Boulton's delayed yield: a different context
1999
Narasimhan, T.N.
In 1954, Boulton proposed a delayed yield idealization to account for the dynamic contribution of water from the zone of desaturation during a pumping test in an unconfined aquifer. During the 1970s, the reasonableness of this idealization was questioned by some on the grounds that the exponential function with a constant parameter, used by Boulton to quantify the drainage process, was too simplistic and that the parameter will, in actuality, vary in time and space. Rather than discussing the merit of the delayed yield idealization in regard to an unconfined aquifer, the present work explores a different physical situation of an aquifer in which Boulton's idealization is realistic. Laboratory evidence exists to suggest that the deformation behavior of some aquifer materials is time dependent (creep or strain accumulation). In turn, this time dependence of deformation leads to a time dependence of the compressibility of the aquifer material, and hence, a time dependence of the aquifer's specific storage. In situations where the specific storage of an aquifer may be characterized by such time dependence, and if such time dependence can be reasonably approximated by an exponential function, then the response of such an aquifer to a constant rate pumping test will be described accurately by Boulton's mathematical formulation.
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