EFFECT OF FRESH FILL ON BEARING CAPACITY OF FOOTING. K. Ramu 1, G.Ch. Satyanarayana 2

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1 5 th IGC 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India EFFECT OF FRESH FILL ON BEARING CAPACITY OF FOOTING K. Ramu 1, G.Ch. Satyanarayana 2 ABSTRACT Generally the plinth level of the building is raised from the ground level, cause in an increase in surcharge pressure at the foundation level results in an increase in bearing capacity. Designers are not able to take into account the above increase, since quantified procedure to consider the above effect is not established. The bearing capacity at foundation level may increase due to the surcharge effect and with the shearing resistance on the failure plane developed in the fresh fill. To quantify the above effect, field experimental study is conducted and is presented in this paper. The field study is conducted by varying the type, density and thickness of soil used as fill and with different combinations to quantify the effect of each parameter on bearing capacity. The experimental results are compared with the computed values using the IS: method. It is observed that with the increase in thickness of the fill, the bearing capacity of the footing increases, but the rate of increase in bearing capacity per unit thickness of the fill is decreasing indicating that the failure plane is not entered into the fresh fill above the ground level but it may be extended along the interface of the ground and the fresh fill. It also observed that the murrum soil fills, results in more increase in bearing capacity than sand. Coarse sand fill has more increase in bearing capacity than the fine sand fills, indicating that the increase in shearing resistance at the interface of the fill and the soil causes an increase in bearing resistance developed at the interface of the fill and the soil. Keywords: Bearing Capacity, Fresh Fill, Field test, Murrum fill, Sand Fill 1 K. Ramu, Professor, Department of Civil Engineering, University College of Engineering, JNTUniversity, Kakinada, Kakinada 5333, India, kramujntu@gmail.com 2 G. Ch, Satyanarayana, Director, SN Associates, Kakinada, satyanarayana.gatti@gmail.com

2 K. Ramu & G. Ch. Satyanarayana

3 5 th IGC 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India EFFECT OF FRESH FILL ON BEARING CAPACITY OF FOOTING K. Ramu, Professor, UCEK, JNT University, Kakinada, Kakinada 533 3, India, kramujntu@gmail.com G. Ch, Satyanarayana, Director, SN Associates, Kakinada, satyanarayana.gatti@gmail.com ABSTRACT: Generally the plinth level of the building is raised from the ground level, cause in an increase in surcharge pressure results in an increase in bearing capacity. Designers are not able to take into account the above increase, since quantified procedure to consider the above effect is not established. To quantify the above effect, field experimental study is conducted and is presented in this paper. It is observed that with the increase in thickness of the fill, the bearing capacity of the footing increases, but the rate of increase in bearing capacity per unit thickness of the fill is decreasing. INTRODUCTION Generally foundations are laid at a certain depth from the ground level, where the bearing resistance is enough to support the structural load. The bearing capacity of the ground depends on shape, width and depth of the foundation, properties of the soil deposits and its variation along the depth. First attempt made [1] to calculate the bearing capacity of a strong layer overlying a weak layer. They assumed that the upper layer served principally to spread the footing load to a larger area on the lower layer of the surface, thus reducing the intensity. Model test results [2] are reported and compared with some approximate solutions given by various authors. Significant research work was published on the ultimate bearing capacity of foundations [3,4]. Punching mode of failure [5,6] are presented to estimate the bearing capacity of a footing on a sand layer overlying clay and compared the results with those of model tests on circular and strip footings and some field observations of foundation failures. Bearing Capacity of layered soils [7,8,9,1,11,12,13,14,15,16,&17] are reported. In homogeneous soils, generally the bearing capacity at foundation increases with depth of foundation. In case of stiff soil overlying soft soil, the bearing capacity may decrease instead of increase with increasing the depth of foundation. Since with the increasing depth of foundation, intensity of pressure on the soft soil increases, results in decrease in bearing capacity. In such cases the depth of foundation may be kept constant and the plinth level of the building may be raised, with fill from the ground level to increase the surcharge pressure at the foundation level resulting in increasing the bearing capacity. But, the designers are not able to consider the increase in bearing capacity with the fill above the ground level, since quantified procedure is not established. Designers have a doubt in their mind about the effect of fresh fill on bearing capacity at foundation level, whether they have to consider the surcharge effect only or the failure plane entered into the fresh fill and extended up to the ground surface or the failure plane extending along the interface of the ground and the fill. Hence, a field experimental study is attempted in this paper to study the effect of the fill above the ground level on bearing capacity of foundation. EXPERIMENTAL STUDY Materials Used Fine sand procured from Ramanakkapeta, coarse sand from Rajahmundry, Murrum from Samalkot and quarry dust from Yeleswaram. The density and strength parameters of the fill materials and existing foundation soil are presented in the table 1.

4 K. Ramu & G. Ch. Satyanarayana Table 1: Properties of the Foundation Soil and Fill Soil Soil Paramet ers Bulk Density (kn/m 3 ) Moisture Content (%) Dry Density (kn/m 3 ) Cohesio n(kpa) Angle of Internal friction, (Deg.) Foun datio n Soil Fine sand murru m Fill Soil Coars e sand Stone dust Preparation of Model Foundation Two chambers of size 1.85 x 1.85 x 1.1 m are constructed with brick masonry to simulate the basement of the peripheral compound wall of the building and to confine the fill soil in position and to avoid the erosion or sliding. The loading arrangement of plate load test is shown in Fig. 1. A Circular extension column made with M2 grade concrete is placed on the 3 cm circular plate concentrically with the help of plumb and bob and leveled by the spirit level. It is also ensured that the centre of the reaction frame coincide the centre of the test plate. The filling material is laid in layers and compacted to a desired height and density without disturbing the plate and loading column. The loading is applied through hydraulic jack placed concentrically over the column through reaction from a crossbeam erected as a platform. as shown in Fig. 1. The two supports for the reference datum rod is placed over the constructed firm brick masonry walls and fixed with three dial gauges of 5mm travel and with a sensitivity of.1 mm, placed at 12 apart on the plate to record settlement. Fig. 1 Test Setup of Plate Load Test A seating load of about 7 kn/m 2 is first applied and released after some time. The load is applied to the plate in cumulative equal increments of onefifth of the estimated ultimate bearing capacity. The load is applied with a hydraulic jack and is measured with a pressure gauge, attached to the pumping unit kept over the pit, away from the testing plate through extended pressure pipes. The load is also applied, without impact, fluctuations or eccentricity. Settlements are observed in each test for each increment of loading after an interval of 1, 2.25, 4, 6, 9, 16 and 25 minutes and thereafter at hourly intervals to the nearest.1mm. As the soil in the foundation, which is under testing is sand, the load increment is kept for not less than one hour or up to a time till the rate of settlement gets appreciably reduced to a value of.2mm/minute. Then the next increment of load is applied and the observations are repeated. The test is continued until a total settlement of 25 mm or the settlement at which the soil fails, whichever is earlier, is obtained. After the load is released, the elastic rebound of the soil is recorded. The load settlement curve is plotted as the load on X-axis and settlement on the Y-axis for each test. From these load settlement curves, the zero correction, which is given by the intersection of the early straight lines or nearly straight line part of the

5 5 th IGC 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India curve with zero deadline is determined and subtracted from the settlement readings to allow for the perfect seating of the bearing plate and other causes. From the corrected load settlement curves, the safe bearing capacities are arriving by the intersection of the early tangent and the tangent drawn at the yield point. RESULTS AND DISCUSSION Bearing Capacity of Footing with Fine Sand Fill at the Ground Surface The stress settlement response curves of the footing on the surface of the ground with cm, 3 cm, 6 cm and 9 cm thick fine sand fill on the ground are presented on the Fig. 2. As expected the bearing capacity of the footing increases with increase in thickness of the fill. From the experimental results, the ultimate bearing capacities of the footing are 221, 285, 381 and 47 kn/m 2 respectively, for cm, 3 cm, 6 cm and 9 cm thick fill on the ground surface, but for the same data, these values are 187, 284, 38 and 476 kn/m 2 respectively. The variation of ultimate bearing capacities of the footing from experimental values with those obtained from the IS code method are -15.4%,.7 %, 1.6 % and % respectively for cm, 3 cm, 6 cm and 9 cm thick fill on the ground surface. The ultimate bearing capacity increases with increasing the thickness of the fill, but the rate of increase in bearing capacity with the increase in thickness of the fill decreases. These phenomena observed may be due complete failure surface may not be developed with the increasing the thickness of the fill. Bearing Capacity of Footing with Murrum Fill at the Ground Surface Variations of settlement with an increase in applied stress on the footing, laid on the ground surface for a murrum fill of thicknesses cm, 3 cm, 6 cm and 9 cm are presented in the Fig. 3. From the experimental results, the ultimate bearing capacities of the footing are 221, 331, 444 and 565 kn/m 2 respectively, for cm, 3 cm, 6 cm and 9 cm thick fill on the ground surface, but for the same data, these values obtained from the IS Code method are 187, 612, 139 and 1465 kn/m 2 respectively. As expected the bearing capacity of the footing increases with increase in thickness of the fill. Settlement (mm) > Surface Foundation Sf +.6 Sand Stress (kn/m 2 ) SF+.3 m fine sand SF +.9 m FS Fig. 2 Stress Settlement Responses of the Footing with the Variation of Thickness of the Fine Sand Fill above the Ground Surface. The variation in ultimate bearing capacities of the footing from experimental values with those obtained from the IS code method are -15.4%, %, 134. % and % respectively for cm, 3 cm, 6 cm and 9 cm thick fill on the ground surface. It is observed that the ultimate bearing capacity increases with increasing the thickness of the fill but the rate of increase in bearing capacity with the increase in thickness of the fill decreases. Also the difference in bearing capacity values from the experiment with those computed increase with increase in thickness of the fill. These phenomena observed may be due to failure surface may not be developed in the fill soil.

6 K. Ramu & G. Ch. Satyanarayana Bearing Capacity of Footing with Different Fills at the Ground Surface Fig. 4 shows the variations of settlement with stress on the footing laid on the ground surface for the fill of thickness of 9 cm for fine sand, coarse sand, murrum and stone dust respectively. From the experimental results, the ultimate bearing capacities of the footing are 47.5, 52, 565 and kn/m 2 for fine sand, coarse sand, murrum and stone dust fills respectively. But for the same data, these values obtained from the IS Code method are , 997.8, and kn/m 2 respectively. Settlement (mm) > Stress (kn/m 2 ) Stress (kn/m 2 ) SF+.9 thick fine sand Sf +.9 Quarry Dust Series2 SF +.9 m murrum Settlement (mm) > Surface Foundation Sf +.6 murrum SF+.3 m murum SF +.9 m murrum Fig. 3 Stress Settlement Responses of the Footing with the Variation of Thickness of the Murrum Fill above the Ground Surface Fig. 4 Stress Settlement Responses of the Footing with the Variation of Type of Fill above the Ground Surface The difference in ultimate bearing capacity values between the experimental and computed are %, 91.8%, % and % respectively for fine sand, coarse sand, murrum and stone dust fill of thickness 9 cm above the ground. Bearing Capacity of Footing with Thickness of the Fills at the Ground Surface The variations of ultimate bearing capacities of the footing for the fills on the surface of the ground with the thickness are presented in Fig. 5 and 6 respectively, for fine sand and murrum fill soils.

7 5 th IGC 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India Ultimate Bearing Capacity (knm 2 ) fine sand Depth of Foundation (m) fine sand computed Fig. 5 Variation of ultimate bearing capacity of the footing with thickness of the fill above the ground surface (Fine sand fill) It is observed that the difference in bearing capacity values with those computed for fine sand fill (Fig. 5) at the surface of the ground are less compared for the murrum fill (Fig. 6). The above phenomena may be due to the two different materials in case of murrum fill, but same type material in case of fine sand. The ground consists of fine sand, hence in case of fine sand fill on the ground surface, the two materials are having nearly same properties, hence the most of the interfacial shear resistance may be developed along the failure surface and hence, the difference in ultimate bearing capacity is very less compared to the other fills. But in case of murrum fill, only part of the interface friction may be developed along the interface, hence, the difference in bearing capacity is very high. The very large difference in bearing capacities between the test results and those computed indicates the failure surface may not be entered into the fresh fill. The increase in bearing capacity with the increase in thickness of fill may be due to the increase in surcharge pressure at the foundation level and with the shearing resistance at the interface of the fill and the ground soil. Ultimate Bearing Capacity (knm 2 ) murrum Depth of Foundation (m) murrum computed Fig. 6 Variation of ultimate bearing capacity of the footing with thickness of the fill above the ground surface (murrum fill) CONCLUSIONS From the field test results, the following conclusions can be drawn. The ultimate bearing capacity of the foundation can be increased by fill at the top surface, but the increase in bearing capacity is about 3% to 8% of the computed values depending on the type of the fill and the type of the soil at the ground surface. The ultimate bearing capacity is increased with the increase in thickness of the fill, but the rate of increase in bearing capacity with the thickness of the fill decreases, indicates that, the failure envelope is not extended into the fresh fill, may be extended along the interface of the ground and the surface of the fill.

8 K. Ramu & G. Ch. Satyanarayana REFERENCES 1. Terzaghi, K. and Peck R.B., (1948) Soil Mechanics in Engineering Practice, Wiley, New York. 2. Kenny, M.J. and Andrawes, K.Z., (1997). The Bearing Capacity of Footings on a Sand Overlying Soft Clay, Geotechnique, Vol. 47, No. 2, Meyerhof, G.G. (1963). Some recent research on the bearing Capacity of foundations, Canadian Geotechnical Journal, 1 (1), pp Meyerhof, G.G. (1965). Shallow foundations, Journal of SMFD, ASCE, Vol. 91, SM2, pp Brown, J.D. & Meyerhof, G.G. (1969). Experimental study of bearing capacity in layered clays, 7 th International conference on Soil mechanics and Foundation Engineering, Mexico, Vol. 2, pp Meyerhof, G.G. (1974). Ultimate bearing capacity of footings on sand layer overlying clay, Canadian Geotechnical Journal. Vol. 11, pp Button, S.J. (1953). The bearing capacity of footings on a two- layer cohesive subsoil, Proceedings of the 3 rd International Conference, S.M.F.E., Zurich, Vol.1, pp Prakash, S. & Saran, S. (1971). Bearing capacity of eccentrically loaded footings, J of SMFE div, ASCE, No. SM1, pp Jacobsen, M., Christensen, K.V. & Sorsen, C.S. (1977). Penetration of thin sand layers, Vagoch Vatten Byggares, Riksforbund, Stockholm, Sweden, pp Khing, K.H., Das, B.M., Puri, V.K., Cook, E.E. & Yen, S.C. (1993). The bearing capacity of a strip foundation on geogrid- reinforced sand, Geotextiles and Geomembranes, 12, pp Michalowski, R.L. & Shi (1995). Bearing capacity of footings over two-layered foundation soils, Journal of Geotechnical Engineering, pp Burd, H.J. & Frydman, S. (1995). Bearing capacity of plane strain footings on layered soils, REPORT No.OUEL:272/95, University of Oxford, U.K 13. Adams, M.T. and Collin, J.G. (1997). Large Model Spread Footing Load Tests on Geosynthetic Reinforced Soil Foundations, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol.123, No.1, January, pp Michalowski, R.L. (22). Collapse loads over two layer clay foundation soils, Soils and foundations, Japanese geotechnical society, Vol. 42, No.1, Michalowski, R.L. (24). Limit Loads on Reinforced Foundation Soils, Journal of geotechnical and geoenvironmental Engineering, ASCE, pp Al-Shenawy, A.O. & Al-Kami, A.A. (25). Derivation of bearing capacity equation for a two layer system of weak clay layer overlaid by dense sand layer, Pertanika J. Sci. & Technol. 13(2): pp Zhu, M. & Michalowski, R. L. Bearing capacity of rectangular footings on two- layer clay, ICSMGE, Osaka.