Calcium carbide slag (CCS), phosphogypsum (PG), and red mud (RM), three types of industrial solid wastes, were employed to improve tunnel muck for assessing the feasibility of their reuse. A series of indoor tests were conducted to investigate the effects of their contents on the physical and mechanical properties of the improved tunnel muck. Microscopic tests were also conducted to reveal the improvement and interaction mechanisms involved. Results indicate that the incorporation of CCS, PG, and RM can significantly improve and enhance the physical and mechanical properties of tunnel muck. The improved tunnel muck containing 2% PG and 6% RM shows higher early strengths as CCS content exceeds 4%. However, after curing for more than 14 days, the unconfined compressive strength (UCS) of the tunnel muck with 4% PG and 4% RM is the maximum regardless of the CCS content. Microscopic analysis shows that reactive substances in industrial solid waste react chemically with soil components, exchanging ions and forming cementitious products such as calcium hydroxide, calcium silicate hydrate (C-S-H), calcium aluminosilicate hydrate (C-A-S-H), and ettringite (AFt). They bind, fill, and encapsulate soil particles, compacting the soil and significantly enhancing the physical and mechanical properties of tunnel muck. Moreover, there is a notable mutual synergy between PG and RM, primarily attributed to their acid-base neutralization and the complementary action of reactive ions. The improved tunnel muck containing 4% CCS, 4% PG, and 4% RM demonstrates the highest enhancement efficiency.

To reuse industrial solid wastes and waste clay with low liquid limit, a kind of soil solidification material by using cement, quicklime and industrial solid wastes such as ground granulated blast -furnace slag (GGBS), silica fume (SF) was developed in this study. Response surface methodology (RSM) based on central composite design (CCD) was used to design the experiment and optimize the mix ratio of GGBS, quicklime and SF under certain cement content conditions (i.e., the content ratio of cement, GGBS, quicklime, and SF was 5: 9.14: 1.7: 2.13). A soil solidification agent named O-QGS was developed to solidify waste clay with low liquid limit. To clarify the solidification mechanism of solidified soil, a series of laboratory experiments such as UCS test, water stability test, and scanning electron microscopy (SEM) test were carried out to capture the mechanical properties, water stability, and microstructure of O-QGS solidified soil and cement solidified soil. For practical purpose of O-QGS, a method for forming prefabricated pile by using O-QGS solidified soil was developed, and a method for strengthening soft foundations with prefabricated O-QGS solidified soil pile was proposed. Based on the results of load tests, the bearing capacity of prefabricated O-QGS solidified soil pile and cement high-pressure rotary jet grouting pile, as well as the composite foundations bearing capacity of prefabricated O-QGS solidified soil pile and cement high-pressure rotary jet grouting pile used for strengthening soft foundations, were analyzed. The feasibility of prefabricated O-QGS solidified soil pile used for strengthening soft foundations was verified in practice. The present study shows that the UCS of O-QGS solidified soil is 7.25 MPa at 28 days, and the water stability coefficient of O-QGS solidified soil is larger than 0.8. Compared with the method of cement highpressure rotary jet grouting pile to reinforce soft foundation, the bearing capacity of prefabricated O-QGS solidified soil pile to reinforce soft foundation is higher, and the cost can be saved by 22.4 %.