Landslides articulation in Wadi Hof area southeast of Cairo, Egypt, based on geological and geophysical investigations

Detailed field geological, geoelectrical, hydrochemical, and microbiological studies were executed in the Wadi Hof area, southeastern Cairo, Egypt to delineate the causes of landslides along the train pathway between the eastern Wadi Hof camp and the western clay quarry. The area under consideration is of a gentle slope from east to west. The Eocene limestone rocks constitute the most common outcrops. Structurally, the investigated area was affected by faulting. Geophysically, 1D Schlumberger Vertical Electrical Soundings in addition to 2D Wenner Electrical Resistivity Tomography (ERT) techniques were performed to characterize the shallow subsurface implications. Eight Vertical Electrical Soundings were measured along with two main profiles-oriented northwest-southeast directions. A Schlumberger configuration with electrode separation starting from electrode separations (AB\2) = 1.5 m. to AB\2 = 200 m. was applied. Three 2D Electrical Resistivity Tomography profiles crossing the train pathway were constructed in a nearly east–west direction. The 1D inversion is applied using WinSev3.4 software and the 2D-ERT profiles were computed using the RES2DINV computer software. Moreover, hydrochemical and microbiological investigations in the form of water samples analysis were conducted east and west of the train pathway. The hydrochemical and microbiological characteristics of the analyzed water found that the water origin is different on the sides, east and west of the railway tracks. The integrated interpretations indicate that the study area consists of three main rock units; surface marl, calcareous sand, and, finally, clay to sandy clay units. The water originates from the Eocene limestone aquifer east of the train route directed towards the clay quarry west of the train path. Additionally, the landslides apparently result from the swelling of thick clay layers which leads to subsequent railway destruction shortly thereafter.


Introduction
The present work aims to find out the water origin that has collected at the floor of the clay quarry to the west of the train pathway and led to landslides and damage of the train path trajectory between the eastern Wadi Hof camp and the western clay quarry, as well as to shed light on the geological conditions prevailing in this particular area. The study area is located to the east of the River Nile, about 16 km to the southeast of Cairo with an average elevation of about 50 m above mean sea level. It extends between latitudes 29º54´31.5ʺ and 29º54´57.5ʺ North and longitudes 31º18´07.1ʺ and 31º18´46.1ʺ East (Fig. 1). The considered area has arid to semi-arid climatic conditions; warm in winter with scarce rain and hot to dry in the summer. Geomorphologically, the study area is subdivided from east to west into the plateau, escarpments, cliffs, terraces, and the Nile Flood plain. The slopes of the Middle Eocene rocks are unstable and represent potential areas of rock failure in many places (Yousif 2000). The Eocene limestone plateau is dissected by several faults and the Nile flood plain and the piedmont plain are formed by a tectonic depression (Henaish and Kharbish 2020). The Gebel Hof Formation was deposited in an open-marine environment and is mainly composed of Nummulitic limestone (Sallam et al. 2015). The groundwater and the surface water of the considered area, except the Nile water, reflect the saline water category (Abdalla and Scheytt 2012). The study area was subjected to three sets of faulting systems orienting mainly in NE-SW, E-W, and NW-SE directions (Moustafa et al. 1985). The general groundwater flow direction is to the northward and westward directions 1 3 195 Page 2 of 12 towards the Nile River. Based on X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM) analyses, Abd-Allah et al. (2008) confirmed that the major clay minerals in the East of Cairo, Egypt are mainly Na montmorillonite and kaolinite.
The huge quantities of clay and limestone exploitation activities are considered as the most important factors of water seepages and landslides (Abd El Gawad and Gamal 2008;Abd El Gawad et al. 2018). Hydrogeologically, the fissured and karstified Eocene limestone aquifer represent the main aquifer in the study area. The quarrying activities in the western parts of the train railway play an important role in the instability of the Middle Eocene rock slopes which represent potential sites of slope failure (Abd El Gawad 2020). In addition, due to systems of vertical and horizontal joints, the water can percolate through open joints, thus creating a seepage trail. Accordingly, the author's field investigations noticed that a considerable landslide under the train pathway and on the floor of the clay quarry, in addition to several water pools, are articulated. (Fig. 2a and b).

Methods and results of studies
Detailed field geological inspections in addition to geoelectrical, hydrochemical, and microbiological investigations were surveyed and executed in and around the area under consideration with a special emphasis on the east and west sides of the train pathway.

Geological fieldwork
According to precise GPS locations of the random distribution of vegetations ( (Fig. 3), it is noted that random distributions of vegetation grew at the top of the clay quarry in addition to the camp. This provides a primary impression about the source of water that drains from the limestone plateau. Additionally, the ground elevations of the aforementioned vegetation range from 42 m, 48 m, 24 m, 22 m, 0 m, 5 m to 1 m above sea level. Moreover, the historical rate of landsliding in the past fifteen years was studied in detail from 2004 to 2018 (Google Earth). This land-sliding rate is based on measuring distances between the train path location and the western border of landslides based on a historical view from Google Earth. The aforementioned measurements showed that the distance between the train path and the western border of landslides decreased gradually and then abruptly from June 2004 to May 2018 which resulted in the complete damage of the train path (Fig. 4).

Electrical resistivity tomography (ERT)
Electrical resistivity tomography has been extensively applied for landslides characterization. Several researchers as Miller and Sias (1998) others have proved the suitability and effectiveness of these methods for landslides characterization. The joint application of vertical electrical soundings (VES), electrical resistivity tomography (ERT, and electromagnetic soundings (TDEM) was applied to Piemonte, Italy that was subjected to a large slow-moving landslide (Godio and Bottino 2001). Time-lapse ERT measurements have proven to be efficient in tracking and monitoring landslide bodies (Perrone et al. 2014). Studies using ERT to monitor landslides under natural conditions over a period of months have been described by Uhlemann et al. (2017). Landslides hazard evaluation have been pronounced by electrical resistivity tomography characterization of slopes prone to mass movements (Huntley et al. 2019).
The geoelectrical resistivity method was executed in the form of 1D Schlumberger Vertical Electrical Soundings (VES) and 2D Electrical Resistivity Tomography (ERT) using a Wenner array. Eight vertical electrical soundings, separated by about 100 m between sounding centers, were measured along with two main profiles-oriented northwestsoutheast directions (Fig. 5). Each profile represents four vertical electrical soundings, the train path located nearly parallel to the two measured profiles and between them. A Schlumberger configuration with an electrode separation starting from (AB\2) = 1.5 m. to AB\2 = 200 m. was applied. Some outcropping geological sections (Fig. 6) near the measured vertical electrical soundings were selected and used for correlations between the electrical resistivity data and the previously mapped geological data. The method used to convert the measured values of electrode separations (AB\2) and apparent resistivities (ρa) into a multi-layer model is that of Zohdy (1989). The 1D inversion was applied using WinSev3.4 software followed by the establishment of the geoelectrical cross-sections. The interpretation results of the measured vertical electrical soundings are shown in Table 1 and illustrated in Fig. 7a and b. The geoelectrical cross-sections can outline the thicknesses and electric resistivities values characterizing the inherited geoelectrical units. Two geoelectrical cross-sections oriented nearly northwest-southeast were constructed to cover the study area. The geoelectrical cross-section A-A′ (Fig. 8) and B-B′ (Fig. 9) are represented by three geoelectrical units as follows: the first (surface) unit is characterized by relatively moderate to high electrical resistivity values and small thickness, which corresponds to the surface marl unit. The second unit is recognized by relatively moderate electrical resistivity values and medium thickness, which corresponds to the calcareous sand unit. The third unit is characterized by relatively low electrical resistivity value and larger thickness which corresponds to the wet claysandy clay unit.
The 2D Electrical Resistivity Tomography (ERT) profiles typically provide better lateral and vertical resolution if they are compared with the 1D vertical electrical soundings due to the typical spatially varying nature of near-surface geologic units. In the area under consideration, three 2D Electrical Resistivity Tomography profiles crossing the train pathway were constructed in a nearly east-west direction (Fig. 10a-c), with an electrode spacing of 4 m., a length of 112 m. and an average depth of penetration of 11 m. The 2D-ERT profiles were computed using the RES-2DINV computer software, which is designed automatically to set up two-dimensional resistivity models of the shallow  (Griffiths and Barker 1993). This software program was developed to invert large data sets collected by a large number of electrodes (Loke 1999 and2004). The three interpreted 2D-ERT profiles present almost the same geoelectrical characteristics with slight variations. These profiles show an upper surface marl unit with relatively high resistivity, overlying a lower resistivity unit of sandy limestone, and finally a lower resistivity layer of clay and sandy clay.

Hydrochemical and microbiological analysis
The chemical characteristics of water play an important role in the geologic history of the enclosing rocks, sources of recharge, velocity, and direction of water movement. Two water samples were taken from the area under investigation; the first one was taken east of the train pathway, from the camp water sewage station, and the second one was taken from the west of the train pathway, from the clay quarry water flowing from the direction of the camp on their way to the clay quarry. These waters were sampled carefully under precautions and analyzed chemically and microbiologically. The chemical analysis includes the major cations (Ca +2 , Mg +2 , Na +1 and K +1 ) and the major anions (CO3 −2 , HCO3 −1 , SO4 −2 and Cl −1 ) expressed in ppm (mg/L) in addition to the total dissolved solids Hydrochemically, the water sample taken from the clay quarry is characterized by relatively high TDS as well as the water type is sodium sulfate (Na +1 > Ca +2 > Mg +2 and SO4 −2 > Cl −1 > HCO3 −1 ) while the water sample taken from the camp is of relatively low TDS and the water type is sodium carbonate. It is important to mention that, the two analyzed water samples are completely different in nature and origin which strongly suggests that the two water samples are from different sources. The microbiological analysis also supports this conclusion because the water samples that were taken from the camp contains fecal bacterial (E. coli) and count 25ˆ10 2 cfu/ml while that of the clay quarry contains no fecal bacterial (Table 2).

Results discussion
The results of the geological, geophysical, hydrochemical, and microbiological studies indicate the following: the field studies showed that there are plant collections that exist at various heights on the mountain and also in the clay quarry indicating that the water comes from the east up the mountain and heads to the west towards the clay quarry. Additionally, the results of the geophysical studies in the study area indicate that there is no contention in the sequences of geoelectrical units horizontally or vertically east and west of the railway tracks. In addition, the presence of a large thickness of clay led to its saturation with water which resulted in landslides along the railway tracks. Moreover, in terms of analyzes of the water samples, hydrochemically and microbiologically, east and west of the train road, the analysis showed that the water sample taken from the sanitation pond of the camp east of the train road is completely different from those of the sample taken from the water west of the train road. This confirmed that the water that falls from the east in the direction of the west is responsible for the landslides and that the water taken from the sewage area does do not leak into the quarry. Furthermore, the landslides which occurred along the tracks of the train appear to have been caused mainly by the Eocene limestone aquifer. Therefore, the expected flow direction (Fig. 11) is from the highlands towards the lowlands (i.e., towards the Nile).

Summary and conclusion
After landslides occurred that caused the collapse of the railway tracks, which are located between Wadi Hof camp and the clay quarry, a question was: who bears the responsibility for this collapse? Is it the water leakage from the sewage station that is located in Wadi Hof's camp? Or is it from the quarrying operations that are taking place in the clay quarry? To understand and solve this problem, the following procedures and conclusions were applied and made: 1. The geological site visits showed the existence of plants at different heights in the camp and the quarry. The water was seen flowing out of the camp towards the quarry. 2. The historical tracking of landslides on Google Earth from 2004 to 2018 shows that the landslides have occurred earlier in time, but they increased dramatically in the past few years. 3. Geophysical studies were conducted including geoelectrical methods (1D and 2D) of the camp and the quarry, to identify the rock units. The result of these studies was that the shallow lithological subsurface sequences are nearly identical east and west of the railway tracks, only the Eocene limestone aquifer is located east of the railway tracks and deepens on the west side 4. The hydrochemical and microbiological characteristics of the analyzed water found that the water origin is different on the sides, east and west of the railway tracks. Consequently, it was concluded that there is no leakage from the station of Wadi Hof, rather the water is coming out from the Eocene limestone, (which is confirmed because that it has no Ecoli bacteria in it).   Thus, the causes of landslides and the reason behind the collapse of the railway tracks have been identified. It is leakage of water from the fissured and karstified Eocene limestone from the east side to the west side, which is from Wadi Hof's camp to the clay quarry. The Eocene aquifer in the eastern parts of the study area represents the main source of water responsible for the landslides along the train track. The rate of landslides is very fast, especially within the last few years. It is recommended to change the path of the railways from this area, to move the electricity towers, and to stop any quarrying operations west of the clay quarry.