AbstractA study of the behaviour of large diameter, bored, cast in-situ piles founded in Keuper marl (Mercia mudstone) is presented. The work is based on instrumented full-scale pile load tests carried out as part of the design of a major Highway communication project in Cardiff, U.K. This research also forms part of an on-going research programme within the Soil/Structure research unit at the University of Glamorgan. The test piles were 0.9m in diameter by 28-32m long and were constructed following the procedure to be used in the actual contract piles. Vibrating wire strain gauges, extensometers and load cells were installed in the test piles at selected locations.
The load test generated extensive data in terms of the strain levels along each pile shaft. All instrument readings were monitored and automatically stored on computer. In addition, a 2m long reinforced concrete column with the same cross-section properties and instrumentation as the test piles was load tested under controlled conditions. The measured stress-strain characteristics of the short column were used to model the deformation parameters of the test piles. Utilising the load test data and the results of a comprehensive site investigation, the initial design of the contract piles has been evaluated. It is established that the design method suggested in the interpretative report of the site investigation, which is partially based on C.I.R.I.A. report No.47, leads to conservative predictions of ultimate shaft resistance. The predicted values are 40-57% of the measured values.
A semi-empirical method is developed which can predict the characteristics of large diameter, bored, cast in-situ piles in Keuper marl, at every stage of loading up to the ultimate state of the pile-soil system. The formulation is supported by load test-data from fully instrumented test piles in Cardiff (South Wales, U.K.) as well as other published pile test data. The analysis is based on separation of shaft resistance and end bearing by formulating the variation in load sharing between the pile shaft and base. The method takes into consideration the influence of non-linear stress-strain behaviour of concrete on pile deformation and the influence on pile settlement of additional compressibility due to any loose soil possibly present at the pile base level.
The proposed method is validated against a large database of full-scale pile loading tests, with a wide range of diameters and lengths, installed in a variety of clays. There are provisions in the model, to accommodate pile conditions with negligible components of either shaft resistance or end bearing. In every case, the predictive capability is judged to be accurate and satisfactory. The improved predictive capability of this method, in pile analysis, is expected to result in a more cost-effective construction. A computer program is written for the complete analysis of a pile using the proposed numerical model. The program can accommodate pile conditions in which the contribution to load resistance of either shaft or end bearing is negligible. The parameters required for input into the numerical model are those that would be available from a standard site investigation, but may also be back-figured from pile test data. These data may then be used to predict the complete load-settlement curve for a pile of different dimensions and material properties under different ground conditions.
|Date of Award
|R. B. Robinson (Supervisor) & R Delpak (Supervisor)