The shear strength of an unsaturated soil is written in terms of two independent stress state variables. One form of the shear strength equation is[Formula: see text]The transition from a saturated soil to an unsaturated soil is readily visible. A second form of the shear strength equation is[Formula: see text]Here the independent roles of changes in total stress σ and changes in pore-water pressure uw are easily visualized.Published research literature provides limited data. However, the data substantiate that the shear strength can be described by a planar surface of the forms proposed. A procedure is also outlined to evaluate the pertinent shear strength parameters from laboratory test results.

A common occurrence in cuts or fills of swelling soils is their reduction in strength with time. At the time of compaction, the clay generally has a high matrix suction. Correspondingly, it has a high strength and will stand at relatively steep side slopes. With time, the soil generally tends towards saturation and the matrix suction reduces towards zero. There is a reduction in total strength and if the gravitational forces are too large, the slope fails.During the past several years, numerous cut and fill slopes have been observed in the Regina area of Saskatchewan. Many of these slopes have remained stable for 4–6 years and then failed. There has been a 20 year history of observations on the Belle Plaine overpass west of Regina. Field and laboratory investigations have been conducted.With a knowledge of the geometry of the slope and failure plane, the simplified Bishop method of stability analysis was used to perform a 'back-analysis' to assess the shear strength parameters. The shear strength parameters from the laboratory program are compared with those calculated from the stability analyses. The analyses indicate that the peak shear strength parameters from triaxial tests on the softened Regina clay (i.e., c' = 5 kPa and [Formula: see text]), with the appropriate pore water pressures, give a factor of safety of 1 for the failed surface. The effect of spring thawing appears to be to produce the condition of most serious pore water pressures.

The stability of slopes at bridge abutments across the Carrot River in east-central Saskatchewan was not influenced significantly by drawdown after flooding in the spring of 1995. Traditional methods of analysis for rapid drawdown predicted the factor of safety of slopes on highly plastic clays of proglacial Lake Agassiz would drop to 0.65 from an initial value of 1.0. Deformation along a well-defined slip plane has persisted at a more or less constant, slow rate since the bridge was constructed in 1975. The river rose approximately 10 m during a flood in the spring of 1995, yet there was only minimal response in piezometers and no measurable increase in the rate of deformation recorded by inclinometers. Pore-water pressures from a steady state seepage model, which was calibrated from piezometer measurements, were integrated into a stability analysis. Changes in pore-water pressures caused by flooding and subsequent drawdown were characterized from a transient seepage model using the flood hydrograph as a flux boundary. The stability analysis integrated with the transient seepage model estimated the factor of safety would drop from 1.0 to 0.91 after drawdown. Field measurements indicated the reduction in factor of safety was even less.

Two parallel, concurrently active slip surfaces of a landslide in clay shale of the Cretaceous Lea Park Formation are causing deformation of a bridge structure across the North Saskatchewan River near Deer Creek, Saskatchewan. The upper slip occurs at the contact between the shale and glacial deposits, which is common in this region. However, the second slip occurs deep in the shale, 24 m below the upper slip zone. This multilevel landslide mechanism, not reported previously in this region, is resulting in a complex deformation pattern where components of the structure are moving at different rates. The multilevel slip mechanism is related to a unique combination of the hydrogeology and geologic structure at this site. Under an upward groundwater gradient, slip surfaces occur at discontinuities in available shearing resistance at different elevations in the shale. The discontinuities are gouge zones in the clay shale, which are the result of a combination of glacial shear and regional tectonism where parameters have been reduced to a residual state ([Formula: see text] and c′ = 0). The pore-water pressures for the slope stability analysis were generated from a site specific finite element seepage model using boundary conditions determined from a regional finite element seepage model. The groundwater models were calibrated from piezometer data and from hydrochemistry of water from farm wells, piezpmeters, and natural surface ponds. The hydrochemistry was used to delineate groundwater, discharge areas from recharge areas. The validity of the landslide mechanism is supported by a stability analysis integrated with the finite element seepage analysis, which demonstrates that two separate parallel slip surfaces at different depths can be at a state of limiting equilibrium concurrently. Key words : bridge deformation, Cretaceous shale, integrated models, residual strength, multilevel slips.