Cross bore incidents are a growing hazard for trenchless installations, as the real estate underground becomes more crowded and trenchless installations become the preferred method. Extra care must be taken, particularly by horizontal directional drilling (HDD) operators, due to the difficulty in locating existing services.
Trenchless Technology has the advantage of not being disruptive to the surface of the ground, roads, traffic and the environment. However, for all its benefits, trenchless methods are made challenging by a lack of visibility, allowing visible identification of where the lines are being installed.
Some challenges associated with trenchless installation, includes:
- Non-metal pipes and cables can be difficult to locate, and are at times unmarked
- Depths of utilities are unknown
- Trenchless methods do not ‘see’ the pipe
- Trenchless installers need information, such as depth, that is not always available through Dial Before You Dig (DBYD).
However, implementing simple, cost effective preventative measures at the time of installation will greatly reduce the cost of location for contractors and the growing rate of infrastructure strikes for utilities.
Cross bore
A cross bore is defined as an intersection of an existing underground utility, or underground structure, with a second utility resulting in direct contact between the transactions of the utilities. A cross bore compromises the integrity of either of the utility lines or other underground infrastructure.
The rising incidence of cross bores has become a safety focus of the North American trenchless industry. A variety of papers on this subject were submitted at the annual Common Ground Alliance (CGA) Conference, held in March 2017, in Orlando, Florida.
Third party strikes on buried infrastructure were tracked by the CGA in its annual Damage Information Reporting Tool (DIRT) report, revealing 317,000 damage incidents on underground infrastructure in the US in 2015 alone. However, the DIRT report states that 45 per cent of these were due to poor practices by installers.
In the US, sewer and stormwater lines are often unmarked and non-metal. Verification projects indicate in high-risk areas suspected of having cross bores that the number of cross bores found per kilometre of mainline sewer inspected is between three and five.
In Australia a similar situation exists, where older sewers are unmarked and newer polyethylene pipes have inadequate location aids, like ineffective detectable tape or no tracer wire. This is also unfortunately the case with a number of communications and gas lines.
The potential for catastrophic incidents from cross bores is acute, especially in the circumstance where a gravity sewer is cross bored by a gas line. Cross bores can remain undetected for decades, allowing for the ingress of root growth in the pipe, which can camouflage the cross bore and block the sewer.
Eventually the sewer backs up, the plumber is called and CCTV inspection is completed before clearing, what they believe to be a simple blockage. All that could be seen from the CCTV vision is a mass of roots, which would likely lead to the deployment of a root cutter to remove the blockage.
The gas line is severed, gas volume builds in the pipe and gravity feeds it back into a building. In this scenario, the potential for explosion is extreme; holes broken into sewers also increase the infiltration and inflow of water, creating structural deficiencies that may eventually create sinkholes and voids that are expensive to repair.
“Cross bores should be no less of a focus in Australia and New Zealand,” says Tapex General Manager Infrastructure and Safety Ed George.
“There are several simple, cheap and effective steps that can be taken to ensure the probability of a cross bore is significantly reduced. One of the these is the implementation of Tapex’s tracer wire system.
Mr George says the best way to reduce occurrences of cross bores is for the utility to make it easier for drillers to identify the location of the existing infrastructure. He says there are several simple, low-cost methods – often not implemented – that can lower the probability of cross bores.
He says the accuracy and speed with which location records are supplied to DBYD must be improved, including depth where possible and the essential update of geographic information system (GIS) data. Logical, visible, physical signage of buried pipelines, especially from the curb to the building, is another measure Mr George says should be implemented.
He says a quality tracer wire system, like those offered by Tapex, should also be in place when using non-metal pipes and cables, so they can be located and their depth logged. This includes at terminations for locators to connect onto tracer wires, like hydrants, gas risers, and waterproof connectors so the system lasts the life of the pipeline.
Although, just as important as the measures outlined above, is backing these systems up with an audit procedure to ensure these measures are being installed, and installed correctly.
This article was featured in the September edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
For more information visit the Tapex website.
If you have a project you would like featured in Trenchless Australasia contact Journalist Chloe Jenkins at cjenkins@gs-press.com.au