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THEORY AND PRACTICE OF ENGINEERING.

Book II.

CHAP. XXX.

railroads.

The steam-engine being the means by which carriages are propelled on a railway, it wouldbe natural that the first men consulted upon the bust method of laying out a line shouldbe those most acquainted with the uses of the new motive power ; but time and experiencehave extended this knowledge, and created many practical civil engineers, whose addedstores of information, skill, and ingenuity, will, it may be fairly hoped, very considerablylessen the expenditure for all future railroads. The first operation in these new nationalworks is an accurate survey of the country to be passed through, which should extend to aconsiderable distance on each side of the intended line : there should be scope enough lefteither way for the engineer to benefit from the levels of the land he is desirous of crossing.The land-surveying and the levelling should progress together, one party mapping whilethe other lays down the rise and fall; the cuttings and embankments must next be calculatedand made to balance as nearly as possible, care being taken that sufficient land is mappedto extend the base of the embankments, and, where deep cuttings are uecessary, that awidth at top is provided, to allow for the talus or slope, as well as for any casualslips or shrinking to which the various earths may be subject. Should the embankmentsnot require all the earth from the cuttings, proper situations must be secured for the depositof the surplus; and, if the cuttings do not afford sufficient for the embankments, the earthmust be obtained at the nearest and most convenient spot: these and various other pointsdependent on locality must be well considered at the time of laying down the map, that,when the period of execution arrives, there may be no obstacle to the pre-arranged plans.Before the engineer can finally determine the width of the respective cuttings, it will benecessary to examine into the nature of the various earths through which his operationsare to be carried, as upon this must depend the quantity to be removed, and consequentlythe widths to be set out. When a cutting is made through clay, the slope in some in-stances is only required to be 1 to 1 ; but if in a constantly wet state, it will slip, evenif 3 to 1 is given; soapy shale will slide when at a much greater slope than 5 to 1 :chalk will stand nearly vertical, and the more it approaches that position, the less it willbe affected by the weather. Sand has been found to stand with a slope of 1 to 1 :on the Newcastle and Carlisle Railway, where the depth of cutting is upwards of100 feet, it stood well with the talus of 1^ to 1. Gravel and dry soils stand wellwith a slope of \\ horizontal to 1 perpendicular, when the height does not exceed 40feet; but in wet seasons the greatest caution is required to make most of the soils stand,whatever slope be given them: 8^ feet in width requires for each mile in length an acre ofground; 16 feet 6 inches, or a rod in width, requires two acres, there being 320 rods to alineal mile, and 160 square rods to an acre. Thus there is no difficulty in ascertaining thequantity contained in the cutting for a general estimate; when the slope and the depthshave been determined on, the mean width multiplied by the depth will give the cubicalcontent. Where the embankments should cease, and the construction of a viaduct begin,or where the open cutting should give way to the tunnel, are points of great importancefor the decision of the engineer.

As it is usual to estimate all earth-works by the cubic yard, it is advisable, whereverpracticable, to take all the dimensions in yards, and their decimal parts; and, by calculating4840 square yards as making an acre, and 1760 yards in length a mile, there would be agreat facility afforded in arriving at the desired quantity. Numerous tables are, however,constructed, by which the quantities are ascertained, without the trouble of calculation. Itmust be remembered that clay, as well as some other earths, will require a larger quantityof cutting to form the embankments than they cube to ; whilst with some common earthsthere is an increase of at least ten per cent, in the quantity, when first used, and before ithas time to become consolidated.

When the railroad is made, it must be entirely free from water: drains must be cuton each side, and conducted to watercourses constructed so as readily to receive all thatpasses by them: in soils like that of the Chatmoss, varying in depth from 10 to 34 feet,and resting on a substratum of clay and sand, considerable difficulty is experienced informing the embankments as well as the cutting- In that instance the operations of theengineer extended over a length of 4 miles ; drains were cut at every 5 yards, and as soonas the'water had passed off, and the moss become dry, it was removed, and the embank-ments formed of it, four times as much being required in the dry state to fill up the same