Assessing Horizontal Strain and Deformation From Extensometer Data

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9902728BurbeyExtensometer data have been used with hydrograph data to determine aquifer and aquitard elastic and inelastic specific storage values. A compressible aquifer experiences virgin or inelastic compression when the preconsolidation stress threshold exceeds the past maximum stress. The component of specific storage associated with stresses less than the past-maximum preconsolidation stress threshold is elastic. These specific storage estimates are based on the assumption that the strain associated with the applied stress is vertical. Consequently any strain associated with an increase stress is directly related to the compaction through the vertical compressibility or specific storage of the hydrogeologic unit.Field data and Biot's theory of three-dimensional consolidation demonstrates that horizontal strain may be significant, and contribute large quantities of water from storage. These results suggest that the classical definition of specific storage may not be applicable for unconsolidated confined aquifers. Calculated drawdowns associated with volume strain tend to be less than those for vertical strain. Hence, estimates of specific storage from extensometer data assuming vertical strain conditions only will likely be too low. The proposed work involves examining stress-strain hysteresis loops from cyclically stressed systems where extensometer and hydrograph data are available. Because extensometer data express the volume strain through the vertical component of deformation, it is hypothesized that the "shape" of the stress-stain loops can be used in conjunction with the hydrograph data to estimate or assess the component of horizontal strain and deformation of the aquifer system. This hypothesis will be tested using both synthetic and existing data. The procedure for examining the validity of this hypothesis will be to simulate first the hysteresis loops with vertical stain models. Secondly, the same systems will be simulated using the volume strain method. The third approach will involve programming a newly developed analytic poroviscous model that accounts for the transient nature of storage and porosity under an applied effective stress. This work will have large implications in our interpretation of storage coefficient from aquifer tests and will greatly help our understanding of aquifer deformation in subsiding basins, particularly where earth fissures are known to result from over-pumping