Effect of Soil Confinement on Ultimate Bearing Capacity of Square Footing Under Eccentric Inclined Load

Transcription

1 Effect of Soil Confinement on Ultimate Bearing Capacity of Square Footing Under Eccentric Inclined Load Vinod Kumar Sing Assistant Manager, RITES, Maduani, India Arun Prasad Lecturer, Department of Civil Engineering, Institute of Tecnology, Banaras Hindu University, Varanasi, India R. K. Agrawal Professor, Department of Civil Engineering, Institute of Tecnology, Banaras Hindu University, Varanasi, India ABSTRACT Tis paper presents te results of laoratory model tests on te effect of soil confinement on te eavior of a model footing resting on Ganga sand under eccentric inclined load. Confining cells wit different eigts and widts ave een used to confine te sand. Te ultimate earing load of a square footing supported on a confined sand ed as een studied. Te studied parameters include te cell eigt, cell widt and te dept from ase of footing to te top of te confining cell, load eccentricity and load inclination. Initially, te eavior of footing for an unconfined case is studied and ten it is compared wit tat of confined soil. Te results are ten analyzed to study te effect of eac parameter. Te ultimate earing capacity improvement due to te soil confinement is represented using a non-dimensional factor, called te Bearing Capacity Ratio (BCR). Te results indicate tat te ultimate earing capacity of square footing can e apprecialy increased y soil confinement under axial load as well under eccentric inclined load. It as een oserved tat suc confinement resists te lateral displacement of soil underneat te footing leading to a significant decrease in te vertical settlement and ence improving te ultimate earing capacity. For small cell widt, te cell soil footing eaves as one unit (deep foundation), wile tis pattern of eavior was no longer oserved wit large cell widt. Te recommended cell eigts, depts, and widts tat give te maximum ultimate earing capacity improvement are presented and discussed. Keywords: Model tests; Coesionless soils; Square Footings; Eccentric-Inclined load; Ultimate Bearing Capacity; Soil Confinement.

2 Vol. 12, Bund. E 2 INTRODUCTION In recent years Civil Engineering professionals ave adopted te practice of improvement of soil y reinforcement, compaction, grouting etc. Te decreasing availaility of good construction sites as led to increased use of sites wit marginal soil properties. In view of tis, te requirement for in situ treatment of foundation soil to improve its earing capacity as risen markedly. Te soil confinement is one suc metod of improving soil capacity. Te more recent advancement in tis field is to provide confinement to te soil y using metalcell, geocell etc. Tis novel tecnique of soil confinement, toug successfully applied in some areas of geotecnical engineering, as not received muc attention in foundation applications. In te last few decades, great strides in te modification of existing forms of foundations ave occurred, along wit te development of new and unconventional types of foundation systems troug consideration of soil and structure interaction. Tis results in a system tat is more realistic in performance, utilizing te form and material strengt. Lateral confinement of coesionless soil is one of tem. Many researcers ave studied te effect of lateral confinement on earing capacity, especially on sandy soil. Tey ave concluded tat y confining te soil tere is a reduction in te settlement, and ence tere is an increase in te earing capacity of te soil. Rea and Mitcell (1978) conducted a series of model plate loading tests on circular footings supported over sand-filled square-saped paper grid cells to identify different modes of failure and arrive at optimum dimensions of te cell. Mamoud and Adrao (1989) presented an experimental study concerning a metod of improving te earing capacity of strip footing resting on sand su-grades utilizing vertical nonextensile reinforcement. Te test results indicate tat tis type of reinforcement increases te earing capacity of su-grades and modifies te load displacement eavior of te footing. King et al. (1993) investigated te laoratory-model test results for te earing capacity of a strip foundation supported y a sand layer reinforced wit layers of geogrid. Puri et al. (1993) studied te ultimate earing capacity of strip and square foundations supported y sand reinforced wit geogrid. Das and Omar (1993) presented te ultimate earing capacity of surface strip foundations on geogrid-reinforced sand and unreinforced sand. Das et al. (2001) investigated te use of vertical reinforcement along wit orizontal reinforcement.te reinforcement consisted of a series of interlocking cells, constructed from polymer geogrids, wic contain and confine te soil witin its pockets. Scimizu and Inui (1990) carried out load tests on a single six-sided cell of geotextile wall uried in te susurface of te soft ground. Mandal and Manjunat (1995) used geogrid and amoo sticks as vertical reinforcement elements and studied teir effect on te earing capacity of a strip footing. Te strengt of confined sand as een studied y Rajagopal, Krisnaswamy and Lata (1999).Tey studied te influence of geocell confinement on te strengt and stiffness eavior of granular soils. Das et al. (2001a) performed an experimental study on te earing capacity of a strip footing supported y a sand ed reinforced wit a geocell mattress. Several autors ave also studied strip foundations ut reinforced wit different materials suc as steel ars (Milovic, 1977; Bassett and Last, 1978; Verma and Car, 1986), steel grids (Dawson and Lee, 1988; Adel-Baki et al.1993 ;), geotextile (Das,1988), and geogrids (Milligan and Love, 1984, 1985; Ismail and Raymond, 1995). Sawwaf and Nazer (2005) studied te eavior of circular footing on

3 Vol. 12, Bund. E 3 confined sand. Tey used confining cylinders wit different eigts and diameters to confine te sand. Study of earing capacity of footing under eccentric or eccentric inclined load as een carried out y many researcers in te past ut witout reinforcement (Meyerof, 1953; Meyerof, 1963; Meyerof, 1965; Prakas and Saran, 1971; Prakas and Saran, 1977) Tis laoratory-testing program attempts to provide a etter understanding of te eavior of square footing resting on confined Ganga sand under eccentric inclined load. LABORATORY MODEL TESTS Footing A square model footing of size 150 x 150 mm and 10mm tick made of mild steel wit grooves at top was used. Te footing ad a groove at te centre and along a line suc tat te eccentricity-widt ratio ecomes 0.1, 0.2 and 0.3. Te ase was made roug y fixing a tin layer of sand onto te ase of te model footing. Te footing was placed on te surface of te sand ed and load was applied on it y a and-operated mecanical jack. Te load transferred to te footing was measured y a pre-calirated proving ring. Te load was applied as per IS: Eac load increment was maintained constant till te footing settlement ad stailized. Te settlement of te footing was measured y dial gauges placed on te footing. Model test tank Model tests were conducted in a test tank, aving inside dimensions of 1500 x 1500 mm in plan and 1000 mm deep. Te size of te tank was decided y te size of te footing and te zone of influence. Te test tank is made of steel and as arrangement to fix te proving ring wit specially faricated inclined load device for applying te axial as well as inclined load to te footing as sown in Fig.1 TEST MATERIAL Ganga sand Locally availale Ganga sand is used as te foundation ed. Te pysical properties of Ganga sand is presented in Tale 1. Te grain size distriution curve of Ganga sand is sown in Fig. 2 and is classified as SP. Confining cells Te confining cells were made of MS plate wit different widt and eigts. Te internal sizes of cells used are 150, 225, 300, 375 and 450 mm. Te eigts of cell used are 150, 225 and 300 mm. Te tickness of te all te confining cells is 10 mm. Te tests were carried out y placing te confining ox initially in position and ten te sand ed was prepared y rainfall tecnique to get te desired relative density of 65%.

4 Vol. 12, Bund. E 4 Figure 1: Experimental setup Figure 2: Particle size distriution curve of Ganga sand

5 Vol. 12, Bund. E 5 Tale 1: Pysical properties of Ganga sand S. No. Caracteristics Value 1 I. S. Classification SP 2 D mm 3 D mm 4 D mm 5 Uniformity coefficient (C u ) Coefficient of curvature (C c ) Minimum void ratio ( e min ) Maximum void ratio (e max ) Void ratio in test (e) Specific gravity (G) Minimum dry density kn/m 3 12 Maximum dry density kn/m 3 13 Density of sand in test kn/m 3 14 Relative density of sand during test (D r ) 65 % 15 Angle of internal friction (ø) 34 o 16 Classification of sand ased on relative density Medium dense EXPERIMENTAL SETUP AND TEST PROGRAM Te footing was placed in position and te load was applied to it y te jack troug te proving ring. Te load was applied in small increments until failure occurred. Eac load increment was maintained constant until te footing settlement ad stailized. Te settlements of te footing were measured y dial gauges. Te geometry of te soil, model footing, and confining ox is sown in Fig. 4. Te test program consisted of carrying out of ten series of tests (A J) on square footing to study te effect of soil confinement on te soil-foundation response as sown in Tale 2. Initially, te test as een carried out under axial load on te footing resting on te unconfined ed. Ten, eac series of te tests were carried out under axial load to study te effect of one parameter wile te oter variales were kept constant. Te studied variales are te cell eigt (), cell widt (B) and te dept from ase of footing to te top of confining cell (u) keeping footing widt () constant for all te cases. Finally, tests were conducted under eccentric-inclined load for one case of confinement. Te confinement condition, for wic te BCR is maximum (B/ = 1.5 and / = 2.0), is cosen for te eccentric-inclined loading. Te e/b ratio for eccentric load is 0.1, 0.2 and 0.3. Te load inclination is 5 0, 10 0, 15 0 and 20 0.

6 Vol. 12, Bund. E 6 Tale 2: Model Test Program A B C D E F G H I J Test Series Constant parameters Variale parameters Test on unconfined sand ed u 2.0 and B =0 1.0,1.5,2.0,2.5,3.0 u B 2.0 and = ,1.5,2.0,2.5,3.0 u 2.0 and B =1 1.0,1.5,2.0,2.5,3.0 u 1.5 and B =0 1.0,1.5,2.0,2.5,3.0 u B 1.5 and = ,1.5,2.0,2.5,3.0 u 1.5 and B =1 1.0,1.5,2.0,2.5,3.0 u 1.0 and B =0 1.0,1.5,2.0,2.5,3.0 u 1.0 and B = ,1.5,2.0,2.5,3.0 u 1.0 and B =1 1.0,1.5,2.0,2.5,3.0 Figure 4: Geometric parameters of confined sand-foundation system RESULTS AND DISCUSSION Te load settlement relationsip and te ultimate earing capacity of te footing wit and witout confinement ave een otained. Te earing capacity improvement due to te soil confinement is represented using a non-dimensional factor, called te Bearing Capacity Ratio (BCR). Tis factor is defined as te ratio of te ultimate earing capacity wit confinement to te ultimate earing capacity in tests witout confinement.

7 Vol. 12, Bund. E 7 Effects of cell widt In order to investigate te effect of cell widt on te footing eavior, five cells wit widt of 150, 225, 300, 375 and 450 mm were used. Fig.5 sows te variation of BCR wit normalized cell widt for different cell eigts wit a constant footing widt of 150 mm. A significant increase in te earing capacity of te model footing supported on confined sand wit te increase of normalized cell widt B/ is oserved up to aout B/ ratio of 1.5; after wic te BCR decreases wit an increase in te B/ ratio. Wile conducting te model tests, it was oserved tat as failure approaced in tests carried out wit small cell widt, te cell and te sand witin te cell eaved as one unit. In tests carried out wit large cell widt, tis eavior was noticed initially, ut as te load was increased it was no longer oserved. Fig.5 also sows tat using soil confinement could result in an improvement in earing capacity as ig as 6.75 times more tan tat witout soil confinement. It is clear tat te est enefit of soil confinement could e otained wit a (B/) ratio etween1.0 to 2.0 wit te maximum improvement in te earing capacity at a (B/) ratio of aout 1.5 for different eigts of confining cells. Wen te footing is loaded, suc confinement resists te lateral displacements of soil particles underneat te footing and confines te soil leading to a significant decrease in te vertical settlement and ence improving te earing capacity. For small cell widts, as te pressure is increased, te plastic state is developed initially around te edges of te footing and ten spreads downward and outward. Te moilized vertical friction etween sand and te inside wall of te cell increases wit te increase of te acting active eart pressure. Tis eavior is oserved until te point wen te system (te cell, sand and footing) starts to eave as one unit. Te eavior is similar to tat oserved in deep foundations (piles and caissons) in wic te earing load increases due to te sear resistance of cell surface. Tis illustrates te increase of te earing load wit te increase of te cell widt and cell eigt.

8 BCR Vol. 12, Bund. E /= 2 / = 1.5 / = B/ u/=0 =150mm Figure 5: Variation of earing capacity ratio wit normalized cell widts (B/) for different cell eigts at u=0 mm Effect of cell eigt In order to investigate te effect of cell eigt on te footing response, tests were carried out using tree different eigts for eac cell widt. Te variation of BCR wit normalized cell eigt (/) is sown in Figure 6 for different normalized cell widts (B/). Te figure sows te same pattern of eavior for te different cell widts. Increasing cell eigts results in a greater improvement in te BCR. Te increase in cell eigt results in te enlargement in te surface area of te cell model footing leading to a iger earing capacity load. Te slope of te BCR versus / curves for B/ ratios of 1.0 and 2.5 are less tan te comparale slopes for B/ ratios of 1.5 and 2.0. Tis trend confirms te previous conclusion tat te greatest enefit of cell confinement can e otained at a B/ ratio of aout 1.5.

9 BCR Vol. 12, Bund. E B/=1.0 B/=1.5 B/=2.0 B/=2.5 B/= u/=o / =150mm Figure 6: Variation of earing capacity ratio wit normalized cell eigt (/) for different cell widts (B/) at u=0 mm Effect of cell dept Te variation of te BCR at different normalized depts to te top of te cell (u/) for cells wit a different cell widts are sown in Fig.7. It is oserved tat te BCR decreases wit te increase in te dept of te cell. Wit an increase in te dept of placement, te soil etween te footing and te cell top deforms laterally and, terefore, te vertical settlements increase and te BCR decreases. Based on tese results, it is oserved tat te maximum earing capacity ratio is otained at u/ ratio of zero i.e., wen te top of cell is placed at te footing level. However, for confined footing te effect of overurden pressure is not significant. Wen te footing is loaded, it settles and te plastic state is developed until te point at wic te system starts to eave as one unit.

10 BCR Vol. 12, Bund. E B/=1.0 B/=1.5 B/=2.0 B/=2.5 B/= /=2 =150mm u/ Figure 7: Variation of earing capacity ratio wit normalized dept to te top of te cell (u/) for different cell widts and constant cell eigt 300 mm Effect of eccentric-inclined load Eccentric-inclined load was applied to te footing for te confined case tat sowed te maximum enefits of confinement. Te ratio of B/ = 1.5 and / = 2.0. Load eccentricity (e/b) used is 0.1, 0.2 and 0.3 and load inclination is 5 0, 10 0, 15 0 and Ultimate earing capacity as een found out from te load-settlement curve. Vertical settlement and orizontal displacement of te footing were recorded y dial gauges. It was oserved tat wit te increase in load eccentricity and load inclination, tere was a reduction in ultimate earing capacity. But tis ultimate earing capacity was found to sow a remarkale improvement wit footing wit confinement. Te orizontal displacement of te footing also increased wit te increase in te load inclination. Te results are presented in Tale 3. BCR ratio for various load inclination and e/b ratio is calculated and sown in te Fig.8. Te ultimate load decreased form 1.35 kg/cm 2 to 0.45 kg/cm 2 wen te load inclination increased from 0 0 to 20 0 for e/b ratio of 0.0. Similarly it decreased from 0.29 kg/cm 2 to 0.07 kg/cm 2 wen load inclination increased from 0 0 to 20 0 for e/b ratio of 0.3. Te vertical settlement and orizontal displacement also sowed a reduction due to confinement.

11 BCR Vol. 12, Bund. E 11 Tale 3: BCR of Unconfined and Confined Footing for Eccentric-Inclined Load Sl. No. e/b i (degree) Footing wit No Confinement Q ult V settl. H disp. (kg/cm 2 ) (mm) (mm) Footing wit Confinement Q ult V settl. H disp. (kg/cm 2 ) (mm) (mm) BCR Variation of BCR wit Load Inclination for Various e/b ratio Load Inclination(degree) e/b = 0.0 e/b = 0.1 e/b = 0.2 e/b = 0.3 Figure 8: Variation of BCR wit load inclination for various e/b ratios (B/= 1.5, / = 2.0)

12 Vol. 12, Bund. E 12 CONCLUSIONS Based on te experimental test results, te following conclusions can e drawn. Soil confinement as a significant effect on improving te eavior of square footing supported on granular soil. Te ultimate earing capacity was found to increase y a factor of 6.75 as compared to te unconfined case. Based on experimental results, soil confinement could e considered as a metod to improve te earing capacity of isolated footings resting on medium to dense sand. Tis type of cells wit different eigts, widts and tickness could e easily manufactured and placed around te individual footings leading to a significant improvement in teir response. In cases were structures are very sensitive to settlement, soil confinement can e used to otain te same allowale earing capacity at a muc lower settlement Te BCR is igly dependent on te B/ ratio (cell widt/footing widt). Te optimum ratio is aout 1.50 eyond wic te improvement decreases as te ratio increase. Increasing te eigt of te confining cells, results in increasing te surface area of te cell-model footing, tis transfers te footing load to deeper dept and leads to improving te BCR. Optimum value of / ratio is 2. To otain maximum improvement, te top of te confining cell sould e situated at top of te foundation ed. Soil confinement as a significant effect on improving te eavior of square footing under eccentric-inclined load. Te BCR in tis case is also REFERENCES 1. Adel-aki,S., G.P. Raymond, and P. Jonson (1993) Improvement of te Bearing Capacity of Footing y a Single Layer of Reinforcement, Proceedings, Vol. 2, Geosyntetics 93 Conference, Vancouver, Canada. pp Bassett, R.H., and N.C. Last (1978) Reinforcing eart elow footing and emankments. Symposium on Eart Reinforcement, ASCE, Pittsurg Binquet, J., and K.L. Lee (1975) Bearing capacity tests on reinforced eart slas. Journal of Geotecnical Engineering Division, 101(12), Das, B.M. (1988) Sallow foundation on sand underlain y soft clay wit geotextile interface, Geosyntetics for Soil Improvement Das, B.M., and M.T. Omar (1994) Te effects of foundation widt on model tests for te earing capacity of sand wit geogrid reinforcement. Geotecnical and Geological Engineering, 12, Das, B.M., V.K. Puri, M.T. Omar and E. Evigin (1996) Bearing capacity of strip foundation on geogrid reinforced sand-scale effects in model tests. Proc., 6 t International Conference on Offsore and Polar Engineering, Vol.12,

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