Staged construction uses controlled rates of load application to increase the foundation stability of structures founded on soft cohesive soils and to improve the slope stability of tailings dams. Because construction causes positive excess pore pressures and because actual failures usually occur without significant drainage, stability analyses should compute the factor of safety against an undrained failure as the most critical and realistic condition. This requires an undrained strength analysis (USA) that treats predicted or measured in situ effective stresses as equal to consolidation stresses in order to calculate variations in undrained shear strength during construction. The recommended USA methodology requires a detailed evaluation of changes in vertical stress history profiles, uses undrained strength ratios obtained from CK0U tests to account for anisotropy and progressive failure, and is more rational than stability evaluations based on UU and CIU triaxial compression testing. Conventional effe...

Stability Evaluation during Staged Construction

The present design practice for embankments on clay foundations is reviewed and its shortcomings are evidenced.A detailed analysis of field observations shows that the actual foundation behaviour d...

The behaviour of embankments on clay foundations

Weathering processes such as softening and freeze–thaw cycling affect the properties of clays. Care must therefore be taken when selecting strength and compressibility parameters for analysis of na...

Effects of freeze–thaw and softening on a natural clay at low stresses

Results of torsional ring shear, direct shear, and triaxial compression tests on cohesive soils reveal that the fully softened shear strength is stress-dependent and related to the type of clay mineral and quantity of clay-size particles. An empirical relationship for the fully softened friction angle is presented that is a function of liquid limit, clay-size fraction, and effective normal stress. Studies of first-time slides, i.e., slopes that have not undergone previous sliding, in stiff fissured clay with a liquid limit between 41 and 130% suggest that the mobilized shear strength along the failure surface can be lower than the fully softened shear strength. Recommendations are presented for estimating the mobilized shear strength in first-time slides based on soil plasticity. Soils with a liquid limit greater than 30% exhibit a large difference between the fully softened and residual friction angles. In these soils, the presence or absence of a pre-existing shear surface should be clarified.

Slope Stability Analyses in Stiff Fissured Clays

The cohesion intercept in the Coulomb shear‐strength equation has a significant influence on the location of the slip surface and the factor of safety. The cohesion intercept results from a linear approximation of a segment of a nonlinear shear strength against effective normal stress relationship. Shear strength on a slip surface, at the instant of a global instability, may range from residual strength to peak strength for stiff clays, and from large strain strength to peak strength for soft clays. Peak strength controls local development of slip surface, but it may not be available during global failure. The critical slip surface for an initial slide is determined using the peak shear strength. However, a mobilized strength equal to or less than the peak strength is used to define the factor of safety. Effective stress‐stability analysis of slope failures has resulted in a reduction factor to the peak strength in order to obtain the mobilized strength for first time slides. For over 60 slope failures in...

Cohesion Intercept in Effective Stress‐Stability Analysis

Case histories of failure of water-retaining structures on highly plastic clay indicate that the intact strength of the clay does not reliably predict the stability condition. A study of the foundation conditions revealed the presence of structural discontinuities such as nugget and blocky structure, joints, fissures, and slickensides. A re-analysis of the failures, using the normally consolidated strength of the soft highly plastic clay, as suggested by Skempton and Hutchinson for stiff fissured clays, gives a more reliable prediction of the in situ stability condition.

Y. Lu

Papers

Shear strength of soft fissured clays