浅层天然气井喷对地层的损伤影响与桩基工程危害分析

针对杭州湾大桥在勘探过程中发现桥址区地层有浅层天然气分布并发生井喷这一现象,基于现场勘探时浅层气井喷特点与影响因素,认为引起井喷的主要原因是起钻抽吸作用和灌浆不及时所致。井喷是含浅层气土体的渗透比降超过其气压临界比降条件下的非饱和渗流破坏过程,对地层的影响表现为对土体的损伤和扰动,按扰动程度可分为剧烈扰动区、严重扰动区、轻微扰动区和含气层压密区。桥址区粘土层中的浅层气不会对桩基工程产生明显的危害,但砂土层中的浅层气可能会引起土体剧烈扰动显著降低桩基承载力、桩基发生不均匀沉降或桩腿下沉、桩基中产生负摩擦力和阻碍桩基础顺利施工等不利影响,应从桩基础合理选型、预先进行不扰动土体控制释放浅层气、充分考虑不利因素的安全设计,以及在施工过程中增高钻孔桩作业平台与增大泥浆比重等几方面进行灾害防治。

Influence of Shallow Natural Gas Blowout on Stratum Damage and Hazard Analysis of Pile Foundation

During the engineering geologic survey of the Hangzhou Bay Bridge, Zhejing province, China, the shallow natural gas was found at the clay and sand stratum of depth 18~60m, which led to the occurrence of gas blowout. Based on the characteristic of gas blowout in field exploration and its effect factors analysis, it is indicated that the main reasons that resulted in gas blowout are the pump action of drill pulling and grouting behind schedule. In fact, the blowout is a process of unsaturated seepage failure when the seepage gradient of soil containing shallow gas exceeds its critical gradient of gas,whose influence on the soil stratum is represented as the form of damage and disturbance. According to the disturbance degree, the influenced soil can be divided into dramatically disturbed zone, severely disturbed zone, gently disturbed zone and compression zone in soil stratum containing gas. In the bridge location zone, the influences of shallow gas in clay stratum and in sand stratum on the pile foundation are different:the former does not cause significant harm disaster, whereas the latter is likely to cause the reduction of pile bearing capacity obviously due to dramatic soil disturbance, non-uniform settlement of pile foundation or pile leg sinking, occurrence of negative skin friction and resistance to security of pile construction. Therefore, the disaster prevention should be carried out by means of reasonable selection of pile types, release of shallow gas under the controlled condition without disturbance soil in advance, safety design under the condition of considering unfavorable factors sufficiently, heightening the drilling construction platform and increasing mud specific gravity during the pile construction.