This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The next stage is the cutting of the trench and the laying of the drain. There are two methods of laying the pipe to the gradient required, the one being by the use of the boning-rod and sight-rails, but the more common way of working is to use an ordinary spirit level and a straight-edge The former is the more reliable and exact method, and is in fact the only one that should be permitted, and it is for the Betting out of the required levels at the various points that the work recorded upon the section is of such value. Before describing this method in detail, some reference may be made to the other system.
It is customary to supply the pipe-layer with a wooden straight-edge of a given length; sometimes it is only long enough to extend over one pipe, or it may be sufficiently long to include several. It may be bevelled on one edge to the gradient required, or, as more usually occurs, the two edges may be parallel, one end having a rough-and-ready contrivance secured to it which has been presumably cut to the gauge, or very often a screw or tack is fastened at one end, projecting in the ratio of the fall required; there are also appliances specially made for "levelling" to various gradients, such as the clinometer. There are many weak points about this system which must Km perfectly obvious to everyone who has had experience of it. The position of each pipe only fixed with reference to the pipe previously laid, so that any error however slight is carried forward from pipe to pipe, and in the case of a long drain it is quite possible on reaching the termination to find an error of several inches either higher or lower than the desired point It is also possible for the pipe-layer- although it would be extremely careless - to reverse the straight-edge, and so error would accumulate. Again, after one pipe is laid, it may be some days before the next pipe is connected to it; in the meantime the pipe last laid may have settled, or may have got slightly misplaced, and there is no means of telling whether it is in correct position or not So long as these things can take place, it is always well to employ contrivances that are less likely to he misused by the ordinary workman. In the case of such small works as house-drainage, it is not always possible to have constant supervision by experts; it is well, therefore, to make everything capable of being effectively done under occasional supervision.
The most efficient appliance for securing a perfectly true gradient in pipes is that known as the boning-rod and sight-rail. The boning-rod is an upright piece of wood cut to any convenient length to suit the depth of the drain. It is fitted with a cross-head exactly like a T - square , and at the other end a shoe is screwed on. which fits on to the invert of the pipe as shown in Fig. 395. This shoe should be secured to the rod by angle-iron fixed with screws, so that whenever the rod is required to be shortened, the change is readily made. The sight-rail, as shown in Fig. 396, consists of a cross-bar considerably wider than the trench, secured to uprights firmly placed in pipes well away from the edges of the trench, so as to be clear of any movement which may be caused by a sudden collapse of the sides of the trench. Two sight-rails are required, one at the point where the drain starts, and the other at the point where the direction or gradient is changed, and intermediate ones can be placed wherever convenient Assuming the distance between manholes Nos. 1 and 3 on the plan to be 100 feet, and the gradient to be 1 in 50, that would be a rise of 2 feet; assuming the reduced level of the invert of the sewer at No. 1 to be 24 feet, and the surface-level to be 32 feet, or a difference of 8 feet, then 11 feet would be a very convenient length for a boring-rod. The sight-rail, therefore, at No. 1 would be fixed on a level of 35 feet, and that at No. 3, 37 feet. We shall now see what assistance we can derive from having taken careful levels in the firet instance. In setting up the sight-rail when the level is fixed, the operator would commence by taking a back-sight upon his nearest temporary bench-mark, and would transfer that level to the upright of the night-rail, adding the difference required. Supposing, for instance, that at No. 1 the bench-mark read 3373, we should require to add 1.27 to give us the value 35, which would lie the height of the sight-rail; in the same way, supposing the benchmark at No. 3 to be 34.86, we should require to add 214 to get the height of the sight-rail at that point, namely, 37. The levels of the sight-rails may also be fixed by placing the level midway between the two points, fixing the boning-rod in the invert of the pipe, and letting the assistant hold the staff on the cross-head of the boning-rod; the operator then takes the reading, and the assistant afterwards holds the staff on the vertical upright to which the sight-rail is to be secured at the next point, raising it or lowering it until tin-same level is read by the operator. Two feet must then be added for the fall which is required for the drain, and the sight-rail securely fixed level across between the uprights. A section illustrating this method of setting up sight-rails is shown in Fig. 397, from which it will be seen that the sight line is exactly parallel to the line of the proposed gradient When the boning-rod is placed on the invert of the pipe, the cross-head should be exactly in Hue when looking over from one sight-rail to the other. As the work proceeds, the boning-rod is held on each pipe in succession, the pipe being adjusted until the cross-head falls exactly into line. The position of each pipe, therefore, is fixed independently of the preceding one. This method is as easy to work as any other, and it is certainly the most accurate.
Boring rod Fig 396._ Shoe for Boring.
Ftg. 396 - Sight-rail for Laying Drains.