Means and methods for construction of geotechnical features are advancing at a rate that is outpacing methods for effective quality assurance, requiring reassessment of commonly used design methodologies. The innovations in bridge foundations (diameter, depth, and materials), earth retention (system innovations), and ground improvement (improved and new solutions) challenge the transportation community to develop solutions through research and guidance updates to address an evolved geotechnical construction industry.

Spotlight Project: Design, Construction, and Long-Term Performance of Geosynthetic Reinforced Soil Abutments and Integrated Bridge Systems

A key success of the Geotechnical Research Program was the development and implementation of the Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) to meet the replacement demand for the “bread and butter” bridges in the Nation’s inventory. The proven performance of bridges constructed with this technology led to the selection of GRS-IBS for the FHWA’s first, second, and third rounds of the Every Day Counts (EDC) initiative.

Since deployment through EDC, research has continued to optimize the design and construction of GRS abutments and IBSs and to evaluate long-term performance of in-service structures. Current research products of those continued efforts include Design and Construction Guidelines for Geosynthetic Reinforced Soil Abutments and Integrated Bridge Systems (FHWA-HRT-17-080), a research report Instrumentation and 5-Year Performance Monitoring of a GRS IBS in St. Lawrence County, NY (FHWA-HRT-20-040), and other external publications. Ongoing research includes the evaluation of large-scale GRS experiments at Turner-Fairbank Highway Research Center.

Image source: FHWA.

The Geotechnical Program continues to advance the state of the practice in geotechnical site and laboratory characterization. This will include two big picture areas of activity. The first is to promote the use of underutilized technologies (e.g., cone penetration testing (CPT), measurement while drilling (MWD), geophysical methods, televiewers, etc.) for conducting site investigations in conjunction with the Every Day Counts Round 5 (EDC-5), the Advanced Geotechnical Methods for Exploration (A-GaME).

Second, the program will be working on advancements related to understanding the value of an adequate geotechnical site characterization (e.g., quantity and type) and quantifying uncertainty in transformations for establishing design parameters for reducing risk in project delivery. This will be accomplished through program sponsored activities.

Spotlight Project: Laboratory Characterization of Open-Graded Aggregates

State and local transportation agencies frequently use crushed, manufactured, open-graded aggregates (OGA) as structural backfill material for retaining walls, bridge foundations, and other ground improvement applications, yet their strength characteristics have not been fully understood or applied to improve design efficiency. To address this knowledge gap, FHWA initiated an advanced laboratory testing program to characterize the engineering properties of OGAs. Results of that program have been published in TechBriefs, research reports, and external manuscripts. Ongoing work includes an interlaboratory study to evaluate the variability for load and resistance factor design calibrations.

Image source: FHWA.

In a collaborative effort, the Geotechnical and Hydraulics Programs have developed a strategic plan called NextScour for advancing the practice in project delivery. Geotechnical research and program activities will focus on developing and validating parameters that better characterize soil and rock under water loads. The challenge facing the NextScour and the national Geotechnical Program is to modify and update scour models to accurately predict the initiation of soil and rock erosion, and identify factors controlling the depth and rate of erosion.

Spotlight Project: Evaluation of Soil Erosion Testing Devices and Transformations to Quantify Soil Erodibility

The Federal Highway Administration’s (FHWA’s) Geotechnical and Hydraulic Research Programs (as part of the FHWA NextScour Program) aim to compare three erosion test devices used to measure/interpret the critical shear stress of soil: the erosion function apparatus (EFA), the ex-situ scour testing device (ESTD), and the field scour testing device (FSTD), which include the in-situ and portable scour testing devices (I/PSTD). The study will evaluate the inherent differences between different ex-situ and in-situ soil erosion testing devices, compare the resulting critical shear stresses between the devices for various engineered (i.e., manmade) soils, and determine appropriate equivalency factors or transformations between the different devices and corresponding test results. The collected test data will also expand on the available dataset of soil parameters and erosion resistance for future research efforts.

Image source: FHWA.

With State departments of transportation (DOTs) adopting transportation asset management (TAM) programs for bridges and pavements, the National Geotechnical Program faces the challenge of contributing to TAM by developing a geotechnical asset management program (GAM) that supports wholistic management of infrastructure assets. Because most geotechnical assets—or elements of assets—are buried, it is not only extremely difficult to manage the condition of most geotechnical assets, it is extremely difficult to develop an approach to understanding system performance as part of a life-cycle analysis.

Spotlight Project: Quantifying the Bump at the End of the Bridge using High-Speed Inertial Profilers

The bump at the end of the bridge (BEB) is one of the most prevalent factors impacting maintenance and ride quality at a bridge’s approach and departure and can be a safety hazard to motorists. The causes of the bumps, along with mitigation strategies, have been well researched, yet the BEB remains ubiquitous, leading to chronic maintenance activities.

The occurrence of the BEB is usually detected qualitatively based on road user feedback, with maintenance strategies subsequently implemented to improve the ride quality. BEB and road roughness can cause many issues beyond user discomfort including: driving safety concerns, vehicle damage, increased maintenance requirements for both automobiles and the bridge infrastructure, higher impact loads, and thus, decreased service life of the bridges.

To assess and quantify the BEB on long-term bridge and pavement performance, FHWA has initiated research to explore potential methods to quantify the roughness at bridge approaches using a commonly used TAM tool, the high-speed inertial profiler, to evaluate appropriate ride indices such as the International Roughness Index (IRI) and Rolling Straightedge (RSE), and to establish performance metrics for the BEB. The most recently completed research efforts have included a technote, Synthesis—The Reduction and Analysis of Pavement Profiler Data to Quantify the Bump at the End of the Bridge (FHWA-HRT-20-021), and the research report Statistical Analysis of Pavement Profiler Data to Evaluate the Bump at the End of the Bridge (FHWA-HRT-21-037).

Image source: FHWA.

In a collaborative effort, the geotechnical program will be providing support to the pavements program to develop proactive solutions for the geotechnical aspects of pavements including the rapid, and cost-effective development of embankments and pavement foundations. The aggregate base supporting all of roads is perhaps the single largest asset in the transportation infrastructure.  This hidden asset is assumed to function for the life of the pavement structure.

Nationally, the issue of contaminated base courses may become even more evident considering the age of the national highway system.  Quality aggregate material is an important natural resource and advances planning and preservation is required to ensure a sustainable supply of aggregates.  The challenge for the National Geotechnical Program is to identify and execute activities that support a broad path to more consistent and significant geotechnical contribution to pavement design and construction.

Spotlight Project: Optimizing Unbound Aggregates for Permeability, Stiffness, and Permanent Deformations

The aggregate base course is an important structural component of a pavement foundation; therefore, the long-term performance of roads relies heavily on the gradation and properties of the aggregates used in this base layer. An ideal aggregate blend would be one with high strength, large stiffness, and good permeability to resist the effect of traffic loading, while also ensuring adequate drainage. However, previous studies have found that there is a tradeoff between permeability and structural stability of nonstabilized aggregate bases (i.e., the higher the permeability, the lower the stability).

The objective of this study is to evaluate a common, well-graded aggregate base material and optimize its permeability and stiffness by varying the percentages of fines, changing the fine sand content, and blending with different quantities of a crushed, manufactured, open-graded aggregate. This work also illustrates the importance of the time to drain of 50 percent time factor for aggregate blends and discusses its implication within the context of current pavement design practice.

Image Source: FHWA

The FHWA Hydraulics Research Program is a part of the National Hydraulics Program that coordinates research with the National Hydraulics Team (NHT), including headquarters, FHWA Resource Center, and Federal Lands Highway offices. Coordination of the program and research also includes the J. Sterling Jones Hydraulics Research Laboratory (Hydraulics Lab) at the FHWA Turner-Fairbank Highway Research Center (TFHRC). Research is conducted in the laboratory and includes collaboration with external partners. The research needs studied are related to the six National Hydraulics Program functional areas.

Hydrology and Extreme Weather

Flood frequency estimation for hydrologic design under changing conditions. Changes to flood frequency and magnitude are still largely unknown. These topics are investigated: (1) Identification of rivers where trends in peak flows are (and are not) present; (2) Attribution of observed peak flow trends; (3) Extrapolation observed or projected peak flow trends to the future; (4) Determination of when and where adjustments are needed. Research is conducted at the Department of the Interior's U.S. Geological Survey Office of Surface Water.

Highway Drainage/Pavement Hydraulics

Hydroplaning Modeling Research. Computational Fluid Dynamics (CFD) Large Eddy Simulation (LES) modeling is used to assess water accumulation to the point of high risk for hydroplaning conditions. Several specific road geometries are assessed for vulnerability to produce conditions for significant hydroplaning risk, and a series of road longitudinal slopes and a couple of cross slopes for multilane roads are tested (Figure1). Research is conducted at Research conducted at TFHRC Hydraulics Lab and the U.S. Department of Energy's Argonne National Laboratory Transportation Research And Analysis Computing Center (TRACC).

Image source: FHWA

Figure 1: Hydroplaning CFD simulation

Culvert Hydraulics

Stream Simulation Design of Culverts for Aquatic Organism Passage (AOP). This research covers best practices for the design and construction of culverts to accommodate aquatic organism passage using principles couched in geomorphic science and hydraulic engineering. Installed projects to demonstrate favorable life-cycle costs compared with alternative design procedures are monitored and documented. This project also includes an innovative approach to estimate channel-forming flow and associated bankfull width using computational fluid dynamics (CFD), flow duration curves, and erosion rate functions. Research is conducted at the Hydraulics Lab and TRACC.

Bridge Hydraulics

Improvement of SRH-2D Hydraulics Modeling Software. The Hydraulics Lab provides technical assistance and conducts physical and CFD experiments to validate sedimentation and river hydraulics (SRH) two-dimentional (2D) model. In addition, CFD modeling is performed for several complex hydraulic case studies to verify the application of 2D modeling and understand when higher levels of analysis with CFD are warranted (Figure 2). Research conducted at the Hydraulics Lab and TRACC.

Image source: FHWA

Figure 2: CFD Streamlines through a bridge opening

Next Generation FHWA Scour Program. NextScour recognizes that the phenomenon of scour consists of two major aspects or components: (a) consideration of water and hydraulic forces (loads) causing (b) erosion resistance of soils, and their associated geotechnical effects (resistance). NextScour seeks to research and develop a design tool using Computational Fluid Dynamics (CFD) that computes hydraulic loads across the bathymetric domain. When linked to NextScour’s geotechnically derived subsurface erosion maps/stratigraphy and information, the design tool produces instantaneous and simultaneous, 3D scour bathymetries for all bridge foundation elements (Figure 3). Research is conducted at the Hydraulics Lab and TRACC.

Image source: FHWA

Figure 3: CFD Scour simulation

Coastal Highways

Scour Estimation for Tsunami at Bridges. The potential hazards to bridges from a tsunami event may include an inundation hazard for the traffic, the force effect on the structure, the erosion of the foundation caused by hydraulic load on soils or rocks, and the erosion of the approach. This study uses a laboratory-validated hybrid numerical scheme that combines CFD with sediment transport analysis to provide a practical evaluation method for scour depth at bridge foundations. Research conducted at Hydraulics Lab and TRACC.