DESIGN OF A PILE GROUP BY USING SOIL PROPERTIES

Transcription

1 1 DESIGN OF A PILE GROUP BY USING SOIL PROPERTIES D.SATYANARAYANA 1, S.CHANDRAKALA 2 1 CIVIL DEPARTMENT. 2 CIVIL DEPARTMENT. (SRI VASAVI ENGINEERING COLLEGE PEDATADEPALLI, W.G.Dt. ANDHRA PRADESDH PINCODE ) Piles are used to transfer the load beyond the zone of ossible moisture changes in collasible soils. INTRODUCTION A ile foundation is a slender structural member made of steel, concrete or wood, which is either driven into the soil or formed in-situ by excavating a hole and filling it with concrete. 1 [email protected], 2 [email protected] Abstract The silty clay soil is a tye of soil which is not caable of suorting a structure, dee foundations are required to transfer the loads. The most common tyes of dee foundations are Piles, Piers and Caissons. By using the soil roerties such as Liquid limit, Saturated density, Otimum moisture content etc,. The ile foundation can be designed. A ile is a slender structural member made of steel, concrete or wood, which is either driven into the soil or formed in-situ by excavating a hole and filling it with concrete. Alication of ile foundation: If the lan of the structure is irregular relative to its outline and load distribution, it can be used to reduce differential settlement. It can resist horizontal forces in addition to suort vertical loads for earthquake rone areas & tall buildings. Piles are used for foundations of some structures such as, transmission towers, off-shore latforms which are subjected to ulift. CLASSIFICATION OF PILE FOUNDATION: Based on the function: End bearing ile Tension or ulift ile Comaction ile Fender ile and dolhins Anchor ile Friction ile Batter ile Based on materials and comosition: Concrete ile Timber ile Steel ile Comosite ile REVIEW OF LITERATURE Pile Foundation: Pile is one of the tyes of dee foundations, which are used when surface soil is unsuitable for shallow foundation. A ile is a slender member made of steel, concrete or wood. It is either driven into soil or formed in-situ by excavating a hole and filling it with

2 2 concrete. These are mostly used for the foundations in case of exansive soils such as black cotton soil and collasible soils such as loess. Design of ile grou Most of the ile foundations consist not of a single ile, but of a grou of iles, Which act in the double role of reinforcing the soil, and also of carrying the alied Load down to deeer, stronger soil strata failure of the grou may occur either by the failure of individual ile or as failure of the overall block of soil the suorting Caacity of grou of vertically loaded iles can, in many cases, be considerably less than the sum of the caacities the individual iles comrise the grou. Grou action in iled foundation could result in failure or excessive settlement' even though loading tests made on a single ile have indicated satisfactory caacity in all cases the elastic and consolidation settlements of the grou are greater than those of single ile carrying the same working load as that on each ile within the grou. This is because the zone of soil or rock which is stressed by the entire grou extended to a much greater width and deth than the zone beneath the single ile. Necessity of ile grous: Pile grous are used when l) Column load is heavy 2) Method of installation of iles is by driving. Classification of ile grous: Pile grous are classified as: l) Free standing ile grou 2) Piled foundation Free standing ile grou: These are used where the foundation soil is exansive in Nature ile ca does not transfer any of the column loads directly to foundation soil. Piled foundation: As ile ca is made to rest on the ground surface, it hels in transfer of art of column load directly to foundation soil these are use in exansive soils.

3 3 Pile sacing and ile arrangement: In certain tyes of soil, esecially in sensitive clays, the caacity of individual Piles within the closely saced grou may be lower than for equivalent isolated ile. However, because of its insignificant effect, this may be ignored in design. Instead the main worry has been that the block caacity of the grou may be less than the sum of the individual iles caacities. As a thumb rule, if sacing is more than 2-3 ile diameters, then block failure is most unlikely it is vital imortance that ile grou in friction and cohesive soil arranged that even distribution of load in greater area is achieved. Large concentration of iles under the center of the ile ca should be avoided. This could lead to load concentration resulting in local settlement and failure in the ile ca. Varying length of iles in the same ile grou may have similar effect for ile load u to 300kn; the minimum distance to the ile ca should be 100mm. For higher than 300kn, this distance should be more than 150 mm. In general, the following formula may be used in ile sacing: As er IS CODE End-bearing: S = 2.5 d Friction iles: S = 3.0 d Piles in loose sand: S = 2.0 d Where: d = assumed ile diameter s = ile centre to centre distance (sacing) Pile grous in cohesion less soils: For driven iles embedded in cohesion less soils, the caacity of the large Equivalent ile (block) will be almost always greater than the sum of the caacities of individual iles, in view of the densification that occurs during driving. Consequently, for design, the grou caacity is taken as the sum of the individual ile caacities or the roduct of the number of iles in the grou and the caacity of the individual ile. This rocedure is not alicable, if the ile tis rest on Comressible soils such as clays; in such cases, the ile grou caacity is governed by the shear strength and comressibility of clay soil, rather than on the Characteristics of the cohesion less soil. Bored iles or cast-in-situ concrete iles are constructed by boring a hole of required diameter and deth and ouring in of concrete. Boring is accomanied invariably by some degree of loosening of the soil. In view of this, the grou Caacity of such iles will be somewhat less than the sum of individual ile Caacities tyically-about two-thirds of it. It may also be taken as the sum of Individual ile caacities aroximately. Pile grous in cohesive soils: When iles are driven into clay soils, there will be considerable re-moulding esecially when the soil is soft and sensitive. The soil between the iles may also have since comaction cannot be easily achieved in soils of such low ermeability. Bored iles are generally referred to driven iles in such soils.

4 4 However, if driven iles are to be used, sacing of iles must be relatively large and the driving so adjusted as to minimize the develoment of ore ressure. The mode of failure of ile grous in cohesive soils deends rimarily uon the sacing of iles. For smaller sacing s, 'block failure' may occur, in other words, the grou caacity as a block will be less than the sum of individual ile caacities. For larger sacing s, failure of individual iles may occur; or, it is to say that the grou caacity is given by the sum of the individual ile caacities, which will be smaller than the strength of the grou acting as a unit or a block. The limiting value of the sacing for which the grou caacities obtained from the two criteria-block failure and individual ile failure- are equal is usually considered to be about 3 iles diameters. Settlement of Grous of Piles: The comutation of settlement of grous of iles is more comlex than that for a Single ile. Settlement of ile grous in sands Settlements of ile grous are found to be many times that of a single ile. The ratio, F g, of the settlement of a ile grou to that of a single ile is known as the grou settlement ratio. F g = S g /S Where F g = grou settlement ratio, S g = settlement of ile grou. and S = total settlement of individual ile Vesic has obtained the relation between F g and B/d, where B is the width of the ile grou (centre to centre of outer most iles), and d is the diameter of the ile (Only ile grous,square in lan, are considered). These results have been obtained for medium dense sand. For sands with other Density indices the results could be different.

5 5 Settlement of ile grous in clay The equation for consolidation settlement may be used treating the ile grou as a block or unit. The increase in stress is to be evaluated aroriately under the influence of the load on the ile grou. When the iles are embedded in a uniform soil (friction and end-bearing iles), the total load is assumed to act at a deth equal to two-thirds the ile length. Conventional settlement analysis rocedures assuming the Boussinesq or Westergaard stress distribution are then alied to comute the consolidation settlement of the soil beneath the ile ti. When the iles are resting on a firmer stratum than the overlying soil (end-bearing iles), the total load is assumed to act at the ile ti itself. If the iles are embedded into the firmer layer in this case, the load is assumed to be transmitted to a deth equal to two-thirds of the embedment from the to of the firmer layer. The rest of the settlement analysis rocedure is alicable. The total ressure may be assumed to be distributed on a sloe of 2 vertical to 1 Horizontal, for the urose of comutation of increment of stress, in an aroximate manner. LABORATORY TESTS AND RESULTS Characteristics of soil considered in Pile foundation design: For design the of ile foundation we considered some of the roerties of soil such as Liquid limit Plastic limit Otimum moisture content Co-efficient of cohesion E o value To find the characteristics of soil we have done some tests of the soil. They are Liquid Limit: We have conducted Casagrande s liquid limit test for a samle of soil.

6 6 Comaction factor: From comaction factor test we can obtain the values of Otimum moisture content, Bulk density, Dry density,d of the soil. We have obtained a liquid limit of 25%from the test. Plastic Limit: Plastic limit is the water content below which the soil stos behaving as a lastic material. The values obtained from comaction factor test are: OMC = 8% Bulk density, = Dry density,d = Tye of soil: The lastic limit of soil obtained is 20%. According to the lasticity chart, corresonding to the values of liquid limit

7 7 =25% and lastic limit = 20%, the soil is inorganic clay with medium lasticity. Name of the exeriment Liquid limit test Plastic limit test Comaction factor test Terminology Liquid limit 25% Plastic limit 20% OMC,, 8%, Result ,

8 8 DESIGN A ile grou to be installed in a saturated silt 2 clay deosit [ C U 40KN / M, L.L = 2.5%, G = 2.7, NMC = 8%, d 24.28Kg / M 26.22Kg / M 3 is designed and estimated 3 A P 40 9 A P Area of the shaft that effective in develoing in skin frictio the settlement of ile grous assuming water table at 2m deth below ground level is erformed. For iles in saturated silt clay C U 40KN / M 2 Assume dia. Of ile = 40 mm A A P 2 d For u 0 A P = M 2 N C 9 40 X 9 X N q KN The ultimate load caacity of the ile grou according to static formula can be calculated as s C a X A s u u s s Ultimate failure load Load carrying caacity of ile Shaft resistance develoed by friction between the ile and shaft Ca Skin friction develoed between the ile surface and clay ' C a c Addition factor = 0.3 (at 6 meters deth) ' c Average cohesion along the length of the shaft ( 50 KN/M 2 ) CN c A P Consider c ' 40 KN/M 2 C = cohesion co-efficient of clay A s = area of the shaft N C Bearing caacity factor for dee foundation A S 2 r l [deends on B D ratio, varies from 6 to 9 ] A S

9 9 A S 7.539m 0.3 x 40 x s 2 A x b g = 6.4 x 4.9 = m 2 s u KN P S F. S. O F. S A X s g = 4 x 6.4 x 6 = m 2 X f g = (360 x 31.36) + (40 x 1536) u u KN Assumed load 1500KN Number of iles required = No. of iles required = 20 = load U Ultimate carrying caacity of 20 iles acting individually = 20 x KN = KN Safe load = ultimate load / factor of safety = / 2.5 = KN Hence safe load from the above two criteria is KN which > 1500 KN The allowable load carrying caacity of ile from block failure criteria is KN Hence individual block failure governs the design Minimum sacing of iles = 3 X d = 3 x = 1.2 m Adot 1.5 m as sacing bit ween iles For iles assuming as single block fg q f A b g C u g A sg Where q f q C u q f 9 40 q f 360KN / M 2

10 10 Design of ile ca: Consider size of ile ca = 7.5 X 6.0 m Bending movement at face of column = (5 x 1500)/20 + ( )/6 = KN-M Factored bending movement = x 1.5 = KN-M Required deth, Bending movement = f ck bd x 10 x 6 = x 20 x 7500 x d 2 d 2 = (142.5 x 10 6 )/ d 2 = But adot, d = 82.9mm Overall deth = 500mm AREA OF TENSION STEEL: Bending moment BM, = 0.87fyAstd = = 0 = Assuming 20mm diameter bar, Area of each bar a st = 2 = 02 = = say 4 nos. Minimum area of steel = 0.12% bd = = 00 2 Hence rovide 4nos. of 20mm bars for 1m width of ile ca. CHECK FOR SHEAR Two-way shear: Shear force (Vu) = = N Nominal shear stress ( V ) = = 22 6 V = n/ 2 Shear strength of 2 concrete = = 0.5+ ( = 1) = = 1.5 >1.0 = 0.25 = = > Hence safe in two way shear. No of bars = =.2 3. =.

11 11 One-way shear: Shear force at column face = 5 2 = 375 KN Factored shear load ( ) = =562.5 KN Shear stress ( ) = = 62. = Permissible shear according to (100 ) =.2 = < Hence deth is safe in one-way shear Settlement of ile grou in saturated clay: Given that liquid limit = 25% Natural moisture content (W) =8% Secific gravity (G) =2.5 = : = = () () = (.) (.) 2. = Water table is at 2m below ground level Comression index Cc=0.009(L.L-10) = 0.009(25-10) = Change in void ratio, = wg

12 12 = = As ile grou is friction ile grou, the load is considered to be sread against 2V:1H Layer = (.5.81) +.5(.5.81) = Cross sectional area at A= (B+ ) (L+ 2 ) = = = Layer = ( ) ( ) = (.5.81) + (.5.81) +.5(.5.81) = Cross sectional area at B= (B+ ) (L+ 2 ) = ( ) ( ) = = = Settlement of soil, = log 1 = 0.15 =0.045m = 0.15 =0.029m.26 Log Log.22.. Total settlement of ile grou = 1 + = = 0.074m RESULTS AND FINDINGS Name of the exeriment Liquid limit test Plastic limit test Comaction factor test Terminology Liquid limit 25% Plastic limit 20% OMC,, 8%, CONCLUSIONS Result , As iles are the main load bearing members in the structures, hence they vital art in construction. The dead load of structures in considerably reduced by using ile foundation by the way of reducing the thickness of walls. Hence different modes of iles are adoted for different tyes of soil based on soil roerties. Hence an attemt is made in this study to evaluate the arameters of design and also to focus the design criteria. REFERENCES: Soil mechanics and foundations by Dr.B.C.Punimia, Ashok Kumar Jain, Arun Kumar Jain. Soil mechanics and foundations by A.R.Arora. Limit state design Dr.B.C.Punimia, Ashok Kumar Jain, Arun Kumar Jain. IS: