Xinbao Yu

Time Domain Reflectometry Automatic Bridge Scour Measurement System: Principles and Potentials

Bridge scour is a major factor contributing to the failures of highway bridges constructed over waterways. During flood events, sediments can be washed away and bridge substructures (piers and abutments) are left inadequately supported. Field monitoring of bridge sour process is necessary to study the scour mechanism, to develop scour-resistant design of bridge piers and abutments, to implement effective scour countermeasures, and to deploy safety warning systems. The current instruments are not completely satisfactory in providing real-time monitoring data during critical flood events. This study introduces the development of an automatic scour monitoring system using time domain reflectometry (TDR) principle. It presents the principles and system design of the TDR sediment scour monitoring system. An analyses algorithm for scour signals has been developed. It was found to be robust and can be implemented to automate scour signal interpretation. Simulated experiments were conducted to validate the performance of the scour monitoring system prototype. The results showed that this TDR technology can accurately measure the scour depth. Besides, the properties of the sediments, such as the porosity and density can be estimated with reasonable accuracy. At the end of the paper, a few important issues associated with field deployments of TDR scour monitoring system are discussed.

Thermal conductivity of quartz sands by thermo-time domain reflectometry probe and model prediction

AbstractThe heat transfer process in soil depends on its thermal conductivity property and is affected by soil compositional and environmental factors, such as soil gradation, mineralogy components, compaction moisture content, and dry unit weight conditions. In this research, thermal conductivities of three quartz sands were studied using a newly developed thermo–time domain reflectometry (TDR) probe. The test results revealed that thermal conductivity of quartz sands increased with compaction moisture content and dry unit weight. Higher quartz content also led to higher thermal conductivity. The new thermo-TDR probe provided measurements of thermal properties similar to those of the standard KD2 probe. The deviation of the two methods was within 5%. An improved thermal conductivity model was also proposed on the basis of the normalized thermal conductivity concept and present experimental data. This improved model exhibited the highest accuracy in thermal conductivity predictions of quartz sands (within...

A new time-domain reflectometry bridge scour sensor

Scour is a major threat to bridge safety. Bridge failures cost millions of dollars each year as the result of not only the direct costs of replacing and restoring these bridges but also the indirect costs related to the disruption of transportation network. Instruments for bridge scour monitoring are necessary to study scour process and support bridge management. Lack of robust and economic scour monitoring devices prevents the implementation of bridge scour monitoring program among bridge owners. This study describes the development of a new time-domain reflectometry scour sensor. The geometry of this sensor was designed to allow for easy installation through common geotechnical boreholes. Coating was applied to the sensor, which provided it with a large range of sensing depths compared to an uncoated metallic rod sensor. The coating also provided corrosion protection. The design of this new sensor was studied by numerical simulations with finite element method. From the results, the effective sampling areas of this sensor were determined. Laboratory evaluations showed that this sensor was sensitive to simulated scour process. An algorithm was developed to determine scour depth from sensor signals. The results indicated that the sensor provided accurate scour depth measurement.

Algorithm for Time Domain Reflectometry Bridge Scour Measurement System

Scour contributes to most failures of highway bridges constructed over waterways in the United States. Sediments are washed away by floods and bridge piers are left inadequately supported. Field monitoring of bridge sour process is necessary to study the scour mechanism, to improve the bridge piers and abutments design, to plan an effective remediation measure, and to develop emergency warning systems. The current tools for bridge scour monitoring are generally not satisfactory for these purposes. This study introduces the development of an automatic scour monitoring system using Time Domain Reflectometry (TDR). After briefly present the theoretical bases of TDR for scour detection, a framework for TDR information analyses is developed. Simulated scour and sediment tests were performed with TDR instrument deployed to acquire signals. The acquired data were analyzed using the developed theory. It was found that TDR signal changes have strong correlation with sediment layer thickness changes, as predicted by the theory. From TDR measured apparent dielectric constant and electrical conductivity, the depth of scour were accurately determined.

Comparison Study of Three Common Technologies for Freezing-Thawing Measurement

This paper describes a comparison study on three different technologies (i.e., thermocouple, electrical resistivity probe and Time Domain Reflectometry (TDR)) that are commonly used for frost measurement. Specially, the paper developed an analyses procedure to estimate the freezing-thawing status based on the dielectric properties of freezing soil. Experiments were conducted where the data of temperature, electrical resistivity, and dielectric constant were simultaneously monitored during the freezing/thawing process. The comparison uncovered the advantages and limitations of these technologies for frost measurement. The experimental results indicated that TDR measured soil dielectric constant clearly indicates the different stages of the freezing/thawing process. Analyses method was developed to determine not only the onset of freezing or thawing, but also the extent of their development. This is a major advantage of TDR over other technologies.

Design and evaluation of a thermo-TDR probe for geothermal applications

A thermo-time domain reflectometry (TDR) probe can function both as a regular probe, for measuring moisture and density, and a dual heat probe, for measuring thermal conductivity, thermal diffusivity, and volumetric heat capacity. This paper describes the design, fabrication, and evaluation of a newly developed thermo-TDR probe. The developed probe was first calibrated for its TDR function in chemicals with known dielectric constants and electrical conductivity. Then the probe was tested in three sands and Kaolin clay at different moisture contents and densities. Available methods for analyses of the thermo-TDR-acquired signals were presented and discussed. The measured thermal properties were also compared against a standard thermal probe KD2 pro. It was found that the thermo-TDR probe has satisfactory accuracy in measurements of soil thermal conductivity, moisture content, and dry density. Based on the test results, recommendations have been provided regarding the use of thermo-TDR probes for geothermal applications.

Implementation of LRFD of Drilled Shafts in Louisiana

This paper presents reliability-based analyses for the calibration of resistance factor for axially loaded drilled shafts. A total of 16 cases of drilled-shaft load tests were collected from the Louisiana Department of Transportation and Development (LADOTD) archives. Only 11 out of the 16 cases met the Federal Highway Administration (FHwA) failure criterion. Because of the limited number of available drilled- shaft cases in Louisiana, additional 15 drilled-shaft cases were collected from a neighboring state, Mississippi, which has subsurface soil conditions similar to Louisiana soils. A database of 26 drilled shafts representing the typical design practice in Louisiana was created for a statistical reliability analysis. Predictions of load-settlement behavior of drilled shafts from soil borings were determined using the FHwA design method through the SHAFT computer program. Measured drilled-shaft axial nominal resistance was determined from either the Osterberg cell (O-cell) test or the conventional top-down static load test. Statistical analyses were performed to compare the predicted ultimate drilled-shaft nominal axial resistance and the measured nominal resistance. Results show that the selected design method underestimates the measured drilled-shaft resistance by an average of 17%. The Monte Carlo-simulation method was selected to perform the LRFD calibration under strength I limit state. Total resistance factors for different reliability indexes (β) were determined and compared with those available in literature. The LRFD calibration showed that the FHwA design method has a resistance factor (ϕ) of 0.60 at a target reliability index (βT) of 3.0. DOI: 10.1061/(ASCE)IS.1943-555X.0000084.

Field testing and analyses of a batter pile group foundation under lateral loading

Much of the new I-10 Twin Span Bridge, built over Lake Pontchartrain between New Orleans and Slidell, Louisiana, is supported by driven batter pile group foundations. To assess the methodology used to design and analyze batter pile foundations and to evaluate their performance under lateral loading, a full-scale lateral load test was conducted on an M19 eastbound pier of the new bridge. The M19 pier foundation consisted of 24 precast, prestressed concrete 110-ft-long batter piles; eight piles were instrumented with in-place inclinometers (IPIs), and 12 piles were instrumented with strain gauges. The test was conducted by pulling the M19 eastbound and westbound piers toward each other with high-strength steel tendons. A maximum 1,870-kip lateral load was applied in increments. A high-order polynomial curve-fitting method was applied for each load increment to fit the measured rotation profiles from the IPIs, and the fitted rotation curves were used to deduce the bending moment, shear force, and soil reaction profiles on the basis of specific mathematical derivations. The calculated moments from curve fitting were compared with the moments calculated from strain gauge measurements, and the results showed good agreement. The soils’ p-y curves at different depths were backcalculated from the derived soil reaction profiles. The resulting p-y curves showed no evidence of group effect. The FB-MultiPier program was also used to analyze the behavior of the M19 pier foundation under lateral loading. Comparison of FB-MultiPier results with measured values as well as values calculated from polynomial curve fitting showed that FB-MultiPier overpredicted the lateral deformations of batter pile group foundations.

Assessment of an automation algorithm for TDR bridge scour monitoring system

Bridge scour is the number one cause of bridge failures. There are high risks associated with scour-induced damages due to their catastrophic nature. Developing real time bridge scour monitoring systems is a pressing task for the research community. Such systems need to be accurate, rugged, and field worthy. Implementing automatic signal interpretation is important to ensure the success of a long term scour monitoring instrument. This paper is part of the efforts to develop a TDR system to monitor the development of scour and sedimentation process. The focus of this paper is on an automation algorithm for TDR signal analyses. It discusses the theoretical foundation of this TDR algorithm for scour depth estimation. The influence of different types of sediments and the conditions of river (including the salinity and the effects of high air entrainment and suspended sediments) on the accuracy are assessed. The study indicates that the scour monitoring algorithm based on extending a physics-based general design equation is accurate under these different conditions. This will help implement a robust algorithm to automate the TDR bridge scour monitoring system.

Effects of Particle Size and Fines Content on Thermal Conductivity of Quartz Sands

The thermal properties of soil have drawn great research attention in recent years. Thermal conductivity is the most important such property, particularly for the design of shallow geothermal systems such as energy piles and borehole thermal energy storage. A newly developed thermo–time domain reflectometry probe was used to measure the thermal conductivity of quartz sands with the peak value method. Three rounded quartz sands with different particle sizes were used in experiments to investigate the effects of particle size and fines content on thermal conductivity. Results revealed that thermal conductivity increased with increasing particle size for uniform sands. The thermal conductivity was lower in fine sands than in coarse sands under dry conditions but higher in fine sands than in coarse sands at low moisture contents. The thermal conductivity of coarse sands was increased to peak value by increasing the fine sands content to the critical fines content corresponding to maximum thermal conductivity. The critical fines content increased with a decrease in the mean grain size ratio of fine sands to coarse sands; the content also was related to the grain shape.