Generally, evaluating the characteristics of strata with low independence is difficult. For this reason, visualization research using various investigations techniques has been advanced. However, these investigations are expensive, and their processes are long. Furthermore, precision is reduced remarkably in unsaturated ground. In this report, we introduce a ground evaluation method that we have devised. Our method employs static insertion of transparent casing using simple, lightweight equipment in a short amount of time as well as a technique for photography inside the casing. This method was applied to a landslide observation point that consists of unconsolidated ground at an existing dam in order to contribute to safety at the site and reduces costs. In addition, it was possible to catch the time change of the ground property by measuring simultaneously, earth temperature and resistivity in the transparent casing. By using this method, we believe that it is possible to easily observe.
Recently, from the perspective of disaster prevention on landslide-prone slopes and poor ground, a variety of visualization studies, including geophysical exploration and borehole observation, have been conducted in recent years. However, direct and accurate observation of the ground is not possible through geophysical exploration, and heavy boring machinery must be transported to exploration sites for drilling surveys. Moreover, steel casing is inserted in ground that has poor consolidation, preventing borehole wall observation. This report deals with our proposal for a ground surveying method that uses photography and various measurements within a transparent polyhedron casing.
The landslide slope of the observation site consists of mudstone, alternating strata of sandstone and mudstone, and conglomerate, all of which belong to the mull belt of Paleogene formation. The bedding has a northeast to southwest strike and a northwestward inclination of 20 to 30 degrees, forming a gentle dip-slope structure along the incline. The field survey conducted prior to preparing the observation plan revealed that the colluvial deposit of the landslide:
Consisted mainly of loose silt containing fine to medium gravel and fine to coarse sand.
Had generally even ground strength to the basement rock surface (confirmed by survey using rod penetration test).
Held no underground water (about 15 to 20% moisture content). For ease of execution, the observation holes on the target slope were located at comparatively large flat spaces, as shown in Photograph 1. In total, two sets of observation holes were constructed, each with three holes: "a" to observe the 0 to 0.7 m range, "b" to observe the 0.7 to 1.4 m range, and "c" to observe the 1.4 to 2.1 m range. These holes were located about 1 meter apart center-to-center as shown in Figure 1.
This ground survey method employs a transparent casing to be inserted into sandy earth of weathered rock in a stabilized ground condition.