The hydraulic hoe or backacter is the machine most widely used for trench excavation for pipelines. In hard ground, rock or roads, the trenching machine might be used which has been described in Section 18.1. Depths for water and gas pipelines are usually the pipe diameter plus 1 m. For sewers, greater depths are often required to maintain falls. When flexible plastic pipes are used, especially in the smaller diameters, pipe joints can be made above ground, the pipe being snaked in. Bottoming of the trench can be achieved by using a straightedged bucket without teeth, and the backhoe can also place soft material or concrete into a trench on which to bed pipes or fully surround them. Provided no men are allowed in the trench, timbering can thus be avoided. When large diameter steel pipes with welded joints have to be laid, a string of several pipes may be welded up alongside the trench, and dozers equipped with side lifting booms can lower the string of pipes into the prepared trench. This reduces the amount of timbering and excavation of joint holes necessary which need only be arranged where successive strings have to be jointed together.
The principal defects occurring on pipelines come from defective joints and pipe fracture due to settlement of a pipe on a hard band, large stone or lump of rock in the base of the trench. The use of the hydraulic hoe makes the preparation of an even bed for the pipe easier to achieve, especially on suitable selected soft granular fill. However, the base of the trench and the bedding along each length of pipe must be carefully boned in before the pipe is lowered to ensure each pipe is fully supported along its body.
For non-flexible pipes of ductile iron, asbestos cement, steel or concrete it will be necessary to joint them after laying. Sufficient access is then required for the jointer to make the joint properly, and support to the trench sides will be essential in every case where there is not absolute certainty there can be no slip of material into the trench. Falls of material into trenches are a major hazard in civil engineering, and adoption of a consistent, rigorously applied safety approach is the only way to prevent accidents. The damaging weight of even a small fall of earth must be borne in mind.
While it will be obvious that gravity sewers must be laid to a fall, it is sometimes not appreciated that pressurized trunk water mains should be laid to a minimum rise or fall. The preferred minimum gradients are 1:500 on a rising grade in the direction of flow; and 1:300 on a falling grade. The former would be to an air release valve, the latter from the air valve to a washout or hydrant. Thus the levels of ground ahead of the pipelaying must be prospected to locate suitable high and low points, and intermediate points where an increase or decrease of grade is necessary. The pipeline between such pre-determined points should follow an even grade. In flat ground it may not be possible to comply with the foregoing grades, but it is still advisable to give uniform rises to air valves and falls to washout positions. In built-up areas pipelines can generally follow the requisite cover below ground surface because branches and connections will release air, and hydrants will be used as washouts.
Backfill to pipes should always be of selected soft or fine granular material to 150 mm above the crown of the pipe. Few contractors in UK would fail to do this, but on some contracts overseas the resident engineer may need to stop the contractor from dozing the excavated hard material straight back into the trench irrespective of the rocks it contains which would at the least damage the sheathing to pipes.