Mechanical properties 1 Mechanical properties Robert (H.R.G.K.) Hack Engineering Geology, ESA, Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Enschede, The Netherlands Definition Physical properties that determine the behavior of ground that is under mechanical stress. Mechanical properties of ground determine the behavior of the ground under stress in a mechanical way; e.g. settlement of ground under a foundation, subsidence due to underground excavation or extraction of gases and fluids, tunnel and slope stability, and ‘breaking’ of rock or cemented soils. Deformation is the change in volume or shape of ground under stress in which rheology and viscosity may be important. Strength commonly denotes a stress condition at which the ground fails (breaks) when a threshold in stress conditions is exceeded. Constitutive models are the aggregate term for relations that describe the chemical- physical- mechanical behavior of ground. Ground materials are diverse and may be gases, fluids, solids (i.e. minerals, grains, and aggregates of grains or minerals), and any mixture of these and also include man-made ground, such as fills and waste dump materials. Ground is commonly differentiated in soil and rock; soil being an aggregate of loose or weakly-bonded particles, and rock consisting of particles cemented or locked together, giving rock a tensile strength. Soil and rock are, by some, differentiated based on a compressive strength difference with soil being weaker than 1 MPa and rock being stronger. A differentiation is made between ‘intact’ and ‘discontinuous’ ground, i.e. ground without respectively with distinct planes of mechanical weakness (discontinuities) such as faults, joints, bedding planes, fractures, schistosity, etc. A groundmass consists of (blocks of) intact ground with discontinuities, if present. Stress, deviatoric stress, deformation, and strain If a normal (σ) or shear (τ) stress is applied on a body of ground, the ground will deform, i.e. will be strained respectively change in shape by an angle rotation. The normal stress minus the hydrostatic stress which is the average of all normal stresses working on a body of ground, is the deviatoric stress. Total and effective stress, and gas and fluid pressure A porous ground consists of a skeleton of solid particles with pores in-between. The pores may contain gases and fluids. When a load is applied on a body of ground part of the load is taken by the skeleton resulting in stress in the skeleton, and part by the gas and fluid. Both skeleton and gas and fluid will deform. Many mechanical characteristics of the ground depend on the stress between particles, therefore normally the load is divided in a so-called ‘effective stress’, being the stress in the skeleton, and the stress (pressure) in the gas and fluid. In case of pores filled by water this is the ‘porewater pressure’. Effective stress together with the gas and fluid pressure is the ‘total stress’ which equals the stress from the outside on the body of ground. In a porous ground without any gas or fluid and in a non-porous ground the effective stress equals the total stress. Total stress situation may be applicable in situations where the pore gas and fluid pressure cannot or not fast enough dissipate, for example, for fast loading of a low-permeable clay (so-called ‘undrained’ situation). In most cases the presence of gas is neglected under engineering conditions, but may be important in, for example, subsidence due to gas and oil exploration. Elastic deformation The relation between normal and shear stress and strain for an intact, homogeneous, isotropic and ideal- elastic body of intact ground is formulated in eqs [1] and [2] (Fig. 1). In elastic deformation, stress and strain are coupled properties; there is no strain without stress and vice versa. Under influence of a normal stress (σ) in a particular direction, the material becomes shorter in that direction and wider perpendicular to the stress direction. The amount of shortening in relation to the stress is expressed by Original published in: Hack, H.R.G.K., 2018. Mechanical Properties. In: Bobrowsky, P.T., Marker, B. (Eds), Encyclopedia of Engineering Geology. Springer, Cham, Switserland. ISBN: 9783319735665. DOI: https://doi.org/10.1007/978-3-319-73568-9_197. pp. 604-618.