Engineering geology

Slope failures evaluation and landslides investigation using 2-D resistivity method

Introduction

Landslides and slope failure are always related to hilly areas and restricted essentially to the urban area. Development for tourism and residential is still rampant in these areas. The prevalence of failures is due to a combination of few factors; i.e. topography, climate, geology and land use. Some countries have similar soils but different rainfall patterns, or similar rainfall but different land use. Urban development including used of steep natural slopes for housing and road are potentially caused unstable areas.

The purpose of resistivity survey is to determine the subsurface resistivity distribution by taking measurements on the ground surface. True resistivity of the subsurface can be estimated (Fig. 1). The ground resistivity is related to various geological parameters such as mineral and fluid content, porosity and degree of water saturation in rock. Variations in electrical resistivity may indicate changes in composition, layer or contaminant levels (Telford and Sheriff, 1984).

Fig. 1. The arrangement of electrodes for a 2-D electrical survey and the sequence of measurements used to build up a pseudo section (Loke, 1999).

Methodology

Six (6) 2-D resistivity survey lines were conducted in Selangor. Lines RL1-RL3 was conducted towards North-South direction as shown in while RL4-RL6 directed towards West-East (Fig. 2). The length for all the 2-D resistivity survey lines were 200 m except for RL1 and RL4 is 300 m each. RL1 was located outside the collapsed area while RL2-RL6 located in the collapsed area. The protocol used for 2-D resistivity was Pole-dipole with 5 m minimum electrode spacing and the data was processed using Res2Dinv software.

Fig. 2. 2-D resistivity survey in Selangor, Malaysia.

Results and discussion

From the 2-D resistivity method, the inversion model resistivity for potential area, RL1 (outside collapsed area) (Fig. 3) shows there are 2 main zones. The first zone is alluvium or highly weathered with resistivity values of 100–1000 Ω m and depth of 70 m. This zone consists of saturated zone (1–100 Ω m) and boulders (1200–7000 Ω m). The second zone with resistivity value of >7000 Ω m which interpreted as granitic bedrock. A lot of small fractures can be seen around the study area as shown in Photo 1.

Fig. 3. Inversion model resistivity in Selangor for RL1 (outside collapsed area).
Photo 1. Small fractures located at the collapsed area in Selangor.

Fig. 4 shows the inversion model resistivity of RL2 and RL3 which has two main zones. The first zone is alluvium or highly weathered with resistivity value of 100–1000 Ω m and depth of 30–70 m. RL2 shows saturated zone while RL3 consists of alluvium or highly weathered zone. Boulders and fractures were located outside the collapsed area with resistivity value of 1200–7000 Ω m. Alluvium may consist of clay, sand and silt. These types of soils indicate that for hillslopes containing both sandy and silty soils, failure can occur above water table under steady infiltration conditions (Lu and Godt, 2008). Rainfall is also one of the factors that may cause to the instability of the slope. Lu and Godt, 2008 present that shallow landslides may occur on the slope under very heavy precipitation conditions. Because soils under these conditions are very nearly saturated, shallow landslides that are generated by this mechanism may mobilize into debris flows (Iverson et al., 1997). Seasonal nut high rate of rainfall, allowing part of the soil possibly to become fully saturated several times during wet seasons of any year. The study area is either decomposed volcanic rock (silt) or decomposed granite (silty sand) and both soils have high strength when unsaturated during dry season but a low strength when fully saturated. The saturated zone, boulders and highly weathered rock will trigger the slope failure.

Fig. 4. Inversion model resistivity in Selangor; (A) LR2 and (B) LR3.

Inversion model resistivity of RL4-RL6 shows two main zones (Fig. 5). The first zone with resistivity values of 100–1000 Ω m was interpreted as alluvium or highly weathered rock with depth of 20–70 m. The zone consists of saturated zone and boulders with resistivity value of 1–100 Ω m and 1200–7000 Ω m respectively. A mass movement can occur any time and the slope may becomes unstable if the process is continuous. It should be noted that the boulders is very close to instability due to change or removing the rock that holds the slope in place (Nelson, 2013, Filip et al., 2017). The movement of mass from its original place may trigger to instability of slope. Second zone is granitic bedrock with resistivity value of >7000 Ω m. In a wet tropical region like Malaysia, deep weathering profile can have a thickness of up to 100 m. Even though the characteristics of weathering profiles differ from place to place, two most common types of profiles are with and without core-stones (Komoo, 1985, Komoo, 1989, Komoo, 1998). Highly to moderately weathered (grades IV and III) materials contain core stones of various sizes (from few cm to few m in diameter). Both types of profiles are prone to rainstorm-induced landslides (Chigira et al., 2011).

Fig. 5. Inversion model resistivity in Selangor; (A) RL4, (B) RL5 and (C) RL6.

Generally the results show that the study area consists of high resistivity value, it might due to sand. It was observed that the topsoil was characterized by isolated low resistivity materials that were identified as saturated zones, highly weathered zone and boulders. The saturated zone might be due to the present of Gapis River as shown in Fig. 6. Such water can seep into fracture (geological discontinuity) present at the study area and replacing the air in the pore space thus increasing the weight of the soil. It leads to increase in effective stress resulting into failure of the slope. The higher the porosity will give higher permeability. Porosity can be a factor in permeability. Water flows quickly through material with high permeability. The Gapis River can be easily flow in the study area due to the sand contained. These low materials overlie the bedrock produce unstable slope which trigger slope failure and may cause to landslides in the study area.

Fig. 6. The arrangement of inversion model of resistivity with underground water from Gapis River.

Conclusion

The 2-D resistivity method has been carried out in Selangor, Malaysia. Six 2-D resistivity survey lines were carried out in order to identify the subsurface characteristic of slope failure. Results of inversion model resistivity indicates that the topsoil was characterized by isolated of low resistivity materials that were identified as saturated zones. The saturated zones, highly weathered zone, highly contain of sand and boulders will trigger slope failure in the survey area that finally may cause to landslides. Table 1 shows summarize of slope failure evaluation using 2-D resistivity method. The 2-D inversion model resistivity has been correlated well with the geological map of Gapis River. Thus, the 2-D resistivity survey has provided valuable information on the subsurface characteristic and the results obtained may be useful for future slope instability investigation.

Table 1. Summarize of slope failure evaluation using 2-D resistivity in Selangor.

Source:

Slope failures evaluation and landslides investigation using 2-D resistivity method

lM.M.Nordiana, I.N.Azwin, M.N.M.Nawawi, A.E.Khalil

NRIAG Journal of Astronomy and Geophysics
Volume 7, Issue 1, June 2018, Pages 84-89

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