[1] The relationship between dielectric permittivity, water saturation, and clay content for the Sherwood Sandstone from NE England was characterized as part of a wider study of the vadose zone moisture dynamics and pollution vulnerability of this aquifer. Dielectric permittivity was measured over the full range of saturation levels, for various lithologies ranging from clean medium-grained sandstone to fine-grained sandstone containing up to 5% clay, using a specially constructed dielectric cell. Dielectric constant, K r , is largely independent of frequency between 350 MHz and 1000 MHz. Below 350 MHz, K r of fine-grained, clay-rich sandstone shows frequency dispersion. Tests on physical models of the sandstone consisting of a fine fraction of Ottawa Sand and montmorillonite clay indicate that the clay minerals within the sandstone are responsible for its frequency dispersive behavior. These tests also show that increasing pore fluid salinity increases dielectric dispersion at the lower end of the frequency range, which indicates that this arises from the interfacial Maxwell-Wagner mechanism associated with platy clay particles. Water saturated sand:clay mixtures show very low dielectric constants at high frequencies (over 650 MHz). This effect is independent of salinity and probably results from the layered geometric arrangement of solids, bound and free water within the swelling clay. The complex refractive index method (CRIM) with a mineral dielectric constant of about 5 provides a good match to the water saturation versus dielectric constant data for all Sherwood Sandstone lithologies at frequencies between 350 and 1000 MHz and for clay-poor sandstone at lower frequencies. Below 350 MHz the presence of a few percent of clay in some Sherwood Sandstone lithologies raises their CRIM best fit mineral dielectric constant substantially.

Numerical investigation of the direct tensile behaviour of laminated and transversely isotropic rocks containing incipient bedding planes with different strengths.

True persistence of rock discontinuities (areas with insignificant tensile strength) is an important factor controlling the engineering behaviour of fractured rock masses, but is extremely difficult to quantify using current geological survey methodologies, even where there is good rock exposure. Trace length as measured in the field or using remote measurement devices is actually only broadly indicative of persistence for rock engineering practice and numerical modelling. Visible traces of discontinuities are treated as if they were open fractures within rock mass classifications, despite many such traces being non-persistent and actually retaining considerable strength. The common assumption of 100% persistence, based on trace length, is generally extremely conservative in terms of strength and stiffness, but not always so and may lead to a wrong prediction of failure mechanism or of excavatability. Assuming full persistence would give hopelessly incorrect predictions of hydraulic conductivity. A new technique termed forensic excavation of rock masses is introduced, as a procedure for directly investigating discontinuity persistence. This technique involves non-explosive excavation of rock masses by injecting an expansive chemical splitter along incipient discontinuities. On expansion, the splitter causes the incipient traces to open as true joints. Experiments are described in which near-planar rock discontinuities, through siltstone and sandstone, were opened up by injecting the splitter into holes drilled along the lines of visible traces of the discontinuities in the laboratory and in the field. Once exposed the surfaces were examined to investigate the pre-existing persistence characteristics of the incipient discontinuities. One conclusion from this study is that visible trace length of a discontinuity can be a poor indicator of true persistence (defined for a fracture area with negligible tensile strength). An observation from this series of experiments was that freshly failed surfaces through pre-existing rock bridges were relatively rough compared to sections of pre-existing weaker areas of geologically developed (though still incipient) discontinuities. Fractographic features such as hackle and rib marks were typical of the freshly broken rock bridges, whereas opened-up areas of incipient discontinuity were smoother. Schmidt hammer rebound values were generally higher for the rock bridge areas, probably reflecting their lower degree of chemical and physical weathering.

A 3D particle-based DEM model was established taking into account the geometries of rock bridges. The model was used to investigate the shear behaviour of incipient rock joints. Fifty-seven direct shear tests were conducted under constant normal load (CNL) boundary conditions using the established model, in which rock bridges with nineteen different geometries and incipient joints with various areal persistence (between 0.2 and 0.96) were involved. Our results show that, for the cases having a single rock bridge, cracks often initiated around the edges of the rock bridges and coalesced first in the middle of the rock bridge areas. While for other cases containing multiple rock bridges, cracks initially appeared at the connection points (located in the middle of the joint planes) of the rock bridges and then propagated to the edges. High crack initiation stresses were measured, which were often more than 60% of the shear strength of the tested incipient rock joints. Sudden failures of the rock bridges subjected to shearing were observed, accompanying dramatic increases in the number of cracks. Another important conclusion derived from this research is that both joint areal persistence and rock bridge geometry played significant roles in the shear failure of the simulated Horton Formation Siltstone joints. The present study has shown that shear strength increased gradually when joint areal persistence was decreased. Interestingly, different shear strength values were measured for rock joints with the same areal persistence (e.g. K=0.5). Shear velocity was also found to have a significant influence on the shear characteristics of the Horton Formation Siltstone joints. A higher shear strength was measured when the shearing velocity was increased from 0.01 to 1 m/s.

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