Deep screening of the ballast is done to ensure that a clean ballast cushion of the required depth is available below the lower half of the sleepers, which is necessary for providing the proper drainage and elasticity to the track. In the absence of a clean ballast cushion of the desired depth, track geometry may get disturbed, affecting the performance of the track.
Deep screening is normally carried out under speed restrictions without obtaining a traffic block. It is desirable to proceed with deep screening in a direction opposite to that of the traffic. An adequate quantity of ballast, the necessary equipment and tools, and the required labour should all be arranged well in time. Work should not be carried out during monsoons. The procedure for deep screening is as follows.
(a) A group of four sleepers are tackled one at a time in sequence. As shown in Fig. 18.2, each of the sleepers (numbered 1, 2, 3, and 4, etc.) is tackled individually.
Fig. 18.2 Deep screening of ballast
(b) The ballast from spaces A and B is removed right down to the formation and wooden blocks are put in its place to support the track. Precautions are taken to avoid digging out the consolidated top of the formation.
(c) The ballast is removed from under sleeper 1, screened, placed under the same sleeper, packed.
(d) The wooden block is removed from space A.
(e) The ballast from space C is removed and placed after screening in space A. If additional ballast is required, it may be taken from the extra ballast trained out in advance along the side of the track.
(f) The wooden block that was removed from space A is placed in space C. Sleeper 2 is provided with wooden blocks on either side in order to support the track.
(g) The ballast under sleeper 2 is removed, a screened ballast is provided in its place, and the sleeper is packed.
(h) Ballast from space D is removed, screened, and placed in space B. Any extra ballast that my be needed is taken from the track.
(i) The wooden block from space B is removed and put in space D to support the rail.
(j) The procedure is repeated till the work is completed.
It can be seen from this procedure that work is done in such a way that when the ballast is being excavated from under one sleeper, there are at least four sleepers between this sleeper and the next sleeper that is being worked upon. The track is also lifted, if required. This should only be done at a rate of 50 mm (2") at a time. It should be ensured that the packing, cross level, and grading-off are satisfactory before closing the day's work.
Deep screening is done under a speed restriction of 15 km/h. This speed can be released to normal speeds in the following ways.
Manual packing By manual packing four times in succession and by picking up slacks over a period of 21 days.
Machine packing Packing with the help of tie machines, by tamping the track thrice followed by picking up slacks over a period of 10 days.
Track drainage can be defined as the interception, collection, and disposal of water from the track. It is accomplished by introducing a proper surface and sub-surface drainage system. This subject has been discussed in detail in Chapter 19.