How can you control water around an excavation
- Dewatering means removal of excess water from saturated soil.
- Dewatering is a necessary process when many construction it comes to projects, particularly construction is for underground when the projects.
Factors such as the type of soil and the nature of the construction site will all influence which dewatering method will be best suited to the project.
1. Sumps and Ditches
2. Shallow Well System
3. Deep Well System
4. Well Point System
5. Vacuum Method
6. Cement Grouting
7. Chemical Process
8. Freezing Process
9. Electro-Osmosis Method
1) Sumps and Ditches
It is the simplest and most commonly used form of dewatering.
In this method, shallow pits, called sumps are dug along the periphery of the area and connected by drains of semicircular in shape and 20 cm diameter.
The water from the slopes flows under gravity and is collected in sumps from which it is pumped out.
2) Shallow well Dewatering
- A hole of 30 cm diameter or more is bored into the ground to a depth not more than 10 m below the pump level. A strainer tube of 15 cm diameter is lowered in the bore hole having a casing tube.
- A gravel filter is formed around the strainer tube by gradually removing the casing tube and simultaneously pouring the filter well so formed.
- The suction pipe from a number of such wells may be connected to one common header connected to the pumping unit.
3) Deep Well Dewatering (Bored Well System)
This system is more suitable when the depth of excavation is more than the 16m or where artesian water is present.
In this, 15 to 16 cm diameter hole is bored and a casing with a large screen is provided. A row of well points is frequently installed at the toe of the side slope of the deep excavation.
A submersible pump is installed at the bottom of the well, of which the casing generally has a minimum diameter of 150 mm. The discharge pipes from the submersible pumps of a number of adjacent wells are connected to a common delivery main. The water is raised from the well by a multi-stage pump.
4) Well Point System
The main components of a well-point system are:
1. well points
2. Riser pipe
3. Swinger arm
4. Header pipe
Single Well Point System
- The well point is perforated pipe 5 to 8 cm in diameter and 1m long covered by cylindrical wire gauge screen known as strainer.
- Pipes are jetted in the ground 1 to 2 meter apart.
- Well point -> riser pipe -> swinger arm -> header.
- It is suitable for lowering water table by 5 to 6 meter in soil.
Multi-Stage Well Point System
- Multi stage well point system is suitable for excavations up to 15 meter.
- When water table is greater than 6 m this method will use.
- In this method 2 or more rows of well point are installed at different elevation.
- In this method wells are installed in 2 stages.
- In 1st stage water table lowered by 5 m.
If required, then 3rd stage of well point can also be installed for the further lower water table. This method is useful for up to 15 m. For up to 15th m deep well system will be used.
Advantages of Well Point System
Installation is very rapid. The equipment is reasonably simple and cheap.
As water is filtered while removing from the ground, soil particles are not washed away. Hence, there is no danger of subsidence of the surrounding ground.
As the water is drawn away by well points from the excavation, the sides of excavation are stabilized and steeper side slopes can be permitted.
Disadvantages of Well Point System
A single-stage well point system is suitable for lowering the water table by 5 to 6 m only. For deeper excavations, where the water table is to be lowered for a depth greater than 6 m, the multi-stage well point system is required.
It is essential to continue pumping once it has been started until the excavation is complete. If it is stopped in between, it may prove to be disastrous.
In the case of the ground consisting of stiff clay, gravel or boulders, well points are installed in drilled holes, which increases the installation cost.
5) Vacuum System
When draining is required for silt or clay which have a size less than 0.05mm. At that time vacuum pump system will require.
The process is as follow:
- The well-points are driven and 25 cm diameter is provided around the well point.
- Installed in the ground by forcing a jet of water under sufficient pressure.
- The sand of medium to coarse size is then forced into this hole as rapidly as possible. This sand forms the filter medium.
- In the uppermost 600 mm to 900 mm, an impervious material such as clay is tamped to form the seal of the upper portion.
The pumping is then carried out by means of equipment capable of maintaining a vacuum in the well-points and the surrounding filter.
In this way, the pressure around the well-points is reduced to a small fraction of the atmospheric pressure. The ground is acted upon by the atmospheric pressure. Thus the soil becomes consolidated under a pressure that is very nearly equal to the atmospheric.
Control of groundwater by Grouting
In highly permeable cohesionless soil, the safety of the side slopes may be endangered through the application of severe pumping. In such cases, especially if control of the groundwater is required permanently, the methods of grouting can be used.
The main idea is to insert fine materials or chemicals around the excavation in order to reduce the hydraulic conductivity of the surrounding soil to a minimum.
In other words, the grouting process creates an almost impervious curtain around the excavation. The grouting is conducted using movable pipes with holes. The grout material is injected under pressure as it flows outside the pipes through the holes to fill the voids of the surrounding soil.
The material used for grouting may be clay, cement or special chemical compounds.
6) Cement Grouting
The material commonly used for grout include:
1) Cement And Water
2) Cement, Rock Flour and Water
3) Cement, Clay and Water
4) Cement, Clay, Sand and Water
6) Clay And Water
7) Chemical Grouting
The desirable properties of chemical grouts:
- It must be able to modify the properties of soil as desired
2) It may either increase the strength or decrease the permeability of soil
3) It must be cheap, non-toxic, non-explosive
4) It must be in the form of a liquid with low viscosity so that it can be readily placed in the soil
5)It must be non-corrosive so that it can be handled with common pumps and piping
6) It must be possible to control the gel time by suitable means.
- Inorganic chemicals
- They are called silicate grouts
- They are cheaper
- Organic chemicals
- They are also called resin grouts
- They possess advantage of low viscosity, precise control of gel time and high strength
- Soils that will not drain using conventional methods Typically a ground freezing system consists of an array of freeze pipes.
- Those are installed into the ground around the perimeter of the excavation, usually in a circular pattern.
- A supermodel brine solution is a pump through to freeze the pipes, which freezes the water-bearing soils around the pipes to create a frozen wall. Extreme care must be taken to make sure that the freeze is complete because any groundwater seepage through the wall or from below the excavation depth will have a sliding effect.
- A refrigeration plant of required installed near the site of work.
- The large pipes of 100 mm to 150 mm diameter.
- The distance between the pipes is about 1 m to 1.50 m.
- The pipes are closed at the bottom.
- The small pipes of 25mm to 50mm diameter are inserted into the large pipes open at the bottom.
- The cold liquid at a temperature of about -23°C to -30°C is then circulated through the circuit.
- The liquid comes through the small pipe and goes up through the large pipe. This causes the ground to freeze around the pipes.
8) Electro-Osmoisis Process
This method is used for fine-grained cohesive soils (such as clay), which can be drained or stabilized using electric current. The method was developed by L. Casagrande (1952).
If the direct current is passed between two electrodes driven into natural soil mass, the soil water will travel from the positive electrode (anode) to the negative electrode (cathode). The cathode is made in the form of wellpoint or metal tube for pumping out the seeping form of well point or a metal tube for pumping out the seeping water.
A steel rod, a pipe or steel piling of excavation can serve as the cathode. The arrangement of electrodes is done in such a way that the natural direction of the flow of water is reversed away from the excavation, thereby increasing the strength of the soil and stability of the slope. The potentials generally used in the process are from 40 to 180 volts, with an electrode spacing of 4 to 5 meters.
9) Vibro Floatation Process
The vibroflot is inserted into the ground and typically can be used to improve soil up to depths of 150 feet. Vibroflotation utilizes water and the mechanical vibrations of the vibroflot to move the particles into a denser state. Typical radial distances affected range from 5 to 15 feet (Bauer Maschinen GmbH, 2012).
The vibraflot is suspended from a crane and seats on the surface of the ground that is to be improved. To penetrate the material, the bottom jet is activated and the vibration begins. The water saturates the material to create a “quick sand” condition (i.e. temporarily liquefying the material), which allows the vibroflot to sink to the desired depth of improvement.
At that point, the bottom jet is stopped and the water is transferred to the upper jet. This is done to create a saturated environment surrounding the vibroflot, thereby enhancing the compaction of the material. The vibroflot remains at the desired depth of improvement until the material reaches adequate density.
The density of the soil is measured by using the power input (via the electric current or hydraulic pressure) as an index. As the material densifies, the vibroflot requires more power to continue vibrating at which point pressure gauge displays a peak in required power.
Once this point is reached, the vibroflot is raised one lift (generally ranging from 1 to 3 feet) and compaction ensues until the peak amperage or hydraulic pressure is reached once again.
- The vibroflotation process can offer the following benefits:
- When the process is done properly, it will reduce the possibility of differential settlements that will improve the foundation condition of the proposed structure.
- It is the fastest and easiest way to improve soil when bottom layers of soil will not provide good load bearing capacity.
- It is a great technology to improve harbor bottoms.
- On a cost-related standpoint, it helps improve thousands of cubic meters per day. It is faster than piling.
- It can be done around existing structures without damaging them .
- It does not harm the environment. It improves the soil strata using its own characteristic
- No excavations are needed, reducing the hazards, contamination of soils and hauling material out from the site.
Suitability of different methods of ground water Conditions
|S.N.||METHOD||CONDITIONS FOR SUITABILITY|
|1.||Sumps and Ditches||For shallow excavations in coarse-grained soils.|
|2.||Well point system||Suitable for lowering water table by 5-6 m in soils|
|3.||Bored Well system||For coarse-grained soils and depth of excavation more than 16 m|
|4.||Vacuum method||Draining silty sands and fine sands|
|5.||Cement grouting||For coarse materials or rocks with cracks|
|6.||Freezing process||Suitable for excavations in waterlogged soils|
|7.||Electro-osmosis process||Suitable for fine-grained cohesive soils such as clays|
I hope this article remains helpful for you.
Happy Learning – Civil Concept