To improve the engineering performance of high-liquid-limit soil, lignin, a by-product of the papermaking industry, was used. First, the influence of lignin content and curing age on the physicochemical and mechanical properties of the improved soil was determined by carrying out pH, Atterberg limits, heavy compaction, unconfined compressive strength (UCS), California bearing ratio (CBR), and resilience modulus tests. Secondly, microscopic images obtained by scanning electron microscopy (SEM) were analyzed. The characteristic and evolution rules of the microstructure were expounded for the improved soil. Finally, combining SEM with energy-dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) analysis, the mechanism of lignin improvement on high-liquid-limit clay is discussed from the perspective of molecular structure. The results showed that the pH value of lignin-improved soil was much lower than that of quicklime-improved soil, which were 7.0 and 11.7, respectively. When the lignin content was 3%, the dry density and mechanical indexes (UCS, CBR, and resilience modulus) of the improved soil all showed the maximum values. From the perspective of microstructure, the connection between soil particles was strengthened through the wrapping and filling of the flocculent cementing materials produced by lignin. The improvement mechanism of lignin on soil was the combined result of ion exchange, hydrogen bonding, covalent bonding, and electrostatic attraction.

To explore the damage evolution law of cement-stabilized weathered sand under the action of freeze–thaw cycles, relevant experimental research was carried out on stabilized weathered sand with a cement content of 3%, including unconfined compressive strength (UCS), scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP) testing. Using the surface appearance, mass, and UCS of the specimen, combined with the changes of microscopic characteristics, the freeze–thaw damage law and microscopic degradation mechanism of cement-stabilized weathered sand were analyzed. The test results showed that with the increase of the number of freeze–thaw cycles, the surface appearance of the specimen continued to deteriorate, the unconfined compressive strength gradually decreased, and the mass of the specimen first increased and then decreased. After nine freeze–thaw cycles, the specimen was seriously damaged, and the strength loss was as high as 55%. From a microscopic point of view, the freeze–thaw cycles caused the pore water inside the specimen to continuously change between ice crystals and liquid. Frost heave and shrinkage weakened the cement bond between the weathered sand particles and made the pores in the specimen develop and expand continuously. With the decrease of the number of micropores and the increase of the number of medium and large pores, the particle skeleton changed from a dense structure to a porous structure, which eventually led to the deterioration of the macroscopic properties of the cement-stabilized weathered sand.

To evaluate the long-term service performance of subgrade with high-liquid-limit soil improved by lignin, a drying-wetting cycle test was carried out for the improved soil with 3% lignin content, and comparison was made with quicklime-improved soil with the same properties in terms of pH, unconfined compressive strength (UCS), appearance, and volume stability. Using a scanning electron microscope (SEM), the microscopic characteristics of the lignin-improved soil were analyzed qualitatively and quantitatively. By analyzing the evolution law of the soil microstructure and pores under the action of drying-wetting cycles, the strength degradation mechanism of the improved soil was discussed at the microscopic level. The test results showed that the soil pH value of the lignin-improved soil was more beneficial to the ecological environment compared with that of the quicklime-improved soil. Furthermore, the stability of the lignin-improved soil was better than that of the quicklime-improved soil due to strength loss and increased volume expansion rate caused by the drying-wetting cycles. In terms of the microstructure, the cementing materials enhanced the agglomeration of the soil by wrapping grains and filling pores. Under the action of drying and wetting, the number of pores inside the soil increased, and the soil structure loosened. Under drying and wetting conditions, the strength degradation of the lignin-improved soil was the result of the interaction of pore development and cementing materials dissolution.

To reduce the harm caused by industrial solid waste to the environment, and solve the problem of excavated soil disposal in buried pipeline management projects, this study proposes a method to produce soil-based controlled low strength materials (CLSM) by using industrial solid wastes (including fly ash and red mud) as partial replacements for cement. The properties of CLSM were characterized in terms of flowability, unconfined compressive strength, phase composition and microstructure. The test results showed that fly ash could significantly improve the flowability of CLSM, while red mud had more advantages for the strength development. When 20% fly ash and 30% red mud were combined to replace cement, the fluidity of CLSM was 248 mm, and the unconfined compressive strength (UCS) at 3, 7 and 28 days was 1.08, 1.49 and 3.77 MPa, respectively. The hydration products of CLSM were mainly calcium silicate hydrate gels, ettringite and calcite. Fly ash provided nucleation sites for cement hydration, while the alkali excitation of red mud promoted the dissolution of SiO₂ and Al₂O₃ in fly ash. The filling and gelation of hydration products make the microstructure dense, which improves the mechanical properties of the mixture.

It is very important for pavement engineers to know which factors are the main reasons for the damage of the paving around manholes. Based on the investigation on the damage of paving around manholes, a vibration model with multidegree of freedom for the vehicle‐manhole cover was established and analyzed. After that, the Matlab software was used to obtain the variation law of impact load over time, and the 95% fourth power of the aggregate force was used as the index to evaluate the pavement damage. Finally, many influencing factors on pavement damage were analyzed by the method of grey correlation entropy. The results indicated that the impact load reached the maximum for the first time when the vehicle reached the top of the manhole cover, which was 1.29 times that of the static load, and the pavement damage coefficient was 2.12 times that of the static load. The influencing factors had different degrees of influence on the pavement damage; from large to small, they were change of road longitudinal slope > driving speed > damping of tire > stiffness of tire > height difference from the pavement damage > height difference from the manhole settlement > stiffness of the manhole cover.