GENERAL

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 :

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.

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 :

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.

REQUIREMENTS OF TRACK DRAINAGE SYSTEMS

1. The surface and underground waters should be well away from a track;
   the banks and cuttings; over or through which they run.
2. The surface water from adjoining land should be prevented from entering the track-formation.
3. The side drains should have sufficient capacity and longitudinal slope to carry away all the possible surface water.
4. Flow of surface water across the track and along the slopes should not cause erosion of the banks and slopes of embankment.
5. Sub-surface water should be efficiently drained off by the sub-surface drainage system.
6. The highest level of ground water table should be well below the level of the subgrade.
7. 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.
8. In water-logged areas; special precautions should be taken especially if detrimental salts are present or if floods are common in the area.
9. 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
     1. wet earth gets into the ballast
     2. ballast sinks into the wet earth.
10. 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.

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.

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
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.

(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:

1. Existence of an underground water-pocket due to hollow basin over thick-impervious layer.
2. Existence of an underground water-pocket over a thin impervious stratum which lies over a fissured strata (i.e.; good pervious soil).
3. Presence of water bearing strata on side-long cuttings and banks i.e.; seepage flow.
4. In case of black-cotton soils or expansive soils.

The following remedial measures are suggested for above special cases :

Case I

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.

Case II

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.

Case III

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.

Case IV

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.

Cross-drainage

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 :

1. wet earth clogs (or gets into) the ballast
2. 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.

1. Only remedy to this defect is to improve the formation. The formation can be improved by two ways:
2. 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).
3. 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.

SUMMARY

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.

SURVEYS FOR TRACK ALIGNMENT

In a railway project from beginning till its construction, four different surveys in stages are required to be conducted. The following survey should be carefully conducted to fix a best possible alignment :

1. Traffic survey
2. Reconnaissance survey
3. Preliminary survey or Survey for initial location
4. Detailed survey or Survey for final location

Before the actual work of survey starts, study of available maps of the area should be made. This desk study helps in drawing the various suitable alignments on the available maps in the office to facilitate easy reconnaissance. For this purpose, generally, the survey maps of India or Aerial maps are used. These maps show the contours, topographical features and physical land utilization like buildings, cultivation, rives, etc.

So, tentative alignments are fixed and drawn in the office which further help in conducting various surveys in a systematic manner. The contour maps are very much helpful to the engineer for fixing gradients within the ruling gradients by providing the suitable length of line within two contours. This study of maps prior to actual survey work is known as map study or paper location.

TRAFFIC SURVEY

The object of traffic survey is to make accurate determination of the potential of available traffic. This is essential for determining the viability of a new line proposed either a branch line or completely a new system, to assure reasonable return on the large investments likely to be involved. Traffic at present and future potential traffic, both passenger and goods traffic, should be considered.

This traffic can further be local as well as of through type. Therefore, the engineer should survey certain features, like the position of traffic generation sources which would furnish control points for general location of the line; the volume of traffic for selecting type of track (B.G./M.G./N.G.) construction which can justify revenues. The design should include sufficient provision for estimated growth of traffic.

For conducting the traffic survey,

These factors should be considered :

1. Census of population with reference to prosperity of the people and the locality, their density and distribution.
2. General resources of production which include agricultural and industrial goods. The nature and weight of different commodities, likely to be transported as a result of a new line, should be studied. The origin and destination of such commodities should be correctly surveyed and assessed.
3. The general character of lands and people of communities must be considered. A rich fertile land is expected to bring a large volume of agricultural products. Mineral and forest wealth also assures the goods traffic from these sources but this traffic will diminish when their natural resources have been exhausted in due course of time. Rural centres have a slow and steady traffic but it is seasonal and fluctuates with crop and market conditions.
4. A single prominent feature may be responsible for building a new line or construction of additional kilometrage as an entry of new line into places with flourishing industries, newly discovered or opened mines or oil fields, etc. should be collected which can attract the traffic.
5. The general information of fairs, recreation centres, business, religious festivals, etc., should be collected which can attract the traffic.
6. It is important to study the traffic history of existing tracks or road transport facilities available or likely to develop, and how much traffic they are likely to attract.

More factors should be considered :

1. The position of traffic, its nature and potential, which would be served by a new line, should be finally estimated.
2. The nature and volume of exports and imports, with centres of their original destination, should be studied.
3. Future possibilities of the development of trade centres, industries and agriculture should be investigated.
4. The station facilities should be properly located with the track alignment as the proper station location will get more business. In addition to above factors, the following practical considerations should also be taken into account.
5. Changes in pricing of products, i.e., freight rates may seriously affect the traffic movements. The purchasers may change in habits and routing.
6. Relocation of industries and populations, due to changing technologies, depletion of raw materials, changes in consumer habits or for purposes of national defense, may change entirely the volume of traffic moving over certain track routes.

Field surveys are carried out to collect the information on above factors and following important maps are prepared for the study –

• Topographical Maps – Showing all physical features viz. land, buildings, rivers, bridges, tunnels, existing routes, contours, etc.
• Agricultural Maps – Showing only the land, fertility value and agricultural products.
• Industrial Maps – These maps show the location of industries, their nature and further development plans.

RECONNAISSANCE SURVEY (Recce)

This is a rough and rapid inspection, both visual and instrumental, of various physical characteristics of the area to determine the suitability of different alignments marked on the available map during map study or paper location. So in this type of survey, the suitable two or three alignments are selected for further preliminary survey.

The objectives of reconnaissance survey are as follows:

1. Acquire the knowledge of physical features of the country like the rivers, valleys, cultivated lands, forests, hills, existing roads, canals, etc., for selecting the proper position of alignment.
2. Collect geological information regarding the following points. (a) Nature of soil, (b) Surface formation of the ground (c) Dip of the existing rocks, and (d) Hill slopes.
3. Collect the information regarding availability of constructional materials, labour and sources of water as permanent facilities for the proposed alternative alignments.
4. Have an idea about possible alternative alignments.
5. Have an idea of rivers and streams which may cross the proposed alignments for determining suitable bridge sites and their bridging requirements.
6. Locate various control points or obligatory points for getting an idea from where the alignment should pass and from where the alignment should not pass.
7. Decide the maximum gradient and curvature for proposed alignment.
8. Prepare rough estimates for different proposed alignments to know most economical, safe and efficient alignment.

An engineer should bear in mind the following factors in reconnaissance survey :

1. The reconnaissance survey should be done for the whole area influencing the railway project, particularly for wide belt on either side of the general direction of alignment rather than for a line only.
2. All the possible alignments marked on the map, during map study, should be examined and improvements made if necessary.
3. All the intermediate points should be very carefully fixed so as to attract maximum traffic and hence more revenue; less construction problems and hence economy in construction. To achieve this, the following points should be considered :

(a) Rivers should be crossed at right angles and at those places where approaches are sound and approach banks are not very high.

(b) Mountain passes should be so located that they could be reached without steep gradients or deep cuttings.

(c) A tunnel may also be proposed if very economical.

(d) The station site should be located within 2 km from the existing town or village and at level stretch of land.

4. The various elements governing the alignment such as gradient, curvature, rise and fall, traffic, etc. already discussed should be thoroughly studied.

Instruments :

The following instruments are used for conducting the reconnaissance survey:

• A Prismatic Compass – This is used to obtain the magnetic bearings of the proposed routes in different directions.
• An Abney Level (or Hand Level or Clinometer) – This is used to determine the gradients or slope angles of the ground upto an accuracy of 10 minutes.
• A Barometer and a Thermometer – This combination is used for accurate determination of altitudes and temperatures of the various points.
• A Pair of Powerful Binoculars or a Telescope – These are used for inspecting the distant physical features to get an idea of the topography of the country.
• A Pedometer – This is used to have rough idea of the distance.

At the end of reconnaissance survey, a map with 6 m to 15 m contour intervals showing all the physical features and two or three suitable alignments is prepared. Out of these alignments, best one is selected further on the basis of preliminary survey.

PRELIMINARY SURVEY (Initial Location Survey)

The object of this survey is to determine the details of different alternative routes as found and drawn in reconnaissance survey and at the same time economies of different routes are studied.

This preliminary survey should be conducted with great precision as the selection of final alignment is based on this survey.

Working of preliminary survey is as follows :

1. First of all, a traverse survey in a belt of about 100 to 150 m width on either side of the centre line, with a transit instrument is done.
2. A techometer is used for plotting the main features ; a chain, prismatic compass and levelling instrument are used for fixing details.
3. Finally drawings and details are prepared for each alignment with respect to following information :

Information :

(a) Length of alternative routes,
(b) Various possible gradients, along the alignment.
(c) Quantity of earthwork.
(d) Maximum heights and lengths of embankments and cuttings.
(e) Characteristic of rivers to be crossed such as H.F.L., L.W.L., flow direction, stream profiles, cost of boring encountered and difficulties in alignment.
(f) Geological information like soil type, rocks and slips, if any.
(g) Details of existing structures viz. bridges, tunnels or culverts.
(h) Details of canal crossing, if any.
(i) In case of level crossing with road, the angle of crossing, longitudinal sections of road for 300 m on either side of crossing and traffic intensity on road.
(j) Availability of materials, labour, drinking water facilities etc.
(k) Details of various affected properties with details of owners to help in acquisition of land.
(l) The climatic conditions of the routes traversed.

4. In the end, a comparative study of various results is made with relative merits and demerits. The route, which is most economical and best from all considerations of an ideal alignment, is selected and plotted on the map.

Instruments :

Various instruments used in preliminary survey are as follows:

• Theodolite – for traversing.
• A Tacheometer – for plotting main features.
• A Dumpy level – for drawing the longitudinal sections and cross-sections.
• Plane table – for plotting interior details.
• A prismatic compass – for magnetic bearings of routes and main points.

The choice of survey instruments will largely depend upon the character of country. In recent times various electronic survey instruments like Total Survey Station are used for collecting all types of survey data. This survey plates can be processed by use of computer soft wares for preparation of various design drawings.

DETAILED SURVEY (Final Location Survey)

The object of final location survey is to transfer or refix the final location of alignment from paper to the ground, in order to carry out the ground survey of this alignment in detail. Before getting the sanction of a railway project, a detailed survey is necessary. Moreover, this survey gives all the required data to the construction engineer such as levels, bench marks, reference pillars, physical features, their measurements, etc. for construction of the track.

The survey work is carried out as below :

1. The centre line of the alignment is marked on the ground by means of fixing about 15 cm long pegs at every 30 metres interval and about 60 cm stout pegs at every 300 metres chainage. All the points, where a change in direction occurs, tangent points of the curve and also the intersection points, are distinctly marked on the ground. Masonry pillars are built around gaps at the tangents points and at around 1500 m chainage. A number of bench marks are established at chainage less than 800 metres to ease the process of checking of levels and to provide gradients. The levels are also marked on the pillars surrounding the pegs, known as reference pillars.
2. The centre line of waterways of culverts, starting and ending points of bridges, the centre line of tunnel, and position of station buildings, station yards, signal cabins, etc., should be clearly marked, on the ground.
3. The demarcating lines of the alignment, according to land width, site of stations, yards etc., should also be marked.
4. Levelling is done along the alignment with the help of precise levels at 30 m, and cross-section are taken at 90 m interval but at closer intervals in case of steep slopes. Longitudinal and cross levelling is done to ascertain the final gradients of alignment.
5. The magnetic bearings of each tangent should be taken at every curve in case of flat-country terrain.

6. The following data is collected for the detailed survey:

• On major bridges, cross-sections are taken at bridge-site and at 0.8 km to 1.6 km upstream and down-stream of bridge-site. In case of minor bridges, cross-sections are taken at bridge-site and at 90 to 150 m upstream and down-stream of bridge site.
• The longitudinal sections of the bed of the stream or water course are taken upto the two extremes of cross-sections on up stream and down stream of water course. The levels, H.F.L. and L.W.L., are shown on the longitudinal sections taken.
• The record of floods in the past i.e. flood history data should be collected and a drawing for river-banks along with the contours on it be prepared.
• The current velocities and depth of scour of the river should be measured.

• The discharge of the river should be determined with the help of velocity of flow and catchment area (either by inspection or by survey map) for that locality.
• Sub-soil boring is carried out along the alignment. The number of bore-holes depend upon the length of bridge. For large bridges, boring at every pillar position and at banks is carried to a depth more than the depth of foundation. In case of small bridges, three borings one at centre and two borings, one at each bank, are carried out to a depth of 15 metres. The data of boring is useful during the construction and maintenance.
• The history of river along with its characteristics such as nature, character, frequency of floods, duration of flood etc. should be recorded.
• The properties of materials, at bed and banks of the river, should be recorded, to assess their erosition characteristics.

7. After collecting the data, the following details should be worked out before starting construction :

(a) Total discharge to be considered, factors affecting discharge, various formulae used and variations in their result.
(b) Calculation and fixation of the waterway.
(c) The type of foundations recommended to be used with reasons and depth of foundations.
(d) Length of bridge and span arrangement adopted with reasons.

8. For detailed survey following instruments are used :

(a) A Theodolite
(b) A Precise level or Dumpy level.
(c) A steel type etc.

In recent days Total Survey Station, an electronic instrument is used for survey work, the data of which can be processed using computer softwares for preparation of construction drawings.

SURVEY DRAWINGS AND PROJECT REPORTS

After conducting all types of surveys, the following construction drawings should be ready to start actual construction work:

(i) General overall map of that area of the country which has been traversed. This is prepared on scale : 1 cm = 20 km. (i.e. 1:20,000,000)

(ii) Index map. scale : 1 cm = 2.5 km. (i.e. 1:2,50,000)

(iii) Index plan and section. Scales : 1 cm = 640 m (Horizontal)
1 cm = 12.5 m (Vertical)

(iv) Detailed plan along the alignment. This consists of longitudinal section and plan.
Scale : 1 cm = 50 m. (Horizontal) (i.e. 1:5000)
1 cm = 5 m. (Vertical) (i.e. 1:500)

(v) Contoured plans and longitudinal sections of all bridges with span 9 m or more to a suitable Scale.

(vi) Plans of station yards : Scale 1 cm = 50 m.(i.e. 1:5000)

(vii) Detailed drawings of buildings and bridges : Scale 1 cm = 1 m. (i.e. (1 : 100)

(viii) Plans of level crossings or grade separated crossings. Scales : 1 cm = 50 m. (i.e. 1:5000)

(ix) Details of all the important features, such as bridges, rivers, roads, canals, air bases, etc. lying within 300 m on either side of the centre line, should be supplied, and drawings should be prepared to a suitable scale.

At the end of the railway project, the engineer must prepare the project report under these headings :

1. Introduction. Historical and geographical background.
2. Alignment Elements. Gauge, obligatory points, gradients, curves, length and levels of different points.
3. Alignment. Basic requirements and factors to be considered for good alignment.
4. Alternate Routes for Alignment. Their merits and demerits with economic considerations.
5. Proposed Alignment. Detailed survey, preparation of drawings and collection of information, etc.
6. Construction. Standard of construction, depending upon type and potential of traffic, locomotive performance and economic considerations, must be described and discussed.
7. Conclusions and Recommendations.
8. Estimation of Railway Project. Finally an estimate of project, considering all capital investments and working expenses, should be prepared.

SUMMARY

The state-of-art of surveying in railway projects is very important. Money spent in survey-work is non-recoverable and every effort should, therefore, be made to conduct survey work as precisely as possible. The four important types of surveys include Traffic Survey, Reconnaissance Survey. Preliminary (or Initial location) survey and detailed (or final location) survey. The instruments used in surveying should be very accurate. Based on the above surveys detailed maps are prepared and information collected is presented in form of drawings and a project report.

DEFINITION

Interlocking is defined as the technique achieved through mechanical or electrical devices or agencies by which it can be ensured that before a signal is taken to “OFF” position, for the route, which the signal controls, is properly set and held, and at the same time all the signals and points, the operation of which may lead to conflicting movements, are locked against the feasibility of such conflicting movements.

NECESSITY AND FUNCTIONS OF INTERLOCKING

In signalling operations, it is absolutely necessary that only that signal should be lowered for which route it is meant and the route has been
advance. Interlocking is the device which helps in proper and safe working of the signals. With an increase in number of points and speed of the trains, it has become essential to eliminate the error on the part of human beings (i.e. cabin-man) and take all the precautions to prevent the possibilities of accidents. This is achieved by interlocking mechanism which makes the arrangements fool-proof.

Interlocking makes the arrangements fool-proof by performing following functions (i.e., Interlocking principles):

1. It must be impossible to take ‘OFF’ a signal for approaching train unless the route (including the over-lap) to which the train is taking, is properly set, locked and held. This means that the points must be set and each facing point is locked so that it finds the passage of the train to withstand the stresses created by the train at the junction of divergence. At the same time, it is necessary that it must be impossible to operate the points (i.e., to unlock or reverse the points) while the train in moving on it; as otherwise, there is danger of the train taking conflicting routes.
2. It must be impossible to take ‘OFF’ position at one and the same time for two fixed signals at the same time which would lead to conflicting movements (i.e., movements which cross each other’s path). So the points and signals should be interlocked against such movements.
3. It must be impossible for loose wagons to interfere with the route for which the points are set and signal has been taken to ‘OFF’ position. For this purpose, it is necessary that levers operating points and signals should be interconnected in such a way that they can be pulled only in a particular sequence or order, and also can be put back in a particular sequence (i.e. reverse sequence or order).
4. The route, for which the points are set and signal taken to ‘OFF” position, should be clear of any obstructions.

METHODS OF INTERLOCKING

The functions (1), (2) and (3) are achieved by mechanical or electrical methods of interlocking of signals whereas function (4) can be achieved by track-circuiting which indicates the portion of the track is whether occupied or clear.

MECHANICAL METHODS OF INTERLOCKING

The mechanical method of interlocking consists of tappets and lock-bars. The tappet bars and lock-bars are placed at right angles to each other. The lock bars have extra metal piece fixed and cut to a shape such that it can be exactly fitted in the notches of the tappet bars.

Following figure shows the simple arrangement of only three levers which explains the principle of interlocking to prevent conflicting movements.

The principle of interlocking is as follows:

1. The normal setting of the points is for main line, where the signal No. 1 indicates the routing signal for main line and signal No. 2 for the branch line (or turnout). The normal setting for the main line is for point No. 3 and signal No. 1 is up.
2. To adjust the track for siding on branch line, the tappet bar for point No. 3 is pulled by lever No. 3 in the shown direction. This pushes the lock bar for point No. 3 out in the direction shown, which makes point No. 3 to be set for branch line.
3. This pushing at the same time releases the lock ‘C’ from tappet bar for signal No. 2.
4. Now, if the tappet bar for signal No. 2 is pulled through lever No. 2, it will prevent the lock bar for point No. 3 to track its path back and in this condition, it is said that signal No. 2 back locks point No. 3.
5. At the same time, lock ‘C’ enters the notch of tappet bar for signal No. 1.
6. So it is concluded that as long as signal No. 2 remains in ‘OFF’ position (i.e., lowered), point No. 3 would keep open for the branch line and signal No. 1 cannot be taken to ‘OFF’ position, (i.e., lowered).
7. Such facilities for interlocking are provided for any number of signals to be operated in the station yard.

MECHANICAL DEVICES FOR INTERLOCKING

The mechanical devices are used mainly to meet the following purposes :

1. A device to ensure that a route is properly set after which only the proper signal can be taken ‘OFF’ and the route cannot be changed after the signal has been taken ‘OFF’.
2. Device to hold a route properly at a diverging point in spite of stresses caused by the train moving over it.
3. A device to ensure that the route cannot be changed while the train is on the point even after putting back the signal.
4. Point and signal levers are provided for locking and releasing one another in different positions so as to ensure correct routing, setting and avoiding conflicting movements.

To meet the above purposes the following mechanical devices are used :

• Detectors
• Stretcher Bar, Lock Plunger and Lock Bar
• Tappet locking
• Slotting of signals
• Connecting devices such as rods, cranks and temperature compensators

(1) Detectors

It is one of the devices to ensure a mechanical relationship between the setting of points and taking ‘OFF’ of the corresponding signal. The detector is so called because it at once detects any defect or failure in the connection between switches and the lever or an obstruction between stock and tongue rail. The signal remains at danger position and cannot be taken to ‘OFF’ position until the defect is set right.

The detectors are used on all points over which signals control the train movements.

The mechanism of a mechanical detector in its simplest form is a box near the points wherein the bars from signal wires are placed at right angles to the bar from the stretcher bars of points. These bars are called slides. Signal slides are placed at right angles to the point slide and all the slides are suitably held, so that there is no vertical movement in them. The point slide
has a number of notches whereas the signal slide has only one notch.

The functions of mechanical detector are as below:

1. When the points are set for the main route, the notch in the signal slide and the notch in the point slide, are face to face and the pull will be transmitted to the main home signal 3.
2. If the points are reversed, the point slide will move to the left, the notch in the slide of signal ‘4’ and notch of the point slide will be face to face and signal 4′ will be taken ‘OFF’ (i.e., lowered).
3. When signal “3” is taken ‘OFF’, signal “4” is locked and when signal “4” is taken ‘OFF’ signal ‘3’ is locked.

(2) Stretcher bar, lock plunger and lock bar

(i) Stretcher Bar

The only movable portions of the permanent way are the tongue rails at the location where two routes meet and due to stresses caused by a train at the divergence, this point is most vulnerable from the point of view of derailments. The points are known as weak links of the track. The two tongue rails are connected to each other by means of two stretchers, which are known as William patent stretchers. The front stretcher extends under the stock rail to prevent “jumping” at switches.

(ii) Point Lock

A point lock is placed in the middle of the track a little in front of the toes of tongue rail. The object of using a point lock is to ensure that each switch is correctly set. It may consist of one of the following types :

(a) When the speed does not exceed 16 km. p.h. a bolt and cotter is individually fitted to each switch rail and padlock or clamp, and a padlock for locking switch rail to stock rail.

(b) When the speed is more than 16 kmph but less than 48 km. p.h. a key of approved design for locking each rail independently.

(c) When the speed is more than 48 km.p.h. a plunger type of facing lock is used as shown in following figure –

The point lock consists of two stretcher blades, connected one to each tongue rail. These blades slide at right angles to the track as shown in above figure. The plunger moves in a plunger casing. The plunger is worked by a rod in the signal cabin is operated, the crank gives motion to both (i.e., lock bar and plunger) but the plunger moves at right angles to the stretcher blades. When the points are set for a particular route, the hole (or slot or notch) in the blade attached to one of tongues comes opposite the plunger rod inside the plunger casing and rod enters the slot, the point remains locked and it is impossible to move the switches.

(iii) Lock Bar

The lock bar is little longer than the longest where base of any vehicle. This is provided for the purpose that the point may not be operated while the train is on it. This is either of an angle iron or T-iron, 12.6 m minimum for B.G. and 12.0 m for M.G. (equal to longest wheel base), is provided near and parallel to the inner side of the rail. The lock bar is attached to the inside face of one of the rails by means of revolving clips.

At the time, when the rod of the point lock is worked from the signal cabin, the lock bar rises slightly above the rail level and then comes down. So, naturally when the train (or vehicle) is on the lock bar, it is impossible for the lock bar to rise above the level due to flange of the wheel. Thus the point cannot be operated when the train is on it.

(3) Tappet-Locking

This has already been described in above article.

(4) Slotting of Signals

Slotting is an arrangement in which though the lever operating a signal is in one cabin but the actual taking ‘OFF’ of the signal requires the releasing of a control by another cabin. Hence, a signal is to be operated and controlled from two different cabins. Besides this, the other requirement of slotting is that each of these cabins can unilaterally put back the signal to danger position. A simple example of slotting is between two non-block cabins at either end of a station say A and B.

In such a case, before one cabin man say A takes ‘OFF the Home signal for a train, he has to get an assurance that the line is clear for an adequate distance beyond the starter while that area is controlled by other cabin say B, and that from cabin B the information is obtained for setting up the points correctly. This is done through a slot from the other cabin. Following figure shows the cross section of a cabin.

(i) Electrical Slotting

This is of two types :
(a) Cabin-type Reversers.
(b) Post-type Reversers.

In Cabin-type Reversers, if the signal lever is pulled without obtaining the slot, the slotted side provided on the opposite side of connecting gear is pulled upwards and the beam swings about the opposite point as the fulcrum without pulling the signal arm. The slotted side has a notch and if the slot is given, an electrically operated block engages (or fits) in the notch and as the slide is fixed, the beam swings with the end of the slide as the fulcrum and thus signal wire gets pulled as shown in following figure.

In Post-type Reverser, the rod connecting the semaphore arm to the weight, has got two portions which get joined together by a wedge operated by the slot from the other cabin.

(ii) Mechanical Slotting

The mechanical slotting is also of two types :

(a) Three-way slot (3-lever slot), (b) Dis-engager

In 3-way slot, it has got three weights that are operated from separate
cabins. The signal arm is coupled to the balance weight. Two weights are on one side and controlled separately from the cabins; the balance weight being on the other side as shown in below figure.

The signal can go to ‘OFF position only when both the weights go up by both the cabins operating the levers. If on the other hand, any of the levers is put back in any of the cabins; the weight connected with it will go down putting the signal to danger i.e., ‘ON’ position.

In dis-engager, the connecting gear of the lever operating the signal has a break, so in this position, the operation of the lever does not move the signal arm. The connecting gear has a slide in the slot lever which gets engaged in the slide of the connecting wire of the signal lever. Thus a connection between the lever and signal is established. In reverse direction both the slots and signal lever will throw the signal to danger-position.

(5) Connecting Devices and Temperature Compensators

(i) Connecting Devices

The connecting devices between the lever in the signal cabin and switch (or signal). Usually consist of 3.8 cm diameter pipe or solid rod. At every change of direction, a suitable crank is used. These pipes move on rollers in frames fixed in concrete at 2 m to 3 m intervals. The distance between a cabin to points or signals (known as rodding signal) should not exceed 275 metres. There is no limit of this distance when points or signals are operated by power.

(ii) Temperature Compensators

Temperature Compensators are used to neutralise the effect of expansion or contraction of rods due to variations in temperature. All the double wire transmissions used for operating signal points, locks and detectors must be provided with compensators. These are provided at about 24 m distance or less in the line of rodding as shown in following figure. A compensator consists of a pair of cranks (one of acute angle and another of obtuse angle) connected by a pipe upto a wire length of about 365 m, only one compensator is used. But two compensators are installed if the length is more than 365 m.

It is desirable to put the compensators in the middle of the length if one compensator is used. The compensators should be placed at proper positions if they are two or more. For two compensators, they are used at quarter points.

The use of compensators at proper position is very essential. Because the compensator by virtue of its arrangement automatically compensates the variation in temperature.

As shown in above figure, whether point ‘C’ or ‘D’ may move to the left or right. But the points A and B will always remain at constant distance. The expansion or contraction of rodding or pipe lengths is adjusted within the compensator (i.e., between C and D).

SUMMARY

With the increasing number of points, signals, etc. to meet the heavy and rapidly increasing traffic demands at high speeds. It is necessary that chances of conflicting movements arising out of human errors are eliminated in order to ensure safety of passengers and goods. This objective is achieved through interlocking of signals and points. So that only points desired for movement are open and all others are locked, till the operation is complete.

Interlocking plays a vital role in preventing conflicting movements and thus accidents, derailments, etc.

Sometimes, we observe defects in track, let us know how to fix them.