The safety of foundation pit construction largely depends on the control of groundwater, so the water-stop curtain, as the main measure to control groundwater, plays a pivotal role in foundation pit engineering.

　　Cement mixing piles are widely used for water-stop and water cutoff in foundation pits because of their convenient construction, low cost, strong adaptability, and ability to meet general water-stop requirements. However, as a temporary measure, cement mixing piles often fail as water-stop curtains due to geological, design, and construction reasons, causing the failure of the entire foundation pit support system and frequently leading to major safety accidents. Using super-long steel sheet piles as support and water-stop curtains has obvious and reliable water-stop effects, conforms to the energy-saving and environmental protection policy orientation of the national 13th Five-Year Plan, and has strong market orientation and vitality compared with traditional cement product support and water-stop systems.

　　The following introduces a typical case of emergency rescue by Guangdong Shouhui Lantian Engineering Technology Co., Ltd. using super-long steel sheet piles after the failure of a deep foundation pit water-stop system.

　　Project Introduction

　　Guangdong Shouhui Lantian Engineering Technology Co., Ltd. is a high-tech enterprise dedicated to project consulting, design, investment, construction, and operation in sponge cities, black and odorous water body treatment, underground parking garages, municipal comprehensive utility tunnels, urban public infrastructure projects, and various geotechnical and hydraulic engineering projects. In 2015, it participated in the design and construction of the foundation pit emergency rescue project for a production research and development building of an institute in Guangdong Province. The project is located next to Kaichuang Avenue in Luogang Economic Development Zone, Guangzhou. The foundation pit has a perimeter of about 320m and an area of about 6400m2. The excavation depth is 17.5m on the west side and 13.5m on the east side. The original support system design scheme of the foundation pit is as follows: support piles: Ø1200 @1400 bored cast-in-place piles; water-stop curtain: single-row Ø1000 @750 cement mixing pile water-stop curtain, penetrating into the relatively impermeable silty clay layer; support system: two concrete corner supports.

　　During actual construction, after the second inner support was poured and the north side of the foundation pit was excavated to the design elevation, it was found that there were multiple continuous water leaks at the northwest corner, mixed with a small amount of fine sand. After the cushion was poured and excavation continued on the south side, a water inrush point suddenly appeared in the middle of the south side, with continuous large water inrush, causing an emergency in the foundation pit. Buildings on the north and south sides settled by 10cm, and the road surface on the west side subsided.

　　Determination of Emergency Rescue Plan

　　Based on the company's geotechnical experts' years of experience in the design and construction of steel sheet pile projects, and with the approval of the expert group, it was finally decided to use an 18-24m super-long IVw steel sheet pile water-stop curtain as the emergency rescue measure for this project.

　　Construction Difficulties

　　Using the steel sheet pile water-stop curtain as the emergency rescue plan for this project faced the following construction difficulties during implementation.

　　1) Complex and harsh surrounding environment. ① There are high-voltage cable trenches and water supply pipelines on the north, west, and south sides of the foundation pit, with the closest distance to the edge of the support piles about 2.2m; ② Multi-story buildings with prestressed pipe piles are adjacent to the north and south sides of the foundation pit, with the closest distance to the pit edge about 4.7m; ③ The foundation pit has been excavated about 10m, and some areas have been excavated to the design elevation, so no construction working face can be provided inside the pit; ④ The steel sheet pile axis is close to the mixing piles on one side and close to the pipe trench on the other; ⑤ Due to severe loss of sand and soil in the stratum, some areas have collapsed.

　　2) Harsh construction geological conditions. ① The design depth of the steel sheet pile toe is located in hard plastic granite residual soil or completely and strongly weathered granite, with developed boulders; ② The underground on the south side of the foundation pit has formed an underground river with the nearby river channel, with severe sand layer loss; the soil layer contains a large amount of construction waste; ③ The steel sheet piles are constructed close to the outside of the mixing piles, and the closing steel sheet piles need to cut into the mixing piles, which increases the difficulty of pile pressing; ④ The design pile length of the steel sheet piles is 18-24m. As a water-stop curtain, the continuity of the steel sheet pile interlock connection must be ensured, so the construction accuracy requirements are very high, and traditional construction techniques cannot meet the requirements.

　　Construction Equipment Selection

　　To solve the above construction problems, based on the selection of many steel sheet pile construction techniques and equipment, the company determined to use the static pressing method for the construction of the steel sheet pile water-stop curtain. The selected equipment is the company's own GIKEN F201 static pile press with auger.

　　Method Features 1) No vibration, no noise, and no impact on surrounding buildings (structures) and residents' lives; 2) Using a new generation control system and advanced information technology, it can automatically recognize the machine posture during construction, reasonably control the load of each action, and accurately guide the construction of steel sheet piles by the static pile press through an infrared guide target, achieving intelligent control and high-precision pile pressing; 3) Due to the lightweight body and small model, it requires little working face on the construction site and can achieve construction in narrow spaces; 4) It can press into hard geology: depending on geological conditions, pre-boring pressing with auger or core removal pressing is used to achieve pressing in hard geology such as gravel layers and pebble layers and strata containing underground obstacles such as reinforced concrete; 5) High safety and strong timeliness.

　　Construction Process and Key Points

　　Process Flow The basic process flow of static pressing steel sheet pile construction is: site preparation → surveying and setting out → trench excavation → static pressing steel sheet piles.

　　Site Preparation Ensure the working face for initial pressing and auxiliary crawler cranes and truck cranes. If roads need to be occupied, traffic evacuation must be done in advance. If there are existing cables or other facilities, protection work must be done.

　　Surveying and Setting Out According to the design, measure and set out control points such as the centerline, corner points, and start and end points of the steel sheet piles. Before starting work every day, set up infrared guide target points and calibration points at about 10m and 5m ahead of the static pile press's forward direction using a total station. The infrared guide target is used to accurately guide the construction of steel sheet piles by the static pile press, and the accuracy of the steel sheet piles is rechecked.

　　Trench Excavation Since the project uses core removal pressing and pre-boring pressing with auger, and according to design requirements, the top elevation of the steel sheet piles is 10cm lower than the crown beam elevation for later foundation pit construction, a trench of 2.0m (width) × 0.5m (depth) should be excavated along the steel sheet pile axis for the static pile press to perform pile pressing construction.

　　Pile Splicing and Corner Piles The standard single length of IVw steel sheet piles in the domestic market is mainly 15m and 18m. To meet the project requirements, existing 15m or 18m steel sheet piles need to be welded and lengthened.

　　To ensure the continuity of the steel sheet pile interlock connection, the steel sheet piles at the corner parts need special treatment to ensure good waterproof effect.

　　Quality and Safety Assurance Measures

　　Quality Assurance Measures 1) The coordinate data of control points must be rechecked when setting out control points. The static pressing method uses a dedicated laser calibrator to accurately guide the construction of steel sheet piles by the static pile press. At the same time, to ensure construction accuracy, the accuracy of the steel sheet piles must be rechecked; 2) For steel sheet piles, material inspection and appearance inspection are generally carried out to correct steel sheet piles that do not meet requirements, reducing difficulties during pile driving.
　　Safety Assurance Measures 1) Strictly operate according to the operating procedures of mechanical equipment; non-operators must not operate mechanical equipment; 2) Safety education coverage rate is 100%. Entering the construction site must comply with the safety management system of the construction site and consciously obey the inspection of safety inspectors; 3) Take corresponding protective measures according to actual conditions during operation. For work at heights, safety belts should be fastened, and a dedicated person should direct safe operations, paying attention to the safe distance between personnel, instruments, and on-site lifting and transportation equipment; when working on roads, personnel must wear yellow safety vests with reflective signs, and safety warning signs should be set up at survey stations, paying attention to passing vehicles.

　　Water-Stop Effect of Steel Sheet Pile Curtain

　　According to the design requirements of the emergency rescue plan, after the construction of the steel sheet pile water-stop curtain was completed using the static pile press, the accumulated water in the foundation pit was pumped out, and earthwork excavation was carried out. It was found that there was no obvious leakage on the side wall of the foundation pit, and the original water inrush point no longer surged. This ensured the safety of the foundation pit and surrounding buildings (structures) and provided a dry construction working environment for the pouring of the bottom slab.
　　
　　Advantages of Steel Sheet Pile Water-Stop Curtain

　　The advantages of the static pressing steel sheet pile water-stop curtain structure are mainly reflected in the following aspects:

　　1) Static pressing construction has no vibration and no noise, and can operate in narrow spaces, achieving green and environmentally friendly construction.

　　2) Fast construction speed and low comprehensive cost. Steel sheet piles can be recycled and reused.

　　3) Wide applicability, and can be constructed under various geological conditions.

　　4) Integrates support piles and water-stop curtain, with obvious comprehensive advantages.

　　5) Can perform high-precision, super-long steel sheet pile construction.

　　6) The interlocks between steel sheet piles engage with each other, providing good sealing and water-stop effect.

　　The successful application of this project has accumulated more theoretical knowledge and practical experience for China in static pressing steel sheet piles. It also shows that the static pressing steel sheet pile support and water-stop curtain has absolute advantages and application prospects compared with similar engineering structures such as cement mixing pile water-stop curtains.