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Trenching - A
Trenching - AAre there any special trenching requirements advised by our customers for PE pipe? No special trenching requirements are normally needed for PE compared to other materials and the preparation of trenches should follow national or local practices. Sharp stones should be removed from the base of the trench and where laying the pipe across rock or granular soil of angular consistency, such as gravel or cobbles, the trench should be excavated below the required depth to allow the pipe to be laid on imported compacted backfill. It is advisable that PE pipe be surrounded by good quality compacted backfill which may need to be imported. Local codes of practice normally advise the nature of pipe bedding material required. The ability of PE pipe to be joined above ground and snaked into the trench allows for the use of narrow trenches. The trench width can be kept to a minimum but should still allow for good compaction during backfilling. Again it is important to consult local and national practices. Are there any limitations on the depth at which PE pipe can be laid? There are normally no depth limitations related to PE pipe material. The depth limitation to which PE pipe can be laid is not governed by the material properties but by the site conditions, soil type, level of water table, etc. This is no different from other pipe materials. The pipe should be designed to resist any additional forces from soil loading associated with deep excavations. Suitable precautions should be taken to prevent collapse of the trench walls for deep excavations and consideration should also be given to trench drainage if the pipe is to be laid below the water table. Reference should be made to health and safety legislation relating to deep excavations to prevent potential injury to operators. PE pipe can be assembled at the surface and placed into a deep trench without site personnel being in the trench for long periods, or possibly not at all. This is a considerable safety benefit where stability of the trench may be a concern. Is trench alignment critical when laying PE pipe? No. The flexibility of PE allows it to be placed with some variations in alignment to suit local terrain if necessary. The alignment of the trench in the vertical plane is dependant on the application for which the pipe is being used. For example, gas and water PE distribution mains can be laid to follow the contours of the ground beneath which the pipe is being laid. PE gravity sewers, however, should be laid to the specified gradient to ensure their correct hydraulic function. In the horizontal plane the pipe can be laid to follow, for example, the alignment of a road or footpath. However there is a limit to the bending of PE pipes. The minimum advisable bend radius at which PE can be laid is dependent on the SDR of the pipe. To avoid any risk of kinking, buckling and overstressing the following minimum bend radii are recommended. These are for pipes without ovality at 20°C
Example ; the minimum radius of bend for placing a 500mm, SDR 11 pipe is;
It should be noted that joints should not be included in bends of the minimum radius in order to avoid high local stresses at fittings or butt fusion joints. Also it is advisable to increase the quoted minimum radius at temperatures below 5°C
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