Rock-steel interface testing and considerations for gravity foundations for tidal energy generators

Abstract

Foundation design of structures subjected to cyclic loading, including stability, cyclic and permanent displacements, soil stiffness for use in dynamic analyses, and soil reactions, requires that effect of cyclic loading is accounted for in the soil parameters. A primary goal of this paper is to provide correlations of these parameters with index parameters for use in practical design. The contour diagram framework for interpretation and presentation of cyclic soil behavior is summarized, as well as pore pressure and cyclic strain accumulation procedures to establish equivalent number of maximum loads, N eq , that gives the same degradation as the irregular cyclic load history. Guidance is given on how the soil parameters can be applied in design, and calculated and measured prototype observations and model test results are used to validate the soil parameter framework. Focus is given to foundation design of structures, but a procedure to analyze the stability of slopes subjected to cyclic loading is also included.

Chalk–steel interface testing for marine energy foundations

“…At these stress levels the shearing is accompanied by observable damage on the chalk surface, seen as a layer of powder (dry samples) or chalk putty (saturated tests) on the steel interface on post-test sample separation. This behaviour is similar to that noted for rock analogues (cement blocks) by Ziogos et al (2015b). For normal stresses from 316 kPa to 1000 kPa (i.e.…”

“…To allow the effect of normal stress and potential for surface degradation and damage to be represented, the range of normalised friction angles obtained are denoted by vertical error bars, as shown in Figure 7 (shown for chalk peak values only for clarity). The results suggest that, over the steel roughness range investigated (R a = 0·4-34 μm), on average the interface becomes stronger as the steel roughness increases, without reaching a 'plateau' as seen in other studies (Barmpopoulos et al, 2010;Jardine et al, 1993;Ziogos et al, 2015b) and as shown for sandstone. However, when increasing normal stress is considered there appears to be a tendency for the chalk-steel interfaces to tend towards the basic chalk-chalk interface properties.…”

“…Scope of testing Interface testing between chalk-steel interfaces at normal stresses relevant to those anticipated at real tidal stream projects (Ziogos et al, 2015b) was carried out in order to obtain the friction properties necessary for the determination of the sliding resistance of a GBS. The UCS of chalk has previously been found to vary significantly with saturation levels, generally showing lower strengths for saturated samples compared to dry ones (Matthews and Clayton, 1993).…”

“…As discussed in the introduction and found in the literature (Ziogos et al, 2015a(Ziogos et al, , 2015b), roughness has a major effect on the interface behaviour, therefore different preparation techniques (polishing and machining) were applied and resulted in three different foundation analogues with a wide range of surface roughness (R a between 0·4 and 34 μm).…”

“…Potential design approaches Previously, Ziogos et al (2015aZiogos et al ( , 2015b have proposed an adhesion factor (α) (shear stress normalised by UCS) type approach for monotonic rock-steel and cement-steel interface strength prediction similar to that developed for rock socket pile adhesion factors (Tomlinson, 2001). In this case, however, the magnitude of shear stress is lower by several orders of magnitude compared to pile applications, due to the unbonded nature of the interface and the CNS conditions for rock socketed piles.…”