Review of through-wall leaking incidents during excavation of the subway stations of Nantong metro line 1 in thick water-rich sandy strata

Highlights

• Hundreds of leaking accidents during excavation in water-rich sandy strata were investigated.

• The temporal and spatial distributions of leaking failures were categorized and characterized.

• Major contributory factors were recognized and corresponding measures were proposed.

Abstract

Through-wall leakages took place frequently during those excavations in water-rich sandy strata, which often resulted in engineering failures (e.g., global instability of retaining structures and damages to adjacent existing structures) and even injuries and/or fatalities. In spite of this, only few comprehensive studies relevant to this topic have been known in literature to date. During the recent construction of Metro Line 1 in the city of Nantong, China, hundreds of through-wall leaking incidents had taken place, which provided an opportunity for comprehensively investigating through-wall leaking in water-rich sandy strata. First, a database comprised of hundreds of leaking accidents collected in Nantong was built in this study; then, these leaking incidents were classified into three major levels by a descriptive statistics method, i.e., minor seepage without soil loss, massive leakage with a handful of soil loss, and massive through-wall ingress of water and soils. The primary contributory factors of these leaking events included exposure of steel bars of diaphragm wall (DW) panels, local bulging of wall faces, flawed joints of DW panels, and complicated geohydrological conditions. Moreover, the characteristics of these leaking events were examined in terms of their temporal and spatial distributions as well as excavation behaviors. Finally, some practical measures and solutions to deal with potential through-wall leaking incidents during excavations in water-rich sandy strata were proposed.

Introduction

Over the past decades, numerous through-wall leaking accidents that occurred in deep excavations have been reported worldwide, which were usually accompanied by economic losses, severe damages to the adjacent urban environments and even injuries and/or casualties. On September 7 of 2005, both severe water leakages through openings in the diaphragm wall (DW) and piping failure occurred at a Taipei Rapid Transit system station box; unfortunately, the implemented emergency grouting plan not only aggravated the piping but also caused noticeable lateral wall movement; as a result, those existing buildings nearby experienced significant sudden settlements. The post-failure investigation revealed that improper concrete casting of DWs had triggered this leaking accident (Ni and Cheng, 2012). In 2008, two through-wall leaking events during excavation of a subway station in Amsterdam of Netherlands caused significant damages to the adjacent historical buildings on timber piles, whose settlements were up to 140−250 mm. A steel stop end and the existence of bentonite inclusions in the DW were blamed for these two accidents, which allowed vast amounts of water and sands to flow through the wall faults, causing catastrophic erosion and loosening of the stratum outside the excavation ultimately (Korff et al., 2011). In September of 2010, massive groundwater gushed into an excavation for an interchange subway station in China through a large hole in the DW; consequently, the axial forces of steel struts reduced suddenly, which posed a threat to the overall stability of retaining systems (Wu et al., 2011). On October 11 and 14 of 2010, massive through-wall leakages and sand boiling occurred during excavation of a subway station in Nanchang, China due to the failure of perimeter waterproof curtains; as a result, large-scale ground collapses occurred behind the excavation (Feng and Lu, 2016). In July of 2013, a sudden massive through-wall water flowing event took place in Shanghai when an excavation went deeply into the confined aquifer; shortly afterwards, the adjacent ground and buildings underwent rapid settlements and visible ground cracks occurred at the ground level. The post-failure investigation disclosed that a flawed slab connector buried in the confined aquifer contributed to this leaking accident (Tan and Lu, 2017). On May 10 of 2019, a destructive seepage failure took place in a deep excavation at the northern bank of the Yangtze River in China; the incident was induced primarily by defects in the waterproof curtains consisting of three-axis mixing piles (Xu et al., 2022).

The leaking cases mentioned above showed that through-wall leaking usually incurred considerable declines in groundwater level and massive ground volume losses outside the excavations, accompanied by ground cave-in failures, pipeline breakage, building cracking/tilting, and other hazards (Bai et al., 2021; Feng and Lu, 2016; Korff et al., 2011; Ni and Cheng, 2012; Tan and Long, 2021; Tan and Lu, 2017; Tan et al., 2021; Wu et al., 2020a, Wu et al., 2020b, Wu et al., 2020c). Through-wall leaking accidents resulted largely from a combination of technological and human factors as well as complex geohydrological conditions. The vast majority of through-wall leaking accidents could have been avoided by exploring their contributory factors and then eliminating them in advance (Di et al., 2022; Di et al., 2023; Hu et al., 2021; Jiang and Tan, 2021; Wang et al., 2022). After reviewing four tremendous leaking incidents in the course of subway construction in soft alluvial deposits, Cheng et al. (2020) pointed out that the vertical joints between adjacent DW panels and the existing seepage-prone weak regions inside soilcrete body were two main pathways for initiating water leaking incidents; hence, ensuring the watertight effectiveness of waterproof curtains was the key point of diminishing its negative impacts. Based on the volume of water and soil loss, the leakage phenomena were roughly classified by Zheng and Diao (2016) into merely groundwater leakage, convergent water-soil loss and continual water-soil loss.

The preceding studies mainly focused on one or several leaking cases of deep excavations in soft clayey deposits. Nevertheless, very few comprehensive studies have been contributed to the diverse through-wall leaking cases in water-rich sandy strata, which are much more vulnerable to groundwater flowing than clayey strata (Wu et al., 2018; Jiang and Tan, 2021, Jiang and Tan, 2022; Long and Tan, 2020; Tan and Long, 2021; Zheng et al., 2016). Within the last four years (2018–2021), hundreds of through-wall leaking accidents have taken place during construction of Metro Line 1 in the city of Nantong, Jiangsu Province, China, which provided a rare opportunity for extensively characterizing through-wall leakages of deep excavations in thick water-rich sandy strata. Thus, drawing a full picture of diverse leaking accidents during excavations, identifying the relevant major contributing factors and then adopting cost-effective measures for reducing disasters and hazards become possible. The findings and lessons learned from these through-wall leaking accidents in Nantong will be practically useful for professionals worldwide to deal with similar underground excavation projects; thus, the objectives of minimizing the high incidence of through-wall leaking events in water-rich sandy strata and protection of densely populated urban environments can be achieved.

In this study, hundreds of leaking accidents occurring in 33 subway excavations of Nantong Metro Line 1 were collected for reviewing first. Then, the major types of these leaking accidents were categorized and the associated causes were recognized. Subsequently, the primary characteristics of these leaking accidents are discussed in terms of their temporal, spatial and location distributions. Finally, countermeasures and remedial measures for mitigating the risk of through-wall leaking incidents in water-rich sandy strata are proposed.