Application of Vertical Electrical Sounding for Subsurface Characterization to Determineslope Instability at Perizie, Nagaland

Vertical Electrical Sounding (VES) technique which employs the Schlumberger depth sounding method was used to investigate the subsurface conditions at the landslidein Perizie colony, Kohima. The interpretation of the VES curves shows differing lithology consisting of weathered shales with associated clay, which is considered an important factor for slope instability. The shear strength of the slope materials, including the country rocks and the soil cover, isalso significantly reduced and easily detached due to the effects of weathering and erosion, causing the landslides. Three to six possible geo-electric layers are delineated from the variations in the resistivity values, with the lithologies consisting of top soil followed by alternate layers of weathered and fractured rocks of varying thickness. The analysis also points to the presence of a weak zoneat a depth of about 10 m, which is not apparent from the surface. The formation of groundwater aquifersin the fractured zones indicates high risk for slope failure as this acceleratesthe weak country rocks to weather. The results obtained by this studycorrespond well with the available borehole data of the area.


Introduction
Kohima, the administrative headquarter of Nagaland, often experiences natural disasters in the form of landslides, which posesa high risk to the residents of this city town.It is situated towards the south of the state hosting several major offices and institutions.
It also facilitates the main artery for the state of Manipur and several other towns, as the Trans-Asian Highway, AH 1, and National Highway, NH 2 pass through this town.Geologically, Kohima town is made up predominantly of the Tertiary Disang Group of rocks consisting of shales with some siltstones and sandstones.][3][4] The shale exposures are dark grey and splintery in nature and are found to be weathered to clay in many areas.
[7][8][9] Kohima, by virtue of being situated in a hilly city town, experiences regularoccurrence of landslides, that are often triggered by external factors like heavy rainfall, seismic and tectonic activities, changes in water level, stream erosion, etc. [10][11][12][13][14] The town receives heavy rainfall, especially during the monsoon season, and the average rainfall recorded for the last 5 years (2017-2021) between the months of May to August ranges from 1000 mm to 1800 mm (Source: Department of Soil and Water conservation, Kohima, Nagaland).Some of the factors that mainly contribute to these mass movements are continuous heavy rainfall and conditions of the slope materials.There is easy infiltration of water specially on bare and exposed slopes hosting weak lithology contributing to weathering of the country rock.As a result of the weathering and erosion process taking place, the slope materials lose their shear strength which is considered an important factor in the occurrence of landslides. 15Landslides are common in areas dominated by rocks, that are jointed, faulted, and sheared.Shales often associated with landslides are mostly weathered shale, which has become clay-rich soil with very low shear strength, especially when saturated with water.Shale and mudstone that are intercalated with minor beds of sandstone are again observed to be more prone to partial or total weathering of the rock mass.The use of the geo-electric method to investigate the subsurface lithology helps in the identification of weakness zones and the possible mass movements in vulnerable areas that are prone to landslides.[18][19][20][21][22][23] Weakness zones are those areas whose mechanical properties are considered as significantly lower compared to the surrounding rock masses.It can occur as fault zones, shear zones or weak rocks and most of these zones are the results of numerous ruptures undergoing over geological time.One significant activity which impacts the properties of the weak rocks is the ongoing weathering process of the rocks as a result of water penetration into these weak zones from the permeable layers.[26][27][28][29] The clay-bearing rocks exhibiting low resistivity on the other hand are the least permeable and considered a waterproof layer, which is capable of acting as the probable layer for the initiation and planeof movement of the weathered materials.[32][33] The presence of high groundwater content on slope areas is another driving force that causes instabilities.The high-quantity rainfall which the area receives during the monsoon season along with favourable geologic conditions, facilitates easy infiltration of water into the weak rocks and leads to an increase of the groundwater table, consequently saturating the weak materials and reducing their shear strength.[36][37] Case studies related to landslides in and around Kohima town were carried out by several past workers, employing various methods to determine the influence of geology on the occurrence of landslides.Sharda and Bhambay 38 prepared the slope classification maps of Kohima town and its geotechnical reports.[41][42][43][44][45][46][47][48] One area within Kohima town that is often plagued by landslides is Perizie colony.It is incorporated in the Survey of India topographicmap, sheet number 83K/2, and lies between 25°41'48'' to 25°41'53'' North latitudes and 94°05'39'' to 94°05'46'' East longitudes (Fig. 1).The study area is densely populated with several buildings, both residential and commercial, offices and roads, including the National Highway-2 (NH-2).The landslide has troubled this colony for the past few decades, damaging both EZUNG et al., Curr.World Environ., Vol.17(3) 657-671 (2022) kutcha houses as well as RCC buildings.Incessant rainfall during the monsoon season acts as the triggering factor for further movements, leading to road blockades, and affecting the colony resident's travel to and from Kohima town.Some researchers have carried out landslide studies in Nagaland.However, most of the earlier workers adopted a geological and geotechnical approach.The electrical resistivity methods for landslide investigations and the corresponding information on the subsurface conditions from such techniques arerare.The present study is probably the first of its kind in this area in which an attempt is made to utilisethe non-invasive method to evaluate the causes of a landslide bydetermining the role of the inherent subsurface conditions of the slope materials.Theresults of this study will therefore contribute towardsa better understanding of the causes of various land instabilities in this part of the country.

Lithology of the Area
Major portion in the area of study consists predominantly of the Disang shales, which are blackin colour.The exposed rocks are sheared, crumpled, weathered, and fractured.Field photographs showing the exposed fractured and weathered shales are given in Fig. 2.

Materials and Methods
The study utilized Vertical Electrical Sounding (VES) method, which givesthe resistivity variations with depth.[51][52][53] One dimensional (1D) VES method measures the electrical resistivity basing on the estimation of electrical conductivity or the resistivity of medium where a transmitting direct current is induced artificially inside the ground between an electrode pair, using galvanic batteries (DC) or low-frequency AC generators, gives the measure of the electric potential between the other electrode pair.The electrical characteristics variation with depth gives the apparent resistivity (ρa) of geologic formation, given as. 54a =K δV/I, where K = π/2 [(L⁄I) 2 -1] is the geometrical spacing factor. .

..(1)
Here, five electrodes are used, where centre of the electrode is kept fixed and measurements are taken for various values of expanding current electrode.The two outer electrodes, the current electrode, act as the source and the other two inner electrodes, the potential electrode, serves as the receiver.The current (I) is introduced between the current electrodes pair, say, A and B and the potential difference (δV) produced is measured with the help of the potential electrodes pair, say, M and N.
All the four electrodes are stretched along a straight line, with the current electrodes spacing kept large in comparison with potential electrodes spacing (Fig. 3).The present study employs the Schlumberger configuration 55 following the spacing of the current electrode and the potential electrode by order of 5, i.e., AB/2 ≥ 5 MN/2.This configuration gives a good resistivity contrast between the saturated and unsaturated layers for analysing the subsurface condition.
The electric topology of the geological bedding of the earth using sounding curves characteristics used for the analysis is classified into the four basic categories for subsurface layers with resistivity ρ1, ρ2, and ρ3, with ρ1 as the first resistivity valuefollowed by ρ2 and ρ3 is given as.ρ1 > ρ2 > ρ3: Q-type ρ1 > ρ2 < ρ3: H-type ρ1 < ρ2 > ρ3: K-type ρ1 < ρ2 < ρ3: A-type The field resistivity data was acquired by using a resistivity meter of the type, Aquameter CRM 500, with varying profile lengths for each VES location due to the surface restrictions (Table 1). 3 (three) VES locations were selected along the colony road to collect the field data (Fig. 4) and analysed for determining the resistivity layers.

Results and Discussion
9][60][61][62][63][64][65] KQ type at VES 1 show resistivity of the type ρ1 < ρ2 > ρ3 > ρ4.The topsoil thickness of the area is 0.498 m with a resistivity value 149 Ωm, followed by a more compact rock layer comprising of shale or shale with sandstone intercalations with resistivity value 739 Ωm.The third layer indicates the presence of weathered shale having resistivity value 112 Ωm with thickness of 7.59 m, followed by the region with fractured shale at greater depth, which may be considered a suitable zone for groundwater reservoir.K type curve is seen at VES 2 showing increasing resistivity to a maximum and then decreasing to lower resistivity values (ρ1 < ρ2 > ρ3).Shale or shale with sandstone intercalation is indicated between the top soil and the lower saturated fractured shale.The firstlayer resistivity comprising of the top soil or shale exposures is 122 Ωm with a thickness of 0.9 m.Shale or shale with sandstone intercalations This layer points to the presence of groundwater (Table : 2).Development of tension cracks can be seen at the VES 3 survey location.These fissures act as the pathways for maximum water infiltration This leads to increased pore water pressure and reduced shear strength of the slope material (Fig. 6).
The VES results are used to construct the subsurface lithologic description diagram, which shows a good correlation with the bore-hole data (Fig. 7)   During the fieldwork, three ring wells were found at different places in the vicinity of the VES survey stations.The following tabulation was made after an inquiry from the local residents (Table : 3).
The depth of water level observed for the three ring wells ranges between 1.5 m to 3 m (approx.)below the ground level, which is in consonance with the VES inferred layer parameters where the potential for groundwater is also inferred at a depth>8 m for VES 1, > 2.2 m for VES 2 and 8 m for VES 3 below the ground level.The comparison between the VES inferred layers at Perizie colony and borehole data at IG Stadium confirms the presence of fractured shale, weathered to highly weathered shale layer as the near-surface materials (Fig. 8).
The resistivity survey was taken during the month of November, which is considered a dry season, and the existence of abundant water in the three ring wells during this season indicates that the area is well-saturated.The VES results have confirmed the presence of viable aquifers; that may be one of the factors for the country rocks to weather, besides generating water pressure in the pore spaces, leading to reduced strength of the slope materials (Fig. 9).7][68] The analysis also shows the dominance of highly weathered and fractured shales, which can be another important factor for the slope failure.The presence of these weak rocks and aquifers can be matched exactly with direct borehole data but since it was not available during the time of data analysis, the comparison was made with the existing borehole data at IG Stadium where the results were approximately matched and were field-confirmed.The presence of this weak zone indicates that the probable sliding of the slope materials might have initiated from this boundary, which led to the slope failure (Fig. 10).

Conclusion
The 1-D resistivity survey, utilizing the VES method and employing the Schlumberger configuration, was carried out at Perizie colony, Kohima.Three VES resistivity survey was conducted to determine the inherent subsurface condition of the slope materials and to correlate the cause of surface instabilities at Perizie colony.Results of the resistivity analysisindicate that the area consists of weathered and fractured rocks that are vulnerable to mass wasting.The weathered rocks were characterized by materials with low resistivity values, identified as the saturated zones.
The pseudo cross-section helped to identify thepresenceofthe weak zone between VES 1 and VES 3 at depthsof 10 m from the surface.During the monsoon season, the surface runoff can cause surface water infiltration into the weathered rock, which can easily seep in within the weakness zones, thereby increasing the weight of the weathered materials that can initiate slope failure.
The presence of abundant groundwater even during the dry season also points to the saturation level of the subsurface materials which can accelerate the weathering of the already weak subsurface materials.
Results obtained by the VES method are in consonance with local geology derived from available borehole data, which confirms the presence of fractured and weathered shale as the near-surface materials.The analysis also demonstrates that viable aquifers and depths of the water table can also be delineated by the VES method.Thus, the 1-D resistivity survey has provided valuable information about the subsurface conditions that might have caused the occurrence of landslides in the study area, and the results obtained may be helpful for future landslide investigations.
is seen in second layer, showinga high resistivity value of 405 Ωm having a thickness of 1.26 m.The third layer has a lower resistivity value of 89.5 Ωmat greater depths, which points to a zone of saturation of the rocks, containing the fractured rocks.These canbe considered as a potential areafor groundwater extraction.VES 3 shows KHK curves type with resistivity values given as ρ1 < ρ2 > ρ3 < ρ4 > ρ5> ρ6.The top soil occupies the first layer as coverage with thickness of 0.691 m and resistivity value 85.48 Ωmand a second layer containing shale is found with a resistivity value of 256.7 Ωm and a thickness of 1.095 m.Weathered to highly weathered shale or clay is found at the third and the fifth layers showing resistivity values ranging between 20.79 Ωm to 23.83 Ωm.The fourth layer with resistivity values of 76.82 Ωm with thickness 4.74 m indicates layers containing fractured shales.

Fig. 4 :
Fig. 4: Google earth image of the study area with VES stations