Hefu Pu

Enhanced CS2 Model for Large Strain Consolidation

AbstractSince original publication in 1997, the CS2 numerical model has been widely used for consolidation analysis and has formed the basis for several more advanced consolidation models. In this paper, an enhanced version of the CS2 model is presented along with simulations to illustrate its new capabilities. The algorithm includes features of the original model with the addition of time-dependent loading, an external hydraulic gradient, and unload/reload effects. Verification checks show excellent agreement with analytical solutions for time-dependent loading under small strain conditions. A variety of numerical examples indicates that the new capabilities describe important considerations for consolidation analysis. The simulations also indicate that failure to correctly account for compressibility and hydraulic conductivity constitutive relationships and self-weight of the soil in particular can lead to significant errors.

CS3: Large Strain Consolidation Model for Layered Soils

AbstractA numerical model, called CS3, is presented for one-dimensional, large strain consolidation of layered soils. The algorithm accounts for vertical strain, soil self-weight, conventional constitutive relationships, changing material properties during consolidation, unload/reload, time-dependent loading and boundary conditions, an externally applied hydraulic gradient, and multiple soil layers with different material properties. CS3 can accommodate equilibrium and nonequilibrium profiles for the initial void ratio as well as variable profiles for preconsolidation stress and applied stress increment. Verification checks show excellent agreement with available analytical and numerical solutions. Several numeric examples are used to illustrate the capabilities of CS3 and highlight errors that may occur when multilayer systems are modeled as a single layer with average properties. Finally, settlement estimates obtained using CS3 are in good agreement with field measurements for the Gloucester test fill.

Assessment of Consolidation-Induced Contaminant Transport for Compacted Clay Liner Systems

AbstractThis paper presents an investigation of the effect of clay consolidation on contaminant transport through compacted clay liner (CCL) and composite geomembrane liner (GML)/CCL landfill bottom liner systems. Numerical simulations were conducted using the CST2 model and consider coupled consolidation and contaminant transport with representative geometry, material properties, and applied stress conditions. Simulation results indicate that, depending on conditions, consolidation can have important effects on contaminant breakthrough time, mass flux, cumulative mass outflow, and concentration distribution within the CCL, not only during the course of the consolidation process but also long after consolidation has completed. In general, the effects of CCL consolidation increase with increasing CCL thickness, increasing magnitude of applied stress, decreasing loading time, increasing CCL organic carbon fraction, and increasing variation of effective diffusion coefficient. Traditional advective-dispersive...

Benchmark Problems for Large Strain Consolidation

AbstractModeling of the one-dimensional consolidation process for soil and other compressible porous media is important for a variety of engineering applications. This paper presents solutions for two benchmark problems for large strain consolidation of a saturated soil layer that can be used to check other analytical and numerical analysis methods. The solutions include effects of vertical strain, soil self-weight, nonlinear constitutive relationships, and changing material properties during the consolidation process. High-accuracy data are presented for each problem and include changes of settlement, void ratio, and excess pore pressure with time.

Consolidation-Induced Solute Transport for Constant Rate of Strain. I: Model Development and Simulation Results

AbstractA numerical model, called CSTCRS1, is presented for coupled one-dimensional consolidation and solute transport under constant rate of strain (CRS) loading conditions. The consolidation algorithm accounts for vertical strain, general constitutive relationships, relative fluid velocity, changing compressibility and hydraulic conductivity during consolidation, and an external hydraulic gradient. The solute transport algorithm accounts for advection, dispersion, linear and nonlinear sorption, and equilibrium and nonequilibrium sorption. Soil compressibility is rate-independent and as such CSTCRS1 is more appropriate for less-structured soils. The model is based on a dual-Lagrangian framework that separately tracks the motions of sold and fluid phases. The development of CSTCRS1 is first described, and is followed by verification checks. Numeric examples are then presented to illustrate the effects of initial contamination distribution, transport conditions, applied strain rate, initial specimen height...

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...

Model for Coupled Large Strain Consolidation and Solute Transport in Layered Soils

AbstractA numerical model, called CST3 (Consolidation and Solute Transport 3), is presented for coupled one-dimensional large strain consolidation and solute transport in layered soils. The consolidation algorithm accounts for vertical strain, soil self-weight, general constitutive relationships, relative velocity of fluid and solid phases, changing compressibility and hydraulic conductivity during consolidation, unload/reload, time-dependent loading and boundary conditions, external hydraulic gradient, variable preconsolidation stress profiles, and multiple soil layers with different material properties. The solute transport algorithm accounts for advection, diffusion, mechanical dispersion, linear and nonlinear sorption, equilibrium and nonequilibrium sorption, porosity-dependent effective diffusion coefficient, and first-order decay reactions. CST3 is based on a dual-Lagrangian framework that separately tracks the motions of fluid and solid phases. The development of CST3 is first described, followed b...

Consolidation-Induced Solute Transport for Constant Rate of Strain. II: Comparison with Incremental Loading

AbstractThis paper presents a numerical investigation of one-dimensional large strain consolidation-induced solute transport for incremental loading (IL) and constant rate of strain (CRS) conditions. Solute transport accounts for advection, diffusion, dispersion, linear and nonlinear sorption, and equilibrium and nonequilibrium sorption, and is consistent with temporal and spatial variations of porosity and seepage velocity in the consolidating soil. Simulations were conducted using material properties for kaolinite clay and indicate that IL and CRS conditions produce significantly different responses during the course of consolidation, including applied stress, rate of settlement, excess pore pressure, and local strain. However, for a given set of initial and boundary conditions, final solute mass outflows and final solute concentration profiles for IL and CRS conditions were generally in close agreement, provided that total elapsed time and final average strain were matched for both loading procedures. ...

Assessment of Consolidation-Induced VOC Transport for a GML/GCL Composite Liner System

AbstractThis paper presents a numerical investigation of the effect of consolidation on the transport of a volatile organic compound (VOC), trichloroethylene (TCE), through a composite liner system comprising a geomembrane liner (GML) overlying and intimate contact with a geosynthetic clay liner (GCL). The numerical simulations were conducted using the model CST2, and considered coupled consolidation and contaminant transport with representative geometry, material properties, and applied stress conditions. The simulation results indicate that, depending on conditions, GCL consolidation can have significant effects on TCE mass flux, cumulative mass outflow, and distribution of contaminant concentration within the GCL, not only during the course of consolidation but also long after consolidation has ceased. Because of the small thickness of the GCL, consolidation-induced advection is insignificant. However, consolidation can significantly impact TCE transport through the GCL via changes in GCL material prop...

Numerical Investigation of Strain Rate Effect for CRS Consolidation of Normally Consolidated Soil

Constant rate of strain (CRS) consolidation tests are commonly used to measure consolidation properties of fine-grained soils. A series of numerical simulations was conducted using the CCRS1 model to investigate the effect of applied strain rate on accuracy of data analysis methods given by ASTM D4186/D4186M-12e1. The simulations consider six normally consolidated soils, each with rate-independent constitutive relationships, constant coefficient of consolidation, and constant coefficient of compressibility or compression index. Results indicate that high values of applied strain rate can introduce significant error for calculated consolidation properties (compressibility, hydraulic conductivity, coefficient of consolidation) using both linear and nonlinear analysis methods. Results also indicate that the normalized strain rate β governs CRS consolidation behavior and that CRS tests generally should be conducted with β ≤ 0.1. Finally, correction factors are proposed for calculated values of coefficient of consolidation from CRS tests on normally consolidated soils.