The safety and stability of the water-stop curtain have a significant impact on surrounding safety. In actual construction, loopholes may occur due to various factors such as scheme design, geological survey, quality control, etc., causing the water-stop curtain to fail, delaying the construction period, or causing safety accidents. Therefore, it is of great significance to study the causes and treatment methods of water-stop curtain failure.
Requirements for Deep Foundation Pits and Water-Stop Curtains
1. Deep foundation pit
A deep foundation pit refers to an excavation project with an excavation depth of 5m or more, or an excavation project with 3 or more basement levels. At the same time, when the geological conditions and surrounding environment are complex, the excavation project can also be called a deep foundation pit. Relevant documents in my country define earth excavation, support, and dewatering projects for foundation pits with an excavation depth exceeding 5m (including 5m); excavation depths not exceeding 5m but the geological conditions, surrounding environment, and underground pipelines are complex or affect surrounding buildings. Safe earth excavation, support, and dewatering projects for foundation pits.
Relevant regulations require that the risk factor of deep foundation pit work is relatively high, and its special plan must be organized by the construction unit to organize expert group members, project leaders, project supervision unit personnel, construction party safety leaders, technical leaders, etc., as well as survey and design units Technical personnel and safety personnel jointly conduct safety demonstrations.
2. Water-stop curtain
Water-stop curtain is the general term for water-stop projects in excavation projects. The significance of its project is to prevent or reduce the penetration of groundwater in the foundation pit to ensure construction safety and prevent settlement of the house after it is put into use.
The water-stop curtain generally consists of three parts. The first part is the retaining pile. Its main function is similar to that of the retaining wall. It has reinforced concrete cast-in-place piles or other forms. There is a certain gap between the piles. The second part is the real water-stop curtain part, which is mainly used to reinforce the soil and block the mutual flow of water inside and outside the foundation pit. Generally, cement mixing piles or compaction grouting technology are used. Part 3 is the support part. Different from common retaining piles, underground continuous walls should be reinforced and maintained in other ways, and are generally used as extra large and deep foundation pits.
What Is the Reason for the Failure of Water-Stop Curtain?
The reasons for the failure of the water-stop curtain need to be analyzed through observation of factors such as foundation pits and pile stirring. The leakage situation of the foundation pit enclosure water-stop curtain is complicated, and the leakage types must be divided and treated separately. Through the construction practice of multiple foundation pits, the author believes that depending on the location of the leakage depth, it can be divided into two situations: leakage above the excavation surface of the foundation pit and leakage below the excavation surface of the foundation pit; depending on the materials used, there are two situations: It can be divided into two types: 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 that there are loopholes in the cold interface with the water-stop curtain during construction, resulting in local unsealing. It is also possible that the quality of the mixing piles between the three layers of silt soil was not up to standard, causing leakage. Due to the tight construction schedule and heavy tasks, after the completion of the construction, the precipitation rate in the foundation pit was too fast and the amount of precipitation was too large, causing water in the soil to flow. , the cement slurry that has not yet solidified will be diluted and transported, so that the quality of pile stirring cannot achieve the expected effect. The last possibility is that the water-stop curtain pile mixing was improperly handled during the design, and large-area excavation work was carried out immediately after the water-stop curtain construction was completed, resulting in the displacement of the supporting structure, causing damage to the mixing pile, and affecting the quality of the mixing pile.
1. There is a stagnant layer with large water pressure at the bottom of the foundation pit.
The depth of the water-stop curtain is located in the stagnant layer but does not completely seal this layer. After excavation of the foundation pit, it is equal to the unloading of the foundation, and the pressure in the soil is reduced. The large well at the bottom of the pit, tool column piles, supporting reinforced concrete cast-in-place piles, etc. Weak parts may produce piping and quicksand. Improper handling may cause the bottom of the pit to bulge, and even endanger the safety of surrounding underground pipelines and buildings.
2. Damage to the water-stop curtain caused by non-underground diving water sources
Leaking underground pipelines, proximity to rivers, and excessive rainfall result in excessive water content in the soil, causing the soil particles to flow in suspension, which increases the active earth pressure on the enclosure structure, resulting in excessive deformation of the enclosure structure. The water-stop curtain breaks, resulting in leakage. If the amount of peripheral water is too large, it may also cause a foundation pit engineering accident.
3. Water seepage from peripheral foundation pits
According to the foundation survey report, there is no aquifer with large water pressure in the underground of this project. However, with the excavation of the foundation pit, the leakage of underground pipes and the surrounding soil have large water content and high groundwater levels, resulting in the leakage from the peripheral foundation pit. Water seeped from the top into the foundation pit, forming an open leak. At the same time, due to the unsatisfactory quality of the cement mixing piles, leakage occurred from the middle part of the foundation pit, and the local leakage was serious, forming a hidden leak; but through observation, the initial leakage carried a lot of sediment, and the later leakage was clean water, without forming a large cavity underground and causing no major impact on the environment.
4. Other causes of damage
With the different excavation depths of foundation pits, the soil-water pressure on the enclosure structure also changes to a certain extent. Some undiscovered quality problems of deep-mixed cement-reinforced soil walls gradually become apparent, such as the phase phase during wall formation. Adjacent groups are not interlocking with each other or the amount of interlocking is small, the joints are not tight or the connection amount is not enough after grouting is broken, the mixing is uneven and debris causes interlayers or blocks, and the cement-reinforced soil is affected by the activity of the soil in the geology and special The influence of geology, poor or no consolidation and other factors can cause leakage of the water-stop curtain. Generally, this kind of leakage flow is small but carries a lot of sediment. Due to the space-time effect, the soil outside the foundation pit forms quicksand. If not treated in time, the soil outside the foundation pit will form a large cavity, endangering the foundation pit and surrounding underground pipelines and pipelines. Building security.
Failure and Leakage Treatment Countermeasures
(1) First use the construction technology of curtain grouting and sealing outside the pit. First, position the overall curtain, and install the first row of grouting outside the previously set water-stop curtain at a position about 20cm deep from the double-shaft deep mixing pile. The second row of grouting holes is set 0.5m away from the first row of grouting holes so that the two rows of grouting holes are arranged in a plum blossom shape. In the process of point layout, grouting should be carried out on the first row of grouting holes to ensure that the grouting pipe remains vertical when pressed in, the grouting should be pressed tightly, and the depth of the grouting should be controlled to 6~8m. 4 layers of clay about 0.5m. Stir cement slurry, cement, and water glass in a ratio of 1:1, and control the grouting pressure at 1MPa.
(2) Carry out pressure reduction treatment for the tube well outside the pit. Due to the influence of the surrounding natural environment during excavation, water may penetrate deep into the soil, causing the soil to contain a large amount of water. When the earthwork is excavated to 7m, there is a large gap between the water heads inside the pit and outside the pit. Therefore, a pressure relief well must be added outside the pit to reduce the pressure difference and reduce the amount of water seepage in the pit, to prepare for the grouting and sealing work outside the pit. Base. At the same time, it is necessary to observe the environment around the curtain at any time, pay attention to humidity changes, and perform intermittent pumping, to pump less and more, and the pumping time should be as short and efficient as possible. Generally, the decompression well is placed in the house on one side of the foundation pit and the west side of the house. The depth of the well should be greater than the depth of the grouting pipe, usually 10~12m. The distance between the two wells is about 20m. Mung bean sand is used as the well. The filter material between the pipe and the well hole.
(3) Carry out shotcrete sealing of the pit external net. Ensure that the net-hung shotcrete sealing work in the pit and the grouting and pressure reduction work outside the pit are carried out at the same time. In the construction of soil-hanging steel mesh between piles in the pit, steel bars with a diameter of 8mm should be used as the main construction material. The distance between steel bars should be controlled at 15cm, with a positive and negative difference of 5mm, so that the steel bars and piles can be reliably connected. The main material of shotcrete is C20, the thickness is controlled at 6cm, and the difference between plus and minus is less than 5mm. During the spraying process, 3cm thick C20 concrete is first sprayed, and after the first layer of the sprayed layer is solidified and cured, the second layer, that is, the remaining 3cm is sprayed, and the curing work is also performed after it solidifies.
(4) Seal the dense concrete in the pit. The elevation of the base is required to be 6.8~7m underground. The main characteristic of the base layer here is the silt layer. The most obvious feature of this type of soil layer is that the permeability is too high. Therefore, it must be done after earth excavation so as not to affect the subsequent As a premise for the project to be carried out, dense concrete is used to pour a concrete retaining wall at the intersection between the base layer and the pile body to prevent moisture and water vapor from entering, effectively completing the penetration and sealing of leakage locations in the pit, and ensuring the quality of the curtain.
(5) Prevent and control the settlement of residential buildings. Grouting is performed on the ground on the corresponding side of the residential building where settlement occurs to effectively reinforce the soil. This is also the key point to ensure that the water-stop curtain prevents leakage. At the same time, grouting holes are added in the courtyards of each resident. The grouting depth is required to be 6m, the positive and negative difference is 0.5m, and the grouting pressure is 0.5MPa. In the grouting process, the grouting pipes are dismantled in sequence. The grouting hole at one end of the grouting pipe is already in the filling range. The grouting pressure is appropriately reduced and the ground conditions are monitored synchronously. When grouting occurs, observers must immediately order the grouting to stop. After the grouting is completed, settlement detection points should be set up or detection modules and equipment should be installed to carry out necessary detection during the grouting process and after the grouting is completed.
(6) Summarize the overall construction process. First, the reinforcement process of compaction grouting is carried out outside the pit. The reinforcement sequence is to reinforce the first row first. After the reinforcement and repair of the first row are completed, the grouting, reinforcement, and maintenance of the second row of grouting holes are carried out. After that, while grouting reinforcement was carried out outside the pit, netting and spray grouting were carried out inside the pit. If leakage is discovered during this period, the leakage location must be found in time, and cement bags should be used to seal the leakage point and carry out reinforcement work. Finally, dense concrete must be poured at the connection between the pile body and the base to form a concrete retaining wall to prevent leakage.
(7) The project effect must be evaluated after the entire work is completed. The first is the detection and assessment of groundwater levels. The detection method is to detect the water level outside the foundation pit through the water level observation hole set on the settlement side of the foundation pit. The test results showed that there was no intermittent water seepage inside the foundation pit from the beginning of the earth excavation to the end of the earth excavation project. The water level outside the protected foundation pit always remains stable, and there will be sudden fluctuations within the controllable range. The reason is natural precipitation. The monitoring results show that the treatment of the water-stop curtain has played its due role in ensuring that the penetration in the foundation pit will not affect the water level and settlement outside the foundation pit. Then monitor the subsidence of the house. The monitoring method is to set up 4 monitoring points or add 4 monitoring equipment to each house in the foundation pit houses on the settlement side. The total amount of equipment is 4 times the total number of houses. Set the system to issue a warning when settlement occurs in the house and record the settlement value at 2cm. From the record analysis, from the construction to the end of the project, the total settlement of the house was controlled within 2cm, indicating that the settlement of the house during the construction period was small, the change was gentle and gradually stabilized.
Construction Management Essentials
The following conclusions are drawn based on engineering practice records and analysis.
(1) The construction quality of water-stop curtains is affected by many factors, so quality monitoring must be strengthened during construction and defective parts must be maintained promptly to ensure the effectiveness of the water-stop curtains.
(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) When the water-stop curtain fails, the cause must be analyzed based on the on-site conditions and dealt with to ensure the safety of the water-stop curtain.
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