Canadian Underground Infrastructure Logo

Ground Penetrating Radar Used In Investigation of Tunnel Deterioration

GPR helps determine reason for serious and costly pavement settling in tunnel

Groundwater inflow in the tunnel.

Groundwater inflow in the tunnel.

Company info

12 Industrial Way
Salem, NH
US, 03079


Phone number:

Read more

Just a few years after the opening of the Cumberland Gap Tunnel, a mountain tunnel that links Kentucky and Tennessee along US25E, highway officials noticed moderate to severe settling of the continuously reinforced concrete pavement, with many voids discovered beneath the pavement surface. The Kentucky Transportation Center (KTC) brought in a research team to find the root causes of the settlement issues by using ground penetrating radar (GPR) and hydro-geochemical water testing (HGWT). The use of the GPR equipment, manufactured by Geophysical Survey Systems, Inc. (GSSI), provided information that slashed the costs for the tunnel repair project by showing more precisely where repairs were necessary. The equipment is now an integral part of the long-term inspection process that will assist in finding other distressed areas within the tunnel if they arise in the future.

Tunnel through the mountain

The Cumberland Gap Tunnel is a twin-bore mountain tunnel in the Appalachian Mountains. Constructed in 1996 at a cost of $260 million, the tunnel is located within the Cumberland Gap National Historic Park on the Kentucky-Tennessee state line. An existing narrow, winding and somewhat hazardous two-lane road goes up and over the mountain, and the state decided to build the four-lane tunnel through the park and the mountain to improve traffic safety without interfering with the scenery.

Located about 1,000 feet below the pinnacle, the tunnel carries about 22,500 vehicles bi-directionally per day. About 10 percent of the traffic was from trucks.

Less than four years after the tunnel opened, highway officials noticed a settlement area and an odd dipping of pavement. The steel-reinforced concrete pavement had settled in various areas of both the north and southbound tunnels. By 10 years after the tunnel’s completion, approximately 7,400 total square feet of pavement surface had voids beneath it that ranged from 0.5 to 40 inches deep. Only the steel reinforcement was keeping the pavement from collapsing.

“GPR gave us a visual on where the problems were located. It was such a large area that I don’t know what else we could have done to visually see where the problems were.” Brad Rister, senior research engineer

Investigation uses GPR and chemical testing

The state made several attempts to shore up the settled pavement areas before initiating an investigative repair in 2007 that attempted to repair the most severely damaged section and get to the root of the causes of the settlement issues.

The effort was led by a team from the University of Kentucky, College of Engineering, Kentucky Transportation Center.

Led by Brad Rister, senior research engineer, the research began with an analysis of the economic impact to the travelling public if the pavement was to fail and the tunnel had to be closed. The study showed that if trucks had to find an alternate route, there would be about a four-hour diversion. User costs for that diversion, along with costs associated with delays for people living in Kentucky but commuting to work in Tennessee, amounted to about $1.1 million per day.

Based on the high level of economic impact, the state made the decision to find a permanent fix for the problem. But first they had to locate the voids and determine why the concrete pavement had settled in various areas throughout the tunnel. To do so, the research team performed GPR surveys and HGWT.

The tunnel was constructed with a 4 to 6 foot layer of crushed limestone-based material placed beneath the concrete pavement. By design, between 500,000 to 1.2 million gallons of groundwater flows beneath the tunnels on any given day through the 4 to 6 foot layer of crushed limestone. The HGWT testing showed that the groundwater inflow in certain areas is aggressive to calcite, so the calcium-rich limestone backfill material placed beneath the concrete pavement was dissolving and leaving the tunnel through the groundwater collection system every day. Laboratory tests confirmed that water samples with a calcium deficiency of less than 0.10 will start to dissolve limestone material. Some of the groundwater entering the tunnel was runoff from coal seams; when water runs through coal it changes chemical properties, becoming more acidic.

The testing results indicated that approximately 0.75 to 1.5 cubic yards of limestone sub-base material left the tunnel every month due to calcium-deficient ground water beneath it. “That’s about a wheelbarrow and a half of material leaving the tunnel in solution, dissolving and washing out through drainage every day,” said Rister. This led to about 70 to 150 square surface feet of new void area opening up beneath the concrete pavement every month.

The team then used GPR technology developed by Geophysical Survey Systems, Inc. (GSSI) to identify the location and size of the voids or cavities. The equipment, SIR-20 and SIR-3000 control units with a 900 MHz antenna and survey wheel, was used to scan from one end of the tunnel to the other on both tunnels, on 2 foot centrelines. Initially they pulled the antenna by walking, but as they grew more experienced, they progressed to attaching the equipment to a vehicle. Each side of the 4,600 foot tunnel takes about 4 to 5 hours to scan, or about 10 hours total.

The first void they found was 4 feet deep and 30 feet long; some of the void areas span across both lanes (30 feet wide) and extend from 1 to 70 feet in length.

“The concrete pavement was essentially performing as a bridge in these void locations,” said Rister. “Structural loading calculations indicate that a concrete slab if designed as a bridge would only be able to span 6 feet before starting to fail. The only reason the pavement structure did not completely collapse is because reinforcing steel is placed inside the concrete.”

Limiting repairs and reducing costs

According to Rister, the GPR equipment was able to delineate where the problems were and gave the team a way to continue to track the problem. They had originally started using the GPR technology in 2002 to conduct scans every six months to identify areas where new voids were appearing. Several costly but temporary fixes had been performed that involved digging a hole and backfilling it with concrete to support the structure. “GPR gave us a visual on where the problems were located,” said Rister. “It was such a large area that I don’t know what else we could have done to visually see where the problems were.”

The study determined that the best remediation strategy was to remove the existing limestone sub-base material and replace it with layers of crushed granite, separated by a geo-grid fabric, and a new 10-inch reinforced concrete pavement. Granite is inert and has properties not affected by the low pH water.

Rister explains that the mountain through which the tunnel goes is a thrust fault – the bottom is limestone, then there is a layer of sandstone and then fieldstone. In Tennessee, the water runs through limestone, while the water coming in from Kentucky was running through fieldstone, so it didn’t have calcium elements. Groundwater with no calcium to neutralize the acid was what dissolved the limestone fill in the tunnel. “Being able to confirm the geologic formation gave us confidence that there were no voids that needed to be fixed on one end of the tunnel, significantly reducing repair costs.”

The initial proposal was to replace approximately 2,800 feet of pavement in each tunnel at an approximate cost of $10 million. The GPR and HGWT results, however, allowed crews to limit repairs to the isolated areas and save considerably. Completed in 2012, the project cost around $3 million. The project took 35 days, with work done 24 hours a day.

As part of the tunnel’s routine maintenance sequence, scans will be performed with the GPR every six months. The equipment is a key component of a longterm inspection process that will assist in finding other distressed areas within the tunnel if they arise in the future.

According to Rister, whose group performs forensics work on failures of bridges; tunnels, and roads, GPR is an important tool available to obtain discernable information, providing an understanding of whether the problem is a sink hole or settling pavement, and if the cause is water trapped beneath a roadway. “Our success goes back to use of the GPR,” explains Rister. “Without it we wouldn’t have had the ability to image what’s going on beneath the surface”

GPR equipment in action.

More from Locating, Inspection

Pole camera from CUES lightweight, portable and wireless

CUES has developed a new lightweight, portable, wireless digital video inspection pole camera that can be operated by one person. The new QuickZoom III (QZIII) camera is designed to provide cable-free safe-viewing in industrial or environmental areas with no man entry. Perform swift inspections and surveys of pipelines, wet wells, manholes, sewer treatment plants, steam generators, tanks, vessels, and other areas that are difficult to reach. QZIII can also be used to locate lateral services or to identify blockages at manholes, access ports, or other entry points without entering the line or structure.

CUES launches new pipeline inspection tool

CUES has developed a new pipeline inspection tool designed to significantly reduce unplanned downtime. The CUES REDI (Remote Evaluation Diagnostic Inspection) Kit is the first pipeline inspection troubleshooting kit available on the market today designed to help diagnose electrical issues, allowing for quicker and more accurate troubleshooting and repairs of problems while in the field. The release of this new product reflects CUES continuing commitment to offer the best customer support services in the business.

Cansel acquires Trenchless Utility Equipment precision locating business

Cansel, a software, hardware, and service provider to engineering, surveying, construction, architecture, government and printing industries announced today that it has acquired the precision location business of Trenchless Utility Equipment Inc. in Ontario and Alberta.  First established in 1992, Trenchless Utility Equipment Inc. was formed to specialize in the relatively new Horizontal Directional Drill Industry. Over the years the company has become a leader in protecting underground utility infrastructure. 

Robust, cost-effective downhole inspection camera from Hoskin

The dipper-See EXAMINER from Hoskin Scientific is a robust and cost effective vertical downhole inspection camera. Designed with portability in mind, this self-contained unit is ideal for viewing in wells, drains, vertical shafts, open bodies of water, boreholes and narrow tubes over 25mm (1") in diameter.

  • Versatile positioning arm
  • High Definition (HD) display screen
  • HD, detachable See-120 camera probe
  • Recordable video/audio
  • Designed to work in the harshest environments
  • Built in hanger - hands-free viewing while in the field

Latest in ground penetrating radar on display at World of Concrete

GSSI, the world's leading manufacturer of ground penetrating radar (GPR) equipment, will be highlighting its latest GPR technology at the 2018 World of Concrete conference, to be held January 23-26, 2018, at the Las Vegas Convention Center in Las Vegas, Nevada, Booth #C3719. On display will be a new affordable UtilityScan GPR System, StructureScan Mini XT all-in-one concrete inspection GPR system, and the new Palm XT miniaturized GPR antenna for the StructureScan Mini XT. 

New Sentinel Portable Inspection System

The new Sentinel Inspection System, from Aries Industries, Inc., is an integrated system with all components operated by an all-in-one controller. The compact, portable unit combines a lightweight reel, a quick set-up tractor and a state-of-the-art camera.


Get our newsletter

Learn more

multiVIEW opens Kitchener office to expand service coverage across Southern Ontario

multiVIEW Locates Inc. a professional services firm that specializes in locating and mapping buried infrastructure has announced the opening of its’ fourth office location in Kitchener, Ontario.  The new location will allow multiVIEW to quickly deploy to deliver private and public utility locating services across the city and surrounding area, bridging the gap between the company’s existing Mississauga and London-based offices.

The Smarter Way to Locate

The Vermeer Verifier™ G3 utility locator by McLaughlin combines trusted precision with a user-friendly interface. Contractors face congested underground conditions, unmarked utilities and increasingly strict regulations, making accurate, timely locating more important than ever.  The Verifier G3 features include a combination peak and null screen, semiautomatic and manual gain adjustment, and automatic depth and current measurement index. The new compass icon simplifies the locating process by automatically calculating the peak signal and informing the user of the direction of the utility path. With the Verifier G3, users can expect the same core qualities of the Verifier G2 including: a durable design with antennas mounted in rubber isolators to standup to the toughest jobsite conditions and the exclusive McLaughlin three-year warranty and weather-proof guarantee. Users new to the locating profession will appreciate the streamlined design and simple interface; advanced operators have the option to capitalize on some of the industry’s most advanced locating technology — all in one device.  

GSSI announces flexible, configurable ground penetrating radar system for utility detection

GSSI, a leading manufacturer of ground penetrating radar (GPR) equipment, announces the release of the UtilityScan Pro, the latest addition to the company’s popular UtilityScan GPR product line. UtilityScan systems are the industry standard for efficiently identifying and marking the location and depth of subsurface utilities, including gas, sewer, and communication lines. Featuring the SIR 4000 control unit, the UtilityScan Pro delivers the configurability and flexibility to address a wide range of utility applications.

multiVIEW opens new office location to meet growing demand across Southwestern Ontario

multiVIEW, a leading professional services firm that specializes in locating and mapping buried infrastructure is pleased to announce the opening of its London, Ontario office. The strategically situated office location will allow multiVIEW to complete local contract-based work and quickly deploy to meet growing demand across Southwestern Ontario for utility locating, hydro-excavation and CCTV camera inspection services.

New guidelines for utility relocations

Utility relocations typically present one of the largest potential risks to delivering major capital infrastructure projects on time or on budget within urbanized areas. Until now there has been little in terms of a standardized process in place regarding the coordination of the utility impacts and the management of the effects of the utility relocations on a project.

Cross bore problem in the crosshairs

They are two words no contractor or municipal engineer wants to hear: cross bore. A cross bore is what happens when one underground utility intersects another. Most worrying is when a gas line punches through a sewer main or lateral. Cross bore risk has risen with the use of trenchless technologies like horizontal directional drilling, because the operator can’t physically see what’s in the immediate vicinity of the drill bit.

Cost-effective site sampling tools for environmental assessments

Geophysical surveys can be the bedrock (pardon the pun) of today’s environmental projects, from locating abandoned underground storage tanks (USTs) and utilities, to complex mapping of geology in remedial investigations, to finding landfill boundaries and other buried unknown problems. In the past few decades, a variety of non-destructive testing methods have gained in popularity over expensive and time-consuming drilling and digging for environmental projects. Among these, the method of pairing ground penetrating radar (GPR) with electromagnetic (EM) induction instruments is one that shows great promise in significantly reducing survey time and costs.


Get Our Magazine

Paper or Digital delivered monthly to you

Subscribe or Renew Learn more

Montreal-based consultancy completes inspection and leak detection of six 1,200-mm steel watermains and one 1,500-mm PCCP main in Montreal

Since September 2013, GAME Consultants has been paving the way for the next generation of watermain condition assessment tools by using existing tools and working to improve the results with every project. The company’s position is to equip itself with a wide range of tools designed exclusively for use in watermains.

GSSI’s new 350 HS Antenna uses HyperStacking technology to improve performance while meeting GPR emissions limits

GSSI has announced its 350 HS Antenna, which greatly improves the depth and data resolution performance of traditional real time sampling technologies. The 350 HS is designed for use with new digital HyperStacking technology, which allows users to see deeper targets and operate in conditions considered too “noisy” for conventional systems. Easily configurable, the 350 HS antenna is ideal for archaeology, geophysics, and utility locating applications.

Trimble launches new version of its smart water management software to streamline utility field operations

Trimble has announced the latest version of its smart water mapping and work management software—Trimble Unity 3.0. Trimble Unity is a cloud-based, (GIS) centric software-as-a-service (SaaS) solution for smart water management. Version 3.0 includes new features and workflows and also offers an App Builder and new integration capabilities to simplify connecting mobile workers with maps and data from back office enterprise systems.

Prevent leaks and bring lead leaching into compliance with ePIPE

Pipe Restoration Inc., Technologies, LLC, (PRT), a world-leading pipe lining innovator and manufacturer of the patented, proven ePIPE pipe restoration technology, offers the fastest, 2 hour return to service barrier coating that prevents water pipe leaks, as well as bringing lead leaching into compliance. In honor of the United States' Environmental Protection Agency's national Fix a Leak Week, and United Nations' World Water Day on March 22, PRT announces that it is helping to combat the global issue of water conservation and access to clean water with the ePIPE patented leak protection process.

“Locatable Rope” from 3M

The Electronic Marker System (EMS) Rope 7700 series uses propriety 3M path marking technology to deliver more accurate and more reliable tracking of underground plastic pipe and fiber optic conduit. Designed as a high performance alternative to the current industry standard solution, the new EMS Rope series is available for use in the electrical utility industry. The EMS Rope Series is also available for gas, telco, wastewater and water utility applications.


Get our newsletter

Learn more

Into the void

It’s not uncommon to use ground-penetrating radar (GPR) to map underground utilities, but some municipalities are using it to find, well, nothing at all.

Lead pipes still a problem in Canada

The shocking story of lead contamination in Flint, Michigan may seem like a one-off situation that most of us living in modern housing don’t need to worry about. But after reading with horror and sadness all the coverage of how officials in Flint failed to protect thousands of mostly poor Americans, including children, from ingesting dangerous levels of lead that leached from corroded water pipes, I decided to look at the situation north of the border. (Flint is only a two-hour drive from London, ON) The conclusion? While the lead pipe problem in Canada isn’t as dire as in Flint, admittedly an extreme example, neither should we turn a blind eye to it, because thousands of Canadians (the exact number is impossible to pinpoint) may also be unknowingly drinking lead-contaminated water because they live in older homes where lead pipes and fittings were commonly installed.

Addressing the challenges of inspecting corrugated metal pipe

Corrugated metal pipe (CMP) is heavily reliant on supporting soil for structural integrity. As such, a condition assessment must include an evaluation of the supporting soil to be considered complete. Previously there was no conclusive and quantifiable way to assess soil condition without damaging the pipe wall – further compromising the structural condition of the pipe.

Robotic inspection goes 1/2-K deep

In June, ASI Marine (ASI) personnel conducted a remotely operated vehicle (ROV) underwater inspection of the Cheves Hydropower Tunnel in Peru and ended up breaking the ASI company record for the deepest flooded tunnel inspection at 570 metres water depth. The Cheves Hydropower Project is being developed by Empresa de Generacion Electrica Cheves S.A., a wholly-owned subsidiary of Statkraft, a Norwegian electricity company.


Get Our Magazine

Paper or Digital delivered monthly to you

Subscribe or Renew Learn more