Question

The majority of new railways are being built for use with electric traction, with many existing railways also being electrified. Summarise:

(i) The benefits of an electrified railway when compared with conventional (diesel) motive power

(ii) The advantages and disadvantages of the two different types of electrification system

(iii) The practical applications of each type of system. Give reasons for your answer.

(b) Describe some common ways of stabilising failed soil slopes. Explain your answer using sketch drawings where appropriate.

(c) Detail and discuss three aspects of water management and control within railway geotechnics and the implications of mismanagement.

Answer 1

Answer a)

(i)The benefits of an electrified railway when compared with conventional (diesel) motive power are as follows:-

• Electrified railways operate at lower operating costs and higher efficiencies.
• They make less noise, they are more strong, robust and responsive.
• They are best suited for city areas and tunnels for transportation because there are no smoke emissions like diesel engines.
• Electricity can be generated without burning natural resources.
• Due to the regenerative braking systems, some electric traction energy can be reused by other trains.
• It is a better and a viable alternative from environmental sustainability point of view.

(ii)The two main types of electric traction systems that exist are as follows:

1. Direct Current (DC) electrification system
2. 3-phase Alternating Current (AC) electrification system

Some of the advantages of DC electrification system are as follows-

• For same operating conditions, DC consumes less energy than AC system.
• The equipment in DC traction system is less costly, lighter and more efficient than AC traction system.
• It causes no electrical interference with nearby communication lines.
• It has less distribution cost.
• Fewer substations are required.
• Lighter overhead current supply wire can be used.
• Reduced weight of support structure.
• Reduced capital cost of electrification.

Some of the disadvantages of DC electrification system are as follows-

• Expensive substations are required at frequent intervals.
• The overhead wire or third rail must be relatively large and heavy.
• Voltage goes on decreasing with increase in length.

Some of the advantages of 3-phase Alternating Current (AC) electrification system are as follows-

• Their construction is simple and robust.
• They have a provision of regenerative braking without additional equipment.
• They have a comparatively better operating efficiency
• Thye give better performance
• There is quick availability and generation of AC that can be easily stepped up or down,
• AC motors can be easily controlled.
• The requirement of number of substations is less ,
• The light overhead catenaries that help to transfer low currents at high voltages.

Some of the disadvantages of 3-phase Alternating Current (AC) electrification system are as follows-

• They have high starting currents
• They have low starting torque..
• They have a complex overhead structure at crossings and junctions.
• Electrification cost is high.
• Maintenance cost is high.
• Overhead wires further limit clearance in tunnels.
• Upgrading needs additional cost especially in case of  brigdes and tunnels.
• When there are faults in the system, it is difficult to interrupt the currents in high voltage

(iii)

• Suitability of DC electrification system

DC electrical system necessitates AC to DC conversion substations relatively at very short distances. That’s why this system is preferred only for suburban and road transport services wherein stops are frequent and also distance between stops is small.In case of heavy trains that require frequent and rapid accelerations, DC traction motors are better choice as compared AC motors.

• Suitability of 3-phase Alternating Current (AC) electrification system

These systems are adopted where high output power is required and also where automatic regeneration braking is needed. However, these systems do not found much favour compared to other systems.

(iv)Some common ways of stabilising failed soil slopes are as follows:-

1. Soil nailing

A soil nail can be defined as a structural member that prevents the collapse of ground material and retains it by virtue of its self-weight, bending strength, and stiffness.

The internal stability of a soil-nailed system is usually assessed using a two-zone model: the active and passive zone, separated by a potential failure surface. The soil-nailed system ties the active zone with the passive zone with its tensile, shear, and bending force. Hence, it is very useful to use soil nails as a remedial measure to treat unstable natural soil slopes or as a construction technique that allows the safe oversteepening of new or existing soil slopes.

2. Drainage Method

One of the slope failure factors is saturation and pore water pressure building up in the subsoil. If drainage system is provided, the chances of building up pore water pressure and saturation of subsoil can be minimized. This method can be very effective.It is easy to maintain the surface drains, but it is difficult to maintain the subsoil drains. Subsoil drain is mostly found in the retaining structure as weep holes method and cut off drain. In general, this method is used in combination with other methods. Surface drain is capable of discharging more water, especially during heavy rain to avoid the effects of large amounts of water absorption by the slope.

3. Retaining Structures Method

This method is generally more costly. However, due to its flexibility in a constrained site, it is always the most commonly adopted method. The principle of this method is to use a retaining structure to resist the downward forces of the soil mass. Ground anchors or other tie back system may be used together with the retaining structures if the driving forces are too large to resist. This method also involves rigid slope surface protection such as shotcrete, masonry and stone pitching. Shotcrete is applying mortar on a slope surface by a certain thickness. Masonry and stone pitching may stabilize the slope to eliminate the failure at face slope. It also may reduce rainwater infiltration and prevent slope erosion of the slope forming materials. A slope will be relatively stable when its profile (section angle) is kept below its angle of repose. Angle of repose is an angle that maintains naturally to a safe equilibrium by the composing material of a slope. This angle deviates from differing materials depending on compaction, particle size and the nature of the material itself.