Pullout Capacity of Model Piles in Sand

A large scale experimental program using vertical and batter model piles in sand subjected to pull out loads has been carried out in a model tank of size 1m x 1m x 1m. Mild steel pipes of varying diameters, lengths, and shapes are used as model piles. A poorly graded river sand having specific gravity G= 2.67, Uniformity coefficient= 3.53, emax = 0.81, emin = 0.54 has been used as foundation medium. The influence of pile inclination, pile length, diameter, surface characteristics and shape were investigated. It is inferred that net ultimate pullout capacity increases significantly with increase in length to diameter ratio. Pullout capacity also increases with increase in diameter. Net ultimate pullout capacity increases with increase in batter angle attains a maximum value and then decreases. Sand coated piles are found to be resisting more pullout forces than smooth piles. It is also found that for piles of varying shape but of constant volume, circular pile will resist more uplift force compared to square or rectangular piles. The experimental values of net ultimate pullout capacities have been compared with predictions made by available theories. Pullout Capacity of Model Piles in Sand K. Kimi Bose and A. Krishnan M.E Student, Department of Civil Engineering, Government College of Technology, Coimbatore [email protected] Asst. Professor, Department of Civil Engineering, Government College of Technology, Coimbatore. Introduction When structures are constructed below the ground water table or if they are constructed under water uplift forces are to be applied on the basement of structures. Also in the case of transmission line towers, mooring systems for ocean surface or submerged platforms, tall chimneys etc are usually subjected to overturning moments due to wind, wave pressure or ship impact etc. These overturning moments are transferred to structures foundation in the form of compression on some elements and pullout on others. The type of foundation usually recommended is a combination of vertical and batter piles. In this paper the behaviour of vertical and batter piles under pull out loads has been investigated. To study the effect of pile inclination, pile length, diameter, shape, surface characteristics and pile tip properties on uplift capacity of piles, laboratory experimental investigation is carried out. Meyerhof(1973) presented an analysis to determine the axial pullout resistance of batter piles. For pile of inclination, with vertical axis α, and vertical depth of embedment, D, ignoring pile weight, the pullout resistance, Pu is given as, ( ) u u s P 'K tan A ο = ρ δ (1) where, As = Embedded pile surface area ρo′ = Average effective overburden pressure= D/2 γ Ku = Uplift coefficient δ = Pile–soil friction angle Awad and Ayoub (1976) studied about the ultimate uplift capacity of vertical and inclined anchors in cohesion less soil. An empirical equation was developed for determining the ultimate uplift capacity of inclined piles. ( ) cos p p cos +tan α ο α = α α (2) Where Po = Net ultimate uplift capacity of vertical pile For vertical piles, o u P =P -W Hanna and Afram(1986) suggested an analytical procedure to evaluate ultimate pullout resistance Pα as ( ) P = Pcos /2 α α (3) Chattopadhyay and Pise (1986) proposed a theoretical analysis for predicting the axial uplift capacity of inclined piles, embedded in sand. They concluded that for equal length piles, the ultimate uplift capacity of inclined pile increases with increase in inclination of pile and decreases after reaching maximum value of αL at α=(15°to 22.5°). Pise and Sharma (1994) carried out extensive work on uplift behaviour of anchor piles in sand under axial pulling loads. A comparison was made between between the experimental and theoretical and experimental values. It was concluded that uplift capacity increases with increase in pile friction angle, depth of embedment and B/d ratio. Different theories regarding behaviour of piles under different loading conditions have been developed over the years. The reliability of the theories can be demonstrated only by comparison of experimental results on model or field piles with the theoretical predictions. Full-scale field tests, though highly desirable, are generally expensive and difficult to perform. In the absence of resources and scope of testing prototype small scale laboratory model test conducted on piles in foundation medium prepared under controlled condition may serve the purpose to some extent. Properly conducted laboratory tests, with known parameters