• Tunnel and Underground Space
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• v.5 no.1
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• pp.70-76
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• 1995

Two types of deformations can occur on the cable during the monitoring of the rock displacement by the time domain reflectometry. One is the impedance model for tensile deformation, and the other is the capacitance model for the shear deformation. The former gives a response signal with a gradual change in the amplitude of the reflected voltage, meanwhile the latter produces a signal with a blunted spike. The resolution of the TDR can be improved to 0.125% using calibration crimps on the cable of 60 meters long. It is recommended that the diameter of the cable should be 18 mm at least in order to induce a better reflected pulse without any open-circuit. The actual TDR technique cannot characterize the type and the magnitude of rock displacement quantitatively. Systematic investigation of the TDR parameters, such as the exact of cable diameter, cable length, number of crimps, combination of shearing and extension, and environment of the TDR equipment, will be able to improve the resolution to 0.01 mm.

• The Journal of Engineering Geology
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• v.18 no.2
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• pp.127-136
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• 2008

Geophysical survey for establishing a wide site for the distribution of water content, wetting front infiltration due to the rainfall, and distribution of groundwater level has been performed by using 8round penetration radar (GPR) method, electrical resistivity method, and so on. On the other hand, a narrow area survey was performed to use a permittivity method such as time domain reflectometry, frequency domain reflectometry, and amplitude domain reflectometry methods for estimating volumetric water content, soil density, and concentration of contaminant in surface and subsurface. The permittivity methods establish more corrective physical parameters than different found survey technologies mentioned above. In this study for establishment of infiltration behaviors for wetting front in the unsaturated soil caused by an artificial rainfall, soil physical parameters for volumetric water content, pore water pressure, and pore air pressure were measured by FDR measurement device and pore water pressure meter which are installed in the unsaturated weathered granite soil with different depths. Consequently, the authors were proposed to a new establishment method for analyzing the variations of volumetric water content and wetting front infiltration from the responses of infiltrating pore water in the unsaturated soil.

• Computers and Concrete
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• v.31 no.4
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• pp.293-306
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• 2023

This study proposes and demonstrates a smart monitoring system that uses transmission lines embedded in a reinforced concrete structure to detect the presence of defects through changes in the electromagnetic waves generated and measured by a time-domain reflectometer. Laboratory experiments were first conducted to identify the presence of voids in steel-concrete composite columns. The results indicated that voids in the concrete caused a positive signal reflection, and the amplitude of this signal decreased as the water content of the soil in the void increased. Multiple voids resulted in a decrease in the amplitude of the signal reflected at each void, effectively identifying their presence despite amplitude reduction. Furthermore, the electromagnetic wave velocity increased when voids were present, decreased as the water content of the soil in the voids increased, and increased with the water-cement ratio and curing time. Field experiments were then conducted using bored piles with on-center (sound) and off-center (defective) steel-reinforcement cage alignments. The results indicated that the signal amplitude in the defective pile section, where the off-center cage was poorly covered with concrete, was greater than that in the pile sections where the cage was completely covered with concrete. The crosshole sonic logging results for the same defective bored pile failed to identify an off-center cage alignment defect. Therefore, this study demonstrates that electromagnetic waves can be a useful tool for monitoring the health and integrity of reinforced concrete structures.

• The Journal of Engineering Geology
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• v.18 no.1
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• pp.83-92
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• 2008

In order to analyze infiltration characteristics of rainfall in soil, two laboratory experiments were conducted using an amplitude domain reflectometry (ADR) sensor and a pore water pressure meter (PWP) in this study. The first experiment is to understand the dependency of volumetric water content and temperature for standard sand and weathered granite soil. The second experiment is a laboratory flume test with changes of rainfall condition. As the results of the dependency experiment, the volumetric water content is increased with increase of the output voltage measured by the ADR sensor in both the standard sands and weathered granite soil. Furthermore, the results also indicate necessity of consideration of the temperature dependency under the condition of high volumetric water contents from 0.15 to 0.45. In the flume test, two measurement devices are detected to the variation of volumetric water content and pore water pressure at the installation point of the flume. In especial, the measured values of ADR4 and PWP3 installed on the lower part of slope are higher than those of the others. It means that the lower part of slope plays a role of a runoff face and a beginning point of slope failure.

• The Journal of Engineering Geology
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• v.18 no.3
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• pp.311-319
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• 2008

Evaluation of an amplitude domain reflectometry (ADR) type soil moisture sensor as ThetaProbe ML2x using the response of frequency impedance was performed in a variety of soil porous media such as Jumunjin standard sand, weathered granite soil at Sangju area, and weathered gneiss soil at Jangsu area. The tested soils were classified with a dried condition and a wetted condition for comparing with soil volumetric water content under different installed depths of the measurement sensor. In the results the part of measurement rod including one signal rod and three shield rod 6cm in length was found to decrease the variation of measurement output voltage with insert 5cm over into the soil porous media. The measurement output voltage was verified to more stable output voltage under weathered granite soils and weathered gneiss soils contained the fine grain materials such as clay and silt minerals than the gradual grain material like as the standard sands. Therefore, measurement values by soil moisture sensor can be offered the more stable values when an contact volume between soil porous media and measurement sensor increase.