Lesson Npte On Introduction to Drainage and Dewatering

Introduction to Drainage and Dewatering

What is drainage?

As the name itself suggests, drainage is the process in which the free water is removed with the help of gravity in order to maintain stable soil conditions.

Drainage can be classified into two categories:

1. Surface drainage
2. Sub-surface drainage

What is Surface drainage?

The water that runs on the surface of the ground is collected and or diverted into a water body.

Sub-surface drainage

It is the collection and disposal of ground water. It is also called as dewatering.

What is dewatering?

The process in which the water is removed from a foundation pit;

• when it is situated below the ground water table or
• when it is surrounded by a coffer dam.
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Why is dewatering done?

Only understanding the definition of dewatering is not enough. It is essential to know the reason as to why it is done.

Some times in some areas, the water table is high. This causes trouble and discomfort during construction. The water is pumped out so as to keep the excavation dry. Excavation has to be kept dry so that concreting can be done.

Sometimes temporary dewatering may suffice. Temporary dewatering is draining out the water at the time of construction and it is then followed by restoration to its original water level as soon as the construction is complete.

What is permanent dewatering and why it is done?

Permanent dewatering is removing the subsurface gravitational water throughout the life of the structure. This is done to protect the structure from moisture and dampness.

Advantages of Subsurface drainage

It helps in improving the properties of the soil

Helps in the stabilization of soils

Let us consider the case of “Fine grained soil”

The water removed from fine grained soil is not much but the improvement in the properties of the soil is significant.

Helps in reducing hydrostatic pressure acting at the base of the structure

Methodology to be adopted for Sub-surface drainage:

The methodology adopted at a site would depend on various factors. They are as follows:

• Characteristics of the soil
• Position of the water table
• Time period.