STUDY ON DAMAGE MECHANISM OF ALKALI ACTIVATED FLY ASH MINERAL POWDER MODIFIED EXPANSIVE SOIL UNDER DRYING WETTING FREEZING THAWING CYCLES

Due to seasonal water supply & shutdown and temperature change, the expansive soil canal of the first phase project of water supply in northern Xinjiang has been in the drying-wetting-freezing-thawing cycles for a long time and is very easy to cause the instability of canal foundation slope. Based on previous research results of alkali activated fly ash mineral powder co curing expansive soil of the canal foundation, unconfined compressive strength, direct shear, compression and SEM scanning electron microscopy tests were conducted on the improved soil after drying-wetting-freezing-thawing cycles. The influence of the drying-wetting-freezing-thawing cycles on the mechanical properties of improved soil and its physical mechanism were emphatically analyzed. The results show that: With the increase of the number of drying-wetting-freezing-thawing cycles, the unconfined compressive strength of the unmodified expansive soil decreases significantly, and the strength decreases by 51.8% after 9 cycles; However, compared with the unmodified expansive soil, the strength of alkali activated fly ash mineral powder improved soil increases by 8~12 times, and the stress-strain curve changes to strain hardening type, showing obvious brittle failure characteristics. With the increase of the number of cycles, the value of the unmodified expansive soil undergoes three stages of extremely sharp reduction-slow reduction-steady change, and tends to be stable after 3 cycles with no obvious change rule; The change value of the improved soil with the number of cycles tends to be constant, but fluctuates up and down, and the shear failure is relatively more stable. After 9 cycles, the fluctuation amplitude is about 6.72%, basically unchanged. With the increase of the number of cycles, the pores of the unmodified expansive soil are obviously affected by the overburden pressure, and the compression coefficient and compression index increase steadily, resulting in larger settlement deformation and higher compressibility; The compression index of improved soil before and after cycling is lower than 0.1, which indicates a low compressibility soil. The compression coefficient first rises and then decreases in the first 5 cycles, and then tends to be stable. Compared with the unmodified expansive soil, the compressibility of improved soil is lower; The drying-wetting-freezing-thawing cycles increase the fine micro pore content and macropores of the unmodified expansive soil, and reduce the compactness of soil particles; However, the improved soil restricts the formation of large holes and the development of cracks, maintains the apparent integrity of the soil mass, inhibits the formation of micro pores of expansive soil and the cutting and damage of soil particles due to cyclic action, reduces the impact on soil pore damage and particle damage, and further significantly increases the strength of the soil mass, indicating that alkali activated fly ash mineral powder solidified expansive soil has obvious advantages in resisting the drying-wetting-freezing-thawing cycles. It can be used as the replacement material for expansive soil canal slope.