. Railway maintenance engineering, with notes on construction . , draining off, with the result that mud pockets are formedand the soil works up into the ballast, destroying its difficulty is overcome by introducing a layer of ballast ofsmall particles between the stone and the grade. Gravel orcinders are suitable for that purpose, and while the stone willenter this layer of sub-ballast to some extent, no harm is done,as the water is not expected to drain off of the surface of thesub-ballast, but passes through it to the surface of the roadbed,which being protected from the sha
. Railway maintenance engineering, with notes on construction . , draining off, with the result that mud pockets are formedand the soil works up into the ballast, destroying its difficulty is overcome by introducing a layer of ballast ofsmall particles between the stone and the grade. Gravel orcinders are suitable for that purpose, and while the stone willenter this layer of sub-ballast to some extent, no harm is done,as the water is not expected to drain off of the surface of thesub-ballast, but passes through it to the surface of the roadbed,which being protected from the sharp particles of the stone bal-last, preserves its original surface. Gravel or cinder is not objectionable for first-class track whenused in this manner, as it is covered with from 6 to 12 ins. of stone,which gives the requisite drainage under the ties and preventsthe dust rising from the finer ballast upon which it rests. 68. Sections.—The sections shown in Fig. 22 of the Penn-sylvania roadway illustrate good practice in the use of stoneballast. BALLAST 175. I v^ c^-^ v.^-^- mxn < mom o c3 O a o O CO o 176 RAILWAY MAINTENANCE With gravel ballast a somewhat flatter slope is is also necessary with cinders^ chats, chert and granulatedslag, where the slope should be about 3 to 1. Fig. 103 shows a composite drawing of the ballast sections ofvarious roads with the sections for single and double trackproposed by the committee on Ballast of the American RailwayEngineering Assn., indicated in heavy lines.* 69. —The general requirements of the specifica-tions for the different kinds of ballast may be summarized asfollows: Stone ballast should be clean and durable. It should breakwith an angular fracture when crushed and the pieces shouldpass in any position through a 2|-in. ring and should not passthrough a |-in. ring. The best grade of gravel ballast should not contain morethan 2 per cent, of dust or 40 per cent of sand. When it con-tains more than
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