As of October 1, 2016, the SWUTC concluded its 28 years of operation and is no longer an active center of the Texas A&M Transportation Institute. The archived SWUTC website remains available here.


SWUTC Research Project Description

Sustainability of Bridge Foundations Using Electrical Resistivity Imaging and Induced Polarization to Support Transportation Safety

University: Texas A&M University

Principal Investigator:
Stefan Hurlebaus
Texas Transportation Institute
(979) 845-9570

Project Monitor:
Dr. Mark Everett
Department of Geology and Geophysics
Texas A&M University

Funding Source: USDOT

Total Project Cost: $43,950

Project Number: 600451-00028

Date Started: 1/7/12

Estimated Completion Date: 12/31/14

Project Summary

Project Abstract:
The goal of this research project is to validate the use of induced polarization and electrical resistivity imaging testing for determining the depth of unknown bridge foundations.  With budget cuts and deteriorating infrastructure, there is a need to seek alternative solutions for nondestructive structural integrity testing that are more robust to limit bridge failures that hinder transportation safety.  The existing methods for unknown bridge foundations in the literature are often hindered by the type of foundation or require the use of a borehole, making them very costly.  As a result, only a few states are working to identify the depth of unknown bridge foundations around them.  In order to solve this national problem, a new and effective method needs to be investigated with full scale bridge testing and disseminated nationwide.  Foundations at a National Geotechnical Testing Site (NGES) with be tested, followed by three bridges with known foundations, and bridges with known foundations unknown to the researchers.  Using as-built plans, a probability of exceedance curve for the results will be created.

Project Objectives:
The objectives of this study are (1) to establish that bridge foundations possess a measureable IP response on known foundations at a NGES, (2) to establish the range of applications for IP and ERI in the field to detect known foundations, (3) to develop a method to obtain the probability of exceedance of an unknown foundation depth with collected data from unknown foundations, (4) to create acquisition protocols for using IP and ERI for unknown foundations to optimize its use nationwide, and (5) to evaluate the method for other subsurface civil engineering infrastructure applications.

Task Descriptions:
Task 1: Literature Review

The research team will compile a literature review, using the resources at Texas A&M University within the Evans Library and limitless availability to online journals that the university holds.  Despite the initial background completion, literature reviews are a constantly evolving project that will continue to grow as the project develops.

Task 2: Foundation Calibration
Task 2 studies the ability of IP successfully image a bridge foundation and the protocol for using the two methods together.  ERI is a common and well understood geophysical method that has been proven for unknown foundations yet it has not been widely applied [2].  The two are used in conjunction with each other because ERI alone is hindered by slender foundations (less than 1 m) and highly conductive formations.  Conversely, IP works extremely well for highly conductive materials.  Because the field setup of the two measurements is very similar, with extra electrodes required for IP, it is advantageous to gather both sets of data in order to get as much information as possible for identifying the unknown foundation.  Task 2 will be carried out at a NGES near Texas A&M University.

This site has both a “sand” site and a “clay” site where different soil conditions can be tested.  Each site also has both shallow and deep foundations.  The sand site, shown below, contains five spread footings and five reaction shafts.  There are two 3 x 3 x 1.2 m3 footings, one 2.5 x 2.5 x 1.2 m3 footing, one 1.5 x 1.5 x 1.2 m3 footing, and one 1 x1 x 1.2 m3 footing along with four 21.3 m long, 0.91 m diameter drilled shafts with underreamed bells and one 5 m long, 0.91 diameter drilled straight shaft.

The general soil layering at the sand site consists of a medium dense tan silty fine sand to 3.5 m below the surface, medium dense silty sand with clay and gravel from 3.5 to 7 m, medium dense silty sand to sandy clay with gravel from 7 to 11 m and very hard dark gray clay from 11 to 33 m.  The water table at the sand site is located 4.9 m below the surface [3].  The general soil stratigraphy of the clay site consist of approximately 5.8 m of stiff, silty clay over a 0.9 m layer of sand and gravel, below which mainly a hard, dark clay exists [4].  Four 0.91 m diameter drilled shafts are located at the clay site with lengths of 23.2 m, 9.5 m, 11 m, and 9.2 m.  The 11 m shaft has a 45° underreamed bell.  Because the known foundations are in the ground without a super structure, controlled field experiments will be conducted here as a supplement for laboratory work.  Research will begin at the sand site beginning with the smallest footing and working up in size to the drilled shafts.

Task 3: Field Calibration
Task 3 includes increasing the complexity of the project by moving from single foundations to full scale bridges.  A database with as-built plans of bridges within the Bryan TxDOT district is available to the researchers with types of substructures.  Of the 511 bridges in this database, 54.0% are concrete piling, 21.5% drilled shaft, 12.1% steel piling, 3.5% spread footing, 2.0% pile cap on steel piling, 0.8% pile cap on concrete piling, 0.6% timber piling, and 0.2% pile cap on timber piling [5].  A minimum of three bridges will be tested with increasing complexity.  Complexity will be introduced with variables such as soil conditions, varying foundation type, gradient terrain, and water.

Task 4: Probability of Exceedance
Geophysical methods are a useful tool for engineers when subsurface properties must be investigated nondestructively.  Unfortunately, these methods typically have not been evaluated from a reliability standpoint so many engineers are wary of their use.  By testing known substructures the accuracy and reliability will be determined for bridge foundations.  This information will be used to create a probability of exceedance curve for bridge foundation depth predictions.  This will be a strong contribution for the engineering community that will make judgment calls on whether or not this approach should be employed for their use.

Task 5: Extension to other applications
Additionally, both ERI and IP will be extended to subsurface investigations for other buried objects that are frequently investigated by engineers like pipeline mapping.  The two methods used together have the potential to be useful for other infrastructure issues beyond unknown bridge foundations.

Task 6: Validation
After the approach has been calibrated with the known foundations, IP and ERI will then be used for testing bridges with foundations that are unknown to the research team.  In order to determine the accuracy that is captured by IP and ERI on different types of substructures, select bridges will be tested before the researchers are given the as-built bridge plans.  This phase of the project will serve as a mock trial for testing the bridges in which the researchers will implement the same approach that they are recommending for national use.  In this way, after the testing is complete, when the plans are seen by the researchers they will be able to view the results with unbiased eyes.  Other current nondestructive testing methods for unknown foundations are expected to be within 1.5 m of accuracy so this is the level that the research team will achieve in order to be considered successful.

Task 7: Dissemination of Results
The final project report will comprehensively document all work performed.  This will include the literature review; acquisition protocols for using IP and ERI for unknown foundations; experimental results at the NGES, full-scale bridges, and additional subsurface infrastructure; and the validation of the method using the probability of exceedance curve for the depth prediction.  The report will be presented in a logical format so that engineers can quickly and easily obtain data from within the report.

Implementation of Research Outcomes:
As of September 2007, there were 67,240 U.S. bridges in the National Bridge Inventory classified as having unknown foundations (FHWA 2008). The bridges spanning rivers are of critical importance due to the risks of potential scour. In fact, it is estimated that 60 percent of all bridge collapses are due to scour (Parola et al. 1997). Not only are these failures costly, they can be deadly for the traveling public. Detecting scour is only part of the assessment that must take place to determine risk of failure and knowing the foundation depth is a critical component of the assessment. The existing methods for unknown bridge foundations in the literature are often hindered by the type of foundation or require the use of a borehole, making them very costly.  As a result, only a few states are working to identify the depth of unknown bridge foundations around them. 

This research explored the feasibility and effectiveness of induced polarization (IP) and electrical resistivity imaging (ERI), near surface geophysical methods, for determining the depth of unknown foundations. With budget cuts and deteriorating infrastructure, there is a need to seek alternative solutions for nondestructive structural integrity testing that are more robust to limit bridge failures that hinder transportation safety.  With this research, an experimental field study was conducted at a National Geotechnical Experimentation Site (NGES) to identify key parameters for the testing design and setup in order to obtain optimal surveys of bridge foundations.  The results showed that IP and ERI can be used in concert with one another to estimate the type and depth of bridge foundations. The results of the experimental field surveys were used to create a probability of non-exceedance curve for future predictions of unknown bridge foundations using the methods described in this research. Finally, the probability of non-exceedance curve was used to validate the method with testing on a foundation unknown at the time of testing.

Products developed by this research include:

Presentation:  Nondestructive Testing of Subsurface Infrastructure using Induced Polarization and Electrical Resistivity, S. Tucker, presented at Kansas State University, Manhattan, Kansas, February 11, 2013.

Journal Article Under Review:  Electrical Resistivity and Induced Polarization Imaging for Unknown Bridge Foundations, S. Tucker, J.-L Briaud, S. Hurlebaus, M.E. Everet, and R. Arjwech, ASCE Geotechnical and Geoenvironmental Engineering, under review.

Impacts/Benefits of Implementation:
The results of this research directly benefit the state of good repair of the nation’s bridge inventory.  For certain types of bridge foundations, it provides a new, less costly and effective method for determining foundation depth.  Researchers also showed that the method can be used for imaging other subsurface structures such as gas lines and other underground utilities.

Web Links:
Final Technical Report