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Subsurface utility engineering

Organisations such as Dial Before You Dig assist in the provision of utility information, however it is important to understand that such information is not always adequate when undertaking detailed design or construction. This lack of accurate information can result in costly conflicts, delays, damages, redesigns, personal injury, and even lost lives. An engineering process known as subsurface utility engineering, which combines civil engineering, geophysics, surveying and non-destructive excavation technologies, has proven to be an excellent solution to providing accurate underground utility information to the engineering and construction industry.

The use of subsurface utility engineering services is fast becoming best practice on infrastructure and construction projects throughout Australia. The benefits of subsurface utility engineering have been recognised in Australia, with the National Capital Authority (NCA) employing subsurface utility engineering practices for the forthcoming improvements to the intersection of Anzac Parade and Constitution Avenue in Canberra. The end result was the provision of accurate information on highly sensitive utilities which had to be avoided.

The process

Using principles of civil engineering, geophysical methods and non-destructive vacuum technology, subsurface utility engineering professionals identify and classify existing subsurface utility data, and map the horizontal and vertical locations of utilities. If a utility conflict does exist, viable alternatives can be found to resolve the conflicts before any damage occurs.

The four major activities of locating, verifying, data collection and data depiction and management can be conducted individually to meet the specific needs of a given project, but are most advantageously used in combination to obtain a 3D understanding of utilities within a project.

Stage one: Gather utility records from all available sources. This is generally done by contacting Dial Before You Dig, however membership is not mandatory and this stage may also include direct communication with utility companies, contractors and site personnel who have a historical understanding of the site. All information obtained is then compiled into a composite drawing and labelled as Quality Level D.

Stage two: A site visit is made to identify visible surface features associated with existing underground utilities (such as hydrants, manholes and valves). At the same time, a topographic survey might be completed for the project. This information is added to the composite drawing and labelled Quality Level C.

Stage three: At this point a decision is made as to which utilities may have an impact on the proposed design, therefore requiring further investigation. Using geophysical techniques including pipe and cable locators and ground penetrating radar, the horizontal position of these critical utilities is determined. This information is compiled into the utility drawing as Quality Level B data.

By taking utility information from the Quality Level B data and referencing it with the proposed design, utility conflict areas are identified and organised in a database known as a conflict matrix. The matrix identifies potential conflicts (such as existing utilities crossing the path of the proposed design) and allows the designers to make educated decisions regarding the relocation of utilities or project redesign. Many times, significant conflicts will appear on the plans, the cross-sections, the drainage profiles and the staging plans.

Stage Four: Once potential conflicts are identified, the final step is to verify utilities at recognised conflicts by vacuum excavation. Data obtained at this stage includes the exact size, material type, depth and orientation of the utilities being investigated. The test hole information is surveyed and included in the utility drawings, which are now certified as Quality Level A information.

The additional data gathered from the completed test holes is added to the conflict matrix. At this point, designers are able to review all options the conflict matrix presents and decide the most economical course of action.

The technology has arrived

The technology is now available to achieve a complete and accurate 3D model of subsurface utilities prior to, and/or at the design phase of a project. In addition to significant cost savings and drastically reduced construction delays, utility owners are able to complete relocations when presented with a utility conflict matrix.

Standards Australia (SA) has begun the process of developing an Australia-New Zealand Standard, in line with the process outlined above, for the practice of subsurface utility engineering. The inaugural meeting of the SA IT 036 Subsurface Utility Engineering Committee to develop the standard was held on 17 August 2011, with the development of a standard normally taking 18-24 months from here. The committee includes representatives of government, private industry, research, academia, users, suppliers and professional organisations.

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