English：

To improve the engineering properties of sludge soil and mitigate its engineering hazards, thispaper conducts an experimental and application study on the modification of sludge soil using a multi⁃component composite admixture consisting of basalt mineral powder, calcium hydroxide, andpolypropylene fiber. First, single⁃factor experiments were carried out to determine the optimal dosages ofbasalt mineral powder (MP), Ca(OH)2 , and polypropylene fiber ( PP) for sludge soil modification.Subsequently, the Box⁃Behnken design (BBD) response surface methodology within the Design⁃Expertsoftware was adopted for multi⁃factor experimental design, a response surface regression model wasestablished to investigate the cross⁃interaction effects of MP⁃Ca(OH)2⁃PP on the engineering propertiesof modified sludge soil, and the optimal mix ratio of the composite admixture was determined. Theexperimental results show that the sludge soil achieves the optimal modification effect with the maximumunconfined compressive strength (UCS) when the mass ratio of MP ∶ Ca(OH)2∶ PP is 16.95 ∶ 5.68 ∶0.25. The measured average UCS value is 797kPa, while the predicted value from the response surfacemodel is 825kPa, with a relative error of merely 3. 51%, indicating the high prediction accuracy of theestablished model. A field application was implemented in a subgrade engineering project in Huzhou,where the sludge soil was paved in layered and compartmentalized sections, mixed with the MP⁃Ca(OH)2⁃PP composite admixture at the optimal ratio, and cured to the specified age. Test resultsdemonstrate that the modified sludge soil subgrade meets the design requirements for compaction inhighway engineering, verifying the feasibility and applicability of the sludge soil modification scheme using the multi⁃component composite admixture.