Transient compaction and expansion of a confined aquifer system is recorded using borehole extensometers. The corresponding hydraulic head fluctuation within the aquifer is monitored in an observation well in Shanghai. A delay time constant was measured to be approximately 0.061 year (or 22 days) for the lower confined aquifer system at Multiple-Extensometer Site F16 in Shanghai. This constant indicates the time difference between the observed compaction and expansion cycles and the observed hydraulic head cycles. The hydraulic head fluctuations register almost no long-term decline. This observed delay time is caused only by recoverable consolidation of the aquitards (clay lenses or interbeds) that lie within the aquifer system. The past maximum preconsolidation pressure is greater than pressures indicated by the recorded fluctuations of head. An equation that relates the delay time constant and the recoverable consolidation coefficient (vertical hydraulic diffusivity) is given by combining Riley's time constant definition and Terzaghi's time factor for an instantaneous load. A material constant coefficient is introduced to this equation for a sequence of nondeclining cyclic loads. This material constant coefficient is simulated under different amplitudes of an assumed nondeclining sinusoidal sequence of cyclic loads using Helm's one-dimensional finite-difference compaction (subsidence) code (COMPAC). This coefficient is found to be approximately 0.074. The recoverable consolidation coefficient is estimated to be 24.8 m²/year for the aquitards (vertical deformation) of the lower confined aquifer system at F16 in Shanghai.

 

Key words: aquifer system, aquitards, recoverable consolidation coefficient, delay time constant, vertical hydraulic diffusivity