Research Journal of Engineering Sciences ___________________________________________ ISSN 2278 – 9472Vol. 4(11), 1-4, November (2015) Res. J. Engineering Sci. International Science Congress Association 1 Comparison of Pile Settlement and Bearing Capacity in Cohesionless Soil Shabbir Ahmed Osmani and Jafor Ahmed Limon Department of Civil Engineering, Leading University, BANGLADESH Available online at: www.isca.in, www.isca.me Received 12th October 2015, revised 10st November 2015, accepted 20nd November 2015 AbstractIn homogeneous cohesionless soil when piles are installed, the soil near the pile gets disturbed to a distance of a few pile diameters around its periphery. In driven piles, this disturbance results into compaction and increased density of the surrounding soil. Again in bored piles the disturbance may result into some loosening of the surrounding soil. The installation of piles results into changes in the surrounding soil density. The bearing capacity of piles should be estimated based on the changed soil properties. However, it is difficult to predict these changes in surrounding soil properties due to pile installation because of the complex interaction between the soil and pile during and after construction. And so that the pile bearing capacity is estimated on the basis of initial strength and different characteristics of deformation of the soil deposit. This paper shows a study area where the total 8 bore holes details would make a comparison on bearing capacity of pile and differences of settlements for a proposed design consideration of deep foundation. Keywords: Bearing capacity, SPT, cohesion, depth, settlement. Introduction Piles are used as a deep foundation to support the loads coming from the entire building and the foundations have to be defined with the soil at a safe stress level and the settlements should be limited to an reasonable value. Whenever any analysis is done for the design of a pile foundation, then the allowable bearing capacity of those piles and their respective settlements are also considered and checked thoroughly prior to finalize the design. A better understanding of settlement is necessary to analyze the Comparison of Pile Settlement and Bearing Capacity.Settlement with an excessive value and the finite movement could produce distortion and cracks in a part or whole structures. Among the different methods, the calculation based on the SPT values is quiet common in our country. Most of the cases pile design in cohesion-less materials (sands, gravelly sands, silty sands, etc.) is based on SPT values. Pile design in cohesive deposits is usually based on unconfined compression. The three empirical methods that can be used to estimate bearing capacity of piles based on field soil tests such as Standard Penetration tests, Static Cone Penetro-meter (Dutch cone with friction sleeve) and pressure-meter tests. In this study the standard penetration tests data are considered to find the values of bearing capacity of single piles and their settlements. Theoretical concept End-bearing Capacity (Q): The ultimate end (point or tip) resistance Q, in tons of driven piles can be as: For sand: =(0.4/ NB)D AP 4 N A (1) where N is the average corrected Standard Penetration Test value near the pile tip and can be obtained as: = C (2) Where the commonly used correction factor (C) is as follows: = 0.77 *log10 (20/); 0.25 tsf Here, N is the average of the observed Standard Penetration Test value near the pile tip. This correction is made for the overburden pressure . D is the depth of pile into granular stratum, which is the pile length (L) in homogeneous cohesion-less soils. B is the width or dia of the pile, and A is the pile area (tip) in ft. Friction Capacity on Perimeter Surface (Q): The friction capacity of a pile can be estimated by using the following relationship: = [(f)(perimeter)(embedment length)] (3) where f is the ultimate unit shaft friction in tons per square feet. For driven piles, this value is given by the following relationship3,5: fs* = N/50 1 tsf (4) Settlement of pile: The settlement prediction of pile foundation is complex and this is due to the changes and disturbances occurred in the soil stress state after the operation of pile installation had been performed and there would have a uncertainty on the exact position of load and the distribution from the superstructure through pile to the soil beneath. The required displacement is small to mobilize skin friction and it may not exceed 0.2 in. and this is for any type of soil, pile and pile dimensions. Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472 Vol. 4(11), 1-4, November (2015) Res. J. Engineering Sci. International Science Congress Association 2 However, Vesic, Sharma and Joshi, found that this value may not exceed 0.4 in (10mm). The displacement required to mobilize pile point resistance is large and this is dependent on the type of soil, the pile type and sizes of pile. Thus, the ultimate skin friction is mobilized much sooner than the point bearing. In addition, the load transfer mechanism also depends on the pile length and the load levels. These and other data presented by Vesic indicate that the load transfer mechanism in piles is not well understood. Since settlements are influenced by load transfer mechanism, only approximate solutions of this problem are available. The following three methods are recommended for estimating pile settlement in cohesionless soils. They are: (1) semi-empirical method, (2) empirical method and (3) pile load test. Semi-empirical Method: For the design purposes, the settlement of a pile can be divided into the following three parts6 = S + S + Sps (5) where: S = the settlement of a single pile top. S = the value of settlement of a pile shaft due to axial deformation. S = the settlement of the base of a pile or point by the load transferred at the base. Sps = the settlement of pile due to the load transmitted along the base of pile shaft. Now S = (Qpa + Qfa) L/)A E (6) where: Qpa = actual base or point load which is transmitted to the base of the pile in working condition. Qfa = shaft friction(actual) load transmitted by the pile in the working stress range (force units). L = length of the pile. A = cross-sectional pile area. E = Young's modulus of elasticity for the pile. = a number which vary with the distribution of skin friction along the pile shaft. Vesic recommended that = 0.5 for the uniform or the parabolic skin friction distribution along the pile shaft. For triangular (zero at pile head and maximum at pile base)skin friction distribution, the = 0.67. Relationships have been established based on theoretical analyses and empirical correlations between soil properties and ultimate point resistance qp for a number of construction sites as reported by Vesic). = Cp pa/(Bq) (7) ps = Cs fa/(D) (8) where: C = empirical coefficient. C = 0.93 + 0.16 * (D/B). Qpa= net point load under working conditions or allowable. Qfa = pile shaft load under working conditions or allowable. q = ultimate end (point)-bearing capacity (force/area). B = pile diameter. D = L = embedded pile length, soil =125lb/ftEmpirical Method: The settlement of a displacement pile for working loads may be estimated by the following relationship = (B/100) + (Qva*L/(A*E) (9) where: S = pile head settlement, in. B = diameter of pile, in. Qva = the load applied on pile, lb. A = cross-sectional are of pile, in. L = length of pile, in. Ep = Young's modulus of elasticity of pile, lb/inStudy area Here 4 plots are attached to each other and the intension is to do the analysis of the soil condition specially to design the foundation of the proposed five storied buildings. A local geotechnical laboratory did the tests on the points as located B.H.01 to B.H.08. The external characteristics of soil are quit same for the full area and there was no any dumping or backfilling record for last 30 years (Source: Sylhet Municipality). So, when survey was started for selection of points for bore hole, first of all the area is divided in 5 columns and 7 rows. And to ease the analysis 8 locations have been selected as shown in figure-1. Figure-1 Study area (Alampur, Dakhshin Surma, Sylhet) Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472 Vol. 4(11), 1-4, November (2015) Res. J. Engineering Sci. International Science Congress Association 3 Figure-2 SPT values for different depth Figure-2 has shown that, the variation of SPT values for different holes are very small at the same depth which depends on the properties of soil. In this case soil properties are almost similar for every case. Figure-3 Allowable bearing capacity for different single pile From figure-3 it has observed the highest bearing capacity is at bore hole 6 and lowest at bore hole 4.It is because of the highest average N values at bore hole six and lowest at bore hole 4. Figure-4 Settlement of pile at preferred locations From figure-4 Sps is maximum at bore hole 2 and minimum at bore hole 6. St depends on Sps, Sp and Ssvalues. Qf and Qp are the governing factors. In this case for bore hole two Qf is higher and Q is lower for bore hole one. Figure-5 Variation of settlement for St(empirical) Figure-5 shows the variation of S and St(empirical). Here in every case St(empirical) is higher than S. The variation is occurred due to the different allowable bearing capacity at every hole.  \n \r  \n  \r\n\n\n  \n  \r\n\n\n  \n  \r\n\n\n  \n  \r\n\n\n  \n  \r\n\n\n  \n  \r\n\n\n  \n  \r\n\n\n  \n  \r\n\n\n          \n \n\n \n  \n \n\n \n  \n \n\n \n  \n \n\n \n  \n \n\n \n  \n \n\n \n  \n \n\n \n  \n \n\n \n                 ! " \r    Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472 Vol. 4(11), 1-4, November (2015) Res. J. Engineering Sci. International Science Congress Association 4 Conclusion In this study the analysis is quite prominent that the data would not be so different from the others. As per the historical background of this plot the interpretation of other boreholes could be analyzed to get the actual data for the analysis. This study could include a lot of other factors to establish a relationship between the frictional resistance and vertical load capacity for the pile and its surrounding areas. Especially in Sylhet region, these methods are quite common to use to find out the bearing capacity and to do the settlement analysis. This output could be used for the design of deep foundations that had been proposed on that plot. Reference1.Gabrielaitis L. and Papinigis V., Design of Deep Foundations on Bored Piles. May 19-21, 2010, Vilnius, Lithuania, the 10th International Conference (2010)2.Salgado, R. Prezzi and M. Seo, H., Advanced modeling tools for the analysis of axially loaded piles. In Proc. Of the International Workshop on Recent Advances in Deep Foundations (IWDPF07): Ed. by Kikuchi, Otani, Kimura and Morikawa. Port and Airport Research Institute, Yokosuka, Japan, Taylor and Francis Group, London, UK, 49-67 (2007)3.Meyerhof G.G., Ultimate Bearing Capacity of Footings on Sand Layer Overlying Clay, Can. Geotech. J., 11(2), 223-229 (1974)4.Peck R.B., Hansen W.E. and Thornburn T.H., Foundation Engineering, 2nd ed. Wiley, New York, 1974 5.Meyerhof G.G., Scale Effects of Ultimate Pile Capacity, J. Geotech. Eng. Diu., ASCE, 109(6), 797-806 (1983)6.Vesic A.S., Design of Pile Foundations, Transportation Research Board, National Research Council, Washington, DC, (1977) 7.Sharma H.D. and Joshi R.C., Drilled Pile Behavior in Granular Deposits, Can. Geotech. J.,25(2), 222-232 (1988)8.Vesic A.S., Load Transfer in Pile-Soil Systems, Proceedings Conference on Design Installation of Pile Foundations, Lehigh University, Bethlehem, PA, 47-73 (1970a)