Chih- Chung

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.

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.

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.

SOME INNOVATIVE DEVELOPMENTS OF TDR TECHNOLOGY FOR GEOTECHNICAL MONITORING

Up-hole time domain reflectometry (TDR) devices that interrogate passive mechanical transducers are advantageous for engineering monitoring in situ. This technology is based on transmitting an electromagnetic pulse through a coaxial cable connected to a sensing waveguide and watching for reflections of this transmission due to changes in characteristic impedance along the waveguide. Depending on the design of the wave guide and analysis method, the reflected signal can be used to monitor various engineering parameters. This paper introduces some recent developments to improve the understanding and extend the usefulness of TDR technology. TDR waveguides for engineering monitoring are grouped into three categories according to the sensing principle, namely interface type, crimp type, and dielectric type. New design concepts and data reduction procedures for each type of waveguide are introduced. These developments improve and lead to a variety of transducers, such as water level, deformation, rainfall, scouring, soil moisture, sediment concentration, and electrical conductivity. The versatility and merits of TDR are highlighted by examples of hybrid economical TDR monitoring systems composed of a single TDR device and multiple sensing waveguides.

Extensive Monitoring System of Sediment Transport for Reservoir Sediment Management

Sedimentation is a serious threat to long-term water resource management worldwide. In particular, reservoir sedimentation is becoming more serious in Taiwan due to geological weathering and climate change in watersheds. Large amount of sediments transport to reservoirs during storm events at hyperpycnal concentration. Full-event monitoring of sediment transport in a reservoir plays an important role in sustainable reservoir management. This chapter begins by reviewing existing surrogate techniques in need for monitoring suspended-sediment transport in reservoirs with high concentration range and wide spatial coverage. More commercially available techniques suffer from particle-size dependency and limited measurement range. This chapter introduces a relatively new technique based on time-domain reflectometry. It possesses several advantages, including particle-size independence, high measurement range, durability, and cost-effective multiplexing. This chapter describes a modified TDR technique for better field applicability and demonstrates its application in an extensive SSC monitoring program for reservoir management through a case study in Shihmen Reservoir, Taiwan. Monitoring stations were installed at the major inflow river mouth and outlet works with fixed protective structures to provide inflow and outflow sediment-discharge records. To capture the characteristics of density currents, a multi-depth monitoring station was designed and deployed on floating platforms in the reservoir. Some of the data collected during typhoons are presented as an example to demonstrate the effectiveness and benefits of the TDR-based monitoring program.

DEVELOPMENT OF SEDIMENT MONITORING DURING HEAVY RAINFALLS

Due to geological weathering and climate change, soil erosion in watershed is becoming a serious problem in Taiwan. Sediments affect water quality during heavy rainfalls and their deposition reduces reservoir capacity. Monitoring of sediment movement is crucial to estimate sediment yield and analyze watershed dynamics related to slope stability. It also plays an important role in reservoir management during heavy rainfalls. While suspended-sediment concentration (SSC) can be observed by manual sampling and lab testing, it is difficult to predict the right sampling timing and mobilize field crew during storms. Currently, there are no effective measuring techniques for automatic SSC measurement, particularly in fluvial environment. Existing methods provide an accuracy much influenced by particle sizes of suspended sediments, function only in a limited range of measurement and are not cost effective for field maintenance and wide spatial coverage. This paper introduces an innovative method based on time domain reflectometry (TDR) that may lead to an effective solution for monitoring of sediment movement. TDR is a monitoring technique based on transmission lines, in which a time domain reflectometer transmits an EM wave and receives a reflected EM wave, and wherein various TDR sensing waveguides can be designed to monitor different physical quantities, such as soil moisture content (based on dielectric permittivity), electrical conductivity, water level, and displacement. A TDR SSC probe is designed and a new travel time analysis method with temperature correction procedure is proposed for accurate determination of SSC. Unlike optical and acoustic method, TDR measurement is shown to be insensitive to sediment particle size. Other advantages of the TDR method include low-cost transducers, durability, and multiplexing capability. Results of laboratory evaluation and field monitoring are introduced.