干湿-冻融循环对碱激发粉煤灰-矿粉改性膨胀土力学特性的损伤机理研究

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

  • 摘要: 由于季节性的通水停水以及温度变化,使北疆供水一期工程膨胀土明渠渠坡长期处于干湿-冻融循环状态,极易引起渠基边坡失稳。基于前期对碱激发粉煤灰-矿粉协同固化渠基膨胀土的研究成果,对经历干湿-冻融循环后的改良土进行无侧限抗压强度、直剪、压缩和SEM电镜扫描试验,着重分析干湿-冻融循环对改良土力学性质的影响规律及其物理机制。试验结果表明:随着干湿-冻融循环次数的增加,未改良的膨胀土无侧限抗压强度下降明显,9次循环后强度下降约51.8%,但碱激发粉煤灰-矿粉改良土相较于未改良的膨胀土强度增幅8倍~12倍,应力应变曲线转变为应变硬化型,呈现出明显的脆性破坏特征;随着循环次数的增加,未改良的膨胀土 c 值经历"极剧减少-降低幅度减缓-平稳变化"3个阶段,在循环3次后趋于稳定, \varphi 值无明显变化规律,而改良土随循环次数的变化 c 值在水平趋势上呈现上下波动,剪切破坏相对更加稳定, \varphi 值9次循环后波动幅度在6.72%左右,基本保持不变;随着循环次数的增加,未改良的膨胀土孔隙受上覆压力影响明显,压缩系数与压缩指数稳步上升,产生较大沉降变形,表现出较高的压缩性,而改良土循环前后压缩指数都低于0.1,属于低压缩性土,压缩系数先上升后下降5次循环后趋于稳定,较未改良的膨胀土表现出较低的压缩性;干湿-冻融循环作用使未改良的膨胀土细-微观孔隙含量以及大孔隙增多,土颗粒间的密实度降低,而改良土限制了大孔洞的生成和裂隙的发展,保持了土体的表观完整性,并且抑制了循环作用对膨胀土微观孔隙的形成和土体颗粒的切割与破损,也减少了对土体孔隙损伤和颗粒破坏的影响,进而显著增强了土体强度,表明碱激发粉煤灰-矿粉固化膨胀土在抵抗干湿-冻融循环作用上具有明显的优势,可作为进行膨胀土渠坡的换填材料。

     

    Abstract: 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.

     

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