Dewatering Process and Its Techniques in Building ProjectsGROUP THREE

Definition

Dewatering is defined the process or term for the control of ground water by pumping to lower ground water level to allow excavation to be done in a dry and stable condition

Also known as Construction Dewatering in Construction Sites

Causes of Surface Water on Construction Sites

Water which accumulate in trenches on the construction sites due to Rain Washing of Plants Water table close to surface Concreting

Factors affecting the choice of dewatering methods

The nature and permeability of the ground. The extent of the area to be dewatered The depth of the water table below ground level and the amount by which it has to

be lowered. The proposed methods of excavation and ground support The proximity of existing structures, the proximity of water courses

Dewatering TechniquesPumping Methods

• Sump Pumping• Shallow well pumps• Well points• Deep wells

Exclusion Methods

• Ground Freezing

• Sheet filling

• Slurry trench cut-off walls with bentonite

• Impervious soil barrier

• Grouted cut-offs

Sumps and sump pumping

Water is collected in deeper parts of the excavation (called sumps) and pumped away

Advantage Simple and cheap method of dewatering

in favorable ground conditions

Disadvantage

The sump takes up space within an excavation

Can lead to water pollution problems due to silt-laden water

Well Point System

A line or ring of small diameter shallow wells (called well points) installed at close spacing (1 to 3 m centres) around the excavation.

Commonly used for dewatering of pipeline trenches

Advantage Can be a very flexible and effective method

of dewatering in sands or sands and gravels

Disadvantage Drawdown limited to 5 or 6 m below level

of pump due to suction lift limits

Deepwells System

Wells are drilled at wide spacing (10 to 60 m between wells) to form a ring around the outside of the excavation

An electric submersible pump is installed in each well.

Advantage Effective in a wide range of ground

conditions, sands, gravels, fissured rocks

Disadvantage Drawdown limited only by well depth and

soil stratification

Shallow Wells

Shallow-bored wells are suitable for sandy gravels and water-bearing rocks for depths not in excess of 5 meters and the principle upon which they operate is similar to that of the well point system.

Advantage It is used to extract large quantities of water from a single hole

Disadvantage The limiting depth to which this method is employed is about 8 m

Ground Freezing

Freezing process involves the circulation of a coolant through a series of pipes inserted into the ground so as to solidify the soil water into ice creating a strong impermeable barrier

Advantage The method is suitable for all types of subsoil with a moisture content in excess of 8% of the

voids

The method will give the soil temporary extra mechanical strength

Disadvantage it can cause ground heave especially in clays and silts

Sheet Piling around the Excavation

The method is used to form a barrier or cut-off wall to the flow of groundwater. The sheet piling can be of a permanent nature, being constructed to act as a retaining wall, or it can be a temporary enclosure to excavation works

 Eductor System (Grout Injection)

Grouts of all kinds are usually injected into the subsoil by pumping in the mixture at high pressure through tubes adequately spaced apart depending on the type of grout and the nature of soil

Advantages Effective in stabilising fine soils (silts, silty sands) by reducing pore water pressures

Suitable when well yields are low

Disadvantages Drawdown generally limited to 25 to 30 m below pump level

Some grouts are only able to reduce the permeability of the soil but will not increase the soil’s stability

Thank you for your attentionGROUP THREE