Back to Table of Contents

40780-AC9
Instability of Geological Materials under Three-Dimensional Stress Conditions

Poul V. Lade, The Catholic University of America

The goal is to study the conditions for instability of geological materials and augment an existing constitutive model for prediction of sandstone behavior. The project involves a combined experimental and analytical research program to investigate the fundamental characteristics and conditions leading to development of shear bands and subsequent sand production in sandstone and other types of rock and concrete.

Results of Studies to Date

The original proposal consisted of three parts, and progress has been and future work will be accomplished as follows:

Part I: Analysis of results has been performed from an experimental program of triaxial compression tests on mortar to determine the stress-strain, volume change (in drained tests) and pore pressure (in undrained tests) in these tests. Specimens with constant density, grain size curve, % cement (12%) and water/cement ratio (0.9) were tested with effective confining pressures from zero to 12,000 kPa. The location and shape of the initial yield surface as well as the conditions for occurrence of instability and shear banding were studied.

Part II: A review of the literature to establish the behavior of cemented soils is in progress. The literature is being reviewed and a pattern of behavior to help model the behavior within the framework of elasto-plasticity is emerging.

Part III: Progress has been recorded mainly in this portion by Mr. Niels Trads, who began his Ph.D.-studies in the Fall 2005 at CUA and he passed the Comprehensive Examination in the Fall 2006. Since then he has devoted full time to research on the present project. His project consists of three portions:

1) Modification of Single Hardening Model for Prediction of Cross-Anisotropic Behavior of Soils and Sandstone.

The single hardening model is an existing elasto-plastic constitutive model for frictional materials such as soils, concrete and rocks. The model has been critically reviewed and several aspects of the isotropic version of this constitutive model have been improved: The plastic potential function has been modified following studies of four candidate functions; the yield function has been improved after testing of several different mathematical formulations; and the switch from hardening to softening has been captured by a more realistic, smooth transition.

The inclusion of cross-anisotropic effects in the model requires experimental results to indicate the necessary modifications to the existing isotropic model. These experiments are explained below.

2) Experimental Program for Determination of Cross-Anisotropic Behavior

To study the cross-anisotropic behavior, artificial sandstone will be fabricated in the laboratory, and specimens will be cored in vertical and horizontal directions and tested in isotropic compression, triaxial compression and extension.

Experimental Equipment: Experimental equipment is available to perform triaxial compression tests with confining pressures up to 2000 psi (13,800 kPa). A 10 tons loading machine has been acquired (with $15,000 from the Department of Civil Engineering) for performance of these tests. All other necessary equipment is also available.

Artificial Sandstone: A rectangular prismatic mold with dimensions of Height x Width x Length = 3.5 x 3.5 x 12 cubic inches has been constructed in which the artificial sandstone will be fabricated by first mixing uniformly graded beach sand with cement and water to produce mortar and then compacting the wet mortar in the mold. Following curing, the artificial sandstone will be removed and cylindrical specimens with height = 3.50 inches and diameter = 1.50 inches will be cored in the vertical and the horizontal directions of the sandstone brick.

Experimental Program: The following drained experiments on cylindrical specimens are planned:

a) 2 isotropic compression tests on vertical specimens.

b) 2 isotropic compression tests on horizontal specimens.

c) 5 triaxial compression tests on vertical specimens with different confining pressures: 2 experiments with low confining pressures to produce failure in the cemented region, 2 experiments to produce failure in the non-cemented region, and one experiment to initiate on the hydrostatic axis outside the initial yield surface.

d) 5 triaxial compression tests on horizontal specimens with similar confining pressures and the same experimental program as discussed for vertical specimens.

e) 3-4 Brazilian tests on vertical specimens.

f) 3-4 Brazilian tests on horizontal specimens.

3) Augmentation of Model and Prediction of Cross-Anisotropic Behavior of Artificial Sandstone

As the experimental results become available, the elasto-plastic single hardening model will be modified to handle cross-anisotropic behavior, and the experimental results will be predicted by the model.

Request for No-Cost Time Extension

Because of the delay in initiation of this project, a no-cost time extension was granted last year for the execution of the present research project. However, in view of the initial delay in finding a suitable Ph.D.-student for the project and the consequent delay in the execution of the project, an additional no-cost time extension of the project of two years is requested. The project currently has $45,421 available for the following two years. This is sufficient to finish the project.

Back to top