Environmental Impact of Ground Deformation Caused by Underground Construction in China
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Japanese Geotechnical Society Special Publication The 15th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering Environmental impact of ground deformation caused by underground construction in China Gang Zheng i) and Yu Diao ii) i) Professor, School of Civil Engineering, Tianjin University, 92 Weijin Road, Tianjin, China. ii) Lecturer, School of Civil Engineering, Tianjin University, 92 Weijin Road, Tianjin, China. ABSTRACT This paper is a review of the study of ground deformation caused by underground construction and the countermeasures to control the environmental impact of underground construction, mainly including deep excavation and tunneling, based on case histories and state-of-art researches in China during the recent decade. According to the mechanism of deformation due to underground construction and its environmental impact, the deformation features and controlling measures were categorized into three classes, i.e., the soil displacement due to stress field change in soil or volume loss caused by construction activities, the ground loss caused by water-soil loss, and large scale ground movement due to the global failure of braced deep excavation or tunnel triggered by local damage of structures or local failure of soil. Furthermore, for each deformation type, the environmental impacts were graded based on the influence magnitude of the excavation and tunneling. Finally, the principles and methods to control the effects of deep excavation and tunneling were summarized. Keywords: impact, countermeasure, excavation, tunnel, deformation, global failure 1 INTRODUCTION interaction during construction. Changes in stress field of soil, fluid field of groundwater and structure internal With the rapid development of urbanization in force can lead to various modes of ground deformation China, more and more underground space has been hence different influences to surrounding environment. explored for public infrastructures and commercial According to the causes of environmental impact, utilization. In the long term, it is estimated that over the the deformation features and controlling measures can next two decades China will build 20,000 to 50,000 be classified into three categories as following. new skyscrapers and more than 170 cities will require (1) The soil displacement caused by stress field mass transit systems by 2025, including 4000 km metro change in soil mass construction activities or ground line. Accordingly, there will be an increasingly number loss due to shield tunneling. Due to excavation, of excavations and metro tunnels projects, 65% of tunneling and dewatering, the soil movements develops which are planned to be constructed in the in eastern during the redistribution of soil stress and pore water China and central China. In most of these areas, the soft pressure. Nowadays the focus of the excavation and ground and high-level groundwater are the primary tunneling is the deformation controlling to minimize the challenges to the control of ground deformations effect on environment, especially in urban area. induced by deep excavation and tunneling. To ensure (2) The ground loss caused by water-soil loss. The the both safety and serviceability of the facilities groundwater may leak through the poor waterproof adjacent to the massive underground structures to be joints of diaphragm wall in deep excavation and built in China, therefore, it is necessary to sum up the segments of shield tunneling. If the fine particles of soil characteristics and countermeasures to environmental pass through the joints with water, the erosion of soil impact of ground deformation based on the successful can lead to large deformation of ground and case histories and state-of-art researches in China underground structures and can consequently cause the during the recent decade. larger opening of the joints of shield tunnel segments. In essence, retained deep excavation and tunneling (3) Large scale ground movement due to global in the above mentioned area establishes a complicated failure caused by local failure. Due to the strict system of soil-water-structure interaction. The initial deformation controlling criterion, the global stability of stress field of soil and groundwater is inevitably the excavation and tunnel seems satisfied, but a global disequilibrated during the process of underground and continual failure can also be triggered by local construction. The ground deformation, as one of the failure of structures or soil. When local damage and most important parameters for the environmental failure occur in the retaining structure of excavation or impact, is the response to the soil-water-structure http://doi.org/10.3208/jgssp.KL-2 10 the lining of tunnel, a domino effect of failure may take Shanghai soft deposits, the deformation behavior of place in an underground structure system without diaphragm walls was analyzed by Xu et al. (2008). It sufficient redundancy, which has not attracted much was found that the location of the maximum lateral wall attention in design. Some case histories show that displacement occurs within H±5m, where H is the global failure of deep excavation and shield tunneling excavation depth. For excavations supported by can extend to a range of more than 100 m long, and the concrete struts, system stiffness has little effect on the width of the zone affected by global failure can exceed normalized maximum lateral displacement. For 100 m, causing large scale ground movement, as excavations braced by steel struts, the normalized showed in Fig.1. maximum lateral displacements decrease with the increasing system stiffness. A method for predicting the maximum lateral wall displacement based on the factor of stability against basal heave was proposed. Based on a database of 35 case histories, the characteristics of ground surface settlement caused by deep excavations in Shanghai soft soils are summarized by Wang et al. (2011).The maximum ground surface settlement generally ranges from 0.1%-0.8%H, with an average value of 0.38%H, where H is the excavation depth. The maximum ground surface settlements Fig.1. Global failure of braced deep exaction in Hangzhou, China increase with the increase of the thickness of soft soils above wall toe, but decrease with the increase of the 2 THE SOIL MOVEMENT CAUSED BY STRESS factor of safety against basal heave. FIELD CHANGE DUE TO CONSTRUCTION Zheng and Li (2012) and Zheng et al. (2014) found ACTIVITIES that the deformation mode of retaining structures has a significant influence on the surrounding structures. It is According to the magnitude of deformation and found that given the same maximum horizontal impact to the adjacent facilities, the soil movements deflection of retaining structure with four deformation because of underground ground construction can be modes including convex, cantilever, composite and categorized as follows. kick-in mode, the displacement fields of soils outside (1) Small deformation. The effect of ground the excavation were considerably different. In practice, deformation on ordinary neighborhood buildings, road besides controlling the maximum horizontal and buried utilities is very slight. But for the existing displacements of the retaining structures, the facilities such us metro tunnel and station, special deformation mode of the retaining structures should be factories, hospitals, only very small deformation (e.g., optimized according to the surrounding environment. less 10 mm), is allowed to occur. However, Zheng and Li (2012) studied the response of conventional construction method and deformation buildings with different angle to excavations (Fig.2). It control techniques can not satisfy the strict deformation is found that the settlement of this kind of building was requirement. Therefore, additional countermeasures smaller than that of building perpendicular to must be adopted. excavations because a part of deformation energy was (2) Normal deformation. The deformation of this torsion energy for the building non-perpendicular to category is within the normal range of magnitude and excavations. When this kind of building is located over adjacent structures can remain within the serviceability the lowest point or the hogging zone of the settlement limit. To ensure that, only the conventional trough, the torsion deformation of the building is the underground construction techniques and deformation most obvious. control methods are necessary. (3) Large deformation. The deformation of this category is beyond the normal range of magnitude and adjacent structures exceeds the serviceability limit due to the large displacement of ground caused by deep excavation or tunneling. Cracks and tilt can happen to the surrounding structures, leakage and breakage occur to underground utilities. However, the stability of the overall stability of deep excavation and tunneling can be maintained. 2.1 Effect of excavation 2.1.1 Ground and building deformation Based on 93 case histories of deep excavations in Fig. 2. Torsion of building non-perpendicular to exactions 11 2.1.2 Tunnel deformation settlement. The result was verified by in-situ monitored The numerical method has been widely conducted ground surface settlements in construction of two to study the effect of excavation on existing tunnel in cross-river tunnels in Hangzhou, China. proximity. Li and Wang (2012) simulated a A simple estimation method of prediction of surface construction of deep excavation adjacent to a subway soil under the horizontal displacement caused