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The present work deals with the stability analysis of rock slopes and study of the physical and geotechnical characteristics of the foundation soils and rocks for the northern part of the upper plateau of Gebel El-Mokattam. This area is located to the east of Cairo, at the northern part of the upper plateau of Gebel El-Mokattam. It is bounded by El Nasr roads from west, Harrfeen Plateau from north. It is bounded from the south by El-Mokattam road and on the east by gentle slope area leading to the New Cairo City. The study area belongs to Duwaiqa and Manshiet Nasser areas, Cairo Governorate. These areas are highly populated areas with randomly built houses
The geomorphic study of the area shows that the area is dissected by several wadies. The downstream direction of these wadies are toward N and NE directions which are compatible with ground gradient. These wadies are controlled by the minor faults and joints in the area. The study area contains small elevations of table lands with little scarps and separated from each other by low elevated lands. These scraps are capped by Ain Musa Bed which is highly jointed bed. These joints are recorded having vertical planes where they have dip in a range between 88°and 90°. Many rocks have been falling along these joints which led to destruction and great damages.
The stratigraphic units exposed in the Study area are all of sedimentary origin. The area is belong to the Middle - Upper Eocene age. The Middle Eocene rocks are composed mainly of white limestones and marls. The Upper Eocene rocks are made up mainly of yellowish sandy dolomitic limestones, marls and claystones.
The beds in outcrops are slightly dipping due to NE by 3°. This dip changes locally around the fault plane. The area is affected by a major fault lying in the northern part of the study area. The fault is oriented in the direction of N55°W, dips toward N35°E and reaches 800 m long. it is dip-slip normal fault and has a relatively small throw which is not obvious due to the extensive cover of buildings in the area. These Eocene rocks are characterized by dense fractures and joints. Joints which measured in the Upper Eocene rock are different than measured in Middle Eocene rocks. Middle Eocene joints are meaured in the ENE-WSW, E-W, WNW-ESE and N-S directions. They dip in a range between 45° and 90°. Upper Eocene joints are oriented in E-W, NE-SW, NW-SE, and N-S trends and having mainly vertical dip.
Five claystone samples which have different geological and mechanical characteristics were selected to analyze with X-ray diffraction. The main identified clay minerals are Montmorillonite and Kaolinite. Montmorillonite clay mineral is the most dominant clay mineral recorded in all the investigated samples. It ranges from 60 % to 75 % with an average of 71%. Kaolinite is the second abundant clay mineral that recorded in the investigated samples where it is percentage ranges from 10 % to 40 % with an average of 29 %.
The geotechnical properties of soil and rock are determined in detail in order to understand the engineering problems could be related to the foundation bedrocks. Fresh soil and rock samples were collected in two measured stratigraphic sections (S1 and S2). These sections represent the complete stratigraphic section of the study area.
The physical and geotechnical properties of the collected nine claystone beds (soil) were determined. The physical properties include the determination of initial moisture content (Wc), bulk density (ɣb), specific gravity (Gs), carbonate content, grain size distribution, the consistency limits, and consistency indices.
The values of initial moisture content of the studied soil samples ranges from 6.62 % to 10.10 % with an average of 8.17 %, while bulk density range from 2.00 g/cm3 to 2.30 g/cm3 with an average of 2.17 g/cm3, specific gravity of these samples ranges from 2.4 to 2.62 with an average of 2.53, the carbonate content range from 0.14 % to 29.99 % with an average of 7.26 % and the clastic content range from 70.01 % to 99.86 % with an average of 92.74 %.The grain size analysis shows that samples have the highest content of silt and clay fractions and the lowest content of sand fraction. This, in turn, reflected in the engineering behavior of soil samples.
The swelling potential of the soil samples was evaluated by direct or indirect methods. The indirect methods include mineralogical analysis, free swell test, colloid content, activity, and plasticity index. The more responsible direct method includes the use of swelling apparatus (Oedometer) to determine the swelling pressure of the soil when having contact with water.
Samples have liquid limit values that range between 53.40 % and 77.10 % with an average of 64.34 %, plastic limit of the studied samples range between 21.90 % and 30.20 % with an average of 27.95 %, shrinkage limit of the studied samples range between 11.90 % and 18.50 % with an average of 14.90 %. These results point to the appearance of highly swelled clay mineral (Montmorillonite) in most studied soil samples. This is confirmed by the results obtained from X-ray diffraction. In spite of these results, the presence of carbonate content in the samples lead to a decrease in the swelling potential.
The values of plasticity index of the studied samples range from 27.90 % to 52.40 % with an average of 39.45 %. This indicates that samples have a medium to very high swelling potential. The values of the liquidity index of the studied samples range from -0.25 to - 0.64 with an average of - 0.45. Therefore, they all can be classified as semi-plastic to solid soils. The values of the consistency index of the studied samples are range from 1.25 to 1.64 with average of 1.45. Therefore, they all can be classified as hard soils.
Analysis of physical and swelling properties of the studied soil samples shows that they have a medium to high swelling potentiality. However, the presence of carbonate content in some soil samples is the cause of the decreasing of their plasticity. The engineering classification of the soil samples is inorganic clay of high to very high plasticity.
The physical and mechanical properties of thirty carbonate rock samples were determined. The physical properties include initial moisture content (Wc), bulk density (ɣb), absorption (A) %, porosity (ɸ) %, and insoluble residue. While the mechanical properties determined by uniaxial compressive strength test for natural samples and water saturated samples, point load test and slake durability test.
The values of initial moisture content of the studied rock samples range between 0.10 % and 2.37 % with an average of 0.71%, while the values of bulk density range between 1.98 g/cm3 and 2.83 g/cm3 with an average of 2.41 g/cm3, water absorption range between 0.74 % and 13.89 % with an average of 3.98 %, porosity ranges between 1.85 % and 32.98 % with an average of 9.38 % and the percentage of carbonate range between 50.75 % and 98.91 % with an average of 79.06 %.
The values of the uniaxial compressive strength of the studied rock samples range from 7.31 MPa to 94.55 MPa with an average of 35 MPa and based on ISRM’s classification, the majority of samples have a weak to medium strong classes, while Young’s modulus values range from 433.73 MPa to 6880.18 MPa with an average of 26180.02 MPa. The Point load index (Is (50)) values range between 0.48 MPa to 1.64 MPa with an average of 0.85 MPa and based on classification of ASTM, the studied samples were classified as medium and high strength samples. The slake durability index after the second cycle (Id2) range between 51.66 % and 99.09 % with an average of 93.87 % and based on Gamble’s classification, the majority of samples have medium-high to high durability.
The different relationships between the physical properties and the uniaxial compressive strength indicate that the uniaxial compressive strength increases by the increase of the bulk density and decreases by the increase of porosity, water content and water absorption. The uniaxial compressive not affected by the increase and decrease of the carbonate content or the clastic (silt/clay) content.
Slope stability for the existing scraps in the study area was investigated whether composed of Middle Eocene or Upper Eocene rocks. This study included measuring and describing of different joint sets that affect the exposed rock units, field measurement of several slope profiles and identifying the different types of rock slope failures (Instability problems).
Three hundred and forty-one joints were measured for Middle and Upper Eocene rocks, repectively at twenty-five stations in the study area. The detailed field study indicates that the majority of these joints are vertical and exist in four main sets of orientation; E-W, NW-SE, NE-SW and N-S. They are open joints with joint walls are rough, undulated and the joint roughness coefficient (JRC) ranges between 14 and 16. The average spacing between joints ranges between 1.5 – 6 m. The spacing between joints leads to the formation of a well-developed block matrix of rocks, these blocks range in size from medium to large size. one of the most critical features in the study area is the presence of tension cracks, which indicating insability of rock slopes.The recorded tension cracks were parallel to the slope trend. They are open, undulating and rough, and their length ranges from 4-7 m and the aperture ranges from 10-50 cm.
The study area is composed of two different rock masses; Middle and Upper Eocene rock masses each one has its different geologic features from the other. Consequently, each rock mass group has a different slope analysis method owing to its mechanical and physical behaviors. Geotechnical properties of joints and slopes description have been determined to understand the different instability problems of both Middle and Upper Eocene slope of the study area.
The slope profiles of the Middle Eocene rocks are steep to very steep form sheer scarps, and have stair-case geometry in few places. Fieldwork shows that almost all the slopes of the Middle Eocene rocks are manmade slopes formed due to the past quarrying activities. Study characteristics of the Upper Eocene slope profiles revealed that all slopes are multimodal i.e. showing three or more slope breakpoints. This is due to the differential weathering occurred between hard limestone beds and alternating soft clay and marl beds. There is a close relationship between the lithological composition of slope and its shape. The steeper parts of slopes are made up of hard limestone beds which have high resistance against the weathering processes. While lithology in the gentle parts is composed of soft clays and marls that have low durability against weathering processes. All slopes are capped by hard, sandy, dolomitic, vuggy limestone beds (Ain Musa Bed) that is dissected by vertical discontinuities and underlain by soft clay bed of high plasticity. This bed forms a vertical cliff and represents the clue of slope instability of the study area.
The present investigation of rock slopes in the study area whether composed of Middle Eocene or Upper Eocene rocks show that these slopes are unstable and the inhabitant’s buildings in that area are vulnerable to high risk of falling rocks (rockfall).
Rockfall problems of Middle Eocene slopes related to the past quarrying activities. These slopes affected by past quarrying cuts in three different ways, which are vertical quarry walls, undercutting of the quarry walls and protruding ridges between adjacent quarries. While, the rockfall of Upper Eocene slopes occur due to lateral clay flow, weakly attached blocks and the formation of undercuttings.
In the present study, modeling and assessment of the rockfall hazards were done by the trajectographic analysis. Rockfall simulations were accomplished using Rocfall software V.4.0 for nine slope profiles of Upper Eocene slopes and one slope profile representative of the Middle Eocene slopes in the study area. These simulations gave a good visualization of the mechanism and behavior of the falling blocks and their relations with the inhabitant’s buildings in the study area. from the trajectories of the falling rock blocks, it can be inferred that the rock blocks are capable of reaching most of the buildings whether by falling or rolling or bouncing, which is very dangerous to the lives of residents of these buildings. These falling blocks have shown high kinetic energy and bounce heigh.
Rockfall risk assessment of these slopes in the study area was done by using the rockfall risk rating system (Saroglou, et al., 2012). The result of this application indicated that the study area has highly rockfall risk need to quickly mitigation measures.
Based on field measurements, geotechnical properties of the soils, mechanical parameters of the carbonate rocks and the rockfall risk analysis; some of important recommendations are suggested to be concerned during the further development in the study area and in similar regions.
6.2.1 Recommendations in case of soil foundation
The soil foundations in the study area are mainly composed of inorganic clay having high to very high plasticity. Hence, one or more of treatment methods should be applied before contraction. The best-fit treatment methods for the studied expansive soils are the soil replacement and the chemical stabilization. Operation of each treatment should be based on the scientific regimes.
6.2.2 Recommendations in case of rock foundation
The Middle and Upper Eocene carbonate rocks have weak to medium strong strengths and they are dissected by many fractures which allow the water to percolate and reduce the strengths of these rocks, so different water types should be prevented to reach these carbonate rocks at the foundation levels.
6.2.3 Recommendations for rock slopes
Analysis of the rockfall of the Middle and Upper Eocene slopes indicates that these slopes are unstable and the inhabitant’s buildings that are located in high-risk zones. Consequently, some mitigations measures have provided to minimize this risk. These mitigations measures include the construction of catch ditch at the toe area of the slopes and barriers lying before the inhabitant’s buildings. Also, re-shaping of the steep parts of the slopes in the form of bench-slope style and removing the hanging weakly and attached blocks that can easy fall down.