Abstract: A novel karst cave filling material with shield waste muck is developed to realize in-situ recycling of shield waste sandy clay. Herein, a case study is conducted on the Shenzhen metro line 3, and the shield tunneling-produced sandy clay is taken as raw material, supplemented with cement and additives, to prepared filling materials for karst caves under the tunnel. Additionally, the effects of various additives, water-to-binder ratios, and clay-to-binder ratios on the performance of the novel material are examined through indoor experiments. The coagulation state and practical performance of the prepared filling materials are further analyzed using scanning electron microscopy and field tests. The research results indicate the following: (1) Aluminum-calcium oxide-based additives significantly improve the strength of the filling material, achieving a 1-hour forming strength of 0.479 MPa and a 28-day strength of 11.048 MPa. Such additives also enhance the water stability and shrinkage resistance of the material, with a 20.5% increase in the water stability coefficient and a 63.4% reduction in shrinkage value at 28 days. (2) Increasing the clay-to-binder ratio and decreasing the water-to-binder ratio enhance the unconfined compressive strength of the filling material at 28 days. The strength of the filling material significantly increases with the rise in the clay-to-binder ratio when the clay-to-binder ratio exceeds 0.3. (3) Scanning electron microscopy tests reveal that under hydration effect, cement and aluminum-calcium oxide-based additives form fibrous and flaky connections within the soil, reducing the overall pore volume and increasing density, resulting in tighter structural connections that yield higher strength for the filling material. (4) Field application tests of the filling material are conducted, with an average core sample strength of 4.16 MPa, meeting engineering requirements.

Key words: waste sandy clay, karst cave filling material, additives, compressive strength, micro-mechanism, field test