Fracture mechanism of rock collapse in the freeze-thaw zone of eastern Sichuan-Tibet Mountains under seasonal fluctuating combinations of water and heat

In the freeze-thaw zone of eastern Sichuan-Tibet Mountains, the phases of water in cracks show strong seasonal variations, which significantly affect the stability of perilous rocks in mountains. However, few works have clearly addressed the role of water/ice in crack development from a fracture mechanics viewpoint to explain the seasonality of rock collapse. In this study, we built physical models from a fracture mechanics viewpoint to calculate water-freezing stress, hydrostatic pressure, and their combinations induced by water/ice in cracks, and show the crack propagation mechanism under temperature fluctuations in different seasons in mountainous regions. Based on the models, we calculate fracture conditions, simulate the crack process, and illustrate the rock collapse mechanism in different seasons by the extended finite element method. The results indicate that different phases of water, which induce stress under spatiotemporal fluctuations of temperature, determine the various propagation styles and influence what kind and when a collapse will occur. The collapse of fractured rocks in different seasons generally results from rock damage accumulation owing to the initiation, propagation, and connection of primary cracks under freezing stress or hydrostatic pressure or their different combinations.

Research on resistance degradation and reliability of pre-stressed concrete structures in freeze-thaw zone

Purpose In the freeze-thaw zone, the pre-stressed concrete of bridge structure will be damaged by freezing-thawing, the bearing capacity of structure will decrease and the safety will be affected. The purpose of this paper is to establish the time-dependent resistance degradation model of structure in the freeze-thaw zone, and analysis the structural reliability and remaining service life in different freeze-thaw zones. Design/methodology/approach First, according to the theory of structural design, a calculation model of the resistance of pre-stressed concrete structures in f freeze-thaw zone is established. Second, the time-dependent resistance model was verified by the test beam bending failure test results done by the research group, which has been in service for 20 years in freeze-thaw zone. Third, using JC algorithm in MATLAB to calculate the index on the reliability of pre-stressed concrete structure in frozen thawed zones, forecasting the s remaining service life of structure. Findings First, the calculation model of the resistance of pre-stressed concrete structures in freeze-thaw zone is accurate and it has excellent applicability. Second, the structural resistance deterioration time in Wet-Warm-Frozen Zone is the earliest. Third, once the pre-stressed reinforcement rusts, the structural reliability index will reach limit value quickly. Finally, the remaining service life of structure meets the designed expectation value only in a few of freeze-thaw zones in China. Originality/value The research will provide a reference for the design on the durability of a pre-stressed concrete structure in the freeze-thaw zone. In order to verify the security of pre-stressed concrete structures in the freeze-thaw zone, engineers can use the model presented in this paper for durability checking, it has an important significance.

Damage Analysis of Freeze-Thaw Processes in Hydraulic Structure Concrete

Based on the theory of damage finite element method, the deterioration process of hydraulic concrete under the freeze-thaw cycles is simulated. The damage evolution equation is deduced and freeze-thaw zone is discussed. The results indicate that the load capacity of structure is degraded under the freeze-thaw cycles. The deterioration of hydraulic concrete under the freeze-thaw cycles, however, is mainly limited at the region of the water fluctuation district and has a little effect to the overall structure. This means that the major loads causing hydraulic concrete failure under the freezing-thawing damage in the coldest region is not those of general design loads, such as gravity load and water pressure load, but the thermal loads, especially by the distinct daily temperature difference. Thus, the freeze-thaw modeling should consider these loads together.

Comparison of thermal and radar active layer measurement techniques in the Leaf Bay area, Nouveau-Québec

Continuous profiles of the frost table for a series of traverses in unconsolidated sediments in the Leaf Bay area, Ungava, were obtained using an all terrain vehicle borne impulse radar system. The results correlate extremely well with data obtained by the insertion of rigid temperature probes at frequent intervals along the traverses. Each of the two systems has its own particular advantages and disadvantages. The radar profiling system provides more continuous information and can be carried out more rapidly than the system using the temperature probes. However, it requires some local ground control as regards soil moisture profiles and values for soil texture and soil temperature. A definite advantage of the rigid temperature probe is that it produces a temperature curve as a by-product and this can be used to assess the effect of different terrain factors on heat flow in the soil. It is, furthermore, a much cheaper and less bulky apparatus than the radar equipment, requiring no auxiliary logistical support for field use. Neither method can be applied universally for thaw zone and active layer determination. The trials in the tidal flats were a failure, the earth materials being too stony for the temperature probe and the salt water in the soil not permitting proper signal propagation for the radar. Neither method is suitable for penetrating bedrock and, in such a case, reliance has to be placed on temperature measurements in drillholes.

Evidence for Headwall Weathering Zones, Boundary Glacier, Canadian Rocky Mountains

Temperature records from several randkluft sites at the margins and headwall of Boundary Glacier (lat. 52° 12′ N., long. 117° 12′ W.) in the Canadian Rocky Mountains are presented. These records indicate that during the ablation season a diurnal freeze-thaw air and rock-surface temperature regime occurs in a 2 m wide zone centred on the randkluft lip. Deeper in the randkluft, stable sub-zero conditions prevail whereas above the randkluft an above-freezing temperature regime prevails. The freeze-thaw temperature regime, observed freezing of melt water and rain water on randkluft rock surfaces, and copious in-situ loose weathering products on the headwall and marginal free faces suggest an active frost-shattering environment. Down-slope migration of the randkluft lip during the ablation season results in a migration of the freeze-thaw zone and thus a seasonal extension of the rock surface area exposed to the favorable weathering environment. Longer-term glacier fluctuations, with attendant thinning and thickening of the ice body, could result in large areas of marginal and headwall rock surfaces being exposed to the randkluft weathering environment over long periods of time. The data and observations from Boundary Glacier support an idea, suggested by Battle (1960), that open and shallow randklufts, rather than closed and deep bergschrunds, are a focus of weathering at glacier margins and thus a potential factor in cirque development.

Evidence for Headwall Weathering Zones, Boundary Glacier, Canadian Rocky Mountains

Temperature records from severalrandkluftsites at the margins and headwall of Boundary Glacier (lat. 52° 12′ N., long. 117° 12′ W.) in the Canadian Rocky Mountains are presented. These records indicate that during the ablation season a diurnal freeze-thaw air and rock-surface temperature regime occurs in a 2 m wide zone centred on therandkluftlip. Deeper in therandkluft, stable sub-zero conditions prevail whereas above therandkluftan above-freezing temperature regime prevails. The freeze-thaw temperature regime, observed freezing of melt water and rain water onrandkluftrock surfaces, and copiousin-situloose weathering products on the headwall and marginal free faces suggest an active frost-shattering environment. Down-slope migration of therandkluftlip during the ablation season results in a migration of the freeze-thaw zone and thus a seasonal extension of the rock surface area exposed to the favorable weathering environment. Longer-term glacier fluctuations, with attendant thinning and thickening of the ice body, could result in large areas of marginal and headwall rock surfaces being exposed to therandkluftweathering environment over long periods of time. The data and observations from Boundary Glacier support an idea, suggested by Battle (1960), that open and shallowrandklufts, rather than closed and deep bergschrunds, are a focus of weathering at glacier margins and thus a potential factor in cirque development.