Loss of matric suction during rain has an important effect on the instability of expansive soil slope. A new device was designed for testing the expansive force in order to propose and determine the equivalent expansive force corresponding to the matric suction. First, the internal relation between the matric suction and the corresponding equivalent expansive force of unsaturated expansive soil was analyzed. Then, numerical algorithm implementation of the equivalent expansive force was discussed, and the equivalent expansive force was introduced into the strength reduction finite element method (FEM). Finally, the equivalent effects of the matric suction and the equivalent expansive force were compared and analyzed by evaluating the stability of an unsaturated expansive soil slope. The results show that the new testing device significantly improves the accuracy of the expansive force test and shortens the testing time. The relation between the matric suction and the equivalent expansive force with the change in initial water content is obvious. The equivalent expansive force can reflect the macro contribution of matric suction to unsaturated expansive soil, and the developed strength reduction FEM based on the equivalent expansive force can be used to evaluate the rainfall-induced instability of an expansive soil slope caused by the decrease in matric suction resulting from the rainfall infiltration.

It is of great theoretical and engineering significance to carry out field rainfall tests and research on double-layer soil slopes in loess areas. Based on the developed rainfall simulation system with slow-moving injection, field rainfall tests were carried out on a natural double-layer loess slope. The characteristics of volumetric water content were monitored, and the rainfall infiltration characteristics and infiltration effect at the interface of the soil layer were analyzed by numerical simulation. The results showed the fastest infiltration at the top platform of the slope, followed by that at the upper surface of the slope, and the slowest infiltration at the lower surface of the slope during rainfall. Under various rainfall intensities, the erosion of the upper silty loess slope was greater than that of the lower clay loess slope, and the erosion patterns were quite different at the end of rainfall. During the infiltration process in the double-layer loess slope, a stagnant transition area was formed near the interface of the soil layer. The equipotential line of water content in the stagnant transition area of the upper region was roughly parallel to the slope surface, and the equipotential line in the lower region was roughly parallel to the interface of the soil layer. With an increase in rainfall intensity, the upper transition area at the interface of the soil layer continued to extend from the slope surface inward, showing the interface infiltration effect that became increasingly significant with the intensification of rainfall. The infiltration effect at the soil layer interface could provide an evaluation basis for rainfall infiltration analyses of multi-layer soil slopes.

When tests are conducted on the field slope under artificial rainfall, because artificial rainfall is often limited to implementation in the mode of local rainfall, there is a boundary constraint effect between the rainfall area and the non-rainfall area, which is manifested in the lateral infiltration of rainwater and the slope deformation retardation of non-rainfall area to the rainfall area. Firstly, a deep-buried isolated trench was proposed to solve these boundary constraints. Then, field cracking tests and the corresponding numerical simulation were conducted under rainfall. In the end, the response of water content and the cracking failure characteristics of the slope were analyzed during rainfall, and the effect of a deep-buried isolated trench on the cracking characteristics of the slope was evaluated. The results indicate that the proposed deep-buried isolated trench measure can effectively eliminate the deformation retardation resulting from the adjacent non-rainfall area so a through-crack parallel to the slope shoulder that extended on both sides of the boundary of the rainfall slope was observed at the slope crest and a cracking failure in the shape of the overall downward cutting was realized. As the crack occurred, the rainwater infiltration further aggravated expansion of depthwise cracks, and a local sliding zone was formed in the upper part of the slope. The deep-buried isolated trench solves the boundary constraints, such as lateral infiltration of rainwater and deformation retardation, and can provide an effective technical measure for the field slope test under artificial rainfall.

To avoid large deformation, resulting from liquefaction, in inclined and deeply deposited liquefiable soil, it is necessary to design economical and reasonable reinforcement schemes. A reinforcement scheme employing subarea long-short gravel piles was proposed, and it was successfully applied in the embankment construction of the Aksu-kashgar highway. To reveal its underlying mechanism and effect on the seismic performance of the highway, the dynamic responses of natural foundation and two kinds of reinforced foundations were analyzed and compared under this scheme, using the program FEMEPDYN. Results showed that both the seismic subsidence and the excess pore pressure ratios were far less in the foundation reinforced with isometric gravel piles and in the foundation reinforced with subarea long-short gravel piles, compared with that in natural foundation. Therefore, the potential hazards of liquefaction were overcome in these two kinds of reinforced foundations. Furthermore, it was obvious that the shielding region only formed within the foundation reinforced with subarea long-short gravel piles. With the shielding effect, the proposed reinforcement scheme employing subarea long-short gravel piles not only eliminated liquefaction in deeply deposited liquefiable soil, but it also demonstrated an outstanding advantage in that the total length of gravel piles used was greatly reduced compared to the total length in the isometric gravel piles scheme and the interphase long-short gravel piles.

The development of an effective evaluation method suitable for loess-tunnel excavation is necessary to avoid the collapse accidents caused by tunnel excavation and any secondary disasters. Although the Fenner formulas and the modified Fenner formulas are widely used in tunnel engineering, a defect still exists in these formulas because the Mohr–Coulomb (M–C) criterion exaggerates the tensile strength of the surrounding rock of the loess tunnel. A newly modified Fenner formula was derived based on joint strength to overcome this deficiency. First, the expressions of stress and the radius of the plastic zone of the surrounding rock of the loess tunnel and the expressions of radial displacement were derived based on the stress-equilibrium equation of the axisymmetric plane and the joint strength. Then, the difference in the modified Fenner formulas based on the two kinds of strength criteria for the loess tunnel were compared. The results showed that the radius of the plastic zone and the radial displacement of the loess tunnel determined by the modified Fenner formula based on joint strength were larger than those determined by the modified Fenner formula based on M–C strength. However, the plastic stress of the plastic zone determined by the modified Fenner formula based on joint strength was smaller. The comparative analysis reveals that the modified Fenner formula based on joint strength can evaluate the stress and plastic-displacement field of the surrounding rock of a loess tunnel more reasonably.