INSITU BEHAVIOUR OF SPLICED PILES AND INTACT PILES

Abstract

Deep deposits of soft saturated clays are usually found in coastal areas, around lakes, ports and harbours. The conventional practices for providing foundations in such deposits are either to use bored piles or driven cast-in-situ piles. Bored concrete piles, many a time suffer from several defects, (Mohan et al 1978). The defects are not always on account of materials and mixes or laxity in quality control. The fault at times lies in selecting a piling system which may not be well suited to highly compressible strata such as soft clays. In such a situation precast driven piles provide a better alternative. In case of precast driven piles also, driving of long piles in single length become inconvenient and costly. Precast piles in smaller segments driven and jointed together with a suitable joint may provide a cost effective and efficient alternative provided the joint between the two sections does not introduce weakness. Though the joints in timber piles are in use for over a century and are common in steel piles also, yet the cost effective and structurally safe joints in concrete piles are yet to be developed, proof tested and standardised for use in the field. In the present investigation, four different types of splices/joints have been designed and developed for the use in precast piles. The response of these splices in concrete piles during driving and subsequently in testing under different loading conditions on full scale piles has been studied in both cohesionless and cohesive soil deposits. Further, the performance of spliced piles vis-a-vis intact has been compared. (v) Four types of splices analogous to Hercules, ABB, Coupler (ring) and square have been selected for the study. These joints have been designated as splice A, splice B, splice C, and splice D respectively. The fabrication of splices/joints is based on indigeneous know how. Using the jointing techniques splices were fabricated, the piles with these splices were precast and driven at two different sites namely, site I at Roorkee cohesionless soil deposit and site Il-at Bombay having a soft cohesive deposit. Flexure tests on two precast piles, 3m long, having two segments joined together with the joint in the middle on all the four splices were also conducted in the laboratory. In addition, one 3 m long intact pile was also tested in flexure. Piles, square in section 200 mm x 200 mm size were cast in two segments, each having 1.5 m length thus when two segments are joined together, a precast pile of 3 m length was obtained. At another site piles of identical section were cast in two segments, each 3m long so as to obtain a 6m long spliced pile. Full scale in situ load tests were carried out on single piles as well as group of four piles to evaluate there performance during driving and also behavour compared with intact piles. Pile foundations in deep deposits of soft saturated clays, many a times are subjected to negative skin friction, a down drag resulting into reduction in load carrying capacity and/or excessive settlements. Such settlements may take place due to (a) the under consolidated nature of the foundation soil (b) remoulding of the soil that occurs during pile driving (c) a freshly placed fill around a pile and (d) ground water lowering. (vi) The Indian sub-continent has abundance of soft seturated clay deposit varying in depth from 6m to 30 m and even more, along the coastal belt, shores of Gulf of Cambay and Bay of Bengal. In such deposits, use of deep piles as foundations is an efficient and cost effective solution for medium to high rise structures. Unless, dully cared for, piles in such deposits, may be subjected to over stressing and/or lead to reduction in factor of safety and ultimately failure of pile foundation. The state-of-the art reveals that the negative drag is reduced or eliminated by using different techniques, e.g. protection piles ; bitumen coating; etc. It has been clearly brought out that the settlement in the ambient ground is the main cause of negative drag on piles which is generated with time. It may therefore be appropriate that if the compressible ambient ground around the pile is transformed into a strata of stiff composite mass having reduced compressibility and high strength by treating the ambient ground, which will result in significant reduction in settlement, thus reducing or eliminating the main cause of negative drag. A field study was also carried out on the precast driven spliced piles in soft clay deposit for the (a) performance of splice pile vis-a-vis intact piles (b) the ambient soft clay around the piles has been transformed into a composite mass of low compressibility by resorting to different ground treatment techniques, which resulted in significant reduction in the settlement of ambient ground and consequently negative drag on piles. * The negative drag on the central precast spliced concrete pile was created by loading the treated/untreated ambient (vii) ground through a specially planned and fabricated steel cap. Insitu load tests on spliced piles were conducted using maintained load method (ISrPart IV-1985). The settlements of the ambient ground and also, the settlement of the central spliced pile were recorded under each load increment. Following the test procedure five full scale insitu tests were carried out at the site, viz., (a) central splices RCC pile and virgin ambient ground around the pile (b) bitumen coated central spliced pile and virgin ambient ground around (c) spliced RCC pile and ambient ground treated with PGP (d) spliced RCC pile and ambient ground treated with MGP and (e) spliced rcc pile and ambient ground treated with SSSSP. The data obtained from the insitu tests at two sites, was analysed and interpreted. Based on the analysis of the experimental data the following conclusions have been drawn :- (1) The performance of the four splices under flexural load in the laboratory have been found satisfactory. The flexure load of splice C was found to very from 71% to 86% and that for splice B varied from 53% to 61 percent of the intact piles. These flexural loads are sufficient to sustain actual bending stresses during installation and also during service condition under actual structures as an alternative to intact piles. (2) The behaviour of spliced piles during driving as compared to intact piles is satisfactory and is in noway inferior to the intact piles. This is true for driving of piles both in cohesionless and soft clays deposits. (viii) (3) The behaviour of spliced piles during load tests under vertical compressive, lateral and pull out loads have been found satisfactory and comparable to intact piles. This is valid for all type of splices in cohesionless soils. (4) The behaviour of spliced piles (with splice B) upto their ultimate capacity under vertical compressive load when compared to intact piles in cohesive soil deposit (Site II) is satisfactory. Further, the piles having splices varying from one to three with same total length of pile in all the cases have almost the same capacity of the piles. It therefore indicates that the number of splices in a pile does not have any effect on the carrying capacity. (5) The behaviour of the splice piles in cluster during driving as well as during load testing in compression is same. It was further noted that the ratio of load of the spliced pile to theat the load perpile in a group was found to vary between 0.96 to 1.15 t against a total settlement between 4.0 mm to 6.0 mm. The behaviour single spliced having more than one splice and behaviour of the same pile in group identical when pile were installed in cohesive soil deposit utilising splice B only. » (6) Ambient clay treated with Plain Granular piles (PGP) or Minigroutd piles (MGP) or Self Setting Soil Slurry Piles (SSSSP) is found to reduce .negative drag significantly. So also the introduction of a bitumen slip layer by coating the pile with bitumen. (ix) (7) All the three methods of ground treatments are found to be effective in transforming the ambient soil mass into improved composite soil mass. However, ambient clay treatment with MGP and PGP is batter as compared to groung treatment using SSSSP. (8) The methods of ambient day treatment are preferable over bitumen slip layer since these may find their applicability on all types including bored cast in situ piles, where as bitumen slip layer method can be used only for prefabricated precast driven piles. (9) Full scale insitu load tests on soft cohesive ambient ground treated with PGP, MPG and SSSSP indicated the improvement in ground as 203%, 178% and 130% respectively as compared to corresponding theoretical improvement as 570%, 427% and 340% respectively when computed by using soil and pile properties. The theoretical predicitions are noted to be on the higher sides as compared with the experimental ones. The improvements in settlement or decrease in compressibility using the ground improvement technique of PGP, MGP and SSSSP is found to be 76%, 76% and 52% respectively. (10) The introduction of spliced pile technology in India is likely to provide a cost effective, Foundation suited to deep deposits of soft saturated clays where the installation, performance and the integrity of driven cast in situ piles and also bored piles has been questionable.