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International Journal of Engineering Research and Development (IJERD)
International Journal of Engineering Research and Development (IJERD)
International Journal of Engineering Research and Development (IJERD)
International Journal of Engineering Research and Development (IJERD)
International Journal of Engineering Research and Development (IJERD)
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International Journal of Engineering Research and Development (IJERD)

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  • 1. International Journal of Engineering Research and Developmente-ISSN: 2278-067X, p-ISSN: 2278-800X, www.ijerd.comVolume 7, Issue 6 (June 2013), PP. 20-2420Behaviour of Raft Foundation with Vertical Skirt UsingPlaxis 2dDr. Sunil S. Pusadkar1, Ms. Tejas Bhatkar2,1Associate Professor, Department of Civil Engg., Govt. College of Engineering, Amravati (M.S) – 444 6042M. Tech. (Civil- Geotech) Scholar, Department of Civil Engg., Govt. College of Engineering, Amravati (M.S)– 444 604Abstract:- Skirted foundation is one in which vertical or inclined wall surrounds one or more sides of the soilmass beneath the footing. Skirted foundation is an alternative approach required for improving the bearingcapacity of raft foundation by using “structural skirts” fixed to the edges of the raft foundation. Construction ofvertical skirts at the base of the raft foundation confines the underlying soil and generates a soil resistance onskirt side that helps the footing to resist sliding. The present work focused on the analysis of two sided and onesided skirted raft with and without vertical skirt using a finite element software PLAXIS 2D. The studiedparameters include the raft sizes,skirt depth and then compared with those of raft without vertical skirt. Theresults indicated that using structual skirts for raft foundation has a significant effect in improving the bearingcapacity. This improvement increases with the skirt depth with varying raft sizes for two sided raft foundationvertical skirt with decrease in settlement. Whereas in one sided raft foundation with vertical skirt there isincrease in both bearing capacity and settlement.Thus finite element analysis presented in this study helped inbetter understanding and identifying the failure pattern of skirted raft with and without skirt for raft foundationwith vertical skirt.Keywords:- Raft Foundation, Structural Skirt, PLAXIS 2D, Bearing Capacity, Settlement.I. INTRODUCTIONThe shallow footing fails due to the shear failure of soil beneath it. When the load gets transferred onthe soil immediately below the strata, the footing moves sideways by shear failure. By confining the soil underthe footing, such failure can be effectively controlled and thus the failure mechanism. Thus by providing skirtwall, the load from the superstructure is transferred to the strata below the bottom of the skirt wall. In thismanner, it may be forecast that a raft laid at ground level with skirt walls at edges to confine the soil beneath, itwill have a load carrying capacity approximately equal to or greater than that of a raft founded at a depth equalto the depth of confinement provided. It may also be forecast that this type of confinement will control the tiltand hence increase the load bearing capacity of footing resting on non-uniform soil condition in which someportion of the soil is weak compared to other. Therefore, it is an alternative approach for improving the bearingcapacity of raft foundation. This approach is based on the use of “structural skirts” fixed to the edges of thefoundation. This method of improvement does not need excavation of the soil, and hence it cannot be restrictedby the presence of a high water table. Structural skirts fixed to the edges of shallow foundations have been usedfor a considerable time, principally to increase the “effective depth” of the foundations in marine and othersituations where water scour may be a problem. However, the use of such structural skirts in conjunction withconventional raft foundations for improving bearing capacity and reduction in settlement has not been widelyemployed. On this basis numerical study have been carried out to study the behaviour of raft foundation withtwo sides and one side vertical skirt and compared with the behaviour of raft foundation without any skirt.II. LITERATURE REVIEWMahiyar and Patel (2000) studied the finite-element analysis of an angle shaped footing under eccentricloading .They concluded that one side vertical projection of footing confines the soil and prevents its lateralmovement. The footing subjected to uniaxial eccentric loads can be designed for no or negligible tilt by givingthe footing an angle shape. The ultimate bearing capacity will be higher when Φ is higher.Al-Aghbari (2007) studied a series of experimental tests to study the settlements of shallow circular foundationson sand with and without structural skirts. The test results indicate that the use of structural skirts reduces thesettlement of footings and modifies the load–displacement behaviour.Salih and Joseph (2010) studied the results for footings with and without skirt, on uniform soilcondition showing that the bearing capacity can be improved by a factor of 1.08 to 1.64 when skirt was providedwith Df/B ratio of 0.25 to 1.0. While comparing the performance of footings with and without skirt, on uniform
  • 2. Behaviour of Raft Foundation with Vertical Skirt Using Plaxis 2d21soil condition with the same Df/B ratio, the improvement factor was found to be decreased from 8% to zero withincrease in Df/B ratio. From the cost analysis, it is evident that skirted raft is more economical than raft.Azzam and Farouk (2010) studied laboratory model tests and numerical study on the behaviour of a strip footingwith structural skirts adjacent to a sand slope with different skirts depth, location of the skirted footing relativeto the slope crest and the slope inclination. They conclude that stabilizing the earth slope using structural skirtswith adequate depth in the conjunction of strip footing adjacent to slope crest has a significant effect inimproving the soil bearing capacity.Nazir and Azzam (2010) studied the behaviour of circular footing resting on partially replaced sandpile with/without skirts and have found that the improvement of load bearing capacity is remarkably increasedand decreased in the vertical settlement using both partially replaced sand piles with/without confinement byskirts.Raft foundations are widely used in supporting structures for many reasons such as weak soilconditions or heavy columns loads. The raft foundation may require more area to transfer heavy load or need toplace at greater depth to keep stress level within permissible bearing capacity.The literature shows that confinement of soil below footing, provision of skirt for strip footing, squarefooting and circular footing improves the bearing capacity and reduces the settlement of foundation underconcentric as well as eccentric loading. In order to give an optimum solution for raft foundation with skirt and toprovide economical foundation, there is need to study of skirted raft foundation with different parametricalaspect. Keeping in view this aspect, this work lead to study parametric effect on behaviour of skirted raftfoundation with vertical skirt using PLAXIS 2D software.III. NUMERICAL MODELINGThe geometry of the finite element soil model adopted for the analysis is 15B X 20B with varying raftsizes B= 10m, 15m, 20m, 30m with skirt depth (Ds) from 0.25B to 3B .Figure 1 shows the geometry model ofraft foundation without skirt and Figure 2 shows raft foundation with two side vertical skirt for one typical caseof 10 m raft size and skirt depth..BSOIL MODELRAFTLOAD15 B20 BFigure 1: Geometry Model of Raft Foundation without SkirtBSOIL MODELRAFTLOADSKIRT15 B20 BBSOIL MODELRAFTLOADSKIRT15 B20 BFigure 2: Geometry Model of Raft Foundation with two sides vertical Skirt and one side vertical skirtIV. SOIL PROPERTIESThe properties of soil used in the analysis are as follows in Table 1.Table 1: Material Properties for studyParameter ValueType of material SandMaterial Model Hardening Soil Model
  • 3. Behaviour of Raft Foundation with Vertical Skirt Using Plaxis 2d22E50(KN/m2) 40000Dry density(KN/m3) 17Power (m) 0.5Cohesion 0.1Friction angle(ᶲ) 32°Angle of dilatancy 2°Interface reduction factor Rinter 0.67Axial stiffness for steel skirtsEA,(KN/m)31500V. REULTS AND DISSCUSSIONUsing above modelling and soil properties analysis was carried out in PLAXIS 2D for raft foundationwithout skirt, with two side and one side vertical skirt. The modelling and results obtained in PLAXIS 2D isbeing shown in Figures 3 for raft foundation without any skirt and for raft foundation with two sided and oneside vertical skirt in Figures 4 and 5 for one typical case of 10 m raft size and skirt depth.(a): Geometry Model in PLAXIS (b): Meshing(c): Deformed Mesh (d): Total Displacement Contours(e): Stresses Contours (f): Load-Displacement CurveFigure 3: Results in PLAXIS 2D for Raft Foundation without Skirt
  • 4. Behaviour of Raft Foundation with Vertical Skirt Using Plaxis 2d23(a): Total Displacement Contours (b): Stresses ContoursFigure 4: Results in PLAXIS 2D for Raft Foundation with Two Side Vertical Skirt(a): Total Displacement Contours (b): Stresses ContoursFigure 5: Results in PLAXIS 2D for Raft Foundation with One Side Vertical SkirtThe results were plotted for bearing capacity obtained from load-displacement curve. Figure 6 showsload displacement curve for 10m raft foundation with two side skirt with different skirt depth. The maximumapplied pressure is 1000 kN/m2load for which settlement is measured and at settlement 100 mm which ispermissible settlement for raft foundation, the corresponding safe bearing capacity noted. Results shown inTable 2 and Table 3 are for 10 m raft with increasing skirt depth for two sided and one sided vertical skirted raftrespectively. The increase in bearing capacity and reduction in settlement for varying raft size and skirt depthcan be seen in Figures 7 for two sides skirt raft foundation. However, for one side skirt raft foundation, bearingcapacity increases with skirt depth but settlement increases for skirt depth of 2B and remain constant as shownin Figures 8.Figure 6: Load –Settlement Curve for 10m Raft Foundation with Vertical Skirt on both side for different SkirtDepthTable 2: Results for one Typical Case of 10 m Raft and Skirt Depth for Two Side Vertical Skirt RaftFoundationDs/B 0 0.25 0.5 1 1.5 2 2.5 3Settlement for 1000 kN/m2(mm) 561 523 527 541 529 490 422 323Bearing Capacity for 100 mm Settlement(kN/m2)166 202 205 232 285 361 434 493Table 3: Results for one Typical Case of 10 m Raft for 10 m Skirt for one Sided vertical Skirt Raft Foundation
  • 5. Behaviour of Raft Foundation with Vertical Skirt Using Plaxis 2d24Ds/B 0 0.25 0.5 1 1.5 2 2.5 3Settlement for 1000 kN/m2(mm) 560 602 634 703 729 736 737 706Bearing Capacity for 100 mm Settlement(kN/m2)168 196 200 208 230 248 272 282Figure 7: Settlement and Bearing Capacity vs Ds/B for two sides vertical skirt raft foundation for different raftsizesFigure 8: Settlement and Bearing Capacity vs Ds/B graph for one side vertical skirt raft foundation for differentraft sizesVI. CONCLUSIONSConstruction of vertical skirts at the base of the footing confines the underlying soil and generates asoil resistance on skirt side. This leads to improvement in the performance of footings. From present researchwork in general, it can be concluded that Increase in skirt depth increases bearing capacity. Two sided vertical skirt raft foundation shows decrease in settlement with increase in skirt depth. One sided vertical skirt raft foundation shows increase in settlement. This increase in settlement maybe due to sliding.REFERENCES[1]. Al-Aghbari M.Y. (2007), “Settlement of Shallow Circular Foundations with Structural Skirts Restingon Sand”, the Journal of Engineering Research Vol. 4, No.1 (2007) 11-16[2]. Azzam W. R. and Farouk A. (2010), “Experimental and Numerical Studies of Sand Slopes Loadedwith Skirted Strip Footing”, Electronic Journal of Geotechnical Engineering, Vol – 15.[3]. Mahiyar H. and Patel A. N. (2000), “Analysis of Angle Shaped Footing Under Eccentric Loading”,Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol - 126, No. 12, pp. 1151- 1156.[4]. Nazir A. K and Wasim A.R (2010), “Improving the bearing capacity of footing on soft clay with sandpile with/ without skirts”, Alexandria Engineering Journal 49,371-377.[5]. Shabana S.K and Joseph M (2010), “Effect of Structural Skirt on Square Footing”http://117.211.100.42:8180/jspui/bitstream/123456789/1063/1/CEGE10.pdf

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