The shield launching is one of the most difficult processes to control and accident-prone in subway tunnelling construction. Whether minus segment can be removed with an advanced length and shield excavation works can be started are fatal to the safety and economic benefit of the project. To research the removal conditions of shield tunnelling minus segments, the software FLAC3D was used to analyze the displacement change of minus segments and the stress change of reaction frame in Beijing subway engineering. The field tested results such as the earth pressures, grouting pressure for segment rings, thrusts of shield jacks, displacement of minus segment rings, and stresses in reaction frame were presented in detail. Compared with the in-situ tested data, the safety removal conditions of shield tunnelling minus segments and reaction frame was suggested. The results indicated that minus segments could be removed when its relative displacement variation was close to zero, the stress of reaction frame tended to be stable, and huge reaction of shield pushing forward was supported by the friction between grouting reinforced soils and assembled reinforced concrete segment rings. The results of field tests agreed well with numerical simulations, it showed that the numerical model was reasonable and the results were reliable.%盾构始发是盾构工法的重要施工工序之一,也是盾构施工中最容易发生事故的环节,关系到盾构隧道能否顺利掘进与及时贯通.以北京地铁为例研究了地铁盾构施工负环管片安全拆除条件问题,采用FLAC3D进行了三维模型计算分析,分析了负环管片的位移及反力架应力变化情况,实测了盾构始发段的土压力、注浆压力、盾构推力、负环管片位移、反力架应力变化规律；通过数值计算与实际监测结果的比较,给出了负环管片及反力架的安全拆除条件建议.结果表明:当负环管片的相对位移变化量趋近于零、反力架应力值基本趋于稳定时,隧道衬砌管片环与壁厚注浆及围岩之间的摩擦力足以平衡盾构推力,方能拆除负环管片与反力架.数值分析与实测结果相吻合,说明数值计算模型合理,结果可靠.
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