软土超深基坑道路临边施工常见问题与应对策略

Common Problems and Countermeasures in Construction of Ultra-deep Foundation Pit near Road in Soft Soil

摘要:

软土地区道路临边超深基坑受交通荷载的影响往往易被忽略，通过上海北横通道黄兴路井及机架段基坑监测数据验证了临边交通荷载的重要性。从场地布置、筹划和组织、围护施工、支撑和土方开挖、地下水控制等五个方面分析了道路临边超深基坑容易忽视的常见问题，探讨主要采取的策略如下：场地条件允许时，尽可能采用一次围挡。场地不足，多次翻交结合栈桥进行土方开挖；应及时备料，无法保证正常运输时，及时与交警部门对车辆疏导；考虑临边道路对围护墙插入深度、支护内力、成槽稳定的影响，建议超载按30 kPa计算，并加强泥浆管理；地下连续墙可采用水泥土搅拌桩进行槽壁加固，钻孔桩则可考虑钢护筒成桩。泥浆护壁时，建议泥浆的比重不小于1.25；空间受限时，地下连续墙接头可采用工字钢，墙缝止水建议采用MJS或RJP；利用时空效应，尽可能选用钢支撑，快挖快撑。重视钢支撑应力松弛，建议采用伺服控制系统。土方开挖时，建议临路侧留土台；采用止水帷幕隔断潜水与坑外的水力联系，尽可能隔断承压水，无法隔断时，建议隔水帷幕深入承压水层不小于9 m，且大于承压水层厚度的一半。

Abstract:

The influence of traffic load on the ultra-deep foundation pit near the road in soft soil area is often ignored. The importance of traffic loads near road is verified through the monitoring data of Huangxing Road Shaft and foundation pit in the machine frame section in Shanghai Beiheng Passageway. The common problems of ultra-deep foundation pit near road easily ignored are analyzed from five aspects of site layout, planning and organization, enclosure construction, support and earthwork excavation, and groundwater control. The main countermeasures are discussed. When the site conditions are allowed, one-time enclosure should be used as much as possible. Due to insufficient space, multiple excavations are carried out for enclosure construction with the trestle. Materials should be prepared in time. When the normal transportation cannot be guaranteed, the vehicles should be promptly guided by the traffic police department. Considering the impact on the insertion depth of the retaining wall, internal force of the support and stability of the groove near road, it is recommended to calculate the overload at 30 kPa and strengthen the mud management. The underground diaphragm wall could be reinforced with cement soil mixing piles. The drilled pile can be considered as steel shield pile. When the wall is protected with mud, it is recommended that the specific gravity of the mud is not less than 1.25. When the space is limited, I-steel could be used for the joint of underground diaphragm wall. MJS or RJP is recommended for water stop of wall seam. When the time and space effect is utilized, the steel support could be selected as much as possible, and quick excavation and quick support are highly recommended. It is paid attention to the relaxation of steel support stress, and it is recommended to use a servo control system. When the earthwork is excavated, it is recommended to leave a soil platform on the roadside. The water stop curtain is used to cut off the hydraulic connection between diving and the outside of the pit, and it is to cut off the confined water as far as possible. When it is hard to cut off, it is recommended that the water stop curtain penetrate into the confined water layer no less than 9 m and greater than half of the thickness of the confined water layer.