Experimental Study on Undrained Shear Properties of Saline Soil under Freeze-Thaw Cycles

The freeze-thaw cycle is an important external factor affecting the hydromechanical characteristics of saline soil in cold regions. Due to the presence of water and salt, it has a greater impact on stability. The construction of various projects, such as ditch fills and road subgrades, has mostly used disturbed soils. Therefore, this article takes remolded saline soil in Qian’an, Jilin Province, China, as the research object to evaluate the action of freeze-thaw cycles on the critical state line, effective stress path, pore water pressure-strain relationship, stress-strain relationship, shear strength index, and other mechanical properties via a freeze-thaw cycle test and a consolidated undrained triaxial shear test (CU). The experimental results show that regardless of whether the soil specimen undergoes a freeze-thaw cycle, its stress-strain relationship shows characteristics of strain hardening, while, as the number of freeze-thaw cycles increases, the shear strength gradually decreases. As both the confining pressure and number of freeze-thaw cycles increase, the pore water pressure increases, as does the pore water pressure coefficient in shear failure. Under the action of freeze-thaw cycles, on the p ′ − q plane of the stress space, the effective stress path gradually moves to the lower left side. Both the effective stress path and the pore water pressure characteristics indicate that the degree of consolidation of the soil specimens continuously decreases as the number of freeze-thaw cycles increases. The position of the critical state line gradually lowers, and the critical state stress ratio decreases. The effective stress strength index can more accurately reflect the comprehensive influence of freeze-thaw cycles and confining pressure on the mechanical characteristics of soils than the total stress strength index. Logistic functions can be used to fit and predict the degradation law of the internal friction angle and cohesion.

Study and Analysis: Mechanism of Vacuum Preloading, Stress of Soil

Several viewpoints of vacuum preloading mechanism were analyzed. Action process of vacuum preloading was compartmentalized three phases. They were seepage consolidation in shallow soil and “surcharge” effect in deep soil by vacuum suction, consolidation stabilization in the shallow soil and transformation of “surcharge” to “negative pressure” in deep soil, seepage consolidation in deep soil under “negative pressure” and to balance. Action mechanism root of vacuum preloading was not negative pressure difference solely, which was produced by seepage field. The root was “positive pressure” and “negative pressure” together during different stage. The root also included the two pressures superposition under the especial phenomenon of vacuum preloading: water level falling. Under the mechanism, stress of soil was analyzed. Effective stress path began from line resembling that under surcharge in q-p plane. It would never reach line and after a short period it would develop paralleling p-axis. Horizontal contractive displacement of soil took place because “negative pressure” intensity was much stronger than “positive pressure” intensity. If surcharge effect equal vacuum preloading effect, soil under vacuum preloading reached yield more difficultly. Last, measured value was analyzed and the analysis of mechanism, stress was validated.

An embankment failure on soft fissured clay

Construction at the Genesee Power Project near Edmonton required 13 m high approach fills on soft clay for a roadway overpass structure. During construction, the north approach fill failed at a height of about 12 m. An investigation of the failure was carried out and a monitoring program instigated for further fill construction and piling for the overpass structure.The in situ mass strength of the soft clay was found to be controlled by defects such as slickensides rather than the intact strength. A translational mode of failure is considered most likely to have occurred, with computations based on c′ = 0 and [Formula: see text] determined from normally consolidated samples yielding the appropriate factors of safety to account for the failure. "Tearing" of the fill and overconsolidated crust due to the translational failure mechanism, with associated reduced average shear strengths, also occurred.Monitoring data for construction after the failure are presented and discussed. The effective stress path/yield envelope approach was used to assess the data. An empirical performance criterion for the site was developed for the overpass structure based on the observed deformation rates and computed factors of safety. Key words: embankment, stability, clay, failure, translational failure, effective stress path, fissured.

Effective stress paths and yielding in soft clays below embankments

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 incorrect. In particular, the transition from small-strain to large-strain behaviour (i.e. yielding) is frequently not taken into account. Recent laboratory testing has demonstrated that the behaviour of soft clays is largely controlled by yielding. The locus of effective stress states causing yield is known as the yield envelope (YE).The effective stress paths (ESP's) in soft clay foundations below the centre of six fills were determined from computed total stresses and measured pore-water pressures. Yield behaviour is clearly indicated by ESP shapes. The yield envelopes inferred from analyses of field data are similar to those obtained from laboratory testing. Effective stress path shapes vary widely, depending on a variety of factors, including imposed stress level, rate of construction, and boundary drainage conditions. This finding contradicts an earlier conclusion that soft clay behaviour can be characterized by a single ESP. Because of the wide range of possible ESP shapes, the parameters [Formula: see text] does not provide an adequate basis for determining the effective stress state in a soft clay.The ESP/YE analyses indicate that yield can occur either during loading or during excess pore-water pressure dissipation following completion of loading. Yield of sensitive soils during loading is usually followed by strain softening. However, in some soils, dilatant behaviour appears to occur. Yield during dissipation of excess pore-water pressure is characterized by a dramatic change in cv and increased compressibility. Key words: soft clay, yield, effective stress paths, field behaviour, strain softening, rate of consolidation.