1. Basic Knowledge of Groundwater

(1) Groundwater

Water buried and moving in aquifers at various depths below the ground surface.

(2) Aquifer

A layered permeable rock formation (soil layer) filled with groundwater, serving as the storage and movement space for groundwater.

(3) Aquiclude

A rock formation (soil layer) that cannot transmit or yield water, or whose permeability and water yield are negligible.

(4) Confined aquifer

An aquifer located between two continuous aquicludes.

(5) Confined water

Water filling an aquifer between two aquicludes (aquitards).

(6) Phreatic water

Water in the first aquifer with a free water surface below the vadose zone.

(7) Groundwater level

The elevation head of the free phreatic surface or the confined aquifer head in the saturated zone.

(8) Hydraulic head

The mechanical energy per unit mass of liquid expressed as the height of a liquid column.

2. Basic Knowledge of Foundation Pit Dewatering
In foundation pit construction, to increase the stability of slopes and pit bottom, reduce the water content of excavated soil, facilitate excavation, or prevent sudden inrush, dewatering of the foundation pit is required. It is divided into gravity dewatering and pressure-relief dewatering.

Application conditions of common dewatering methods

(1) Common foundation pit dewatering methods
Open sump pumping

A drainage method in which sumps are excavated outside the pit to collect phreatic water, construction water, and precipitation, and pumps are used to discharge the collected water out of the pit.
Light wellpoint

Small-diameter wellpoints are sunk into the aquifer below the pit bottom around or along one side of the foundation pit. The upper part of the wellpoints is connected to a header pipe, and pumping equipment continuously extracts groundwater from the wellpoints through the header pipe, lowering the original groundwater level below the pit bottom.

Light wellpoint dewatering

Ejector wellpoint

In ejector wellpoint dewatering, a special ejector is installed inside the wellpoint pipe. A high-pressure water pump or air compressor supplies high-pressure water (water-jet wellpoint) or compressed air (air-jet wellpoint) through the inner pipe to the ejector, forming a water-air jet that draws groundwater out through the gap between the outer and inner pipes of the wellpoint.
Electro-osmosis wellpoint

The wellpoint pipes (light or ejector wellpoint pipes) themselves serve as cathodes and are arranged around the foundation pit. Steel pipes (φ50-75mm) or rebars (φ25mm or larger) are used as anodes and buried vertically inside the wellpoints. The cathodes and anodes are connected by wires to form a circuit, and a strong direct current is applied to the anodes.
Tube well (deep well)

A dewatering method in which tube wells are installed to the design depth through boreholes, and pumps placed inside the tube wells discharge groundwater out of the pit.

Tube well dewatering

(2) Gravity dewatering
Purpose of gravity dewatering

a. Effectively lower the groundwater level within the excavation depth range;
b. Effectively reduce the water content of the excavated soil to improve slope stability, increase the consolidation strength of the soil in the pit, facilitate mechanical excavation, and provide dry working conditions inside the pit.
Types of gravity dewatering

a. Closed-type gravity dewatering: When a water-stop curtain is installed around the foundation pit to cut off the hydraulic connection of groundwater between aquifers inside and outside the pit, in-pit gravity dewatering is generally adopted;
b. Open-type gravity dewatering: When no water-stop curtain is installed and large-slope excavation is adopted, gravity dewatering both inside and outside the pit is generally adopted;

c. Semi-closed gravity dewatering: When the water-stop curtain around the foundation pit is insufficient in depth and only partially cuts off the hydraulic connection between aquifers inside and outside the pit, in-pit gravity dewatering is generally adopted.

Operation control of gravity dewatering

a. Before正式 dewatering begins, the elevation of each wellhead and the ground surface must be accurately measured, the static water level determined, and pumping equipment, cables, and drainage pipes arranged for a trial run of dewatering. The purpose is to check whether drainage and electrical circuits are normal and whether the pumping system is intact, ensuring the normal operation of the entire dewatering system.
b. The extracted groundwater should be discharged into municipal pipelines or other drainage facilities outside the site. Re-infiltration of extracted groundwater on site should be avoided to prevent adverse effects on dewatering performance.

c. Dewatering operation should coordinate with foundation pit excavation construction. Pre-dewatering should be carried out before excavation, generally requiring about 2 weeks of pre-dewatering time before excavation. During excavation, water accumulated in the pit due to rainfall or other factors should be promptly discharged out of the pit to minimize infiltration of atmospheric precipitation and accumulated water.

d. For foundation pit projects with strict environmental protection requirements around the pit and strong hydraulic connection between the dewatered aquifer inside the pit and groundwater outside the pit, the dewatering operation principle of "dewatering on demand" should be strictly implemented to avoid excessive lowering of the groundwater level.

e. Groundwater level monitoring should be carried out both inside and outside the foundation pit. Where conditions permit, automatic groundwater level monitoring should be adopted for full-process tracking and monitoring.

f. During the dewatering operation stage, damaged pumps should be replaced promptly. Dewatering well pipes can be gradually cut off as excavation proceeds.

g. After the foundation pit is excavated to the design depth, appropriate well-sealing measures should be taken to effectively seal the dewatering wells according to the recharge conditions or water level recovery characteristics of the groundwater in the pit.

(3) Pressure-relief dewatering
Purpose of pressure-relief dewatering

To promptly reduce the confined water head of the lower confined aquifer, prevent sudden inrush at the pit bottom, and ensure the stability of the pit bottom during construction. (Sudden inrush is abrupt and can cause serious engineering accidents, huge economic losses, and severe negative social impacts.)
Types of pressure-relief dewatering

a. In-pit pressure-relief dewatering: The depth of the bottom of the relief well filter must not exceed the depth of the bottom of the water-stop curtain to be truly in-pit pressure-relief dewatering. Otherwise, a large amount of extracted groundwater comes from horizontal radial flow below the water-stop curtain, causing increased ground deformation outside the pit. The in-pit pressure-relief dewatering scheme is adopted when one of the following conditions is met:

① When the water-stop curtain partially penetrates into the confined aquifer, the length L of the curtain below the top of the confined aquifer is not less than 1/2 of the confined aquifer thickness, or not less than 10m;

② When the water-stop curtain penetrates into the aquitard or aquiclude below the top of the confined aquifer, the curtain has completely cut off the hydraulic connection between the confined aquifers inside and outside the pit. 

Schematic diagram of in-pit pressure-relief dewatering structure

b. Out-of-pit pressure-relief dewatering: The depth of the bottom of the relief well filter must be not less than the depth of the bottom of the water-stop curtain to ensure the effectiveness of out-of-pit pressure-relief dewatering. Otherwise, a large amount of extracted groundwater comes from horizontal radial flow outside the pit, resulting in slow water level decline outside the pit or dewatering failure, and also causing increased ground deformation outside the pit. The out-of-pit pressure-relief dewatering scheme is adopted when one of the following conditions is met:
① When the water-stop curtain does not enter the lower confined aquifer targeted for dewatering;

② The length L of the water-stop curtain below the top of the target confined aquifer is much less than the confined aquifer thickness and does not exceed 5m. 

Schematic diagram of out-of-pit pressure-relief dewatering structure

c. Combined in-pit and out-of-pit pressure-relief dewatering: When site conditions cannot fully meet the selection conditions for in-pit or out-of-pit pressure-relief dewatering, a reasonable combined out-of-pit and in-pit pressure-relief dewatering scheme can be selected by comprehensively considering site construction conditions, hydrogeological conditions, water-stop curtain characteristics, and surrounding environment characteristics and protection requirements.
Operation control methods for pressure-relief dewatering

a. The principle of "pressure-relief dewatering on demand" should be strictly followed. Considering environmental factors, the relationship between safe confined water level depth and foundation pit construction conditions, the phased confined water level control standards for each construction section should be determined, and a detailed pressure-relief dewatering operation plan should be formulated.
b. During dewatering operation, the pressure-relief dewatering operation plan should be strictly implemented. If foundation pit construction conditions change, the dewatering operation plan should be adjusted or modified promptly.

c. Water extracted from all relief wells should be discharged outside the influence range of the foundation pit or into nearby natural water bodies. The on-site drainage capacity should account for the discharge when all relief wells (including standby wells) are fully activated. A water volume metering device and a check valve should be installed at the pump outlet of each relief well.

d. After all relief wells are completed and the on-site drainage system is installed, a group well pumping test or trial run of pressure-relief dewatering should be conducted to comprehensively inspect the power system (including backup power), drainage system, pumps in wells, measurement system, automatic monitoring system, etc.

e. Dewatering operation should be continuously monitored without interruption. For major deep foundation pit projects, automatic water level monitoring systems should be considered for full-process tracking and monitoring of confined water levels, so that changes in confined water levels inside and outside the pit during dewatering operation are under constant monitoring.

f. Within 1 week before the official start of dewatering, environmental background values should be measured, including initial confined water levels inside and outside the pit, initial ground settlement values of adjacent ground around the pit, initial deformation of protected objects, and deformation of the pit enclosure. Monitoring items that overlap with foundation pit design requirements may use existing foundation pit monitoring data. During dewatering operation, monitoring data should be promptly organized, relevant curves plotted, and potential problems predicted and handled in a timely manner.

g. When environmental conditions are complex and surface settlement outside the pit caused by dewatering exceeds environmental control standards, measures such as controlling the dewatering amplitude, artificial groundwater recharge, or other effective environmental protection measures can be taken.

h. After dewatering is stopped, reliable well-sealing measures should be taken for the dewatering tube wells.