Abstract
In order to develop the evaluation method for the drainage effect by drain tunnel, monitoring of water content using neutron moisture logging in the drainage boring pipe for 19 months and environmental isotope concentrations (Tritium and Radon: 222Rn) in groundwater from the drainage boring pipes for five years were carried out after the construction of drain tunnel. The drain tunnel is 1.25m (diameter) × 500m (length) at 50m underground with drainage boring pipes, which are expected to drain superfluous water around the sliding surface of an active landslide in Tertiary mudstone area, Niigata Prefecture. Consequently, the moni-, toring of water content showed a remarkable decreasing with about 8% of water content around the sliding surface for the first four months and subsequently slight decreasing. The tendency of decreasing of water content corresponds with the fluctuation pattern of discharge water volume from drainage boring pipes. The monitoring of tritium and 222Rn concentrations revealed that the groundwater around the tunnel was recharged from rain before 10 years and the unsaturated zone around tunnel was occurred. These change of groundwater flow after construction of drain tunnel certifies that the monitoring techniques are useful methods for evaluation of the effect by the drain tunnel.
