Thursday 3 June 2010

HIGHWAY DRAINAGE

The main drainage system consists of:

· Foul water drainage to carry away safely, and without causing unacceptable pollution.

· Storm water drainage to limit or prevent flooding


Foul Water Drainage

Three systems are in existence:

· Totally Separate

· Partially Separate

· Combined System


Totally Separate

· Two sewer networks are required

· One carrying foul sewage and the other storm water.

· Reduces pollution of watercourses to a minimum.

Partially Separate

· Compromise system

· One carrying storm water

· The other foul water plus the storm water off house roofs and backyards.

· Provides a flushing of the foul sewer by the roof water

· The foul sewer must be provided with storm overflows

· Possibility of pollution of watercourses remains

Combined System

· Carries both foul and storm water

· Found in many old built-up areas

· Was the cheapest and quickest way of providing the drainage required under the Public Health Acts.

· The storm water flow is many times greater than the quantity of foul sewage carried

· Sewer is designed as a storm water drain.

· Size of pipe required tends to be very large

· When the storm flow exceeds a predetermined limit, the overflows discharge he excess flow

· Overflows are designed to operate when the flow in the sewer exceeds six times the dry weather flow

· Mixture of foul and storm water causes pollution

· Today unacceptable


Foul Drains

· Emphasis in roadwork is on storm drainage

· Basic knowledge of foul drainage is needed

· A foul sewer is normally designed to carry the foul sewage of the area which it serves

· Allowance for future developments

· Liquid wastes from Industry must also be allowed for in the design

· Even with a totally separate system there will be some storm water getting into the foul drain, yard gullies in factories and garages should be connected to it, not to the storm sewer.


Storm Water Drains

· Estimation of the quantity of storm water to be carried is difficult to determine. Affected by:

· Variation of rate of intensity of rainfall during a storm.

· The unpredictable direction of movement of the storm.

· The degree of impermeability of the area on which the rain falls

· The storage capacity of the sewer system itself.

· The time taken for rain water to get into the sewer.


Road Drainage Systems

Main drainage requirements fall into two categories:

· Sub-soil Drainage

· Drainage of the Carriageway


Sub-soil Drainage

· Must be a provision of sub-soil drainage to cope with water in the ground and to ensure that the water table can be kept low enough

· Sub-soil drainage also helps to prevent frost damage to the road structure by keeping it drained.

· Usually associated with the construction of new roads

· Care must be taken to keep the water table below the road formation level.

· May require a system of land drains, collector drains and a long outfall pipe before the ground can be drained sufficiently for the Works to commence.

· Before excavation for cut and fill begins, it is good practice to lay French drains


Drainage of the Carriageway

· Surface of the road must be kept clear of standing water

· Roads should be cambered when straight

· Laid to crossfalls on bends

· Adequately provided with gullies or grips to dispose of water

· In many cases, the drainage of surface water and sub-soil water are connected to the same system


Drainage Theory

Aim of Drainage:

· Provide drains/sewers with sufficient capacity to deal with he most severe storm conditions

· Laid to falls so that he water entering the sewers will be conducted away from the road, discharging into watercourse

· Outfall to the watercourse must high enough to be clear of the highest water level of the river or stream to prevent backing up


Self-cleansing Velocity

· Laid uphill from the outfall point with the gradient of the pipeline being sufficient for any water to move with at least ‘self-cleansing velocity’

· Gradient to be such that the minimum velocity of flow will allow solid particles to remain in suspension in the water


Gradient

Gradients at which the various sizes of pipes are self- cleansing are approximately as follows:

· 100 dia 1 in50

· 150 dia 1 in75

· 225 dia 1 in 112

· 300 dia 1 in 150

NB Self-cleansing can be determined by dividing the pipe diameter (measured in millimetres) by 2.


Manholes

· All sewer pipelines must he laid in straight lengths to a constant grade

· Manholes are required at every change of:

· a) direction ;

· b) gradient;

· c) pipe size;

· d) pipe type;

· For long straight lengths, manholes or inspection chambers are often provided at not more than 100 m intervals.

· Manholes are provided to enable the sewer to be rodded in the event of it becoming blocked


Gullies

· Connections to storm sewers made by laying junction pipes along the sewer pipeline.

· Not less than 100 mm dia.


Drainage Excavation

Reasons for Collapse

· Failure of the soil to support its own weight.

· Steeply angled bedding planes which encourage slips

· Water sheds or seepages

· Breakdown of the cohesion of the soil by frost or heavy rain.

· Changes in the type of soil such as weak material underlying sound rock or layers of sand and clay

· Failure due to trenching on or near the position of an earlier excavation.

· Vibration due to the close passage of vehicles and plant.

· Failure due to loads placed too near the edge of the face.

· Impact of heavy loads such as pipes, striking the sides of a trench when being lowered.

· Inadequate timbering in supporting a face.

· Lack of cohesion (or shearing) of soil

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