Abstract The development of overpressure in continental margins is typically evaluated with hydrogeological models. Such approaches are used to both identify fluid flow patterns and to evaluate the development of high pore pressures within layers with particular physical properties that may promote slope instability. In some instances, these models are defined with sediment properties based on facies characterization and proxy values of porosity; permeability or compressibility are derived from the existing literature as direct measurements are rarely available. This study uses finite-element models to quantify the differences in computed overpressure generated by fine-grained hemipelagic sediments from the Gulf of Cadiz, offshore Martinique and the Gulf of Mexico, and their consequences in terms of submarine slope stability. By comparing our simulation results with in situ pore pressure data measured in the Gulf of Mexico, we demonstrate that physical properties measured on volcanic-influenced hemipelagic sediments underestimate the computed stability of a submarine slope. Physical properties measured on sediments from the study area are key to improving the reliability and accuracy of overpressure models, and when that information is unavailable, literature data from samples with similar lithologies, composition and depositional settings enable better assessment of the overpressure role as a pre-conditioning factor in submarine landslide initiation.

Abstract Peat is a highly compressible geological material whose time-dependent consolidation and rheological behaviour is determined by peat structure, degree of humification and hydraulic properties. This chapter reviews the engineering background to peat compression, describes the distribution of peat soils in the UK, provides examples of the hazards associated with compressible peat deposits and considers ways these hazards might be mitigated. Although some generalizations can be made about gross differences between broad peat types, no simple relationship exists between the magnitude and rate of compression of peat and loading. Based on examples described here, land failures resulting from peat compression are locally generated, but due to the sensitive nature of peat these can result in runaway failures that pose great risk. Understanding the geological hazards associated with compressible peat soils is challenging because peat is geotechnically highly variable and the mapped extent of peat in the UK is subject to considerable error due to inconsistencies in the definition of peat. Mitigating compression hazards in peat soils is therefore subject to considerable uncertainty; however, a combination of improved understanding of the properties of compressible peat, better mapping and land use zoning, and appropriate construction will help to mitigate risk.

ABSTRACT Shales are enigmatic rock types with compositional and textural heterogeneity across a range of scales. This work addresses pore- to core-scale mechanical heterogeneity of Cretaceous Mancos Shale, a thick mudstone with widespread occurrence across the western interior of the United States. Examination of a ~100 m (~328 ft) core from the eastern San Juan Basin, New Mexico, suggests division into seven lithofacies, encompassing mudstones, sandy mudstones, and muddy sandstones, displaying different degrees of bioturbation. Ultrasonic velocity measurements show small measurable differences between the lithofacies types, and these are explained in terms of differences in allogenic (clay and sand) and authigenic (carbonate cement) mineralogy. Variations in ultrasonic velocities can be related to well log velocity profiles, which allow correlation across much of the eastern San Juan Basin. A quarry block of Mancos Shale from eastern Utah, USA, a common target for unconventional exploration and ultrasonically, compositionally, and texturally similar to the laminated muddy sandstone (LMS) lithofacies of the San Juan core, is examined to sublaminae or micro-lithofacies scales using optical petrographic and electron microscopy. This is mapped to results from axisymmetric compression (ASC) and indirect tensile strength testing of this facies at the core-plug scale and nanoindentation measurements at the micron scale. As anticipated, there is a marked difference in elastic and failure response in axisymmetric and cylinder splitting tests relating to loading orientation with respect to bedding or lamination. Shear bands and Mode-I fractures display contrasting fabric when produced at low or high angles with respect to lamination. Nanoindentation, mineralogy distribution based on MAPS (modular automated processing system) technique, and high-resolution backscattered electron images show the effect of composition, texture phases, and interfaces of phases on mechanical properties. A range of Young’s moduli from nanoindentation is generally larger by a factor of 1–4 compared with ASC results, showing the important effect of pores, microcracks, and bedding boundaries on bulk elastic response. Together these data sets show the influence of cement distribution on mechanical response. Variations in micro-lithofacies are first-order factors in determining the mechanical response of this important Mancos constituent and are likely responsible for its success in hydrofracture-based recovery operations as compared with other Mancos lithofacies types.