Preventing utility damage through non-destructive technologies

Despite growing awareness around safe excavation processes, hundreds of thousands of damaged utilities are reported each year across Canada and the U.S. During the design phase of a construction project, it can be difficult to achieve an accurate picture of subsurface utilities, which creates unnecessary risk for all stakeholders involved. Utility records are often a composite of accurate records, outdated records and at times, incomplete records. In fact, a study conducted in the mid-90’s revealed that existing records and visible feature surveys are typically 15 to 30 percent off the mark, and in some cases, considerably worse (Stevens and Anspach 1993).

Adding to the challenge is the fact that over the past 50 years there’s been a shift towards building subsurface utilities out of durable PVC materials rather than the more traditional cast iron piping and ductile iron. These non-conductive, “nontoneable” assets are not always traceable through commonly used utility detection methods.

Conventional locating methods depend on the ability to detect electrical currents that flow on metal conduits, sheaths or wires running along buried utilities. Plastic, concrete and other nonmetallic pipes, cables and conduits don’t carry electrical current, making these features essentially impossible to detect. To address this challenge, construction practices commonly require tracer wires to be laid out with buried utilities, but this is not always a fool-proof solution. For example, tracer wire may not follow the actual pipe location, may not be properly installed or joined and could become cut or corroded.

Detecting the undetectable

In response, project engineers are increasingly leveraging non-destructive technology such as Ground Penetrating Radar (GPR) to accurately identify even those buried assets traditionally considered undetectable. GPR works by transmitting high frequency radio waves into the ground or structure and analyzing the reflected velocity and energy to create a profile of the subsurface features. Reflections are caused by a contrast in the electrical properties of subsurface materials making it highly effective for locating non-conductive utilities. This contrasts from standard electromagnetic induction equipment which operates by locating either a background signal or a signal introduced into the utility line using a transmitter.

Though effective for any project requiring the identification of non-conductive utilities, GPR has become particularly essential to Subsurface Utility Engineering (SUE) – a civil engineering process conceptualized in the U.S. that is slowly gaining momentum across Canada.

Mitigating risk and eliminating surprises

In 2011, a Purdue University Study found that for every dollar spent on SUE during the initial stages of a construction project, an average of $4.62 was saved. Based on the American Society of Civil Engineers (ASCE) Standard 38-02, SUE provides a framework to evaluate the quality of data related to existing utility records, according to four quality levels. Whether a new building is being constructed, a road is being rehabilitated or a new subway track is being installed, these quality levels provide a solid foundation during the development stage of a project so that there are no surprises later down the road.

Leveraging a wide intersection of technology including electromagnetic instruments, Ground Penetrating Radar (GPR), GPS and vacuum excavation, the SUE process follows four main steps to essentially eliminate the risk of finding or hitting an unknown buried utility during excavation. This in turn, reduces project delays due to utility relocates while minimizing the chance of liability related to utility service disruptions or an environmental hazard.

Known as Quality Level D (QL-D), the first step in a SUE project is to gather existing utility records during project planning activities. QL-C then involves surveying visible above ground utility facilities, such as telephone boxes or maintenance hatches, and correlating this information with existing records. From there, QL-B involves applying geophysical methods such as GPR to determine the existence and horizontal position of virtually all subsurface utilities within a project’s limits.

This information is correlated with Level C & D to provide a comprehensive subsurface utility dataset that includes abandoned lines and other discrepancies. QL-A, commonly referred to as “Daylighting” then involves hydro-vacuuming or hand digging to verify the precise vertical and horizontal position of underground utilities along with their type, size, condition and material.

Once this process is completed, project stakeholders are provided with a digital composite of the subsurface utilities within their project area along with a utility conflict matrix that allows them to visualize conflicts between existing utilities and their proposed plans.

Though the concept of SUE originated in the U.S., its growing relevance north of the border became apparent when the Canadian Standards Association (CSA) released Standard S250 Mapping of Underground Utility Infrastructure in 2011. The Standard was created to compliment and extend ASCE 38-02 by setting out requirements for generating, storing, distributing, and using mapping records to ensure that underground utilities are readily identifiable and locatable. Accuracy levels expand upon QLA, prescribing a finer level of detail to define the positional location of infrastructure.

Empowering responsible design for Ontario’s transportation infrastructure

GPR has been leveraged to successfully carry out SUE projects across the province of Ontario and has played a major role in several major transit projects. For example, in 2011 GPR was applied to carry out sections of utility mapping along approximately 1,500 metres of the TTC’s active railway. All existing underground utilities were identified and mapped including electrical cabling, switch gears, firewater lines, drainage and various communication cabling.

Following the survey, detailed layered drawings were delivered that depicted the entire underground infrastructure within the highly travelled subway area. This allowed the consulting engineering firm assigned to the project to accurately design the construction of the new track, effectively remove the existing tie and ballast track and install the concrete slab-on-grade.

SUE also played an integral role in the Union Station Revitalization. As part of the revitalization, GO Transit is expanding its services and infrastructure which includes a reconfiguration of track between the Don River and Strachan Avenue. Existing tracks will be interconnected to various platforms, allowing more riders to access trains.

Because the track had been subject to change and resurveyed multiple times over the years, concerns arose over the accuracy of existing data particularly for subsurface assets. Project engineers were dealing with infrastructure that was 50-70 years old and they were uncertain of the location of underground utilities. They were also working with disparate datasets scattered across multiple platforms.

The SUE process was implemented to get an accurate depiction of all existing public and privately owned utility services including gas, hydro, water, fibre optics, telecommunications and signal cabling. Non-destructive geophysical inspection methods including GPR and electromagnetic induction were leveraged to detect buried cables and utilities that crossed the corridor. The vertical and horizontal positions of selected subsurface assets were then confirmed through vacuum excavation and referenced against existing maps to reveal anomalies. Information was stored in a central database to be referenced by Go Transit years down the line for future rehabilitation projects.

A final thought

Today’s critical dependence on the undisrupted availability of telecommunications, power distribution, water and sewer networks along with transportation systems demands a greater focus on damage prevention. The hope is that organizations will continue to become aware of the ROI that is being achieved through the integration of non-destructive testing on major construction projects, particularly in the design phase. This awareness will support a trend towards safer excavation practices, reduced liability, optimal project design, and a subsequent reduction in incidents of damaged utilities across the country.