深埋岩溶区基岩面漏失致钻孔坍塌机理与预防

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

摘要:

针对深埋岩溶区钻探施工中钻遇基岩面时因泥浆失返性漏失引发钻孔坍塌甚至地面塌陷的工程难题，本文结合北京市通州区3起钻孔坍塌事故案例，采用理论分析与工程验证相结合的方法，系统研究了坍塌机理与防治措施。基于朗肯土压力理论，建立了泥浆漏失前后孔壁应力平衡模型，推导了保持孔壁稳定的临界深度计算公式。结果表明：泥浆失返导致液柱压力下降，无法平衡地层侧向土压力，是孔壁失稳的根本原因；坍塌形式分为局部坍塌与整体坍塌，前者因临界深度以上地层液柱压力不足而自上而下失稳，后者需同时具备基岩顶板破碎、溶洞或地下河发育、覆盖层以非黏性土为主等极端地质条件。工程实践表明，合理设计孔口管深度、小径掏眼、提前下套管等预防措施可有效避免坍塌事故。研究成果对深厚覆盖层岩溶区钻探安全具有重要指导意义。

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.