Effects of thawing-induced softening on fracture behaviors of frozen rock

Due to the presence of ice and unfrozen water in pores of frozen rock,the rock fracture behaviors are susceptible to temperature.In this study,the potential thawing-induced softening effects on the fracture behaviors of frozen rock is evaluated by testing the tension fracture toughness(KIC)of frozen rock at different temperatures(i.e.-20℃,-15℃,-12℃,-10℃,-8℃,-6℃,-4℃,-2℃,and 0℃).Acoustic emission(AE)and digital image correlation(DIC)methods are utilized to analyze the microcrack propagation during fracturing.The melting of pore ice is measured using nuclear magnetic resonance(NMR)method.The results indicate that:(1)The KIC of frozen rock decreases moderately between-20℃ and-4℃,and rapidly between-4℃ and 0℃.(2)At-20℃ to-4℃,the fracturing process,deduced from the DIC results at the notch tip,exhibits three stages:elastic deformation,microcrack propagation and microcrack coalescence.However,at-4℃e0℃,only the latter two stages are observed.(3)At-4℃e0℃,the AE activities during fracturing are less than that at-20℃ to-4℃,while more small events are reported.(4)The NMR results demonstrate a reverse variation trend in pore ice content with increasing temperature,that is,a moderate decrease is followed by a sharp decrease and-4℃ is exactly the critical temperature.Next,we interpret the thawing-induced softening effect by linking the evolution in microscopic structure of frozen rock with its macroscopic fracture behaviors as follow:from-20℃ to-4℃,the thickening of the unfrozen water film diminishes the cementation strength between ice and rock skeleton,leading to the decrease in fracture parameters.From-4℃ to 0℃,the cementation effect of ice almost vanishes,and the filling effect of pore ice is reduced significantly,which facilitates microcrack propagation and thus the easier fracture of frozen rocks.

Effects of confining pressure and temperature on strength and deformation behavior of frozen saline silty clay

Buildings are always affected by frost heave and thaw settlement in cold regions,even where saline soil is present.This paper describes the triaxial testing results of frozen silty clay with high salt content and examines the in-fluence of confining pressure and temperature on its mechanical characteristics.Conventional triaxial compression tests were conducted under different confining pressures(0.5–7.0 MPa)and temperatures(-6℃,-8℃,-10℃,and-12℃).The test results show that when the confining pressure is less than 1 MPa,the frozen saline silty clay is dominated by brittle behavior with the X-shaped dilatancy failure mode.As the confining pressure increases,the sample gradually transitions from brittle to plastic behavior.The strength of frozen saline silty clay rises first and then decreases with increasing confining pressure.The improved Duncan-Chang hyperbolic model can describe the stress-strain relationship of frozen saline silty clay.And the parabolic strength criterion can be used to describe the strength evolution of frozen saline silty clay.The function relation of strength parameters with temperature is obtained by fitting,and the results of the parabolic strength criterion are in good agreement with the experimental results,especially when confining pressure is less than 5 MPa.Therefore,the study has important guiding significance for design and construction when considering high salinity soil as an engineering material in cold regions.

Hysteresis in the ultrasonic parameters of saturated sandstone during freezing and thawing and correlations with unfrozen water content

Determining the mechanical properties of frozen rock is highly important in cold-area engineering.These properties are essentially correlated with the content of liquid water remaining in frozen rock.Therefore,accurate determination of unfrozen water content could allow rapid evaluation of mechanical properties of frozen rock.This paper investigates the hysteresis characteristics of ultrasonic waves applied to sandstone(in terms of the parameters of P-wave velocity,amplitude,dominant frequency and quality factor Q)and their relationships with unfrozen water content during the freeze-thaw process.Their correlations are analysed in terms of their potential for use as indicators of freezing state and unfrozen water content.The results show that:(1)During a freeze-thaw cycle,the ultrasonic parameters and unfrozen water content of sandstone have significant hysteresis with changes in temperature.(2)There are three clear stages of change during freezing:supercooled stage(0℃to-2℃),rapid freezing stage(-2℃to-3℃),and stable freezing stage(-3℃to-20℃).The changes in unfrozen water content and ultrasonic parameters with freezing temperature are inverse.(3)During a single freeze-thaw cycle,the ultrasonic parameters of sandstone are significantly correlated with its unfrozen water content,and this correlation is affected by the pore structure.For sandstones with mesopores greater than 50%,there are inflection points in the curves of ultrasonic parameters vs.unfrozen water content at-3℃during freezing and at-1℃during thawing.It was found that thermal deformation of the mineral-grain skeleton and variations in the phase composition of pore water change the propagation path of ultrasonic waves.The inflection point in the curve of dominant frequency vs.temperature clearly marks the end of the rapid freezing stage of pore water,in which more than 70%of the pore water freezes.Consequently,the dominant frequency can be used as an index to conveniently estimate the unfrozen water content of frozen rock and,hence,its mechanical properties.

A model of unfrozen water content in rock during freezing and thawing with experimental validation by nuclear magnetic resonance

The unfrozen water content of rock during freezing and thawing has an important influence on its physical and mechanical properties.This study presented a model for calculating the unfrozen water content of rock during freezing and thawing process,considering the influence of unfrozen water film and rock pore structure,which can reflect the hysteresis and super-cooling effects.The pore size distribution cu rves of red sandsto ne and its unfrozen water conte nt under different temperatures during the freezing and thawing process were measured using nuclear magnetic resonance(NMR) to validate the proposed model.Comparison between the experimental and calculated results indicated that the theoretical model accu rately reflected the water content change law of red sandstone during the freezing and thawing process.Furthermore,the influences of Hamaker constant and surface relaxation parameter on the model results were examined.The results showed that the appropriate magnitude order of Hamaker constant for the red sandstone was 10J to 10J;and when the relaxation parameter of the rock surface was within 25-30 μm/ms,the calculated unfrozen water content using the proposed model was consistent with the experimental value.

Effect of the frozen layer on the stability of cut soil slopes during seasonal freezing and thawing

Research on the stability of soil slopes in seasonally frozen regions has mainly focused on slope failures during the thawing window.There are few studies on slope stability during the freezing window and its subsequent influence on slope failure in the next thawing window.In this paper,soil strength was tested during freezing and thawing to obtain temperature-dependent strength parameters for the simulation of slope stability.Then,the slope's temperature field over an entire year was accurately simulated so that characteristics of the frozen layer could be determined at any time.Based on the above results,the progressive failure modes of frozen soil slopes are discussed.The results show that:1)during the freezing window,depth of the frozen soil layer increases,as does the slope's safety factor,while a yield zone propagates towards the slope shoulder.(2)During the thawing window,the frozen soil layer shrinks in depth while the yield zone continuously expands,which decreases the safety factor.Comprehensive analysis of these results indicate that the frozen layer provides a“toe-locking effect”that increases the safety factor during the freezing window,while it also provides a“dragging effect”that propagates the yield zone towards the slope shoulder.During the thawing window,the“toe-locking effect”gradually diminishes;a continuous sliding surface is formed,which lead to a landslide.The frozen soil layer of the freezing window accelerates the slope sliding in the thawing window.