Constant Rate Of Strain Consolidation Tests Research Articles

Analysis of prefabricated vertical drains with surcharge preloading of Hiep Phuoc clay

Finite-difference analysis of soft ground treatment with prefabricated vertical drains (PVDs) was performed using a newly developed consolidation model. The model uses the concept of multi-compression indices and coefficients of consolidation (CoCs) to investigate the behaviour of a soft ground deposit (pore water pressure dissipation, surface settlement and sub-layer ground settlement) and to validate the newly developed Consopro finite-difference procedure. The pre-consolidation pressures of the soft ground deposit were characterised using a combination of piezocone penetration profiling and constant rate-of-strain (CRS) consolidation tests (0.02%/min) on undisturbed samples that were retrieved from the Saigon premier container terminal in the south of Vietnam. After comparison of the back-calculated results with field-observed data, a correlation between the CoCs determined by CRS consolidation tests and the CoCs from piezocone dissipation tests, which were carried out after soil improvement, was determined.

Simplified Apparatus for CRS Consolidation Testing of Soils

This article presents the details of a simplified Constant Rate of Strain (CRS) consolidation apparatus for determining consolidation properties of soils. The apparatus has a loading piston with peripheral O-rings that slides through the consolidation ring. The loading piston has provision for back pressure application. The excess pore water pressure developed during the consolidation test can be measured at the base of the specimen. The apparatus is validated by performing CRS consolidation tests on four types of soil specimens of varying plasticity characteristics and on an undisturbed soil specimen. The results from the proposed apparatus were compared with the results obtained from the consolidation tests using the conventional incremental loading oedometer apparatus and the standard CRS consolidation apparatus. The test results prove that the proposed CRS consolidation apparatus can be conveniently used for determining the consolidation properties of soils.

Constant Rate of Strain Consolidation Test Using Conventional Fixed Ring Consolidation Cell

This paper describes a modified conventional fixed ring consolidation cell to perform constant rate of strain (CRS) consolidation test for determining consolidation properties of soils. The modified consolidation cell is similar to the conventional oedometer cell with an additional provision for measuring pore water pressure at the bottom of the soil specimen. Detailed description of the modifications required in the conventional consolidation cell is provided. The performance of the apparatus is validated by performing CRS consolidation tests on four types of reconstituted soil specimens and one undisturbed soil specimen. The test results obtained from the proposed CRS apparatus were compared with the results obtained from the incremental loading consolidation tests. The test results prove that the modified consolidation cell is very well suited for performing CRS tests to determine the consolidation properties of soils.

Numerical Model with Finite Differences Approach for CRS Consolidation Test

The Constant Rate of Strain (CRS) consolidation test is extensively used in last time to estimate the settlement of clayey soils in many geotechnical laboratories. Different theoretical solutions and numerical models have been developed to estimate consolidation parameters from CRS consolidation test data, and investigate the strain rate effect on the CRS consolidation results. In this study, a new numerical model is developed to simulate CRS consolidation test for small and large strain conditions and for both linear and nonlinear soils. This numerical model is based on the solution of Terzaghi’s classical consolidation equation by finite differences approach, with taking into account the variation of sample height with test time. Results of this numerical model indicate that applied vertical load at the top boundary of sample and excess pore pressure at its base are dependent on the applied strain rate. Evaluation of the consolidation parameters from numerical results of this model with small and large theoretical solutions shows excellent agreement between all methods in small strain level, and when large strain conditions are reached only use of large strain theories can produce good convergence with model results. However, when great strain rates (approximately β ≥ 0.1) are applied, a significant error can be observed in consolidation parameters calculation by using both small and large solutions. Finally, simulation of some experimental CRS tests reported in literature with this numerical model provides comparable consolidation parameters to those evaluated from the experimental CRS tests data.

Numerical Simulation to Select Proper Strain Rates during CRS Consolidation Test

Constant rate of strain (CRS) and incremental loading (IL) consolidation tests are extensively used to measure consolidation properties of clayey soils. However, results of CRS test are usually strain rate dependent. In this study, a finite differences simulation of CRS test based on Terzaghi’s theory of consolidation is performed. The numerical simulation permits to evaluate a strain rates range satisfying a preset relative pressure criterion. Results of simulation show that the required relative pressure criterion can be verified, during the steady state stage of CRS test, only for particular types of soils and particular range of strain rates. Subsequently, a simple method is proposed to select, for a soil sample defined by its initial height, initial void ratio and liquid limit, an appropriate strain rates range satisfying the ASTM 4186-06 criterion. Comparison of previsions and experimental results reported in literature shows good agreement.

Determination of the continuous stress-dependent permeability, compressibility and poroelasticity of shale

The continuous changes of the stress-dependent permeability, compressibility and poroelasticity of a tight oil and gas shale were characterized by using the Constant Rate of Strain (CRS) consolidation test. The CRS consolidation test compresses a thin disk test specimen at a given constant rate of strain under one-dimensional consolidation with one-sided drainage condition and measurement of excess pore pressure at the undrained end. Permeability is calculated from the one-dimensional consolidation equation assuming oedometric loading, incompressible solid grains, and an idealized excess pore pressure distribution in the sample. The CRS test method has been widely used for the determination of the stress-dependent permeability of soft sediments, but has not been utilized for very stiff and low permeability shale as well as to obtain their stress-dependent compressibility and poroelasticity parameters. To test its appropriateness to shale, CRS tests were performed on thin disk samples of Mancos shale using a high pressure and high temperature triaxial cell under high isotropic confining stresses. Two modifications were done to make the CRS test applicable to determination of the continuous stress-dependent hydro-mechanical properties of tight shale: (1) the test method was modified for isotropic loading, and (2) nonlinear poroelastic effects were accounted for in the solution of the pore pressure dissipation equation. The permeability values from the CRS tests with the modified analytical solution were found to be in agreement with those obtained from the Constant Pressure Gradient Permeability test, and Pressure-pulse Decay Permeability test using nitrogen as pore fluid and corrected for Klinkenberg and non-Darcy flow effects.

Evaluation of Data Analysis Methods for the CRS Consolidation Test

AbstractThe constant rate-of-strain (CRS) laboratory consolidation test is used to measure consolidation properties of fine-grained soils. Although the CRS test offers many advantages over the conventional incremental-loading consolidation test, uncertainties associated with the method of data analysis have presented an obstacle to more widespread use of the CRS test in practice. This paper presents results from a numerical investigation of the accuracy of linear and nonlinear data analysis methods for the CRS consolidation test. Numerical simulations were conducted using a validated large strain consolidation model for CRS loading conditions, published material properties for two reconstituted clay soils, and three applied strain rates. Based on the numerical results, recommendations are provided for analysis of CRS consolidation data, including changes to the ASTM D4186 equations for nonlinear data analysis. The most appropriate analysis method for soils with linear compressibility is the current ASTM D...

Investigation on the Characteristics of Pore Water Flow During CRS Consolidation Test

Several experimental methods have been proposed for consolidation test such as: constant loading rate, constant gradient and constant rate of strain (CRS). Unfortunately, there are no unique criteria for performing the CRS consolidation test. Also, there are considerable differences among the standards proposed for the test. In the present paper, authors have tried to find the reasons for differences among standards, by studying the basic assumptions made on the characteristics of the pore water flow during consolidation process. In the course of study, CRS consolidation tests were carried out under different strain rates on remolded samples. The results of the tests have indicated that the Darcy’s law is not valid throughout the CRS test and therefore, any consolidation equation based on Darcy’s law would not provide accurate results. The results of the current experiments also showed that with regard to the applied strain rates, there are three different flow regimes governing the process named as: pre-linear (non-Darcy flow), linear (Darcy flow) and post-linear (non-Darcy flow). Experimental results also showed that distinction between boundaries of linear and nonlinear flow is possible from the excess pore pressure developed during the tests.

Strain Rate Effect on Long-Term Consolidation of Osaka Bay Clay

The consolidation characteristics of clay, based on the isotache concept in which the strain rate effect is considered, have been studied by many researchers. Most of these studies are aimed at calculating the secondary consolidation with high accuracy in order to evaluate the long-term settlement of large structures. In this study, as the first step toward improving the accuracy of the evaluation of long-term settlement at the Kansai International Airport, the consolidation characteristics of Osaka Bay clay are examined and organized based on the isotache concept. This study proposes a simplified model based on the isotache concept by using a compression curve and the relationship between the consolidation yield stress and the strain rate. The former and the latter are obtained from the constant rate of strain consolidation (CRS) tests and long term consolidation (LT) tests, respectively. The latter is expressed by an equation with three isotache parameters. This model is very practical because it requires a minimum of only one CRS test and one LT test. It is widely applicable to the Osaka Bay clay. The isotache parameters used in this model can be commonly determined for the Osaka Bay clays retrieved from various depths at the Kansai International Airport.

Constant Rate of Strain Consolidation Equipment and Procedure for Stabilized Soils

Abstract Stiffness, that is, the relationship between applied stress and resulting strain, is a function of applied stress and, in the case of stabilized soils, age. For that reason the standard odometer test may be unsuitable as it can take several days to complete a test and in that time the stiffness will change regardless of stress changes. The constant rate of strain consolidation (CRS) test may be the preferred test to assess the stiffness of stabilized soils. A simple CRS cell has been developed to operate with a standard triaxial loading frame. Comparisons between the results of oedometer tests and CRS tests show that the test procedure and equipment is valid. Tests on stabilized soils show the importance of minimizing the time of a test since there is an increase in stiffness with age.

Consolidation and Permeability Behavior of High Porosity Baltic Seabed Sediments

Abstract The consolidation and permeability (hydraulic conductivity) characteristics of the high-porosity surficial seabed sediments from a test site in the Eckernförde Bay, Baltic Sea, were studied using a constant rate of strain (CRS) consolidation and hydraulic conductivity testing system employing flow pumps. These silty clay sediments are characterized by high void ratios, high organic content, and variable concentration of methane gas. Results from CRS consolidation tests show appreciable apparent overconsolidation of the sediments at the upper 200 cm with a gradual transition to normally consolidated states below 250 cm in depth. The highly compressible nature of the material is reflected in the large values of the compression index. The hydraulic conductivity of the box core samples ranges from 2.5 × 10−7 to 3.0 × 10−6 cm/s for void ratios ranging from 4.5 to 7.0 with a linear relationship between void ratio and log of hydraulic conductivity. The engineering behavior of these sediments is of practical significance in geoacoustic modeling of the seafloor for studies related to mine countermeasure programs.