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地下ダム施工における基盤評価装置および基盤評価方法の開発

近藤 高弘*1・大塚 寿次*2・平山 哲也*3

地下ダムの施工は,離島などの水資源確保を目的に施工される。
この様な離島の多くは厚く堆積した琉球石灰層の下部に不透水層地盤があり,確実に基盤層まで根入れ掘削する事が重要である。
しかしこの琉球石灰層,不透水層地盤ともに軟岩に近く,実施工では調査ボーリングデータを基に掘削深度が計画されている。
この基盤層まで削孔し到達したかの判定評価手段として,従来は削孔機の電流値の変化等を施工経験に基づいて判断されてきた。
そこで削孔機本体に三軸加速度振動速度センサを設置し波形解析を行う事で,到達判定評価を客観的かつ総合的に判定評価する手法と基盤評価装置の開発を行った。

キーワード:  地下ダム,基盤層,判定評価,波形解析,振動加速度,相関

*1 技術センター 土木技術開発部 土木技術開発プロジェクト室
*2 土木本部 土木技術部
*3 土木本部 機械部

Development of Equipment and Method for Evaluating Bedrock for Underground Dam Construction

Takahiro KONDO*1, Hisatsugu OTSUKA*2 and Tetsuya HIRAYAMA*3

Underground dams are constructed to secure water resources on isolated islands, and so on.
Most such isolated islands, have an impermeable ground formation in the lower part of the thickly accumulated Ryukyu lime layer. Knowing precisely where the bedrock layer begins is important during excavation.
The Ryukyu lime layer and the impermeable ground formation have hardness values similar to that of nearby soft rock.
During construction, the excavation depth is , determined from soil survey data obtained from boreholes.
Conventionally, the construction stager determines when the bedrock layer has been reached by observing the change in the electric current value of the boring machine, and so on.
For this study, a three axis acceleration vibration velocity sensor was installed in the main unit of the boring machine. By analyzing the waveform, the point at which the bedrock layer was reached could be objectively determined by synthetic means.
This technique as led to the development of the bedrock evaluation equipment.

Keywords:  underground dam, bedrock layer, evaluation, waveform analysis, vibration acceleration, correlation

*1 Civil Engineering development Section, Civil Engineering Research Institute, Technology Center
*2 Civil Engineering Technology Section, Civil Engineering Headquarters
*3 Machinery Dept. Civil Engineering Headquarters