A vertical cut off wall for water barrier was constructed at the final landfill disposal site in Oita prefecture of Japan to prevent the inflow of groundwater from the surroundings of the landfill site and leakage of leachate (sewage) to the outside. Due to existence of hard rock formations existed in the target ground to construct the cut off wall for water barrier, a Trench cutting Re-mixing Deep wall (TRD) construction method using a pre-drilling by a double rock auger in combination was planned for excavation of cut off wall for water barrier, initially. However, if this plan was adopted, there should be tight schedule for the construction due to pre-drilling. The alternative with CSM (Cutter Soil Mixing) construction method which does not require pre-drilling even in hard rock formation was proposed and the excavation was carried out with CSM method against relatively less hard rock formations. Also, a new machine imported from overseas was used for CSM construction method. Since, in Japan, there is no previous experience for drilling of hard formation layer with the new CSM machine, a pre-test was carried out. In the pre-test, the quality of the soil mixing wall and its drilling capacity against the hard rock formation were checked. The pre-test result was included in the construction cycle for CSM method with the new machine.
From this point of view, case study of construction with CSM method which was applied to vertical cut off wall for water barrier of waste landfill waste disposal site in Japan is described and the pre-test performance of rock drilling is summarized in this paper.
The final landfill disposal site of waste in Oita prefecture of Japan was developed for final landfill site of general disposals such as burning residual, non-inflammables and bulky garbage choice residual. Its landfill area and volume are 14,200 m2 and 71,000 m3 respectively, and its landfill has been completed already. However, this disposal site was designated as an inappropriate disposal site, and required to take a countermeasure in order to satisfy the national landfill closing criteria with the state aid business. Measures were mainly planned to construct a vertical water barrier for preventing the inflow of groundwater from the surroundings to the landfill site and the outflow of leached water (wastewater) to the downstream groundwater, and to perform final earth capping for controlling the penetration of rainwater into the landfill site. Initially, the cut off wall for water barrier was planned with a Trench cutting Re-mixing Deep wall (TRD) method using a pre-drilling by a double rock auger in combination due to existence of hard rock formations in the target ground. However, in the actual construction, both the TRD method which is the in-situ stirring and mixing method, and the CSM (Cutter Soil Mixing) method which does not use the supplementary method, were used in combination to shorten the construction schedule. This change of the construction method was attributed to the corroboration of the pre-test results of the CSM machine for the applicability to hard rock carried out when it was introduced from overseas, and the results of the pre-test at the actual construction site. In this report, based on these construction progress, we report a track record in which the CSM method was used for of the vertical cut off wall for water barrier of the final landfill disposal site of waste firstly in Japan, and a pre-test results of rock drilling performance, approximately 10 years ago.