4. Manual for investigating the deterioration degree of the ground behind slope protection work

In the maintenance of aged slope protection work, it is important to easily and cost-effectively assess the deterioration degree of the ground behind slope protection work (which cannot be visually inspected), and to perform repairs and other actions according to its condition. Therefore, we developed the “Free directional dynamic rod penetration test” (Figure 1), in which a rod is driven into the ground to estimate its strength, for cases where the slope ground is sound part or deteriorated part.
The testing equipment is compact and lightweight. The rod of the testing equipment is driven into the slope ground with a shockless hammer through a small-diameter hole on the slope protection work, and the penetration resistance values are determined from the number of strikes required to penetrate 10 cm of the rod to determine the distribution of loosening in the slope ground (Figure 2).
By measuring the striking force, we introduced a function to correct test errors caused by variations in hammering, thereby improving measurement accuracy. Additionally, we developed the “SE-VP test” (Figure 3), which can be applied when the slope ground is bedrock.
In this test method, an electric drill is used to drill through the bedrock from the side of the slope protection work, and the drilling resistance SE is measured from the power consumption per drilling depth, and the elastic wave velocity Vp of the ground is measured from accelerometer installed inside the borehole and on the ground surface. SE has a strong correlation with the compressive strength of the rock, and Vp has a strong correlation with the fracture state of the rock. The combination of both test values is used to assess the deterioration degree (Figure 4).
These tests can be performed at 1/10 to 1/4 the cost of conventional boring investigations, allowing investigations to be conducted at many sites and improving the safety of the slope protection work.
In addition, a manual for investigating the deterioration degree of the ground behind the slope protection work has been prepared.
This includes points of focus for visual inspection and the test methods described above in order for appropriate inspection according to the type of slope ground.