Chih-Ping Lin

Theoretical model of a multisection time domain reflectometry measurement system

A multisection model is presented to simulate electromagnetic wave propagation in an unmatched time domain reflectometry (TDR) probe and layered soil system. The model uses a linear-time-invariant feedback system to model each section and links each section in a bottom-up fashion. Multiple sections can be incorporated in this model by a simple extension of a single-section system. An unmatched TDR probe system is modeled by dividing it into equivalent sections and matching the simulated waveform with the actual waveform. The excellent match between the simulated and the recorded waveforms verifies the model. This model eliminates the requirement of a matched 50-V probe handler in probe design for dielectric measurement. It can also be used to assist probe design and TDR data interpretation. The reflected waveforms in layered soil specimens are modeled, and the simulated waveforms are compared to those obtained from experiments. The method of using apparent dielectric constant to obtain average volumetric water content is examined for the layered soil system. The results show that apparent dielectric constant is applicable for the measurement of average volumetric water content for a layered soil, but caution has to be used when interpreting the waveforms.

Comprehensive wave propagation model to improve TDR interpretations for geotechnical applications

Time domain reflectometry (TDR) is becoming an important monitoring technique for various geotechnical problems. Better data interpretation and new developments rely on the ability to accurately model the TDR waveform, especially when long cables are used. This study developed an efficient, complete, and general-purpose TDR model that accounts for all wave phenomena including multiple reflection, dielectric dispersion, and cable resistance all together. Inverse analysis based on the TDR wave propagation model is proposed to calibrate the TDR system parameters and determine the TDR parameter that changes with the physical parameter to be monitored. Calibration of TDR cable and data interpretations for various geotechnical applications were demonstrated with laboratory experiments. The excellent match between the simulated and measured waveforms validates the TDR wave propagation model. The results show that the proposed numerical procedure is a relatively simple, efficient and high-resolution tool for probe design, parametric studies, data interpretation, and inverse analyses. This study should provide a sound theoretical foundation for further TDR developments in geotechnical monitoring.

Development of TDR Penetrometer Through Theoretical and Laboratory Investigations: 1. Measurement of Soil Dielectric Permittivity

A TDR penetrometer was introduced to allow simultaneous measurements of dielectric permittivity and electrical conductivity during cone penetration. This study focused on the theoretical development and experimental evaluation of electrical conductivity measurement using the TDR penetrometer. Theoretical development takes into account the cable resistance and nonconducting cone shaft, leading to a new data reduction equation and calibration procedure. Measurement sensitivity and spatial sampling bias were experimentally evaluated for various probe configurations. The results show that the measurement sensitivity of interest may be controlled by the geometric factor of the probe. The complication and implications of the spatial weighting bias are discussed. A prototype TDR penetrometer was calibrated and used to perform simulated penetration tests in a soil. Experimental results do not show significant errors in electrical conductivity due to the penetration disturbance, verifying the effectiveness of the TDR penetrometer for electrical conductivity measurement.

Development and Calibration of a TDR Extensometer for Geotechnical Monitoring

Up-hole reflectometry devices that interrogate passive mechanical transducers are increasingly used in geotechnical instrumentation. One of such a monitoring technique is the time domain reflectometry (TDR). This study utilizes the principle of TDR to devise a TDR extensometer using a waveguide with an impedance mismatch interface inside it. The movement of the impedance mismatch interface is coupled with the displacement of interest such that the time shift in the reflected signal becomes a measure of the displacement. The accuracy of the TDR extensometer is better than ± 0.5 mm and is not affected by measuring range and cable length. It can be used for applications that require both high accuracy and large displacement range, and is suitable for long-term monitoring in a harsh environment where humidity and lightening surge may cause problems for electronic sensors. The devised extensometer can be combined with existing TDR transducers for groundwater pressure and shear deformation to form an integrated TDR monitoring system for slope stability.

Assessment of Ground Improvement with Improved Columns by Surface Wave Testing

ABSTRACT Soft ground is often improved to increase strength and stiffness by methods such as jet grouting and stone column which result in heterogeneous ground with improved columns. Experimental methods (Standard Penetration Test, sampling and laboratory testing, etc.) used to assess such ground improvement are subjected to several limitations such as small sampling volume, time-consuming, and cost ineffectiveness. It’s difficult to assess the average property of the improved ground and the actual replacement ratio of ground improvement. The use of seismic surface wave method (i.e. multi-station analysis of surface wave, MASW) for such a purpose seems to be a good candidate. But the surface wave method is essentially a 1-D method assuming horizontally-layered medium. What MASW measures in the highly heterogeneous improved ground remains to be investigated. This study evaluated the feasibility of MASW in the heterogeneous ground with improved columns and investigated the homogenization of shear wave velocity measured by MASW. The lateral sampling space of the surface wave testing was also investigated by field testings of different survey line locations relative to the improved columns. The engineering information that can be extracted from the improvement rate of shear wave velocity obtained by surface wave testing is discussed.

A novel TDR signal processing technique for measuring apparent dielectric spectrum

Conventional time-domain reflectometry (TDR) signal interpretation generated single-valued apparent dielectric constant, while sophisticated, inconvenient measurement system and analysis is needed for measuring spectral complex dielectric permittivity (CDP). Niching between these approaches, a novel phase velocity analysis (PVA) method is developed to efficiently measure apparent dielectric spectrum (ADS) directly from TDR signals in a simple, quick, model-free, and inversion-free manner. The proposed PVA method extracts the two reflections from the top and end of the sensing probe by proper window selection and calculates their phase shift at each frequency, from which the phase velocity and corresponding apparent dielectric constant can be determined. Numerical and experimental results demonstrated that PVA is capable of measuring ADS in a frequency band, typically from 100 MHz–1 GHz. The effect of signal truncation was identified as the main cause of poor results outside the effective frequency band. Factors that affect the frequency band of effective ADS were numerically investigated. For highly dispersive materials, the end reflection pulse does not fully develop before the arrival of subsequent multiple reflections, resulting in severe truncation error and decreasing the upper frequency of effective ADS. Methods to estimate and extend the frequency band of effective ADS were further proposed. The simple procedures behind PVA, and its computationally efficient nature, make this method suitable for field monitoring. The resultant ADS is a potential improvement to current applications utilizing a travel time approach.

An Improved Modeling of TDR Signal Propagation for Measuring Complex Dielectric Permittivity

Time domain reflectometry (TDR) is a measurement technique based upon transmission line theory. The solutions of transmission line equations are reformulated in terms of independent physical properties, instead of coupled per-unit-length circuit parameters. The complete TDR response is effectively modeled by a non-uniform transmission line using the non-recursive ABCD matrix approach. Approaches to calibrate line parameters and perform TDR measurements based upon such model are introduced with an example on dielectric spectroscopy. TDR modeling in terms of decoupled physical parameters and non-recursive algorithm allows more convenient calibration of line parameters and facilitates interpretation of TDR measurements.

TDR Method for Compaction Quality Control: Multi Evaluation and Sources of Error

The use of the time domain reflectometry (TDR) method, designated ASTM D6780 (ASTM D6780, 2005, “Standard Test Method for Water Content and Density of Soil in Place by Time Domain Reflectometry (TDR),” Annual Book of ASTM Standards, Vol. 04.09 ASTM International, West Conshohocken, PA), to measure water content and soil density is relatively new with both successful and unsatisfactory results being reported. This paper reexamines the TDR method from various aspects, including laboratory investigation of the relation between TDR-measured electrical properties and soil phase parameters, full scale laboratory evaluation of the standardized methods, real world performance evaluation at construction sites of earth dams, and theoretical reexamination. Laboratory calibration reveals the effect of soil type on soil electrical properties, and its implication on the TDR method is discussed. The full scale laboratory evaluation supports the validity of both the one-step and two-step TDR methods. But the field evaluation at construction sites of earth dams indicates otherwise, particularly in the measurement of soil density. The errors in the two-step method are attributed to the penetration disturbance of the field multipole resonance probe (MRP). An additional source of error in the One-Step method is found to be a theoretical flaw in the current empirical adjustment process for correcting the variation of pore-fluid conductivity from the calibration test to field measurements. Remediation for the two-step method and updating of the ASTM D6780 to overcome the shortcomings of the one-step testing are in progress.

Improved TDR Method for Quality Control of Soil-Nailing Works

AbstractTime domain reflectometry (TDR) has become an effective nondestructive testing method for soil-nailing inspection. Previous studies utilized a preinstalled, single-core, electric wire alongside the rebar within the soil nail. Measurements may be affected by possible grout defects and excessively overestimate the rebar length if the wire is coiled around the rebar. An improved TDR waveguide and a corresponding decoupled data reduction method (for both soil-nail length determination and grout condition inspection) are proposed herein. The feasibility and advantages of the new approach were experimentally verified with two types of TDR device. The proposed methodology makes it possible to use a portable, low-cost, and low-speed TDR device as a quick and economical tool for quality control of soil nailing.

Performance of 2-D ERT in Investigation of Abnormal Seepage: A Case Study at the Hsin-Shan Earth Dam in Taiwan

ABSTRACT There is a great demand for effective non-destructive methods to examine the interior of reservoir structures, such as dams. The present study was aimed at assessing the performance of electrical resistivity tomography (ERT), a popular non-destructive testing method, to investigate leakage at an earthen dam. Several abnormal leaks appeared on the downstream face after the dam was reconstructed to raise the maximum reservoir water level. Three 2-D ERT surveys were deployed on the left abutment, dam crest, and downstream shell. Periodic measurements were also collected on the downstream shell for time-lapse measurements. To gain confidence and avoid over interpretation, the 3-D effects on 2-D ERT were examined and the results of the 2-D ERT were appraised by forward modeling and synthetic inversion. Integration of ERT results with geotechnical monitoring data revealed the likely mechanism of abnormal seepage. Relational interpretation of time-lapse measurements further supported the hypothesized me...