Pipe Material	Nominal Diameter
Mild Steel (MS)	600 mm and above
Ductile Iron (DI)	80 mm to 600 mm
Asbestos Cement (AC)	100 mm to 450 mm
uPVC (uPVC)	100 mm and below
Un-lined Galvanized Iron (GIU)	100 mm and below
Lined Galvanized Iron (GIL)	100 mm and below
Polyethylene (PE)	150 mm and below

Photos showing the above mentioned types of pipe materials are attached at Appendix I for reference.

• Durable in withstanding internal water pressures, but relatively more brittle and weaker than MS and DI pipes in withstanding excessive external load or pressure.
• Use of AC pipes has been discontinued for many years, but there are still large quantities of these pipes in the existing water mains systems.
• AC pipe bursts are seldom due to aging effects, but are often the result of being disturbed and damaged.

• Relatively weak in resisting direct impact.
• Laid outside carriageways, mostly used in small diameter salt water supply systems.

• Strong in resisting impact and tensile stress, but prone to corrosion.
• Used only in small diameter fresh water supply system and commonly in exposed manner especially in rural areas.
• GIU pipes are no longer in use for new works since early 1995. GIL pipes have then been adopted for use as a standard practice as they are more resistant to internal corrosion.

• Adopted for full scale use since October 1998 to phase out buried GIU, GIL and uPVC pipes.
• Corrosion free and strong in withstanding excessive stresses.

1. direct damage of water mains and valves by indiscriminate or reckless use of construction plant;
2. ground settlement or movement caused by excavation work;
3. disturbance of lateral or vertical support to water mains; and
4. excessive stress or loading caused by piling, blasting, heavy mechanical plant or construction traffic.

Circulate layout plans with relevant details to Water Supplies Department (WSD) to request indication of the alignment of existing water mains in the vicinity, before commencing any excavation for roadworks (see Clause 17(d) of the Conditions of Excavation Permit issued by Highways Department (HyD)).

Check the marked-up plans or mains record plans returned from WSD to see if any existing water main may be affected near the work, before starting any excavation. The alignment of the water mains shown in the returned plan(s) is indicative only. WSD will endeavour to provide the contractor with the most up-to-date information, but it takes time to incorporate all recently laid mains into the central records and hence such information may not be shown in the returned plan(s).)

Make use of suitable pipe detector to ascertain as accurately as possible the alignment and depth of buried water mains near the work, before digging trial holes.

Dig trial holes by hand tools to ascertain the exact positions and depths of water mains, before using mechanical plant for road breaking and excavation.

Contact the relevant Regional Inspector/(Inspection) of WSD as listed in Appendix II for assistance, whenever there are uncertainties or problems with regard to the locations and protection of existing water mains. (A sketch showing the boundaries of the four WSD Regions is attached at Appendix III)

Execute the work safely by strictly observing the following requirements :

1. Careful Operation of Heavy Mechanical Plant
   Be careful in operating heavy mechanical plant, especially in close proximity to existing water mains and valves. Excavation close to or around water mains should be carried out by hand digging method.
2. Protective Measures
   Implement all necessary protective measures to adequately support and protect the exposed water mains, including the associated thrust blocks and valves.
   (A sketch showing the suggested method for support of water main is attached at Appendix IV for reference. The proposed method is by no means exhaustive and the details shall depend on site conditions and the type, size and length of the water main to be supported.)
3. Adequate Site Supervision and Control
   Provide adequate site supervision and control to ensure that all the above requirements are closely observed, and remind the workers from time to time of the importance of avoiding damage to water mains and the full implications of the damage.

Notify WSD staff to inspect the waterworks installations before backfilling, and backfill the excavation with suitable materials in layers with adequate compaction to prevent ground settlement adjacent to or below existing water mains. (See paragraph 6 also)

Apart from taking those precautionary measures mentioned in paragraph 5.2.1 to 5.2.7, the contractor should also avoid the following malpractices during excavation to prevent damage to water mains :

• DON'T pass heavy plants, such as excavator or compaction roller, over buried water mains after the road surfacing is removed when the cover to the water mains is much reduced. This is to prevent possible pipe damage due to excessive loading.
• DON'T disturb or expose any thrust block without WSD's prior agreement. Exposure of a thrust block without providing proper support may result in its movement and detachment of the associated pipe joints and eventually a burst. WSD should therefore be consulted on all cases where exposure of thrust blocks or water mains are required to enable suitable protective measures to be taken.
• DON'T just use steel wire ropes or chain blocks to support the exposed water mains. I-beams/R.S.J. should be used to provide proper vertical and lateral support of water mains as suggested in the sketch at Appendix IV.
• DON'T undermine water mains when excavating trench alongside or across it. This is to prevent settlement of ground adjacent to or below existing water mains.
• DON'T stockpile excavated materials above valve covers or adjacent to hydrants. This is to ensure that the valves and hydrants can be accessed and operated at all times. All valve covers should be kept clear of excavated materials during trench excavation or reinstatement. They should not be buried under stockpile of excavated materials or paved over by road reinstatement. Any construction debris that falls into the valve pits should be cleared immediately.
• DON'T expose the valves above general excavation level without protection. Proper fencing with warning signs should be provided to prevent accidental damage to the exposed valves by the swinging parts of the excavators.
• DON'T carry out blasting at a distance of less than 6 metres from water mains. This is to avoid causing excessive ground vibration and movement that may exceed the tolerable limits of water mains.

1. The excavation shall be backfilled with suitable materials in compacted layers. The backfill materials shall not contain any broken concrete, bricks, clay, bituminous material, materials susceptible to spontaneous combustion, perishable materials or debris, and shall not exceed 75 mm maximum particle size.
2. Backfill material around water pipes should be carefully deposited by hand in layers not exceeding 100 mm thick to a level of 300 mm above the crown of the pipes. The material in this zone shall be carefully compacted by hand-rammers or manually operated power equipment.
3. The material on top of the 300 mm level shall be deposited in layers not exceeding 150 mm thick and then compacted adequately with a power rammer or vibratory plate/roller. The compacted backfill shall obtain a relative compaction of at least 98% for levels within 150 mm of road formation level and at least 95% for other levels. The relative compaction test shall be carried out in accordance with the procedures specified in Test No. 12 & 15 of BS 1377:1975.

The minimum cover to a water main (e.g. 900 mm under carriageway and 450 mm under footpath) as stipulated in the Excavation Permit should not always be taken as the actual cover. In areas with congested utilities, some short sections of water mains may have been laid with a shallower cover. There are also cases in which the road levels have been lowered or the footpath has been converted into a carriageway, but due to various factors, the water mains existing there have not been re-laid at a greater depth, thus leaving the covers much less than the minimum requirement.

The minimum cover to a water main should be 700 mm measured from the existing or future sub-grade (i.e. site formation level), whichever is the greater and should always be observed. In case this cannot be achieved, WSD should be consulted to enable suitable measure to be taken.

Deep excavation such as construction of basement can often cause ground movement that may either fracture the pipe body or dislocate/pull out the pipe joints. The ground movement depends on the type of retaining wall, bracing system, construction method, and whether temporary dewatering or lowering of ground water level is involved. Its effect on a water main depends on the magnitude of the differential settlement, the type of pipe material and jointing as well as the age and condition of water main. MS, DI and uPVC pipes are flexible, but AC and CI pipes are rigid and brittle. Joints for MS pipes (welded/flanged joints) and UPVC pipes (solvent-cement joints) are however rigid, whereas DI and AC pipe joints (push-in joints) can take up certain amount of differential settlement. In this regard, it is advisable to locate and expose the pipes and joints for an accurate analysis of the likely settlement figures and the limits that the pipes can tolerate. Pipe settlement markers should be installed to gauge the movement of the pipe foundation and be closely monitored. In case the calculations or in-situ measurements indicate an unacceptable settlement level, the water mains should either be diverted if feasible or be supported using mini-piles for instance.

WSD has set up dedicated Roadworks Inspection Teams in September 1993 as a positive measure to step up the surveillance of all road work activities which may affect water mains and to give advice and/or warning to the concerned contractors for proper protection of water mains against damage.

In case that main burst or leak is discovered in the vicinity of works, the contractor should report immediately to the Customer Telephone Enquiry Centre of WSD at the 24-hour hot-line (tel: 2824 5000). WSD emergency gang will attend the incident immediately.