PIPE INSTALLATION

Extremely reliable and satisfactory installations can be achieved without difficulty using PRINCE u.PVC Pipes. Provided the special properties of this material are fully understood and taken into account. These are the most important factors to consider.
 
(a) Due allowance must be made for the co-efficient of thermal expansion
(b) Pipe work must be given adequate support, particularly in respect of heavy
      fitting.
(c) In the design of special components, abrupt changes in cross section must
     be avoided, since the material is notch - sensitive.

PRINCE Pipes are generally laid with the crown about one meter below the surface. Allowance is to be made for the relatively high co-efficient expansion of PVC, particularly when laying on a hot day. Such pipes should be allowed to cool off in the trench before making the final connections and completing the backfill. Air release valves should be incorporated at all high points. These values may be screwed into saddles, which can be used for ferrules for service connection. Valves and other heavy fittings should be properly anchored.

EXCAVATION

The trench should not be opened too far in advance of pipe laying and should be backfilled as soon as possible. The width of the trench at the crown of the pipe should be as narrow as practicable, but not less than the outside diameter of the pipe plus 300 mm (12 in.) to allow proper compaction of the side fill. Above the crown of the pipe the trench may be of any convenient width.
 
The trench should be excavated to the depth below the invert of the pipe will allow necessary bedding. Before placing this bedding, the trench bottom should be prepared. All soft spots should be hardened in by gravel or broken stone. Rock projections should be removed. In fine grained soils such clays, silts or fine sand, disturbance and erosion of the bottom of the trench should be prevented by placing a layer of bedding material 75 mm (3 in) thick on the virgin surface on the bottom before permitting traffic.

MATERIAL FOR BEDDING AND SIDE-FILL

Clay should never be used immediately around the pipe, for bedding, sidefill or backfill. It is not possible to compact it sufficiently, and it is liable to swell, shrink and erode.
 
Some other soils, however, as excavated from the trench (such as free draining coarse sand, gravel, loam and soils of friable nature)•may be suitable, but must be capable of being compacted sufficiently to provide support for the pipe. Fills such as hard chalk, which break up when wet, should not be used.
 
Should the material excavated from trench be unsuitable, then granular material must be imported. The most suitable is gravel or broken stone from 10 mm to 5 mm (3/8 to 3/16) in size, since it requires little tamping, but coarse sand, or sand and gravel from 20 mm (3/4 in) down as it comes from the quarry, is acceptable. An excess of fine particles makes the mixtures more difficult compact when damp.

BEDDING AND SIDE FILLING

With flexible pipes it is of great importance that the side fill should be very firmly compacted between the sides of the pipe and the soil sides of the trench sheeting should be partially withdrawn to allow this to be done. Before backfilling ,any leveling pegs or temporary packing should be removed. The thickness of the bedding under the barrel of the pipe should not be less than one-third of the diameter, and a minimum of 100 mm. (4 in) thick. In very soft or wet conditions or where the bottom of the trench is very irregular, this thickness should be increased as necessary, to give a suitable bed.
 
The bedding should be thoroughly compacted in layers not more than 150 mm. (6 in) thick to give a uniform bed, true to gradient, on which the pipe may be laid. Pipes should be laid directly on this bedding. Bricks or other hard material must not be placed under the pipes for temporary support. After the pipes have been laid and tested. further bedding material should be placed around the pipe and thoroughly compacted in 75 mm. (3 in) layers by careful tamping up to the crown of the pipe, eliminating all cavities under the two lower quadrants of the pipe.

The same material should then be placed over the crown of the pipe for not less than two-third of the diameter, with a minimum height of 100 mm. (4 in) and a maximum of 300 mm. (12 in) and be thoroughly compacted. The process of filling and tamping should proceed equally on either side of the pipe, so as to maintain an equal pressure on both sides.

BACK FILLING

Normal filling of the trench should then proceed in layers not exceeding 300 mm. (12 in) in thickness, each layer being well rammed. Heavy mechanical rammers should not be used until the fill has reached a depth of 300 mm. (12 in) above the top of the pipe.

Special consideration and selection of backfilling material will be necessary if the risk of surface subsidence is an important consideration: for example, tinder roads.

USE OF CONCRETE SPECIAL CASES

Generally, the use of concrete with PVC pipes is wasteful, since it converts a flexible pipeline into a beam of negligible flexural strength, which will fracture under minor ground movement. More than 2 Ft. of coyer concrete is normally unnecessary.

Less than 450 mm. (18 in) of cover, elsewhere than under roads, narrow concrete slabs on a cushion of filling material above the pipe should be used as a protection against picks, etc. At shallow depths under roads, etc, special consideration should be given to all the engineering factors involved, such as the class of roads, its construction, and the proximity of other services.
At or above ground level concrete surround should be used to protect the pipe.