Foreword

FastTIMES Special Issue

Exploration Geophysics and Nondestructive Evaluation (NDE): Adding Value to Transportation Infrastructure Condition Assessment from Project Delivery to Asset Management

Hoda Azari, Steven Cooper, Derrick Dasenbrock, Julie Johnston, Monica Jurado, Benjamin Rivers

INTRODUCTION/FORWARD

Transportation infrastructure exists all around us- providing mobility, connecting us for work and recreation, for health and culture, for entertainment and community, providing a means for the efficient and safe movement of people and goods. While transportation infrastructure includes railroads, seaports, waterways, aviation, transit systems as various modes and components within the U.S. transportation system, the network of surface roads is ubiquitous, covering over 4-million road-miles, 617,000 bridges, and hundreds of tunnels across the United States connected by some 164,000 miles of national highway arterials.  Effective use of resources to manage highway system performance, improvements and maintenance requires a sound basis on which to inform decisions, including understanding and assessing existing conditions.  Furthermore, the need to minimize disruptions to mobility; accelerate project delivery for rehabilitation and new construction; provide safe and reliable performance for roads, bridges, tunnels and drainage structures; and make effective use of limited resources have led to advancements and increased use of geophysical and non-destructive testing and evaluation technologies within the transportation industry.

In transportation engineering, there are four broad use cases where sensing and monitoring are commonly applied: 

1. Site Characterization for New Construction: Prior to construction when locations for future assets are being considered and where design plans are prepared, sites are investigated to determine the composition, stratigraphy, material properties, behavioral characteristics and variation of in-situ site conditions affecting performance and constructability. Identifying the locations and proximities of existing man-made subsurface structures and facilities, such as utilities, are also critical to evaluate construction impacts, prevent disruptions and maintain safety.  
2. Material and Construction Testing: During construction, testing is performed to assess product quality assurance, acceptance and payment.
3. Asset Condition Assessment for Rehabilitation Evaluation and In-Service Inspection: Conditions of infrastructure components and materials are evaluated to determine in-service integrity or performance as an integral part of maintaining our infrastructure while assessing and considering life-cycle repair and replacement options, budget and scheduling.
4. Forensic Investigation:  In instances where performance of facilities and component elements experience failure, repair solutions and preventative measures require investigations to determine causation. 

Purpose and Need

The purpose of each of these applications is similar – assessing the quality or condition of the site, constructed works, or structural elements. The evaluation purposes differ in resolution and reliance of measurement, when evaluations are conducted, and the depths and spatial areas or volumes of interest. Due to the different materials being considered and the different times when assessments are made, the evaluations may be conducted by a variety of different transportation engineering functional areas: Geotechnical, construction, pavement, structures, maintenance, and other disciplines often play a role. Use cases are expanding as Transportation Asset Management (TAM) moves beyond more traditional visual and qualitative inspection into areas such as formalized geotechnical performance monitoring and structural health monitoring programs. 

The geotechnical and geological community uses geophysical methods to explore in-situ soil and rock in or around transportation features to evaluate the conditions of the natural environment adjacent to and supporting constructed works. These methods provide a valuable complement to traditional drilling and sampling as much larger areas and volumes of earth and rock can be economically assessed. Some geophysical and non-destructive evaluation techniques can be applied to constructed works themselves, including wall backfill, embankment earthwork construction, pavement subgrade, and foundation elements. The pavement and structural engineering communities apply non-destructive evaluation (NDE) methods to constructed features such as pavements and bridge superstructure and substructure components at the time of construction and evaluated in-service over time. The physical principles and measures used for exploration geophysical methods are the same used for non-destructive evaluation and generally include electrical, electromagnetics, mechanical-wave and nuclear methods. In addition to the non-invasive nature of the techniques, exploration geophysics and nondestructive evaluation methods often allow comparatively large volumes of material to be examined efficiently, and not exclusively at the discrete locations. The tests are also comparatively rapid allowing minimal disruption to the traveling public and providing condition feedback quickly. 

Uses within Highway Infrastructure

There are many transportation engineering applications for the combined suite of technologies:

• Site Characterization of Embankments, Slopes and Foundation Sites 
  • Determination of location, extent, and size of voids/caverns/anomalies
    • Natural (karst/landform)
    • Man-made (mining, excavation, abandoned utilities)
    • Natural inclusions (boulders)
    • Man-made inclusions (tanks, chambers, ordinance)
  • Stratigraphy
  • Material type and classification
  • Integrity of embankment fill or wall backfill

• Roadway (pavement)
  • Pavement quality assurance and structural and functional conditions 
  • Subsurface condition survey (pavement support/material quality/location below pavement)
  • Pavement Life Cycle decision making/rehabilitation timeline 

• Bridge and Tunnel Structures
  • Foundation Element Evaluations
    • Integrity of piles and drilled shafts (shape, strength) at time of installation
    • Deterioration or corrosion detection
  • Evaluation of Cable Strands and Grout
  • Evaluation of Fracture Critical Elements
  • Evaluation of Anchored Systems
  • Evaluation of Tunnel Lining and Panel Systems
  • Bridge Deck Evaluations
    • Deterioration Detection
  • Bridge Beam Evaluations
    • Deterioration, loss of section
    • Corrosion
    • Cracking
  • Structural Health Monitoring (SHM)
    • Long-term evaluation of important or critical structures
• Utilities
  • Conflict avoidance (time, money, safety)
    • Avoid interruption of service
    • Avoid damage and repair costs
    • Avoid loss of life due to conflict with electrical or gas utility
    • Avoid loss of critical power or communication 
• Geohazards
  • Groundwater, dewatering, settlement, liquefaction
  • Slope movements
  • Landslide/Rockfall/debris
  • Karst, mining, and other subsidence considerations

Geotechnical Applications

While some project sites are reasonably uniform, ground conditions influencing design and construction can vary considerably across even across small project sites. Variability becomes a more significant consideration when a project corridor is being considered or when geologic conditions, such as mountainous terrain or bodies of water add complexity. 

Basing geotechnical project designs on limited information can result in design and construction complications when actual site conditions differ from those documented in the geotechnical reports and contract documents. Constructability issues, cost escalations, and associated delays can easily result from:

• Foundation material cost or installation time overruns due to construction difficulty

• Discovery of conditions which make planned means and methods inappropriate

• Unanticipated groundwater, seepage, and dewatering

• Misclassified or mischaracterized subgrade, soil, or rock properties

• Mischaracterized site stratigraphy, top of rock, or other location inaccuracies

• Unanticipated cobble, boulder, or rock conditions

An increasing number of agencies are updating design manuals and standards of practice to include exploration geophysics as scoping or supplemental investigation techniques. Educational and training materials are being updated to reflect the advantages and are increasing use of multiple exploration techniques and multiphase investigations as part of comprehensive and optimized site characterization strategies. Drivers include more complex and high-value projects, alternative delivery procurement processes, and a recognition that additional information lowers risk and provides a substantial return on investment. As agencies adapt their practices, geophysical techniques are being allowed and encouraged to help deliver projects more quickly and with improved quality. 

Geophysical data are now digital in their native form and can be easily recorded in the field, transmitted, processed, analyzed, reported, archived, and visualized. The information can be easily exchanged among stakeholders and used to inform decision making for more expensive invasive investigation, sampling and testing. Improved initial site characterization can provide a more comprehensive basis for understanding the measurements from geotechnical instrumentation and monitoring programs used as part of research projects, assessing project warranty compliance, or as part of a long-term geotechnical asset inventory and monitoring program. 

“Every Day Counts” (EDC) is an initiative of the Federal Highway Administration (FHWA) designed to identify and deploy innovations aimed at reducing the time it takes to deliver highway projects, improving quality, enhance safety, saving money or adding value to improve project delivery more broadly. It is a state-based model that identifies and supports proven, but underutilized, innovations. As part of the (2019-2020) EDC-5 suite of technologies, newer geotechnical tools are highlighted through “Advanced Geotechnical Methods in Exploration,” or the “A-GaME.” More common subsurface exploration methods, such as soil borings, provide relatively limited data for project design, which can result in constructability issues and increased cost. Advanced geotechnical exploration methods, such as Exploration Geophysical methods, offer solutions for generating more accurate geotechnical characterizations that improve design and construction, leading to shorter project delivery times and reducing the risks associated with limited data on subsurface site conditions. Among the wide variety of geophysical methods, electrical resistivity and seismic methods were featured as representative examples of useful methods for transportation applications as part of EDC-5. 

Geophysical Method Application (Electrical and Seismic Examples)

Seismic mechanical wave propagation techniques provide direct measurements of very small strain (1×10-6) soil properties (GMax, EMax) useful for evaluating load-displacement behavior, seismic hazard susceptibility, and seismic soil-substructure interaction modelling. These techniques also aid in establishing stratigraphic changes, boundaries, and anomalies within subsurface profiles over large areas. When used in conjunction with point-specific in-situ test data, stratigraphy changes between points can be more accurately inferred and confirmed as necessary. This technology can therefore result in improved spatial accuracy of subsurface models, and an improved understanding of the degree of variability at a site.

Electrical resistivity techniques can identify stratigraphic boundaries and anomalies providing subsurface profiles over large areas; these techniques are particularly useful for distinguishing materials having dissimilar conductivity, making them useful in identifying clay, voids, and water-bearing layers within more granular soils or bedrock. Related techniques such as induced polarization and self-potential are particularly useful in identifying variations due to mineralogy, groundwater conditions, and groundwater flow. Applying electrical resistivity techniques in conjunction with other tools such as cone penetration testing with an electrical resistivity module to verify point-specific data can greatly improve site characterization including variations in groundwater levels across the site.

NDE Method Application

Visual inspection methods have been the primary nondestructive method under the National Bridge Inspection Standard (NBIS), representing the highest percentage of the inspection procedures. The use of NDE methodologies and noninvasive techniques have been widely increased by advancements in computational devices, sensor technologies and other interdisciplinary fields. NDE is now a relatively mature field emphasized with the most incredible progress leading to smaller, lighter and smarter instruments providing the ability to detect problems that the bare eye cannot properly identify or effectively characterize. These NDE improves transportation agencies plan for, design, build, and operate the nation’s transportation systems. On the other hand, effective NDE technologies in terms of reliability, sensitivity, accuracy and benefits within the highway infrastructure realm requires understanding the problem; technology integration and development; and partnerships across government, academia, and industry sectors. Input from these groups is essential to the success of the NDE Programs which can be credited to the findings of a number of national projects (e.g., the second Strategic Highway Research Program (SHRP2)) and close collaboration and cooperation among asset owners, researchers, technology providers, and policy makers. SHRP2 has produced numerous solutions that have been implemented in the field to solve various transportation challenges to lead NDE toward inventing lower cost methods and designing instruments with greater reliability, sensitivity, user friendliness and high operational speed.

The SHRP2 program have given agencies:

• Cost-effective bridge designs for faster, longer-lasting replacement
• Pavement preservation techniques for high-traffic roadways
• Methods to improve operations and extend capacity
• Innovative strategies for managing large, complex projects
• Planning techniques for conserving green spaces and protecting the environment
• Training for fast, multi-agency response.

In addition, nondestructive evaluation is crucial to operate and maintain high-value infrastructure assets to optimize performance, extend asset lifecycles and reduce operational downtime and costs. While many technologies offer potential benefits in a quantitative infrastructure asset-management plan, their use presents challenges. NDE provides insights that help asset owners to make better decisions, enhance efficiency, perform preventive maintenance and maximize investments in assets. With monitoring, maintenance, computer vision, safety and reliability, NDE is enabling the next generation of asset management. With that regard, asset owners are gaining more confidence in using NDE technologies as the state of the practice for inspection and evaluation of the transportation infrastructure for quality assurance and quality control of new bridges as well as condition assessment of in-service bridges and forensic analysis when appropriate.

The trend toward a global market led to a growing recognition of the value of international standards for test procedures and personnel qualification, that are being gathered by engaging relevant professional organizations and technical committees, such the American Association of State Highway and Transportation Officials (AASHTO) on Bridges and Structures, Maintenance, Materials and Pavements, and Construction; Federal Highway Administration (FHWA) NDE program; the American Society of Nondestructive Testing (ASNT); and the Transportation Research Board; among others.

NDE Method Application (Ground Penetration Radar and Infrared thermography Examples)

NDE Instruments are now commercializing on platforms using common nondestructive methods such as acoustic and electromagnetic waves, electromagnetic radiation, and methods developed based on electrical and magnetic properties of materials. In addition, data acquisition and software packages are available to collect and analyze data obtained from various NDE methods. The increased processing speed and improvement in hardware is allowing real-time imaging of all the NDE methods such as ground penetration radar, infrared thermography, radiography, ultrasonic, shearography, etc. Nondestructive test procedures deploy computers, electronics and robotics hardware in interactive processes with analytical and numerical analysis to develop graphical outcomes. Furthermore, efforts are being made to integrate and fuse several methods. The complimenting capabilities offer greater detectability and the overlapping ones enhance the reliability. In parallel, data fusion techniques are being developed to allow effective data-acquisition and processing, providing sound interpretation of test parameters in relation to the material integrity.

Concrete degradation and steel corrosion are major concerns in highway bridges, deteriorating the structural integrity and service capability of impacted bridges overtime. The most common NDE methods used by State department of transportations, primarily on bridge decks, pavement, and tunnels, are ground penetrating radar (GPR), infrared thermography (IR), and impact echo. Among non-destructive testing methods to evaluate bridge condition, GPR is known as multifunctional nondestructive test method that examines large areas in a short time, together with providing information on the depth and spacing of reinforcement. GPR uses electromagnetic wave pulses in the microwave band of the radio spectrum to image the subsurface features in bridge decks. Radar reflections from inside the material are then imaged for in-depth characteristics of subsurface layers and can be used for detecting honeycombing areas, delaminations, voids, cracks and active corrosion in real-time. GPR can be deployed by hand or on push carts (“ground coupled”) for a slow, more detailed inspection, or attached to a vehicle at traffic speeds (“air coupled”), but with some lost resolution in exchange for speed.

Infrared thermography technique is a full-field nondestructive tool that widely used by State DOTs. Infrared thermography has a potential to detect subsurface delaminations before spalling develops, and enhancing the visual inspection of concrete bridges. Infrared thermography is a valuable, non-contact, not time consuming and user-friendly nondestructive tool that uses thermal imager to detect radiation (heat) coming from the surface of the structure. Infrared sensors detect the thermal energy that is radiated from objects in the infrared band of the electromagnetic spectrum converting it to temperature and displaying an image of the temperature distribution called thermograms. Infrared thermography uses a thermal imaging camera that record the temperature of an object at multiple points across a large area, creating two-dimensional thermographic images. Flaws in the bridge deck, pavement, or tunnel liner heat and cool at a different rate, providing a surface image of the location of the flaw.

Non-destructive evaluation (NDE) has also been widely used in pavements as a rapid test tool for construction quality assurance and structural condition evaluation of existing pavement for the purposes such as, (i) estimation of its present sufficiency, (ii) estimation of remaining service-life and (iii) decision on the choice of rehabilitation measure. The nondestructive nature and the rapidity of NDE minimize disruption to traffic, provide a better indication of the variations in the pavement, and may reduce the cost of testing.

An article showcasing the pavement NDE program of the Minnesota Department of Transportation (MnDOT) is provided in this issue as an example.  MnDOT has a wide range of NDE equipment, which provide its capabilities for comprehensive and complete pavement evaluations.  The article highlights the following MnDOT NDE equipment and its capabilities along with examples of each equipment:

• Road Doctor Survey Van (RDSV): The RDSV collects continuous surface and subsurface measurements by integrating state-of-the-art hardware and advanced software to process, synchronize, and visualize large and complex data.
• Density Profiling System (DPS): The DPS measures the Dielectric Constant, which is correlated to Density, or percent of maximum compaction (% Gmm) of the placed asphalt pavement mat
• A1040 MIRA Device: The MIRA is an ultrasonic tomographic device that provides imagining of the internal structure of concrete pavements.  
• Ground-Coupled Ground Penetrating Radar (GPR) used for Sink Hole Analysis: The GPR directly map the void surface extent by analysis of the increase in direct reflection.   
• MnROAD Digital Faultmeter: The Digital Faultmeter Digital Faultmeter is an electronic digital device that is used to measure joint faulting in concrete pavements.
• Lightweight Inertial Surface Analyzer (LISA): The LISA is used to measure the longitudinal pavement profile and used to calculate the International Roughness Index (IRI).
• Automated Laser Profile System (ALPS): The ALPS is used to collect surface profiles on both Portland cement concrete (panel warp/ curl) and asphalt concrete pavements (rutting).

In addition to applications to pavements, subsurface utility engineers and geophysical service providers need the ability to detect, locate, and characterize subsurface utilities but face numerous challenges in doing so. Underground utilities can be made from many different types of materials and can be located at various depths in soil conditions ranging from silty clay to sandy loam. Because of these constantly changing conditions, it may take several different technologies to locate and identify unknown utilities.

The best overall practice is to employ multiple types of geophysical technologies, deployed in multiple channel modes when possible. Using digital geophysical mapping in conjunction with common pipe and cable locating tools enhances utility detection and data interpretation. This combined approach produces more complete mapping and supports a more targeted and less expensive test hole program.

Preface to Articles

As the demands on surface transportation infrastructure systems are growing and expanding, the need to adapt, develop, and implement innovative technologies and solutions to meet new challenges involving system performance, improvement, maintenance and management are likewise evolving and changing process workflow and engineering practice.

Minimizing transportation disruptions; accelerating project delivery for rehabilitation and new construction; providing safe and reliable performance from roads, bridges and drainage structures; and making effective use of our limited resources have all prompted increased reliance on geophysical and Non-destructive testing and evaluation technologies in the transportation industry. 

The Federal Highway Administration has long championed the development, evaluation and implementation of these technologies for transportation application in partnership with state and local highway agencies and industry through multiple programs and efforts.  However, please keep in mind that the U.S. Government does not endorse any specific products or manufacturers. Trademarks or manufacturers’ names that appear in this article and throughout this publication do not reflect a preference, approval, or endorsement of any one product or entity by the U.S. Government. The articles that follow showcase effective examples of applying Geophysical and NDE methods and technologies within transportation – from project delivery to managing our transportation infrastructure – which was the objective.  We sincerely thank all the authors who have contributed to this issue on the use of geophysical and NDE technologies within highway infrastructure.