立坑掘削影響試験における地下水流動影響予測解析

Analitical prediction of groundwater flow in the shaft excavation effect experiment

柳澤 孝一; 今井 久; 斎藤 章; 大澤 英昭  ; 中島 誠

Yanagizawa, Koichi; Imai, Hisashi; not registered; Osawa, Hideaki; not registered

立坑掘削影響試験において、立坑掘削の地下水流動への影響を定量的に評価するために立抗掘削位置を中心とした約300m四方の区域を調査対象領域として、地下水挙動を把握するための調査計測システムを設定し、気象観測や河川流量などの表層水収支観測、定期的な間隙水圧分布計測、詳細な透水試験などを実施している。今回、この調査計測システムから得られた掘削前の生データ・知見を示し、そのデータ・知見の水理地質構造モデルおよび数値解析諸条件設定に反映させる考え方・プロセスを示し、その妥当性の検討をおこなった。またこのモデル・条件設定に基づく立坑掘削の地下水流動への影響予測解析を掘削開始より3年間を対象におこない、この結果以下のことが予測された。1)立坑掘削から離れるにつれピエゾ水頭圧の低下量は小さく、低下の顕著な範囲は半径約100m以内であること。2)掘削に伴う立坑内への湧水量は月吉断層を貫き、高透水性部に到ると急激に増加し最終掘削時に最大の約30l/minとなること。3)既存坑道内への湧水量は立坑掘削に伴った顕著な低下は示さず、掘削終了近くなり緩やかな低下を示すこと。低下量は約2l/min。この予測解析結果は、今後掘削に伴い得られるデータとの比較検討を通じ、今回行った一連の地下水流動評価方法の妥当性の検討を行い、さらに精度の高い地下水流動評価システムの確立に反映させていくことが大切である。

A hydrogeological measurement system was set up in order to understand the shaft excavation effect on groundwater flow quantitatively in the area of 300 meters square at the center of which the shaft is to be located. The meteorological and hydtological investigations have been performed, based on the data obtained by the measurement of river discharge rate, permeability tests and periodical monitoring of distribution of pore pressure. The present paper, first of all, describes the data and informations obtained from the measurement system before the excavation, secondly the handling of the data and the process for modelling of the hydrogeological structure and setting of some conditions for numerical simulation of the groundwater flow, and thirdly the examination of the model and the condition for calculation. Finally it referrs to the predictive simulation of the excavation effect on the groundwater flow for the period of the following three years. As the results of the simulated calculation based on the above model and conditions, estimations on the groundwater flow are summarized as follows: (a)Degree of decrease of pore pressure is smaller in accordance with distance from the shaft. The remarkably effected area is restricted within 100 meters during the next three years after the excavation. (b)When the excavated shaft passes through the Tsukiyoshi fault and reaches a highly permeable zone, the outflow into the shaft increases rapidly. The maximum outflow becomes up to about 30 L/min at the final stage of the excavation. (c)Decrease of the outflow into the existing shaft and drift is not distinct. However, the outflow decreases so gradually, when the excavation gets near to the final stage. Decrease of the outflow is about 2L/min. Method for the predictive analysis of groundwater flow developed in this study is required to be examined through a comparison between the results of prediction and the measurements during and after the excavation. It is ...