Track drainage involves the interception and removal of water from; upon or under the track and is accomplished not only by surface interception and drainage arrangements; but also where necessary by a sub-surface drainage system.
Sources of Moisture in a Railway Track
Various sources of moisture affecting a railway track are the following :
- Surface water due to train, dew or snow.
- Moisture sucked up by capillary action resulting in increase of moisture in the subgrade or embankment.
- Seepage-water from adjacent area.
- Hydroscopic water or held-water.
Surface water is one which is free to move under the action of gravity; such gravitational water is removed and diverted from the track and adjoining land by side drains (Fig. 1, D1 and D2); and is termed as surface-drainage.
The excess of moisture due to capillary action or water seeped from adjacent area is removed and diverted by lowering the water table and is termed as sub-surface-drainage.
Hydroscopic water which is retained in the pores of soil mass or on surface of soil particles as an integral part of the soil due to surface tension and adsorptive forces cannot be drained off by normal gravitational methods.
SIGNIFICANCE OF TRACK DRAINAGE
The bearing power (or strength) and stability of soil (i.e., resistance to shear of soils) are greatly reduced due to presence of excess moisture. The variation in bearing power and stability depends upon the percentage of moisture, soil type and mode of stress-application.
Track-drainage is important due to following reasons :
- The modern-track embankments; which are subjected to heavy and fast
loads; get settled due to presence of excess-moisture.
- 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.
- 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.
- 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.
- 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.
- Further more; due to dynamic loads on wet-soils; the slush is formed which is forced out and clogs the ballast.
- The erosion of soil from the bank; slopes of embankment; cut and hill side is caused due to surface water.
- In the rainy season; the presence of a badly drained track formation; is the main cause of accidents due to derailments.
REQUIREMENTS OF TRACK DRAINAGE SYSTEMS
- The surface and underground waters should be well away from a track;
the banks and cuttings; over or through which they run.
- The surface water from adjoining land should be prevented from entering the track-formation.
- The side drains should have sufficient capacity and longitudinal slope to carry away all the possible surface water.
- Flow of surface water across the track and along the slopes should not cause erosion of the banks and slopes of embankment.
- Sub-surface water should be efficiently drained off by the sub-surface drainage system.
- The highest level of ground water table should be well below the level of the subgrade.
- The track alignment should be made to rest on pervious; naturally drained and suitable soils. Coarse-textured soils are more permeable; retain less capillary moisture and respond more readily to drainage system.
- In water-logged areas; special precautions should be taken especially if detrimental salts are present or if floods are common in the area.
- Special measures should be taken in the following cases :
- Existence of an underground water pocket due to hollow basin over thick impervious layer.
- Existence of an underground water pocket over a thin impervious stratum which lies on fissured strata (i.e. good pervious soil).
- Presence of water bearing strata on side with long-cuttings and banks i.e.; seepage flow.
- In case of black-cotton soils; or expansive soils.
- In case of track-drainage problems where either
- wet earth gets into the ballast
- ballast sinks into the wet earth.
- The drains or pipes should be kept at closer spacing to keep the water table well below the formation to prevent capillary rise of water.
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.
The construction engineer should provide a properly designed drainage system or alternatively; the bank and formation structures must be strengthened and made proof against weakening by damp and through the use of a better bank-material. The method of drainage is adopted on the basis of economic considerations in light of soil characteristics and drainage problems.
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.
So; the surface water is first collected in well designed side-drains and cross-drains which is further disposed off at the nearest stream or natural water course which is connected to side drains. Cross-drainage structure; like culverts and bridges may be necessary for disposing of the surface water. When warranted; the use of perforated pipes, pipes with loose joints, boulder drains etc. can be made.
The only economical method to prevent the entry of water is to provide
turfing on the side slopes of embankment or cutting and on the cess (i.e.; portion of formation beyond the space occupied by ballast).
Surface water from the embankment can also be drained by use of sand
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.
(i) Drainage of Capillary Water :
The best method of preventing the capillary rise is to provide pervious layer in the embankment as shown in Fig. 4. The rise can also be prevented by providing a blanket or inverted filter of pervious material below the ballast as shown in Figs. 2 and 3.
(ii) Drainage of Seepage Water :
In case of track in cuttings; the water seeps from adjacent area to subgrade as shown in Fig. 4.
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.
The surface water entering the subgrade is prevented by providing catch water drains at the top of cutting and side-drain as shown in Fig. 4. Further to check seepage through catch water-drains; the drains should be paved. The water from catch water drains is finally disposed off in natural river or water course.
In case of formation in cutting; the side drains are also provided with perforated pipes (or blind-drains) underneath to lower the ground water-table as shown in Fig. 5.
Underground drains are also proposed below the subgrade for lowering the ground-water table. Blind drains with inverted filter provide underground drainage which improves the stability of formation by lowering the water table.
However; the rise of capillary water can also be controlled by this method because for a particular soil; there is limit upto which such capillary rise can take place.
TRACK DRAINAGE UNDER SPECIAL CASES
Special situations may arise in any one of the following cases:
- Existence of an underground water-pocket due to hollow basin over thick-impervious layer.
- Existence of an underground water-pocket over a thin impervious stratum which lies over a fissured strata (i.e.; good pervious soil).
- Presence of water bearing strata on side-long cuttings and banks i.e.; seepage flow.
- In case of black-cotton soils or expansive soils.
The following remedial measures are suggested for above special cases :
As shown in Fig. 6; the water pocket has been formed because of the presence of ridges on all the sides; and moisture cannot escape. Moreover it is too deeply located to evaporate quickly. This type of soft soil with impounded water in a pocket seriously affects the stability of the track and embankment.
A simple remedy to this problem is to provide a cut or pipe or channel for draining away the water pocket as shown in Fig. 7. This cut or pipe can be taken to nearest outfall which further can not adversely affect the track and embankment.
In such cases; when the water pocket is held up by a thin impervious layer which lies over a good pervious soil or fissured strata as shown in Fig. 8.
A simple remedy for this is; by drilling or puncturing through the thin impervious stratum and to provide an easy escape for impounded water. This will stabilize the whole subgrade as shown in Fig. 9.
In case of presence of water bearing strata on side long cutting and banks, where seepage flow takes place.
A remedy to this is to take due precaution during construction. The source of water bearing strata should be located and water should be diverted to outfall and taken away to have no adverse effect as shown in Fig. 4 and has already been discussed under article on drainage of seepage water.
The problem of drainage in case of black cotton soils is very acute as they swell with excess of water; and shrink and crack on drying due to shortage of water.
A remedy to this problem of drainage is to improve the quality of sub-grade soil. Quick lime can be mixed with soil to make the soil granular in structure which will improve the undesirable property of swelling and shrinkage. This mixed granular soil should be used in the top layer of sub-grade and the banks should be laid to proper slopes. This process is known as stabilization of soil.
A new method for treatment of low bearing soils with poor drainage characteristics using Geotextiles has been developed and is being practiced in advanced countries as well as in India. In this method a layer of Geotextiles is generally laid either directly ‘below the ballast or Sand inched between layers of Sand. In U.S.A.; these geotextiles have been extensively used for soil stabilization and laid directly underneath the ballast. The unique property of Geotxtiles is that they allow the water to pass through but not the foil fines. As a result Geotextiles not only work as separators and filters but also drain the water and provide reinforcement to the weak-soil bed. This is best method to improve weak subgrade or formation under railway track ‘or’ even under a road.
Whenever streams or water courses have to cross the track; facility for cross-drainage has to be provided. The water from the side-drains is taken across by these cross drains in order to divert the water away from the track. Generally, the cross-drainage structures consist of drain pipes, culverts or the bridges.
The choice of the type of a bridge will depend upon several factors like span, loads, etc. However; the cross-drainage system can be designed and used depending upon the requirements and economic considerations. R.C.C. and steel bridges are very common these days.
TRACK DRAINAGE PROBLEMS
The bad drainage results in either of the following two problems :
- wet earth clogs (or gets into) the ballast
- ballast sinks into the wet earth
1. When wet earth clogs the ballast
Due to bad drainage; it is observed that wet earth gets into the ballast which causes mud, pumping around the sleeper and also causes the formation of ridges along the cess and in between the tracks. The rise of wet earth may also check the functioning of existing drainage system on the track. This finally results in constant lifting of the track which is evident from mud-pumping. This occurrence of mud-pumping is clear indication of the entry of subgrade soil into the ballast.
- Only remedy to this defect is to improve the formation. The formation can be improved by two ways:
- By providing an interposing layer of sand between the ballast and the formation of an inverted filter between the ballast and the formation (as shown in Fig. 2).
- The formation is also improved lowering the ground water level (as shown in Fig. 5).
Through soil stabilization using Geotextiles. This method is already covered under case IV above. The use of geotextiles for soil stabilization not only improves the soil strength but also improves the drainage. This method is being adopted on South Central and South Eastern Railways in India to control mud-pumping formations on track. This method is widely used in U.S.A. for tacking drainage problem and improving the characteristics of poor quality formation soil.
2. When ballast sinks into the wet earth :
This happens due to rupturing of wet formation under heavy loads and goes on increasing in size under repetition of loads. (Fig. 10). Finally these ruptures (or cracks) become wide enough to give way to the ballast. As a result; ballast goes on sinking into wet earth formation and results in formation of water pockets along with lifting of the track. Due to loss of ballast and formation of water pockets; trouble gets aggravated and may result in the failure of formation if not properly curbed at the right time.
TRACK DRAINAGE REMEDIAL MEASURES
The following remedial measures depending upon cause of occurrence are recommended to prevent sinking of ballast into wet soil :
1. Use of pervious cess :
If trouble in track performance due to formation of water pocket is not excessive; in such cases pervious cess can be provided as shown in Fig. 11.
2. Use of perforated pipes and trench drains :
Water pocket can also be successfully drained by use of perforated pipes and trench drains. These perforated pipes are laid with loose joints so that water which surrounds them can enter inside after it has seeped to the rubble filled trench. Perforation pipes with perforations of 20 cm to 25 cm diameter would serve the purpose best (as shown in Fig. 12).
3. Use of inverted filter blanket :
If water pocket formation causes excessive trouble then the method is to first screen the ballast; drain off water pockets; and then provide 20 cm to 30 cm thick inverted filter blanket. If use of inverted filter is not permitted by cost considerations then sand layer of the same thickness can be used between the ballast and formation level as shown in Fig. 13 or even use of Geotextiles can be made as filter.
4. Cement grouting :
The trouble of water pocket is also controlled by grouting cement in the water pocket and stabilizing the formation. This method is very effective but costlier one; so can be adopted on important tracks; with high speed and heavy traffic.
5. Combination of pervious cess and inverted filter :
Once the water pocket is formed; pervious cess is provided to drain off this water. But in case it is inadequate and still accumulation of water continues. In that case; to check the further accumulation of water; the inverted filter blanket can be used on the formation.
6. Use of sand piles :
In case a pervious soil strata is existing below cohesive soil; the sand piles can be used. Water pocket is formed in cohesive soil and water from it can be collected in the sand piles as shown in Fig. 14 which further passes it to pervious layer and formation gets stabilized.
7. Use of counterfort drains :
Sometimes, the drains of special shape; as shown in Fig. 15 known as counterfort drains are provided at suitable intervals along the track. The cross-section of the drain may be 20 cm x 30 cm or any other suitable cross-section depending upon drainage requirements.
8. Use of Capillary break :
If the trouble of water pocket is caused due to capillary rise; then capillary break is the best remedy. But use of capillary break in embankment is not possible when embankment is built. So while constructing formation this should be borne in mind. This capillary break is the pervious layer of coarse grains as shown in Fig. 16.
The sources of moisture in a railway track are surface water, soaked water, seepage and hydroscopic water. The presence of excessive water reduces the track stability; erodes the banks of the embankment and in some cases may even results in accidents or derailmerts. Therefore; a well designed system providing adequate drainage facilities is essential for a railway track.
In some locations special drainage problems are posed; which must be solved as dictated by the cause; situation and circumstances. Black cotton soil is most troublesome for drainage and should be handled very carefully.