Effect of Reinforcement on Bearing Capacity and Settlement of Sand

Transcription

1 Effect of on Bearing Capacity and Settlement of Sand Dr. M. S. Dixit 1 and Dr. K. A. Patil 2 1. Associate Professor, Department of Civil Engineering, Maharashtra Institute of Technology, Aurangabad (Maharashtra State), India, address: manishsdixit@gmail.com 2. Associate Professor, Department of Civil Engineering, Government College of Engineering, Aurangabad (Maharashtra State), India, address: kapatil67@gmail.com ABSTRACT The present investigation was undertaken to study the behavior of reinforced sand in improving the bearing capacity and settlement resistance under square footing. Locally available river sand was used along with geogrid as a reinforcing material. The tests were conducted at a density of 18kN/m 3. The parameters selected were depth of the top layer of reinforcement below the footing and D/B ratio of the reinforcement. Relationships between intensity of loading and settlement have been presented to determine the influence of the above parameters on the bearing capacity and settlement. It can be concluded that by a suitable arrangement of the reinforcing geogrid, the bearing capacity and settlement resistance of sand is improved as compared to the unreinforced sand. The estimation of load carrying capacity of footing is the most important step in the design of foundation. The number of theoretical approaches, in-situ tests and laboratory model tests are available to find out the bearing capacity of footing. The reliability of any theory can be demonstrated by comparing them with the experimental results. The results from laboratory model tests on square footings resting on sand with and without reinforcement are presented. The effect of bearing capacity of sand below the footing for square plate with variation in size, depth to width ratio and the effect of permissible settlement is evaluated. A steel tank of size 900mm 1200mm 1000mm is used for conducting model tests. KEYWORDS: Square footing, model test, sand reinforcement, geogrid, ultimate bearing capacity, settlement. INTRODUCTION The practice of building houses and roads on fiber reinforced earth with different types of reinforcing intrusions is an older concept. Rope fibers and bamboo were used to strengthen rural road bases and the soil low-cost low-rise buildings, although such practices were in existence, the reinforced earth concept was not studied or explained until the work of Henri Vidal (1967) of France who demonstrated its wide application and developed rational design procedures. The concept of reinforcing the earth was mainly pertaining to the metal reinforcement in early days. In recent times, this concept has been extended to the other materials, like fabrics, geotextile, geogrid, geocell, geomembranes, often termed as Geosynthetic. (Narain and Ratnam, 1985; Datye and Nagaraju, 1985). There are various techniques of ground improvement depending on the type of soil, nature of improvement desired, time involved in the treatment, availability of materials and economic viability (Sreekantiah, 1987 ). The main purpose of ground improvement techniques are to reduce

2 Vol. 19 [2014], Bund. E 1034 the settlement and to increase the bearing capacity. The present investigation deals with the behavior of reinforced earth in improving the bearing capacity and also settlement resistance under square footing. The bearing capacity of foundation is an important factor for designing the type of foundation and depth of foundation. Bearing capacity problems are focusing attention of the researchers for improvement of soil by using new material into the soil in various forms. One of the recent inclusions in bearing capacity problem is that of reinforcement. The bearing capacity of reinforced soil depends on various factors like length of reinforcement, number of reinforcement layers, placement of layers, angle of internal friction of soil and interface friction between soil and reinforcement. In the developing countries there is a growing need for research to be undertaken aimed at channelising local technology to the design and construction of low cost housing projects. It is expected that the locally available metal strips may provide a good reinforcement to the foundation soil in particular to long-term construction involving heavy loads over inferior foundation soil condition. Use of geosynthetics for improving the performance of shallow foundations has been studied by engineers over the past two decades. In the cases of poor marginal ground conditions, geosynthetics reinforcement is proved to be cost-effective solution. Among the range of geosynthetics available in the market, geogrids are the most preferred type of geosynthetic material for reinforcing the foundation beds. The beneficial effect of a geosynthetic inclusion is largely depends on the form in which it is used as reinforcement. For example, the same geosynthetic material, when used in planar layers or geocells or discreet fibers comprising exactly the same quantity of materials, will give different strength improvements in different forms. This difference in strengths achieved is mainly due to the different mechanism of failure in soil reinforced with geosynthetics in different forms. EARLIER STUDIES CARRIED OUT A shallow foundation is load carrying structures that transmit loads directly to the underlying soil. Shallow is a relative term, a foundation with a depth to width ratio less than or equal to four (D/B 4) is simply called a shallow foundation (Das, 1999). A foundation must satisfy two fundamental requirements: ultimate bearing capacity and settlement of foundations. The bearing capacity of soil can be defined as the foundation s resistance when maximum pressure is applied from the foundation to the soil without arising shear failure in the soil. The load per unit area of the foundation at which shear failure takes place is called the ultimate bearing capacity. By taking into account these two criteria, there are many theories and many approaches in laboratory and in situ studies to determine the ultimate bearing capacity. Firstly, Prandtl (1921), and thereafter Reissner (1924) presented theories based on the concept of plastic equilibrium. Later, the formulation was modified by Terzaghi (1943), Meyerhof (1963), Hansen (1968), Vesic (1973) and others. Many researchers have carried out the study for the beneficial effects of using planar forms of reinforcement in soil, such as Benquiet and Lee(1975), Akinmusuru and Akinbolade(1981), Fragaszy and Lawton(1984), Guido et al.(2005), Gosh et al.(2005), Patra et al. (2005), Patra et al.(2006), Basudhar et al.(2007), EL Sawwaf(2007), Ghazavi and Lavasan(2008), Sharma et al.(2009). In recent years, soil reinforcement in the form of geocell has been utilized successfully in many areas of geotechnical engineering such as Bathurst and Jarrett (1989), Bush et al.(1990), Cowland and Wong(1993), Krishnaswamy et al.(2000), Dash et al. (2001 a, b), Sireesh(2005), Zhou and Wen(2008). Wasti and Butun(1996) reported the possibility of using randomly distributed fiber and mesh element to increase the bearing capacity of subgrades. Dash et al. (2004) compared the relative performance of different forms of reinforcement (i.e. geocell, planar and randomly distributed mesh elements) in sand beds under strip footing. To compare the performance of geosynthetic materials in different forms, Latha and Murthy (2007) carried out a

3 Vol. 19 [2014], Bund. E 1035 systematic series of triaxial compression tests on sand reinforced with geosynthetic in freeform (planar, discrete fiber and cellular) using the same quantity of reinforcement EXPERIMENTAL STUDY Materials used for the testing: sand The test sand used in this study was dry sand collected from Godavari river near Paithan about 50 km from Aurangabad in Maharashtra State of India. The aim of this work is to study the effect of variation in size, depth of sand cushion below the footing (Dsc) and permissible settlement of footing on bearing capacity of sand for square footing. The specific gravity of sand is found to be 2.65, coefficient of uniformity (Cu) 3.20, coefficient of curvature (CC) 0.96 and effective particle size (D10) 0.42mm. Grain size distribution curve is shown in figure 1. The maximum and minimum dry unit weights of the sand were determined according to IS: 2720(1983), (part 14). The maximum dry unit weight of sand obtained is 18kN/m³ and the minimum dry weight obtained by pouring into the loosest state is 15.3kN/m³. The friction angle of the sand at 82.90% relative density (Dr), as determined from direct shear test on dry sand sample, is found to be 40 ⁰. Figure 1: Grain size distribution of sand Geosynthetics: Fiberglass Geogrid Fiberglass Geogrid is used to reinforce sand bed in the modal testing. The load elongation behavior of the fiberglass geogrid is determined from standard multi-rib tension test (ASTM Standard D 6637, 2001) and is shown in figure 2. Table 1 presents the properties of fiberglass geogrid and photograph 1 shows fiberglass geogrid

4 Vol. 19 [2014], Bund. E Tensile load (kn/m) Axial Strain (%) Figure 2: Load elongation behavior of fiberglass geogrid Photograph 1: Fiberglass geogrid Table 1: Properties of Fiberglass geogrid Sr.No. Property Fiberglass Geo-grid 1 Ultimate tensile strength (kn/m) 25 2 Failure strain (%) Secant modulus (kn/m) at 5% strain 14 4 Mass per unit area (g/m 2 ) Aperture size (mm) 34 X 28

5 Vol. 19 [2014], Bund. E 1037 Test set-up Laboratory model tests The load tests were conducted in a rectangular steel tank of 900mm 1200mm in plan and 1000mm in depth. The side walls of the tank were made smooth by painting with an oil paint to reduce the boundary effects. The model footings used for the tests were square in shape. The footings were made of 10mm thick rigid steel plate of sizes 100mm 100mm, 120mm 120mm, and 150mm 150mm. A hydraulic jack attached to the proving ring was used to push the footing slightly into the bed for proper contact between the sand and footing. A schematic diagram of the test set-up is shown in figure 3. In the present study the intensity of loading will be denoted by q and depth of sand cushion below the footing will be denoted by Dsc. Preparation of test bed Pluviation i.e. raining technique was used to place the sand in test tank. The height of fall to achieve the desired relative density was determined by performing a series of trials with different heights of fall. In each trial, the densities were monitored by collecting samples in small aluminum cups of known volume placed at different locations in the test tanks, based on the minimum and maximum void ratios of the sand for each height of fall, the relation between the height of fall and the corresponding relative density was developed. The size of geogrid squarely shaped reinforcement is kept equal to 6 times the width of the footing. Figure 3: Typical Sketch of Prototype Model of size 900mmx1200mmx1000mm Testing procedure Initially the sand bed of 900mm depth was prepared in the steel test tank using the sand raining technique. After preparing the bed, the surface was levelled, and the footing was placed exactly at the centre of the loading jack to avoid eccentric loading. The footing was loaded by a hand operated hydraulic jack supported against a reaction frame. A pre-calibrated proving ring was used to measure the load transferred to the footing. The load was applied in small increments. Each load increment was maintained constant until the footing settlement was stabilized. The footing settlements were measured through the dial gauges whose locations are as shown in the figure 3. Table 2 shows the effect of intensity of loading and D/B ratio on settlement at depth of

6 Vol. 19 [2014], Bund. E 1038 sand cushion below the footing (Dsc = 900mm), with and without reinforcement, for 100mm square plate. The effect of reinforcement on the bearing capacity is quantified in terms of Bearing Capacity Ratio (BCR). BCR = Ultimate bearing capacity for reinforced sand Ultimate bearing capacity for unreinforced sand = q ur / q u Testing program Model test is conducted on square footing resting on unreinforced sand. After that model tests were carried out on footings resting on geogrid reinforced sand by variation in D/B ratio as 0.3, 0.4 and 0.5. The objective of the test is to compare the performance improvement due to reinforcement for D/B ratio. Figure 4: Layout of geogrid in the model test Table 2: Effect of q and D/B ratio on settlement in mm at D sc = 900 mm with and without Geogrid for 100mm square plate Intensity of loading (q) (kn/m²) Depth of sand cushion below the footing (D sc = 900mm) out (D/B = 0.3) (D/B = 0.4) (D/B = 0.5)

7 Vol. 19 [2014], Bund. E 1039 Settlement in mm Intensity of loading kn/m out ( D/B = 0.3) ( D/B = 0.4) ( D/B = 0.5) Figure 5: Effect of q and D/B ratio on settlement in mm at D sc = 900 mm with and without Geogrid for 100mm square plate Figure 5 shows effect of intensity of loading and D/B ratio on settlement for depth of sand cushion below the footing (Dsc). It is seen from the figure 5 that settlement decrease with introduction of sand reinforcement and the reduction in settlement is appreciable for the intensity of loading of 65 kn/m 2 onwards. Log Load Log Settlement Figure 6: Log Load V/s Log Settlement for 100 mm square plate at 900mm depth without reinforcement

8 Vol. 19 [2014], Bund. E 1040 Log Load Log Settlement Figure 7: Log Load vs Log Settlement for 100 mm Square Plate at 900mm depth with reinforcement (D/B = 0.3) As shown in figure 6 and figure 7 log load vs log settlement curve is plotted, from the curve point of intersection will give the ultimate load and it is seen from the graphs that ultimate load increases with addition of geogrid. Table 3: Effect of reinforcement on ultimate load for 100mm square plate Ultimate Load kn/m 2 without reinforcement at D sc = 900mm Ultimate Load kn/m 2 with reinforcement at D sc = 900mm D/B = 0.3 D/B = 0.4 D/B = (1.26) 100 (1.24) (1.21) Table 3 shows the ultimate bearing capacity of unreinforced sand and reinforced sand and it is seen that with the addition of reinforcement there is increase in the ultimate bearing capacity and the values in the parenthesis indicate the values of bearing capacity ratio (BCR). Table 4: Effect of q and D/B ratio on settlement in mm at Dsc = 900 mm with and without Geogrid for 120mm square plate Intensity of loading (q) (kn/m²) Depth of sand cushion below the footing (D sc = 900mm) out (D/B = 0.3) (D/B = 0.4) (D/B = 0.5)

9 Vol. 19 [2014], Bund. E Settlement in mm Intensity of Loading kn/m out reinforcement Renforcement (D/B = 0.3) Renforcement (D/B = 0.4) Renforcement (D/B = 0.5) Figure 8: Effect of q and D/B ratio on settlement in mm at D sc = 900 mm with and without Geogrid for 120mm square plate Figure 8 shows effect of intensity of loading and D/B ratio on settlement for depth of sand cushion below the footing (Dsc). It is seen from the figure 8 that settlement decrease with introduction of soil reinforcement and the reduction in settlement is appreciable for the intensity of loading of 65 kn/m2 onwards. Table 5: Effect of reinforcement on ultimate load for 120mm square plate Ultimate Load kn/m 2 without reinforcement at D sc = 900mm Ultimate Load kn/m 2 with reinforcement at D sc = 900mm D/B = 0.3 D/B = 0.4 D/B = (1.07) (1.04) (1.02) Table 5 shows the ultimate bearing capacity of unreinforced sand and reinforced sand. The values in the parenthesis indicate the values of BCR. The values of BCR falls in the range of 1.02 to 1.07.

10 Vol. 19 [2014], Bund. E 1042 Table 6: Effect of q and D/B ratio on settlement in mm at D sc = 900 mm with and without Geogrid for 150mm square plate Intensity of loading (q) (kn/m²) Depth of sand cushion below the footing (D sc = 900mm) out (D/B = 0.3) (D/B = 0.4) (D/B = 0.5) Settlement in mm Intensity of Loading kn/m out (D/B = 0.3) (D/B = 0.4) (D/B = 0.5) Figure 9: Effect of q and D/B ratio on settlement in mm at D sc = 900 mm with and without Geogrid for 150mm square plate

11 Vol. 19 [2014], Bund. E 1043 It is seen from the figure 9 that settlement decreases for reinforced sand in comparision with unreinforced sand. The reduction in the settlement is appreciable for the intensity of loading of 65 kn/m2 onwards. Table 7: Effect of reinforcement on ultimate load for 150mm square plate Ultimate Load kn/m 2 without reinforcement at D sc = 900mm Ultimate Load kn/m 2 with reinforcement at D sc = 900mm D/B = 0.3 D/B = 0.4 D/B = (1.06) (1.03) (1.01) RESULT AND DISCUSSION The tests were conducted both on unreinforced sand and on reinforced sand. The ultimate bearing capacity (qu) for the unreinforced sand has been compared with the ultimate bearing capacity of the reinforced sand (qur ) and the bearing capacity ratio (qur / qu) has been computed for all the test. It is observed that the maximum value of the bearing capacity ratio (BCR) was obtained for D/B ratio in the range of 0.3 to 0.4. The results indicate that there is optimum value of D = 0.3 B for geogrid reinforcement at which the BCR value becomes maximum. For depth of sand cushion of 900mm for square plate of sizes 100mm, 120mm and 150mm, ultimate bearing capacity values are found to be 80.6kN/m², 91.20kN/m² and kN/m² respectively. As compared to 100mm square plate, the percentage increase in the ultimate bearing capacity for 120mm and 150mm square plates are found to be 13.15% and % respectively. Thus it indicates that as plate size increases, ultimate bearing capacity goes on increasing. introduction of sand reinforcement (geogrid) the percentage increase in ultimate bearing capacity for 100 mm square plate with D/B ratio of 0.3, 0.4, and 0.5 are found to be 26.94%, 24.06%, and 21.24% respectively. introduction of sand reinforcement (geogrid) the percentage increase in ultimate bearing capacity for 120 mm square plate with D/B ratio of 0.3, 0.4, and 0.5 are found to be 7.14%, 4.7%, and 2.32% respectively. introduction of sand reinforcement (geogrid) the percentage increase in ultimate bearing capacity for 150 mm square plate with D/B ratio of 0.3, 0.4, and 0.5 are found to be 6.09%, 3.68%, and 1.32% respectively. CONCLUSIONS Based on the studies carried out the different conclusions drawn are as given below. 1. For square plates of size 100mm, 120mm and 150mm with depth of sand cushion of 900mm, ultimate bearing capacity values for unreinforced sand are found to be 80.6 kn/m², kn/m² and kn/m² respectively. Thus, it indicates that as plate size increases, ultimate bearing capacity also increasing.

12 Vol. 19 [2014], Bund. E For square plates of size 100mm, 120mm and 150mm with depth of sand cushion of 900mm and D/B ratio of 0.3 ultimate bearing capacity values for reinforced sand are found to be maximum. 3. For square plates of size 100mm, 120mm and 150mm with depth of sand cushion of 900mm and D/B ratio of 0.3 settlement values are reduced by 35%. 4. There is a considerable reduction in the settlement for reinforced sand in comparison with unreinforced sand for the intensity of loading 65 kn/m 2 onwards. 5. A proper placement of geogrid reinforcement is required to obtain significant load settlement and bearing capacity improvement. 6. Apart from the tensile strength of reinforcement, its layout and configuration plays a vital role in increasing the bearing capacity. REFERENCES 1. Adams, M.T., Collin, J.G., Large model spread footing load tests on geosynthetic reinforced soil foundation. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 123, Akinmusuru, J.O., Akinbolade, J.A., Stability of loaded footings on reinforced soil. Journal of Geotechnical Engineering Division ASCE 107(6), ASTM Standard D 6637, Standard test method for determining tensile properties of geogrids by the single or multi-rib tensile method. American Society for Testing and Materials, Pennsylvania, USA. 4. Basudar P.K, Saha S., Deb K., Circular footings resting on Geotextilereinforced sand bed. Geotextiles and geomembranes 25 (6), Binquet, J., Lee, K.L., Bearing capacity test on reinforced earth slabs. Journal of Geotechnical Engineering Division, ASCE 101 (12), Dash, S.K. Sireesh, S., Sitharam T.G., Model Studies on circular footing supported on geocell reinforced sand underlain by soft clay Geotextiles and Geomembranes 21 (4), Datye, K.R., and Nagaraju, S.S Ground Improvement, Proceedings of the Indian contribution to Geotechnical Engineering, Vol.1, IS: Method of Load-Tests on Soil, Indian Standards Institutiin, New Delhi. 9. Narain, J. and Rathnam, M.V., Geotextiles, Proceedings of the Indian Contribution to Geotechnical Engineering, Vol.1, Sreekantah, H.R., An Investigation of a Rectangular Footing on Reinforced sand, Proceedings of the Indian Geotechnical Conference, Vol.1, Abdullah, H., Bandyopadhyay, A. and Singh, S., Laboratory assessment of shear strength of sand, Proceeding of INDOROCK2009, second Rock conference, Feb , Delhi ASTM D Standard Test Method For Bearing Capacity of Soil for Static load and Spread Footings, Annual Book of ASTM Standards, ASTM International, West Conshohocken PA 13. Bolton., M.D. and Lau, C.K., Scale effect in the bearing capacity of granular soils, International proc. of the 12 th International conference on Soil Mechanics and Foundations Engineering, Rio De Janeiro, Brazil,

13 Vol. 19 [2014], Bund. E Bolton, M.D. and Lau, C.K., Vertical bearing capacity factors for circular and strip footings on Mohr-Coulomb soil, Canadian Geotechnical Journal, 30, Cerato, A.B., Scale effect of shallow foundation bearing capacity on granular material, Ph.D. Dissertation, University of Massachusetts Amherst, Cerato, A.B. and Lutenegger, A.J., Scale effects of shallow foundation bearing capacity on granular material, Journal of Geotechnical and Geo-environmental Engineering, ASCE, 133(10), Clark, J.I , The Settlement and bearing capacity of very large foundations on strong soils:1996 R.M. Hardy keynote address, Canadian Geotechnical Journal, 35, Das, Braja Principals of Foundation Engineering, International Thomson Comp., De Beer, E.E., Bearing capacity and settlement of shallow foundations on sand, International proc. of the Bearing Capacity and Settlement of foundations Symposium, Duke University, Durham, NC, G. Madhavi Latha, and Amit somwanshi Effect of reinforcement form on the bearing capacity of square footings on sand, Geotextile and Geomembranes, S.N. Moghaddas Tafreshi and A.R. Dawson Comparison of Bearing capacity of a strip footing on sand with geocell and with planar forms of geotextile reinforcement, Geotextile and Geomembranes, Hansen, J.B Revised and extended formula for bearing capacity, Danish Geotechnical Institute, Copenhagen, Bulletin, 28, Ingra, S.T. and Baecher, G.B Uncertainty in bearing capacity of sand, Journal Geotechnical Engineering, ASCE, 109(7), Jacek, T. and Ivo, H., Class A prediction of the bearing capacity of plane strain footings on sand, soils and foundations, 39(5), Jain, A., Effect of loading Mechanism on Plate load test results, M.Tech thesis IIT Bombay 26. Jain, A., Dasaka, S.M. and Kolekar, Y.A., Lessons learned from failure of a field load test, In proceedings of International Symposium on forensic approach to Analysis of Geo-hazard Problems, 14-15December 2010, Mumbai, CD-Rom proceedings 27. Junhwan, Lee and Rodirigo, Salgado., Estimation of Bearing Capacity of Circular Footings on Sands Based on Cone Pentration Test, Journal of Geotechnical and Geo-environmental Engineering, Vol. 131, No.4, Kumar, J. and Khatri, V.N., Effect of footing width on bearing capacity factor N γ, Journal of Geotechnical and Geo-environmental Engineering, ASCE, 134(9), M. veiskarami, M. Jahanandish and A. Ghahramani., Prediction of the bearing capacity and load-displacement behaviour of shallow foundations by the stress-levelbased ZEL Method, Scientia Iranica, Meyerhof, G.G., Some recent research on the bearing capacity of foundations, Canadian Geotechnical Journal, 1,

14 Vol. 19 [2014], Bund. E Prandtl, L., On the penetrating strengths (hardness) of plastic construction materials and the strength of cutting edges, Zeitschrift fu Angewandte Mathematik and Mechanik, (1), Perkins, S.W. and Madson., Bearing Capacity of Shallow Foundations on Sand- A Relative Density Approach, Journal of Geotechnical and Geo environmental Engineering, Vol.126 No.6, Reissner, H., Zum Erddruck problem(concerning the earth pressure problem), International proceedings of the first International Congress of Applied Mechanics, Delft, (Germany), Steenfelt, J.S., Scale effect on bearing capacity factor N γ, International proceedings of the 9 th International Conference on soil mechanism and foundation engineering, Tokyo, Japan, 1, Terzaghi, K., Theoretical soil mechanics, John Wiley and sons, New York, USA. 36. Vanapalli, S.K., Sun, R. and X.Li., Bearing Capacity of an Unsaturated sand from model footing tests, Taylor and Francis Group, London, ISBN , Vesic, A.S., Analysis of ultimate loads of Shallow foundations, Journal of the soil mechanics and foundations Division, ASCE, 99, ejge