*************************************************

Following is a question by the Hon CHAN Wing-chan and an oral reply by the Secretary for Works, Mr Lee Shing-see, in the Legislative Council meeting today (May 17) :

Question:

In the early hours of the 4th this month, an underground potable water pipe in Cheung Sha Wan burst. The outpour of water and mud caused serious flooding and as a result, traffic in the vicinity was disrupted for several hours during the peak hours in the morning. In this connection, will the Government inform this Council:

(a) of the reasons for the bursting of the relevant water pipe;

(b) whether the Water Supplies Department staff providing emergency services outside office hours are on shift duty or on-call at home; and

(c) whether underground potable water pipes are regularly inspected, maintained and replaced; if so, of the details; if not, the reasons for that?

Reply:

Madam President,

(a) All potable water pipes currently in use in Hong Kong are properly designed and are durable under normal circumstances. Experience shows that pipe burst may be caused by road works and public utility trenching works, deep trenching works, excessive loading of vehicles, vibration, underground soil movement, pipe corrosion induced by underground soil and aging of the pipe itself.

The potable pipe that burst underneath Cheung Sha Wan Road on 4 May was an asbestos cement pipe 300mm in diameter laid around 20 years ago. While a pipe can generally last up to 50 years under normal conditions, older designs, such as asbestos cement ones, may have a service life of barely 20 years, if conditions are less favourable.

Although asbestos cement pipes can withstand the pre-designed water pressure, they are more vulnerable than ductile iron pipes or steel pipes if they have problems with the foundation or when they are subject to external interference. For those reasons, the Water Supplies Department (WSD) has ceased to use asbestos cement pipes for many years and has been phasing out the existing ones.

It is very difficult to pinpoint the real cause of the pipe burst because gushing water washed mud from the ground and there is no way to find out what the underground conditions were before the bursting occurred. However, in the light of past experience and upon examining the mode of pipe failure, we believe that it was probably caused by the aging of the pipe itself or its connection, and by the partial loss of support as a result of soil loosening. The burst section has been replaced with stronger ductile iron pipe, while the loosened soil was removed and replaced by better soil to support the pipe.

(b) WSD has emergency standby gangs stationed at designated spots for call duties, both during and outside normal office hours.

Outside normal office hours, ten standby gangs are stationed at WSD's regional offices and depots throughout the territory, namely two at the Hong Kong and Islands Regional Office, two at the Mong Kok Office, and one at each of the following: Mainland South East Regional Office, Fanling Depot, Tai Po Depot, Yuen Long Depot, Mui Wo Waterworks Depot and Lamma Island Depot.

All the emergency standby gangs are properly accommodated at WSD's depots and regional offices, and the size of each gang is determined according to the service area and the location of WSD facilities.

Upon receipt of a report of pipe burst, WSD's Customer Telephone Enquiry Centre will notify the region concerned. The staff on duty will arrive at the scene as soon as possible to turn off the gate valves so that the outflow of water can be stemmed without delay. In the process, WSD staff will have to locate the burst pipe and the exact location of the gate valves. Besides, they have to take into account the area involved and the number of households affected by the suspension of water supply. The number and size of gate valves that need to be turned off will depend on the actual situation. Sometimes the valve pit covers are submerged in water and debris and thus cannot be opened. Under such circumstances, the workers will have to take a longer time to turn off more valves that are further away and a larger area will therefore be affected.

After the flow has been stemmed, the maintenance term contractor of WSD will immediately carry out the repair works under the instruction of WSD staff. The contractor will resume the supply of water as soon as possible and restore the scene back to normal condition.

Before carrying out emergency remedial works, WSD engineers will examine the severity of the burst pipe and the extent of the damage to its surroundings. Consideration will also be given as to whether the burst section has to be replaced. If the suspension of water supply exceeds 6 hours or a large number of households are affected, WSD will activate the public standpipe or mobilize a water truck to provide temporary water supply to the affected households within 3 hours.

In this incident, an emergency standby gang had just finished handling a case of pipe burst in Hamilton Street, Mong Kok. Upon receipt of a report of pipe burst in Cheung Sha Wan Road, the gang polished off the work and rushed to the scene in 25 minutes. As some of the valve pit covers were covered with water and debris and thus could not be opened, several main gate valves at more distant locations had to be turned off to put the inflow under control. The water stored at the downstream side of the water mains flowed backwards because the system itself was filled with water. Under such circumstances, the gate valves of other branch distribution mains had to be closed to completely stem the flow.

(c) It is WSD's current practice to replace old water pipes annually to reduce the risk of bursting and leakage. WSD staff normally conducts inspection on underground pipes to decide whether old pipes should be replaced when excavation works are being undertaken under other projects. Such a measure allows redeployment of resources for replacement works in a more cost-effective way. Of course, those pipes with high frequency of bursting will be included in the replacement programme. WSD also regularly monitors the progress of other works, including road works and underground utility works (such as drainage, gas pipes and electric cables), undertaken in the vicinity of its pipes. If WSD staff detects any works which may cause damage to the water pipes nearby, they will give warning and ask the authorities concerned to take adequate protective measures.

WSD has produced a number of educational and promotional publications such as "Guidelines for Excavation near Water Mains" for distribution to contractors and the authorities concerned, reminding them to take appropriate measures to protect water pipes at the work sites when road works are undertaken.

Underground pipes are subject to regular leak tests while main valves are under frequent inspection and operation. Any defects found will be rectified and leaking pipes will be replaced as soon as possible.

Leak tests on pipes are also conducted regularly throughout the territory by the Leakage Detection Unit of WSD. The testing frequency depends on the leakage record of individual pipe sections and their respective monitoring zones. Leak tests are normally carried out on an average of once every 12 months in each monitoring zone. For pipes with a longer service age or monitoring zones with a higher leakage record, leak tests will be conducted more frequently. Rehabilitation and improvement works will be undertaken for pipes with high frequency of bursting or leakage.

WSD investigates and examines the cause of each pipe burst incident so that preventive measures may be taken. If the burst is caused by external factors, it is not necessary to replace the whole pipe, and reinforcing the protection for the affected section will suffice, such as enveloping the pipe with concrete. Such practice aims at redeploying resources for replacement works in a more cost-effective way. On the other hand, if the burst is due to aging of the pipe, WSD will consider including the affected section of the pipe in the replacement programme.

In Hong Kong, there is an underground pipework network of about 6,100 km in length supplying potable water and salt water to the whole territory. Along with the development of the urban area and the new towns, about 42% of the pipes were laid 30 years ago or even earlier and their design life will soon expire.

WSD completed the Underground Asset Management Study in late 1997. It was proposed in the study that aged water pipes of about 3,050 km in length should be replaced/rehabilitated in phases within 20 years to prevent further deterioration of the water supply network. The estimated cost of the whole project is around $11 billion. It is expected that the number of pipe failures can be reduced by 60% after the completion of the programme.

End/Wednesday, May 17, 2000

NNNN