Towards leak-free drainage systems
Leaks to or from drain pipes that are not watertight are often insidious. A clean water supply pipe is normally under pressure, and failure will often be rapidly apparent as surface water bubbling up, loss of mains pressure, and visible contamination. Drains by contrast often have little head, and may be partially or wholly empty for appreciable periods of time. Damage can thus build up unobserved, making even apparently good drainage systems unsustainable.
Infiltration can overload sewage/treatment systems and reduce the wastewater treatment efficiency, for example if an underground spring finds access to a drain easier than its normal run-off route. But it can also compromise ‘grey water’ systems (where water is non-potable but still suitable for industrial processes), or allow pollution into stormwater drains that are intended to discharge into streams untreated.
Exfiltration can contaminate groundwater, streams and lakes raising health and environmental issues. In dry climates, loss of water that would otherwise be treated and reused can be significant. Ground saturated by leakage from drainage systems that are not watertight may be more liable to frost heave, and leaks tend to create voids in many types of ground. (Once established, the ‘pumping’ action from road traffic can cause voids to expand catastrophically).
Why do pipes leak?
There have been relatively few large-scale comparative performance studies of installed drainage pipes. One of the most important is still the huge 2003 country-wide study of the installed German sewage network. (The study found broadly similar results in Dutch and Swedish studies).
The study found defect rates of plastic pipes, which tend to deform rather than break under pressure, to be lower for virtually all types of defect (although the rigid pipes in the study will on average have been older than the plastic ones). Connections that were not watertight accounted for around half the defects, particularly intruding and defective connections and displaced connections. The rest are made up of fissures and breaks, surface damage, obstacles and deformations.
Importantly, a major cause of defects was faulty installation combined with poor supervision.
Figure 1. Mean defect rates of defective sections showing the defect codes: BAG (intruding connections), BAH (defective connections), etc. After Prof. Dr.-Ing. Stein & Partner GmbH.
Leaks thus impact many parts of the water and sewerage system (Figure 2).
Figure 2: Impacts on the different parts of the sewage system. (Source: Prof. Dr.-Ing. Stein & Partner GmbH).
Watertightness by good design and planningThoughtful design and planning of drainage runs can prevent many leaks, and reduce any remedial costs. One objective is to achieve straight pipe runs with adequate downfall and the minimum number of connections.
Plastic pipe sections are generally longer than rigid ones, and so have fewer connections and thus less risk of leakage, although allowed lengths vary. Sharp bends promote blockages and should be avoided. Junctions are available in a limited number of defined geometries, and with rigid materials alignments have to be very accurate in all three dimensions. Plastic piping is usually more forgiving and can accommodate minor deflections, but abusing this facility can place excessive stress on connections.
Connections should be planned for ease of installation and maintenance – it is hard to guarantee a watertight connection if the installer is bent double and working largely by touch. If future connections may be needed, it is wise to incorporate the necessary junctions from the start. Adequate venting of sewer gases is also a consideration – these can over time corrode or perforate almost any pipe material.
Planners should also consider tree locations, both existing and planned. Roots can rapidly convert a slight seepage from a connection or perforation into a full-blown leak, often accompanied by blockage. If trees cannot be avoided, devices like Rootboxes can be planned in. These are infiltration units installed near trees to create a water tank where the roots can find water without having to enter pipes to find it. Rainwater can be led by a perforated pipe near the tree to the Rootbox. The tube delivers water to the tree roots, and once it is full, any excess water is passed to a conventional rainwater drainage system.
InstallationRegardless of material, the pipe supplier will have recommendations for installation, especially the depth of trench and thickness and nature of bedding material. The latter is not optional – any ground will move over time and adequate bedding is essential to cushion the resulting stresses.
An adequate trench depth is important since the backfill absorbs surface loading (and in some climates gives some frost protection). Plastic, being deformable, is better than most pipe materials under impact. Plastic pipes will stay watertight at well above 10% deformation. They therefore resist sagging and surface collapses. Shifts in the ground will tend to bend plastic pipes but crack rigid pipes, which can then leak (Figure 3).
Figure 3: Rigid pipes (left) receive the full stress of any changes in ground loading, while plastic pipes (right) can deform to absorb the stresses. [No text in field]
With trenching, digger operators are your allies – they should report any voids or lengths of unusually soft ground, also any hard areas such as intrusive boulders or old building foundations. Any of these can create stresses leading to failure in the pipe and need to be rectified.
Accurate connections are key to watertightness. Pipe manufacturers in all materials put much effort into devising types of connections that give the installer some form of positive feedback that a good connection has been made:
- Many wastewater systems use male and female socketed clay pipes, with the female end often pointing upstream. They may use spigot or socket joins, and are now often joined by flexible plastic couplers.
- Ductile iron pipes are now available with ‘pushfit’ type connectors.
- Concrete pipes have recently increased the quality of their connections, although this comes with extra costs.
- For plastic pipes, chemical methods like butt- or electrofusion welding are strongest and are best for pumped (pressurized) sewage, hazardous fluids, and wastewater pipes near a drinking water source. It can however be difficult to ensure a reliably watertight weld in awkward working situations so socket and seal connections may be preferred. PE pipes are generally welded, PP pipes can be welded but are mainly joined by socket and seals, and PVC pipes can be joined by socket and seals but also solvent welded (glued).
Materials and components of drainage systems in whatever material must be exhaustively tested – these include performance under pressure, high and low temperatures, and impact. Wastewater engineering products often need to be certified both to European and local standards.
EN 1610 "Construction and Testing Of Drains And Sewers" covers drains and sewers that are to be buried and/or operating under pressure (rehabilitated sewers are tested to EN 752-5). EN 1610 tests to the standard watertightness pressure of max 0.5 bar (a 5-metre water column) and -0.3 bar (in lab conditions only, 3-m water column). The 5-m water column simulates the outward pressure of a pipe full of water, while the -3-m column simulates the inward pressure from permanent groundwater.
The finished drainage system will also need to be tested using smoke (primarily to spot any misconnections) and pneumatically and/or hydraulically to test for watertightness and leaks –we normally advise water-pressure rather than air-pressure testing.
These tests are carried out only once the trench has been fully backfilled – failure at this point is expensive (and re-excavating the trench risks causing further damage). Also, it is not unknown for a trench to be accidentally back-filled before a good connection has been made, and this only comes to light on system test – an expensive mistake. Sewer systems are first generally flushed with high-pressure water, and then (plastic pipes only) deflection or out-of-round tested. Then follows the leak test. All openings including branches and inlets are closed off and the pipe is slowly filled with water from the lowest point so the air can escape.
Manholes have their own test requirements. They must be stiff enough to keep their shape under load at both high and low ground temperatures for 50 years plus. They must remain leak-tight (watertight) despite any movement of the pipes feeding into them, and must withstand impacts at low temperatures and continual traffic loads. These requirements are confirmed by the EN 13598-2 manhole tests.
Inspection and maintenance
Post-development ground settlement is always possible, along with frost heave or desiccation during extreme weather events, and natural changes in underground watercourses and springs. There can also be corrosion or chemical attack, blockages and high pressures from tree roots or ‘fatbergs’ (and the last two can develop remarkably quickly).
Regular inspection is important both inside the pipe and where possible along the surface line of the drain (looking for unexpected damp areas, signs of subsidence etc). It can nip many problems in the bud, and prevent small leaks from developing into catastrophic ones.
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