Pipe pile driving into rock

Impact driving technique is commonly used in offshore environments to install pipe piles. This technique consists in striking the pile head with a hammer to reach the required installation depth. The quest for offshore wind energy demands that pipe pile foundations be installed in increasingly challenging geotechnical conditions. To reach final installation depth, the pile has now to be driven through very dense soil with high resistance and even into rock layers. This increases the difficulty of pile driving while avoiding its damage during hammering. The objective of this thesis is to portray stresses in a pipe pile during driving into rock layer and to assess the structural integrity of the pile toe. An analytical approach has been attempted first to solve the problem for the static case. On one hand, an original analytical solution of a pipe pile subjected to non-axial static loading has been developed based on the Airy stress function. This solution gave results approaching those of Boussinesq’s plane strain solution for the case of large diameters. On the other hand, two novel closed-form solutions for cylindrical and spherical cavity expansion in a rock mass governed by a particular case of the Hoek-Brown yield criterion were developed and validated numerically. Two numerical methods were implemented to simulate dynamic pipe pile driving into rock: the Lagrangian method consisting in an axisymmetric modelling based on the “zipper type” technique and the Coupled Eulerian Lagrangian method (CEL-3D). Stresses in tubular piles during impact driving were evaluated, allowing the structural integrity of the pile toe to be assessed. To validate the proposed numerical models, a 1g reduced scale model was built, featuring different synthetic rock resistances. The developed experimental model is able to simulate pipe pile driving into rock. Comparison between numerical simulations (CEL-3D) and experimental tests showed similar results in terms of strains at the pile toe level. Based on numerical simulations and experimental results, the following conclusions can be drawn: • The power form of the Extended Drucker-Prager yield criterion can emulate the mechanical behaviour of rock mass governed by the Hoek-Brown yield criterion. • The “zipper-type” technique is applicable to quasi-static cone penetration into dense sand and fully coring penetration of pipe pile undergoing driving. • Pipe pile driving into good quality rock mass (σc>20 MPa) entails a significant risk of damaging the pile toe; the use of high steel grade is recommended.