The rigid pile composite foundation method is the most commonly used method for strengthening weak soil foundations. In this method, piles usually need to pass through weak soil layers, and the pile end falls on a bearing layer with good bearing capacity. Under existing technical conditions, the thicker the weak soil layer, the longer the pile body, and the more difficult it is to ensure the construction quality of the pile. In response to this issue, some scholars have adopted the rigid pile composite foundation method with a thick cushion layer for reinforcement treatment. This article uses PLAXIS 3D (V20.04.00.790) software to establish a finite element model of rigid pile composite foundation with a thick cushion layer and simulate the process of foundation reinforcement. The influence of parameters such as thickness, compression modulus, and shear strength index of the cushion layer on foundation settlement and pile–soil stress distribution is studied, and the reasonable range of these parameters is analyzed under the condition of considering reinforcement effect. Through comparative analysis, it can be concluded that for deep and weak soil areas, the thickness of the cushion layer can range from 0.5 to 2.6. The thickness and compressive modulus of the cushion layer have a significant impact on the settlement of the foundation, the pile–soil stress ratio, and the stress of the pile body, while the shear strength index of the cushion layer has a relatively small impact on these parameters. Reasonably selecting the geometric and mechanical parameters of the cushion layer can effectively reduce stress concentration at the pile top and better play the role of the soil between piles.

This study systematically investigates the engineering characteristics of lead-contaminated red clay stabilized by calcium lignosulfonate and ordinary Portland cement composite binders. A series of experiments were conducted to evaluate the effects of lignosulfonate contents (0%, 0.25%, 0.5%, 1%, 2%), cement content (4%, 6%, 8%, 10%), and lead ion concentration (0%, 0.1%, 1%) on the mechanical properties, permeability characteristics, and leaching behavior. Key findings include the following. (1) Based on the highest mean UCS values observed in this study, the best-performing formulations were 1% lignosulfonate + 4% cement for uncontaminated soil, 0.5% lignosulfonate + 4% cement for 0.1% lead, and 0.25% lignosulfonate + 10% cement for 1% lead. (2) The permeability coefficient initially decreases and then increases with lignosulfonate addition, with maximum reductions of 65.9% and 44.4% for 0.1% and 1% lead contamination under their respective best-performing formulations under these specific test conditions. (3) The leaching concentration of 0.1% lead-contaminated soil met the national standard (<5 mg/L). Critically, however, the 1% lead-contaminated soil failed the TCLP test, with a leaching concentration of 37.3 mg/L, vastly exceeding the regulatory limit. This constitutes a treatment failure for environmental safety purposes, rendering the concurrent mechanical strength improvement irrelevant. (4) Microstructural and X-Ray Diffraction analyses (SEM and XRD) suggest that lignosulfonate improves soil structure by promoting the formation of C-S-H gel and ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O), whereas high lead concentrations inhibit ettringite formation. This research provides a theoretical foundation for the multi-criteria evaluation and application of lignosulfonate–cement composites in lead-contaminated soil remediation.

To address expansive soil swelling–shrinkage distress and the limitations of single carbide slag (brittle failure), pure fiber (limited strength), and cement (high carbon emissions), this study develops a multi-indicator optimization method for carbide slag–polypropylene fiber composite improvement. Twenty-five full-factorial tests were conducted under three schemes: single slag, single fiber, and composite improvement. Six indicators (USR, LSR, VSR, cohesion, internal friction angle, UCS) were used to construct a CRITIC–entropy–TOPSIS model. The new insight brought by this research lies in coupling combination weighting with TOPSIS to eliminate the deviation of one-sided optimal formulas biased toward swelling inhibition or strength growth obtained by conventional single weighting and conventional range analysis, filling the gap of a multi-objective evaluation system balancing indicator conflict and dispersion, and realizing multi-index balanced optimization. The optimal C8P0.3 (8% slag + 0.3% fiber) has a relative closeness of 0.93. Compared to raw soil, USR drops by 77.07%, and cohesion and UCS increase by 164.30% and 71.91%; compared to C8P0, cohesion and UCS increase by 8.56% and 10.16%, relieving brittleness. XRD/SEM reveal that slag hydration forms rigid C-S-H networks while fibers create flexible 3D networks, jointly achieving rigid–flexible synergy that fills pores and bridges cracks. This work provides an optimization approach for low-carbon expansive subgrade stabilization.

A sustainable controlled low-strength material (CLSM) was developed using ground granulated blast-furnace slag, steel slag, and flue gas desulfurization gypsum in combination with cement to synergistically utilize engineering excavated soil and industrial solid wastes for stabilizing low liquid limit clay. However, the long-term durability evolution of this material under harsh and coupled environmental conditions—particularly freeze–thaw cycles, wet–dry cycles, and prolonged drying—has not been systematically investigated. In this study, systematic freeze–thaw cycling (up to 11 cycles), wet–dry cycling (up to 11 cycles), and natural drying (until mass stabilization) tests were conducted on specimens with binder contents ranging from 8% to 16%. The evolution of mechanical performance was evaluated via unconfined compressive strength (UCS) tests, while microstructural changes were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results revealed a distinctive “S-shaped” fluctuation in UCS under freeze–thaw cycles. High-binder (16%) specimens maintained strengths of 1783–2395 kPa with intact surfaces and no visible cracking—significantly outperforming low-binder specimens. Under wet–dry cycles, strength initially increased after the first cycle and then declined progressively, with the lowest strength loss observed at 10% binder content. During drying, both water loss rate and drying shrinkage strain decreased with increasing binder content: from 8% to 16% binder, the water loss rate dropped from 34.16% to 29.03%. Microstructural analysis revealed that higher binder content promoted the formation of a dense, interwoven network of C–S–H gel and ettringite, which effectively filled intergranular pores and encapsulated soil particles, thereby enhancing macroscopic durability. This study provides a sustainable material solution for utilizing industrial solid wastes in the stabilization of low liquid limit clay for CLSM applications under severe environmental conditions, supporting the broader adoption of waste-to-resource strategies in construction engineering.