Aiming at the geological characteristics of loess overlying red-bedded mudstone in the Gansu area, two kinds of industrial solid wastes, namely steel slag and mineral powder are adopted to carry out the research on the mechanical properties and the constitutive model of the modified loess-red bedded mudstone mixed fill by compacting and triaxial CU test. The results show that: (1) The maximum dry density of the mixed fill is the largest when the mass ratio of loess: red mudstone is 1:3. (2) The shear strength of steel slag is maximum when the mass dosing of steel slag is 7.5%. The larger the mass dosing of mineral powder is, the larger the shear strength is. (3) It is verified that the strain hardening process of soil samples before and after steel slag improvement could be accurately fitted by the Duncan-Zhang model. (4) For the problem that the hyperbolic model promoted by Shen Zhujiang can not fit the strain hardening well, this paper proposes to use the staged fitting method with positive and negative solutions, and it is verified that this method can accurately fit the strain hardening process of the mineral powder amended soil.

The water absorption and softening and volume expansion characteristics of the red mudstone are prone to cause the phenomenon of slurry mud and uneven settlement of the roadbed and other diseases, so it should be improved when used as highway roadbed filler. In this paper, fly ash was chosen as an improver to analyze the compositional composition and ionic content of red-bedded mudstone and fly ash through indoor X-ray diffraction tests, chemical composition tests and easily soluble salts. The physical and mechanical properties of the specimens before and after improvement were analyzed by unconfined compressive strength tests, indoor variable head penetration tests and consolidation tests. The effects of different fly ash admixtures on the shear stress-strain gauge shear strength of the modified specimens were investigated by conventional triaxial UU tests; the effects of the age of maintenance on the shear strength parameters of the modified tests when the fly ash admixture was 10% were analyzed. The results show that the red mudstone in the area is a low liquid-limit clay with the highest SO₄^2- ion content and alkaline pH in its labile salt solution. In triaxial shear test, both modified and unmodified specimens showed plastic damage, and both the amount and age of fly ash admixture of the specimens contributed to the increase of shear strength of the specimens, and the increase of cohesion was much larger than the internal friction angle. The cohesive force of the specimens at the age of 28 d with 10% fly ash was 37.9% and 6.9% higher than that of the unmodified unmaintained specimens and 10% unmaintained specimens, respectively, and the internal friction angle increased by 1.2° and 0.7°. The hydrostatic experiments of the specimens all verified the reliability of the fly ash improvement.