Dobroslav Znidarcic

The Theory of One-Dimensional Consolidation of Saturated Clays: III. Existing Testing Procedures and Analyses

The successful calculation of the progress of consolidation requires that both the theory used to model the field problem and the material properties used by the theory must be appropriate and suitable. Thus the testing procedure must provide reliable and consistent information on the material behavior. Further, the test procedure must be accompanied by methods of analysis that will produce values of the material properties that are appropriate to the theory. This paper presents a summary of the existing laboratory methods that are used to determine the consolidation properties of soils. Emphasis is placed on the analysis of the test data. It is shown that all the methods that are currently used contain inherent simplifying assumptions. These methods are restricted in their applicability to problems where linear or constant material properties or both are good approximations to real behavior.

Air Entrapment Effects on Hydraulic Properties

The purpose of this paper is to show that the soil-water characteristic and unsaturated hydraulic conductivity functions derived for soils with continuous air channels, in which the air is assumed to exist at constant atmospheric pressure, should not be used for simulating infiltration process. Ongoing experimental work is oriented towards investigating the influence of the entrapped air on the hydraulic constitutive relations of a compacted fine-grained soil imbibing liquid. Testing is being conducted in two phases to determine (1) soil-water characteristic function and (2) unsaturated hydraulic conductivity function for systems involving discontinuous air phase. Hydraulic constitutive functions for soils with discontinuous air phase are proposed using the obtained experimental results.

Centrifuge modeling for undergraduate geotechnical engineering instruction

A small, simple, and economical instructional centrifuge has been developed at the University of Colorado at Boulder to assist in undergraduate geotechnical engineering education. Centrifuge experiments on stability of slopes and retaining walls have been developed. These experiments are conceptually simple, yet fundamental, and do not require elaborate instrumentation and data acquisition. Classical failure patterns discussed in the class can be reproduced in the models. Experimental results can be used to verify such theories as undrained slope stability analysis and Rankines or Coulombs lateral earth pressure theories. Each of the tests can easily be conducted up to four to five times in a 2-h laboratory session. Comprehensive laboratory reports can be generated by students discussing both qualitative and quantitative aspects of the tests in relation to the theoretical concepts taught in the classroom. In addition to the experiments on slope stability and lateral earth pressures, demonstration experiments on footings and reinforced earth slopes have also been conducted using the instructional centrifuge.

A New Centrifugal Testing Method: Descending Gravity Test

This study describes a new centrifugal test, called the descending gravity test (DGT), which combines in situ shear strength tests with a new material preparation technique. The material preparation technique provides soil profiles of constant overconsolidation ratio and varying void ratio in cohesive soils. When the DGT is repeated for different acceleration levels during centrifuge testing, it provides the void ratio-undrained shear strength relationship for various magnitudes of overconsolidation ratio. Therefore it renders the observation of the uncoupled effects of void ratio and overconsolidation ratio on the generation of shear strength feasible. The DGT has been successfully conducted on a clay sample in laboratory centrifuge facilities. The method requires the unloading of the clay cake, and since swelling of a sample is more rapid than its consolidation, the test takes a relatively short time.

Determination of Tailings Impoundment Capacity via Finite-Strain Consolidation Models

Mine designers are often faced with the problem of predicting the capacity of tailings impoundments for different production schedules and for different stages of mine development. The finite-strain consolidation model developed by Gibson is commonly used to describe the settlement of soft tailings materials as it allows for the non-linearity of material properties. This paper presents the upper and lower bound solutions for a 3-dimensional tailings deposition problem based on Gibsons theory. The presented solutions can be readily applied to determine the required impoundment capacity for various deposition rates and impoundment geometries. The proposed methods rationally account for the tailings compression as the height of the impoundment increases and provide predictions of the short-term and long-term tailings settlements.

A Mass-Conservative Numerical Solution for Finite-Strain Consolidation during Continuous Soil Deposition

Consolidation of accreting soft soils is a topic of significant interest for engineers dealing with mine tailings deposition, hydraulic fills, dredging deposits, and wetland construction. In these cases, the classical Terzaghis theory often fails to produce satisfactory solutions due to variations in compressibility and permeability under large deformations. The finite-strain theory introduced by Gibson allows for the non-linearity of material properties and provides an accurate description of soft soil deposits undergoing large displacements. Numerical accretion models based on Gibsons theory need to account for both the non-linearity of the governing equation and the continuous domain change. This paper investigates the numerical performance of several one-dimensional finite-difference schemes with different time-stepping algorithms and mesh discretization procedures. An optimal mass-conservative scheme is selected and implemented into a numerical model. The presented field example demonstrates that the employed mapping technique ensures both mass and water conservation, essential for water balance and storage capacity prediction in slurry disposal projects.

Flow Pump System for Unsaturated Soils: Measurement of Suction Response and the Soil–Water Retention Curve

Characterization of the suction response and the soil–water retention curve (SWRC) is important for geotechnical engineering applications and is a factor linking unsaturated soil mechanics with geotechnical practice. Therefore, a reliable and convenient method is needed to obtain parameters for models describing flow phenomena in unsaturated soils. Many SWRC measurement methods, including those using flow pump systems, have been developed. This paper describes the operation and limitations of an improved automated flow pump system for accurate SWRC measurement over drying and wetting cycles. We measured the transient suction response to obtain the SWRC using an equilibration method during both drying and wetting cycles. The system was successfully tested using a sand, a silt, and a silty sand. Characteristic curves for both drying and wetting cycles with proper flow rates were obtained within 1–3 weeks of testing. The proposed method is, therefore, very effective for routine application in engineering practice.

COMPARISON OF TWO MODELING APPROACHES FOR SIMULATING THE CONSOLIDATION AND DESICCATION BEHAVIOR OF DREDGED FILL

This paper presents a comparison of two modeling approaches that can be used to predict the consolidation and desiccation behavior of dredged material, using a case study of Poplar Island, Chesapeake Bay. As part of the Site Development Plan (SDP) for Poplar Island, it is necessary to accurately predict the behavior of hydraulically placed dredged material soon after placement, as well as several years following placement. The accuracy of the prediction is critical because the success of the intertidal wetland portion of the site is highly sensitive to final elevations. This paper will present the results of comparative analyses using the U.S. Army Corps of Engineers’ computer model PSDDF, and the University of Colorado computer model CONDES. In the PSDDF analyses, the desiccation was modeled simultaneously with the consolidation, but the settlement results are computed separately for each process. In the CONDES analyses the settlements from each component cannot be separated in a single run. Thus, in order to facilitate the comparison, CONDES analyses were performed initially for consolidation only and subsequently for consolidation and desiccation simultaneously. The results indicate that both models, PSDDF and CONDES, are comparable tools to predict the consolidation and desiccation processes of fine-grained dredged material. When the same conditions are imposed in both models the results are quite similar. This is not a surprise for the consolidation portion of the process, as both models are based on the same theory. For the desiccation analysis the crucial parameter is the effective desiccation rate. As long as that rate is the same for both models, similar results should be obtained. The effective desiccation rate in PSDDF is obtained as a result of the water balance calculation with a number of empirical factors affecting the outcome. In CONDES, the effective desiccation rate is the top boundary condition and must be specified as input data. For the CONDES analyses the site conditions affect only the boundary conditions for the layer, while the quantities within the layer (void ratio and porewater pressure) are obtained in the solution process.