<p>Unlike traditional karstification by total removal, saprolite karstification is treated here as a dual-stage process: in situ saprolitization of soluble bedrock followed by saprolite removal. Excavation faces in the Dacun Tunnel (Yunnan, China) expose extensive dolostone saprolitization and widespread infilling of subsurface spaces; however, saprolite karstification’s contribution to karst-space enlargement remains unclear. Field surveys, grain-size analysis with post-sieving clast identification, and acid-dissolution tests indicate that subsurface infill derives from surface residual–colluvial deposits. Abundant surface materials together with pre-existing negative landforms and feeder conduits establish sustained sources and pathways for subsequent filling of newly enlarged voids. In addition, the infill’s low permeability and hydraulic retention can sustain slowly varying humidity in adjacent rock, consistent with environmental requirements for physically driven saprolitization. Taken together, the supply-and-connectivity evidence and the humidity-buffering property of the infill establish the preconditions for saprolitization; combined with observations of multi-episode creep and staged evolution of infill–saprolite intermingled bands, they delineate a positive-feedback pathway for subsurface karst-space enlargement: the infill creates conditions favorable for saprolitization → saprolite is removed, generating new voids → the newly formed space is subsequently occupied by creeping infill, promoting continued enlargement. This “saprolite-first, removal-later” two-stage karstification subtype complements karstification by total removal and provides a new mechanistic perspective on subsurface evolution and water–rock interactions in karst regions.</p>