Introduction

With the rapid development of economic construction, the ground space is becoming increasingly tight, and the underground space has become the development space of public transportation and basic engineering. Most of the construction of these underground spaces adopt deep foundation pit excavation (Lu 2015; Wu et al. 2015; Tan and Lu 2017; Lyu et al. 2018; Xu et al. 2018). With the large-scale construction of urban underground rail transit and major transportation hubs, deep foundation pit engineering has become more and more complex. The control of groundwater level and the protection of surrounding environment have become the difficulty and focus of construction. In the construction of deep foundation pit, it is very necessary to reduce the groundwater level by pumping. However, with the decrease of groundwater level, the effective gravity stress of the soil below the original water level in the foundation will increase, which will lead to the consolidation of the foundation soil. In this way, uneven settlement, inclination and cracking of ground buildings within the influence range of precipitation will be caused, which will endanger their safety and normal use, and have an adverse impact on the groundwater system (Lu et al. 2006; Peng et al. 2011; Pujades et al. 2017; Wang et al. 2017) (as shown in Fig. 1).

Fig. 1
figure 1

Geological disasters caused by deep foundation pit pumping: a Surface cracking; b Building settlement; c Ground collapse; d Spring water cut-off

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For the study of groundwater, Theis (1935) proposed the unsteady well flow formula of intact confined water wells, which laid the foundation for the theory of unsteady groundwater runoff. Based on Theis formula, Jacob (1946) established the stable flow solution of confined aquifer in infinite area considering the vertical recharge of aquifer. Li and Neuman (2007) proposed a five-layers aquifer system, and obtained the closed analytical solution by using Laplace transform. Malama et al. (2007; 2009) deduced the analytical solution considering multi-aquifer system according to different hydrogeological conditions. However, analytical calculation cannot analyze the reinjection operation process in complex strata.

Numerical calculation can simulate different geological conditions, different construction processes and different operating conditions. He et al. (2008) verified that in the case of recharge, inserting the filter pipe into the soil layer with good water permeability can improve the precipitation effect by the finite element method. Niu et al. (2013) found that the ground settlement around the foundation pit decreased significantly after recharge, and the ground settlement near the recharge well decreased more significantly by the Processing-Modflow. Huang and Xu (2014) conducted three-dimensional simulation on the dewatering and recharge process of deep foundation pit, and found that the recharge amount was directly proportional to the recovery of water level. However, numerical simulation only provides a guidance for recharge design, which is lack of practical significance. Lu et al. (2015) studied the makeup well layout and injection rate through the field pumping recharge test before construction, which was a simple reinjection equipment. Wang et al. (2012) determined the overall design scheme of pumping and recharge through recharge test, and controlled the settlement of subway column pile within 10mm. Li and Wang (2022) calculated the hydraulic conductivity and permeability coefficient of deep confined aquifer according to the pumping test, and calculated the maximum recharge volume of the single well recharge test. Guo et al. (2022) carried out the recharge test of Beijing subway, realizing the equivalence of pumping capacity and recharge capacity.

Based on the existing recharge design and recharge system in deep foundation pit, it can be found that there is a lack of a highly automatic system of dewatering and recharge. Therefore, this paper introduces a self-developed integrated system of dewatering and recharge for deep foundation pit and its application in a subway station under construction in Jinan.

Development of the integrated system of dewatering and recharge

As illustrated in Fig. 2, the integrated system of dewatering and recharge is composed of seven parts: a pumping system, an assembled water tank, a variable frequency pressure system, a cleaning and filtering system, a recharge pressure tank, a recharge well system and a central control system. The pumping system is used to extract groundwater and transfer it to the assembled water tank. And the assembled water tank is used to hold the extracted groundwater and plays a transitional role. The groundwater recharge power is provided by the variable frequency pressure system. The cleaning and filtering system can clean and filter groundwater to improve recharge quality. The recharge pressure tank is used for the diversion, and balance of water and pressure to avoid the frequent opening of the pump system. After being pressurized, filtered, and diverted by the equipment, the groundwater is injected into the recharge well system, which then seeps into the underground aquifer. In addition, we can control the opening state and operation mode of the whole system through the central control system, and can view and process the recharge data. In the following section, we will introduce each part of the system in detail.

Fig. 2
figure 2

The structure diagram of the integrated system of dewatering and recharge: a The system diagram; b The system photos

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The pumping system

The pumping system is used to extract groundwater. A reverse circulation drilling rig is used to drill a dewatering well inside the deep foundation pit, and the bottom of the dewatering well is located in the aquifer. As is shown in Fig. 3, a pumping pump is placed in the dewatering well, the outlet of the pumping pump is connected with the dewatering branch pipe, multiple branch pipes are collected into the dewatering main pipe, and the dewatering main pipeline is connected with the assembled water tank. Each dewatering branch pipe is equipped with a switch valve and a one-way valve, so that the groundwater can only flow into the main pipe from the branch pipe, which ensures that there is a large water pressure in the main pipe and can inject the groundwater into the assembled water tank.

Fig. 3
figure 3

The pumping system diagram

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The dewatering well should be inserted to a depth of at least 1m below the foundation pit floor. In this way, the groundwater level formed after groundwater extraction will be below the foundation pit floor, which is convenient for engineering construction. Besides, the dewatering well should be placed in the aquifer or fractured rock mass, so that the dewatering effect will be obvious.

The assembled water tank

The external dimension of the assembled water tank is 3.5m × 2m × 2m, which is assembled from several small pieces of stainless steel plates, which is convenient for transportation and modification (see Fig. 4). The main pipe (the inlet pipe) is installed at the top of the assembled water tank, and the outlet pipe is installed at the bottom of the water tank. An overflow pipe is installed on the upper part behind the assembled water tank, and the position of the overflow pipe is 20cm lower than that of the inlet pipe, so that the maximum liquid level in the assembled water tank is lower than that of the inlet pipe to ensure that backflow will not occur.

Fig. 4
figure 4

The structure diagram of the assembled water tank

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There are three dummy plates installed inside the assembled water tank, with a height of 1 m, which can play a role in blocking and filtering sediment deposition, ensuring that the water flowing out of the water tank does not contain large soil particles and sludge. The inspection hole is opened at the top of the water tank to observe the internal conditions. Three drain pipes are installed at the bottom of the water tank to discharge the precipitated sludge and mud. In addition, a water gauge is installed on the side to observe the water level and water quality in the assembled water tank. The assembled water tank acts as a transitional and temporary storage, and it has no effect on recharge volume.

The variable frequency pressure system

The selection of water pump shall fully consider the total drainage of the foundation pit. The water inflow of the foundation pit is greatly affected by geological conditions, water level drawdown and seasons. The pumping of the foundation pit will decrease with the increase of time, and reach a relatively stable value in the later stage. According to many years of dewatering and recharge experience, the pumping capacity in the initial stage is generally 5 times of the stable value.

As shown in Fig. 5, in order to ensure the continuity and economic operation of the water pump, the water pump with flow of 150 ~ 200m3/h and lift of 60m is selected. There are two control modes for the water pump, one is pressure control, which can set the water pump pressure and recharge pressure, and the other is frequency control, which can control the frequency of the water pump. The frequency control mode has the advantages of flexibility, high precision, strong reliability, multiple functions, and fast reflection speed.

Fig. 5
figure 5

The variable frequency pressure system diagram

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The cleaning and filtering system

The water from the assembled water tank is injected into the cleaning and filtering system through the variable frequency pressure system for secondary treatment (see Fig. 6). The selection of cleaning and filtering system shall meet the requirements of recharge water quality to avoid secondary pollution and blocking the filter pipe of recharge well. The system adopts the difference of inlet and outlet water pressure or time control to realize full-automatic cleaning. After electric brushing and filter screen filtration, the recharge water is further clarified and filtered. At the same time, the blowdown valve is opened to realize the automatic blowdown control. The cleaning time, cleaning mode and cleaning force can be adjusted.

Fig. 6
figure 6

The structure diagram of the cleaning and filtering system

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The recharge pressure tank

The recharge pressure tank is used for diverting, balancing water volume and stabilizing pressure of the recharge system. It can avoid frequent opening of the variable frequency pressure system, and ensure the stable operation of the whole system. The recharge pressure tank is a cylindrical hollow tank with a diameter of 1.8 m and a height of 2.5 m. Its rated pressure is 1MPa, and the safety performance meets the basic requirements of engineering safety technical specifications.

The recharge well system

The recharge well system can inject the treated water into the underground aquifer. This system is mainly divided into three parts: the pipe part, the aboveground part of recharge well and the underground part of recharge well (see Fig. 7). The pipe part is divided into the recharge main pipe and the recharge branch pipe. The main pipe is connected to the pressure system, and each branch pipe is connected to the main pipe. The branch pipe is equipped with a recharge valve to control the opening and closing of the recharge well.

Fig. 7
figure 7

The structure diagram of the recharge well system

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The aboveground part of the recharge well is mainly used to encapsulate the wellhead, which is sealed and connected by flange, and the flange cover is equipped with the manometer, the air evacuation valve and the measuring hole. The manometer can check and record the recharge pressure of the recharge well in real time. And the air evacuation valve is used to discharge the gas in the pipe and well to make them full of water. We can measure the groundwater level in the recharge well by passing the measuring rope through the measuring hole.

The depth of the recharge well should be the same as that of the dewatering well, so as to meet the homogenous and homologous recharge. The number of recharge well should be 20% more than that of dewatering well. The same number of recharge well can achieve the same amount of recharge. The extra recharge wells can be used for emergency. In case the recharge well becomes clogged, the backup recharge well can be activated immediately.

There are three materials around the recharge well pipe: the filter material, the clay ball and the concrete. The filter material is located at the lower part of the recharge well. The type and particle size of the filter material are determined according to the formation parameters, which is mainly used to adjust the recharge capacity and prevent the recharge well from silting. And the concrete is located at the upper part of the recharge well and is used to fix the well. In addition, the clay ball is in the middle to form a soil layer between the filter material, and the soil layer can prevent the filter material from being polluted by the concrete.

The central control system

The central control system adopts advanced frequency conversion control technology, and it has soft start and protection functions, such as overload, short circuit, overvoltage, undervoltage, phase loss and overheating. This system can realize manual and automatic operation functions to meet the requirements of different working conditions. Certainly, the system can automatically adjust the water flow according to the water volume, and can also carry out signal alarm, self-inspection and fault judgment under abnormal conditions. Wireless remote control is realized, and the status and operation parameters of the whole system can be monitored remotely (see Fig. 8).

Fig. 8
figure 8

The central control system. a The operation interface; b The wireless remote control interface

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Operation and technical advantages

The operating principle

The operation process of the integrated system of dewatering and recharge is as follows:

  1. (a)

    The pumping system is opened to inject water into the assembled water tank, and the central control system records the inlet flow.

  2. (b)

    The water in the assembled water tank passes through three partitions for preliminary water quality treatment, and the precipitated water enters the water pump system.

  3. (c)

    According to the inflow flow, the central control system controls the operation frequency and pressure of the variable frequency pressure system.

  4. (d)

    The variable frequency pressure system sends the water into the cleaning and filtering system for secondary water quality treatment, and the treated water quality has met the requirements.

  5. (e)

    The water after secondary treatment is diverted through the recharge pressure tank, and then flows into the main recharge pipe.

  6. (f)

    The recharge valve on the recharge branch pipe is opened, and the water enters the recharge well and then flows into the underground aquifer.

As shown in Fig. 9, the central control system is the core of the whole system. It controls the normal operation of the whole system by detecting the flow and pressure of water passing through each subsystem. This system realizes the dynamic balance between the inflow of the pumping system and the recharge flow of the recharge system, and ensures the basic stability of the water surface in the assembled water tank, so there is no need to manually control the operation of the system. If the pumping volume is relatively large at the initial stage of foundation pit excavation, the system will increase the reinjection volume. And if the pumping volume is relatively small at the later stage of foundation pit excavation, the system will reduce the recharge pressure or adopt the natural reinjection mode. Therefore, the operation of the whole system does not require staff to operate on site, and it can be remotely monitored and adjusted by computer or mobile phone.

Fig. 9
figure 9

The operation diagram of the whole system

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The technical advantages

Groundwater recharge is generally far away from the building area, and it is shallow recharge. It is unnecessary to consider the coupling effect of groundwater dynamics with soil stresses and strains when concrete is used for fixing around the recharge well. The key consideration is the recharge volume under the condition of safe reinjection pressure.

The integrated system of dewatering and recharge has the following technical advantages:

  1. (1)

    Intelligent control. This system can realize intelligent automatic control according to the monitoring data, including the switch of the variable frequency pressure system, the adjustment of the recharge pressure, the adjustment of the pump speed, the switch of the cleaning and filtering system. Through the central control system, the operation of the precipitation and recharge is ensured in a stable, visual and controllable state.

  2. (2)

    Recharge diversion. After water purification, pressurization and other treatment, the water pumped from the foundation pit is transported to multiple recharge wells through the recharge main pipe, so that the recharge wells and recharge water are evenly distributed. In this way, the groundwater level in a certain area will not rise excessively or the pressure will be too high, which will affect the stratum structure.

  3. (3)

    Recharge groundwater has high requirements for water quality. Through two water quality treatments, it is ensured that groundwater will not be polluted by recharge.

  4. (4)

    Wireless remote control. The operation of the whole system does not require staff to operate on site, and it can be remotely monitored and adjusted by computer or mobile phone.

Engineering application

Engineering background

This integrated system of dewatering and recharge for the deep foundation pit has been applied to a subway station under construction in Jinan. This station area belongs to alluvial inclined plain, with relatively flat terrain. Within the exploration depth, the stratum mainly includes artificial fill, silty clay, silt, pebble and gravel sand. And Groundwater has high bearing capacity. In order to protect the safety of the foundation pit and surface buildings, and protect the underground spring system after a large amount of pumping in the foundation pit, it is necessary to recharge the groundwater extracted from the foundation pit into the stratum. Therefore, the integrated system of dewatering and recharge for deep foundation pit is adopted.

Single well recharge test

Three recharge wells were selected for the recharge test: HG-44, HG-42 and HG-40. Here, the depth of HG-44 is 55 m, and the depth of HG-42 and HG-40 is 22 m. The steps of single well recharge test are: (a) Record the groundwater level of the adjacent observation well and start the system. (b) Then, carry out natural recharge test (no pressure recharge test), set the recharge pressure to 0, and record the groundwater level and recharge volume in the observation well at intervals of 5 min, 10 min, 15 min, 30 min and 1 h, respectively. (c) Adjust the recharge pressure to 0.02 MPa, 0.03 MPa and 0.04 MPa, respectively, and record the data according to step (b). (d) After completing a recharge test, wait until the groundwater level returns to the original, and then carry out the next recharge well test.

The results of single well recharge test are shown in Fig. 10. From Fig. 10a, we can see that with the increase of recharge pressure, the recharge volume gradually increases, but the increasing trend is smaller and smaller, and the initial effect is obvious. When the recharge pressure increases to a certain amount, the recharge volume gradually tends to be stable. At the same time, we can find that the recharge effect of deep wells is much better than that of previous shallow wells. This provides a good guidance for the recharge well design. It can be seen from Fig. 10b that there is a logarithmic function relationship between the groundwater level and the recharge volume. The groundwater level rises obviously at the initial recharge stage, and the groundwater level changes little at the later recharge stage, which is mainly due to groundwater seepage in the underground aquifer.

Fig. 10
figure 10

Results of single well reinjection test: a Relationship between recharge pressure and recharge flow. b Relationship between recharge volume and groundwater level

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Multiple wells recharge test

A total of 44 recharge wells are arranged in this station. In order to verify the recharge capacity of the whole system, we carried out multi well recharge test. Firstly, the natural recharge capacity of the system is tested under no pressure. The recharge wells were gradually opened one by one, and the total amount of reinjection after each additional recharge well was recorded. The test interval was 30 min. Then, the recharge tests were carried out under the recharge pressure of 0.03 MPa, 0.04 MPa and 0.05 MPa, respectively.

As shown in Fig. 11, we can see that the overall recharge volume increases gradually with the increase of recharge wells. Under the condition of no recharge pressure, with the increase of recharge wells, the recharge volume gradually increases, but the increase degree gradually decreases. After opening 27 recharge wells, the recharge volume is stable at 340 m3/h. When the recharge pressure is 0.02 MPa, the recharge volume remains stable after opening 20 recharge wells, about 590 m3/h. When the recharge pressure is 0.03 MPa, the recharge volume remains stable after opening 16 recharge wells, about 600 m3/h. When the recharge pressure is 0.04 MPa, the recharge volume remains stable after opening 13 reinjection wells, about 605 m3/h. Therefore, it can be seen that increasing the recharge pressure can improve the overall recharge volume, but it also causes the waste of recharge wells. When the recharge pressure is 0.02 MPa, the maximum recharge volume can be achieved by opening 20 recharge wells, so there is no need to set up 44 recharge wells, which causes a lot of waste of human and material resources. Because the recharge capacity of the whole system is affected by the number of recharge wells, the diameter of reinjection pipes and the maximum recharge capacity of the variable frequency pressure system. So blindly increasing recharge wells may not improve the overall reinjection volume, but the relationship between them needs to be reasonably coordinated.

Fig. 11
figure 11

The results of multiple wells recharge test

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Conclusions

We developed an integrated system of dewatering and recharge for deep foundation pit. And it can realize automatic dewatering and recharge of deep foundation pit. Based on a subway station under construction in Jinan, we carried out the single well recharge test and Multiple wells recharge test, verified the recharge capacity of the system, and explored the influencing factors of the recharge capacity of the system. The main conclusions were summarized as followed:

  1. 1.

    An integrated system of dewatering and recharge for the deep foundation pit is developed, and it is mainly composed of a pumping system, an assembled water tank, a variable frequency pressure system, a cleaning and filtering system, a recharge pressure tank, a recharge well system and a central control system.

  2. 2.

    This system can realize intelligent automatic control according to the monitoring data. The operation of the whole system does not require staff to operate on site, and it can be remotely monitored and adjusted by computer or mobile phone.

  3. 3.

    For single recharge well, with the increase of recharge pressure, the recharge volume gradually increases, but the increasing trend is smaller and smaller, and the initial effect is obvious. The relationship between the groundwater level and the recharge volume is a logarithmic function.

  4. 4.

    The overall recharge volume of this system increases gradually with the increase of recharge wells. But blindly increasing recharge wells may not improve the overall reinjection volume, and the relationship between the number of recharge wells, the diameter of reinjection pipes and the maximum recharge capacity of the variable frequency pressure system needs to be comprehensively considered.