DRAINAGE SYSTEM, SOURCE AND REQUIRMENTS

DRAINAGE SYSTEMS

The efficiency of a modern track carrying heavy and dynamic loads depends entirely on preservation and retention of its stability and elasticity by proper drainage.

Sources of Moisture in a Railway Track

Various sources of moisture affecting a railway track are the following :

1. Surface water due to train, dew or snow.
2. Moisture sucked up by capillary action resulting in increase of moisture in the subgrade or embankment.
3. Seepage-water from adjacent area.
4. Hydroscopic water or held-water.

 1. Surface Drainage :

Due to rain; dew and snowfalls; the moisture moves into the embankment under the action of gravity. 

This movement of water is resisted by the permeability of soils. So it is desirable that good soils should be used for bank and formation.

 The best soil would be a well-graded material of high-internal friction having high-cohesion without any characteristic of detrimental shrinkage i.e.; when it dries without any expansive tendencies; when it gets damp with small capillarity; should possess good elasticity and even with fairly large water content not of too much plasticity.

 When compacted; such an ideal material must remain stable both when wet and dry i.e.; under all weather conditions. However; such soils satisfying such characteristics are very rarely available in actual practice.

Surface water from the embankment can also be drained by use of sand

piles.

2. Sub-surface Drainage :

Changes in moisture content of subgrade or formation in embankment or in cutting are caused; mainly due to fluctuations in movement of capillary water; seepage water from adjacent area; ground water table and percolation of rain water. 

The object of sub-surface drainage is to keep these fluctuations of moisture as minimum as possible. 

The different sub-surface drainage systems used under different conditions are discussed in the following paragraphs.

3. Drainage of Seepage Water :

In case of track in cuttings; the water seeps from adjacent area to subgrade. 

A water bearing strata impounds its water because it has no escape. In such cases; the construction engineer should determine the source which feeds this water bearing strata and should divert it to the surface of interception. 

This water is further collected and carried away to some point of outfall where it can have no further adverse effect on the track.

Track-drainage is important due to following reasons :

1. The modern-track embankments; which are subjected to heavy and fast
   loads; get settled due to presence of excess-moisture.
2. The presence of excess sub-surface moisture reduces the track-stability and results in ballast pockets, dirty ballast, low joints pumping sleepers, unstable formation; and slipping and subsiding banks.
3. The presence of surface water and ground water if not properly drained results in recurrent soft spots; unstable banks and cuttings; bank-slips and landslides.
4. Excess of water in banks and under banks in most of the cases is the source of all troubles; as moisture in contact with soil is a cause of weakening of all its supporting power. A high percentage of moisture content breaks down its bearing power; causes heaving up due to swelling and shrinkage of soil on drying.
   
5. The bad-soil at wet bank when it dries results in shrinkage and cracking up of bank and formation; which further allows ballast from ballast section to run into these cracks and get lost the utility of the ballast.
6. Further more; due to dynamic loads on wet-soils; the slush is formed which is forced out and clogs the ballast.
7. The erosion of soil from the bank; slopes of embankment; cut and hill side is caused due to surface water.
8. In the rainy season; the presence of a badly drained track formation; is the main cause of accidents due to derailments