This paper presents an overview of several stress-wave based nondestructive testing methods which can be used to assess the condition of concrete structures such as dams, buildings, and foundations. The specific methods to be presented include the use of the impact echo (IE) and spectral analysis of surface waves (SASW) methods in the assessment of dam concrete condition (including freeze-thaw damage assessment), the use of ultrasonic pulse velocity tomography (UPV tomography) in the 2 dimensional imaging of concrete defects in walls and foundations, and the use of the crosshole sonic logging (CSL) method for rapid, accurate, and cost-effective quality assurance of drilled shaft foundations. Included in this paper are summary descriptions of each of the NDT methods used (including some underlying theory), along with brief case histories of the application of each of these methods to real-world problems. Case histories presented include the evaluation of the Rogers' Dam spillway for freeze-thaw damage and overall concrete condition, the use of the CSL method in quality assurance testing of foundations for the LA Metro Green Line, and the use of tomography for imaging a defect in a deep foundation.
Remote monitoring of a deterioration process has been studied through radar measurements of laboratory size concrete specimens representing conditions of concrete dams. The investigation includes manufacturing of concrete specimens which model different deterioration stages of a dam, radar measurements, and imaging of the specimens. The specimens have a delamination filled with air or water, which represent different physical conditions of a deteriorating dam. The change of the returned signals from the specimens is related to the condition change inside the concrete. The results show the feasibility of using a radar for monitoring the condition change of concrete dams at a distance.
The nation's failing infrastructure points to the need to develop new techniques and equipment for the diagnostic evaluation of concrete in locks, dams, bridges, pavements, etc. An ultrasonic pulse echo (UPE) system for the non-destructive testing of concrete structures was developed at the U.S. Army Corps of Engineers, Waterways Experiment Station (WES) in the late 1980s. Because of the overwhelming amount of information in ultrasonic pulse echo (UPE) signals from concrete, learning to interpret raw data can be confusing. Although a human has the ability to recognize and calculate the complex signal information it can take months or years to master interpretation. The application of digital signal processing (DSP) algorithms for computerized signal interpretation can help reduce these problems. Computer processing of signals is also needed so that there is less dependency on highly experienced personnel and more objective diagnostic decisions can be made. This paper explores several DSP techniques aimed at computer interpretation. Three techniques were investigated: split- spectrum processing (SSP), ray-based modeling (RAM), and artificial neural networks (ANN).
Mill Creek Dam, near Walla Walla, Washington has experienced anomalous seepage since its first filling in 1941. Various attempts to abate and control the seepage, including construction of a concrete wall, have not been completely successful. Construction of the cutoff wall reduced the seepage by about 30 percent, from 33 cubic feet per second to 22 cubic feet per second, and downstream saturated farmland was reduced by 56 percent. However, there are indications of increased seepage pressures in a conglomerate formation in the right abutment. A comprehensive, integrated geophysics investigation of the right abutment area of the dam was conducted to detect and map anomalous conditions and assist in the evaluation of remedial measures. The geophysics program consisted of microgravity, ground penetrating radar, seismic reflection, electromagnetic conductivity, and electrical resistivity surveying. Results of the program indicate anomalous conditions extending from the reservoir area through the right abutment. The aspects of the program planning leading to technique selection and field procedures are emphasized, as well as the role of different geophysical techniques in defining the nature of anomalous condition.
Most conventional seismic geophysical means of determining material stiffness (e.g., shear modulus) for engineering studies have important limitations when used at dams and landfills because they are intrusive and the results are adversely affected by the inherent non-isotropic state of stress beneath slopes. One exception is the spectral-analysis-of-surface-waves (SASW) method which provides a non-destructive and non-intrusive means of measuring Rayleigh wave dispersion which can then be used to determine the vertical variation of material stiffness. The SASW method was developed under `level ground' assumptions and has proven to be an effective method for layered media. The SASW method was recently found to be an appropriate method to determine stiffness for sloping ground conditions. The potential application of the SASW method to slopes of dams and landfills is addressed by presenting the results of this theoretical study of surface wave propagation in sloping ground and a validation study at a sloping ground site.
An intensive crosshole seismic survey was done across a 700-foot-long stone-masonry dam. It involved measurements on six connected panels each approximately 100 feet in width extending completely across the dam from abutment to abutment. The objective was to provide tomographic images of P-wave velocity and dynamic elastic moduli of the dam and foundation materials along the axis of the dam. Field seismograms were recorded with an airgun source and hydrophone detectors. Data analysis included interactive time-picking, plotting of common source gathers, and tomographic imaging using an iterative back-propagation technique. Color-coded tomograms of velocity and dynamic Young's modulus were produced and correlated with geological and geophysical data measured on drill core samples. Low values of velocity and dynamic elastic modulus correlated with low RQD and high fracture frequency. The tomograms showed significant variations of mechanical properties in the stone masonry dam and its foundation. The colored tomograms were useful in highlighting zones of weak rock possibly requiring remedial action. They also assisted engineering evaluation of the dam by providing a detailed two-dimensional distribution of mechanical properties which can be used as ground truth data for numerical modeling of stress-strain fields.
The behavior of engineering systems is highly dependent on the distribution of material parameters. The goal of tomographic imaging is to invert boundary measurements to determine the field of a parameter. This paper reviews difficulties in tomographic imaging, summarizes inversion methods and ray tracing algorithms, and presents a comparative study of matrix-based inversion methods for simulated, laboratory, and field data. The effect of errors in ray-paths and travel times is assessed.
An in situ geophysical investigation consisting of crosshole and downhole shear wave (S-wave) and surface vibratory tests was performed at Sardis Dam, located on the Tallahatchie River in northwest Mississippi. Tests were conducted in a pile test section at the downstream toe of the dam. The purpose of the investigation was to determine changes in S-wave velocities, which are related to soil strength, in the test section due to pile driving activities. Comparison tests were conducted three times: prior to driving the piles, immediately after, and three months after the piles had been driven. The S-wave crosshole and surface vibratory tests indicated a significant velocity increase, averaging more than 20 percent, due to the driving of piles in the test section. Of the geophysical tests conducted, crosshole testing was the most sensitive in denoting velocity changes. The downhole S-wave test did not detect any significant velocity increases. The scope of this paper is limited to results of crosshole testing.
The use of the spectral-analysis-of-surface wave (SASW) method is becoming more common as a tool for integrity testing of lifeline and infrastructure components. The method, which is based upon a fundamentally correct theoretical background, can be utilized at several scales. So far, the method has been used in numerous projects dealing with diverse topics from the integrity of dam foundations, to the soundness of concrete slabs, to the suitability of structural members. In this paper, the efforts by the authors towards developing automated procedures at three different scales are described. First, an automated procedure for reducing data for large scale tests (depth of penetration up to 100 m) is presented. In the second section, a new trailer- mounted device, that in less than 40 sec automatically collects and interprets SASW data, is described. Finally, a new hand-held device, that in less than fifteen seconds, collects, reduces, and archives the results over a distance of 15 cm, is presented.
To aid in determining the depth of unknown bridge foundations, two artificial neural networks were trained to predict the length of piles from sonic mobility test data. Synthetic mobility data generated using a one-dimensional model of the foundation system was used to train the networks. To simulate bridge bents and pile caps, a massive element atop the pile was included in the model. The source and receiver positions were on top of the simulated bent or cap. The first network was trained using `raw' mobility data; the second network was trained using `enhanced' mobility data in which the response of the bent or cap was subtracted from the response of the whole foundation system to emphasize resonances associated with the pile tip. Both networks yielded accurate predictions of pile length when tested with additional synthetic data. The networks also yielded reasonably accurate predictions of pile length when tested with experimental mobility data from two bridges. improved artificial neural networks would likely result if they were trained using experimental, rather than synthetic, data.
Evaluating existing pile foundations is becoming more critical as the nation's infrastructure ages. Limited or non existent pile installation records are common making it very difficult to assess an existing foundation's capacity, evaluate pile integrity, and determine pile penetration in scour conditions. Low and high strain nondestructive testing are cost effective tools to determine pile length, integrity, and bearing capacity of existing pile foundations. This paper provides a general overview of low and high strain testing as applied to testing existing piles incorporated in structures.
Many tall reinforced concrete structures in electricity generating plants have been in service for over a quarter of a century. Like may others in our infrastructure, they are showing signs of deterioration. However, their relative size and height make the task of their inspection a difficult and costly one. This paper introduces a new and cost-effective approach for their evaluation, including a blend of nondestructive testing, sampling, and laboratory testing. Case histories are presented describing this evaluation system applied to chimney stacks, fly-ash silos, and cooling towers.
This paper presents an overview of a new nondestructive testing (NDT) system that allows rapid nondestructive assessment of many types of structural materials. The new system is based on scanning impact echo (IE), using a rolling receiver, digitally controlled impact source, and a distance measurement wheel integrated into a system that is capable of performing over 3000 IE tests per hour. The system has been successfully used on both concrete and wood for condition assessment. Previously, impact echo testing has been limited to point-by-point testing at rates of typically 30 - 60 points per hour. The new system is usable on any flat, relatively smooth surface such as floor slabs, pavements, walls, columns, beams, etc. In addition to IE scanning, the new system has recently been expanded to allow the performance of spectral analysis of surface waves (SASW) scanning on concrete and wood. The SASW method allows the measurement of material stiffness (modulus) versus depth, and therefore can give a profile of the material condition versus depth. Included in this paper are brief discussions of the IE and SASW methods, the scanner system hardware, and the software which was developed to enable efficient processing, analysis, and display of the test data and results. Also included are sample data plots and a case history presentation of the use of the system in the field, including one in which 23,000 IE tests were performed on an elevated floor slab in approximately 16 hours of testing time.
Ground-penetrating radar (GPR) has been used to evaluate the condition of a group of bridge abutments that were built in the 19th century in New England. Thickness and configuration of the abutments could be determined, and small voids suggesting open joints were detected within the abutments. Use of low and high frequency transducers is recommended in bridge abutment surveys.
State-of-the-art ground penetrating radar (GPR) technology was used successfully in tunneling through the former L.A. City Oil Field to search for uncharted, abandoned oil wells. A magnetometer probe was previously used for this purpose, because it was felt abandoned oil wells with steel casings may exist ahead of tunneling. These wells were suspected to contain methane gases which could be released into the tunnels. Studies revealed the abandoned wells could be wooden-cased or uncased open holes, indicating they would not be detected using a magnetometer probe. GPR was therefore selected as a geophysical technique more capable of detecting both steel-cased and uncased oil wells. After some initial testing from inside the tunnel, a commercially available GPR system was selected. Procedures were developed for conducting the surveys and evaluating the data profiles for possible oil wells. The profiles were obtained by moving the radar antenna across the smoothed tunnel face. During tunnelling of the oil field area abandoned oil wells were not encountered. However, the GPR surveys did detect anomalous radar reflections that the machine operator was alerted to as possible oil wells. Review of the data indicates that other changes in ground conditions were detected, such as transitions from soft- to hard-ground conditions and zones of oil bearing sands. These results suggest GPR could be useful for other exploratory applications during mining. GPR was also used as an investigative tool to check for possible shallow subsurface voids from the ground surface. Air-filled cavities or voids beneath city streets can sometimes be formed as a result of deeper tunneling-induced ground movements, resulting in dangerous sink-hole forming conditions. The GPR surveys were conducted from the street surface above the tunnels in areas where geotechnical data measured greater ground movements. These surveys helped rule out the possibility of voids beneath the street pavement in an area where over nine inches of ground settlement was measured.
The safety of constructed facilities is determined by the condition of the materials and the total system. Deterioration has been occurring at a faster rate than the repair and retrofit activities to remedy the situation. One of the methods to evaluate the existing conditions of a structure is the use of nondestructive testing equipment. There are many types of equipment which require standards for their adoption by the industry as a commercial product. It is imperative that researchers and industry representatives cooperate to prepare and promulgate the required standards.
The high-resolution acoustic mapping (HRAM) system was developed in response to a stated need by the United States Army Corps of Engineers to evaluate the floor of a navigation lock that could not be dewatered. Navigation Lock #26 on the Mississippi River was constructed on piles and mats over a sandy bottom. Over the years the footings had shifted, probably damaging the floor. The lock could not be dewatered because of leakage around the lock-wall footings. Based on our work in ultrasonic inspection; using B-scan, C-scan and holographic imaging to display hidden faults in metal; we were asked to propose a solution to the navigation-lock imaging problem. The C-scan ultrasonic method employs a single ultrasonic transducer stepped over a regularly spaced grid to collect a set of data that can be displayed on an oscilloscope screen to evaluate the material being inspected. It is well suited for the inspection of large flat areas. We proposed to build in essence a large C-scan system. A boat supporting several ultrasonic transducers would move in a regular X-Y pattern over the floor of the lock. The data from the scan would be computer processed to provide a plot of the surface of the floor. We called this a 3D plot since it would be a nearly three-dimensional view of the inspected surface. I should point out that this work was proposed in 1975 and conducted in 1976, when small high-powered computers were still well in the future. The results of the program were spectacular. The 3D plot of the Lock 26 floor showed that individual concrete slabs had broken, some slabs had been tilted, the entire river side of the lock floor had settled nearly two meters, and piles of silt had built up in front of the lock gates.
Time domain reflectometry (TDR) techniques can be deployed to measure water pressures and relative dam abutment displacement with an array of coaxial cables either drilled and grouted or retrofitted through existing passages. Application of TDR to dam monitoring requires determination of appropriate cable types and methods to install these cables in existing dams or during new construction. This paper briefly discusses currently applied and developing TDR techniques and describes initial design considerations for TDR-based dam instrumentation. Water pressure at the base of or within the dam can be determined by measuring the water level within a hollow or air-filled coaxial cable. The ability to retrofit existing porous stone-tipped piezometers is an attractive attribute of the TDR system. Measurement of relative lateral movement can be accomplished by monitoring local shearing of a solid polyethylene-filled coaxial cable at the interface of the dam base and foundation materials or along adversely oriented joints. Uplift can be recorded by measuring cable extension as the dam displaces upward off its foundation. Since each monitoring technique requires measurements with different types of coaxial cables, a variety may be installed within the array. Multiplexing of these cables will allow monitoring from a single pulser, and measurements can be recorded on site or remotely via a modem at any time.
Geophysical investigations were conducted at Coursier Lake Dam, British Columbia, Canada, for detection of seepage flow within the embankment and/or its foundation. The geophysical investigation consisted of self-potential (SP), electrical resistivity, and magnetic surveys, as well as telluric monitoring. The results are reported.
I am pleased to welcome you to this timely and informative conference organized by The International Society for Optical Engineering (SPIE). As Conference Chair, I am very pleased, but not at all surprised, by your interest and enthusiastic response: Over 300 participants are contributing over 210 technical papers to six topical conferences.
The infrastructure in the United States and the world is aging. There is an increasing awareness o the need to assess the severity of the damage occurring to our infrastructure. Limited resources preclude the replacement of all structures that need repairs or have exceeded their lifetimes. Methods to assess the amount and severity of damage are crucial to implementing a systematic, cost effective approach to repair and/or replace the damaged structures. The challenges of inspecting aging structures without impairing their usefulness rely on a variety of technologies and techniques for nondestructive evaluation. This paper will briefly describe several nondestructive evaluation technologies that re required for inspecting a variety of systems and structures.
the role of the National Laboratory in technology transfer is to help industry implement the latest technology. The National laboratory must have a unique capability that does not compete with tax paying companies. Typically a National Laboratory will demonstrate the benefit of applying NDE to a company's production line. The company then contracts with a vendor to supply a production inspection system based on the laboratory's demonstration. Both the company and the vendor benefit from the participation with the National Laboratory. The company improves the quality of its products and the vendor has a new product to market. The purpose of this paper is twofold: (1) to present a brief overview of NDE capabilities and application activities within the National Laboratory System and (2) to identify NDE point-of- contacts at each laboratory.
There is no doubt that our aging or inadequate infrastructure is in trouble. Since engineers believe that we're dealing with 'a series of accidents just waiting to happen,' significant failures in the past support this theory. The Hartford Civic Center, the Kansas City Hyatt Regency, and the Mianus River Bridge are just a few examples of past failures that were all unexpected. These and other catastrophic failures confirm the problems that exist in our infrastructure due to poor design, deterioration, lack of inspection, overloading, inadequate maintenance, and the 'low-bid' syndrome. Finding and correcting all the problems amy be an impossible task, but through the use of current nondestructive evaluation methodology, the risks of failure can be minimized.
Construction is the largest industry in the world, amounting to 10% of the world's gross domestic product. Civil infrastructure systems are generally the most expensive investments/assets in any country. In the U.S. it is estimated at 20 trillion dollars. In addition, these systems have long service life compared with any other kinds of commercial product and are rarely replaceable once they are erected. Yet the feedback and controls ont he 'state of health' of constructed systems are practically nonexistent. Nondestructive evaluation is an essential part of this feedback and monitoring systems for infrastructures. NSF and NIST NDE initiative as well as workshops and recent awards/projects are described in this paper.