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

6

0m

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 z

one

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.