The safety and stability of the waterproof curtain have a significant impact on surrounding safety. In actual construction, various factors such as scheme design, geological exploration, and quality control may lead to loopholes that cause the waterproof curtain to fail, resulting in delays or safety accidents. Therefore, research on the causes and treatment methods of waterproof curtain failure is of great significance.
　　Requirements for Deep Foundation Pits and Waterproof Curtains
　　1. Deep Foundation Pit
　　A deep foundation pit refers to an excavation project with a depth of 5m or more, or a basement of 3 or more floors. At the same time, when geological conditions and surrounding environment are complex, the excavation project can also be called a deep foundation pit. According to relevant Chinese documents, it is defined as: earthwork excavation, support, and dewatering engineering for foundation pits with an excavation depth exceeding 5m (including 5m); and earthwork excavation, support, and dewatering engineering for foundation pits with an excavation depth not exceeding 5m but with complex geological conditions, surrounding environment, and underground pipelines, or affecting the safety of surrounding buildings.
Relevant regulations require that deep foundation pit work has a high risk coefficient, and its special plan must be organized by the construction unit to conduct safety demonstration together with expert group members, project managers, project supervision unit personnel, construction safety officers, technical directors, as well as technical and safety personnel from survey and design units.
　　2. Waterproof Curtain
　　The waterproof curtain is a general term for waterproofing works in excavation projects. Its significance lies in preventing or reducing the infiltration of groundwater into the foundation pit to ensure construction safety and prevent building settlement after use.
　　The waterproof curtain generally consists of three parts. The first part is the retaining pile, which functions similarly to a retaining wall and can be reinforced concrete cast-in-place piles or other forms, with certain gaps between piles. The second part is the actual waterproof curtain part, mainly used for soil reinforcement and blocking the mutual flow of water inside and outside the foundation pit, generally using cement mixing piles or compaction grouting technology. The third part is the support part. Unlike ordinary retaining piles, underground continuous walls use other forms for foundation pit reinforcement and maintenance, generally used for extra-large and extra-deep foundation pits.
　　What are the causes of waterproof curtain failure?
　　The causes of waterproof curtain failure need to be analyzed through observation of factors such as the foundation pit and mixing piles. The leakage situation of the waterproof curtain for foundation pit enclosure is complex, and leakage types must be classified and treated separately. Through multiple foundation pit construction practices, the author believes that according to the depth and location of leakage, it can be divided into two situations: leakage above the excavation surface and leakage below the excavation surface; according to the materials used, it can be divided into reinforced concrete gap leakage and cement-soil gap leakage.
　　At the same time, the causes of waterproof curtain failure may be loopholes in the treatment of cold joints during construction, resulting in local non-sealing. It may also be due to unqualified quality of mixing piles between the three layers of silty soil causing leakage. Because of tight schedules and heavy tasks, after construction is completed, the dewatering speed of the foundation pit is too fast and the dewatering volume is too large, causing water flow in the soil layer, diluting and transporting the not-yet-set cement slurry, so that the quality of the mixing piles cannot achieve the expected effect. The last possibility is improper design treatment of the waterproof curtain mixing piles, and large-scale excavation immediately after the completion of the waterproof curtain construction, causing displacement of the support structure, damaging the mixing piles and affecting their quality.
　　1. Perched water layer with large water pressure at the bottom of the foundation pit
　　The depth of the waterproof curtain is located in the perched water layer but does not completely seal this layer. After excavation, it is equivalent to foundation unloading, and the pressure in the soil decreases. Weak parts such as large-diameter wells, tool column piles, and supporting reinforced concrete cast-in-place piles at the pit bottom may all produce piping and flowing sand. Improper treatment may also cause heaving of the pit bottom, and even endanger the safety of surrounding underground pipelines and buildings.
　　2. Damage to the waterproof curtain by non-phreatic water sources
　　Leakage of underground pipelines, proximity to rivers, and excessive rainfall cause excessive soil moisture content, and soil particles suspend and flow, increasing the active earth pressure on the enclosure structure. This causes the waterproof curtain to fracture due to excessive deformation of the enclosure structure, resulting in leakage. If the external water volume is too large, it may also cause foundation pit engineering accidents.
　　3. Seepage from the peripheral foundation pit
　　According to the geological survey report, there is no perched water layer with large water pressure underground in this project. However, as the foundation pit is excavated, leakage from underground pipelines and high water content in surrounding soil and high groundwater level cause water to seep from the top of the peripheral foundation pit into the pit, forming visible leakage. At the same time, due to the unsatisfactory quality of cement mixing piles, leakage occurs in the middle of the foundation pit, and local leakage is quite serious, forming hidden leakage. However, through observation, the initial leakage carried more sediment, and later leakage was clear water, without forming large cavities underground, and did not cause significant impact.
　　4. Other damage causes
　　As the excavation depth of the foundation pit varies, the soil and water pressure on the enclosure structure also changes to some extent. Quality problems of some undiscovered deep mixing cement-reinforced soil walls gradually emerge, such as adjacent groups not interlocking or with small interlocking amount during wall formation, poor joint sealing or insufficient connection after grout interruption, uneven mixing with debris causing interlayers or inclusions, and poor or no consolidation of cement-reinforced soil due to the activity of soil in the geology and special geological influences. These factors can all cause leakage of the waterproof curtain. Generally, such leakage has a small flow rate but carries more sediment. Due to time-space effects, local soil outside the foundation pit forms flowing sand. If not treated in time, large cavities may form in the soil outside the pit, endangering the safety of the foundation pit and surrounding underground pipelines and buildings.
　　Countermeasures for Failure and Leakage Treatment
　　(1) First, adopt the construction technology of curtain grouting outside the pit for sealing. First, position the overall curtain. Outside the previously set waterproof curtain, install the first row of grouting holes at a depth of about 20cm from the biaxial deep mixing piles. Set the second row of grouting holes at a distance of 0.5m from the first row, arranging the two rows in a quincunx pattern. In the layout step, grout the first row of grouting holes, ensuring the grouting pipe remains vertical when pressed in, and compact the grouting, controlling the pressing depth at 6-8m, entering about 0.5m into the fourth layer of clay. Mix cement slurry, cement, and water glass in a 1:1 ratio, with grouting pressure controlled at 1MPa.
　　(2) Conduct pressure reduction treatment with tube wells outside the pit. Due to the influence of the surrounding natural environment during excavation, water may penetrate deep into the soil layer, causing the soil to contain a large amount of water. When earthwork is excavated to 7m, there is a large difference in water head between inside and outside the pit, so pressure-reducing wells should be added outside the pit to reduce the pressure difference and lower the seepage volume inside the pit, laying a foundation for grouting and sealing outside the pit. At the same time, observe the surrounding environment of the curtain at all times, pay attention to humidity changes, and conduct intermittent pumping, achieving less pumping but more frequent pumping, with pumping time as short and efficient as possible. Generally, pressure-reducing wells are placed in the houses on one side of the foundation pit and on the west side of the houses. The well depth should be greater than the grouting pipe depth, generally 10-12m, with a distance of about 20m between two wells, using green bean sand as the filter material between the well pipe and the well hole.
　　(3) Conduct hanging net shotcrete sealing outside the pit. Ensure that the hanging net shotcrete sealing work inside the pit is carried out at the same time as grouting and pressure reduction outside the pit. In the construction of the steel mesh hanging between piles inside the pit, steel bars with a diameter of 8mm should be used as the main construction material, with a spacing of 15cm, with a tolerance of 5mm, so that the steel bars are reliably connected to the pile body. The main material for shotcrete is C20, with a thickness of 6cm, and the tolerance should be less than 5mm. During spraying, first spray 3cm thick C20 concrete. After the first sprayed layer solidifies and is cured, proceed with the second layer, i.e., the remaining 3cm, and similarly cure after solidification.
　　(4) Seal the already compacted concrete inside the pit. The base elevation is required to be 6.8-7m underground. The main property of the base layer here is silty soil, and the most obvious characteristic of this type of soil layer is excessive permeability. Therefore, after earthwork excavation, on the premise of not affecting subsequent engineering, apply dense concrete pouring at the junction of the base layer and the pile body to build a concrete barrier wall to block moisture and water vapor, effectively completing the seepage sealing of the leakage location inside the pit and ensuring the quality of the curtain.
　　(5) Prevent settlement of residential buildings. Grout the ground on the corresponding side of the settled residential buildings to effectively reinforce the soil, which is also the key to ensuring the anti-seepage of the waterproof curtain. At the same time, add grouting holes in the courtyards of each resident, requiring a grouting depth of 6m, with a tolerance of 0.5m, and grouting pressure of 0.5MPa. In the grouting process, remove the grouting pipes in sequence. The grouting holes at one end of the grouting pipe are already in the fill range. Appropriately reduce the grouting pressure and monitor the ground conditions simultaneously. When grout leakage occurs, the observer must immediately order to stop grouting. After grouting is completed, set settlement monitoring points or install monitoring modules and equipment, and conduct necessary testing during and after grouting.
　　(6) Summarize the overall construction process. First, conduct compaction grouting reinforcement outside the pit. The reinforcement sequence is to reinforce the first row first. After the first row is reinforced and repaired, carry out grouting, reinforcement, and maintenance of the second row of grouting holes. Then, while grouting and reinforcing outside the pit, carry out hanging net shotcrete inside the pit. If leakage is found during this period, promptly locate the leakage position and use cement bags to seal the leakage point and reinforce it. Finally, pour dense concrete at the connection between the pile body and the base to form a concrete retaining wall to prevent leakage.
　　(7) Evaluate the engineering effect after the entire work is completed. First, detect and evaluate the groundwater level. The detection method is to detect the water level outside the foundation pit through the water level observation holes set on the settlement side of the foundation pit. The detection results show that from the start of earthwork excavation to the end of the earthwork excavation project, there was no intermittent seepage inside the foundation pit. The water level outside the protected foundation pit remained stable, with sudden fluctuations within a controllable range in the middle, caused by natural precipitation. The monitoring results indicate that the treatment of the waterproof curtain played its due role, ensuring that the seepage inside the foundation pit would not affect the water level and settlement outside the pit. Then monitor the settlement of houses. The monitoring method is to set 4 monitoring points or install 4 monitoring devices in each house within the foundation pit on the settlement side, with the total number of devices being 4 times the total number of houses. Set the system to issue a warning when house settlement reaches 2cm and record the settlement value. From the record analysis, from the start of construction to the end of the project, the total settlement of the houses was controlled within 2cm, indicating that the house settlement during construction was small, with gentle changes and gradual stabilization.
　　Key Points of Construction Management
　　Based on engineering practice records and analysis, the following conclusions are drawn.
　　(1) The construction quality of the waterproof curtain is affected by many factors, so quality monitoring must be strengthened during construction, and defective parts must be maintained in a timely manner to ensure the effectiveness of the waterproof curtain.
　　(2) Foundation pit dewatering should be carried out after the curtain is closed and reaches a certain strength, to avoid affecting the quality of the mixing piles.
　　(3) In the case of waterproof curtain failure, the cause should be analyzed in combination with the site conditions before treatment to ensure the safety of the waterproof curtain.