To improve the poor characteristics of low strength and high compressibility of weak silty soil, a series of samples with different cement dosage, fiber content, and fiber length was prepared in this experiment, and unconfined compressive strength (UCS) tests, triaxial tests, and scanning electron microscopy (SEM) tests were carried out to explore the influence of polypropylene fiber on the strength of cement-stabilized soil and analyze the curing mechanism of fiber-reinforced cement soil. The test results show that the factors affecting the UCS of the sample from high to low were: cement dosage, fiber content, and fiber length. An orthogonal test found that the optimal ratio of the sample was cement dosage of 18%, fiber content of 0.4%, and fiber length of 3 mm, and the UCS of the sample can reach 1.63 MPa. The triaxial test shows that when the cement dosage is 15% and the fiber length is 9 mm, the incorporation of fiber can significantly improve the toughness and strength of soil. When the cement dosage is 15%, the UCS with 0.4% fiber content is 1.6 times that without fiber. With the increase of fiber content, the peak stress and axial strain of fiber-cured soil are increased, and the cohesion and internal friction angle are also increased. The failure mode and SEM test of fiber-reinforced cement soil show that when the cement dosage is 15% and the fiber length is 9 mm, the addition of fiber can improve the deformation ability of cement soil and slow down the development of cracks. With the increase in fiber content, the number and width of cracks are significantly reduced, and the failure mode changes from brittle failure to ductile failure.

Microbially induced calcium carbonate (CaCO₃) precipitation (MICP) is an emerging soil-treatment method. To explore the effect of this technology on granite residual soil, this study investigated the effects of the mechanical properties and disintegration resistance of microbially cured granite residual soil under different moisture contents by conducting direct shear and disintegration tests. The curing mechanism was also discussed and analyzed. Results showed that MICP can be used as reinforcement for granite residual soil. Compared with those of untreated granite residual soil, the internal friction angle of MICP-treated granite residual soil increased by 10% under a moisture content of 30%, while its cohesion increased by 218%. The disintegration rate of the MICP-treated granite residual soil stabilized after a maintenance time of 5 days under different water contents. Therefore, we provide the explanation that the improvement of the shear strength and disintegration resistance of granite residual soil is due to CaCO₃ precipitation and the surface coating.

Fibers are applied in construction work to improve the strength and avoid brittle failure of soil. In this paper, we analyze the impact mechanism of fiber type and length on the immobilization of microorganisms from macroscopic and microscopic perspectives with fibers of 0.2% volume fraction added to microbial-induced calcite precipitation (MICP)-treated sand. Results show the following: (1) The unconfined compressive strength (UCS) of MICP-treated sand first increases and then decreases with increasing fiber length because short fiber reinforcement can promote the precipitation of calcium carbonate, and the network formed between the fibers limits the movement of sand particles and enhances the strength of the microbial solidified sand. However, the agglomeration caused by overlong fibers leads to uneven distribution of calcium carbonate and a reduction in strength. The optimal fiber length of polypropylene, glass, and polyvinyl alcohol fiber is 9 mm, and that of basalt fiber is 12 mm. (2) The UCS of the different fiber types, from small to large, is basalt fiber < polypropylene fiber < glass fiber < polyvinyl alcohol fiber because the quality of the fiber monofilament differs. More fibers result in more a evident effect of interlacing and bending on sand and higher strength in consolidated sand.

To study the influence of sand particles on the strength and dry–wet cycle characteristics of cement-solidified South China coastal soft clay, a series of cement-solidified South China coastal soft clay samples with different contents and sizes of sand particles were prepared, and unconfined compression strength (UCS) tests, dry–wet cycle tests under seawater and freshwater conditions, scanning electron microscopy tests, and X-ray diffraction tests were carried out. The test results show that the UCS of cement–sand-solidified soil (CS-SS) samples at 7, 14 and 28 d increase as sand particle content increases, and their 28 d strength increase as sand particle size decreases; the average mass loss rate of sand–cement samples ranges from 14% to 61%. The properties of strength and dry–wet cycle durability improve as the sand particle content increases and the sand particle size decreases. Moreover, the anti-dry–wet cycle ability of samples under freshwater conditions is better than those under seawater conditions. The scanning electron microscopy (SEM), X-ray diffraction (XRD), and ion chromatography test results show that the stiffness replacement effect (the high stiffness of sand particles replaces the low stiffness of soft clay), the interface cementation effect between the sand particles and cemented soil, and the microcrack expansion-obstructing effects of sand particles contribute to the improvement of the strength and dry–wet cycle durability of cement–sand solidified South China coastal soft clay.