Large scale and intensive metro construction through dense urban area increases sharply the impaction on risk control and environment protection. Three typical cases of shield crossing building above ground (SCBA), shield crossing tunnel from above (SCTA) and shield crossing tunnel from below (SCTB) are studied, respectively, based on field measurements and site investigations of actual projects in Shanghai soft ground. The risks of shield crossing sensitive buildings and subways, ground movement prediction and its control regulations, the settings of shield driving parameters such as earth pressure, driving speed, postures and grouting are demonstrated and summarized in detail. It is proposed that stringent stipulations on controlling ground volume loss (GVL) ratio (GVLR) and strengthened monitoring measures are necessary and substantial for eliminating/reducing potential construction risks. It is urgently decisive to improve the performance of shield machine and to make it more flexible for counteracting complications of geology and environment, as refer to the present status of shields in Shanghai, most of them being overused or out of date.
۱٫ Background

With rapid development of Shanghai, the accelerated urbanization has made the development and utilization of land as well as underground space become the very important factor affecting the sustainable development of urban society. Since 30 years of open policy, huge urban railway transport systems have been planned in such cities as Beijing, Shanghai, Guangzhou, serving as 3D frame of underground space utilization, and now the ambitious plan is being put into construction at ever highest speed. As for Shanghai, eight metro lines with total length of 230 km have been completed within past 20 years. By 2010 before Shanghai EXPO, 11 metro lines with total length of 400 km will be in operation in which 9 lines run through urban central areas and most of them are tunnels constructed by shield machines. Meanwhile, a total number of 14 large scale vehicle tunnels crossing Huangpu River have been planned in which 8 tunnels have been completed and 6 tunnels being under construction. Most of them are/will be built by shield machines.

The shield tunneling technology has developed quickly and used widely these years to meet the demands of large scale tunneling projects and has achieved good results featuring high speed, safety, small disturbance to ground surface. But with a large number of tunnels driving closely across buildings and running subways, intensive and difficult construction has produced unforeseen risks and potential problems. More and more frequently the high sensitive and unstable ground have been/will be met which results in sharp increasing risks and heavy impaction on environment. The settlement allowed for these closely located facilities are usually a few millimeters. Although normal techniques and experiences from past two decades could guarantee the ground settlement to be controlled within (10 to 30) mm, they are now becoming no longer so feasible and practicable in Shanghai. It is in urgent need that an effective way of controlling shield driving be developed and studied.

The typical Shanghai soft ground is composed of littoral deposits featuring with saturated, flow to soft plastic clay with high compressibility and sensitivity and low strength, long stabilizing time and big settlement after being disturbed. In order to counteract the difficulties from geological condition, advanced shields have been developed in both domestic and abroad. The ground volume loss ratio (GVLR) by disturbance of shield has been minimized to about 5‰ by application of advanced shield and relevant construction techniques in developed countries of the world (Mair and Taylor, 1998; van Hasselt et al., 1999). While in Shanghai, the GVLR has been controlled within 1% under normal conditions. With the development of Shanghai metro network, the frequency of close crossing below/above running subways and nearby residential buildings will be increased a lot and lead to a huge surge in risks. In very complicated geology and environment, the GVLR produced by shield has been controlled within 1–۵‰ in order to protect the sensitive residential buildings and to ensure the safety operation of running subway tunnel by taking very special measures. It has been realized in common that stringent regulation on GVLR not only forces the construction contractors to strengthen their administration but also the performance of shield equipments, and consequently leads to elimination/reduction of various risks and to satisfy the requirements of environment protection in spite of changing conditions.