IGC. 50 th INDIAN GEOTECHNICAL CONFERENCE BEARING CAPACITY OF SQUARE FOOTING RESTING ON GEOGRID- REINFORCED SAND

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1 INDIAN GEOTECHNICAL CONFEENCE BEAING CAPACITY OF SQUAE FOOTING ESTING ON GEOGID- EINFOCED SAND. Sahu 1, C.. Patra 2, B.P. Sethy 3 ABSTACT Since the original work by Binuet and Lee (1975), a number of studies have been conducted to evaluate the possibility of constructing shallow foundations on reinforced soil to increase their load-bearing capacity and reduce settlement. esults of several laboratory model test studies have been published relating to the improvement of the load-bearing capacity of shallow foundations supported by sand reinforced with various materials such as geogrids, geotextiles, metal strips, metal bars, rope fibres etc. The use of geogrid for soil-reinforcement has increased greatly, primarily because geogrids are dimensionally stable and combine features such as high tensile modulus (low strain and high load), open grid structure, positive shear connection characteristics, light weight and long service life. The open structure provides enhance soil-reinforcement interaction. Several works have been done relating to the estimate of the ultimate bearing capacities of shallow foundations, supported by geogrid reinforced sand. Few experimental studies have been made on the evaluation of bearing capacity of shallow foundations on geogrid-reinforced sand under eccentric load. This research study aims at investigating the potential benefits of using the reinforcement to improve the bearing capacity and reduce the settlement of shallow foundations. To implement this objective, a total of sixteen number laboratory model test were conducted by using suare surface foundation over the reinforced sand bed. The model footing used for the model tests in the laboratory is of size 10cm x 10cm. The average relative density maintained during all the tests is 69%. The reinforcing material used in the experiment is TGB40 in 2, 3 and 4 number of layers. The load eccentricity is varied from 0 to 0.15B with an increment of 0.05B.. The vertical distance of first 1 Sahu., Civil Engineering, esearch Scholar, ourkela, India, roma.sahu.civ@gmail.com 2,Patra, C., Civil Engineering, Professor, ourkela, India, crpatra19@yahoo.co.in 3 Sethy, B. P, Civil Engineering, esearch Scholar, ourkela, India, barada.jeetu@gmail.com

2 oma Sahu 1, Chittaranjan Patra 2, Barada Prasad Sethy 3 geogrid layer from base, distance between the consecutive geogrid layers, and width of the geogrid has been kept constant. Parametric studies have been made to evaluate the influence of load eccentricity on bearing capacity of the foundation. Based on the model test results, an empirical non-dimensional reduction factor has been developed. This reduction factor will compute the ultimate bearing capacity of footing subjected to eccentrically loaded suare footing by knowing the ultimate bearing capacity of footings under centric vertical load for reinforced condition. Keywords: Ultimate bearing capacity, Geogrid, Sand, Eccentric loading, Suare foundation, eduction factor Assumed failure mode under an eccentrically loaded surface suare foundation on geogrid-reinforced sand

3 INDIAN GEOTECHNICAL CONFEENCE BEAING CAPACITY OF SQUAE FOOTING ESTING ON GEOGID- EINFOCED SAND oma Sahu 1, esearch Scholar, National Institute of Technology, ourkela, Chittaranjan Patra 2, Professor, National Institute of Technology, ourkela, Barada Prasad Sethy 3,esearch Scholar, National Institute of Technology, ourkela, ABSTACT: In this paper laboratory model tests was reported on a suare foundations underlying by a reinforced sand bed. All the tests were conducted in dense sand. The load eccentricity ratio (e/b) was varying from 0 to 0.15, with an increment of The number of geogrid layers was varied from 2 to 4. All the tests were conducted at a particular relative density and angle of internal friction. Based on the laboratory model test results, an empirical relationship called reduction factor has been developed. This reduction factor is the ratio of the ultimate bearing capacity of an eccentrically loaded foundation to the ultimate bearing capacity of the centrally loaded. Keywords: Ultimate bearing capacity, Geogrid, Sand, Eccentric loading, Suare foundation, eduction factor INTODUCTION In Civil engineering practice the load from super structure is to be transferred to a soil layer through footing that is capable to withstand this load with adeuate factor of safety under tolerable settlement. When a structure is built, in many cases, these footings are subjected to eccentric loading such as footings subjected to vertical load and moments due to earthuake, water, wind, earth pressures etc. Due to this load eccentricity, the overall stability of the foundation decreases along with differential settlement, tilting of the foundation, heaving the supporting soil which reduces the bearing capacity. This might be avoided either by constructing the footing with larger dimensions to reduce the contact pressure which lead to uneconomical design or by increasing the bearing capacity of the supporting soil by using soil reinforcement techniue. During the last four decades, the results of a number of studies have been published relating to the ultimate bearing capacity of shallow foundations supported by multi-layered geogrid-reinforced sand. The results were mostly obtained from small-scale laboratory model tests (Omar et al., 1993 [4]; Das and Omar, 1994 [1]; Omar et al., 2006 [5]; Patra et al., 2006 [7]; Sadoglu et al., 2009 [9]; Shin et al., 2002 [10]; Dash 2012 [2]). Most of the experimental studies given above were conducted for strip foundation condition. Omar et al., (1993) [4]; Kumar and Walia, (2006) [3]; Madhavi Latha and Somwansi (2009) [6] provided the results of suare foundations in different aspects. None of the above published studies address the effect of load eccentricity on the ultimate bearing capacity of suare foundation. The purpose of this investigation is to present some laboratory model test of surface suare foundation resting over geogrid-reinforced sand with B/L ratio 1. MATEIAL AND METHODS All the experimental test program were carried out in the Geotechnical Engineering laboratory of the Civil Engineering Department of NIT ourkela, India. All the model tests were conducted in a steel tank with dimensions of 1.0m (length) 0.504m (width) 0.655m (height). The vertical and bottom edges of the tank were stiffened using angle

4 oma Sahu 1, Chittaranjan Patra 2, Barada Prasad Sethy 3 sections to avoid lateral yielding during soil placement and loading. The two length sides of the tank are made of 12 mm thick high strength fiberglass. The inside walls of the tank were smooth enough to minimize side friction. The boundary distances were greater than the footing length, width and depth during the tests, it was observed that the extent of failure zones was not more than the footing geometry, and the frictional effect was insignificant to affect the results of model tests. All the load tests were carried out with footing size of 100 mm x100 mm (B/L=1). It was made out of a mild steel plate with a thickness of 30 mm. Locally available sand was used for the present model tests. The sand used for the tests had 100% passing 0.7 mm size sieve and 0% passing 0.3 mm size sieve. All the model tests were conducted by using a poorly graded sand with effective grain size D10 = 0.33 mm, uniformly coefficient Cu = 1.42, and coefficient of curvature Cc = 1.37, average unit weight of sand γ = kn/m 3, relative density of sand Dr = 69% and friction angle of sand = Biaxial geogrid (TGB 40) was used for the present tests. The physical properties of the geogrid are given in table - 1. To achieve the desired average unit weight, sand was poured into the test tank in layers of 25 mm from a fixed height using a sand raining techniue. In order to achieve the desired density, the height of fall was fixed by making several trials in the test tank prior to the model tests. Geogrid layers were placed in the sand at desired values of u/b and h/b. All the load test was conducted in surface case with different centric and eccentric load. Static vertical loads were applied to the model foundation by an electrically operated hydraulic jack. Load on the model foundation was measured by a proving ring, and the settlement was measured by two dial gauges attached to the foundation diametrically opposite to each other. Load was applied on the model foundation with small increment and the resulting deformation was recorded so that the entire load-settlement curve could be obtained. For the present experiment, the following parameters were adopted for the geogrid layers: u/b = 0.35, h/b = 0.25, b/b = 4.5. Table 1 Properties of the geogrid Parameters Polymer Aperature size (MD/CMD) Junction thickness Tensile strength at 2% elongation Tensile strength at 2% elongation (CMD) Tensile strength at 5% elongation Tensile strength at 5 % elongation Elongation at maximum load Elongation at maximum load (CMD) Ultimate tensile strength Ultimate tensile strength (CMD) Quantity Polyster 25 mm/25 mm 9 mm 7.5 kn/m 7.5 kn/m 14 kn/m 14 kn/m 15 % 15 % 40 kn/m 40 kn/m ESULTS AND DISCUSSION Purkayastha and Char (1977) [8] carried out stability analysis of an eccentrically loaded continuous foundations supported by sand using the method of slices proposed by Janbu (1957). Based on that analysis, they proposed that k where K k = reduction factor = ultimate bearing capacity of eccentrically loaded continuous foundations = ultimate bearing capacity of centrally loaded continuous foundations k k k a d B u u e d, B B e d 0, B B b e B c where, a, b and c are constants. (1) (2) (3) (4) (5)

5 INDIAN GEOTECHNICAL CONFEENCE Fig. 1 Assumed failure mode under a centrally loaded surface suare foundation on geogridreinforced sand Where, u = Location of the top layer of reinforcement measured from the bottom of the foundation; N = Number of reinforcement layer; h = vertical distance between two consecutive layers; b = width of reinforcement layer. d = u+ (N-1) h d = depth of reinforcement measured from the bottom of the foundation. Fig. 2 Assumed failure mode under an eccentrically loaded surface suare foundation on geogrid-reinforced sand The ultimate bearing capacity of all suare foundation tests are determined from experimental model tests which are conducted in the laboratory are given in Table - 2 (col.5). As discussed in above in order to uantify certain parameters like e/b, d/b all the model test results have been analysed using Nonlinear egression Analysis Program (NLEG). NLEG performs statistical regression analysis to estimate the values of parameters for linear, multivariate, polynomial, logistic, exponential, and general nonlinear functions. The regression analysis determines the values of the coefficients that cause the function to best fit the observed data that is being provided. The following procedure is adopted to analyse the test results and develop the reduction factor. For a given d/b regression analyses is performed to obtain the magnitudes of a, b and c. egression analysis has been done to determine the values of a, b and c for each depth of reinforcement layer (d). k d a B b e B c (6) The values of a, b and c which was obtained from analyses are 2.97, 0.6 and 1.01 respectively. The experimental values of K defined by E. (4) are shown in Col. 4 of Table - 2. In computing the experimental k values, the first row values for each depth of reinforcement layer (where d /B= 0.6, 0.85, 1.1 and e/b= 0) are used as the reference values (i.e. denominator). For comparison purposes, the predicted values of the reduction factor k obtained using E. (6) are shown in Col. 5 of Table - 2. The deviations in between predicted values of k and experimentally obtained k are shown in Col. 6 of Table - 2. In most cases the deviations are ±1% or less. Thus E. (7) provide a reasonable good and simple approximation to estimate the ultimate bearing capacity of suare foundations subjected to eccentric loading. 0.6 d e 2.97 B B 1.01 k (7)

6 oma Sahu 1, Chittaranjan Patra 2, Barada Prasad Sethy 3 Table 2 Model test results Experimental u (kn/m 2 ) (3) Predicted k (5) Deviation (%) = (col.5-col.4) /col.5 (%) (6) d/b (1) e/b (2) Experimental k (4) Table - 3 Comparison of Ultimate Bearing capacity obtained from E. 10 Predicted k (3) Experimental u (kn/m 2 ) (5) Predicted u (kn/m 2 ) (6) Deviation (%) = (col.6- col.5) /col.6 (7) d/b (1) e/b (2) (1-k)Pred (4) k e d u, B B (8) e d u 0, B B The values of a, b and c was obtained from regression analyses. The predicted values of u defined by E. (10) are shown in Col. 6 of Table-3. In computing the predicted u values, the centric e d u, B B 1 (9) k e d u, B B e d u 0, B B e d u 0, B B 1 k pred (10) experimental u values for each depth of reinforcement layer (where, d/b= 0.6, 0.85, 1.1 and e/b = 0) are multiplied with (1-k)Pred. the predicted values of u are shown in Col. 6 of Table

7 INDIAN GEOTECHNICAL CONFEENCE 3. The deviation in between experimental ultimate bearing capacity and predicted ultimate bearing capacity will be lie in between ±6%. CONCLUSIONS The results of laboratory model tests conducted to determine the ultimate bearing capacity of a suare foundation supported by geogrid reinforced sand and subjected to eccentric load. The eccentricity ratio e/b was varied from 0 to Based on the model test results, the following conclusions can be drawn: I. An empirical relationship for reduction factor in predicting ultimate bearing capacity has been proposed for eccentrically loaded suare foundation resting over geogrid-reinforced sand bed. II. For similar reinforcement conditions, the ratio of the ultimate bearing capacity of eccentrically loaded foundations to that loaded centrally can be related by a reduction factor. The reduction factor is a III. function of d/b and e/b. A comparison between the reduction factors obtained from the empirical relationships and those obtained from experiments shows, in general, a variation of ±1% or less. Similarly, a comparison in between the experimental ultimate bearing capacity and predicted ultimate bearing capacity will be lie in between ±6%. The developed reduction factor also give well agreement with the experimental value Soil, Geotech. and Geol. Engg, 24, Omar, M. T., Das, B. M., Puri, V. K. and Yen, S.C. (1993), Ultimate bearing capacity of Shallow Foundations on Sand with Geogrid einforcement, Can. Geotech. Jl, 30(3), Omar, M. T. (2006), Ultimate bearing capacity of eccentrically loaded strip foundation on geogrid-reinforced sand, Unv. of Sharjah Jl. of Pure and app. Sci, 3(2). 6. Madhavi Latha, G. and Somwanshi, A. B. (2009), Bearing capacity of Suare Footings on Geosyntetic einforced Sand, Geotext and Geomembr, 27, Patra, C.., Das, B.M., Bhoi, M. and Shin, E.C. (2006), Eccentrically loaded Strip Foundation on Geogrid-einforced Sand, Geotext and Geomembr, 24 (4), Purkayastha,. D. and Char,. A. N. (1977), Stability analysis for eccentrically loaded footings, Jl.of Geotech. Engg. Div, ASCE 103 (6), Sadoglu, E., Cure, E., Moroglu, B. and Ali Uzuner, B. (2009), Ultimate loads for eccentrically loaded model shallow strip footings on geotextile-reinforced sand, Geotext and Geomembr, 27, Shin, E. C., Das, B. M., Lee, E. S. and Atlar, C. (2002), Bearing capacity of strip foundation on geogrid-reinforced sand, Geotech. and Geol. Engg, 20, EFEENCES 1. Das, B. M. and Omar, M. T. (1994), The effects of foundation width on model tests for the bearing capacity of sand with geogrid-reinforcement, Geotech. and Geol. Engg, 12, Dash, S. K. (2012), Effect of geocell type on load-carrying mechanisms of geocellreinforced sand foundations, Int. Jl. of Geomech, ASCE, 12(5), Kumar, A. and Walia, B. S. (2006), Bearing Capacity of Suare Footings on layered