soil of RD 9% and the corresponding friction angles were 33 and 37 respectively Properties of Reinforcing Material The thickness and aperture size of the selected geogrid material were measured using the vernier caliper. For the determination of tensile strength, geogrid sample of 1mm length and 2mm width is used and determined by wide width grip method. Some of the commercially available geogrid materials are listed below in Table 1 and for the present investigation; the geogrid of CE 111 was selected. The characteristic ultimate tensile strength of proposed reinforcing material at maximum strain of 1% given by the manufacturers is tabulated in Table1. Table 1 Properties of Reinforcing Material Materials Thickness Aperture size Ultimate tensile strength CE mm 2.8mm 2.KN/m CE mm 3.2mm 7.68KN/m CW131.mm.mm.8KN/m 2.4. Fabrication of Geogrid Cages The depth of influence for bearing capacity is B and for settlement is 2B for an isolated footing. The geogrid circular pipes of 4 cm diameter are prepared and used as vertical reinforcement by varying the spacing as shown in figure. 2. The length of one geogrid cell used is mm and they are placed at.3b depth from the base of footing where B is the width of the footing. The number of rows of geogrid reinforcement is 4 and the number of columns of reinforcement is 4. Figure 2 Geogrid Cages editor@iaeme.com

4 Amala Raju Arul. A and Madhumathi R.K 3. RESULTS AND DISCUSSIONS Tests were conducted on model footing placed on sand bed of loose density (14.4 kn/m 3 ) and their performance was studied in order to understand the effect of reinforcing the sand. During the tests, load intensity versus settlement behaviour of isolated footing placed over reinforced sand was recorded. The results obtained from the 1g model tests are presented and discussed below Performance of Isolated Footing on Unreinforced Soil The load verses settlement response obtained for unreinforced sand is shown in Figure. 3. The curves indicate that as the load increases the settlement also increases rapidly. The increase in load beyond certain limit, increases the settlement rapidly and leading to failure. Load (KN) Figure 3 Load vs. settlement relation for unreinforced sand 3.2. Performance of Isolated Footing on Reinforced Soil Load (KN) Without Geogrid with geogrid at cm spacing Settlement mm Load KN. 1 without geogrid with geogrid at 2cm spacing Figure 4 Load vs Settlement curve for reinforced soil sample with geogrid placed at cm vertical spacing Figure Load vs Settlement curve for reinforced soil sample with geogrid placed at 2cm vertical spacing Figure 4 shows the load vs. settlement relation for soil sample with and without geogrids. In this case the geogrid cage spacing is maintained as cm.the comparison shows that initially as the load increases the settlement also increases gradually with increase in rate of settlement. From the findings it is also inferred that the reinforced soil offers more resistance to applied load than the soil without reinforcement editor@iaeme.com

5 Study on Improvement in Bearing Capacity of Soil Using Geogrid Reinforcement For example at a load of.kn the soil without geogrid exhibits more settlement than soil with geogrid. Similar observation is illustrated in Figure for soil reinforced using geogrid cages of 2cm centre to centre spacing Effect of Vertical Spacing of Geogrid Reinforcement Relation between loads vs. settlement for different spacing of geogrid is presented in Figure. 6. From the results it is observed that as the vertical spacing between the geogrid cages decreases the settlement occurred for particular load intensity also decreases resulting in the increase of the ultimate bearing capacity of the soil. Even though the initial portions of the response curve illustrate minimal improvement, the bearing capacity improvement is significant in latter portion of curve for less spacing of reinforcement. The percentage increase in the bearing capacity of soil due to the reinforcement provided by geogrids placed at cm, 1cm,1cm,2cm spacing are %, 44%, 3% and 29% respectively. Hence this response proves that vertical spacing of geogrid cages contribute appreciably in enhancement of bearing capacity Load (kn) Without Reinforcement At cm Spacing At 1cm Spacing At 1cm Spacing At 2cm Spacing Figure 6 Relation between load vs. settlement for different spacing of geogrid 4. NUMERICAL MODELLING A finite element model using Plaxis 2d is simulated to investigate the effect of geogrid reinforcement in improving the bearing capacity of granular soil. The footing is discretised using plate elements whereas the soil medium (i.e. test medium) is discretised using 1 node wedge elements.. The material of footing is idealized with linear elastic material property. Mohr-Coulomb plasticity material is used to idealise elastoplastic response of sand medium. Figure 7 shows the typical model simulated for numerical analysis. Figure 7 Finite Element Model editor@iaeme.com

6 Amala Raju Arul. A and Madhumathi R.K. COMPARISON BETWEEN EXPERIMENTAL AND NUMERICAL ANALYSIS Figure. 8 presents the comparison between the load-settlement response curve obtained through numerical analysis and from the laboratory tests for unreinforced case. It is clear that for particular load intensity, plaxis underestimates the settlement by approximately 8%. But load settlement pattern is almost same. This shows that the results obtained from numerical analysis holds reasonably good with the experimental results. However numerical analysis predicts lower settlement values for all load intensities. Figure 8 Comparision of 1g model and numerical results for footing placed on unreinforced soil bed Load (kn) Plaxis Exp Figure 9 Comparison of 1g model and numerical results for footing placed on reinforced soil bed Figure 9 shows the comparision ion of experimental and finite element results for soil reinforced with geogrids cages of vertical spacing cm. Results shows that the settlement increases exponentially with increase in load, which compares reasonably well with the experimental results. This response confirms that the material model considered ed and conditions simulated in the FE model predicts the response of footing reasonably. 6. CONCLUSION An in-situ method of stabilization using geogrid in the form of geogrid cell was established and the improvement in the bearing capacity is observed from both the experimental and numerical study. The findings of the study are summarized below editor@iaeme.com

7 Study on Improvement in Bearing Capacity of Soil Using Geogrid Reinforcement The ultimate bearing capacity of the reinforced soil increases with decrease in vertical spacing between the geogrid cages. The maximum percentage increase in the bearing capacity of the reinforced soil is observed as % for geogrids placed at cm spacing and 29% for geogrids placed at 2cm spacing. The increase in the load carrying capacity of the reinforced soil is due to the additional adhesive shear resistance mobilized between reinforcement. The results of FEM analysis agrees well with the results of model tests conducted. REFERENCE [1] Al-Sinaidi Rahman and Ali Hassan Ashraf. Improvement in bearing capacity of soil by geogrid - an experimental approach. International Association for Engineering Geology and environment (IAEG) 26; pp 1-. [2] Das B.M and Shin E.C. Strip foundation on geogrid-reinforced clay behavior under cyclic loading. Geotextiles and Geomembranes. 13(1),1998; [3] Dong Y.L, Han J, and Bai X.H. A numerical study on stress-strain responses of biaxial geogrids under tension at different directions. GeoFlorida Conference, 211, pp [4] Das B.M. Shallow foundation on sand underlain by soft clay with geotextile interface. Geosynthetics for Soil Improvement pp [] Moayedi H, Kazemian S, Parasad A. and Huat B.K. Effect of Geogrid Reinforcement Location in Paved Road Improvement. Electronic Journal of Geotechnical Engineering, 29. pp 1-11 [6] Huabao zhou and Xuejun wen. Model studies on geogrid or geocell-reinforced sand cushion on soft soil. Geotextiles and geomembranes. 26, 28., pp [7] Madhavi Latha G. Design of geocell reinforcement for supporting embankments on soft ground. 12th international conference of international association for computer methods and advances in Geomechanics (IACMAG. 28. Chu Bo and Choa. Improvement of ultra-soft soil using prefabricated vertical drains Geotextiles and geomembranes. 24, 26. pp [8] Amalendughosh, Ambarish Ghosh and Ashis Kumar Bera. Bearing capacity of square footing on pond ash reinforced with jute-geotextile. Geotextiles and geomembranes. 23,2. pp [9] Andre R.S, Fahel, Ennio M, Palmeria and Ortigao, Behaviour of Geogrid reinforced abutments on soft soil in the BR 11-sc highway brazil. Proc ASCE special Geotechnical publication. 2. [1] Binquet J. and Lee K.L. Bearing Capacity Tests on Earth Slabs. Journal of the Geotechnical Engineering Division., ASCE. Vol.11(12), 197., pp [11] Shahid Bashir Bhat, Danish Kunroo and Zahid Ahmad. Comparison of Maximum Dry Density Optimum Moisture Content and Strength of Granular Soils Using Different Methods of Compaction, International Journal of Civil Engineering and Technology (IJCIET), 6 (2), 21, pp. 1. [12] Abhishek Singh, B. R. Phanikumar and Ram Prasad. Effect of Geogrid Reinforcement on Load Carrying Capacity of A Coarse Sand Bed, International Journal of Civil Engineering and Technology (IJCIET), 7 (3), 216, pp editor@iaeme.com