James Blatz

Suctions, stresses and strengths in unsaturated sand–bentonite

Abstract This paper presents suction and strength characteristics for a dense, compacted, unsaturated sand–bentonite mixture under a variety of preparation and stressing conditions. Suctions were determined experimentally using thermocouple psychrometers and the filter paper method. They are shown to be related to water contents, saturation, dry densities and osmotic agents. The influence of the initial suctions on strength was evaluated using quick undrained triaxial compression tests, here called ‘constant-mass’ tests. Examination of the suction response to applied external stresses was carried out using stress-controlled triaxial tests along selected stress paths. These tests measured suctions in tests where the mean stress p and deviator stress q were changed systematically to give a series of constant values of Δ q /Δ p . The results showed that suction decreased as mean stress loading increased. Suction changes appear to be produced only by the mean stress component of the stress tensor and not by the shear stress component. In this compacted sand–bentonite material, for the pressure range at which the tests were performed ( p ′≤3 MPa), changes in suction produced by changes in mean stress are largely reversible.

Evaluation of the influence of boundary confinement on the behaviour of unsaturated swelling clay soils

Swelling soils are found in many regions throughout the world. Damage caused to infrastructure by these types of soils is measured annually in billions of dollars. These excessive damages are, in part, due to the lack of proper design, resulting from a need for better tools for practitioners to assess the impact of swelling soils in typical design applications. This paper presents an experimental testing program with interpretations to provide a framework for predicting the behaviour of swelling soils under general stress and volume state conditions for practical applications. The experimental testing adopted a new automated triaxial apparatus that controls boundary stress and strain while applying liquid infiltration conditions at the perimeter or center of triaxial specimens. Results demonstrate the influence of a range of boundary conditions on the behaviour of swelling soil during liquid infiltration. The range of boundary conditions examined in the experimental testing include constant mean stress (C...

Development of a hydraulic conductivity apparatus for bentonite soils

Measurement and interpretation of hydraulic conductivity in porous media is a complicated process, and many laboratory apparatuses exist for different soil types and conditions. To use models for interpretation and prediction of hydraulic conductivity, accurate test measurements are required. A new hydraulic conductivity apparatus is presented that includes simultaneous control of volume and stress states. The apparatus includes the ability to automatically control volume to apply selected displacement boundary conditions while imposing radial flow conditions. The capabilities of the system are displayed using two selected hydraulic conductivity tests on an unsaturated sand–bentonite mixture, which is a swelling soil. Hydraulic conductivity on the order of 10−13 m/s was measured using the new system and compared closely with previously measured values using a similar material. Post-test measurements displayed internal water content, density, and saturation changes that occurred during testing.

Numerical modeling of sand drain performance — a case study of the Salter Street Bridge construction

Several bridge crossings proposed for the Red River Floodway expansion project were recently constructed using vertical sand drains to accelerate excess pore-water pressure dissipation and settleme...

Evaluation of Yielding in Unsaturated Clays Using an Automated Triaxial Apparatus with Controlled Suction

Research in the field of unsaturated soil mechanics for high plastic clays is very active. One area of considerable current interest is development of general constitutive models for unsaturated clay based sealing materials in a frame work that can be implemented in numerical modeling tools (Alonso et al. 1990, Delage and Graham 1995, Toll 1990). In particular, more quantitative information is required to define the features of yielding, failure and strain hardening for predictive modeling applications. Soil suction must be controlled and independently measured in laboratory tests. This will allow examination of behaviour along any stress path that can be expected to occur in engineering applications which will provide the necessary material information to calibrate and validate proposed constitutive frameworks.

A capillary-tube model for two-phase transient flow through bentonite materials

Swelling mechanisms occurring on the pore scale or at the molecular level of high plasticity, unsaturated soils often control macroscopic behaviour. In this paper, a new capillary tube model is proposed. The model is used to represent laboratory-scale infiltration tests on a bentonite-rich soil. The goal is to develop a greater understanding of bulk behaviour by closely examining microscopic behaviour. A decrease in hydraulic conductivity with increasing water content has been observed in laboratory studies on flow through shrink–swell materials. The proposed mechanism causing a decrease in conductivity is a change in pore-size distribution. The unique feature of the new capillary-tube model is that, as water flows down the tube, the tube’s cross-sectional area contracts to restrict flow, thus representing the change in pore-size distribution observed in the physical tests. Flow data from the capillary tube are used to model the laboratory results, and new insight is gained into bulk flow behaviour. Final...

Evaluating shear mobilization in rockfill columns used for riverbank stabilization

The major rivers within the City of Winnipeg are founded in glacial Lake Agassiz clay and silt sediments that have low shear strength. As such, riverbank instabilities are a common issue along many stretches of the rivers. The use of rockfill columns has become an increasingly utilized approach for stabilizing failing banks. Recent cases in Winnipeg have shown that movements can occur following installation of rockfill columns. Uncertainty regarding the magnitude of these movements that is required to mobilize shearing resistance in the rockfill columns has resulted in situations where the stability of riverbanks following remediation has been questioned. This has provided a need to improve our understanding about how much movement a stabilized slope must undergo before sufficient shear resistance of the rockfill column will be mobilized. The results of experimental testing conducted to assess the shear mobilization of rockfill column materials using a large-scale direct shear test apparatus are presented...

Natural Processes and Strength Degradation

Many engineering projects are designed on the basis of laboratory tests using so-called ‘undisturbed’ samples of clay taken from the field. There is a tendency to test only intact specimens and discard specimens that appear disturbed, fissured or otherwise weaker. It is known, however, that natural processes such as wetting-drying, freezing-thawing, desiccation, heating-cooling, and alterations in chemistry can affect the structure of clays and significantly change their compressibilities, hydraulic conductivities and strengths. For example, plastic clays that have been fissured by desiccation or freezing cannot reliably provide peak strength resistance in slopes and under engineered embankments. The paper shows examples of projects where natural processes degraded the strengths of natural and reconstituted clays. The case histories in the paper provide a reminder of the importance of recognizing natural processes and the limitations of laboratory measurements when selecting appropriate parameters for numerical modeling.