Experimental Research On Mechanical Characteristics Of Rock Under The Coupling Effect Of Pre-stress And Freezing And Thawing

Freezing and thawing is an important threat against the rock engineering in cold regions. There are many factors affecting the freeze-thaw effect on rock which domestic and foreign scholars have been investigating for decades, and have achieved fruitful results.However, the experimental study of freezing and thawing in the past are almost all for stress-free rock, without considering the stress existing in rock mass in the natural environment. To investigate the impact of the existing stress on the freezing and thawing effect, a series of laboratory freeze-thaw experiments considering the coupling effect of the pre-stress are carried out. That is to say, rock samples are suppressed by a pre-stress during the freezing and thawing. With the research on some macroscopic physical changes and strength change law of rock, the influencing mechanism of the pre-stress in freezing and thawing is revealed by analyzing the stress distribution on the pore wall based on some assumptions. At last, we amended a slope mass rating system suitable for cold regions and proposed a new idea of using pre-stressed anchor cable(bolt) to prevent the freeze-thaw damage to rock engineering in cold regions. Generally speaking, we have done the following research:1) A pre-stress equipment was designed and manufactured to exert pressure on rock samples. The equipment has the following key functions: 1, accurately providing high stress; 2, maintaining the stress in a lone time; 3, being adapt to harsh environment such as freezing and thawing; 4, the stress can be monitored; 5, small and exquisite, compatible with the freeze-thaw equipment.2) The freeze-thaw cycle test of saturated sandstone under 5 levels of stress(0MPa,0.75 MPa, 1.6MPa, 3MPa, 5MPa)was conducted. The freezing and thawing were carried out in 3 batches(20times, 40 times, 60times)。The dry mass, saturated mass, height,diameter and longitudinal wave velocity of the sample were measured before and after freeze-thaw cycle,these changes can reflect the degree of freezing and thawing of rocks.3) Uniaxial compression tests were carried out on the rock samples of each pre-stress level and each batches of the freeze-thaw cycle. The strength and deformation characteristics of rock under different pre-stress were studied after different times of freeze-thaw cycles. And the variation law of the frost resistance coefficient of the rock with the size of the pre-stress is studied. In order to investigate the longitudinal strength characteristics of the specimens, Splitting test was carried out on the rock after 40freeze-thaw cycles under the action of each pre-stress. In addition, in order to verify theresults of strength characteristics of rock under the three stress state are consistent with that under uniaxial compression, selecting the value of pre-stress as 0MPa and 3MPa after 60 cycles to do the three axial compression tests. Confining pressure 5 levels: 0, 5, 10, 15,20 MPa, the results of fitting criterion of Mohr-Coulomb and Hoek-Brown have been compared.4) According to the results, based on the assumption that the pore is the circular channel and the material is elastic, the stress analysis on the pore wall is carried out, giving failure condition of interaction of pre-stress and frost force pore wall. And then reveals the mechanism of the influence of pre-stress on the freezing and thawing of rocks.5) The concept of the threshold value of frost resistivity is put forward. It is pointed out that when the pre-stress reached the threshold value, the freeze-thaw damage will stop.And based on the study of the relationship between pre-stress and frost resistance coefficient, the freezing and thawing coefficient of TSMR system was adjusted. The rock quality evaluation system(Y_TSMR) in cold region considered the effect of stress state on the freezing and thawing of rocks is put forward. At last, the writer tries to put forward the new idea of using pre-stressed anchor cable(bolt) for prevention of frost damage in Engineering. This is a breakthrough to the fixed thinking of past that has relied on the prevention of frost damage of waterproof and thermal insulation. That may help find a new technology for the prevention of freeze damages.