Foundation Pit Dewatering

It refers to the dewatering work carried out when excavating a foundation pit, where the groundwater level is higher than the excavation bottom and groundwater continuously seeps into the pit. It is done to ensure that the foundation pit can be constructed under dry conditions and to prevent slope instability, foundation piping, pit bottom heave, pit bottom boiling, and reduction in foundation bearing capacity.

Dewatering construction is an important construction step in deep foundation pit construction. Most foundation pit accidents are related to groundwater. Therefore, effective groundwater control must be carried out in deep foundation pit construction. Foundation pit dewatering is an important step to ensure foundation quality.

Methods of foundation pit dewatering:

Open ditch with sump pit dewatering, light well point dewatering, jet well point dewatering, electro-osmosis well point dewatering, deep well point dewatering, etc.

I. Commonly Used Empirical Parameters

1. When the foundation pit width is less than 6 meters, a single-row linear well point layout can be arranged along the long side of the foundation pit; when it is greater than 6 meters, well points need to be arranged on both sides or in a ring layout. The single-row linear well points should be arranged in the upstream direction of groundwater flow.

2. After the dewatering wells have been running for a period of time, the groundwater will form a stable dewatering cone. The slope of the dewatering cone is about 1:10, which means that when the groundwater level at the well point drops by 1 meter and stabilizes over a long period, the groundwater level within about 10 meters from the well point will be affected, and the farther from the well point, the smaller the dewatering amplitude.

3. It is generally required that the groundwater level be lowered to 0.5 to 1 meter below the bottom elevation of the foundation pit, so as to maintain the dry state of the foundation pit during excavation and also ensure the stability of the foundation pit slope during excavation.

4. The spacing of well points can generally be selected as 0.8 meters, 1.2 meters, or 1.6 meters, depending on the groundwater level and the depth of the foundation pit. In special parts, the well points can be appropriately densified to ensure the dewatering effect.

5. The punching depth should be about 0.5 meters lower than the filter pipe. When backfilling the well pipe, use coarse sand to fill up to at least 1 meter above the filter pipe, and seal the top at least 0.5 meters below the ground surface with clay.

II. Dewatering Methods

1. Sump Pit Dewatering

Open ditch with sump pit dewatering has the characteristics of convenient construction and low cost, and is most commonly used on construction sites.

In foundation pit slope support projects in high groundwater level areas, this method is often used as an auxiliary drainage measure for other dewatering methods. It mainly removes phreatic water, construction water, and rainwater.

When the groundwater storage is small and the geological conditions are good, open ditches and sump pits can be used to remove accumulated water in the foundation pit.

However, in areas with abundant groundwater, if only this method is used for dewatering, due to the large amount of seepage from the foundation pit slope, the working surface will be muddy, which is unfavorable for structure construction. Therefore, this dewatering method is generally not used alone in foundation pit slope support in high groundwater level areas, and is usually used in combination with dewatering well points or cutoff curtains.

2. Cutoff Curtain

A cutoff curtain cannot be used alone as a dewatering scheme; it is generally used in combination with open ditches or well point dewatering.

Cutoff curtains are generally used in situations where groundwater is very abundant, groundwater recharge is very fast, or special control of slope instability and uneven settlement of surrounding buildings is required.

Common types include cutoff walls, curtain grouting, steel sheet piles, etc. While cutting off groundwater seepage into the foundation pit, they also provide some support to the foundation pit slope.

At the same time, due to the presence of the cutoff curtain, the impact of foundation pit dewatering on groundwater outside the curtain is greatly reduced, and the stability of surrounding buildings is effectively guaranteed.

Of course, the construction of cutoff curtains requires a large site and generates significant noise, so there are some restrictions when constructing in densely built-up areas and near residential areas.

3. Light Well Point

Light well point is a widely used dewatering method in China. It is simpler, faster, more economical, and safer to construct than other well point systems, and is particularly suitable for situations with small dewatering areas and small groundwater storage.

This method generally lowers the water level by 3 to 6 meters. The applicable soil permeability coefficient for light well points is 0.1 to 50 m/d. When the soil permeability coefficient is relatively small, it is necessary to seal the top of the well point pipe with clay and ensure good airtightness at all connections of the well point system, thereby enhancing the pumping capacity by increasing the vacuum degree of the entire well point system.

4. Pipe Well Point

Pipe well points are suitable for sand and gravel layers with large permeability coefficients, strata with abundant groundwater, and occasions where light well points are not easy to solve.

It has the characteristics of simple construction and large water output. The water output flow of each pipe well can reach 50 to 100 m3/h, and it can lower the groundwater level by about 3 to 5 meters.

This method is generally used for phreatic water dewatering, and is most effective when the soil permeability coefficient is in the range of 20 to 200 m/d.

5. Jet Well Point

The pumping system and jet well pipe fittings of jet well points are relatively complex, with a relatively high failure rate during operation and large energy loss. Compared with other well point dewatering methods, it has the characteristics of large dewatering depth and high operating cost. The jet well point system can generate a vacuum of 250 mm mercury column at the bottom of the well point, and its water level lowering depth is generally between 8 and 20 meters. Its applicable soil permeability coefficient is the same as that of light well points, generally 0.1 to 50 m/d.

6. Electro-Osmosis Well Point

Electro-osmosis well points are suitable for geological conditions with very small permeability coefficients, such as clay, silty clay, silt, and mucky clay with permeability coefficients less than 0.1 m/d. It needs to be used in combination with light well points or jet well points. During the dewatering process, voltage, current density, and power consumption should be measured and necessary adjustments made, which is relatively cumbersome.

7. Deep Well Point

Deep well points are a dewatering method commonly used in foundation pit support. Its advantages are large drainage volume, large dewatering depth, and large dewatering range.

The applicable soil permeability coefficient for deep well points is 10 to 250 m/d, and the water level lowering depth exceeds 15 meters. It is often used to lower confined water.

Using deep well points to lower the confined water level helps reduce pressure and ensure the safety of the foundation pit. However, due to reasons such as large dewatering depth, large water output, and steep water level drawdown curves, it inevitably causes a large impact range and degree of impact, and thus easily causes uneven settlement of buildings around the foundation pit.

III. Issues Needing Attention

1. Operation Timing and Emergency Plans

Foundation pit excavation and dewatering operations should be carried out in seasons with little rainfall and low groundwater levels. By reasonably arranging the construction organization plan, the difficulty of dewatering should be minimized as much as possible, and at the same time, the construction compactness of the structure at the bottom of the foundation pit should be increased, so that the structure can meet the requirements for backfilling or waterproofing and flood prevention as soon as possible, thereby shortening the dewatering operation time.

In order to ensure the safety and compactness of construction, emergency plans must be designed, such as backup facilities and backup power, rainproof measures and seepage prevention measures, slope stability and settlement monitoring, etc.

2. Layout of Dewatering Well Points

Well points can be evenly arranged around the foundation pit. Omitting one well point due to the setting of access roads to the foundation pit will not have a great impact on the dewatering effect.

However, for large-area foundation pits, sometimes some dewatering well points need to be set inside the foundation pit to ensure the dewatering effect.

The minimum distance between the well point location and the periphery of the foundation pit is generally not less than 2 meters to ensure the stability of the foundation pit slope.

3. Conditions for Stopping Dewatering

It is not that dewatering can be stopped once the structure at the bottom of the foundation pit is formed. Generally, it should be considered whether the structure can be submerged or can prevent submergence, and at the same time, the strength of the structure bottom slab and the overall weight of the structure should be calculated to withstand and resist the buoyancy caused by the rise of groundwater.

4. Monitoring of Groundwater Level

During the dewatering process, the groundwater level should be monitored to ensure that the groundwater level remains at a relatively stable elevation after a long period of dewatering, avoiding excessive lowering of the groundwater level causing waste of resources and uneven settlement of structures.

At the same time, if the groundwater level is found to rise or fall abnormally, the cause should be quickly identified and hidden dangers eliminated.

5. Prevention of Uneven Settlement

Uneven settlement of buildings around the foundation pit is mainly due to the lowering of the groundwater level. After the foundation soil under the building is dehydrated, its bearing capacity decreases, and it is compressed and consolidated under the action of gravity.

To prevent such uneven settlement, it is necessary to control excessive and rapid lowering of the groundwater level. Measures usually taken include slowing down the dewatering speed, installing cutoff curtains, and drilling wells for groundwater recharge.

Among them, the groundwater recharge scheme is relatively simple and effective. It is only necessary to drill wells and recharge groundwater between the building and the dewatering wells near the building, which can largely prevent building settlement.

6. Quality of Well Point Construction

During the well point construction process, its quality should be strictly controlled. In particular, during the process of placing filter pipes and filling sand and gravel, it is necessary to prevent the well wall from collapsing and blocking the filter pipe to form a "dead well".

At the same time, correctly filled sand and gravel can filter groundwater and prevent a large amount of underground mud and sand from being pumped out during drainage, which would damage the water pump and easily block the pipeline.

Moreover, a large loss of underground mud and sand may cause excessive ground settlement or even cracking and collapse.

Foundation pit dewatering is an important technical measure in earthwork engineering and foundation and foundation engineering construction. It can remove water from the foundation pit soil, promote soil consolidation, and improve foundation strength.

At the same time, it can reduce lateral displacement and settlement of the soil slope, stabilize the slope, eliminate piping, reduce heave of the base soil, enable the construction of foundation and foundation engineering below the natural groundwater level to avoid the influence of groundwater, provide relatively dry construction conditions, and also reduce earthwork volume, shorten the construction period, improve project quality, and ensure construction safety.

In engineering practice, adopting a reasonable dewatering scheme can facilitate construction organization, reduce costs, shorten the construction period, and produce considerable economic benefits.