LONG-TERM MONITORING AND SAFETY EVALUATION OF A METRO STATION DURING DEEP EXCAVATION

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1 Available online at Procedia Engineering 14 (2011) The Twelfth East Asia-Pacific Conference on Structural Engineering and Construction LONG-TERM MONITORING AND SAFETY EVALUATION OF A METRO STATION DURING DEEP EXCAVATION L. RAN a, X. W. YE b, and H. H. ZHU c a Hangzhou Metro Group Co., Ltd., Hangzhou, China b Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China c School of Earth Sciences and Engineering, Nanjing University, Nanjing, China Abstract The subway systems play a vital role in alleviating the urban traffic congestion problem. Recently, lots of underground railway transportation networks have been opened to operation or are being constructed in major cities of China. Due to the complexity and uncertainty inherent in excavation activities, metro station excavations brings a challenge to civil engineering communities and poses threat to the public safety in metropolitan regions. The stability of deep excavation and adjacent buildings has gained highlighted concerns during metro station construction. A viable and practical way to ensure the construction safety is by executing real-time monitoring strategy with the aid of advanced sensing and signal processing technologies. In this paper, the design and implementation of a long-term monitoring and safety evaluation system for the deep excavation of a metro station has been addressed. A software platform has been developed for analyzing and processing monitoring data based on the concept of dynamic construction inverse analysis, which includes the database system, the dynamic construction feedback system, and the deformation forecasting system. Field monitoring results in various categories during deep excavation are presented. After examining the field measurement results, the following conclusions are drawn: (i) the deformation of the diaphragm wall and ground surface settlements increased with the excavation depth; (ii) the location of the maximum horizontal displacement moved downward to the excavation face during excavation; (iii) the axial forces of struts transferred from the first row to the others during excavation; and (iv) the monitoring results indicate that the braced excavation remained overall stable at the construction stage Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Presenter: rl@hzmetro.com Corresponding author: cexwye@polyu.edu.hk Published by Elsevier Ltd. Open access under CC BY-NC-ND license. doi: /j.proeng

2 786 L. RAN et al. / Procedia Engineering 14 (2011) Keywords: Metro station, deep excavation, monitoring, field instrumentation, safety evaluation. 1. INTRODUCTION Life-cycle structural health monitoring has gained rapid progress with the aid of advanced technologies in sensor and sensing network, data acquisition and communication, signal processing, data and information management. Large-scale monitoring systems for intelligent infrastructures have been designed and implemented worldwide (Barke and Chiu 2005; Ko and Ni 2005; Wong 2007; Ni et al. 2009). However, for underground structures, the application of monitoring systems during construction and operation is scarcely put into practice (Okundi et al. 2003; Bhalla et al. 2005; Wright 2010). Nowadays, more and more metro systems have been constructed in the metropolises of China. Owing to complex geological condition, harsh construction situation, and immature computational methodology, construction of metro systems is often subjected to considerable sources of uncertainties. To ensure the safety of adjacent building structures, it is a vital necessity for monitoring deep excavations of metro stations at their in-construction stage. In the construction of metro systems, deep excavations with large excavation areas, great excavation depths, and complex shapes and geological conditions remain a challenging and high risky task in metropolitan regions (Dunnicliff 1993; Ou 2006). The deformation and stability of deep excavation are highly affected by soil characteristics, groundwater variation, surcharge condition, etc. In the past several decades, several cases of excessive deformation or instability in braced excavations of metro stations occurred in China. Considering this problem, field instrumentation and performance evaluation of deep excavations are of great importance for quality control and safety assurance (Phienwej and Gan 2003; Leung and Ng 2007; Shao and Macari 2008). This paper introduces the instrumentation work of the deep excavation of a metro station. The main monitoring results of deformation induced by excavation and loading in the lateral support system are presented and analyzed. The results are used as guidelines on performance evaluation and risk assessment of the deep excavation activity. 2. DESCRIPTION OF THE METRO STATION The metro station under study is an island platform and the main structure is constructed by open excavation sequential operation method. The total length of the excavation is m and the typical excavation width is 44.5 m (see Figure 1). It is formed by double-layer six-span rectangular reinforced concrete frame structure. There are totally eight passageways and one fire evacuation port in this metro station and the access channels for import and export is a single-layer box-shaped structure. Both sides of this station are shield tunnels. The minimum distance between adjacent buildings and the station is larger than the excavation depth. There are no municipal pipelines in the excavation region. The metro station is situated in the coastal plain of Qiantang River, with flat terrain and simple topographical features. Main formation of this site from top to bottom consists of the layers of mixed fills, silty clay, clayey silt, sandy silt, and silty sand. The phreatic water exists in the upper layers of fills, silt, and sand. In the shallow phreatic zone, the groundwater level varies from 0.5 m to 3.4 m. The deep confined aquifers are mainly distributed in the underlying layers of cobble gravel. The main support system of this excavation is an 800 mm-thickness diaphragm wall. Four rows of steel struts are constructed from top to bottom and each strut has a diameter of 600 mm and a thickness of 16 mm. The vertical spacing of the struts is 3 m.

3 L. RAN et al. / Procedia Engineering 14 (2011) Zone A (192.4 m) Zone B (80.5 m) Zone C (171 m) N 44.5 m Figure 1: Photo and plan view of metro station excavation. 3. MONITORING ITEMS AND INSTRUMENTATION SCHEME 3.1. Arrangement of monitoring points Aiming to ensure excavation safety, the monitoring points are designed to be capable of capturing the deformation properties of the excavation and surrounding environments. Table 1 lists the total monitoring items and related equipments. According to the excavation characteristics and surrounding conditions, the instrumentation mainly includes the following contents: (1) monitoring of horizontal displacements of the diaphragm walls at different depths; (2) monitoring of axial forces in struts; (3) monitoring of ground surface settlements outside the excavation; (4) observation of groundwater levels; and (5) monitoring of bottom heaves in the excavation. Table 1: Monitoring items and related equipments No. Monitoring Item Equipment Amount 1 Horizontal displacements of the diaphragm wall Inclinometer 40 2 Horizontal displacements of surrounding soils Inclinometer 8 3 Axial forces in struts Axial-force transducer 85 4 Reinforcement stresses of the diaphragm wall Stress gauge 56 5 Groundwater levels Water-level tube 19 6 Bottom heave Settlement gauge 20 7 Displacements of the top of the diaphragm wall Level sensor and theodolite 40 8 Vertical displacements of center posts Level sensor 20 9 Vertical displacements of adjacent buildings Level sensor Settlements of surrounding soils Level sensor 25 Figure 2 shows the instrumentation plan of the excavation. The acceleration-type inclinometers are used to monitor the horizontal displacements of the diaphragm wall and surrounding ground soils. The vertical displacements of the wall top and settlements of surrounding ground soils are measured by level sensors and theodolite. The bottom heave within the excavation is measured by settlement gauges. The axial forces in the struts and the reinforcement stresses in the diaphragm wall are recorded based on the readings of axial-force transducers and vibration-wire stress gauges, respectively. The water-level tubes are installed around the excavation to measure the groundwater levels.

4 788 L. RAN et al. / Procedia Engineering 14 (2011) Horizontal displacement of the diaphragm wall and vertical displacement of the wall top Horizontal displacement of ground soil Groundwater level Reinforcement stress in the diaphragm wall Bottom hea ve Settlement of ground soil (a) Zone A (b) Zone B (c) Zone C Figure 2: Instrumentation of the metro station excavation. 3.2.Setting of alarming indices Setting reasonable alarming indices is of great importance in monitoring the excavation. If the alarming values are set to be too large, it will fail to achieve the desired safety warning, while too small warning values will restrict the progress of project implementation. Therefore, the alarming values should be determined based on the analysis of actual field situations. The alarming indices are typically expressed by the amount of accumulative variation and variation rate. The amount of accumulative variation should not exceed the design value. Table 2 lists the alarming indices used in this project Integrated monitoring and evaluation system Data analysis is essential in safety monitoring of geotechnical structures. As the complexity and specificity of geotechnical engineering, it is not easy to carry out assessment and prediction based on the monitoring results. In this aspect, therefore, how to extract the useful information from the huge amount of data is of particular importance. To ensure safety construction and to reduce the impact on the surrounding environments, informational construction technology is applied in excavation of the metro station by tracking and monitoring the whole construction process. For the purpose of analyzing the monitoring data timely and effectively, an integration framework consisting of database system, dynamic construction feedback system, and deformation prediction system is designed. Figure 3 illustrates the schematic diagram of the operating principle of this integrated monitoring system.

5 L. RAN et al. / Procedia Engineering 14 (2011) Table 2: List of alarming indices Alarming Index Item Daily variation (mm) Accumulative variation (mm) Displacements of adjacent buildings ±3 ±20 Inclinations of adjacent buildings 3 Settlement of surrounding soils ±3 2.4 H Vertical displacements of the top of the diaphragm wall ±3 3.2 H Lateral displacements of the diaphragm wall ±3 3.2 H Reinforcement stresses of the diaphragm wall 110 MPa Axial forces in the struts 0.8Fy Bottom heave ±3 ±30 Vertical displacements of center posts ±3 ±20 Groundwater level Remarks: H is the excavation depth; Fy is the design value of axial force. Data Acquisition System Data Analysis Dynamic Construction Feedback and System Monitoring Inverse Analysis Result Comparison Deformation Prediction System Figure 3: Schematic of operating principle of integrated monitoring system. 4. MONITORING RESULTS AND ANALYSIS 4.1. Analysis of horizontal displacements of diaphragm wall The typical monitoring data measured by inclinometers are shown in Figure 4. The monitoring results show that the horizontal displacement of the diaphragm wall increases gradually with the increase of excavation depth. The location of maximum displacement moved down during the excavation process. The maximum horizontal displacement of the diaphragm wall measured by inclinometers ranges from 24 mm to 82 mm around the excavation. The southern wall of Zone A had significantly larger later deformation compared to other sidewalls. The limit value of horizontal displacement of diaphragm walls proposed by Clough and O Rourke (1990) is 0.5%H (1) h max

6 790 L. RAN et al. / Procedia Engineering 14 (2011) where h max is the maximum horizontal displacement of the diaphragm wall, and H is the excavation depth. In this case, h max equals to 85 mm. The measured maximum displacement did not exceed but is very close to this limit value. The health condition of the braced excavation was paid special attention at excavation stage. When the excavation depth increased, the maximum horizontal displacement of the diaphragm wall occurred in different depths. The monitoring results of inclinometers indicate that the maximum horizontal displacement appears below the excavation face when the ratio of excavation depth to the height of diaphragm wall H/Ho is less than 0.5; while if H/Ho is equal to 0.5, the maximum horizontal displacement normally appears at the excavation face. Horizontal displacement (mm) ) Depth (m Week 1 Week 2 Week 3 Week 4 Week 5 Week 8 Week 9 Figure 4: Horizontal displacements of the diaphragm wall measured by Inclinometer CX Analysis of axial forces in struts The axial forces of the steel struts were measured by axial-force transducers. Figure 5 shows the axial force time histories of corner brace ZL2 and horizontal strut ZL3. The axial forces of struts were demonstrated to vary with excavation depths. The lateral supporting loads transferred from the first row of struts to lower rows when the excavation proceeded. Durng excavation, the axial force in the first strut decreased to nearly zero while the others remained in a stable loading condition Analysis of ground surface settlements Figure 6 shows the settlements of ground surface induced by the excavation of Zone A. The monitoring results indicates that the maximum settlement occurred 4 m outside the excavation. During excavation, the ground surface settlements increased gradually. The settlement profile can be simplified by a parabola curve. This phenomenon is consistent with Clough and Schmidt s finding (Clough and Schmidt 1981).

7 L. RAN et al. / Procedia Engineering 14 (2011) Axial force kn) ZL2-1 ZL2-2 ZL2-3 ZL Time (d (a) Corner brace ZL2 Axial force kn) ZL3-1 ZL3-2 ZL3-3 ZL Time (d (b) Horizontal strut ZL3 Figure 5: Axial forces of a corner brace and a horizontal strut measured during excavation. 0-1 Distance from excavation edge (m) Settlement (mm) th week 8th week 9th week 10th week 11th week 12th week Figure 6: Settlement profile of the ground surface induced by excavation.

8 792 L. RAN et al. / Procedia Engineering 14 (2011) CONCLUSIONS A real-time monitoring and safety evaluation system has been designed and implemented for the excavation of a metro station during construction. An integration framework for analyzing the measurement data is designed, consisting of database system, dynamic construction feedback system, and deformation prediction system. The monitoring data show that the deformation of the diaphragm wall and ground surface settlements increased with the excavation depth. The location of the maximum horizontal displacement moved downward to the excavation face. The axial forces of struts are seen to transfer from the first row to the others during excavation. The monitoring results indicate that the excavation remained overall stable at the construction stage. ACKNOWLEDGMENTS The authors wish to express their thanks to the Hangzhou Metro Group Co., Ltd. for permission to publish this paper. REFERENCES [1] Barke D, and Chiu WK (2005). Structural health monitoring in the railway industry: a review. Structural Health Monitoring. 4(1), pp [2] Bhalla S, Yang YW, Zhao J, and Soh CK (2005). Structural health monitoring of underground facilities: technological issues and challenges. Tunnelling and Underground Space Technology. 20(5), pp [3] Clough GW, and O Rourke TD (1990). Construction induced movements of in-situ walls. Design and Performance of Earth Retaining Structures, Geotechnical Special Publication, ASCE, Vol. 25, pp [4] Clough GW, and Schmidt B (1981). Design and performance of excavations and tunnels in soft clay. Soft Clay Engineering. Elsevier Scientific, pp [5] Dunnicliff J (1993). Geotechnical Instrumentation for Monitoring Field Performance. John Wiley & Sons, Inc. [6] Ko JM, and Ni YQ (2005). Technology developments in structural health monitoring of large-scale bridges. Engineering Structures. 27(12), pp [7] Leung EHY, and Ng CWW (2007). Wall and ground movements associated with deep excavations supported by case in situ wall in mixed ground conditions. Journal of Geotechnical and Geoenvironmental Engineering, ASCE. 133(2), pp [8] Ni YQ, Xia Y, Liao WY, and Ko JM (2009). Technology innovation in developing the structural health monitoring system for Guangzhou New TV Tower. Structural Control and Health Monitoring. 16(1), pp [9] Okundi E, Aylott PJ, and Hassanein AM (2003). Structural health monitoring of underground railways. Proceedings First International Conference on Structural Health Monitoring of Intelligent Infrastructure, Tokyo, pp [10] Ou CY (2006). Deep Excavation - Theory and Practice. Taylor & Francis, Inc. [11] Phienwej N, and Gan CH (2003). Characteristics of ground movements in deep excavations with concrete diaphragm walls in Bangkok soils and their prediction. Journal of the Southeast Asian Geotechnical Society. 34(3), pp [12] Shao Y, and Macari EJ (2008). Information feedback analysis in deep excavations. International Journal of Geomechanics, ASCE. 8(1), pp [13] Wong KY (2007). Design of a structural health monitoring system for long-span bridges. Structure and Infrastructure Engineering. 3(2), pp [14] Wright P (2010). Assessment of London underground tube tunnels investigation, monitoring and analysis. Smart Structures and Systems. 6(3), pp