The design and performance of piles socketed into weak rock

The research described in this thesis has been undertaken to obtain an understanding of the behaviour of rock socketed piles in weak rock, viz. Melbourne Mudstone, and to develop a practical and rational method of design for such piles. The strength and compressibility properties of Melbourne Mudstone have been considered through a programme of laboratory and field tes ts, with the emphasis b on the drained properties. It has been possible to d correlations of the drained strength and compressibility parameters with moisture content. Other properties considered include mineralogy, particle size distribution, permeability and coefficient of consolidation. The laboratory work included a special study to model the action of side resistance. This was achieved with the development of a constant normal stiffness direct shear machine which modelled the stiffness of a rock mass and thus allowed for normal s tresses to develop as shearing and dilation occurred. These results have been considered with concepts relating to the shear strength of rock joints and have assisted in developing an understand of side resistance in rock socketed piles. The performance of rock socketed piles has been considered through the analyses of 49 field e tests, of which 43 were made during the course of this project. More than half of these tests were made on prototype sized piles, with the remainder being made on smaller piles. The tests included piles designed to carry load by side or base resistance only, piles designed for side and base resistance to act together, and piles designed to investigate construction practices. The pile tests were made at different sites to allow variation in rock strength, modulus and joint frequency to be assessed. Some piles were instrumented with strain gauges to enable load distributions to be calculated. Whenever practicable, the tested piles were excavated to reveal the failure mechanisms. The results of the laboratory and field tests have been summarized in a set of design curves. These curves and the concepts contained in them have been used to develop a design method which reflects the observed behaviour of rock socketed piles. Particular attention has been given to the distribution of load between side and base resistance and to the non-linear load-settlement characteristics, including the peaking of the side resistance component and the work strengthening of the base resistance component. The design method has been based on an initial linear elastic analysis, followed by the relaxation of stresses to allow for non-linear effects. It has thus been based on a combination of theoretical elastic solutions, laboratory tests and field tests, and includes the effects of construction practice. The design method may be applied to designs for side or base resistance only piles, or to piles with side and base resistance acting together. The design curves have been developed primarily for Melbourne Mudstone, and it is suggested that further work is needed to develop the appropriate curves for other rocks. The main design concepts are considered to be applicable to a wide range of conditions.