Abstract
A technical solution for tidal turbine foundation in granite seabed consists of grouted steel piles. The piles would be subjected to cyclic loading due to the severe service conditions. The mechanical behaviour at the interface between the pile and the surrounding media is one of the key points that determine the bearing capacity of the foundation system. Experimental research work has been carried out in the laboratory to study the grouted pile-to-rock connection (GPRC) and focused more precisely on the pile to grout connection when the pile is equipped with shear-keys that enhance the interface capacity. Monotonic and cyclic shear tests were performed using a specific direct shear test device (BCR3D), allowing application of sample confining conditions close to the in-situ conditions, namely constant normal stiffness conditions. Cyclic tests - either under one way or two-way shear loading application - were performed on several samples, under both constant volume and various constant stiffness boundary conditions. This study has shown that the strength of the interface is highly dependent on the combination of mean load, cyclic amplitudes and number of cycles applied to the interface. Cyclic failure was observed with less than 30 cycles for two-way and one-way cyclic loading.
Highlights
Shearing deviceThe grout-shear keys interface behaviour was studied using a 3-D servo-controlled direct shear machine named “BCR3D” (for its French acronym Boîte de Cisaillement pour joints Rocheux 3D)
The interface stiffness, knormal, is maintained constant by adjusting the normal stress with respect to the normal displacement of the interface. This boundary condition is intermediary between the Constant Volume boundary condition (CV), which represents infinite normal stiffness, and the Constant Normal Load (CNL), where no lateral stiffness is imposed allowing for larger deformations and softer interface responses
The x-axis and y-axis represent the ratio between the mean cyclic load Qmean and the half cyclic load Qcyclic respectively and the tensile static capacity Qs
Summary
The grout-shear keys interface behaviour was studied using a 3-D servo-controlled direct shear machine named “BCR3D” (for its French acronym Boîte de Cisaillement pour joints Rocheux 3D). The X and Y loading axis is composed of two independent jacks connected to two consecutive 1/36 reduction gears Their combined use, in addition to the vertical jack which imposes the confining normal stress to the sample, allows for the development of fully three-dimensional solicitations to the sample. Each one of these boxes consists of two independent parts (bottom and top boxes), with a spacing of 15 mm between them to simulate the interface between grout and shear keys. The metal plates with the shear keys that simulate the surface of an offshore steel pile are carefully placed inside before filling with fresh grout. Example of BCR3D bottom box with a solid steel plate with shear keys
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