International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volu...
Upcoming SlideShare
Loading in …5
×

The effect of soil improvement on foundation super structure design

1,245 views

Published on

Published in: Technology
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
1,245
On SlideShare
0
From Embeds
0
Number of Embeds
3
Actions
Shares
0
Downloads
57
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

The effect of soil improvement on foundation super structure design

  1. 1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME258THE EFFECT OF SOIL IMPROVEMENT ON FOUNDATION & SUPERSTRUCTURE DESIGNThulaseedharan V1, Narayanan S.P21(Department of Civil Engineering, Government Engineering College,Thrissur, Kerala, India)2(Department of Civil Engineering, University Technology PETRONAS, Malaysia)ABSTRACTFor a particular arrangement of superstructure, soil strength is the most important factoraffecting the design of its supporting raft and hence the cost of construction. Folding of raftor folded plate foundations can be used to reduce the material consumption in a raft design.The beneficial effects to foundation and superstructure design by the selective soilimprovement below a raft or folded plate foundation were studied in this paper with the helpof Winkler and continuum methods of soil modelling.Key words: Raft foundation, folded plate foundation, Winkler model, coefficient of subgradereaction, continuum analysis, Mohr-Coulomb model, soil improvement.1.0 INTRODUCTION1.1 OverviewRaft foundations are provided in situations where soil is not that weak to need pilefoundations but isolated footings cannot be recommended due to the possible higherdifferential settlements. The design of a raft depends on the stiffness of superstructure (SS),column spacing, and projection of raft beyond the outer lines of boundary columns (PR), raftthickness, soil stiffness, strength of concrete and yield strength of steel. Among the variousparameters listed above, strength of concrete is decided considering the site specific needsand difficulties in achieving the maximum strength of concrete. Column spacing is decidedby the architectural needs. The SS and substructure are provided with sufficient sizes to givea safe and economic structure. The soil related term is the least controllable among the above.In the conventional method of raft design, a constant bearing pressure is assumed below thefoundation which is possible only in the case of very soft soils almost in a fluid state. Whenanalyzed using commercial software, the soil media may be represented by a system ofINTERNATIONAL JOURNAL OF CIVIL ENGINEERING ANDTECHNOLOGY (IJCIET)ISSN 0976 – 6308 (Print)ISSN 0976 – 6316(Online)Volume 4, Issue 2, March - April (2013), pp. 258-269© IAEME: www.iaeme.com/ijciet.aspJournal Impact Factor (2013): 5.3277 (Calculated by GISI)www.jifactor.comIJCIET© IAEME
  2. 2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME259identical but mutually independent, closely spaced, discrete and linearly elastic springs andthis method is based on Winkler’s hypothesis (1867). The elastic constant of the springs isobtained from the modulus of subgrade reaction, ks and is defined asks =௤ఋ(1)Where q is the load per unit area (or contact pressure) and is the settlement under theloaded area. The base pressure below a raft may vary from point to point depending on theload and moment, rigidity of SS and type of soil. As load is applied, the raft settles and thecontact pressure under the raft is re-distributed depending upon the stiffness of foundationand SS. The settlement of the raft also depends on factors such as the increase in stress-strainModulus (E) with depth of soil below raft and the consolidation of soil. Since ks is a variabledepending on the several factors listed above, its computation is very difficult. The commonmethods used for the determination of ks are Plate load test, Triaxial Test, Consolidation Test,CBR test and Empirical relations (Bowles, 1997). Plate load tests are conducted using smallplates of size varying from 30 cm to 76 cm. The stress increase in the soil due to loading onthe plate is felt over a small area whereas that under a raft influences a large area and henceTerzhagi (1955) suggested an expression for correction for size effect of foundation. Bowles(1997) suggested the ranges for ks for sandy soil as given in Table 1. Scott (1981) proposedempirical expression connecting ks with standard penetration resistance (N) for sandy soils.The simplifying assumptions of Winkler model itself cause some errors (Terzhagi, 1955).The springs are neither elastic nor independent. The settlement due to the applied load at onepoint in the raft is felt at the adjacent areas and hence a uniformly loaded raft may exhibit adish shaped settlement, unlike the uniform settlement predicted by Winkler (Coduto, 2001).Hence efforts were made to couple the springs so that the effect of vertical load is transmittedin the lateral direction also (Horvath, 2011). In continuum methods the soil media isrepresented by 3D finite elements. However continuum analysis is time consuming andgetting the representative soil properties for assigning to the model is very difficult. Incontrast, the Winkler foundation is very simple and large numbers of software are availablebased on this method, capable of analysis and design of rafts meeting different countryspecific codes of practices. The difficulty in computing ks led ACI committee 336 (1988) torecommend that the raft designs be carried out varying the value of ks over a range of onehalf to 5 or 10 times the furnished value. The furnished value in a soil report is hereaftercalled the designated value of ks.Table 1 Values of ks for sandy soils. Table 2 Stress –Strain Modulus (Bowles, 1997)Thulaseedharan and Narayanan (2013a) studied the impact of varying ks in the designof raft foundations. The maximum settlement and the maximum values of top and bottombending moments (BM) in a raft generally reduce as ks increases. The comparisons ofType of Soil ks in kN/m2/mLoose Sand 4800-16000Medium Dense Sand 9600-80000Dense Sand 64000-128000Clayey Medium DenseSand32000-80000Type of soil E in kN/m2Sand-Silty 5000 to 20000Sand-Loose 10000 to 25000Sand Dense 50000 to 81000
  3. 3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME260Winkler and Continuum methods showed slightly lower maximum top moments and moremaximum bottom moments in the later method. It was also concluded that ks based methodsare sufficient for design purpose of flat rafts. For unsymmetrical rafts the design for a rangeof ks varying from half to four times the reported value for site resulted in an increase of 2 to2.5 % in the total reinforcement required. This was further validated using continuummethod. For symmetrical rafts, the increase in reinforcement was around 1% at bottom withslight reduction in top reinforcement. Designing the raft for such a range of ks or E valuesgives a conservative design at a small additional expense. The raft models incorporatingsuperstructure showed lower settlement values compared to the models withoutsuperstructure implying that the superstructure stiffness helps to re-distribute base pressure.PR is a possible variable which may be restricted depending on the site conditions.Calculations were carried out on rafts with varying PR from zero to 1.5 m. The settlementpattern changes with PR and a dish like settlement (Central part of raft settling more) wasobserved for nearly 1.5 m PR in Winkler method. With continuum modeling using Plaxis 3D,a dish shaped settlement of raft was observed for low values of PR of nearly 0.25 m. As soilstrength is reduced, the settlement pattern changed with more deflection at the edges andcorners of the raft for the same PR. An increase in PR reduces the maximum top moments inthe raft- which usually occurs in the outer-spans of raft. The effect of top moments in a raft isfelt over a large area and hence requires reinforcement to be given for a greater area.Generally the design bending moments at bottom at the face of columns are much more thanthe top moments. However the bottom moments reduces to minimum value within a smallarea around the load transfer area of column and rest of the raft in the bottom portion is givenonly nominal reinforcement for satisfying crack width requirements. In general the settlementcomputations by Winkler method are not comparable to continuum methods, especially whenthe soil is soft. Hassan (2011) studied the variation of raft deflection with ks at variouslocations in a raft and the influence of column spacing and raft thickness on settlement ofraft. In the current research, column positions are fixed and raft thickness is to be reduced asmuch as possible. Hence these aspects are not given much importance. Gupta (1997)compared the analysis results of rafts using conventional method and Winkler foundationsand concluded that the former is generally on the conservative side. He also reported thatbending moment in a raft may vary several times depending upon the raft size and soilproperties under the raft. This variation increases further as the deviation from symmetry ofthe shape or loading of the raft increases. However a variation from 10 kNm to 70 kNm inBending Moment (BM) in a raft is an increase by 7 times which may not require an increasein member size or reinforcement. We are more interested in finding out the change requiredin the size of member or area of steel.Folded plates are widely used in SS for spanning large areas. Due to its folding,bending moments are reduced, which reduces the required concrete and reinforcement.Folding is done in straight lines and form work can be placed very easily. In foundations, ifthe folding is done in such a way that steep slopes are not provided, then form work can beavoided. The construction of folded rafts are easy compared to beam and slab rafts and theadditional space created at the basement level by folding a raft can be used for storage ofwater or for using as cable trenches. Hanna and Rahman (1990) investigated on thegeotechnical aspects of triangular strip footings and concluded that there is 40 % increase inbearing capacity when such structures are used as foundations with consequent reduction insettlement. Thulaseedharan and Narayanan (2013b), compared raft and folded platefoundations, giving importance to material savings that could achived by using the later. The
  4. 4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME261design was arrived after conducting analysis for a wide range of soil properties in order to geta conservative design of both raft and folded plate foundation. Nearly 40% savings inconcrete, 30 % savings in steel and overall 35% savings in cost of construction could beachieved by folding the raft. Between folded and flat rafts, folded raft structure requiresslightly lower reinforcement for columns and beams. In continuum method, the settlementsunder corner and edge columns were more in folded rafts compared to flat rafts, especiallywhen the raft projections were low. This is found to be due to the heavy lateral loads at thosesupports and due to which folded raft was deflecting in the fold direction. This increases thereinforcement in the outer spans of folded raft and in the corner and edge columns at theground floor of the building. BM in the outer beams of ground floor also showed someincrease compared to flat raft. As E of soil increases, the BM in raft, column reinforcementand BM in beams reduces. The increase in PR brings significant changes to top momentvalues in the raft. Column and beam reinforcement were slightly reduced as PR increases. Italso reduces reinforcement required and settlement of both flat and folded rafts. In generalWinkler method is sufficient for the design of flat rafts and in the case of folded plates, lateralstiffness of soil need to be considered in the analysis. The column designs obtained by givingfixed supports to the columns and sometimes even that obtained from Winkler method maynot give a conservative design pointing the need for elaborate soil structure interactionstudies including continuum analysis for important structures. The designs of folded platesare affected by the central rise or fall of fold.(In foundations, the folded part is going belowground level and hence the term fall is used). With increase in the fall, reinforcement requiredcan be further reduced. As the fall of the folded raft increases, settlement reduces due to theincrease in stiffness of foundation. Reinforcement required in folded raft also reduces.Similarly column and beam reinforcement in the superstructure was slightly lower comparedto one with less fold height.1.2 Objectives of studyThe present paper compares the performance of folded plate and raft foundation underidentical SS stiffness and loading when soil strength parameters are varied below it atdifferent locations. The positions of maximum settlement in a raft were identified usingWinkler and continuum methods and soil strength was then increased at those places.Numerically, this strengthening was accounted with increase in ks and E values. Winklermethod is very popular and easy to use for soil modeling and its inherent disadvantages weretaken care in this study with the help of continuum methods. There is uncertainty in thedetermination of ks and E values and hence the impact of its variation over a range infoundation and SS design were also studied.1.3 Importance of the studyThe increased soil stiffness reduces settlement of raft and with several other benefitsto substructure and superstructure. The localized improvement in soil strength (LISS) canreduce differential settlements along with considerable reduction in material requirements ofsteel and concrete. Folded plate foundations are provided to reduce material consumption andlocalized strengthening of soil may further improve the advantages.
  5. 5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME2622.0 MODELING AND STRUCTURAL IDEALIZATION2.1 Winkler and continuum modeling:StaadPro (2008) and SAFE (2009) were used to model the Winkler Foundation. Theks of the proposed site is given as 25000 kN/m2/m and this value is hereafter referred to asdesignated ks. The analysis was carried out with and without the SS. The SS was modeledusing line and plate elements and the substructure with quadrilateral plates. Several analyseswere carried out taking typical situations. All designs were carried out using BS 8110 andBS 8007. Continuum modeling was carried out using Abaqus (2011), Plaxis 3D (2004) andStaadPro (2008) . Solid elements were used to model the soil mass. In Abaqus and Plaxis,interface option is possible, where as in StaadPro, rough contact is assumed. In StaadPromodelling, only E is varied. Mohr- Coulomb model was used for the study purpose in Plaxisand Abaqus as the present comparison is made for a site containing sandy strata. Mohr-Coulomb model in Plaxis requires 5 parameters as input namely the Cohesion (C), the angleof Internal Friction ( θ), the Modulus of Elasticity (E), the dilatancy and the Poisson’s ratio.Here the exact evaluation of E is very difficult and hence a range of values are takenconsistent with the known soil properties. In this study the E values were varied from 15000to 60000 kN/m2and from 30 to 43 degrees. A very small value of cohesion is given to aidcomputation as recommended in Plaxis 3D foundation user manual (2004). Therecommended range of variation of E values is given in Table 2 (Bowles, 1997). Dilatancyvalue is given as zero and the Poisson’s ratio was varied from 0.35 to 0.4. Abaqus was usedonly for comparing the results obtained from the structural software Staad for the foldedplate.The comparison of designs were done between flat slab raft and folded raft. The raftswere analyzed for 43 service load combinations and 53 ultimate load combinations. M40concrete and Steel of grade 460 was used. The comparison was made on a raft consisting of 4equal spans of 8 m in the X direction and 7m in the Y direction. Two projections of raft in Xdirection, 0.3m and 1m were considered in the present studies. Raft projection in the Ydirection is kept at 0.3 m. A 3D view of the folded raft is given in Fig. 1. Folding isintroduced in the X direction in such a way that the inclination of the surface is less than 32degrees. The building is seven storied with column sizes of 600x600 mm. Seismic forces aregenerated as per UBC (1997) for zone 2A and soil classification Sc as per the siterequirements.3.0 ANALYSIS AND DESIGN RESULTSThe flat and folded raft models were analyzed using Winkler soil model with 3 valuesof ks ( 12500, 25000 50000 kN/m2/m). The continuum analysis was carried out in two steps.In one step, E is the only variable with values of 15000, 30000 and 60000 kN/ m2. In thesecond case, E and were varied for analysis with Mohr-Coulomb model. To study theeffect of soil improvement, ks or E values are varied at select locations below the raft; theincrease was 4 times in ks or E values compared to the adjacent area. The impact of the sameon structural design was worked out. Analysis and design results are given in eight sub-parts.Variation of settlement below raft, base pressure, BM in rafts, Shear and Impact of raftprojections are covered in the first five parts. Part 6, contains the design of raft and foldedplates. Part 7 deals with the impact of soil stiffness on SS design and the eighth part is acomparison of material and cost savings. In each section, a comparison on the performance offlat and folded plate foundations are given for both Winkler and continuum modeling.
  6. 6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME263Fig.1 View of the raft folded in X- direction.3.1 Comparison of settlementThe maximum settlement of the foundation was reduced as ks or E increased in bothWinkler and continuum methods. When PR is considerable, the raft may settle in a dishshape with more deflection at centre. With LISS, the settlement pattern is changing inaddition to reduction in settlement. Without any increase in PR, the location of maximumsettlement of raft shifts to centre from the edges.
  7. 7. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME264Fig.2 Variation of maximum settlement below flat and folded rafts in continuum method withchange in E for LISSIn Fig 2, the maximum settlement is more below folded raft in continuum method comparedto flat raft. This is due to the very high lateral forces applied and consequent lateraldisplacement at the supports of corner and edge columns in X direction (Fig.1). The LISSchanges the settlement pattern of the folded raft. Even for low PR values, dish like settlementpattern is obtained. The maximum settlement is reduced as shown in fig.2.3.2. Variation of base pressure below the raft and the folded platesBase pressure below the raft was more under the column load transfer area. Forsmaller PR values, the maximum base pressure occurred under corner and edge columns. AsPR increases, the area of raft increases and maximum base pressure below the raft is reducedsignificantly. At interior columns, the base pressure under column load transfer point andadjacent areas were almost uniform for low values of ks. As ks increases, the differencebetween maximum and minimum base pressure adjacent to the load transfer area alsoincreased. The increase in ks leads to load transfer through a small concentrated area belowload point. The same trend was observed below the folded rafts with much less variationbetween the maximum and minimum base pressure anywhere. This may be due to the higherstiffness of folded raft re-distributing base pressure. Continuum modeling also gave similarresults for both flat and folded rafts. LISS results in very high base pressure at the soilimproved area and the values go on reducing as the ks or E in the central area increases.3.3 Variation of maximum design momentsThe maximum value of bottom moments at the face of columns in the raft is taken asthe design bottom moment in a raft. As ks or E increases, the bottom and top moments arereduced (Fig. 3a and 3b). With LISS, for the flat raft, considerable reduction in maximumtop moment occurs along with increase in bottom moments for low E values. For folded raftswith LISS at the boundary area, maximum top moments were slightly increased in the outermost fold portion compared to the case without LISS as shown in fig.3 (a). Continuummethods showed a reduction of 67% for the maximum top moment in X-direction comparedto flat raft. Bottom moment (Mx) was reduced by 50%.024681012141618200 10000 20000 30000 40000 50000 60000 70000SettlementinmmYoungs Modulus E in kN/m2Flat RaftFolded RaftFlat Raft with SoilimprovementFolded Raft withSoil improvement
  8. 8. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME265Fig.3 a and 3b Variation of top and bottom maximum BM in folded and flat rafts usingcontinuum methodsThe top bending moment Mx in the fold portion was reduced by 80% and goes onreducing with increase in fall of the folded portion. BM in y direction is concentrated in anarrow width of folded raft supporting the columns and for the rest of the area BM wasreduced by 90%.The increase in fall of the folded portion increases the stiffness and reducessettlement and which in turn reduces BM and hence the reinforcement needed in a raft.3.4 Influence of shearThe thickness of the folded raft is increased for a small cross-section supporting thecolumns as shown in Fig. 1. No other special care was required in comparison to the flat rafts.3.5. Impact of projections of the raft (PR) beyond the outer line of boundary columnsBy providing projections to the raft, some other advantages were also observed. Therewas a reduction in the total and differential settlements with change in deflection pattern andreduction in reinforcement in the substructure and SS. The studies on folded rafts withcontinuum model also gave similar results. If PR could be increased, then LISS is not01002003004005006007008009000 10000 20000 30000 40000 50000 60000 70000TopmomentinkNmYoungs Modulus E in kN/m2Flat RaftFolded RaftFlat Raft with SoilimprovementFolded Raft with Soilimprovement02004006008001000120014000 10000 20000 30000 40000 50000 60000 70000BottommomentinkNmYoungs Modulus E in kN/m2Flat RaftFolded RaftFlat Raft withSoilimprovementFolded Raftwith Soilimprovement
  9. 9. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME266required at the boundary area. However if the central area of the raft is subjected to moresettlement, then LISS at that area is beneficial.3.6 Design of Flat and folded RaftsMinimum reinforcement was decided considering the crack width limitations. For 900mm raft the value of this moment is 375 KNm for Service load in main direction and 325kNm for secondary moment for a reinforcement of 20mm@ 200 mm c/c, for a crack width of0.3mm. Similarly the other ranges are worked out for different diameters of extra bars to beprovided like 20@200 mm c/c, 25@ 200 mm c/c etc. After finding out ranges of BM fordifferent combinations of reinforcements, the raft BM at different locations were grouped into these ranges and reinforcement was provided accordingly. Then the raft was investigatedfor one way shear and punching shear. At few locations, the bottom reinforcement isincreased to give additional shear capacity for avoiding shear links. After completing thereinforcement design, the raft was further analyzed changing the ks values to 12500KN/sqm/m and locations where reinforcement requires modification were identified. Thenanalysis was repeated after varying the ks to 50000 Kn/m 2/m and design was reviewed andreinforcement detailing done incorporating all the cases of maximum moments. Afterworking out the total quantity the design was checked for continuum modeling. There was noincrease in reinforcement required for variation in ks, may be due to the symmetry of thestructure. Checking using Continuum method resulted in an increase of 1% reinforcement inthe bottom area. There was a decrease in reinforcement in the top area and was neglected.For folded rafts, the same design procedure was followed, though the reinforcement requiredand ranges of moments were different. The thickness of folded portion is 350mm and it ismore at column supports. The folded portion was subjected to much less moments and hencemuch less thickness and reinforcement were needed. The impact of varying ks values fromhalf to two times the designated value was found to be insignificant as far as structural designwas concerned. However continuum modeling required more reinforcement upto 9%. Thisshowed the need for modifying the Winkler Method giving stiffness to soil in the lateraldirections also. With LISS, there was a small increase in reinforcement required at the bottomand reduction in reinforcement required for the top in the case of flat raft, the later beingmuch more. In the case of folded raft, the top reinforcement needed increased in the outerspans of fold area and reduced at several other places at the bottom. The net effect was thatthere was no significant savings in reinforcement for the folded raft but other advantageswere there in superstructure design along with reduction in raft settlement.3.7 Impact on SS design.For Winkler model, it is observed that the column reinforcement decreases withincreasing ks values for both flat and folded rafts. Continuum analysis of flat rafts alsoshowed similar results. With soil improvement there is further reduction in columnreinforcements as shown in Fig. 4a. Buildings supported on folded raft required morereinforcement for edge and corner columns at Ground floor (Fig.4b). The figures 4 and 5 alsocontain comparisons with cases involving fixed end condition to columns. For the interiorcolumns, reinforcement required was slightly lower compared to building with flat raftfoundation, for all floors. LISS at the boundary reduces column reinforcements as shown inFig. 4a and 4b for both flat and folded plate foundations. In general beam bending momentsare also reduced with soil improvement with occasional local variations as shown in Fig 5aand 5b.
  10. 10. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME267(a) Winkler Method(b) Continuum MethodFig. 4. Variation in corner column reinforcement with soil strength(a) Winkler Method15002000250030003500400045000 20000 40000 60000AreaofSteelinmm2Modulus of Subgrade reaction in kN/m2/mFlat Raft withoutsoil improvementFlat Raft with soilimprovementFixed Support15002000250030003500400045000 20000 40000 60000 80000AreaofSteelinmm2Youngs Modulus in kN/m2Flat Raft without SoilimprovementFolded Raft without soilimprovementFlat Raft with SoilimprovementFolded Raft with soilimprovement5405505605705805900 20000 40000 60000Max.MomentinbeaminkNmModulus of subgrade reaction in kN/m2/mFlat Raft without soilimprovementFlat Raft with soilimprovementFixed Support
  11. 11. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME268(b) Continuum MethodFig.5. the variations in a GF beam BM in Winkler and continuum method.3.8. Comparison of material consumption and cost of execution.For a flat raft, LISS reduces the maximum top moment and hence reinforcementconsiderably and it more than compensates the small increase in bottom reinforcement. There is asmall reduction in BM of the beams for all floors of the building with LISS and the variation wasvery small and may not affect design. For folded rafts, the top BM in the outer spans wereslightly increased in fold direction. In superstructure design, there was an overall reduction inbeam moments and column reinforcements. If the cost of LISS is not substantial, considering theoverall advantages- especially in settlement reduction- it is a better option.4.0 CONCLUSIONSA raft and folded plate foundation were designed for varying ks values from half totwo times the designated values. Soil improvement was done at the boundary of the raftbelow columns. For flat rafts, the bottom maximum BM increased and the top maximum BMreduced in the case of LISS compared to the case without soil improvement. Continuummethods showed similarity in the BM values obtained with those computed using ks withslightly higher bottom moments and lower top moments. The top reinforcement requiredreduces and the decrease is much more than the increase in bottom reinforcement. Comparedto the design made for the superstructure of flat raft without soil improvement below it,column reinforcement required reduces with LISS. Similarly the bending moments arereduced for the beams throughout the building due to LISS, the variation being very small.LISS also helps to reduce the total and differential settlements. Between folded and flat rafts,folded raft structure requires slightly lower reinforcement for columns and beams. WithLISS, the reinforcement required further reduces in both cases. In continuum method, thesettlements under corner and edge columns were more in folded rafts compared to flat rafts,especially when the raft projections were low. This is found to be due to the heavy lateralloads at those supports and due to which folded raft was deflecting in the fold direction. Thisincreases the reinforcement in the outer spans of folded raft and in the corner and edgecolumns of ground floor. Hence Winkler methods may not give conservative results as far ascolumn designs are concerned. The effect of LISS reduces such local high values ofsettlement and the increase in column reinforcement. In folded rafts, slight increase in top andbottom reinforcement was needed with LISS at certain areas and there was a reduction insome other areas. Top reinforcement increases for the outer spans and bottom reinforcement5255305355405455505555600 20000 40000 60000 80000Max.MomentinbeaminkNmYoungs Modulus in kN/m2Flat Raft without soilimprovementFolded Raft without soilimprovementFlat Raft with soilimprovementFolded Raft with soilimprovementFixed Support
  12. 12. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME269in the central area. Overall there was not much change in the reinforcement required. BM in theouter beams of ground floor also showed some increase in BM compared to that with the flat raft.As E of soil increases, the BM in raft, column reinforcement and BM in beams reduces. Atlocations where settlement of the raft is computed to be more, with LISS, there are severaladvantages in the overall performance of sub-structure and superstructure. However, the costsavings depends on the cost of LISS and may not be substantial.REFERENCES[1] Winkler. E, (1867), Die Lehre von Elastizitat und Festigkeit (On Elasticity and Fixity),Prague, p.182.[2] Bowles, J.E (1997), Foundation Analysis and Design, 5th Edition, Mc GrawHillInternational Edition, Singapore.[3] Terzaghi, K. (1955), “Evaluation of Coefficients of subgrade reaction”, Geotechnique, 4,297-326.[4] Scott, R.F. (1981), Foundation analysis, Prentice Hall Inc.[5] Coduto, D. P (2001), Foundation design: principles and practices, Prentice Hall Pvt Ltd,pp.352-369.[6] Horvath, J.S and Colasanti R.J (2011), “Practical Subgrade Model for Improved Soil-Structure Interaction Analysis: Model Development”, International Journal of Geomechanics, 11,1, February 1.[7] ACI Committee 336.2R (1988), Suggested design procedure for Combined Footings andMats, Detroit, pp-13.[8] Thulaseedharan, V. and Narayanan, S.P (2013a), “Impact of coefficient of subgradereaction on raft foundation design”, International Journal of Applied Engineering Research,8, 2, pp. 187-201 © Research India Publication. ISSN 0973-4562[9] Plaxis 3D Foundation User Manual, 2004 PLAXIS bv, Netherlands.[10] Hassan, I.M. (2011), “Influence of structural and soil parameters on Mat deflection”,International Journal of Civil and Structural Engineering, 2, 1.[11] Gupta, S.C. (1997), Raft Foundation Design and Analysis with A Practical Approach,New Age International, New Delhi.[12] Hanna A and Mohamed Abd El- Rahman (1990), “Ultimate bearing capacity of triangularshell strip footings on sand”, Journal of Geotechnical Engineering, ASCE, 116, 12.[13] Thulaseedharan, V. and Narayanan, S.P (2013b), “Comparison of Raft and folded PlateFoundations”, International Journal of Applied Engineering Research, (accepted forpublication) © Research India Publication. ISSN 0973-4562[14] StaadPro V8i, 2008, Technical references, www.bentley.com[15] SAFE v12, Analysis reference Manual, 2009, Computers & structures, Inc., Berkeley.[16] BS 8110 Part 1 (1997), Structural Use of Concrete: Code practice for design andconstruction.[17] BS 8007 (1987), Code of Practice for design of concrete structures for retainingaqueous liquids.[18] Abaqus User Manual (2011), Dassault Systems Simulia Corporation, Providence, RI,USA.[19] UBC 1997, Uniform Building Code, International Council of Building Officials.[20] M. Alhassan and I. L. Boiko, “Effect Of Vertical Cross-Sectional Shape Of FoundationAnd Soil Reinforcement On Settlement And Bearing Capacity Of Soils” InternationalJournal Of Civil Engineering & Technology (IJCIET) Volume 4,Issue 2,2013,Pp: 80 –88, Issn Print : 0976 – 6308, Issn Online : 0976 – 6316.

×