Behaviour of Square Footing Resting on Reinforced Sand Subjected to Static Load

IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 02 | July 2016 ISSN (online): 2349-6010

Behavior of Square Footing Resting on Reinforced Sand Subjected to Static Load Hanamant Chavan UG Student Department of Civil Engineering Dr. P.G. Halakatti College of Engineering & Technology, Vijayapur, Karnataka, India.

Kumar Nivaragi UG Student Department of Civil Engineering Dr. P.G. Halakatti College of Engineering & Technology, Vijayapur, Karnataka, India.

Ravirajgouda Patil UG Student Department of Civil Engineering Dr. P.G. Halakatti College of Engineering & Technology, Vijayapur, Karnataka, India.

Sagar Hipparagi UG Student Department of Civil Engineering Dr. P.G. Halakatti College of Engineering & Technology, Vijayapur, Karnataka, India.

Basavaraj Hotti Assistant Professor Department of Civil Engineering Dr. P.G. Halakatti College of Engineering & Technology, Vijayapur, Karnataka, India.

Abstract A series of laboratory model test has been carried out to investigate the bearing capacity of the square footing resting on reinforced sand bed. The geo-grid as a reinforcement material has been used. In the present study an attempt has been made to study the bearing capacity of square footing on sand reinforced bed. The effect of different parameter like the depth of the upper most layer of reinforcement from the base of the model footing (u), for different densities of the sand (γ = 1.60, 1.70, and 1.80gm/cc) the test has been carried out. Three numbers of layers, has been fixed. The test results showed that the beneficial use of geo-grid reinforcement in terms of increasing in the bearing capacity and minimizing the settlement, at an optimum depth of reinforcement, however for the higher density of the soil gives maximum bearing capacity. Therefore, for effective utilization of geo-grid reinforcement, the optimum depth should be (u = 0.40B) which is found to be good agreement with the past researchers, and the foundation soil should be in higher density.

Keywords: Soil reinforcement, Model tests, Sand, Shallow foundations, Geogrid, Static loading _______________________________________________________________________________________________________ I.

INTRODUCTION

During the past three decades, results of several studies have been published that relate to the evaluation of the ultimate and allowable bearing capacities of shallow foundations supported by sand reinforced with multi-layered geo-grid. The technique of reinforcing soil, which in its present form French architect Henri Vidal in 1965, is one of the more recent and fast-growing techniques of soil improvement in the field of geotechnical engineering. The concept of reinforced soil is based on the existence of tensile strength of reinforcement and soil reinforcement interaction due to frictional, interlocking and adhesion properties. The early structures built using this technique were earth retaining structures, but it was eventually realized that the technique is also useful in foundation problems. The method of reinforcing soil with layers of individual reinforcements placed horizontally as described here is best attempted in conjunction with fills that are required to support shallow foundations such as footings.

In the cases of poor to marginal ground conditions, geosynthetic reinforcement is proved to be a cost-effective solution and in some cases geosynthetics open up the possibility of constructing shallow foundations in lieu of expensive deep foundations.

Among the range of geosynthetics available on the market, geo-grids are the most preferred type of geosynthetic materials for reinforcing the foundation beds. The beneficial effect of a geosynthetic inclusion is largely dependent on the form in which it is used as reinforcement.

These synthetic materials and products generally have a long life, but are costly and may not create environmental problems in the future. All of these studies have been conducted for surface foundation conditions II. BACKGROUND Terzaghi’s(1943) theory of bearing capacity is widely used in practice. Khing, et al., (1992) ) carriedout laboratory-model tests for determining the bearing capacity of a strip foundation Adams & Collin (1997)they concluded the performance of a 0.61 m footing on one layer of geo-grid at a depth (z) below the bottom of the footing of 225 mm, which corresponds to a z/B ratio of

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Behavior of Square Footing Resting on Reinforced Sand Subjected to Static Load (IJIRST/ Volume 3 / Issue 02/ 052)

0.375 performed best in terms of ultimate bearing capacity, Patra, et al., From the Laboratory model test results for the ultimate bearing capacity of a strip foundation supported by multi-layered geo-grid reinforced sand are presented, Hatafe et al., (2000) A series of laboratory model test has been carried out to investigate the bearing capacity of shallow footings on reinforced sand.

Madhavi latha & Amit (2009a) Results show that the effective depth of reinforcement is twice (2B) the width of the footing and optimum spacing of geosynthetic layers is half the width of the footing III. MATERIALS AND EXPERIMENTAL STUDY In the present study Bhima river sand with symbolic representation SP is filled in the testing tank of size 600mm x 600mm x 600mm using raining technique for densities 1.60gm/cc, 1.70gm/cc 1.80gm/cc with geo-grid (SG-200) as reinforcement placed at U/B ratios 0.2, 0.4 and 0.6. is used. A steel plate of 100 mm x 100 mm x 10 mm is used as footing.

All model tests were conducted using the setup shown in Fig.1. The vertical load was applied on the model footing using screw jack, which provides vertical displacement. Proving ring and two dial gauges placed diagonally on the footing were used for measuring load and settlement respectively.

Fig. 1: Line diagram of Experimental Setup

The U/B ratio corresponding to depth first reinforcement was given in table 1.

Table – 1 U/B ratio corresponding first reinforcement depth U/B ratio First reinforcement depth(U) 0.2 2cm 0.4 4cm 0.6 6cm

IV. RESULTS AND DISCUSSION Static Loading on Un-Reinforced Sand: The Load-Deformation Curve for the Density 1.60 Gm/Cc, 1.70 Gm/Cc and 1.80 Gm/Cc are Plotted and Shown in the Fig. 2.

From the Fig.2 it is observed that (i)the increase in the density of the foundation bed leads to increase in the bearing capacity and decrease in the deformation, (ii) the load carrying capacity of the reinforced sand bed of density 1.60 gm/cc, 1.70 gm/cc and 1.80 gm/cc are respectively 36, 96, 108 kN/m2.

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Behavior of Square Footing Resting on Reinforced Sand Subjected to Static Load (IJIRST/ Volume 3 / Issue 02/ 052)

Fig. 2: Pressure - Settlement curves for unreinforced sand for different densities

Effect of U/B on Ultimate Bearing Capacity of the Square Footing on Reinforced Sand Bed for Static Loading: The pressure-settlement curves for the densities 1.80 gm/cc for U/B = 0.2, 0.4 and 0.6, for geo-grid SG-200 are shown in Fig.3.

The value of ultimate bearing capacity of the footing of reinforced sand for different U/B ratios and densities is exclusively given in Table 3.

Table – 2 Ultimate bearing capacity of reinforced sand for different densities and U/B ratio.

Ultimate bearing capacity (kN/m2) γ=1.60 γ =1.70 gm/cc γ =1.80 gm/cc gm/cc U/B = 0.2 145 162 178 U/B = 0.4 156 171 180 U/B = 0.6 112 126 150

It can be seen that, the pressure v/s footing settlement response of reinforced sand bed is far better than the un-reinforced case.

The footing resting on the soil-reinforcement composite will carry more loads. This shows that strength improvement is totally depends on the position of the reinforcement within the sand bed. The response of the reinforced sand bed is seen to improve as the depth ratio U/B= 0.4 and thereafter shows a decreasing trend.

For γ = 1.80gm/cc, at U/B = 0.4, there is a maximum ultimate bearing capacity of 180 kN/m2 is observed when compared with densities 1.60gm/cc and 1.70gm/cc the values are 156 kN/m2 and 171 kN/m2 respectively.

Fig. 3: Pressure - Settlement curves for 1.80gm/cc density at different U/B ratios.

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Behavior of Square Footing Resting on Reinforced Sand Subjected to Static Load (IJIRST/ Volume 3 / Issue 02/ 052)

Variation of Density with Constant Depth Ratio (U/B): The pressure-settlement curves for U/B = 0.2, 0.4 and 0.6 and densities 1.60 gm/cc, 1.70 gm/cc and 1.80 gm/cc using geo-grid 200 are shown in Fig. 4 – 6. From figures obtained it is found that The interfacial frictional resistance increases with increases in soil density. Therefore, with the increase in density of the soil, the frictional resistance between the geo-grid and the sand increases, thereby increasing the resistance to downward penetration of sand below the geo-grid and hence a higher improvement in overall strength. This is due to the frictional resistance at the interface of the sand and reinforcement which would have prevented the soil mass from shearing under vertical applied load (8).

The maximum value of ultimate bearing capacity obtained for U/B ratio 0.2, 0.4 and 0.6 for density 1.80gm/cc, is 178 kN/m2, 180kN/m2 and 150kN/m2 respectively. The ultimate load carrying capacity of reinforced sand bed increases up to U/B ratio 0.4 and afterworlds decreases

Fig. 4: Pressure - settlement curves for U/B ratio 0.2 at different densities

Fig. 5: Pressure - settlement curves for U/B Ratio 0.4 at different densities

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Behavior of Square Footing Resting on Reinforced Sand Subjected to Static Load (IJIRST/ Volume 3 / Issue 02/ 052)

Fig. 6: Pressure - settlement curves for U/B Ratio 0.6 at different densities

V. STRENGTH IMPROVEMENT RATIO In order to get a quantitative assessment of the extent of soil improvement, the improvement due to the provision of geo-grid reinforcement can be shown in non–dimensional strength improvement ratio which is define as� the ratio of the ultimate bearing capacity of the reinforced sand to the un-reinforced sand which is same as bearing capacity ratio� .

Ultimate bearing capacity of reinforced sand SIR = Ultimate bearing capacity of un − reinforced sand Calculation of the strength improvement ratio for đ?›„ = 1.7 gm/cc, u/B = 0.4and for SG 200.

171 đ?‘†đ?‘Ąđ?‘&#x;đ?‘’đ?‘›đ?‘”đ?‘Ąâ„Ž đ?‘–đ?‘šđ?‘?đ?‘&#x;đ?‘œđ?‘Łđ?‘’đ?‘šđ?‘’đ?‘›đ?‘Ą đ?‘&#x;đ?‘Žđ?‘Ąđ?‘–đ?‘œ = = 1.781 96 The rest of the calculated values is attributed in the Table 4.2.

Fig. 4.10 – 4.12 depicts the curve of strength improvement ratio v/s depth ratio (U/B) from which it is observed that there has been maximum increase in ultimate bearing capacity compared to the non-reinforced.

The peak of the curve is obtained at U/B = 0.4 because of the reason that for U/B<0.4. the overburden pressure was not sufficient to develop frictional resistance at the interface of the reinforcement and the sand. With the increase of U/B, bearing capacity reaches its maximum value at optimum value of U/B = 0.4 as the overburden pressure was just sufficient to develop the maximum frictional resistance. Thereafter the value of strength improvement ratio gradually decrease because of the reinforcement was depressed or placed at the deeper region.

The value of the optimum depth of the reinforcement obtained in the present study is in fairly good agreement with the values reported in the literature Yetimoglu et al., (1994) have carried out experiments on geo-grid reinforced sand using model rectangular footing of 127 mm x 101.5 mm size. Maximum beneficial effect was obtained in this study by placing the reinforcement at a depth of about 0.3B. Comparable values of optimum embedment depth (0.315B & 0.3125B respectively) have been reported by Hataf and Razavi (2000) [tests on model square footing (B = 140 mm) resting on waste tire material reinforced sand beds] and Gosh et al., (2005) [model square footing (B = 800 mm) underlain by pond ash reinforced with jute geotextile].

The studies reported in the literature as well as the present study clearly demonstrate that the optimum location of a geo-grid layer of reinforcement is about 0.4B. TABLE 4.2 Strength improvement ratio for different density and u/B ratio.

On the other hand, for the Îł = 1.6 gm/cc, u/B = 0.4 and for geo-grid SG 200 there is a maximum value when it is compared with the depth ratio of (U/B = 0.4and 0.6). The same trend is observed for the rest of two densities (i.e., = 1.7 and 1.8gm/cc).

Type of reinforcement U/B = 0.2 U/B = 0.4 U/B = 0.6

Strength improvement ratio Density = 1.6 Density = 1.7 Density = 1.8 gm/cc gm/cc gm/cc SG 200 SG 200 SG 200 4.027 1.687 1.648 4.333 1.781 1.667 3.111 1.312 1.388

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Behavior of Square Footing Resting on Reinforced Sand Subjected to Static Load (IJIRST/ Volume 3 / Issue 02/ 052)

Fig. 4.10: Variation of strength improvement ratio with depth ratio for density 1.6 gm/cc for geo-grid SG 200

Fig. 4.11: Variation of strength improvement ratio with depth ratio for density 1.7 gm/cc for geo-grid SG200

Fig. 4.12: Variation of strength improvement ratio with depth ratio for density 1.8 gm/cc for geo-grid SG 20

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Behavior of Square Footing Resting on Reinforced Sand Subjected to Static Load (IJIRST/ Volume 3 / Issue 02/ 052)

Settlement Reduction Factor The improvement due to the inclusion of a geo-grid reinforcement layer in sand, in terms of reduction in footing settlement, can be known through the parameter settlement reduction factor which is defined as: Settlement reduction factor = So â&#x2C6;&#x2019; Sr /So Where in, So is the settlement of the non-reinforced sand bed at a given pressure and Sr is the settlement of the sand bed strengthened by geo-grid reinforcement at the same pressure.

Calculation of settlement reduction factor for đ?&#x203A;&#x201E; = 1.6 gm/cc, U/B = 0.4 for the geo-grid SG 200.

17.8â&#x2C6;&#x2019;24.6 đ?&#x2018;&#x2020;đ?&#x2018;&#x2019;đ?&#x2018;Ąđ?&#x2018;Ąđ?&#x2018;&#x2122;đ?&#x2018;&#x2019;đ?&#x2018;&#x161;đ?&#x2018;&#x2019;đ?&#x2018;&#x203A;đ?&#x2018;Ą đ?&#x2018;&#x;đ?&#x2018;&#x2019;đ?&#x2018;&#x2018;đ?&#x2018;˘đ?&#x2018;?đ?&#x2018;Ąđ?&#x2018;&#x2013;đ?&#x2018;&#x153;đ?&#x2018;&#x203A; đ?&#x2018;&#x201C;đ?&#x2018;&#x17D;đ?&#x2018;?đ?&#x2018;Ąđ?&#x2018;&#x153;đ?&#x2018;&#x; = = 16.417 17.8 The rest of the calculated values is attributed in the Table 4.3.

Table - 4.3 Settlement reduction factor for different density and U/B ratio.

Settlement reduction factor Density = 1.6 gm/cc Density = 1.7 gm/cc Density = 1.8 gm/cc Type of reinforcement SG 200 SG 200 SG 200 Without reinforcement 36 96 108 U/B = 0.2 16.659 21.147 24.216 U/B = 0.4 16.417 21.103 24.115 U/B = 0.6 16.519 21.138 24.2

Due to the inclusion of geo-grid at a depth of 0.4B the bearing capacity increased significantly which is quantified by strength improvement ratio and reduces settlement.

This settlement reduction factor is same as the parameter used by P.vinod et al,. (2009) in their study on braided coir rope reinforced sand bed. It is observed that both strength improvement ratio and settlement reduction factor are heavily dependent on embedment depth.

Performance improvement of the sand increases almost linearly from a reinforcement embedment depth of about 0.2B up to a depth of about 0.4B. It is also seen from Fig. 4.13 - 4.15 that increase in embedment depth beyond 0.4B leads to reduction in strength improvement ratio as well as settlement reduction factor, even though at a smaller rate.

It indicates that embedment depth of about 0.4B is the optimum value from both settlement and shear failure considerations.

Fig. 4.13: Settlement reduction factor with strength improvement ratio for different depth ratio for geo-grid SG 200.

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Behavior of Square Footing Resting on Reinforced Sand Subjected to Static Load (IJIRST/ Volume 3 / Issue 02/ 052)

Fig. 4.14: Settlement reduction factor with strength improvement ratio for different depth ratio for geo-grid SG 200.

Fig. 4.15: Settlement reduction factor with strength improvement ratio for different depth ratio for geo-grid SG 200

VI. CONCLUDING REMARKS Laboratory model test results for a square footing supported by geo-grid reinforced sand of different densities have been presented. The ultimate bearing capacity and settlement obtained from these tests have been presented. Based on the present tests, the following conclusion is drawn.

Concluding Remarks 1) Provision of the geo-grid reinforcement layers improves the load carrying capacity of the model footing.

2) Effective depth of the zone reinforcement below a square footing is twice the width of the footing (2B).

3) In the case of the geo-grid reinforced both ultimate bearing capacity and bearing capacity at any settlement of square footing are maximum at U/B = 0.4.

4) From the overall performance point of the view of the model footing (i.e., both strength and settlement aspects), the optimum location of the geo-grid reinforcement is about 0.4B below the base of the footing, within the effective reinforcement zone.

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Behavior of Square Footing Resting on Reinforced Sand Subjected to Static Load (IJIRST/ Volume 3 / Issue 02/ 052)

5) Strength improvement ratio also increases up to a depth ratio U/B = 0.4, thereafter it decreases with further increases in depth ratio. This is due to the fact the magnitude of the mobilized frictional resistance at the interface of sand and the reinforcement because of the smaller or the lower overburden pressure over the reinforcement.

6) Use of the geo-grid reinforcement leads to better performance from the point of view of strength improvement as well as settlement reduction.

7) The experimental study results suggest the possibility of developing a predictive model for strength improvement due to use of geo-grid reinforcement.

8) As the density of the soil increase the load carrying capacity increases. In order to get the effective utilization of geo-grid reinforcement, the soil should have higher density therefore the stiffness between the soil and geo-grid reinforcement increases.

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Vinod p, Ajithabhaskar and Sreehari, “Behaviour of cyclic square model footing on loose sand reinforced with braided coir rope”, Geotextiles and Geomembranes, Vol. 27, No. 5, 2009, pp. 464-474 A Mostafa and A.K. Nazir, “Behavior of repeatedly loaded rectangular footings resting on reinforced sand”, Geotechnical Journal, Vol. 23, 2010, pp. 435– 440.

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