There has been a number of incidents where gas pipes bored through stormwater drains and sewers have been cut when clearing blockages. As a result, natural gas has escaped, causing a risk of explosion in and near domestic houses.
Several councils found gas pipes and other utility connections (including electricity and communication cables) that had been bored through their underground drainage assets. This becomes a hazard when the drain becomes blocked either at the bored pipe or further past this point. If there is a blockage, drainage crews are required to unblock the drain to prevent flooding.
The current practice to unblock a drain involves using a small locating head that is propelled into the drain via water pressure to locate the blockage. This allows the distance from the access pit to the blockage to be determined. A high-speed rotating cutting head is then propelled down the drain, cutting through any debris or obstructions creating the blockage and pumping excess water and debris from the drain.
If a gas or other utility pipe has been bored through the drain either before or at the blockage, there are limited ways of detecting it. Many gas and other utility pipes have been cut without drainage workers realising. As a result, natural gas has been released into the drain causing the risk of explosion and fatalities or serious injuries to workers.
WorkSafe expects employers to develop and implement safe systems of work to identify and unblock stormwater drains and sewers that have gas or utility pipes bored through them. This system of work should include locating intruding assets before cutting any pipes.
Employers should also:
- devise and implement suitable emergency procedures based on site conditions
- assess the drain/sewer to see if a pipe- locating camera can be used to view the blockage. This method can be used where there is not already a build up of excess water and mud
- consider the development of a pipe-locating dummy head to locate intruding pipes crossing stormwater drains prior to the use of a high-speed cutting head (see picture below)
- ensure suitable equipment is available to detect utility pipe intrusions such as dummy heads, electronic underground pipe locators, pot holing tools etc
- use a dummy head that could be a modified bullet with adjustable legs and rollers fitted to its feet to make it easier to retrieve from drains (for pipes with six to 15-inch diameters)
- (if a gas/utility pipe is located) pothole the identified area to determine the service and contact the relevant utility provider to remove/relocate the intruding asset.