A method for calculating the inflow of water into a confined aquifer foundation pit considering a water-stop curtain.

Abstract

This invention discloses a method for calculating the inflow volume of a confined aquifer foundation pit considering a cutoff wall. The method includes: considering the effect of a suspended cutoff wall, dividing the seepage field near the cutoff wall into a flow around the wall and a non-flow around the wall, and introducing a seepage path length influence coefficient for the flow around the wall; obtaining a calculation formula for the inflow volume of the confined aquifer foundation pit based on the seepage path length influence coefficient according to Darcy's law; obtaining the foundation pit inflow volume through numerical simulation, and obtaining the relationship between the seepage path length influence coefficient and the cutoff wall insertion ratio; establishing a fitting formula between the seepage path length influence coefficient and the cutoff wall insertion ratio, and substituting the relevant values ​​into the calculation formula for the inflow volume of the confined aquifer foundation pit to obtain the inflow volume of the confined aquifer foundation pit considering the cutoff wall. This invention overcomes the problem that conventional calculation methods do not consider the cutoff wall, introduces the seepage path length influence coefficient and its determination method, and provides accurate results for the calculated inflow volume of the confined aquifer foundation pit, providing a theoretical basis for similar projects.

Description

Technical Field

[0001] This invention relates to the field of foundation pit engineering technology in civil engineering, and in particular to a method for calculating the inflow of water in a confined aquifer foundation pit considering a water-stop curtain. Background Technology

[0002] In recent years, with the increasing utilization of urban underground space, the number of deep foundation pits has increased dramatically, with many designed depths exceeding 30 meters. These deep foundation pits often have confined aquifers at their bottom, making them highly susceptible to problems such as sudden surges of confined water, structural uplift, and ground subsidence during construction. Therefore, the design requirements for water-stopping and dewatering in deep foundation pits are becoming increasingly stringent. The "wall-well" system, combining water-stop curtains with pressure-reducing wells, is one of the commonly used dewatering measures. Water-stop curtains act as a water barrier, and by designing a reasonable total pumping capacity for the pressure-reducing well group, good dewatering results can be achieved with safe and simple construction.

[0003] The current "Technical Specification for Foundation Pit Support" (JGJ120-2012) provides formulas for calculating the inflow of water in intact and incomplete wells under general conditions in both unconfined and confined water. However, these recommended formulas do not consider the influence of the cutoff wall on the seepage field of the foundation pit. In actual engineering, the cutoff wall has a significant impact on the seepage field of the foundation pit: on the one hand, groundwater outside the foundation pit bypasses the bottom of the cutoff wall and enters the foundation pit, which prolongs the groundwater seepage path and reduces the hydraulic gradient difference between the inside and outside of the foundation pit; on the other hand, the cutoff wall inserts into the aquifer, which reduces the seepage cross-sectional area. Therefore, the inflow of water in the foundation pit using the "wall-well" system will be smaller than the result calculated according to the standard formula. If the design calculation is performed according to the standard, the final foundation pit dewatering construction scheme will be unreasonable. Currently, many scholars have studied the inflow of water in deep foundation pits considering the cutoff wall, but existing numerical calculation and simulation methods for suspended curtain foundation pits have problems such as low accuracy, overly complex calculation formulas, and weak practicality. Therefore, a method for calculating the inflow of water into a confined aquifer foundation pit that takes into account the water-stopping curtain is needed to provide a theoretical basis for calculating the inflow of water into such foundation pits and for arranging water-stopping and dewatering facilities. Summary of the Invention

[0004] The main objective of this invention is to overcome the aforementioned deficiencies in the prior art and propose a method for calculating the inflow of water into a confined aquifer foundation pit that considers a water-stop curtain. This method takes into account the influence of the bypass zone formed by the suspended water-stop curtain on the hydraulic gradient and seepage cross-section of the groundwater in the foundation pit. It overcomes the situation where conventional calculation methods do not consider the water-stop curtain. The modified well method based on bypass and non-bypass zones theoretically conforms to the seepage principle. By introducing the influence coefficient of seepage path length and its determination method, the calculated inflow of water into the confined aquifer foundation pit is accurate and can provide a theoretical basis for similar projects.

[0005] The present invention adopts the following technical solution:

[0006] A method for calculating the inflow of water into a confined aquifer foundation pit considering a water-stop curtain includes:

[0007] Considering the impact of the suspended water-stop curtain, the seepage field near the water-stop curtain is divided into a flow around the flow zone and a non-flow around the flow zone, and the influence coefficient of the seepage path length in the flow around the flow zone is introduced.

[0008] Based on Darcy's law, a formula for calculating the inflow of water into a confined aquifer foundation pit is obtained, which is based on the influence coefficient of seepage path length.

[0009] The water inflow of the foundation pit was obtained through numerical simulation, and the relationship between the influence coefficient of the seepage path length and the insertion ratio of the water-stop curtain was obtained. Among them, the water-stop curtain insertion ratio is equal to the ratio of the depth of the suspended water-stop curtain inserted into the confined aquifer to the thickness of the confined aquifer.

[0010] A fitting formula for the influence coefficient of seepage path length and the insertion ratio of the water-stop curtain is established. The relevant values ​​are substituted into the formula for calculating the inflow of water into the foundation pit of the confined aquifer, and the inflow of water into the foundation pit of the confined aquifer considering the water-stop curtain is obtained.

[0011] Preferably, the zoning of the seepage field is based on the blocking effect of the water-stop curtain on seepage; there are seepage areas in both vertical and horizontal directions around the water-stop curtain, which are called the flow bypass area; there are seepage areas in only the horizontal direction that are not affected by the water-stop curtain, which are called the non-flow bypass area.

[0012] The preferred formula for calculating the inflow of water into a confined aquifer foundation pit based on the influence coefficient of seepage path length is as follows:

[0013]

[0014] Where Q is the inflow rate (m³) of the confined aquifer foundation pit considering the water-stop curtain. 3 / s); K is the permeability coefficient of the confined aquifer (m / s); s0 is the drawdown of the design groundwater level in the pit (m); r0 is the radius of the circular pit (m); M is the thickness of the confined aquifer (m); M r 1. Distance from the bottom of the stop curtain to the impermeable base plate (m); R is the radius of influence of the pit dewatering (m); D is the thickness of the stop curtain (m); α is the influence coefficient of the seepage path length in the flow around the flow zone; L is the depth of the suspended stop curtain inserted into the confined aquifer (m); π is pi.

[0015] Preferably, the water inflow rate of the foundation pit is obtained through numerical simulation calculation, and the relationship between the seepage path length influence coefficient and the cutoff wall insertion ratio is obtained, specifically including:

[0016] Establish a numerical model for calculating the seepage flow in the foundation pit; among which, the top and bottom plates of the confined aquifer are impermeable boundaries, the inner and outer sides of the cut-off wall are impermeable boundaries, the outer boundary of the model is a constant head boundary with the initial water head, and a single complete pumping well is set at the center of the foundation pit for dewatering;

[0017] Based on the designed permeability coefficient, aquifer thickness and the groundwater drawdown designed for the foundation pit, simulate and calculate the variation relationship between the pumping volume of the foundation pit and the insertion ratio of the cut-off wall through the established numerical model;

[0018] Obtain the relationship between the influence coefficient of seepage path length and the insertion ratio of the cut-off wall through the formula for calculating the water inrush volume of the foundation pit in the confined aquifer based on the influence coefficient of seepage path length.

[0019] Preferably, the fitting formula between the influence coefficient of seepage path length and the insertion ratio of the cut-off wall is as follows:

[0020] When 0.1 ≤ L / M ≤ 0.6:

[0021]

[0022] When 0.6 < L / M ≤ 0.9:

[0023]

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] The present invention provides a method for calculating the water inrush volume of a foundation pit in a confined aquifer considering a cut-off wall, establishes a formula for calculating the water inrush volume of a foundation pit in a confined aquifer based on the influence coefficient of seepage path length, the influence coefficient of seepage path length varies with the insertion depth ratio of the cut-off wall, and a relational expression between the influence coefficient of seepage path length and the insertion depth ratio of the cut-off wall is obtained by fitting, which can make the calculation of the water inrush volume of the foundation pit in the confined aquifer simple, convenient and more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flowchart of the method for calculating the water inrush volume of a foundation pit in a confined aquifer considering a cut-off wall according to an embodiment of the present invention;

[0027] Figure 2 It is a calculation model of the confined aquifer with a suspended cut-off wall according to an embodiment of the present invention;

[0028] Figure 3 It is a finite element numerical simulation model of the seepage flow in the foundation pit according to an embodiment of the present invention;

[0029] Figure 4 It is a relationship diagram of the water inrush volume Q of the foundation pit and the insertion depth ratio L / M of the cut-off wall according to an embodiment of the present invention;

[0030] Figure 5This is a graph showing the relationship between the infiltration path length influence coefficient α and the curtain insertion depth ratio L / M in an embodiment of the present invention. Detailed Implementation

[0031] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0032] Example 1

[0033] See Figure 1 As shown, a method for calculating the inflow of water into a confined aquifer foundation pit considering a water-stop curtain includes:

[0034] S101, considering the influence of the suspended water-stop curtain, the seepage field near the water-stop curtain is divided into the flow around the flow zone and the non-flow around the flow zone, and the influence coefficient of the seepage path length in the flow around the flow zone is introduced.

[0035] S102, Based on Darcy's law, the formula for calculating the inflow of water into a confined aquifer foundation pit based on the influence coefficient of seepage path length is obtained;

[0036] S103, the water inflow of the foundation pit is obtained through numerical simulation calculation, and the relationship between the seepage path length influence coefficient and the water-stopping curtain insertion ratio is obtained; whereby, the water-stopping curtain insertion ratio is equal to the ratio of the depth of the suspended water-stopping curtain inserted into the confined aquifer to the thickness of the confined aquifer.

[0037] S104. Establish a fitting formula for the influence coefficient of seepage path length and the insertion ratio of the water-stop curtain. Substitute the relevant values ​​into the calculation formula for the inflow of water into the foundation pit of the confined aquifer to obtain the inflow of water into the foundation pit of the confined aquifer considering the water-stop curtain.

[0038] In this embodiment, before S101, the method further includes: studying the seepage field of foundation pit dewatering under the influence of suspended water-stop curtain based on the large well method, adding the boundary conditions of suspended water-stop curtain, and establishing a calculation and analysis model.

[0039] Specific calculation models such as Figure 2 As shown, where H is the height of the confined aquifer outside the foundation pit (m); h1 is the groundwater head value at the maximum drawdown depth outside the cutoff wall after dewatering (m); h2 is the design value of the groundwater head inside the foundation pit (m); L is the depth of the suspended cutoff wall inserted into the confined aquifer (m); r0 is the radius of the circular foundation pit (m); R is the radius of influence of the foundation pit dewatering (m); M r M is the distance from the bottom of the water-stop curtain to the waterproof base plate (m); M is the thickness of the confined aquifer (m); D is the thickness of the water-stop curtain (m).

[0040] The basic assumptions of the model include: (1) the precipitation process is a three-dimensional steady flow; (2) the water supply of the overlying unconfined aquifer is not considered; (3) the confined aquifer is homogeneous, isotropic and of equal thickness; (4) the pumping point is located at the top of the confined aquifer, the well group in the pit is considered for precipitation, and the confined head line in the pit is simplified to a horizontal line; (5) the foundation pit is equivalent to a large-diameter circular confined aquifer incomplete well.

[0041] Furthermore, S101 is specifically as follows.

[0042] Due to the presence of the water-stop curtain, seepage occurs in both vertical and horizontal directions around it; this area is called the flow-around zone. As the impermeable layer approaches the bottom, the blocking effect of the water-stop curtain gradually decreases until it becomes negligible. The area unaffected by the water-stop curtain experiences only horizontal seepage; this area is called the non-flow-around zone.

[0043] Most theoretical analytical solutions, when calculating the seepage path in this region, typically take the average of the longest and shortest seepage paths, i.e., (l max +l min ) / 2, but the streamlines in the flow region are not linearly distributed, so directly using (l max +l min The calculated value of ) / 2 differs significantly from the actual situation. To make the calculation of water inflow in the foundation pit more accurate, this embodiment introduces the influence coefficient α of the seepage path length in the flow around the pit. This influence coefficient α varies with the insertion depth ratio of the water-stop curtain L / M, which can reflect the uneven distribution of streamlines in the flow around the pit.

[0044] Therefore, the length of the equivalent seepage path in the flow around the flow zone can be expressed as:

[0045]

[0046] (1) In the formula, α is related to L / M, which can be expressed as:

[0047]

[0048] Furthermore, S102 is specifically as follows.

[0049] The average hydraulic gradient of groundwater entering the foundation pit from outside the cutoff wall is as follows:

[0050]

[0051] According to Darcy's law, the groundwater runoff at the bottom of the cutoff wall can be obtained, which is the total inflow of water into the foundation pit:

[0052]

[0053] Combining formulas (3) and (4), we can obtain:

[0054]

[0055] It is assumed that vertical seepage around the foundation pit occurs only at locations infinitely close to the water-stop curtain outside the foundation pit, and the flow in other areas is still horizontal. Therefore, the water flow outside the foundation pit can be calculated using an unconfined well in a confined aquifer without considering the water-stop curtain, that is, calculated using the recommended formula in the Technical Code for Building Foundation Pit Support JGJ120 - 2012. The drawdown s of the groundwater level outside the foundation pit can be obtained as follows:

[0056]

[0057] The designed drawdown of the groundwater level inside the foundation pit is s0. Then:

[0058] s0 = H - h2 = H - h1 + h1 - h2 = s + h1 - h2 (7)

[0059] Combining formulas (5), (6), and (7), we can obtain:

[0060]

[0061] Furthermore, the S103 is as follows specifically.

[0062] Refer to Figure 3 as shown. A numerical model for calculating the seepage in the foundation pit is established. The top and bottom plates of the confined aquifer are impermeable boundaries, the inner and outer sides of the water-stop curtain are impermeable boundaries, the outer boundary of the model is a constant head boundary with the initial water head, and a single fully penetrating pumping well is set at the center of the foundation pit for dewatering. The selected hydraulic conductivity K h = K v = 10 m / d, the thickness of the aquifer M = 30 m, and the designed drawdown of the groundwater level in the foundation pit s0 = 15 m.

[0063] Since the influence coefficient α of the seepage path length in the seepage around area is related to the ratio L / M of the insertion depth of the water-stop curtain, the Midas finite element software is used to calculate the water inrush volume of the foundation pit. The change of the pumping volume Q of the foundation pit with L / M is simulated through the numerical model. The results are shown in Figure 4 as shown. On the premise that the water inrush volume Q, L, and M of the foundation pit are known, the influence coefficient α of the seepage path length can be calculated through formula (8), and then the variation law of α with L / M can be obtained. The specific relationship change is shown in Figure 5 as shown.

[0064] Furthermore, the S104 is as follows specifically.

[0065] When 0.1 ≤ L / M ≤ 0.6, as L / M increases, α approximately linearly increases; when 0.6 < L / M ≤ 0.9, α approximately exponentially increases. According to the variation law of α in the figure, the functional relationship between α and L / M can be simulated as follows:

[0066] (1) When 0.1 ≤ L / M ≤ 0.6:

[0067]

[0068] (2) When 0.6 < L / M ≤ 0.9:

[0069]

[0070] Substitute formula (9) or (10) into formula (8). Under the condition that each parameter (K, s0, r0, M, M r , R, D, α, L, and π) on the right side of formula (8) is known, the water inflow of the foundation pit in the confined aquifer considering the cut-off curtain can be obtained.

[0071] Example 2

[0072] This example further elaborates on the present invention in combination with two foundation pit projects.

[0073] Project A is a certain subway ventilation shaft project. The outsourcing length of the project is 49.0 m, the width is 22.5 m, and the foundation pit excavation depth is 28.15 m. The diaphragm wall is adopted as the retaining structure for this project. The width of the diaphragm wall is 1.2 m and the depth is 48.60 m. The strata of this project mainly include the following seven layers. The first layer is miscellaneous fill, with a thickness of about 1.82 m; the second and third layers are gray sandy silt, with a thickness of about 12.30 m; the fourth layer is gray silty clay, with a thickness of about 6.00 m; the fifth - 1 layer is gray clay, with a thickness of about 4.00 m; the sixth layer is dark green - straw yellow silty clay, with a thickness of about 3.20 m; the seventh layer is straw yellow sandy silt, with a thickness of about 4.70 m. Below the seventh layer is straw yellow - gray fine sand, with a thickness of more than 33.00 m, and occasionally there are thin layers of clay. The shallow groundwater in the project area belongs to the phreatic water type and is mainly stored in the second, third, and fourth layers. The confined aquifer related to the project is the seventh layer of sandy (silty) soil. This project includes the sixth layer of hard clay layer, which has a good water - blocking effect.

[0074] Project B is a foundation pit project for a subway station. The project site is bordered by a river to the east and a high-rise residential area to the south. The excavation depth is approximately 39.80 meters, with a large excavation area, forming an arc shape with a radius of 350 meters, extending 236 meters, and covering an area of ​​approximately 2,290.7 square meters. To ensure the stability of the foundation pit, a diaphragm wall is used for the long double tunnel. The diaphragm wall is 65.00 meters deep and 1.2 meters thick. The width of the foundation pit is 19.0-23.0 meters. This project presents the following challenges: the foundation pit is located above a weak unconfined aquifer, with a groundwater level between 0.5 and 1.2 meters, and the water volume is small, making drainage easy; however, there are high-head confined aquifers at the bottom of the foundation pit, including the Upper Pleistocene 1st and 2nd confined aquifers and the Middle Pleistocene 3rd confined aquifer. These three confined aquifers are interconnected, forming a composite confined aquifer with a top elevation of -28.45 meters, a bottom elevation of -146.65 meters, and a thickness of approximately 118.2 meters. The static water level is approximately -5.00 meters, which is extremely detrimental to the stability of the foundation pit's bottom slab. Therefore, the impact of the high-head confined aquifer on the foundation pit needs to be considered in the engineering project, and corresponding measures must be taken to ensure the stability of the foundation pit.

[0075] Based on the engineering geological and hydrological conditions, the simplified model of the foundation pit is shown in Table 1.

[0076] Table 1 Soil layer distribution and cutoff wall depth

[0077]

[0078]

[0079] The calculation parameters for the two projects are shown in Table 2. The inflow rate of the confined aquifer foundation pit, considering the water-stop curtain, was calculated using the method proposed in this invention. The theoretical formula was used to calculate the inflow rate Q1. The analytical solution of the inflow rate was compared with the measured value, and the results are shown in Table 2. Where Q2 is the actual pumping volume on site, and Δ is the deviation between the calculated pumping volume Q1 and the actual pumping volume Q2, Δ=(Q1-Q2) / Q2×100%.

[0080] Table 2 Calculation parameters for water inflow in confined aquifer foundation pit considering water-stop curtain.

[0081]

[0082] Analysis of the table data reveals that the theoretical formula for calculating the water inflow in the foundation pit is largely consistent with the actual field measurements, with an error within 5.0%. This indicates that the theoretical formula for calculating the water inflow in the foundation pit is highly accurate, and this method can be applied to similar foundation pit projects.

[0083] The above are merely specific embodiments of the present invention, but the design concept of the present invention is not limited thereto. Any non-substantial modifications made to the present invention using this concept shall be considered as infringing upon the protection scope of the present invention.

Claims

1. A method for calculating the inflow of a pressure-bearing aquifer foundation pit considering a water stop curtain, characterized in that, Including: Considering the influence of the cut-off effect of the suspended cut-off curtain, the seepage field near the cut-off curtain is divided into a bypass flow area and a non-bypass flow area, and the influence coefficient of the seepage path length in the bypass flow area is introduced; According to Darcy's law, the calculation formula for the water inflow of the foundation pit in the confined aquifer based on the influence coefficient of the seepage path length is obtained; The water inflow of the foundation pit is obtained through numerical simulation calculations, and the relationship between the influence coefficient of the seepage path length and the insertion ratio of the cut-off curtain is obtained; where the insertion ratio of the cut-off curtain is equal to the ratio of the depth of the suspended cut-off curtain inserted into the confined aquifer to the thickness of the confined aquifer; A fitting formula for the influence coefficient of the seepage path length and the insertion ratio of the cut-off curtain is established, and the relevant values are substituted into the calculation formula for the water inflow of the foundation pit in the confined aquifer to obtain the water inflow of the foundation pit in the confined aquifer considering the cut-off curtain; The calculation formula for the water inflow of the foundation pit in the confined aquifer based on the influence coefficient of the seepage path length is as follows: ； Where Q is the inflow rate (m³) of the confined aquifer foundation pit considering the water-stop curtain. 3 / s); K is the permeability coefficient of the confined aquifer (m / s); s0 is the drawdown of the design groundwater level in the pit (m); r0 is the radius of the circular pit (m); M is the thickness of the confined aquifer (m); M r 1. Distance from the bottom of the stop curtain to the impermeable base plate (m); R is the radius of influence of the pit dewatering (m); D is the thickness of the stop curtain (m); α is the influence coefficient of the seepage path length in the flow around the flow zone; L is the depth of the suspended stop curtain inserted into the confined aquifer (m); π is pi.

2. The method for calculating the inflow of a pressure-bearing aquifer foundation pit considering a water-stop curtain according to claim 1, characterized in that, The zoning of the seepage field is based on the cut-off effect of the cut-off curtain on seepage; there are vertical and horizontal seepage areas around the cut-off curtain, which is the bypass flow area; the seepage area that is only horizontally oriented without being affected by the cut-off curtain is the non-bypass flow area.

3. The method for calculating the inflow of a pressure-bearing aquifer foundation pit considering a water-stop curtain according to claim 1, characterized in that, The water inflow of the foundation pit is obtained through numerical simulation calculations, and the relationship between the influence coefficient of the seepage path length and the insertion ratio of the cut-off curtain is obtained, specifically including: A numerical model for calculating the seepage of the foundation pit is established; where the top and bottom plates of the confined aquifer are impermeable boundaries, the inner and outer sides of the curtain are impermeable boundaries, the outer boundary of the model is a constant head boundary with the initial water head, and a single complete pumping well is set at the center of the foundation pit for dewatering; Based on the designed permeability coefficient, aquifer thickness, and the designed groundwater drawdown of the foundation pit, the relationship between the pumping volume of the foundation pit and the insertion ratio of the cut-off curtain is simulated and calculated through the established numerical model; Through the calculation formula for the water inflow of the foundation pit in the confined aquifer based on the influence coefficient of the seepage path length, the relationship between the influence coefficient of the seepage path length and the insertion ratio of the cut-off curtain is obtained.

4. The method for calculating the inflow of a pressure-bearing aquifer pit considering a water-stop curtain according to claim 1, characterized in that, The fitting formula for the influence coefficient of the seepage path length and the insertion ratio of the cut-off curtain is as follows: When 0.1 ≤ L / M ≤ 0.6: ； When 0.6 < L / M ≤ 0.9:

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