This paper investigated the influence of dry–wet cycles and sulfate attack on the performance of magnesium potassium phosphate cement (MKPC) as well as the effect of waterglass on MKPC. X-ray diffraction (XRD), TG-DTG, and scanning electron microscopy (SEM-EDS) were used to examine the phase composition and microstructure of MKPC. The results showed that the flexural and compressive strength of an MKPC paste increased initially and subsequently decreased in different erosion environments. The final strength of the M0 paste exposed to the SK-II environment was the highest, while that of the M0 paste exposed to the DW-II environment was the lowest. The final volume expansion value of MKPC specimens under four corrosion conditions decreased in the following order: DW-II, M0 > SK-II, M0 > DW-II, M1 > SK-I, M0 > DW-I, M0. Compared to the full-soaking environment, the dry–wet cycles accelerated sulfate erosion and the appearance of damages in the macro and micro structure of the MKPC paste. With the increase in the number of the dry and wet cycles, more intrinsic micro-cracks were observed, and the dissolution of hydration products was accelerated. Under the same number of dry–wet cycles, the strength test and volume stability test showed that the durability in a Na₂SO₄ solution of the MKPC paste prepared with 2% waterglass (M1) was superior to that of the original M0 cement. The micro analysis indicated that waterglass can improve the compactness of the microstructure of MPC and prevent the dissolution of struvite-K.

Vacuum–surcharge preloading is widely used in soft soil foundation reinforcement projects. To study the characteristics of the changes in pore water pressure and settlement of the soft soil foundation as a result of the effect of vacuum–surcharge preloading, in this paper, a consolidation model was established for large–sized sand–drain fluid–solid coupling along the depth direction while taking the sequence of the actual surcharge and vacuum into consideration by using FLAC^3D, and a soft foundation’s consolidation process was calculated and analyzed in detail. The results showed the following: (1) the sand drain could be used as a load boundary condition and a vacuum load could be realized by adjusting the pore pressure at the nodes. (2) For the effect of a single vacuum, the settlement and pore water pressure were evenly distributed on the horizontal plane, and the maximum calculated value of the surface settlement was 90.92 cm, but the phenomenon of a “pot–bottom shape” of the settlement and pore pressure gradually became obvious with the increase in the surcharge, and the maximum calculated value of the surface settlement was 153.01 cm. (3) In the numerical calculations, it was found that the pore pressure and settlement distinctly fluctuated once each level of surcharge was applied. The changes in the settlement were basically consistent with the measured values after the adjustment of positive and negative pressure, and the variation trend of the pore pressure was consistent with the measured value. (4) The depth affected by the vacuum could reach at least 16 m. All of these findings are expected to provide references for the calculation of consolidation in vacuum–surcharge preloading.

The rapid development of expansive soil fissures and the attenuation of strength under the action of repeated atmospheric wetting-drying cycles have a very adverse impact on the shallow stability of expansive soil slope. In this study, shear strength test and fissure observation test were performed on typical residual expansive soil specimens at various wetting-drying cycles and low stress. The results show that the shear strength index of residual expansive soil should be obtained according to the high and low stress sections, respectively. The cohesion of the three residual expansive soils decreased to a similar low value range (0~5 kN) at multiple wetting-drying cycles and low stress; the attenuation of cohesion is the main cause of shallow slope collapse. The fissure rate of the specimen surface increases with the increasing number of wetting-drying cycles, and increasing the dry density can significantly inhibit the development of fissures. The repeated wetting-drying cycles leads to the rapid development of slope fissures from the surface to the inside, destroys the internal natural structure of undisturbed expansive soil, reduces the cementation between soil particles, and leads to the decrease of soil cohesion. For slope stability analysis of residual expansive soil, the strength parameters of the specimens with multiple wetting-drying cycles and internal fissure network should be selected according to the overburden pressure of the slope sliding surface. The research results provide a theoretical basis for the stability analysis of shallow landslide of expansive soil slope and slope protection design.

In order to study the flexural performance of the combined structure of steel-pipe and steel slag powder ultra-high-performance concrete (UHPC), nine round steel-pipe beams filled with steel slag powder UHPC of different types were fabricated according to the orthogonal test method with the steel pipe type, coarse aggregate content, steel fiber admixture, and curing system as parameters. The broken ring morphology, deformation characteristics, deflection distribution, and flexural bearing capacity of the steel-pipe–UHPC beams were analyzed via a pure bending test and a finite element simulation. The results show that the damage morphology of the round steel-tube–UHPC beams prepared by using steel slag powder UHPC as the inner filling material was “bow damage” under the pure bending load, and the load capacity was higher. When the cross-sectional deflection reached L/30, the external load was still not reduced, and the steel-tube–steel-slag powder-UHPC beam had a better plastic deformation capacity and a later flexural bearing capacity. The type of steel tube had a significant influence on the flexural bearing capacity of the steel-tube–UHPC beam, and the larger the diameter of the steel tube section and the thicker the tube wall, the higher its flexural bearing capacity. The calculated ultimate flexural bearing capacity by the finite element software and the test results had a stable error between 5.6% and 11.2%, which indicates that the model was reasonably established. The research results can provide a reference for the application of steel pipe UHPC engineering.