Study on the Improvement of the Rock Fall Detection System using Steel Wire Cable Sensor-On the Effects of Signal Cable Insertion to Measurement Circuit and Grouping of the Sensors-

Abstract

Steel wire cable sensor (SWCS) stretched along a rock slope is effective to detect the sign of a big scale rock fall and slope failure, because the sensor could detect the collision vibrations of rock particles and blocks that appear preceded the big failure. In the conventional SWCS system, electric signals generated in SWCS are once amplified by amplifiers placed on the slope and are transmitted to the room for measurement. It is considered that practical applicability will be increased if the signals could be transmitted directly from SWCS to measurement house without amplifying on the slope, because the accidents of measurement circuit occurs mainly around the amplifiers placed on the slope. Moreover, practical applicability will be increased economically, if the number of channels of measurement instrument is decreased, grouping of plural strings of SWCS. In this study, it was examined how much length of coaxial signal transmission cable could be inserted between SWCS and amplifier in the measurement house without losing the signal characteristics. Then it was confirmed theoretically and experimentally if the output level of signal of SWCS was kept for practical use, when 4-13 strings of SWCS were grouped in parallel or in series to a channel. Results obtained from these examinations are as follows: 1) Length of coaxial signal transmission cable which could be inserted between SWCS and measurement house without pre-amplifying on the slope surface is 120m; 2) According to the field tests, rock fall behaviors could be interpreted clearly from the changes of signal waveforms, even when the coaxial signal transmission cable was inserted between SWCS and amplifier placed in the measurement house; 3) Rock fall characteristics could be identified, when the 4-13 strings of SWCS are grouped both in parallel and in series to a channel in addition to the insertion of coaxial signal transmission cable.