Mechanism and prevention of borehole collapse induced by mud loss at bedrock surface in deep buried karst area

Abstract:

To address the engineering challenge of borehole collapse or even ground subsidence induced by total mud loss when drilling into the bedrock surface in deep-buried karst areas， this study systematically investigates the collapse mechanism and preventive measures through a combination of theoretical analysis and engineering verification， based on three borehole collapse accident cases in Tongzhou District， Beijing. Based on Rankine’s earth pressure theory， a stress balance model of the borehole wall before and after mud loss is established， and a formula for calculating the critical depth to maintain borehole wall stability is derived. The results indicate that the fundamental cause of borehole wall instability is the decrease in hydrostatic pressure caused by total mud loss， which fails to balance the lateral earth pressure of the formation. The collapse modes can be classified into local collapse and overall collapse. The former occurs from top to bottom above the critical depth due to insufficient hydrostatic pressure， while the latter requires extreme geological conditions such as a fractured bedrock roof， well-developed karst caves or underground rivers， and an overburden dominated by non-cohesive soils. Engineering practice demonstrates that preventive measures， including the rational design of conductor casing depth， small-diameter pilot drilling， and advanced casing running， can effectively avoid collapse accidents. These research findings provide important guidance for drilling safety in karst areas with deep overburden.