Abstract
The development and construction of offshore wind farms requires the correct estimation of the friction that can be mobilised at the rock/grout interface. In conventional studies, the shear behaviour of a joint is usually investigated with laboratory tests under constant normal load/stress (CNL), however, in engineering practice, direct shear testing under constant normal stiffness (CNS) has been proved to be more realistic in the assessment of the development of the side shear resistance in rock grouted pile design. In this work, an extensive experimental campaign on the shear response of a weak carbonate rock (limestone) interface with grout is presented, in the frame of offshore wind turbines. First, basic mechanical testing is performed on the two interface materials in order to evaluate their mechanical properties. The output of these tests reveals not only the contrasting properties of the two interacting materials, but also the decreased response of the limestone in the presence of water. A series of monotonic shear tests (both under CNL and CNS conditions) on wet rough limestone/grout interfaces reveal the high impact of adhesion between the two materials to the mechanical response. Based on the monotonic results, a number of CNS shear tests under cyclic loading takes place, where different failure modes are observed dilatant and contractant response. The variability of the failure mode is strongly related not only to the adhesion created with the cast grout, but also to the limestone’s micro-structural heterogeneity that manifests already after consolidation. The post-shear morphological state of the interface is analysed, while the variability of the failure surface and the presence of water gouge creation do not allow a clear correlation of the morphologfy to the mechanical response. Overall, the response of this type of weak rock interface where the properties of the grout are significantly higher, is governed by the behaviour of the rock.
Highlights
The design and construction of offshore wind turbines in weak to moderately strong carbonate rock formations in the north of France, is considered and studied since a few years
The shear behaviour of a bonded rough limestone/grout interface is studied with lab-scale shear tests under constant normal stiffness (CNS) in wet conditions
The basic mechanical properties of the studied limestone are identified in both dry and wet conditionsrevealing a nonnegligible impact of the water to the response; lower tensile and compressive strength in wet conditions
Summary
The design and construction of offshore wind turbines in weak to moderately strong carbonate rock formations in the north of France, is considered and studied since a few years. Seidel and Haberfield (2002) discussed the importance of realistic roughness modelling for representative laboratory testing, by comparing the response of interfaces with regular triangular asperities to the response of irregular profiles produced based on the fractal model of Seidel and Haberfield (1995) Their experimental results on unbonded rock/concrete interfaces show a more brittle response of higher shear resistance for regular triangular asperities, compared to a ductile response of lower peak shear stress for the corresponding irregular profile. The shear behaviour of a bonded rough (regular triangular asperities) limestone/grout interface is studied with lab-scale shear tests under constant normal stiffness (CNS) in wet conditions (offshore applications). The failure mechanisms are explored taking into account the contrasting mechanical properties of the two materials composing the interface, as well as the post-shear geometrical profile of the samples
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