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.
As the excavation proceeds, the sides of the trench should be supported by proper timbering fixed by "experienced and competent timbering". Fig. 403 shows a cross-section of a trench fully timbered The walings of 3-ineh spruce planks are first set up and secured by stretchers of good strong square or round d fir or (in narrow trenches) of larch. The poling boards are of elm or spruce cut into 3-feet-f,-ineh lengths, with the toes sharpened and driven in with a mall behind the walings as close as the nature of the ground requires. The lov timbers are inserted in the same manner as the work proceeds. In case runninng sand is met with, the poling boards should be wedged close together and additional stretchers inserted, and behind the poling hoards, litter, rushes, or brackm should be packed as tightly as possible, and added as required. Occasionally this running sand is so difficult to deal with that a second and inside course of poling boards is required to resist the pressure. The timber within each length of walings is called a " setting", and in difficult ground the work should proceed by settings, one being opened out as the preceding one is being filled up. This is naturally a costly process, but it is a safe one. In certain cases, to support property, some of the timber will require to l>e left in the trench, and when this-is done, such timber should be accurately measured; but if the necessity for leaving it has arisen from carelessness or neglect on the part of the contractor, any timber so ordered to be left should not be paid for. The timbering should be strong enough to carry the staging necessary to hold the weight of mate rial thrown up at each "lift" and the men working upon it. Tin- stagings are usually from 4 feet to 5 feet apart vertically. Water and gas pipes and smiliar things will require supporting across the trench so as to prevent their fracture. These are sometimes slung to the top stretchera by chains, bo that these walings must not be struck as the trench is being refilled. Extra timbering will likewise be required in the shoring up of walls and buildings.
Fig. 403- - Method of Timbering Trench.
Before laying the pipes, the trench must be neatly formed to the shape of the drain to be laid, proper socket-holes being cut so that each that may be truly and firmly bedded throughout its entire length, all irregularities resulting from bad workmanship being filled up with gravel or concrete. To obtain a true alignment of the pipes, the centre of the trench should be carefully notched on each sight-rail, and a stout cord connected to each mark from sight-rail to sight-rail. From this a plummet can be suspended over each pipe as required. Whenever the pipes require slightly raising so as to be true to the gradient, the packing must be done with the very finest material. If the foundation appears liable to give way when the trench is refilled a foundation of concrete should be formed, and carried up at least half the height of the pipes, so that these may be securely bedded in it, and the sides neatly levelled as shown in Fig. 404. The concrete must be lowered to the bottom of the trench in buckets, and properly placed in position; for if it is shot from the top of the trench, the stone will tend to separate from the mortar.
Fig. 404. -Concrete cradle for pipes.
Special care must be taken that drains are not laid in newly-made ground, as they are sure to subside along with it, thus fracturing the joints. The excavation in this kind of ground should be carried down to the solid earth, and a substantial wall of concrete or of dry rubble should be built up to the requisite level in order to ensure a firm foundation for the drain, and the pipes, after being placed in position and jointed, should be securely wedged on each side to prevent them rolling when the trench is being refilled.
After the pipes have been adjusted to the exact line and gradient, the jointing of the pipes should be proceeded with, according to whatever process is adopted, but no pipes should be covered up until they have been examined end tested for water-tightness. The drains should be kept as free as possible from rubbish and water daring construction, and iron drains should always have every length brushed out before jointing, and upon completion they should all be cleansed and left entirely free from rubbish of every kind.
In refilling the trench, the best material should be selected to go in first- such as sand or fine gravel, - and this must be carefully placed and compactly rammed round the pipe without disturbing the line or fracturing the joint. When there is not less than a foot of cover over the pipes, the water used in testing may be let off, the discharge being observed in order to l>e sure that all the pipe- were fully charged, and that the test was a fair one. The trench can then be refilled in layers not more than 9 inches thick, and well punned, two rammers following one filler, but no clay should l>e brought within 12 inches of the surface. The supports for gas and water pipes will require to be left until the trench has become quite consolidated. It is better not to reinstate the surface-materials immediately, but to cover off with good ashes or gravel. Settling will probably take place at the rate of 1 inch per foot in depth, even with the best rammed refilling, and with clay the rate will be even more, as it is very difficult to get it compactly together unless it has "fallen". When consolidation appears to have been effected, the surface-materials should be reinstated and made good.
The first paragraph of By-law 62 prohibits the passage of drains under buildings except where no other mode is practicable, and the next paragraph prescribes that such a drain shall be laid in a direct line for the whole distance beneath the building, and be completely embedded in and covered with solid concrete at least 6 inches thick all round. It has already been pointed out that in certain cases, especially in town buildings, some drains must of necessity be laid under the building, and where there is no access to a back road the drains of terrace-houses are bound to be carried from back to front. It frequently happens that where drains are laid under a building, through some failure of a joint the sewage escapes and runs underneath the pipes into the subsoil, causing very serious mischief indeed, - and this stats of things is quite likely to occur even where the joints have been made in cement. It does not always follow that the drain should be laid under the Hour of the basement; there are many installers in which economy may be practised by carrying the drain on brackets built into a wall of the basement, and thence into the road to be connected to the sewer. But it is obvious that earthenware pipes would not be suitable for a drain in such a position. Iron pipes only can be used; such pipes completed with a run-lead joint will not only convey the sewage adequately, but will be both water-tight and gas-tight. The fewness of the joint, and the rapidity with which they can be made, are very important advantages in favour of iron pipes, particularly when they have to be laid under a Moor. Iron pipes, as they can be satisfactorily worked under pressure, may be of a smaller diameter than stone-ware pipes, provided no inlets are required for waste pipes about the level to which the drain has to be laid.
This proposal of the Model By-laws to embed the pipes in concrete is a clumsy method, and it would be especially expensive to apply it in the case of an existing building on account of the excessive amount of material requiring to be got out, and also of the cost of the concrete, every ounce of which would have to be earned down in buckets from the outside, and in case a drain required relaying under business premises, the disturbance to business caused by the undue length of time taken up by such a process would be a serious matter. The proposal further leaves out of consideration any improvements which may have been effected in pipes themselves, or in the methods of jointing, during the last twenty years. I have myself had Hassall's double lined-joint pipes laid beneath buildings, and seeing that such a joint can be made to stand a pressure of 100 lbs. to the square inch, it is surely safe enough in the interior of a building, more especially when the ground is covered with a layer of concrete before the floor is put down.
The further proviso in the model code for adequate means of ventilation to be provided in the case of a drain passing under a building is a right one, and no exception can be properly taken to it; if the terminal of the drain is not at a soil-pipe (which could be used for ventilating purposes), a special vent-pipe should in every case be provided.