Abstract: A series of large sinkholes developed within a residential suburb in southwestern Iran, causing significant damage to properties and necessitating partial evacuation of the area. A comprehensive investigation program revealed that the primary cause of these incidents was the presence and progressive enlargement of karstic cavities within the limestone bedrock of the Asmari Formation. The overburden consists of a thick layer of dense to very dense quartz sand, which gradually ravelled into the underlying cavities, triggering ground surface subsidence and collapse. To mitigate the risk of future sinkholes, an extensive treatment program was implemented with the aim of stabilizing the upper-level cavities and reducing the likelihood of further collapse. The adopted solution relied on cement-based grouting to fill and seal the karst voids. Two distinct grout systems were developed and applied: (i) a sand-rich cavity-filling mortar grout and (ii) a cement–bentonite permeation grout. The performance of these mixes was evaluated through systematic testing of compressive strength, workability (slump and flow), thermal conductivity, thermal resistance, bleeding, air content, and setting time. Following the treatment, a control program involving drilling of verification boreholes and coring of hardened grout was carried out to assess in-situ grout properties and compare them with laboratory specimens. The results confirmed that the selected grout mixes satisfied the specified performance criteria and provided effective stabilization of the treated karst cavities, offering a practical framework for similar ground improvement projects in karstic regions of Iran. Keywords: Karst; Sinkholes; Cement Grouting; Cavity Treatment; Permeation Grout; Ground Improvement; Iran.