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Interference of adjoining rectangular footings on reinforced sand
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Interference of adjoining rectangular footings on reinforced sand

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  • 1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME TECHNOLOGY (IJCIET)ISSN 0976 – 6308 (Print)ISSN 0976 – 6316(Online)Volume 3, Issue 2, July- December (2012), pp. 447-454 IJCIET© IAEME: www.iaeme.com/ijciet.aspJournal Impact Factor (2012): 3.1861 (Calculated by GISI)www.jifactor.com © IAEME INTERFERENCE OF ADJOINING RECTANGULAR FOOTINGS ON REINFORCED SAND Dr. S. S. Pusadkar1, Sachin S. Saraf2 1 (Associate Professor, Department of Civil Engineering, Government College of Engineering, Amravati-444 604, India, pusadkar.sunil@gcoea.ac.in) 2 (M. Tech (Civil-Geotech) Scholar, Government College of Engineering, Amravati-444 604, India, sachinces2007@rediffmail.com) ABSTRACT The influence of two adjoining rectangular footings on bearing capacity and settlement resting on reinforced sand is discussed in the paper. The parameters include sizes and spacing’s of the footing. The model tests were conducted for simulating the various conditions of footing and showing the effect on the bearing capacity and settlement. It has been observed that providing continuous geogrid reinforcement layer in the foundation soil under the closely spaced rectangular footings improved the bearing capacity. Keywords: Bearing capacity, Geogrid, Interference effect, Rectangular footing. 1. INTRODUCTION Many townships are developed and lot many are proposed with higher construction density. As a common practice several storied buildings are constructed in a series keeping very small spacing between adjacent corner footings. Due to heavy loads and the non availability of good construction sites, engineers are often required to place footings at close spacing’s. Therefore, the footings in the field generally interfere with each other to some extent and are rarely isolated. The general scenario is to design the footings as an isolated footing. The interfering as well as spacing effect is not considered while designing the footings. 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. Therefore in the preset study, the influence of two adjoining rectangular footing resting on reinforced sand on bearing capacity and settlement was carried out. The effect was studied for various sizes and spacing’s of the footing on reinforced sand. 447
  • 2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME2. LITERATURE REVIEW The study on ultimate bearing capacity of two interfering strip footings using themethod of stress characteristics shows that the efficiency factor ξγ decreases continuouslywith an increase in spacing [1]. The ultimate bearing capacity of number of strip footingsusing the lower bound limit analysis in combination with finite elements shows that thefailure load for a footing in the group becomes always greater than that of a single isolatedfooting [2]. The effects of multiple-footing configurations in sand on bearing capacity usingfield plate load tests and finite element analyses shows that the load responses of multiplefootings are similar to those of the single footing at distances greater than three times thefooting width [3]. The interference of surface model footings resting on sand shows that theinterference between footings was observed to cause an increase in bearing capacity anddecrease in settlement with reduction in spacing [4]. The interference effect on the ultimatebearing capacity of two closely spaced strip footings placed on the surface of dry sand byusing small scale model tests shows that an interference of footings leads to a significantincrease in their bearing capacity [5]. The numerical examination of bearing capacity ratio forrough square footings located at the surface of a homogeneous sandy soil reinforced with ageogrid was shows the bearing capacity of interfering footing increases with the use ofgeogrid layers depending on the distance between two footings [6]. The effect of spacingbetween the footings, size of reinforcement and continuous and discontinuous reinforcementlayers on bearing capacity and tilt of closely spaced footings was investigated by performingtotal 74 tests. It shows considerable improvement in bearing capacity, settlement, and tilt ofadjacent strip footings by providing continuous reinforcement layers in the foundation soilunder the closely spaced strip footings [7]. The interference effect of two nearby stripfootings on reinforced sand shows that the bearing capacity of single footing on thereinforced soil decreases with increase in D/B [8]. The literatures, shows work on theinterfering effects of different sizes footings on unreinforced and reinforced sand. However,the interfering effect of rectangular footing on bearing capacity and settlement is notavailable for reinforced sand. This revels that the influence of two adjoining footings onbearing capacity and settlement for various sizes and spacing of the footings on reinforcedsand is need of the future. In order to evaluate the effects of two adjacent footings onreinforced sand, laboratory experiments to simulate the various conditions of footing wasperformed. In each case, different sizes and spacing of footing were applied for the purposesof comparison among all of the results for development of knowledge base in this regards.3. MATERIAL The material used for the study is as follows.3.1 Foundation Material For the model tests, cohesionless dry sand was used as the foundation material. Thestudy was carried out on Kanhan Sand as foundation material. This sand is available inNagpur region of Vidharabha, Maharashtra. The test sand has angular shape, uniform yellowcolour with small proportion of flint stone of black colour. The particle size of sand decidedfor the test was passing through IS sieving 2 mm and retaining on 450 micron IS sieve. Theproperties of sand used are as shown in Table 1. 448
  • 3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME July Table 1 Properties of the Sand Used 1: Properties Value Specific Gravity 2.67 Bulk Density (KN/m3) 14.91 Angle of Internal Friction 28° Coefficient of Uniformity Cu 2.29 Coefficient of Curvature Cc 1.09 Effective Size D10 0.513.2 Model Footing Rectangular model footings of dimensions 3cm x 6cm, 4cm x 8cm and 5cm x 10cmwere fabricated by using cast iron material as shown in Fi 1. Every footing has a little t Fig.groove at the center to facilitate the application of load. The footings were provided with thetwo flanges on two sides of footings to measure the settlement of footing under the action of measureload with the help of dial gauges. Figure 1: Model Footing3.3 Geogrid Commercially available continuous biaxial geogrid was used for reinforcing thesand bed. The size of biaxial geogrid reinforcement used was five times the size of thefooting as shown in Fig. 2. The biaxial geogrid reinforcement was placed at the location ofthe desired layer of reinforcement i.e. D/2 or B/2 from bottom of footing. The top surface ofthe sand will be leveled and the biaxial geogrid reinforcement will be placed. eled 449
  • 4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME July Figure 2: Biaxial Geogrid4. EXPERIMENTAL SETUP The experimental setup used for studying the performance of adjacent footing onreinforced sand is shown in Fig. 3. The assembly for the model plate load test setup consist of inforceda tank of size 0.5m x 0.5m x 0.6m. A loading frame for applying the load to the models isassembled over the tank. The load was applied with manually controlled hydraulic jack andmeasured with the help of proving ring. Dial gauges were placed on each flanges of each Dialfooting to measure the settlement. HYDRAULIC JACK PROVING RING LOAD CELL MAGNETIC DIAL STAND GAUG TEST TANK Figure 3: Experimental Setup5. TEST PROCEDURE The sand was poured in the tank by rainfall technique keeping the height of fall as 35cm to maintain the constant relative density throughout the bed. The sand was poured up tothe location of the desired layer of reinforcement, then the top surface of th sand made theleveled and the biaxial geogrid reinforcement was placed at depth 0.5D below footing. Again,the sand was filled over this geogrid reinforcement layer in the tank. A manually controlledhydraulic jack with activated loading piston, installed between the sliding beam and strong between 450
  • 5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEMEreaction beam as shown in Fig. 3 was used to provide the required load on the footings. Boththe footings will be simultaneously loaded vertically. The vertical displacement of each testfooting was measured by taking the average of two dial gauges readings. By graduallyincreasing the load, a series of tests was carried out so as to monitor the complete load-deformation plots till the ultimate failure occurs. Each test was carefully controlled byobserving the displacement of each footing through dial gauge reading.6. TEST RESULTS Load settlements for each testing were plotted. The curves, in general, show a linearvariation in the initial portion and become non-linear thereafter. Fig. 4 shows average loadsettlement curve for isolated rectangular footings. Load(kN) 0 0.2 0.4 0.6 0 1 Settlement(mm) 2 3 3cmx6cm 4 5 4cmx8cm 6 5cmx10cm 7 Figure 4: Load settlement curve for isolated footings. Load settlement curve for adjoining rectangular footings placed reinforced sand bed atdifferent spacing to width ratio (S/B) are shown in Fig. 5 - 7. Load(kN) 0 0.02 0.04 0.06 0.08 0.1 0.12 0 1 2 Settlement(mm) S/B=1 3 S/B=2 4 5 S/B=3 6 7 8 9 Figure 5: Load Settlement curve for 3cm x 6cm rectangular footing. 451
  • 6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME 0 0.1 Load(kN) 0.2 0.3 0 1 Settlement(mm) 2 S/B=1 3 S/B=2 4 S/B=3 5 6 7 8 Figure 6: Load Settlement curve for 4cm x 8cm rectangular footing. Load(kN) 0 0.1 0.2 0.3 0.4 0.5 0.6 0 1 2 Settlement(mm) 3 S/B=1 4 S/B=2 5 S/B=3 6 7 8 Figure 7: Load Settlement curve for 5cm x 10cm rectangular footing. The ultimate bearing capacity was obtained by using tangent intersection method.Tables 2 show the bearing capacity of corresponding model footing. Table 2: Ultimate bearing capacity of footing for Different S/D Ratio. Ultimate bearing capacity(KN/m2) S/B Ratio 3cm x 6cm 4cm x 8cm 5cm x 10cm 1.0 51.11 71.87 90.80 2.0 42.77 66.56 81.75 3.0 36.66 60.62 76.20 Isolated 33.88 51.00 70.80 452
  • 7. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME7. DISCUSSIONS AND INTERPRETATION OF RESULT The bearing capacity of adjoining footing resting on reinforced sand was studied. Thebiaxial geogrid was kept at 0.5D below the footing. The adjoining footing was spaced tostudy the interference effect on reinforced sand. Fig. 5-7 shows that with increase in S/B thebearing capacity decreases and the settlement was observed to be increase. The ultimatebearing capacity was observed to be more than that for isolated footing. The increase in theultimate bearing capacity may be due to existing footing acts as a surcharge for the adjacentfooting and at wider spacing no interference takes place and each footing acts as anindividual (isolated) footing.7.1 Efficiency Factor (ξγ) The efficiency factor (ξγ) is the ratio of average pressure on an interfering footing of agiven size associated with either an ultimate shear failure or a given magnitude of settlementto the average pressure on an isolated footing of a given size associated again with either anultimate shear failure or the same magnitude of settlement. Table 3 shows the efficiencyfactor for different S/B ratio for 3cm x 6cm, 4cm x 8cm and 5cm x 10cm dimensionrectangular footings. Table 3: Efficiency Factors for Different S/B Ratio Efficiency Factors (ξγ) S/B Ratio 3cm x 6cm 4cm x 8cm 5cm x 10cm 1.0 1.50 1.40 1.28 2.0 1.26 1.30 1.15 3.0 1.08 1.18 1.07 From Table 3, it can be seen that, the efficiency factor decreases with increase in S/Bratio. This indicates that the bearing capacity is greatly influenced by spacing between them.As the spacing decreases, the bearing capacity is observed to be increased.8. CONCLUSIONS From the present study following conclusions are drawn • Bearing capacity of rectangular footings increases as the size of footing increases. • Bearing capacity of interfering footing is more than that of isolated footing. • Bearing capacity of interfering footing increases as spacing between them decreases. • The settlement was observed to be increase as spacing is decreased. • The efficiency factor decreases with increase in S/D ratio. 453
  • 8. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308(Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEMEREFERENCES [1] J. Kumar and P. Ghosh (2007), “Ultimate bearing capacity of two interfering rough strip footings”, Int J Geomech ASCE 7(1), 53–62. [2] Kumar and P. Bhattacharya (2010), “Bearing capacity of interfering multiple strip footings by using lower bound finite elements limit analysis”, Computers and Geotechnics 37, 731–736. [3] J. Lee and J. Eun (2009), “Estimation of bearing capacity for multiple footings in sand”, Computers and Geotechnics 36, 1000–1008. [4] I. N. Khan ,K. C. Bohra ,M. L. Ohri and Alam Singh (2006), “A Study on Interference of surface Model Footing Resting on Sand”, The Institution of Engineers, Malaysia, Vol. 67, March 2006, 15-23. [5] J. Kumar and M.K. Bhoi (2009), “Interference of two closely spaced strip footings on sand using model tests”, J Geotech Geoenviron Eng ASCE 2008, 134(4), 595–604. [6] M. Ghazavi and A. A. Lavasan (2008), “Interference Effect of Shallow Foundation Constructed on Sand Reinforced with Geosynthetics”, Science Direct Geotextiles and Geomembranes 26(2008), 404-415. [7] A. Kumar and S. Saran (2003), “Closely Spaced Footings on Geogrid-Reinforced Sand”, J Geotech Geoenviron Eng ASCE 1090-0241(2003), 129:7(660). [8] P. Ghosh and P. Kumar (2009), “Interference Effect of Two nearby Strip Footing on Reinforced Sand”, Contemporary Engineering Science, Vol. 2, 2009, No.12, 577-592. [9] Ravin M. Tailor, M. D. Desai and N. C. Shah, “Performance Observations For Geotextile Reinforced Flexible Pavement On Swelling Subgrade: A Case Of Surat, India” International Journal of Civil Engineering & Technology (IJCIET), Volume3, Issue2, 2012, pp. 347 - 352, Published by IAEME[10] P.A. Ganeshwaran, Suji and S. Deepashri, “Evaluation Of Mechanical Properties Of Self Compacting Concrete With Manufactured Sand And Fly Ash” International Journal of Civil Engineering & Technology (IJCIET), Volume3, Issue2, 2012, pp. 60 - 69, Published by IAEME 454