GRANULAR ANCHOR PILE SYSTEM UNDER AXIAL PULLOUT LOADS

Abstract

The demand of construction of the variety of structures such as bridges, transmission lines, chimneys, radar-microwave and TV towers, facilities for storage of petroleum products, tall buildings etc. has increased significantly, particularly during the last decade. Normally foundations of such structures are required to transmit compressive forces safely to the subsoil. However, many a times these may be accompanied with moments in addition to the lateral forces causing uplifting of foundations. In such situations, piles provide an appropriate solution. However, the problem becomes difficult when foundations are required to be laid in weak sub-soil deposits. This necessitates the adoption of some suitable ground improvement foundation technique such as the proposed granular anchor pile (GAP) that may serve as a better alternative solution comparing with the conventional methodologies that are in practice. Fig.1 demonstrates various components of the proposed GAP-system. , Ground Level Sranular Pile Steel Tie Rod - Surrounding Soil Steel Anchor Pedestal GAP-System The present investigation is concerned with the study of behaviour of "Granular Anchor Pile" (GAP) system under vertical pullout loads. Thus, 'GAP-system' may be defined as the modified granular pile which is reinforced with a suitable steel tie bar protruding above the pile head and the lower end of the tie bar is fixed with a circular steel plate embedded into a concrete pedestal at the bottom of predrilled hole, which is followed by compaction of granular material with an internally operating hammer provided with a hole for the tie bar to pass through. A glance of the literature reveals that various studies on anchors has been carried out by many investigators [Meyerhof and Adams (1968); Das (1975,1983); Nene (1983); Ranjan, Saran and Nene (1985); Bouazza and Finlay (1990); Ghaly, Hanna and Ranjan (1991); Hanna and Ghaly (1992); Rao and Prasad (1993)]. Parameters such as - failure mechanism, anchor size, embedment depth etc. have been studied. Subsequent developments suggested piles as an alternative to deep anchors, particularly when the construction becomes difficult. Based on extensive studies on piles carried out so far, it has been noticed that design and analysis of piles under compressive loads can be carried out with more confidence under normal operating conditions. Few authors contributed earlier for the development of analytical procedures in estimating pullout capacities of piles. These authors are - Prakash (1962); Pise (1969); Ranjan (1970); Sowa (1970); Vesic (1970 &1972); Meyerhof (1973); Poorooshasb et.al (1982); Subbarao and Venkatesh (1985); Madhav (1987); Chattopadhaya and Pise (1987); Hanna et.al. (1992); Datta et.al. (1994); Reddy (1996); Reddy et.al. (1998); Pise and Patra (2001). Granular piles are relatively recent development and may be considered an alternative solution to piles of more lengths, and are therefore economical. Many authors have ably demonstrated the use of granular piles for resisting compressive loads (Hughes and Withers 1974; Rao 1982; Ranjan 1989; Bergado 1991; Madhav and Impe 1994; Bouassida et.al. 1995; Poorooshasb and Meyerhof 1996). The available research work on granular piles indicates that its application to pullout loads has not attracted desired attention. Thus, keeping its advantages in view and to bridge the knowledge gaps, the granular pile is modified to the proposed GAP-system. So far no systematic study has been carried out on GAPsystem as done herein. Though a similar foundation technique - 'granular pile anchor' was suggested (Kumar 1995) with a limited scope i.e. to control 'heave', which usually occurs in swelling soils (expansive soils). Further, an attempt (Kumar and Ranjan 2000) has also been made subsequently to apply the 'modified granular pile' to resist uplift loads. In the present study, both laboratory and field investigations were carried out for proper understanding of the behaviour of single GAP and group of GAPsystems. Tests on the proposed GAP-system(s) individually and on its groups installed in varying soil conditions have been conducted to arrive at ultimate pullout capacities. Experimental investigation is divided in two parts as follows- (a) Laboratory Tests (Cohesionless Soil) (b) Field Tests (Cohesive Soil and Cohesionless Soil) The laboratory study consisted of 16 tests, which included single GAP- systems with varying diameters (d) and lengths (L). Two diameters (d) namely, 50 mm and 100 mm and L/d ratios varying as 7.5, 10, 12.5 and 15 have been studied. For 100mm diameter of GAP-systems, the groups of 2 and 4 GAP-systems having same L/d ratio of 7.5 and varying spacings (S) to diameter (d) expressed as (S/d) of 2, 2.5, 3 and 3.5 have also been studied. The recorded data in terms of applied pullout loads (P) and corresponding displacements (A) have been plotted. Influences of parameters such as length, number of GAP-systems and spacings on the pullout capacity were studied. The intersection tangent method is considered to determine the ultimate pullout loads. Similarly, for application of GAP-systems in the field, varying subsoil conditions at two different sites have been considered. Overall 17 prototype tests were These are further divided into two parts namely 6 tests at Site-I and 11 tests at Site-I I respectively. The influence of length and number of GAP-systems on the pullout capacity has been studied in the field also. The diameter of GAP as 300 mm and the spacings (s) as 3 times the diameter were kept constant for both the sites in the field. At Site-I having Cohesionless soil - single GAP-systems having diameter as 300 mm. and lengths varying from 2m, 3m, 4m and 6m respectively have been tested. In addition, two tests one each on the group of 2 and 4 GAPsystems have also been conducted. At Site-ll also, which was predominantly having soft Cohesive soil - 11 tests, including single with varying lengths as 2m, 4m and 6m and also the groups of 2,3 and 6 GAP-systems having two different lengths as 4m and 6m have been conducted. The diameter at this site was also kept similar to that of Site-I as 300 mm. In addition, two group tests having 2 and 3 GAP-systems of same lengths as 6m and different diameter i.e. 350 mm have also been conducted. Cost Analysis per meter length of the GAP system used in the field tests have been carried out, and then compared with the cost of concrete pile of same dimensions. It was found that cost of a GAP system is about half of the corresponding concrete pile. Based on the laboratory study carried out on single and groups of 2 and 4 GAP - systems in loose to medium dense cohesionless soil, the major conclusions drawn may be summarized as- (a) The Ultimate Pullout Capacity of single GAP-systems increases with the increase in length (L) to diameter (d) ratios. This holds good for both the diameters (d) i.e. 50 mm and 100 mm in the laboratory study. However, the rate of this increase of ultimate pullout capacity of single GAP-systems having 50 mm and 100 mm diameters were observed as 10% and 60% respectively, IV which indicates that the increase in Ultimate Pullout Capacity is a function of diameter of the GAP. (b) The Ultimate Pullout Capacity of the group of 2 and 4 GAP-systems are also observed increasing with the increase in spacings(s), i.e. S/d ratios. Though various S/d ratios such as - 2, 2.5, 3 and 4 have been considered for the present study, the spacing of '3d' may be considered optimum (where d is the diameter of GAP). Beyond it, the increase in Ultimate Pullout Capacity is marginal. Similarly, for field investigations also, the significant conclusions drawn are - (a) At Site-I (Cohesionless soil), the trend of increase in Ultimate Pullout Capacity of single GAP-systems has been observed similar to the laboratory tests. The influence of L/d ratios on the Ultimate Pullout Capacity up to 10 is observed significant beyond which it is marginal. It is in order with the observation drawn from laboratory tests. Whereas, for groups of 2 and 4 GAP systems, the Ultimate Pullout Capacities are almost equal to its value for single GAP-system multiplied by the number of GAP-systems in that group. (b) At Site-ll (Cohesive soils) also, the test results follow the same trend as that of results of the Site-I. This indicates the effectiveness of the GAP-systems in cohesive soils also. In general, the analysis of both laboratory and field test data indicate that the GAP-system is a cost-effective foundation system specifically for structures subjected to uplift forces. On the basis of present study it may also be concluded that the proposed GAP-system is found useful for varying soil conditions, such as tested in this investigation for Cohesionless soil and Cohesive soils as well. This foundation system may provide resistance to compressive forces also in addition to the uplift resistance.