Inelastic constitutive properties and shear localization in Tennessee marble

The inelastic response of Tennessee marble is modelled by an elastic plastic constitutive relation that includes pressure dependence of yield, strain‐softening and inelastic volume strain (dilatancy). Data from 12 axisymmetric compression tests at confining pressures from 0 to 100 MPa are used to determine the dependence of the yield function and plastic potential, which are different, on the first and second stress invariants and the accumulated inelastic shear strain. Because the data requires that the strain at peak stress depends on the mean stress, the locus of peak stresses is neither a yield surface nor a failure envelope, as is often assumed. Based on the constitutive model and Rudnicki and Rice criterion, localization is not predicted to occur in axisymmetric compression although faulting is observed in the tests. The discrepancy is likely due to the overly stiff response of a smooth yield surface model to abrupt changes in the pattern of straining. The constitutive model determined from the axisymmetric compression data describes well the variation of the in‐plane stress observed in a plane strain experiment. The out‐of‐plane stress is not modelled well, apparently because the inelastic normal strain in this direction is overpredicted. In plane strain, localization is predicted to occur close to peak stress, in good agreement with the experiment. Observation of localization on the rising portion of the stress–strain curve in plane strain does not, however, indicate prepeak localization. Because of the rapid increase of mean stress in plane strain, the stress–strain curve can be rising while the shear stress versus shear strain curve at constant mean stress is falling (negative hardening modulus). Copyright © 2001 John Wiley & Sons, Ltd.     

Coupled deformation-diffusion effects on water-level changes due to propagating creep events

Water-well-level fluctuations associated with episodic creep are studied using a coupled deformation-diffusion solution for the pore pressure produced by a plane-strain shear dislocation moving steadily at a speedV in a linear elastic, saturated porous medium. For largeVr/2c, wherer is distance from the dislocation andc is diffusivity, the solution approaches the form of the uncoupled elastic solution used by Wesson (1981) to analyze water-level changes due to creep events. The differences between the two solutions are significant within 10 diffusion lengths (20c/V) from the fault plane. More specifically, the pore pressure predicted by the coupled solution reverses sign behind the dislocation and is much smaller in magnitude than that predicted by the uncoupled solution. For an undrained Poisson ratio of 0.3, Skempton's coefficient of 0.8 and a shear modulus of 30 GPa, the coupled solution predicts a peak pore-pressure change of 13.7 kPa (137 mbar) per millimeter of slip forV=1 km/day andc=1.0 m²/sec. The spectrum of the coupled solution is limited to a band of frequencies, centered at a value proportional toV and approximately inversely proportional to the distance from the observation point to the fault plant. Thus, close to the fault plane the frequency band occupied by the coupled solution may lie above the range at which water wells can respond. The coupled solution is used in interpreting the same creep-associated water-level change observed by Johnson (1973) and modeled by Wesson (1981) using the uncoupled solution. Although there are uncertainties in properties of the rock material and in the speed of the creep event, the coupled solution predicts a water-level change comparable in magnitude to the observed change.     

A Linear Analysis of the Interaction Between the Atmosphere and an Underlying Compliant Plant Canopy

We summarise the results of a theoretical investigation of the interaction between the lower atmosphere and a compliant plant canopy. In this investigation, plant collisions were modelled as simple spring or dashpot interactions between adjacent plants. Linearised canopy models show that dashpot collisions and spring collisions are effective at reducing the amplitude of a canopy’s response to wind forcing, and may thus serve to stabilise plants against lodging and windthrow. Both types of collisions can also induce shifts in the stand resonance frequency and may thus serve to stabilise stands even further by shifting the canopy resonance frequency away from the most energetic frequencies in the atmospheric power spectrum. We also include a linear analysis of the shear layer instability over a waving canopy, and show that canopy drag has a strong effect on the instability. However, stem stiffness and inter-plant collisions have only minor effects on the wavenumber, growth rate, and frequency of the most unstable wave.     

Variability in surface moisture content along a hillslope transect: Rattlesnake Hill, Texas

Surface soil moisture content exhibits a high degree of spatial and temporal variability. The purpose of this study was (a) to characterize variations in moisture content in the 0–5 cm surface soil layer along a hillslope transect by means of intensive sampling in both space and time; and (b) to make inferences regarding the environmental factors that influence this variability. Over a period of seven months, soil moisture content was measured (gravimetric method) on a near-daily basis at 10 m intervals along a 200 m downslope transect at the Rattlesnake Hill field site in Austin, Texas. Results indicate that significant variability in soil moisture content exists along the length of the transect; that variability decreases with decreasing transect-mean moisture content as the hillslope dries down following rain events; and that the dominant influences on moisture content variability are dependent upon the moisture conditions on the hillslope. While topographic and soil attributes operate jointly to redistribute soil water following storm events, under wet conditions, variability in surface moisture content is most strongly influenced by porosity and hydraulic conductivity, and under dry conditions, correlations are strongest to relative elevation, aspect and clay content. Consequently, the dominant influence on soil moisture variability gradually changes from soil heterogeneity to joint control by topographic and soil properties as the transect dries following significant rain events.