GROUNDWATER CONTROL DURING DEEP EXCAVATION IN THE SOFT DEPOSIT OF CHINA: AN OVERVIEW Yong-Xia Wu1 , Shui-Long Shen2,a , Ye-Shuang Xu2,b and Wen-Juan Sun3 1 Department
of Civil Engineering, Shanghai Jiao Tong University and State Key Laboratory of Ocean Engineering, No.800 Dongchuan Rd., Shanghai 200240, China. E-mail:
[email protected] 2 State
Key Laboratory of Ocean Engineering, No.800 Dongchuan Rd., Shanghai 200240, China. E-mail: a
[email protected], b
[email protected] 3 The
Via Department of Civil and Environmental Engineering, Virginia Tech., Blacksburg, Virginia, 24061, USA. E-mail:
[email protected] The Quaternary soft deposits along the coastal region of China are generally multiaquifer-aquitard system (MAAS) with characteristics of very high compressibility of aquitards and high groundwater level in both phreatic and confined aquifers. Anthropogenic activities, e.g. construction of subway stations and/or high rise buildings in coastal areas, usually require deep foundations. During excavation of foundation pits, groundwater needs to be pumped down to a certain level to ensure both the project safety and the protection of surrounding environment. In engineering practice, dewatering is usually conducted through pumping of groundwater with various types of wells to lower groundwater table during excavation. However, dewatering from both phreatic and/or confined aquifer may induce ground subsidence in the surrounding area. To control groundwater and to protect the surrounding environment, the combination of cutoff wall and wells located both inside and outside the foundation pits is widely adopted in excavations. This paper introduces the MAAS hydrogeological formation of several coastal cities in China, such as Tianjin, Shanghai, Hangzhou, Ningbo, Fuzhou, and Guangzhou. Then, the dewatering patterns are identified according to the relationship between the MAAS and the depth of cutoff walls. Some typical cases with special dewatering patterns are presented at the end of this paper.
Keywords: Soft deposit, Deep excavation, Dewatering patterns, Case histories. 1. INTRODUCTION
The coastal line of China is bounded by the Bohai Sea, the Yellow Sea, the East China Sea, and the South China Sea. Construction of high-rise buildings and underground infrastructures, such as metro tunnel, underground parking garage, underground department store, etc., in coastal cities requires for the deep excavations in soft deposit. Since the Quaternary soft deposits along the coastal region in China are very complicated and groundwater
Proceedings of the International Conference on Ground Improvement and Ground Control Edited by Buddhima Indraratna, Cholachat Rujikiatkamjorn and Jayan S. Vinod Copyright © 2012 by Research Publishing Services. All rights reserved. ISBN: 978-981-07-3560-9 :: doi:10.3850/978-981-07-3560-9 07-0702
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table is very high, dewatering is required in underground engineering construction. The purposes of dewatering are as follows (Wu 2003): 1) to lower water content in the soil inside the foundation pit; 2) to ensure the stability of foundation pit; 3) to provide convenience for construction operation of mechanical diggers and workers; 4) to prevent confined water inrushing from the excavation face. However, it is very common that withdrawal of groundwater induces land subsidence along the coastal region (Chai et al., 2004, Shen et al., 2011, Shi et al., 2008). Reasonable dewatering schemes should be achieved with the consideration of the geological characters of coastal region of China before excavation. This paper presents the geotechnical conditions of several coastal cities in China at first. Then, concurrent foundation pit dewatering patterns used in coastal cities are summarized with the discussion of merits and demerits of each dewatering pattern. 2. GEOLOGICAL CONDITION IN COASTAL CITIES OF CHINA
Figure 1 depicts the coastal regions in China. The area of coastal regions in China is about 1,370,000 km2 , including ten provinces, three municipalities and two special zones. Tianjin, Shanghai, Hangzhou, Ningbo, Fuzhou, and Guangzhou are six typical cities from north to south. According to the forming environment (e.g. marine and/or continental phases), the Quaternary deposit is generally marine-terrestrial interlaced phase with sand, gravel, clay and silty clay in coastal regions in China. Generally, the hydrogeologic formation in coastal cities is composed of a phreatic aquifer and several confined aquifers. Aquifers are separated by aquitards, forming so-called multi-aquifer-aquitard system (MAAS). The soft soil in MAAS is characterized with high void ratio, high compressibility and low intensity. Table 1 tabulates the thickness of the Quaternary deposit and physical/mechanical properties of soft soil in coastal cities (He 1984, Sun et al., 1984, Zhu 1998, Yan et al., 2001, Xing et al., 2005, Wang 2006, Xu et al., 2007, Shen et al., 2010, Ye et al., 2010, Wang et al., 2011). N
N
Liaoning Province Beijing Tianjin
Bohai Sea
Hebei Province Shandong Province 0
150 300km
Coastal Area
Yellow Sea Jiangsu Province Shanghai Hangzhou Bay
Hangzhou
Ningbo
Zhejiang Province
East China Sea
Fuzhou
Fujian Province Guangxi Province
Guangdong Province
Taiwan
Guangzhou
Macao
North Bay
Hongkong
South China Sea
Hainan Province
Figure 1.
Plan view of coastal region of China.
Groundwater Control During Deep Excavation in the Soft Deposit of China: An Overview
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Table 1. Thickness of the Quaternary deposit and physical/mechanical properties of soft soil in coastal cities of China (modified from Sun and Zheng 1984). City
Hd (m)
D (m)
w (%)
e
w L (%)
wP (%)
k (cm/s)
Cv (cm2 /kg)
Tianjin Shanghai
100–430 200–300 100-300 50-110
Fuzhou
40-70
Guangzhou
26–48
0.97 1.37 1.07 1.34 1.02 1.42 1.08 1.87 1.17 1.60
36 43 34 41 33 39 36 54 41 46
19 23 21 22 18 22 21 25 20 27
0.051 0.097 0.065 0.117
Ningbo
34 50 37 47 35 50 38 68 42 73
1E–07 6E–07 2E–06
Hangzhou
7–14 6–17 1.5–6,>20 3–9 9–19 2–12 12–29 3–19 1–3,19–35, 0.5–10
3E–08 7E–08 8E–06 5E–07 3E–08
0.095 0.072 0.203 0.070 0.0118
Note: Hd = thickness of Quaternary deposit; D = depth of soil layer; w = water content; e = void ratio; w L = Liquid limit; w P = Plastic limit; k = Hydraulic conductivity; Cv = coefficient of consolidation. Table 2. Dewatering types of excavation in coastal cities of China. Dewatering types
Hydrogeological characteristics
Light well point Ejector well point Perched water and Electro-osmotic small phreatic water well point Phreatic water Tube well confined water
D
Dewatering mode
Dewatering well location
/ / Drainage well In the pit / / / Drainage well In the pit No Around the pit portion In or around the pit most relief well In the pit Cutoff In the pit
Dewatering depth (m) Single stage
