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Water test is not intended to reassess the individual pipe performance because this should have been done in the manufacturing process. Instead, it serves to check the occurrence of faults during pipe laying process and to ensure that the pipeline can resist the internal hydrostatic service pressure it will be subjected to.
Autogenous healing is common in underground drainage pipes because of the presence of water on either side of the pipes. The non-moving cracks in concrete pipes are sealed by calcium carbonate crystals from carbon dioxide in air and calcium hydroxide in concrete in the process called autogenous healing. The healed cracks are impermeable and behave even stronger than the original.
Small diameter precast concrete pipes are normally manufactured by spinning method. The spinning method basically makes use of the principle of centrifugal forces which diminishes towards the centre of precast pipe. Hence, problems like the presence of voids and variation of dimension occur frequently and remedial works like filling of voids by cement mortar has to be carried out depending on the severity of deficiency.
Concrete surround is normally adopted for rigid drainage pipes to resist high traffic loads (e.g. under shallow covers) and to allow for using pipes with lower strength. Moreover, the use of concrete surround can minimize settlement of adjacent structures. In addition, the highest possible accuracy in levels and gradient can be achieved by using concrete surround as considerable settlement is expected in other types of beddings like granular bedding.
By using standard precast units of Y-junction branch pipelines, it is beyond doubt that joints between branched pipelines and main pipelines are properly formed and the quality of joints is relatively less dependent on workmanship. However, it suffers from the problem that with fixed precast units of Y-junctions, sometimes it may be difficult for contractors to determine the precise orientation of specific angles of Y-junctions with respect to gullies. (e.g. gullies are connected through side branches to carrier drains)
In the middle third of the base of precast concrete pipes, the bedding layers are recommended to be left uncompacted because it helps to reduce the reaction force at the invert of the pipes and intensifies the effect of shear forces. Moreover, the bending moment at pipe invert is increased by the compaction of bedding layer. The general rule for this region of bedding layer is that it should be firm enough for the pipes to rest on.
In reclamation areas drainage pipes are usually laid at flatter gradients when compared with upstream stormwater pipes. The fact that the nature of flow in stormwater drain is by gravity makes the downstream pipes in reclamation areas relatively deep below ground surface. It is preferable to have outfall of drains above the tidal influence level and this accounts for the relative flatter gradient of drain pipes in reclamation area.
Pipes used in drainage works are normally of spigot and socket type which is flexibly jointed. For this kind of pipe connections, it allows small amount of rotation and hence it could withstand certain degree of uneven ground settlement.
From the design point of view, it is preferred to minimize the width of pipe trenches because of the following reasons:
(i) Higher cost of excavation is associated with wider pipe trenches.
In considering the loads on buried pipeline, there are normally two scenarios: narrow trench condition and embankment (wide trench) condition.
For narrow trench condition, when the pipe is laid in a relatively narrow trench with backfill properly compacted, the weight of fill is jointly supported by both the pipe and the frictional forces along the trench walls. For embankment condition, the fill directly above settles less than the fill on the side. Consequently, loads are transferred to the pipeline and the loads on pipeline are in excess of that due to the fill on pipeline.
