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

In accretionary wedges, often morphologically similar sedimentary intrusions, when observed by remote geophysical means, may have one of two quite different driving mechanisms and a highly variable significance for the regional hydrogeologic picture. For example, mud diapirs are driven by buoyancy forces that arise from bulk density contrasts. In them, mud and pore fluid upwell en masse and fluid migration is a related (sometimes important) but generally subsidiary process. In contrast, diatremes contain sediments fluidized during rapid fluid advection and are forcibly and directly driven by the hydrogeologic system. The nature of fluid input from local and exotic source regions can, therefore, strongly affect sedimentary intrusive processes and vice versa. This complicates the process of defining the main features of the hydrogeological systems operating in accretionary wedges. Focused vertical advection through steep sided (piercement) mud diapirs requires conduit systems, otherwise flow will be diffuse and directed more horizontally out of the low-permeability mud mass. However, where the permeability of the overburden is less than that of the diapir, the whole diapir may act as a conduit. Apart from this special case, conduits will be associated with highly anisotropic scaly fabrics that can sometimes develop in the marginal shear zone of diapirs. Scaly fabrics form during deformation and compaction of a mud matrix under conditions of constant or increasing effective stress. However, the effective stress path can be complex as it is both controlled by the relative rates of upward intrusion and burial (by sedimentation and/or structural thickening) and the hydrogeologic system. Due to this, it appears likely that even in a geographically related group of diapirs, effective stress histories will vary widely between intrusions so that some can form advective pathways for fluids and some cannot. Mixed systems of behavior may also be present with local diatremes developing within diapirs above the terminations of conduit systems and rapidly expanding methane gas pockets. The potentially heterogeneous near-surface behavior may be why the surface manifestations of sedimentary intrusions are so variable when observed in the field. Diatremes can also form separately as large primary features above any structural or stratigraphic conduit that rapidly expels water or gas into the base of an unlithified sediment column. When active, large diatremes require enormous quantities of fluid (water or gas) to drive them, particularly if they are long lived features and hence are a direct indication of at least an episodically vigorous hydrogeologic system.

The nature and hydrogeologic significance of mud diapirs and diatremes for accretionary systems

A method of interpreting conventional oedometer test data using work per unit volume as a criterion for determining both in situ effective and yield stresses in clay is presented. This technique wa...

Work as a criterion for determining in situ and yield stresses in clays: reply

Yielding is an important feature of the stressstrain behavior of lightly overconsolidated clays. This paper describes tests on 36mm triaxial samples of natural Mexico City clay. The specimens are c...

Yielding of Mexico City clay and other natural clays

A global analysis of the consolidation of natural clays is realized considering the consolidation process to be a combination of the effects of compressibility and of permeability. The compressibil...

Tenth Canadian Geotechnical Colloquium: Recent developments in consolidation of natural clays

Current methods of predicting the response of soft clays to surface loading are often unsuccessful because the assumed constitutive relationships, including effective stress path behaviour, are inc...

J. H. A. Crooks

Papers

Effective stress paths and yielding in soft clays below embankments