GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE Prof. J. N. Mandal Department of cvl engneerng, IIT Bombay, Powa, Mumba 400076, Inda. Tel.022-25767328 emal: cejnm@cvl.tb.ac.n
Module - 9 LECTURE - 48 Geosynthetcs for ground mprovement
Recap of prevous lecture.. Excel program for PVD drans Geosynthetc encased stone columns Encasements Axsymmetrc fnte element model (fem) of encased stone column
Fnte Element Modellng of Encased Stone Columns Axsymmetrc fnte element model (FEM) of encased stone column (After Dutta et al., 2012)
- 15-node trangular elements to model the deformatons and stresses n the sol - Un-draned (B) Mohr-Coulomb model for soft clay - Draned Mohr-Coulomb model for stone column. - The geogrd elements are modeled as elastc materal. FEM fne mesh generaton - Short term plastc analyss as calculaton procedure.
MATERIAL PROPERTIES Propertes of clay and stone: Parameters Propertes Clay Stone Elastc modulus, E' (kpa) 4000 50000 Posson s rato, ν' 0.4 0.3 Coheson, c u (kpa) 10 0 Angle of nternal frcton, φ 0 45 Propertes of geogrd: Length of geogrd: 2D, 4D and full length (5D) D = dameter of column Stffness of geogrd: 200 kn/m, 300 kn/m, 1000 kn/m and 2000 kn/m
The analyss has been carred out keepng n vew: - Radal deformaton of stone column wthout and wth encasement, - Relatve shear stress dstrbuton, - Pressure - settlement response, Effect of length, and Effect of stffness of encasements
Radal Deformaton: - A huge radal dsplacement of about 13 mm n case of ordnary stone column (OSC) - Full length encasement of stffness 300 kn/m reduce the maxmum radal dsplacement to 4 mm. Radal deformaton (a) ordnary stone column (b) encased stone column (300 kn/m)
Effect of encasement stffness on radal deformaton Full length Encasements Stffness 1000 kn/m maxmum radal deformaton = 2 mm Stffness 2000 kn/m maxmum radal deformaton = 1.4 mm - More hoop tenson force gets developed n the stffer encasement at the same appled load resultng more confnng pressure to the stone column. - For encasement stffness of 200 kn/m, 300 kn/m, 1000 kn/m and 2000 kn/m, hoop tenson s 18.13 kn/m, 23.81 kn/m, 44.49 kn/m and 56.21 kn/m respectvely. Radal deformaton of encased stone column wth stffness (a) 1000 kn/m and (b) 2000 kn/m
Relatve Shear Stress Dstrbuton - Relatve shear stress s defned as the rato of moblzed shear stress to the maxmum shear stress. Relatve shear stress dstrbuton (a) ordnary stone column (b) encased stone column (2000 kn/m)
Influence of stffness of encasement Pressure - settlement response of stone column wthout and wth encasement of dfferent stffness values
Influence of length of encasement Pressure - settlement response of encased stone column wth dfferent encasement lengths For the same prescrbed dsplacement, the maxmum hoop tenson developed n the partal encasements of 2D and 4D length are 7.462 kn/m and 44.64 kn/m respectvely whle the maxmum hoop tenson s 56.21 kn/m n the full length encasement
Influence of stffness on length of encasements Influence of encasement stffness on ts varous lengths
CONCLUSIONS Encasng the stone column wth sutable full length encasement can ncrease the bearng capacty of clay many tmes than that of the ordnary stone column. Encasement of hgher stffness provdes more confnng pressure to the stone column due to the generaton of hgher hoop tenson force n t. When the stone column s encased, the appled load s dstrbuted to entre length of the column whereas the ordnary stone column fals due to the lateral bulgng of the stones wthn 2D length of the column from the top.
The moblzed shear stress zone s more n encased column when compared wth ordnary stone column for the prescrbed settlement. For an encasement wth certan stffness, the load carryng capacty gets mproved wth the ncrease n length of the encasement. The effect of encasement stffness s neglgble when the encasement length s very short. However, as the length of encasement s ncreased, the stffer encasement produces better results and t s more pronounced for full length of encasements.
Analytcal Study on Encased Stone Column R = ntal radus of the column H = ntal length of the column R f = fnal radus of the column after deformaton H f = fnal length of the column Unform lateral deformaton of stone column (smplfed assumpton) (Wu and Hong, 2009) It can be assumed that the load s transferred unformly causng a unform lateral deformaton along the entre length of the column. For more smplcty, t can also be assumed that the volume always remans constant whle the unform lateral deformaton takes place.
f l H H H 1 f 1 H H f H R H R 2 f 2 l f l f f f 1 1 R H 1 H R H H R R l l l f f r 1 1 1 R R 1 1 R R R R R 2 ) R (R 2 If axal stran of the fully encased stone column = ε l If volume s constant at the tme of deformaton, Therefore, At any stage of loadng, f crcumferental stran or hoof stran = ε r - For the calculated hoop stran, tensle strength of geogrd can be determned from tensle strength versus stran curve obtaned from tensle strength test.
Consderng the encasement as a thn cylndrcal element, generaton of crcumferental hoop stress n the encasement s shown below. Hoop stress generaton n a cylndrcal geogrd encasement If confnng pressure = P c and hoop stress = P h, P h x t x Lx 2P c x D f x L P h P D f = fnal dameter of the stone column L = length of the encasement T = tensle strength of the geogrd per unt length at the correspondng hoop stran t = thckness of the thn cylndrcal encasement c x D 2t f T t
GROUND IMPROVEMENT USING GEOCELL In Inda, half of the subcontnents consst of black cotton sols whch are hghly plastc and swellng n nature. It s a very serous problem for the engneers to construct embankments or renforced sol retanng walls on swellng sols. The conventonal methods are excavaton and replacement wth good qualty fllng materals or plng whch s not economcal and practcal. Alternatvely, geocell mattress can be used as an effectve ground mprovement technque.
What s Geocell? Geocell s a three dmensonal honeycomb structures made of a seres of nterlockng cells. Jute geocell mattress Unaxal geogrd Geocell mattress
Geocell may be made of Geogrd Non-woven and woven geotextle Plastc and Geofoam The geometry of cell may be trangular, square, rectangular and hexagonal. It provdes very good confnng effects. The deployment of geocell renforced mattress ensure stff platform and drastcally mprove the bearng capacty of weak sol.
Mhaskar and Mandal (1996) conducted plate load test on soft saturated marne clay subgrade to fnd out the effcency of geocell, effect of ts cell geometry as well as effect of relatve densty of the backfll materal. They reported consderable mprovement n load carryng capacty as well as reducton n settlement when the clay s renforced wth geocell. It was found that the ultmate bearng capacty as well as bearng capacty rato of marne clay ncreases wth decreasng wdth to heght rato of geocell. A three-dmensonal fnte element analyss was also carred out usng 'ANSYS' to valdate the expermental results. The fnte element results were found n accord wth the expermental results.
Renforcement Mechansm - Unlke the unrenforced base, geocell-renforced base can provde lateral and vertcal confnement, tensoned membrane effect, and wder stress dstrbuton. - As the geocell s a three dmensonal structure, t can provde lateral confnement to sol partcles wthn cells. Geocell mattress can provde the lateral and vertcal confnement n followng ways: Lateral confnement: (1) As the load over the mattress ncreases, hoop stress s generated n the cell wall resultng n the lateral confnement to the nfll materal. (2) Lateral confnement from the adjacent cells to prevent the lateral expanson of any cell.
Vertcal confnement: (1) The frcton between the nfll materal and the geocell wall, and (2) The geocell-renforced base acts as a mattress to restran the sol from movng upward outsde the loadng area. Unrenforced and geocell-renforced bases (After Pokharel et al., 2010)
Desgn Consderatons The renforcng effect can be ncreased by ncreasng the heght of cell, wdth of mattress and decreasng the equvalent dameter of cell. Effect of heght Effect of wdth Effect of equvalent dameter of the sngle cells Effect of tensle stffness Effect of nfll sol Depth of embedment Effect of basal renforcement
Area of applcatons: Under pavements Beneath the embankments Over column treated grounds Over bured ppes Landfll
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Prof. J. N. Mandal Department of cvl engneerng, IIT Bombay, Powa, Mumba 400076, Inda. Tel.022-25767328 emal: cejnm@cvl.tb.ac.n