ASCE 65-17

Abstract

Scope: This is a standard guideline for calculating the saturated hydraulic conductivity (Ksat ), permeability (k), and porosity (n) of finegrained, isotropic, and homogeneous soils using (1) strain-stress data from the incremental loading of a soil sample in a standardized consolidometer (step-load test), (2) 1D vertical consolidation theory relating Ksat to the coefficient of consolidation (cv), (3) the relation between Ksat and k, and (4) the relation between porosity and the void ratio of a soil undergoing primary consolidation. An undisturbed soil specimen is inserted in a consolidometer and subjected to incremental vertical loads allowing axial (vertical) drainage. The vertical axial deformation of the soil specimen is measured as a function of elapsed testing time through several cycles of incremental loading until the end of the consolidation test. The measured deformation is graphed as a function of the log10 of time or the square root of time for each increment of vertical load. These graphs are used to calculate the coefficient of consolidation for each load increment applied to the soil specimen. (Casagrande’s unpublished work of 1938 and Taylor’s 1948 graphical methods are commonly used.) Terzaghi’s (1925) 1D vertical consolidation theory relating Ksat to cv is then used to calculate Ksat for each increment of vertical load applied to the soil specimen. The permeability and porosity of the soil sample decrease with increasing applied vertical load according to known laws. The permeability and porosity of the tested soil specimen are calculated for each loading increment as well. The calculated Ksat , k, and porosity yield a series of pairs of values (Ksat , vertical effective stress; k, vertical effective stress; and porosity, vertical effective stress) that are graphed to produce relations between (1) Ksat and the vertical effective stress, (2) permeability and vertical effective stress, and (3) porosity and vertical effective stress. The classical theory of 1D (vertical) consolidation (Terzaghi 1925) assumes that Ksat , k, and porosity remain constant as soils consolidate under increasing vertical effective stress caused by external loads or groundwater extraction. In fact, the poroelastic theory of 3D consolidation makes the same assumption concerning Ksat of a deforming soil (Loáiciga 2013). Yet, the experimental evidence from 1D consolidation tests indicates that Ksat , k, and porosity decrease when a soil consolidates under increasing vertical effective stress as discussed in this document. The increase in vertical effective stress caused by declining pore water pressure during sustained groundwater extraction; the changes effected by groundwater withdrawal on Ksat , k, and porosity; and the associated land subsidence are the primary foci of this standard guideline. This standard guideline demonstrates the relation between 1D consolidation and land subsidence driven by groundwater withdrawal, highlighting the relevance of strain-stress phenomena observed in consolidometer testing to the groundwater-induced phenomenon of land subsidence (consolidation of compressible strata by groundwater withdrawal). This standard’s methodology can be applied to calculations of land subsidence, groundwater flow predictions, and transport of dissolved solutes moving in groundwater through fine-grained soils.