Dewatering Methods - Groundwater Control

Groundwater Control

When rainwater collects in surface depressions, it is called surface water; when it percolates down into the permeable subsoil and collects in tiny soil voids, it is called groundwater; the upper surface of the groundwater is called the water table.

Following some problems of water in the subsoil effect the construction work:

1. A high water table could cause flooding during wet periods.
2. Subsoil water can cause problems during excavation work by its natural tendency to flow into the voids create by the excavation activities.
3. It can cause an unacceptable humidity level around finished buildings and structures.

Civil engineers are must restrict the major problems of ground water and keep the excavation to safe. Therefore, they introduce many dewatering methods for controlling the ground water.These methods can be classified into two groups:

• Permanent exclusion of groundwater
• Temporary exclusion of groundwater

Permanent Exclusion of Groundwater

This can be defined as the insertion of an impermeable barrier to stop the flow of water within the ground.

1.Diaphragm walls

Diaphragm Wall

Diaphragm walls are suitable for all types of soil, and are usually of in-situ reinforced concrete installed using the bentonite slurry method or constructed using precast concrete method .

This form of diaphragm wall has several advantages:

•  low installation noise and vibration;
•  can be used in restricted spaces;
•  can be installed close to existing foundations:
• suitable for basement underground car park and similar structure.

However, this method is generally uneconomic unless the diaphragm wall forms part of the permanent structure.

2.Thin Grouted Membranes

Thin Grouted Membranes

These are permanent curtain or cut-off non structural walls or barriers inserted in the ground to enclose the proposed excavation area. They are suitable for silt and sand and can be installed rapidly but they limitation is the depth to which the formers can be driven and extracted.

3.Secant Piling or Contiguous Piling

Contiguous Piling

Secant piling is an alternative method to the reinforced concrete diaphragm wall, consisting of a series of interlocking reinforced concrete-bored piles.Ensuring that the piles interlock for their entire length. This will require special cutting tools to form the key in the alternate piles for the interlocking intermediate piles. The pile diameter selected will be determined by the strength required after completion of the excavations to one side of the wall. The usual range of diameters used is between 300 and 600 mm. Contiguous piling can be faced with a reinforced rendering or covered with a mesh reinforcement, sprayed with concrete to give a smooth finish (called shotcrete or gunite). An alternative method is to cast, in front of the contiguous piling, a reinforced wall terminating in a capping beam to the piles.

4.Grout Injection

cement grouting method

Grouts of all kinds are usually injected into the subsoil by pumping in the mixture at high pressure through tubes placed at the appropriate centres, according to the solution being used and/or the soil type. Soil investigation techniques will reveal the information required to enable the engineer to decide on the pattern and spacing of the grout holes, which can be drilled with pneumatic tools or tipped drills. The pattern and spacing of the injection pipes will depend on the grout type and soil conditions.

1. Cement Grout: Suitable for coarse grained soil and fissured and jionted rock strata
2. Chemical Grout: Suitable for medium to coarse sands and gravels
3. Resin Grout: Low viscosity then Chemical Grout and penetrate into silty fine sands.
4. Bituminous grout: Unsuitable for underpinning work and penetrate into fine sands for decrease the permeability of soil.

4.Freezing

Freezing

Freezing is suitable for all types of subsoil with a moisture content in excess of 8% of the voids. The basic principle is to insert freezing tubes into the ground and circulate a freezing solution around the tubes to form ice in the voids, creating a wall of ice to act as the impermeable barrier. This method will give the soil temporary extra mechanical strength, but there is a slight risk of ground heave, particularly when operating in clays and silts. The circulating solution can be a brine of magnesium chloride or calcium chloride at a temperature of between -15 and -25 °C, which would take between 10 to 17 days to produce an ice wall 1.000 m thick, depending on the type of subsoil. For works of short  duration where quick freezing is required, the more expensive liquid nitrogen can be used as the circulating medium.

Temporary Exclusion of Groundwater 

This can be define as the lowering of the water table and within the economic depth range of 1.5m can be achieved by subsoil drainage method. For deeper treatment a pump or pumps are usually involved. 

1.Sump Pumping

Simple Sump Pumping

Sump pumping is suitable for most subsoil and, in particular, gravels and coarse sands when working in open shallow excavations. The sump or water collection pit should be excavated below the formation level of the excavation and preferably sited in a corner position to reduce to a minimum the soil movement due to settlement, which is a possibility with this method. Open sump pumping is usually limited to a maximum depth of 7.500 m because of the suction lift limitations of most pumps. An alternative method to the open sump pumping is the jetted sump, which will achieve the same objective and will also prevent the soil movement. In this method a metal tube is jetted into the ground and the void created is filled with a sand medium, a disposable hose and a strainer.

2.Well-point System

Well-point systems are popular for water lowering in non-cohesive soils up to a depth of between 5.000 and 6.000 m. To dewater an area beyond this depth requires a multi-stage installation. The basic principle is to water-jet into the ground a number of small diameter wells, which are connected to a header pipe attached to a vacuum pump

3.Shallow-Bored Well

Shallow-bored wells are suitable for sandy gravels and water-bearing rocks. The action is similar in principle to well-point pumping, but is more appropriate for installations that have to be pumped for several months, because running costs are generally lower. This method is subject to the same lift restrictions as well-point systems and can be arranged as a multi-stage system if the depth of lowering exceeds 5.000 m

4.Deep-Bored Wells

Deep-bored wells can be used as an alternative to a multi-stage well-point installation where the groundwater needs to be lowered to a depth greater than 9.000 m. The wells are formed by sinking a 300 to 600 mm diameter steel lining tube into the ground to the required depth and at spacing to suit the subsoil being dewatered. This borehole allows a perforated well liner to be installed with an electro-submersible pump to extract the water. The annular space is filled with a suitable medium, such as sand and gravel, as the outer steel lining tube is removed.

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