# Issues in Interpretation of Constant Rate of Strain Consolidation Test Data

23 Jul 2018-pp 69-83

TL;DR: In this article, a series of CRS consolidation tests have been performed on reconstituted samples of eight different clayey soils with different plasticity indices, and the test data obtained from these tests were analyzed through the method of interpretation given in ASTM: D4186-06 (Standard Test Method for One-Dimensional Consolidation Properties of Saturated Cohesive Soils Using Controlled-Strain Loading).

Abstract: Constant rate of strain (CRS) consolidation test is a fast test method to characterize consolidation behavior of fine grained soils. In this test method, the test specimen is deformed at a constant rate of deformation, and pore water pressure at the base of the test specimen and axial reaction load are measured at successive interval during the test. This test method has several advantages over traditional incremental loading consolidation test. In the present research, a series of CRS consolidation tests have been performed on reconstituted samples of eight different clayey soils with different plasticity indices. The suitable strain rate at which the test is to be performed has been decided considering the criterion given in the literature. The test data obtained from these tests have been analyzed through the method of interpretation given in ASTM: D4186-06 (Standard Test Method for One-Dimensional Consolidation Properties of Saturated Cohesive Soils Using Controlled-Strain Loading. 2008). The present study shows that the existing method of interpretation yields unreliable results for a significant duration at early stage of the test despite of performing the test at recommended strain rate. Analysis of present experimental data and some of the available recommendations to select suitable strain rate indicates that the theory for CRS consolidation which forms the basis for the existing method of interpretation is unable to describe rapid evolution of pore-water pressure at early stage of the test. Because of this, the existing method of interpretation fails to interpret the consolidation parameters accurately throughout the test.

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TL;DR: In this article, the authors made the following observations: 1) The HYDRAULIC GRADIENT must inDEED be CONSTANT at the DRAINAGE SURFACE to atTAIN a constant rate of STRAIN with LINEAR MATERIAL PROPERTIES.

Abstract: IN RESPONSE TO QUESTIONS RAISED BY THE AUTHORS' ORIGINAL PAPER, ENTITLED "CONSOLIDATION AT CONSTANT RATE OF STRAIN," THE FOLLOWING OBSERVATIONS ARE MADE: 1. THE HYDRAULIC GRADIENT MUST INDEED BE CONSTANT AT THE DRAINAGE SURFACE TO ATTAIN A CONSTANT RATE OF STRAIN WITH LINEAR MATERIAL PROPERTIES. THIS DOES NOT MEAN THAT THE GRADIENT MUST BE CONSTANT THROUGHOUT THE SAMPLE. IN THE ACTUAL SOLUTION, THE GRADIENT VARIES LINEARLY FROM THE SURFACE OF THE SAMPLE TO THE IMPERVIOUS BOTTOM, LEADING TO A PARABOLIC VARIATION OF THE EXCESS PORE PRESSURE FOR THE LINEAR MATERIAL. 2. THE PORE PRESSURE AT THE BASE MUST REMAIN CONSTANT FOR A LINEAR MATERIAL. FOR THE NONLINEAR MATERIAL, IT WILL NOT REMAIN CONSTANT. IN PRACTICE, NORMALLY CONSOLIDATED SAMPLES WILL HAVE GRADUALLY INCREASING PORE PRESSURES WITH TIME AT THE BASE. THIS IS PRECISELY WHAT IS PREDICTED BY THE NONLINEAR THEORY. 3. THE FORM OF THE STRESS/STRAIN RELATION HAS TO BE INDEPENDENT OF TIME OR STRAIN RATE. THE PROPOSED CONSTANT RATE OF STRAIN CONSOLIDATION TEST PROVIDES A STRAIGHTFORWARD MEANS OF OBTAINING VALUES OF VOID RATIO VERSUS EFFECTIVE STRESS FOR PREDETERMINED STRAIN RATES. THESE COULD, THEN, BE USED IN VISCOELASTIC ANALYSES. IN CONCLUSION, IT IS EMPHASIZED THAT ALTHOUGH THE SOLUTIONS PRESENTED START AT TIME ZERO, THEY SHOULD BE USED IN AN INCREMENTAL MANNER ONCE A STEADY STATE HAS BEEN REACHED; THE DATA IN THE ORIGINAL PAPER WERE HANDLED IN PRECISELY THIS MANNER.

158 citations

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TL;DR: In this article, a general purpose consolidometer for studying the one-dimensional consolidation behavior of soils is described, which can be used to measure residual pore pressures and swell pressures during back pressure saturation at constant volume.

Abstract: A new general purpose consolidometer for studying the one-dimensional consolidation behavior of soils is described. The test specimen can be loaded incrementally, as in the conventional test, at a constant rate of stress, or at a constant rate of strain. The apparatus can also be used to measure residual pore pressures and swell pressures during back pressure saturation at constant volume. Pore pressures are measured at the impervious base, and free drainage occurs from the top surface. A general solution to the consolidation equation is derived for conditions of constant rate of strain assuming constant \Ic\dv\N. While particular solutions are given for both a linear soil having constant \Im\dv\N and a nonlinear soil with constant \IC\dc\N, it is shown that for most purposes the linear solution is sufficiently accurate for interpreting constant rate of strain (CRSC) test results. The CRSC test is shown to be a simpler and much faster method for measuring the consolidation characteristics of soils than is the conventional test.

134 citations

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TL;DR: In this paper, a theoretical solution was developed to enable the determination of the coefficient of consolidation, the compression index, and the preconsolidation pressure from this testing procedure, within established limits.

Abstract: Consolidation tests on three materials were conducted by inducing six different constant rates of strain on the samples. The total load on the sample, the pore pressure at the base and the deformation were measured. A theoretical solution was developed to enable the determination of the coefficient of consolidation, the compression index, and the preconsolidation pressure from this testing procedure. Within established limits, it is shown that the consolidation parameters determined by this method agree with parameters determined by the conventional testing method. The advantages of the proposed procedure are that a much shorter time is required to determine the consolidation parameters and that this procedure can be used to study the strain rate sensitivity of cohesive soils to consolidation loads.

106 citations

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TL;DR: Controlled-gradient (CG), constant-rate-of-strain (CRS), and conventional incremental-loading (STD) consolidation testing are compared and evaluated in this paper.

Abstract: Controlled-gradient (CG), constant-rate-of-strain (CRS), and conventional incremental-loading (STD) consolidation testing are compared and evaluated. Undisturbed samples of three soils common to Kentucky were used in the testing program. Results of 15 CG, 14 CRS, and 32 STD consolidation tests are evaluated. The feasibility of these new test methods for routine testing is briefly discussed and recommendations are made for refinements in testing procedures.

58 citations

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