FHWA engineers are using geophysical investigations to characterize subsurface conditions.

Transportation engineers often need to learn about conditions beneath the surface when they plan to rehabilitate existing roads or build new ones. In Hawaii, for example, subsurface lava tubes could collapse and bring a roadway down with them, or in Colorado, abandoned lead mines could similarly undermine construction. Complicating matters, test borings and excavations themselves can be intrusive, especially in environmentally sensitive areas.

As the Federal Highway Administration (FHWA) strives to uphold its commitment to environmental stewardship, the agency increasingly is using geophysical imaging and geophysical testing methods for site characterization and geotechnical investigations before and during highway construction. These methods represent environmentally sensitive options for collecting timely information that can influence design quality and roadway performance. Nondestructive geophysical methods enable engineers to peer into the unknown and gain detailed knowledge of highly variable subsurface conditions to improve safety during construction and mitigate project risks and costs associated with "change of conditions" claims, when contractors encounter different and more expensive subsurface conditions than specified in their contracts.

"Geophysics involves the use of nondestructive methods to determine the nature, characteristics, and extent of natural or manmade materials below the ground's surface or within manmade structures," says David Lofgren, engineering geologist with FHWA's Western Federal Lands Highway Division (WFLHD). "Geophysical testing is based on the ability of certain types and frequencies of manmade energy, such as radio frequency, seismic vibration, and acoustic and electrical energy, to penetrate the ground and certain manmade materials such as concrete, and to measure the change in behavior of the energy that occurs when it passes through, or is reflected or refracted by, those materials."

FHWA's Office of Federal Lands Highway (FLH) has been deploying a number of geophysical imaging and other geophysical methods in innovative ways. When off-the-shelf solutions have not been effective, FLH engineers have designed and developed new approaches to overcome engineering challenges.

In 2003, the FLH publication Application of Geophysical Methods to Highway Related Problems (FHWA-IF-04-021) shared with highway engineers a basic knowledge of geophysics and methods for enhancing geotechnical investigations. The document provides a broad range of practical methods for evaluating the physical properties of soil and rock, such as seismic methods used to estimate the depth to bedrock or locate underground voids. Below are highlights of selected geophysical investigation techniques and successful applications thereof on recent FLH projects.

Types of Geophysical Methods

Many applications in highway engineering could benefit from the use of geophysical methods, including evaluation of the thickness and condition of pavement structures; detection of subsurface voids (gaps), caves, and abandoned mine openings; and determination of the location and extent of voids in reinforced concrete structures such as bridge piers and foundations.

Two of the more familiar types of nondestructive geophysical testing are seismic refraction and reflection, which are based on measurements of velocity changes that occur when manmade acoustic (seismic) waves encounter materials of differing material properties below the ground's surface. Another technique is ground penetrating radar (GPR), which utilizes the reflection of radiowave to microwave electromagnetic frequencies in materials of varying dielectric permittivity (and low electrical conductivity) to map natural and manmade subsurface features. Yet another technique is electrical resistivity, which uses an electrical current sent through the ground to determine subsurface water conditions; evaluate aquifers, wells, and plumes; detect voids; and evaluate environmental aspects of landfills.

Approaches to employing geophysical methods vary widely depending on the experience and knowledge of the operators and agencies using them. In the past, some engineers were reluctant to employ geophysical techniques on projects due to complications associated with the cost of equipment rental, mobilization, and deployment; complexity of the technology; extensive data refinement and analysis requirements; perceived application limitations; and difficulties interpreting survey findings. Today, however, geophysical imaging and geophysical methods are finding renewed acceptance in the transportation community after advances in rapid, user-friendly data acquisition; simplified data analyses; improved survey presentation; and lower deployment costs.

Engineers have adapted seismic methods for condition evaluation of pavements, concrete slabs, and walls, producing a technique known as the ultraseismic test. In this method, engineers transmit acoustic energy through a concrete structure itself, instead of the ground, with sound reflection coming from either the bottom of the structure, as in a bridge foundation, or a defect zone. Similarly, engineers now routinely use the GPR method, originally developed for high-resolution imaging of the subsurface, for structure condition evaluation. Highway engineers use these types of geophysical investigations in the transportation and infrastructure systems to evaluate new structures for quality assurance and in-service structures for forensic and quality control purposes.

3-D Tomography Using Seismic Reflection Or Refraction

For subsurface characterization, volumetric imaging provides information that engineers can use to more fully evaluate site conditions. Standard practice in geophysical surveys involves using seismic refraction techniques to produce two-dimensional (2-D) cross sections of the subsurface. The state of the practice for nearly three decades has been to process refraction data with layer reconstruction techniques using the generalized reciprocal method, time-intercept, and similar approaches. In the past decade, however, advances in computer technology and development of tomographic modeling algorithms have greatly increased the ability to detect subsurface anomalous features, increase lateral and vertical resolution, and provide more accurate graphical presentation of data.

Tomography refers to imaging by sections or slices. Typically, engineers create 2-D tomographs to image areas, while three-dimensional (3-D) tomographs help them visualize volumes. Using raw seismic data collected with sensors, engineers can create 3-D tomographs they can then analyze to measure quantities of subsurface materials.

Recently, FLH engineers have shown that 2-D finite element modeling of seismic refraction data can successfully image discrete anomalies such as voids. Advances such as 3-D seismic refraction tomography provide substantial enhancements over traditional line surveys, which makes the method a cost-effective supplement to any conventional drilling program.

Seismic Reflection at A Mountain Tunnel

In July 1930, workers completed the 1.1-mile (1.8-kilometer)-long Mount Carmel Tunnel, which provides direct access to Utah's Bryce Canyon National Park and Arizona's Grand Canyon National Park through Zion National Park in Utah. Although it was the longest tunnel in the United States at the time of construction, it is not currently large enough to accommodate two-way recreational vehicle traffic. As traffic volumes increase, FLH is working with the National Park Service (NPS) to study several alternatives to improve traffic flow through Zion. These alternatives include enlarging the existing tunnel and constructing a parallel tunnel.

To appraise preliminary alternatives for technical and economic feasibility, FLH needed to characterize the ground conditions around the existing tunnel. To make these measurements with minimal interruptions to traffic, the team employed a proprietary seismic reflection/holography technique. This technology analyzes the seismic signals for multiple source and receiver locations, identifying reflector zones within the rock mass that might correspond to voids, fracture zones, or significant changes in geologic structure. The team verified the tomography technique by comparing the images to known ground supports and voids.

"The nondestructive seismic reflection method provided rapid characterization of large tunnel segments as well as identification of structural 'targets' warranting more detailed investigation," says Khamis Haramy, geotechnical engineer with the Central Federal Lands Highway Division (CFLHD). "The information generated from this technique enabled engineers to compare cost-benefit analyses of various alternatives."

Seismic Refraction and Compaction Grouting

In Colorado's Rocky Mountain National Park, NPS needed to rehabilitate a historic, 50-foot (15-meter)-high, dry-stack wall (rock wall constructed without mortar) on Trail Ridge Road. After consultation with NPS, the design team selected compaction grouting as the preferred solution. Compaction grouting is a ground treatment technique by which low-mobility cement grout is injected into the ground mass under pressure. The grout fills voids or rock fractures and displaces loose soils in the subsurface, consolidating the treated ground mass in place. Engineers normally apply this technique on a grid pattern, and workers inject the grout vertically through grout tubes, forming grout columns. Engineers often use this treatment in highway applications to improve stability by filling voids in rock walls or armored slopes.

FLH used a new high-definition seismic imaging (HDI) system to gauge the subsurface conditions before and after grouting. This imaging technique, using seismic tomography, involves state-of-the-art algorithms to produce 3-D images of subsurface conditions. This relatively inexpensive seismic method proved effective in producing accurate images before and after grouting. The technique also produced difference tomograms that depict the effects of grouting, including the resultant grout columns and ground densification characteristics, which demonstrate that the majority of the wall voids are filled.

"The HDI method can be an effective tool for use with grouting methods to enhance the quality assurance methods and monitor volumetric grout injection with the improved soil mass," says Haramy. "Accurate images are produced in near real time, providing immediate confidence in survey results and allowing adjustments of receiver placement to zero in on regions of interest."

3-D Tomography Using Crosshole Sonic Logging

As the number of projects using deep foundations (such as bridge piers and abutments constructed on drilled shaft foundations) increases, highway agencies are increasingly interested in detecting construction defects occurring during concrete placement. Obtaining accurate and timely information on the integrity of concrete structures, such as drilled shaft foundations, is essential to avoid structural instability and other safety issues.

Crosshole sonic logging (CSL), also known as ultrasonic testing, is an indirect, nondestructive imaging method for detecting defects inside the rebar cage of a drilled shaft or diaphragm wall element (an underground structural element constructed using slurry trench technique). CSL has become a standard test for most transportation agencies and currently is performed on most drilled shafts in the United States and other developed countries. Before accepting CSL, FLH engineers performed quality assurance testing on a limited number of drilled shafts, primarily using the sonic echo and impulse response test.

"The gamma-gamma density logging tests are gaining popularity within the DOTs as a secondary test to the CSL test for identifying anomalies within drilled shafts," says Haramy. "CSL is mainly used to characterize anomalies that are located in the concrete between tubes, while gamma-gamma density logging identifies anomalies within a small zone around the tubes. These two methods complement one another to adequately evaluate the conditions of the drilled shaft in the subsurface."

Several variations of the CSL equipment and techniques exist, including a source (pulse transmitter) and receiver simultaneously lowered in the same tube (the single hole ultrasonic test, or SHUT), a source and receiver lowered in adjacent tubes, and a source and multiple receivers lowered in separate tubes. Researchers derived the CSL method from the ultrasonic pulse velocity test. The basic principle of the CSL test is that the velocity of an ultrasonic pulse through concrete varies proportionally with material density and elastic constants (which are relationships that determine the deformations produced by a given stress system acting on a particular material). A known relationship between fractured or weak zones and measured pulse velocity and signal attenuation is fundamental for these tests. Research shows that weak zones reduce velocities and increase attenuations.

During CSL tests, instruments are used to measure and record the apparent signal travel time between transmitter and receiver. By measuring a pulse's travel time along a known distance (between transmitter and receiver), engineers can calculate the approximate velocity as a function of distance over time. If they make a number of measurements and compare them at different points along the concrete structure, they can assess the overall integrity of the concrete.

The FLH study, Velocity Variations in Cross-Hole Sonic Logging Surveys: Causes and Impacts in Drilled Shafts (FHWA-CFL/TD-08-009), identifies conditions that affect the load-bearing capacity of drilled shafts by modeling various conditions and analyzing them with numerical methods. The analysis first identifies design criteria and construction procedures, and reviews nondestructive evaluation (NDE) techniques. This analysis uses results based on principles and theorems from engineering mechanics, geotechnical engineering, concrete chemistry, and geophysical engineering, which researchers analyze numerically using a proprietary software tool dubbed the Geostructural Analysis Package (GAP). GAP is a geotechnical engineering application that performs postprocessing for the CSL data, combining the numerical methods of the discrete element method, particle flow method, material point method, and finite differencing, together with engineering mechanics constitutive models, concrete chemistry models, thermodynamics models, and geophysical tomography and holography. Highway and mining engineers also have used GAP for ground characterization.

"Our study explores many concerns recently raised for drilled-shaft design, construction, and maintenance," says Haramy. "Our recommendations offer engineers a better understanding of drilled-shaft foundations to revolutionize foundation design, concrete mix design, construction techniques, NDE measurement, and defect evaluation to improve performance and efficiency."

Vibration Monitoring Case Study

Recent construction along the General Hitchcock Highway (Catalina Highway) through the Coronado National Forest, northeast of Tucson, AZ, raised the possibility of harmful vibrations on natural geologic formations, including rock pinnacles and a natural rock bridge. "To characterize the constructability of the roadway without damaging these protected natural resources, an FLH study defined the natural vibration properties and construction vibration response of selected pinnacles and natural bridges along the project," says Matthew DeMarco, geotechnical engineer with CFLHD. "What added complexity to the study was the fact that these features ranged in height from a few meters to upwards of 20 meters [66 feet], possessed various slenderness ratios, were composed of more than one distinct rock unit, and were subject to a wide range of mechanical and blast-induced vibrations associated with road construction," DeMarco says.

Altogether, engineers investigated 20 pinnacles and one large natural bridge for their vibration response parameters -- natural frequency and vibration damping. The engineers also documented the induced vibrations from the onsite hoe ram (hydraulic hammer), compactors, normal forest traffic, and several test blasts, and analyzed them with spectra analyses to characterize the vibration sources. The engineers also derived induced vibration attenuation functions for one compactor, the hoe ram, and a test blast. They then combined analyses of pinnacle response, equipment vibration, and ground attenuation to identify pinnacle features at risk from planned construction activities.

Based on the low damping characteristics measured, the study identified only two pinnacles that showed potential for instability. Both had natural frequencies generally below the higher amplitude vibrations induced by the compactors; however, the smaller pinnacle seemed to be affected by lower mode vibrations associated with compactor harmonics. The study concluded that the pinnacles were robust, with minor exceptions, and not overly susceptible to construction vibration damage. FLH modified plans (because of minor exceptions) and constructed the road without harming the natural geological features.

Detecting Subsurface Voids

Lava tubes are natural conduits through which lava travels beneath the ground surface. The tubes form when lava channels crust over after cooling. Depending on the thickness of the material above them, lava tubes beneath a roadway could pose a significant risk to the long-term stability of a roadway and public safety.

At Lava Beds National Monument in northern California, NPS staff were concerned that lava tubes posed significant risks to roadway construction, long-term stability and maintenance of the roadway, and public safety. To address these concerns, FLH used several geophysical techniques for locating near-surface voids that could affect roadway stability.

The main objectives of the study were to detect subsurface voids in specific geological settings, detect and characterize the vertical and horizontal extent of the voids, determine the most economical and time-efficient geophysical method to use during roadway site investigations, and identify the range of applications of such methods nationwide. FLH researchers collected geophysical data at the site using GPR, magnetics, high-resolution shear wave seismic reflection, electrical resistivity, and electrical conductivity. The researchers knew the geometry and location of voids at each test site, and used the information to assess the accuracy of each geophysical method they applied to detect the voids.

"The results of the investigation indicated that some of the geophysical methods were effective in detecting voids, while other methods were limited due to the localized geological setting and void geometries," says CFLHD's DeMarco. "Depending on site conditions, such as subsurface geology or void size and depth, when a combination of methods was used, there was a greater chance of effectively delineating the location and orientation of the voids."

The researchers determined that the combined GPR and magnetic methods were the most economical and least time consuming for detecting voids with depths of 0-30 feet (0-9 meters). They recommend that engineers perform magnetic surveys first as a reconnaissance tool to locate the position of magnetic anomalies that might indicate large potential voids. The next step would be to conduct a focused GPR survey to evaluate each magnetic anomaly and determine the depth and lateral extent of the features.

Electromagnetic Induction And Clay Seam Mapping

The presence of clay beneath a roadway poses problems for rehabilitation design and construction. Roads constructed over clay could be subject to differential settlement and deformation due to changes in volume caused by swelling or shrinking, low shear strength, high moisture content, and clay structure, including dipping or horizontal bedding. To understand the behavior of soil in situ, geotechnical engineers typically take soil borings at 0.25- or 0.5-mile (0.4- or 0.8-kilometer) intervals.

Although direct soil sampling provides the best information in terms of soil type and Atterberg limits (a series of thresholds observed when the water content of a soil is steadily changed), the boring intervals could miss critical clay-rich zones, and the geologic interpolation between borings might not be representative, missing large expanses of clay. Geophysical techniques such as electromagnetic induction (EMI), however, could provide a better understanding of overall type distribution of soil behavior.

The frequency domain EMI method for mapping clay beneath roadways fills the gap between soil sampling locations. Soil conductivity information derived through EMI methods can provide valuable qualitative information for evaluating road-base materials during the design phase. This information can guide soil boring by targeting the most likely locations with potential swelling clay problems.

FLH engineers used various EMI instruments on State Route 537 (SR-537), in Rio Arriba County, NM, to map the extent of clay soils. The EMI also was used for a more detailed investigation at the Natchez Trace Parkway in Mississippi to locate clay-rich zones beneath long stretches of roadway.

The SR-537 investigation concluded that frequency domain EMI profiling would be the only cost-effective, rapid way of mapping in sufficient detail the lateral extent of clay soils in the road base. The Natchez Trace study demonstrated EMI's efficiency in mapping the spatial variation of soil conductivity within the road base.

FLH's geophysical investigations demonstrated the effectiveness of EMI, which can focus drilling programs during project site investigations, road rehabilitation, and construction. The method also can provide significant financial savings by avoiding construction cost overruns.

Leading the Way

To build and maintain safe roadway infrastructure, engineers need the ability to "see" inside rock masses before blasting, "see" that grout has in fact filled voids, and "see" that no clay seams are located under roadbeds. As technology evolves and provides better tools for exploring below the ground's surface and inside manmade structures, FLH engineers now are "seeing" where previously they could only estimate. Using geophysical technologies is likely to become standard practice in the future. In the meantime, FLH is pioneering the way for others to follow.

Amit Armstrong manages the technology deployment program at WFLHD in Vancouver, WA. He has been with FHWA for 7 years, coordinating deployment of new, innovative, emerging, and underutilized technologies in design and construction of roads on Federal lands projects. He has more than 20 years' experience in numerical simulation and visualization of natural systems, and is a licensed professional engineer. He earned a doctorate in civil engineering from Texas Tech University.

Roger Surdahl joined FHWA in 1987, after receiving a master's degree in civil engineering from Montana State University. He is a registered professional engineer in Colorado. As the technology delivery engineer for CFLHD, he brings a wide range of experience in highway materials, contract administration, and innovative solutions to transportation problems.

H. Gabriella Armstrong is an information technology consultant. When she is not writing in code, she freelances on food, culture, and travel. She received a bachelor's degree in comparative literature with a minor in Spanish from the University of Colorado.

For more information, contact Amit Armstrong at 360-619-7668 or amit.armstrong@fhwa.dot.gov, Roger Surdahl at 720-963-3768 or roger.surdahl@fhwa.dot.gov, or Gabriella Armstrong at 503-504-0340 or support@yourtechvalet.com.