Deep foundation pit excavation dewatering and drainage construction method

Abstract

The invention provides a deep foundation pit excavation dewatering and drainage construction method which is characterized in that a deep foundation pit is close to a river, a deep foundation pit geological layer comprises an upper loam layer and a lower fine sand layer, an upper layer berm, a lower layer berm and an excavation base face are arranged in an excavation mode, an upper dewatering pipe well is arranged on the excavation face, a lower dewatering pipe well is arranged on the upper layer berm, and a lower dewatering pipe well is arranged on the lower layer berm. A reverse filter layer is arranged on the slope surface of the fine sand layer, a water collecting well is arranged at the slope toe of the fine sand layer, a pump is matched with a dewatering pipe well and the water collecting well for dewatering, water level gauges are arranged in the water collecting well and a river, and by obtaining the corresponding water level in real time and controlling the pump to operate, combined dewatering and drainage are achieved, the operation energy consumption of the pump is reduced, and the dewatering and drainage construction efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the field of engineering construction, and particularly relates to a construction method for dewatering during deep foundation pit excavation. Background Art

[0002] For civil engineering construction, deep foundation pit excavation is a conventional process for foundation treatment. During deep foundation pit excavation, it is necessary to ensure the stability of the foundation pit slope while dewatering the groundwater to ensure dry construction of the foundation excavation surface. For deep foundation pit excavation, setting up dewatering pipe wells is an effective dewatering construction method. By using dewatering pipe wells in combination with water pumps to pump water, the groundwater level can be effectively controlled to ensure the groundwater level during construction.

[0003] For deep foundation pit excavation, especially when the excavation depth is greater than 10m, it is generally necessary to set up multiple berms and conduct combined dewatering. For fine sand layers, the water contained is confined water and needs to be treated separately. Currently, for combined dewatering under complex geological conditions, there is a lack of reasonable intelligent decision-making. If excessive measures are taken, the construction cost will increase; if insufficient measures are taken, there will be construction risks in foundation pit excavation. Especially when the water level of the outer river changes, it will correspondingly cause changes in the groundwater level. If the dewatering can be reasonably controlled according to the water level of the outer river, there is room for optimized control. Summary of the Invention

[0004] Aiming at the problems of the prior art, the present invention provides a construction method for dewatering during deep foundation pit excavation, which is applicable to deep foundation pits and fine sand layers located at the bottom of the foundation.

[0005] The present invention provides a construction method for dewatering during deep foundation pit excavation. The deep foundation pit is close to a river. The upper part of the geological layer of the deep foundation pit excavation is a loam layer, and the lower part is a fine sand layer. The characteristics are as follows: the excavation is provided with an upper berm, a lower berm and an excavation base surface. The geological layer above the lower berm is a loam layer, and the geological layer between the lower berm and the excavation base surface is a fine sand layer. An upper dewatering pipe well is arranged on the excavation surface, a lower dewatering pipe well is arranged on the upper berm, an anti-filter layer is arranged on the slope surface of the fine sand layer, and a catchment well is arranged at the toe of the fine sand layer slope. The upper dewatering pipe well is connected to a first pump, the lower dewatering pipe well is connected to a second pump, and the catchment well is connected to a third pump. The first pump, the second pump and the third pump respectively pump out the water bodies in the upper dewatering pipe well, the lower dewatering pipe well and the catchment well. A water level gauge is arranged in both the catchment well and the river to respectively obtain the water level Hs1 of the catchment well and the water level Hs2 of the river. The construction method includes the following: S1: When the water level Hs1 of the catchment well ≥ H1m, the first pump, the second pump and the third pump work at powers P1n, P2n and P3n respectively; S2: When the water level Hs1 of the catchment well < H1m, the following judgment is made: S21: When Hs2 < H2min, the second pump operates at power P2m, and the first pump operates at power P1m; S22: When H2min ≤ Hs2 < H2m, the second pump operates at power P2n, and the first pump operates at power P1m; S23: When Hs2 ≥ H2m, the second pump operates at power P2n, and the first pump operates at power P1n; S241: When Hs1 < H1min, the third pump does not operate; S242: When H1min ≤ Hs1 < H1m, the third pump operates at power P3m.

[0006] Preferably, the powers of the first pump, the second pump, and the third pump are all adjustable. P1n, P2n, and P3n are the rated powers of the first pump, the second pump, and the third pump respectively, and P1m, P2m, and P3m are all less than their rated powers.

[0007] Preferably, P1m, P2m, and P3m are respectively 80% of the rated powers of the first pump, the second pump, and the third pump.

[0008] Preferably, the length of the descending water pipe well is less than that of the ascending water pipe well. The ascending water pipe well and the descending water pipe well are both annularly distributed, and the bottom elevations of the ascending water pipe well and the descending water pipe well are the same, both more than 3 m below the excavation base surface.

[0009] Preferably, the sump is arranged at the corner of the excavation base surface and is set to be one. A drainage ditch is arranged at the toe of the slope of the excavation base surface. The drainage ditch is inclined, and the elevation of the drainage ditch at the toe of the slope of the sump is the highest.

[0010] Preferably, a soil retaining ridge is arranged outside the ascending water pipe well on the excavation surface.

[0011] The working principle of the present invention is as follows: For the fine sand layer structure, its water body is confined water. If the groundwater level is too high, the sand in the fine sand layer will precipitate, which will affect the stability of the slope. In order to prevent quicksand piping in the fine sand layer, a filter layer can be set on the slope of the fine sand layer. By using the filter setting, the stability of the slope is protected. At the same time, for the water body precipitated from the filter layer, slope toe open drainage is formed by using the drainage ditch, and then it is pumped out through the sump in cooperation with the pump.

[0012] During the installation of the dewatering pipe wells, the control of the groundwater level is affected by the water level of the external river. If designed according to the highest water level of the external river, it will cause an increase in the length and drainage volume of the dewatering pipe wells. If designed according to the normal water level of the external river, when the water level exceeds the designed water level (the water level is higher than the designed level), there may be insufficient dewatering capacity, resulting in water bodies flowing into the foundation pit. For the fine sand layer, when the water level exceeds the designed level, the water should be drained through the filter layer and drained in a timely manner through the sump to ensure drainage and slope stability under extreme conditions.

[0013] The lengths of the dewatering pipe wells set at different berms are different. During dewatering, the second pump should be preferentially used for operation and drainage. At the same time, dewatering should be closely related to the water level of the external river. When the water level of the external river is low, ensure that both the first pump and the second pump operate at a low power. At this time, the foundation pit is affected by the water level of the external river, and the overall groundwater level is low. When the water level of the external river rises to a certain height, the power of the second pump is preferentially increased, that is, the dewatering pipe wells and the second pump set on the upper berm. When the water level of the external river continues to rise, both the first pump and the second pump drain water at the rated power to meet the maximum dewatering design requirements.

[0014] For the water level of the sump, if the water level of the sump is too low, it means that no water is released from the fine sand layer, and at this time the third pump does not work. If the water level of the sump exceeds a certain level, the third pump starts to work at the power of P3m. If the water level of the sump is too high, at this time, the first pump, the second pump and the third pump all work at the rated power (even if the water level of the external river is not high, the first pump and the second pump should also work at the rated power to lower the groundwater level where the foundation pit is located).

[0015] When the water level of the sump is less than H1m (even if the water level is very low, such as less than H1min), only rely on the water level Hs2 of the external river to judge and adjust the power of the first pump and the second pump accordingly. For details, see steps S21 - S23. Through this adjustment, the matching of the water level of the external river with the power of the first pump and the second pump can be ensured, preventing the pump power from being insufficient when the water level of the external river is too high, resulting in the rise of the groundwater level. Through the judgment of the water level of the external river and the sump, the pump is controlled with segmented power, which can reduce the power consumption of the pump, effectively control the groundwater level, ensure the optimized operation of the pump, and through the corresponding sensing and control equipment, the scientific and intelligent level of dewatering can be improved.

[0016] For the pump, different segmented power controls can also be adopted to combine the water level of the external river and the sump for segmented optimization decision-making to further improve the scientific rationality of the decision-making.

[0017] The advantages of the present invention are as follows: (1) For deep foundation pit excavation, combined dewatering and drainage is adopted. Dewatering pipe wells are set in the loam layer, and an anti-filter layer and toe ditch drainage are set in the fine sand layer to form combined dewatering and drainage. (2) For different forms of dewatering pipe wells, pumping control is carried out by using a pump. The water levels of the sump and the river are collected, and the operation of the pump is optimized by using the water levels to improve the operation efficiency of the pump. (3) Through the optimized control and regulation of the combined dewatering and drainage, on the basis of ensuring the control of the groundwater level, the operation of the pump is optimized to reduce energy consumption waste. Description of the Drawings Figure 1 It is the construction profile view of the present invention; Figure 2 It is the layout schematic diagram of the drainage ditch and sump on the excavation base surface; Figure 3 It is the control flow chart of the construction method.

[0019] Detailed Embodiment: The following is a specific explanation of the content defined by the present invention.

[0020] The present invention provides a dewatering and drainage construction method for deep foundation pit excavation. The deep foundation pit is close to a river. The upper part of the geological layer of the deep foundation pit excavation is a loam layer, and the lower part is a fine sand layer. It is characterized in that: the excavation is provided with an upper berm 1, a lower berm 2 and an excavation base surface 3. The geological layer above the lower berm 2 is a loam layer, and the geological layer between the lower berm 2 and the excavation base surface 3 is a fine sand layer. An upper dewatering pipe well 5 is set on the excavation surface 4, a lower dewatering pipe well 6 is set on the upper berm 1, an anti-filter layer 7 is set on the slope of the fine sand layer, and a sump 8 is set at the toe of the fine sand layer slope. The upper dewatering pipe well 5 is connected to a first pump, the lower dewatering pipe well 6 is connected to a second pump, and the sump 8 is connected to a third pump. The first pump, the second pump and the third pump respectively pump out the water bodies in the upper dewatering pipe well 5, the lower dewatering pipe well 6 and the sump 8. Water level gauges are set in both the sump 8 and the river to respectively obtain the water level Hs1 of the sump 8 and the water level Hs2 of the river. The construction method includes the following: S1: When the water level Hs1 of the sump 8 ≥ H1m, the first pump, the second pump and the third pump respectively work at powers P1n, P2n and P3n; S2: When the water level Hs1 of the sump 8 < H1, the following judgment is made: S21: When Hs2 < H2min, the second pump works at power P2m, and the first pump works at power P1m; S22: When H2min ≤ Hs2 < H2m, the second pump works at power P2n, and the first pump works at power P1m; S23: When Hs2 ≥ H2m, the second pump operates at power P2n, and the first pump operates at power P1n; S241: When Hs1 < H1min, the third pump does not operate; S242: When H1min ≤ Hs1 < H1m, the third pump operates at power P3m.

[0021] Both the upper and lower drain wells 5 and 6 are provided in multiple numbers. Each drain well corresponds to a pump. The sump 8 is provided as one, and the third pump is also provided as one. The multiple first pumps are connected to the control circuit 1, and the multiple second pumps are connected to the control circuit 2. The control circuit 1 and the control circuit 2 can synchronously adjust the power of the first and second pumps, that is, the multiple first and second pumps do not support adjusting the power of one or several of them, but only support synchronous power adjustment. Since the dewatering operation runs through the construction process, therefore, both the first and second pumps need to work continuously. The form of synchronous adjustment is convenient for the layout of the construction site and also convenient for control adjustment.

[0022] The excavation depth of the deep foundation pit is not less than 10m.

[0023] Preferably, the powers of the first pump, the second pump, and the third pump can all be adjusted. P1n, P2n, and P3n are respectively the rated powers of the first pump, the second pump, and the third pump. P1m, P2m, and P3m are all less than their rated powers. The first pump, the second pump, and the third pump are all connected to a frequency converter for frequency adjustment.

[0024] Preferably, P1m and P2m respectively take 80% of the rated powers of the first pump, the second pump, and the third pump. For the settings of P1m and P2m, it can be determined by simulation according to the hydrogeological conditions of the deep foundation pit where it is located. Generally, P1m and P2m should meet the requirement that when the river water level is H2min, this power can control the groundwater level in the area where the foundation is located below the excavation foundation surface by no less than 1.5m.

[0025] Preferably, the length of the lower drain well 6 is less than that of the upper drain well 5. The upper drain well 5 and the lower drain well 6 are both distributed in a ring shape. The bottom elevations of the upper drain well 5 and the lower drain well 6 are the same, and are both more than 33m lower than the excavation base surface.

[0026] Preferably, the sump 8 is arranged at the corner of the excavation base surface 3, and there is only one sump. A drainage ditch 9 is arranged at the toe of the slope of the excavation base surface 3. The drainage ditch 9 is inclined, and the elevation of the drainage ditch 9 at the toe of the slope diagonally opposite to the sump 8 is the highest. This setting form can ensure the timely drainage of the seepage water on the slope between the lower dike platform 2 and the excavation base surface 3. The drainage ditch 9 can be locally reinforced with concrete.

[0027] Preferably, a soil retaining ridge is arranged outside the upper downcomer well 5 on the excavation surface 4. The setting of the soil retaining ridge can protect the deep foundation pit excavation and prevent external water sources from entering the foundation pit. Especially in the rainy season, it can effectively avoid rainfall from flowing into the foundation pit and causing an increase in the difficulty of foundation pit dewatering and drainage.

[0028] Taking a certain pumping station project as an example, this project is adjacent to a certain river, which is a river for water diversion project. Due to its engineering characteristics, its water level often fluctuates, resulting in the fluctuation of the groundwater level during the foundation pit excavation. Through on-site exploration, the buried depth of the groundwater level in the pumping station foundation pit is 2.9 - 3.8m, the maximum excavation depth of the foundation pit excavation is 13.2m, and the water level drawdown is about 10.7m. According to the geological exploration results, the strata lithology involved in the slope mainly includes the first to fourth layers of heavy silty loam, the first - 1 layer, the fifth layer of sandy loam, and the sixth layer of silty fine sand.

[0029] The silty fine sand layer is located at the bottom of the deep foundation pit excavation. It has medium water permeability, and the groundwater in the layer has pressure. Considering this geological structure, the construction unit adopts the double dike platform layout form, arranges an anti - filtration layer in the silty fine sand layer, sets tube well dewatering wells at the excavation layer and the first dike platform position respectively, and sets a drainage ditch at the toe of the slope, forming a combined dewatering and drainage construction method of "circular dewatering tube well + open drainage at the toe of the slope". Combining the monitoring of the external river water level and the sump water level, the pump power is adjusted, and obvious construction effects have been achieved. There are no problems of foundation pit instability and leakage during the construction period, ensuring the smooth progress of the project construction.

[0030] The above - mentioned implementation manners are only the preferred implementation manners of the present invention. The protection scope of the present invention should not be regarded as limited to the specific forms stated in the embodiments. The protection scope of the present invention also includes equivalent technical means that those skilled in the art can think of according to the concept of the present invention.

Claims

1. A construction method for dewatering during deep foundation pit excavation. The deep foundation pit is close to a river. The upper part of the geological layer where the deep foundation pit is excavated is a loam layer, and the lower part is a fine sand layer. It is characterized in that: The excavation is provided with an upper berm, a lower berm and an excavation base surface. The geological layer above the lower berm is a loam layer, and the geological layer between the lower berm and the excavation base surface is a fine sand layer. An upper dewatering well is arranged on the excavation surface, a lower dewatering well is arranged on the upper berm, an anti-seepage layer is arranged on the slope surface of the fine sand layer, and a catchment well is arranged at the toe of the fine sand layer slope. The upper dewatering well is connected to a first pump, the lower dewatering well is connected to a second pump, and the catchment well is connected to a third pump. The first pump, the second pump and the third pump respectively pump out the water bodies in the upper dewatering well, the lower dewatering well and the catchment well. Water level gauges are arranged in both the catchment well and the river to obtain the water level Hs1 of the catchment well and the water level Hs2 of the river water respectively. The construction method includes the following: S1: When the water level Hs1 of the catchment well ≥ H1m, the first pump, the second pump and the third pump work at powers P1n, P2n and P3n respectively; S2: When the water level Hs1 of the catchment well < H1m, the following judgment is made: S21: When Hs2 < H2min, the second pump works at power P2m, and the first pump works at power P1m; S22: When H2min ≤ Hs2 < H2m, the second pump works at power P2n, and the first pump works at power P1m; S23: When Hs2 ≥ H2m, the second pump works at power P2n, and the first pump works at power P1n; S241: When Hs1 < H1min, the third pump does not work; S242: When H1min ≤ Hs1 < H1m, the third pump works at power P3m.

2. The deep foundation pit excavation dewatering construction method according to claim 1, characterized in that: The powers of the first pump, the second pump and the third pump can all be adjusted. P1n, P2n and P3n are the rated powers of the first pump, the second pump and the third pump respectively, and P1m, P2m and P3m are all less than their rated powers.

3. The construction method for dewatering during deep foundation pit excavation according to claim 2, characterized in that: P1m, P2m and P3m respectively take 80% of the rated powers of the first pump, the second pump and the third pump.

4. The deep foundation pit excavation dewatering construction method according to claim 1, characterized in that: The length of the lower dewatering well is less than that of the upper dewatering well. The upper dewatering well and the lower dewatering well are both distributed in a ring shape, and the bottom elevations of the upper dewatering well and the lower dewatering well are the same, both more than 3m lower than the excavation base surface.

5. The deep foundation pit excavation dewatering construction method according to claim 1, characterized in that: The catchment well is arranged at the corner of the excavation base surface and is set to be one. A drainage ditch is arranged at the toe of the slope of the excavation base surface. The drainage ditch is inclined, and the elevation of the drainage ditch at the toe of the slope diagonally opposite to the catchment well is the highest.

6. The dewatering construction method for deep foundation pit excavation according to claim 1, wherein: A soil retaining ridge is arranged outside the upper dewatering well on the excavation surface.

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