Evaluation of the joint distribution in the exploratory adit

Abstract

Dong Nai 4-2 exploratory adit is one in four exploratory adits for Dong Nai 4 hydropower dam, 80m length, 2m diameter, and 140° direction. In exploratory adit, the joints are complex and most of the joint spacing is less than 0,6m. Most of the joint roughness is smooth, but the joint separation is small causing the infilling such as sandy silt, calcite, and quartz that control entirely the shear strength of the joint, that obstructs collapse of the exploratory adit. Howe ver, from these characteristics don’t yet accurately assess the stability of exploratory adit, it also depends on an important factor that is the orientation of joints in the exploratory adit. Therefore, in order to evaluate the stability of the exploratory adit, the paper used rose diagram and steronet method to statistic the joints, that easily find out the main development and the pole density of the joints. Therefrom, evaluating their effect on the excavation direction. Considering the 5-10m section of the Dong Nai 4-2 exploratory adit, it’s through 30 joints, with 6 joint systems. From the rose diagram and stereonet method show that the axis of the 5-10m section is perpendicular to the main development direction of the joints, but the excavation direction drive against the dip of these joints, with density > 50%. According to Bieniawski (1989), the 5-10m is unfavorable. Especially, in the case where many joints cross. So, there should excavate another direction to ensure stability.