Cross bores are defined as “an intersection of an existing underground utility or underground structure by a second utility resulting in direct contact between the transactions of the utilities that compromises the integrity of either utility or underground structure”. As early as 1976 the US National Transportation Safety Board investigated an explosion of a cross bore of a natural gas line into a sanitary sewer line that resulted in two deaths.

Ploughs, percussion moles and horizontal directional drills are trenchless technologies that offer the advantage of minimal disruption to the surface and streets while providing cost effective installation. These methods do not allow for an operator to observe the installation path.

If an energised utility is inadvertently installed into a sewer, it can be cut by rotating drain cleaning equipment or occasionally by high pressure jetting equipment. Such electrical lines and gas lines can result in immediate danger to construction, drain cleaning personnel and the public. When gas lines are encountered and cut by cleaning tools, the sewer becomes pressurised, forcing gas into the structure. Catastrophic explosions are then the likely result.

Proactive utilities and contractors have developed elimination processes for new construction and legacy installations. Legacy cross bore elimination projects to identify and remove gas lines from sewers have found an average of two to three cross bores per mile inspected. Cross bores have been found at homes, schools and a hospital.

Cross bores damage other utilities resulting in increased maintenance and repairs. State regulations in over 40 US states require some level of sewer operator marking to identify location of sewers. These requirements are not uniform, do not provide depth and have been variably enforced. Contractors have supported enhanced marking requirements. Sewer operators have frequently resisted expanded marking of sewers.

Continuing accounts of damage, injury or death from explosions resulting from cross bores and damages of many types of utilities have led to action by the construction industry, governmental regulators and utilities. The Cross Bore Safety Association (CBSA), www.crossboresafety.org, has been offering an opportunity to provide awareness of the cross bore issue and share the best concepts for cross bore elimination.

Education and training of drain cleaners, construction personnel and utility managers is essential. Better safety instructions provided with equipment will help. Improved locating of existing utilities and legacy inspection projects will decrease cross bores.

Prevention efforts are cost effective while also being morally responsible

Recently the Minnesota Office of Pipeline Safety issued requirements to gas distribution companies to prevent creation of and verify that cross bores have not been created during new construction. Most utilities and contractors have adopted visual means, potholing or CCTV cameras, as effective methods. Mapping and GIS implementation are receiving higher attention as means to store inspection information and verify that areas have been inspected and cleared.

GPR, acoustic and seismic have been used or considered, but have not yet found significant favour in cross bore elimination projects. Continuous wave signal devices have been recently proposed as a method for cross bore identification. Forward looking horizontal directional tooling has been under research for several years. The current best technologies and procedures now in use offer the opportunity to allow continued use of Trenchless Technology for safe installation of utilities.