To develop efficient methods for maintaining landslide prevention facilities, particularly groundwater drainage works

1.Introduction

  Landslide is phenomena which a slope moves slowly towards the bottom of the slope under the influence of groundwater, etc. Therefore, to prevent reactivation of landslide, it is important that the remedial works of groundwater drainage work etc. is maintained completely.

  However, more than 50 years have passed since the enforcement of the Landslide Prevention Act (March 31, 1958), when the construction of such structures began in earliest, and it has been found that the water collection boreholes are blocked at about 30% of groundwater drainage facilities.

  Accordingly, the landslide research team and the Snow Avalanche and Landslide Research Center are researching to establish efficient methods for maintaining and evaluating the soundness of groundwater drainage works.

2.Groundwater drainage works, particularly drainage wells

  Groundwater drainage works drain groundwater from within of a landslide slope to the outside of the slope. A typical example is drainage well.

  Drainage wells are built by constructing vertical wells in relatively large landslide slopes in regions where there is abundant groundwater, and then executing water collection and drainage boring from the wells to collect and drain the water in and near the slip surface.

　　　

                                                    Structure of water collection works

  The important functions of this work are collecting and draining groundwater, but it is known that over the years, iron in the groundwater turns into ferric oxide, blocking the bore holes. Therefore it is important to inspect and find in its early stages whether the bore holes are blocked, and if they are, to clean them to restore the function of the work.

             Example of adhesion of blocking material at the

         outlet of water collection borings of a drainage well

                 (Must be cleaned to restore its functions)

3.Developing the inspection method

  Such inspections are now generally done by a worker who climbs down directly into the well to visually examine the state of blocking of the outlets of water collection and drainage boring works to the bottom of the well. However, this work is dangerous because of the possibility of toxic gases, insufficient oxygen, or collapse of deteriorated elevation equipment. It also takes time to prepare for and conduct the inspection. We are therefore developing a remote inspection method that will allow observation by an inserted camera from outside of the well.

          Camera for observing drainage well                                    Image obtained by camera

  We are also studying methods to make inspections more efficient by specifying locations for priority inspections by predicting bore holes that are susceptible to blocking based on the chemical constituents of the groundwater.

4.Evaluating the decline of function and performance, and the stability of a landslide

  In order to evaluate the function whether deteriorate or not from time by time, the landslide research team also measures the quantity of drained water in the drainage wells. The team is also collecting cases (in cooperation with the national government and prefectures) to clarify the state of deterioration of functions and performance of groundwater drainage works according to various environmental conditions and to evaluate the stability of landslides. The team plans to research rational methods of inspection and improvement.

      Observing the quantity of drainage in a drainage well

　

(Contact:Landslide research team)