Fully weathered phyllite is widely encountered along railway corridors in China, yet its suitability as subgrade fill remains insufficiently documented. This study provides an integrated laboratory and field evaluation of both untreated and low-dosage cement-stabilized phyllite for sustainable transport constructions. Laboratory investigations covered mineralogy, classification, compaction, permeability, compressibility, shear strength, and bearing capacity, while large-scale field trials examined the influence of loose lift thickness, moisture content, and compaction sequence on subgrade quality. Performance indicators included the degree of compaction and the subgrade reaction modulus K₃₀, defined as the plate load modulus measured with a 30 cm diameter plate. A recommended cement dosage of 3.5% (by weight of dry soil) was established based on preliminary trials to balance strength development with construction reliability. The results show that untreated phyllite, when compacted under controlled conditions, can be used in lower subgrade layers, whereas cement stabilization significantly improves strength, stiffness, and constructability, enabling reliable application in the main load-bearing subgrade layers. Beyond mechanical performance, the study demonstrates a methodological innovation by linking laboratory mix design directly with field compaction strategies and embedding these within a life-cycle perspective. The sustainability analysis shows that using stabilized in-situ phyllite achieves lower costs and approximately 30% lower CO₂ emissions compared with importing crushed rock from 30 km away, while promoting resource reuse. Overall, the findings support circular economy and carbon-reduction objectives in railway and road earthworks, offering practical guidance for low-carbon, resource-efficient infrastructure.

Extreme rainfall events pose a growing threat to slab track subgrades by triggering mud pumping through fines migration and structural voids. This study introduces two innovations to enhance climate resilience in high-speed railway infrastructure: (i) the Rain Intensity Ponding (RIP) method, which links regional rainfall statistics with axle-pass thresholds to predict mud pumping potential; (ii) an optimized drainage retrofit using permeable shoulders and blind ditches. Physical model tests reveal that mud pumping occurs only when structural gaps, ponding, and cyclic loading coincide. The RIP method correctly identified a 71% exceedance in the critical ponding duration (52 min) on a representative high-speed line in Eastern China, explaining recurrent failures. Parametric analyses show that the proposed drainage retrofit—using shoulder fill with ka > 23 mm/s and blind ditches with kg > 23 mm/s—reduces ponding time by up to 90% under 1-year recurrence storms. This study establishes a physics-based, region-specific strategy for mud pumping mitigation, offering guidance for climate-adaptive slab track design and operation.

Loess is highly water-sensitive and collapsible, and its conventional use as backfill in road and railway infrastructure may lead to settlement and deformation after wetting. Converting excavated loess into controlled low-strength material (CLSM) provides an alternative for backfilling; however, its engineering performance is strongly dependent on the water-to-solids ratio (W). In field construction, variations in the natural moisture content of loess cause the actual W to deviate from its design value, whereas conventional CLSM mix designs generally assume a fixed W, which may result in insufficient flowability or strength under actual field conditions. A Box–Behnken design was first employed to investigate the effects of the cement-to-soil ratio (C), water-to-solids ratio (W), and polycarboxylate superplasticizer content (P) on flowability (f), bleeding rate (Bᵥ), wet density (ρ), and 28-day compressive strength (qᵤ), based on which regression-based predictive models were developed. The sequential least squares programming (SLSQP) algorithm was then used to optimize mix proportions for backfilling behind abutments, culverts, and retaining walls and for subgrade backfilling. The optimization minimized the material cost per unit volume while requiring all performance indicators to satisfy the specified criteria under variations in W (ΔW = 0%, ±1%, ±2%, and ±3%). The results show that W is the dominant factor governing f, Bᵥ, and ρ, whereas C has the greatest influence on qᵤ. Increasing P markedly improves f but also increases Bᵥ. For backfilling behind abutments, culverts, and retaining walls, when the mean field soil moisture content is overestimated and the actual W is consequently lower than its design value, the mix optimized for ΔW = −3% satisfies the flowability criterion even when the measured flowability is up to 21.3 mm below the design value because of the lower actual soil moisture content. Conversely, when the mean field soil moisture content is underestimated and the actual W exceeds its design value, the mix optimized for ΔW = +3% satisfies the compressive-strength criterion even when the measured strength is up to 0.09 MPa below the design value because of the higher actual soil moisture content. Accounting for possible variations of up to ±3% in the water-to-solids ratio under field conditions, the optimal mix proportions are C13.14W37.64P0.069 for backfilling behind abutments, culverts, and retaining walls and C15.00W34.28P0.097 for subgrade backfilling. These findings provide a basis for the performance control and mix design of loess-based CLSM subjected to variations in the water-to-solids ratio.

To enhance the strength and water stability of stabilized expansive soil, this study investigates the use of cement, montmorillonite adsorption modifier (MAM), and their composite system. Laboratory tests evaluated compaction characteristics, swell–shrink behavior, and mechanical performance. The results show that MAM more effectively regulates compaction by reducing optimum water content and increasing maximum dry density; 6% MAM increases maximum dry density by ≈0.04 g/cm³ and reduces optimum water content by ≈2%. In terms of swell–shrink behavior, MAM reduces both swelling and linear shrinkage more effectively than cement. The incorporation of 5% MAM reduces the free swelling ratio by 40% and the equilibrium moisture absorption by 2.7%, lowering the swelling classification to non-expansive. Furthermore, 5% MAM decreases the unloaded and loaded swelling ratio by 14.7% and 5%, respectively, while increasing MAM from 2% to 6% further reduces linear shrinkage by 1.12%. Cement significantly enhances compressive strength, with 7–28 d values reaching 2.2–2.7 times those of untreated soil at 9% content; however, its water stability under wet–dry cycles is limited. In contrast, the cement–MAM composite system achieves balanced improvement by simultaneously suppressing swelling and enhancing both strength and water stability. These findings provide a reference for the treatment and engineering application of expansive soils.