Study of Landslides using electrical resistivity tomography (ERT) in the Kiasar-Semnan road, Iran

Landslides may be considered a common natural hazard. The events in many cases lead to significant economic losses as well as fatality damages. Therefore, the Investigation of landslides in order to reduce damage in the preliminary studies of construction projects, especially linear structures in areas with landslide potential is of great importance. Electrical tomography or electrical resistivity tomography (ERI) is a geophysical technique for imaging sub-surface structures (sliding surface in this case) from electrical measurements made at the surface or boreholes. In recent decades, geophysical methods have been widely used in landslide investigations. Among the geophysical methods, ERT is very useful for landslide investigation. In this study, after the landslides in the Langar and TelmaDarreh regions which two sections of Kiasar-Semnan road were destroyed by these landslides, field surveys were conducted on the sites. In this study, a total of six ERT profiles were carried out for landslides (four profiles on Langar and two profiles on TelmaDarreh landslides) investigation. All ERT profiles were performed normally in the direction of the landslide. The Langar Landslide (located in the Langar region in Mazandaran province) occurred on the Kiasar-Semnan road at longitude 53 36 02 and latitude 36 13 03. Due to this landslide, about 300 meters of the Kiasar-Semnan main road was completely destroyed. The TelmaDarreh landslide (located in the TelamaDarreh region in Mazandaran province) also occurred on the Kiasar-Semnan road at latitude 53 41 38 and latitude 36 14 59. After the landslide, about 20-30 meters of the Kiasar-Semnan main road sunk and has been repaired. In this study, the first stage in our electrical tomography was sending an electric current into the ground based on different arrays (dipole-dipole, pole-pole arrays, and Vertical Electrical Sounding) and then measuring the response of the earth in voltage. Bad data points were easily viewed as they appeared as stand out points because the values were displayed in the form of profiles for each data level. These bad data points could be due to the failure of the relays at one of the electrodes, poor electrode ground contact due to dry, sandy, or stony ground, attaching electrodes to wrong connectors, or shorting across the cables due to very wet ground conditions. In the next step for building the inversed resistivity model, the algorithms of well-known IPI2win and Res2dinv software were used.
It should be noted that geoelectrical techniques (such as ERT), like other geophysical methods, are used as a complement to other geotechnical methods such as drilling and sampling. Therefore, is noted that the results obtained are based only on the interpretation of electrical resistivity data. In the Langar landslide, according to the results of ERT models and the geology of the region, the existence of four main layers in sections was determined. The electrical resistivity tomography technique showed that the first layer is characterized by very low electrical resistivity (less than 20 ohmmeters) on the sections. This layer of clay is saturated with water. The second layer is with low electrical resistivity (20 to 100 ohmmeters). This layer is most likely a sand-clay layer with water-saturated sand. The third layer is with medium electrical resistivity (100 to 300 ohmmeters). This layer is composed of dense wet sand. The fourth layer has high electrical resistivity (more than 300 ohmmeters). This layer is the bedrock of the area. The depth of the bedrock increases along with the landslide mass from the landslide crown to the landslide heel so that the depth at the landslide center (middle of the valley) varies from 25 m (GH profile center) to 60 m (AB profile center) from the ground surface. The bedrock is located in the center of the landslide from the floor of the old road at a depth of about 50 to 60 meters. It is likely that the presence of a water-saturated clay layer on a layer of dense wet sand caused the Langar landslide in the area. Probably the rupture level starts from about 12 meters in the center of the GH profile and continues with 15 meters in the center of EF and reaches about 25 meters in the center of AB. The depth and topography of slip surfaces have been determined and expressed in all sections.
In the landslide of TelmaDarreh at the southern shoulder of the road, the depth of the bedrock starts from at least 2 meters in the center of the landslide and reaches about 6 to 8 meters so that the separation surface between the clay mass and saturated sand with the bedrock is broken. In this place, the depth of rupture in the center of the landslide is about 8 meters from the road surface. Due to the slope of the mountain, this depth reaches about 20 meters above the road at the bottom of the road and at the location of the CD profile.