Proceedings of Indian Geotechnical Conference December 15-17,2011, Kochi (Paper No. D-324) TIME DEPENDENT SETTLEMENT OF PILED RAFT FOUNDATION S.P. Bajad , Ph.D.(Engg.), Lecturer, Applied Mech.Dept, Govt. PT, Amravati -444603, INDIA,sp_bajad@yahoo.co.in R. B. Sahu , Ph.D.(Engg.),Prof, C EDept., Jadavpur University, Kolkata – 700032, INDIA, rbsahu_1963@yahoo.co.in ABSTRACT: Generally, for piled raft foundation predictions are made of pile load sharing and settlements at the end of construction only. However, the time-dependent settlement of piled raft foundation has important role to play in design of piled raft resting on soft clay. In this paper, the results of laboratory model study on time dependent settlement of piled raft in soft clay were analyzed. This study aims to establish the importance of the consolidation settlement in the design of piled raft foundation on soft clay. Model tests were performed to evaluate the influence of raft size, pile spacing and length on the time dependent settlement of piled raft foundation. The result shows that settlement reduction ratio of piled raft reduces with the increase in pile to raft area ratio as well as slenderness ratio of the piles. Ratio of total settlement to immediate settlement increases with the increase in pile to raft area ratio. INTRODUCTION The use of piled raft foundations has become more popular in recent years. The combined action of the raft and the piles can increase the bearing capacity, reduce settlement, and the piles can be arranged to reduce differential deflection in the raft. In the early 1970’s several buildings supported on piled raft foundations were monitored in UK (Hooper, 1973). In the 90’s, investigations carried out during the construction of several tall buildings in Frankfurt, Germany area Katzenbach et al., (2000), provided new stimulating data. Such observations led to a deeper insight into the mechanisms which govern the behavior of piled raft foundations. Mandolini & Viggiani (1997) and Mandolini et al. (1998) collected 42 well- documented case histories of the settlement of piled foundations. Some cases reported significant increase in settlement at the end of construction, due to primary consolidation in fine-grained soils (Hooper, 1979; Katzenbach et al., 2000) and creep in coarse-grained soil (Mandolini & Viggiani, 1997). This aspect deserves some attention, as the long-term settlement is most probable cause of damage to services, claddings and architectural finishes. As pointed out by Poulos (1993) the relative magnitude of short term and long-term settlement depends on the geometry of foundation and the nature of the soil. Hooper & Wood (1977) compared settlement of a raft and piled raft in London clay and observed that the raft settlement was about 50% of the final settlement at the end of the construction, while settlement of the piled raft was very close to the final settlement. The data collected by Morton & Au (1974) for seven buildings on London clay show a ratio between the settlement at the end of construction and the final settlement ranging between 0.4 to 0.7, irrespective of piled or unpiled foundation.; out of both cases, the highest observed ratio is that of a piled foundation. The present study is focused on the consolidation settlement and settlement reduction of piled raft foundation with piles embedded in artificially consolidated soft clay bed by using small scale laboratory model tests. Two series of model tests were conducted to carry out a parametric study. The effect of raft size, pile length and spacing on the reduction of the consolidation settlement and its contribution in the total settlement of the piled raft has been investigated. EXPERIMENTAL STUDY Model tests were carried out in a cylindrical mild steel tank of 600 mm diameter and height of 500 mm. Schematic diagram of the model test set up is given in Fig.1. Locally available alluvial type clayey silt / silty clay having liquid limit as 58% and plastic limit of 26% was artificially consolidated to prepare the soil bed. Oven dried soil was mixed thoroughly with water at water content of 55% byweight. The soil was placed in the tank in three layers each about 200 mm thick and artificially consolidated under the consolidation pressure of 30 kPa using dead weight blocks. After placing the soil into the test tank, a layer of jute geotextile followed by 50 mm layer of uniformly graded sand was placed over it for facilitating drainage during consolidation. Consolidation period was determined by recording time-settlement data of the clay bed during some preliminary tests. Consolidation period for the first two layers was set as 48 hours and for the third layer it was set to 7 days. Undrained shear strength of the consolidated clay as measured by unconfined compression tests / vane shear tests after the model tests was found to be 8±0.5 kPa. Model piles were made of 10 mm diameter mild steel solid rods. The pile was glued with fine sand to simulate roughness of prototype concrete piles. Two different raft sizes were chosen as 100 mm and 200 mm square size and 10 mm thickness. Model rafts were made of mild steel plates. After preparation of the clay bed model piles of specified lengths and arrangement were driven in the clay bed vertically using wooden templates. Then, the model raft was placed over the piles and bolted to act monolithic 225