【Excerpts】Deep foundation pit water stop curtain failure cause analysis and treatment measures, detailed analysis!

       Water curtain safety and stability has a significant impact on the surrounding security, in the actual construction, due to scheme design, geological exploration, quality control and other factors cause water curtain failure, make delay or cause safety accidents, so the water curtain failure reason and treatment research is of great significance.

Deep foundation pit, water stop curtain requirements

1、 Deep foundation pit

       Deep foundation pit refers to the excavation project with the excavation depth of 5m and 5m above, or 3 floors and above the basement. At the same time, when the geological conditions and surrounding environment are more complex, the excavation project can also be called deep foundation pit. According to relevant documents in China, earthwork excavation, support and dewatering works of foundation pit with excavation depth exceeding 5m (including 5m); earthwork excavation, support and dewatering works with excavation depth not exceeding 5m, but complicated geological conditions, surrounding environment and underground pipelines or affecting the safety of surrounding buildings.

       According to the relevant regulations, the risk coefficient of deep foundation pit work is relatively high, and the special plan shall be organized by the construction unit by the expert group members, the project leader, the personnel of the project supervision unit, the safety responsible person of the construction party, the technical leader and the technical personnel and safety personnel of the survey and design units to jointly conduct safety demonstration.

2、Stop the water

       Water stop curtain is the general term of the suspended water project of the excavation project. The significance of the project is to prevent or reduce the infiltration of groundwater in the foundation pit, so as to ensure the safety of construction and prevent the settlement of the house after being put into use.

       The water stop curtain is generally composed of three parts. The first part is the retaining pile, whose main function is similar to the retaining wall, with reinforced concrete cast-in-place pile or other forms, and there is a certain gap between the pile bodies. The second part is the real water stop curtain part, which is mainly used to reinforce the soil, partition the mutual flow of water inside and outside the foundation pit, and generally apply cement mixing pile or compaction grouting technology. Part 3 is the supporting part. Different from the common retaining pile, the underground continuous wall is strengthened and maintained in other forms, and is generally used as a large deep foundation pit.

What is the cause of the failure of the water-stop curtain?

       The reasons for the failure of water stop curtain need to be analyzed after the observation of factors such as foundation pit and stirring pile. The leakage of the curtain of the foundation pit enclosure is complicated, so the leakage type must be divided and treated separately. Through multiple pit construction practice, the author believes that according to the different depth of leakage, it can be divided into two types: leakage above the excavation surface and excavation surface, according to the different materials, reinforced concrete gap leakage and cement soil gap leakage.

       At the same time, the reason for the failure of the water stop curtain may be loopholes in the treatment of the cold interface with the construction suspended water curtain, and the partial unsealing occurs. It is also possible that the quality of the mixing pile between the three layers of silt leads to leakage, because the construction period is tight and the task is heavy. After the completion of the construction, the precipitation speed of the foundation pit is too fast and the precipitation is too large, leading to the water flow in the soil layer, the dilution and handling of the cement slurry that has not yet solidified, so that the quality of the stirring pile can not achieve the expected effect. The last possibility is that the water stop curtain stirring pile is improperly handled in the design, and the large area excavation is immediately carried out after the completion of the water stop curtain construction, resulting in the displacement of the supporting structure, causing damage to the stirring pile and affecting the quality of the stirring pile.

1、The stagnant water layer at the bottom of the foundation pit with large water pressure

       Water curtain depth in the stagnant layer but not completely closed this layer, foundation pit excavation is foundation unloading, the pressure in the soil, the bottom of the hole hole, tool column pile, supporting reinforced concrete pile and other weak parts are likely to produce pipe gushing and sand, improper handling will cause pit uplift, and even endanger the safety of the surrounding underground pipelines and buildings.

2、The destruction of water stop curtain by non-underground diving water source

       Underground pipe leakage, distance near the river, excessive rainfall cause soil water content, soil particles suspended flow, make the envelope by the active soil pressure, the retaining structure deformation caused by large water curtain fracture, leakage, if the peripheral water is too large, and may cause foundation pit engineering accident.

3、Water seepage occurs in the peripheral foundation pit

       There is no stagnant water layer with large water pressure in the project, but with the leakage of the underground pipe in the foundation pit and the surrounding soil with high water content and the high groundwater level in the project, water leakage from the top to the foundation pit has formed. However, in the early stage, the leakage is clear water, forming no large hole underground, with no great influence.

4、Other causes of damage

       With the difference of foundation pit excavation depth, the retaining pressure has changed, some undiscovered deep mixing cement reinforced soil into wall quality problems gradually reflected, such as a wall between adjacent groups not bite or occlusion on small amount, slurry joint lax or cohesion is not enough, uneven mixing with debris interlayer or clamp, cement reinforced soil by geological soil activity and the influence of special geological consolidation or no consolidation factors, can cause water stop curtain leakage. Generally, this kind of leakage flow is small but the sediment is more. Due to the space-time effect, the local soil outside the foundation pit forms quicksand, and the soil outside the foundation pit forms a large hole, which endangers the safety of the foundation pit and the surrounding underground pipelines and buildings.

Countermeasures for failure and leakage treatment

（1）The first pit outside curtain grouting plugging construction technology, first to the overall curtain, set before the water curtain of the outside, the biaxial deep mixing pile about 20cm deep location of the first row of grouting hole, the 2 row grouting hole position set with the first row grouting hole is 0.5m position, make the two rows of grouting holes in plum shape. In the distribution link, grouting should be carried out on the first row grouting hole to ensure that the grouting pipe remains vertical when pressed in, and the grouting should be compacted, the pressure depth should be controlled at 6~8m, and the fourth layer clay is about 0.5m. The cement slurry, cement and water glass are stirred in a ratio of 1 ∶ 1, and the grouting pressure is controlled at 1MPa.

（2）Step-down treatment of the pipe well outside the pit. Due to the influence of the surrounding natural environment during the excavation, the water body may go deep into the soil layer, resulting in a lot of water in the soil. When the earthwork is excavated to 7m, there is a large gap between the water head in the pit and outside the pit, so the step-down well should be set up in the pit to reduce the pressure difference, reduce the water seepage in the pit, and lay a good foundation for the grouting and plugging work outside the pit. At the same time, the surrounding environment of the curtain should be observed at any time, pay attention to the changes in humidity, conduct intermittent pumping, do less pumping and more pumping, and the pumping time should be as short and efficient as possible. Generally, the step-down well is placed in the house on the side of the foundation pit and the west side of the house. The depth of the well is greater than the depth of the grouting pipe, generally 10~12m, and the distance between the two Wells is about 20m. Mung bean sand is used as the filter material between the well pipe and the well hole.

（3）Spray plugging of the net outside the pit. Ensure that the work of net grouting in the pit is carried out at the same time as the grouting and pressure reduction outside the pit. In the construction of soil-hanging steel mesh between piles in the pit, the steel bar with a diameter of 8mm should be taken as the main construction material, the distance between the steel bars should be controlled at 15cm, and the positive and negative difference is 5mm, so that the steel bar and the pile are connected reliably. The main material of shotcrete is C20, the thickness is controlled at 6cm, and the positive and negative difference value should be less than 5mm. During the spraying process, 3cm thick C20 concrete is sprayed first, and after the first injection layer is solidified and cured, the second layer, that is, the remaining 3cm is sprayed, and the curing work is carried out after solidification.

（4）Conduct the plugging of the dense concrete in the pit. Requirements the base elevation in 6.8~7m underground, the main characteristics of the foundation layer, the most obvious feature of the kind of soil layer is that the permeability is too large, so after the earthwork excavation, on the premise of not affect the subsequent engineering, in the base and pile transition site application of dense concrete pouring a concrete barrier wall, block moisture, water into, effectively complete the pit seepage plugging, ensure the quality of the curtain.

（5）Prevention and control of residential building settlement. Grouting on the ground of the corresponding side of the residential building where the settlement occurs, to effectively reinforce the soil, which is also the key to ensure the leakage prevention of the water stop curtain. At the same time, a grouting hole should be added in the courtyard of each resident. The grouting depth should be 6m, the positive and negative difference value should be 0.5m, and the grouting pressure should be 0.5MPa. In the grouting link, the grouting pipe should be removed in turn. When the grouting hole at one end of the grouting pipe is early in the filling range, the grouting pressure should be reduced appropriately, and the ground conditions should be monitored synchronously. When the grouting situation occurs, the observer should immediately order the stop of the grouting. After the grouting is completed, settlement detection points or detection modules and equipment should be installed, and necessary testing should be carried out in the grouting link and after the grouting.

（6）Summarize the overall construction process. First of all, the reinforcement treatment of compaction grouting is carried out outside the pit. The reinforcement order is to reinforce the first row first. After the reinforcement and repair of the first row, the grouting, reinforcement and maintenance of the second row grouting hole are carried out. After that, while the grouting outside the pit, the pit net grouting. If the leakage phenomenon is found during this period, it is necessary to find out the leakage location in time, and use a cement bag to block the leakage point, and conduct the reinforcement work. Finally, the dense concrete should be poured at the connection between the pile body and the foundation bottom to form a concrete retaining wall and prevent leakage.

（7）The project effect should be evaluated after the whole work is completed. The first is the detection and evaluation of the underground water table. The detection method is to detect the water level outside the foundation pit by detecting the water level observation hole set on the settlement side of the foundation pit. The test result is that from the beginning of earthwork excavation to the end of earthwork excavation project, there is no interval of water seepage inside the foundation pit. The water level outside the protected foundation pit is always stable, and it will suddenly fluctuate within a controllable range in the middle, due to natural precipitation. The monitoring results show that the treatment of the water stop curtain plays its due role and ensuring that the penetration in the foundation pit will not affect the water level and settlement outside the foundation pit. After that, the house settlement phenomenon is monitored. The monitoring method is to set up 4 monitoring points or 4 additional monitoring equipment in each house in the foundation pit houses on the settlement side, and the total amount of equipment is 4 times the total amount of the houses. The setting system gives a warning at 2cm to record the settlement value. From the record analysis, the total settlement of the house is controlled within 2cm from the construction to the end of the project, indicating that the settlement of the house during the construction is small, the change is gentle and gradually stable.

Key points of construction management

The following conclusions are drawn according to the engineering practice records and analysis.

（1）The construction quality of water curtain is affected by many factors, so it is necessary to strengthen the quality control in the construction, and timely maintain the defective parts to ensure the effectiveness of water curtain.

（2）The foundation pit precipitation should be carried out after the curtain is closed and reaches a certain intensity to avoid affecting the quality of the mixing pile.

（3）In the case of failure of water stop curtain, the causes should be analyzed according to the site situation to ensure the safety of water stop curtain.

——Source: Piling World

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