SUBSOIL DRAINAGE SYSTEM

Submitted By:

M.Vinod Kumar,

09011BB047.

What is Subsoil Drainage?

Subsoil drainage systems are used in architectural, sewage and irrigation applications to remove excessive water from the soil surrounding a building, provide water to crops or drain sewage away from a building.

Subsoil drainage systems are governed by necessary restrictions dependent on the intended purpose of the system.

PURPOSE OF SUBSOIL DRAINAGE

Increase the stability of the

ground and footings of

buildings.

Reducing foundation

movements due to the

variations in the soil

moisture content.

Mitigate surface water ponding and water logging of soils by lowering watertables.

Increase soil strength by reducing the moisture content.

LAYOUT

The layout is directly related to:

• The topography,

• The location of buildings and access points,

• The geology (nature of subsoil and level of groundwater) and

• Area of a property.

Main subsoil drains often follow natural depressions. Subsoil drains should connect to a storm water pit or a point of connection, and be consistent with the layouts for the site storm water drain and the external storm water drainage network.

These layouts may involve branch subsoil drains connecting to a main subsoil drain. Manholes should be provided at main junction points to facilitate inspection and cleaning.

Water Table Drawdown to Single and Multi-Drain Systems

Typical Single Drain Drawdown Values

General parallel drain lateral spacing and depths for different soils.

DESIGN CONSIDERATIONS

The design and investigation of subsoil drainage systems are uncertain processes. Only in a very limited number of situations can the need for subsoil drainage be identified without detailed subsurface investigations involving excavations, field observations, and soil tests.

Favoured design

30-45% angle

Small laterals into larger main

Other Design Considerations

In evaluating the need to remove subsurface water, consideration must be given to surface water runoff from buildings, road pavements and subgrades, and other impervious surfaces.

In clay soils, subsoil drains can alter long-term soil moisture regimes so that building foundations can be adversely affected by removing water, or some cases, by introducing water.

Consideration should be given to the possible effects of intermittent or permanent reduction in groundwater levels on adjacent lands when designing subsoil drainage.

In soils with clay content exceeding 20%, lowering watertables can cause soil shrinkage and damage to structures. It is recommended that subsoil drains should not be placed too close to buildings on clayey sites.

Size Specifications

In most applications, the specifications on size and shape of subsoil drainage systems are similar.

To prevent excessive clogging, the drainage pipes are required to be 3 inches or more in diameter.

Trench widths shall be a minimum of 300 mm where circular pipes are used. A minimum width of 450 mm is required where human access is required. Where a trench is deeper than 1.5 m, shoring as specified by relevant construction safety acts and regulations shall be used.

Shape and Material Specifications

Most applications, such as simple water drainage for home placement and sewage uses, require the use of "D" shaped drainage pipes rather than circular pipes.

The pipes can be made of clay, cement or plastic; this is dependent upon the intended use.

Drain Envelopes

Drain envelopes are placed around a subsoil drain such as a pipe, for several purposes. These purposes include :

to prevent excessive movement of soil material into the drain;

to increase the permeability and hydraulic performance of the material near the subsoil drain;

to increase the effective surface area of the drain;

to stabilise the soil in which the drain is being placed; and

to support and provide structural bedding for the pipe or conduit.

Filter Material

Filter material consisting of natural clean washed sands and gravels and screened crushed rock should be:

well graded with a mix of different sizes of sand particles and an adequate permeability with:

• natural sand, less than 5% passing a 75 urn sieve.

• screened crushed rock, sizes 3 mm to 20 mm.

chemically stable and inert to possible actions of soil and groundwater.

Monitoring Wells

On large or important installations, provision should be made to monitor subsurface water levels by installing monitoring wells or piezometers. Monitoring wells allow both the level and water quality of the subsurface water to be checked.

Piezometers can consist of a length of plastic tube, 25 mm to 38 mm in diameter, with holes to admit water. Both monitoring wells and piezometers should be capped to exclude surface water.

Queries?

THANK YOU..!