Mechanical properties of composite improved phyllite soil under uniaxial compression

To address the issue of insufficient strength of phyllite soil as a subgrade filler, a composite improvement method using cement and red clay to improve phyllite soil was proposed. Through unconfined compressive strength tests, the stress-strain curves of composite improved phyllite soil under soaked and unsoaked conditions were studied, and the variations of unconfined compressive strength and deformation modulus with red clay mixing ratio and cement content were analyzed. Experimental results show that with the increase of red clay mixing ratio, the stress-strain curve of red clay modified phyllite soil exhibits typical elastoplastic characteristic when cement content is 0. When cement content is not 0, the stress-strain curve of composite improved phyllite soil approximates linear elastic characteristic, and the uniaxial compressive strengths and deformation moduli of red clay modified phyllite soil and composite improved phyllite soil increase. Under soaked condition, red clay modified phyllite soil disintegrates to result in a reduction of unconfined compressive strength and deformation modulus to 0. For composite improved phyllite soil, the unconfined compressive strength and deformation modulus decrease, with softening coefficients of 0.45-0.62 and 0.71-0.93 for cement contents of 3% and 5%, respectively, showing better water stability. When phyllite soil is improved by using cement and red clay, the increase in uniaxial compressive strength is better than the sum of the increments achieved by using cement and red clay alone, realizing an improvement effect of "1+1>2", that is, synergy. Therefore, it is advisable to adopt the composite improvement scheme to improve phyllite soil. Cement content of 3% and red clay mixing ratio of 20% can be used as an economic composite improvement scheme to meet the strength requirement of 350 kPa. In order to enhance water stability, rapid filling and full play of synergy, the composite improvement scheme of cement content of 5% and red clay mixing ratio of 40% can be considered.