Long-term Settlement Of Metro In Soft Ground And Its Influence On Safety

Differential settlement during operation period is one of the main types of damage for metro tunnel in soft ground. Large long-term settlement and differential settlement are suggested to reduce the safety, durability and waterproof of the structure and segment joint of metro. In addition, intensive differential settlement can reduce the regularity for track, comfort for custom, and safety for metro. Therefore, Based on the settlement data of Nanjing Metro Line West Extension Line tunnel during operation period, long-term settlement of metro tunnel in soft ground and its influence on safety are studied. Main conclusions can be drawn as follows:(1) Analysis the data of settlement for Nanjing Metro Line West Extension Line tunnel during operation period suggests that severe settlement has been occurred. There are four settlement troughs and the length for them occupies about40%of total length of tunnel. In addition, the settlement rate is quick, and only17%areas are stable at present.(2) Combined with metro construction experiences of other cities and Nanjing Metro Line West Extension Line tunnel, the causes of long-term settlement are analyzed qualitatively. The geology (including distribution, consolidation and creep characteristics of soft ground,), vibration load of train, and the seepage of tunnel are major factors to induce the long-term settlement during operation period, and the factor of geology effect the long-term settlement basically. In addition, although the construction of projects nearby the metro has no effect on long-term settlement at present, it would be pay more attention on this factor in future. (3) The ultimate settlement of Nanjing Metro Line West Extension Line tunnel is predicted by using hyperbolic functions. The value of long-term settlement (150-190mm) is obviously higher than do at present, which indicate a serious problem for settlement in Hexi area, and the attention and manage is urgent need.(4) The statistical results for width, length and density of cracks between settlement troughs and non settlement troughs suggest that the structural cracks are dry shrinkage cracks rather than temperature cracks and settlement cracks. The changes of widths with time by analyses of50typical cracks suggest that the major factor is temperature rather than differential settlement. However, the monitoring location for cracks, monitoring precision and the distance between the monitoring location and settlement location in this study may also affect the results.(5) The results for displacement of pump room, statistics for seepage in tunnel and long-term settlement suggest that more serious leaking would increase the volume of water of pump room and the settlement rate for metro tunnel. Such results also indicate that the rate for leaking in Hexi area is obviously higher than do designed. The class of waterproof is only3-4. The damage induced by s is serious.(6) The value of differential settlement in Hexi area does not reach the permissible value, though it has not damaged the settlement joints. However, the potential safety hazard for waterproof is still existed.(7) The metro in Hexi area have show the voids of solid bed, and in some area such voids is serious. In this study, we also summarized the characteristics of voids and essentials for grouting reinforcement in Hexi area.(8) We suggest the Shanghai thumb technique show high precision and are easy to use, contrasted by three common calculated methods for radius of curvature of tunnel. The results indicate that most radius of curvature induced by long term settlement are over15000m, and only three areas in Hexi area show higher radius (1000-3000m) than do standard value for metro.(9) The long-term settlement effect on the tunnel slope length and slope is studied. The results showed that the soil settlement has little effect on the West Extension Line slope length in the operation period; and it also caused the small variation of the overall tunnel slope, which change is less than0.5‰accounting for87.9%of the full range; while only2%of the section (located at c3), its slope change is greater than1‰(maximum of1.2‰), which caused a certain influence on the drainage of the tunnel.(10) Combined with the Nanjing Metro Line West Extension Line tunnel, which preliminary studied the long-term settlement of the cut-and-cover tunnels in soft ground pre-warning model, including alarm object, alarm indicators, alarm values, alarm level, the model of functional modules and its mutual relations.