Railway Track bed formation foundations and formation of the track bed can affect the actual track quality and performance of railways.
Usual forms of cross-sections:
The Naturally occurring soil is known as subgrade and when it is prepared to receive the ballast and track , it is called ‘Formation’.
When a formation is raised on bank of earth , it is called an embankment. When it is made after cutting the ground , it is called ‘cutting’. In case of cutting the track is laid below ground level thus required portion to be excavated.
Normally a railway line will be constructed on a flat ground or on embankment or cutting or in a combined section.
Features of Rail Track bed level:
When the formation is to be made on embankment or cutting , various features should be carefully considered.
- Width of formation : With normally depends on number of tracks , gauge of tracks , centre to centre distance between the tracks , width of ballast layer , width of trenches if needed.
2. Slopes of sides: Stability of the earthwork depends mainly on two factors namely ‘cohesion’ and ‘friction’. For temporary stability , cohesion is useful and reliable but permanent stability is achieved only by friction which keeps the natural angle of repose of the material.
Sloped to be provided to the sides of the formation should be slightly flatter than angle of repose of the material. The slopes in cutting vary from 1.5 to 1 or steeper.
3. Drains: The accumulation of water reduces the friction in all sort of soils. In case of embankments , the rain water is easily drained off but in cutting, drains to be provided. The side drains are constructed along the track at a depth of aprox. 1200mm from the rail level.
Size of the drain depends on the rainfall data.
Stabilisation of track on poor soil
Sometimes it becomes unavoidable to lay tracks on a very poor (or undesirable) soil. In such cases it becomes necessary to improve and strengthen the nature of soil by some suitable methods. Under such circumstances, the following methods are used.
- Layer of Moorum
- Cement Grouting
- Sand Piles
- Use of Chemicals
Layer of moorum:
This method is widely used and is adopted if a poor quality soil comes across a track such as black cotton soil which is a fine black loomy soil. This soil has the tendency of expanding (or swelling) when moist and of caking and cracking heavily when dry.
Tracks laid on formation of maintain. In rainy season, the soil fills up ballast interest less, the track in the worst places gets sodden and spongy track is reduced. In hot weather, the cracks are formed and the ballast is lost in filling up these cracks. Thus, the alignment as well as level is disturbed and with mud filling the interstices, the track loses. Its resiliency, therefore, for these very reasons, a layer of moorum varying in thickness from 300mm to 600mm is laid under the ballast. This layer distributes the pressure of the load and prevents the ballast from being lost in the cracks of the soil.
Instead of moorum, other materials such as ashes, concrete, slabs, rubber, unserviceable sleepers etc are also used and are found quite satisfactorily
In this method, steel tubes of 30mm in diameter and 1.5m long are driven into the formation at every alternate sleeper and near their ends as shown in figure. The tubes are driven into the foundation at an angle such that the end of tube is nearly under the rail. The cement grout is forced under a pressure of 0,7 N/mm2 through these tubes. The proportion of cement grout depends on the type and condition of formation. The concert grout spreads through the poor soil and consolidates it. The steel tubes are then gradually taken out.
This method of strengthening the track laid on poor is most widely used in development countries like America. In this method, a vertical bore about 300mm diameter is made in the ground by driving a wooden pile. The wooden pile is then withdrawn and the space is filled with sand and is well rammed. The sand piles are driven in the pattern as shown.
It is also arranged that cross sectional area of the sand piles is about 20% of the formation area. Thus, the top section of the formation is covered with sand which makes the track stable on poor soil.
Use of chemicals
In this method, chemicals are used in place of cement grout to consolidate the soil. For example, silicate of soda followed by calcium chloride is effective for sandy soils containing less than 25% of silt and clay.