A slope is defined as a ground surface that positions at an angle to a horizontal level. Slopes may be creating from the natural or usually by people. An earth slope is an unsupported, inclined surface of a soil mass (Salunkhe et al., 2017). Each slope has singular soil properties and geometric features, which try to resist soil gravity or failure. Slope collapse cause the soil body to transfer downward and outward usually happening slowly or rapidly without attention. Slides generally start from hairline tension cracks, which propagate inside the soil layers (Abbas, 2015).
Slope stability problems are frequently encountered in the construction of roads, canals and dams as well as some natural slopes are may become unstable due to the presence of water which weakens the soil characteristics or due to an excavation. The slip of a slope can be catastrophic and causes human losses in addition to considerable natural damages. (Fawaz et al., 2015).
Instability related issues in engineered as well as natural slopes are common challenges to both researchers and professionals. In construction areas, instability may result due to rainfall, increase in groundwater table and change in stress conditions. Similarly, natural slopes that have been stable for many years may suddenly fail due to changes in geometry, external forces and loss of shear strength (R. Desai & R. Joshi, 2019). The failure of slopes takes place mainly due to the action of gravitational forces, and Seepage forces within the soil (Salunkhe et al., 2017). In addition, the long-term stability is also associated with the weathering and chemical influences that may decrease the shear strength (Abramson et al., 2002; R. Desai & R. Joshi, 2019).
The engineering solutions to slope instability problems require good understanding of analytical methods, investigative tools and stabilization measures. One says, “The primary aim of slope stability analyses is to contribute to the safe and economic design of excavation, embankment and earth dams” (R. Desai & R. Joshi, 2019).
The rock-socketed pile is a large diameter pile, widely employed as an important form of bridge foundation in regions characterized by complex engineering geology, such as rivers, lakes, and valleys (Yong et al., 2012).
In this study, large diameter bored rock-socketed piles are being used to carry heavy loads from Conveyor Belt (Fig. 1). The total length of the Conveyor Belt is about seven kilometers for Dangote Cement Factory to quarry area in order to transport limestone from quarry to the Factory. The vertical, long, bored, end bearing and reinforced concrete pile were used for this study.
1.1 Location and Accessibility
The case study area is located in regional state of Oromia, at Mugher town, Ethiopia (Fig. 2). The area is accessible by Asphalt road leading to Mugher cement enterprise, via Mugher town, about 90km west from Addis Ababa. The area is bounded by geographical coordinate latitude 9˚15’00”N to 9˚30’00” N and 38˚20’00”E to 38˚30’00”E, in the Northwestern Ethiopian Plateau (Amberber, 2018).
1.2 Geology of Study Area
Mugher valley is part of Blue Nile Basin situated in the Northwestern Ethiopian Plateau. The E and SE part of the area is bounded by the tectonic escarpment of the uplifted western flank of the Main Ethiopian Rift. It is also bounded by the Axum–Adigrat and Ambo-Nekemte lineaments to N and S directions respectively. The Blue Nile Basin contains about 1400m thick section of Mesozoic sedimentary rocks that un-conformably overlying NeoProterozoic basement rocks and un-conformably overlain by Early-Late Oligocene and Quaternary volcanic rocks (Gani et al., 2009).
1.3 The Type of Soil and Rock on Study Area
In the study area, there is huge amount of surface soil covering the bed rocks. The stiff light brown gravelly silty soils are found from depth of 0–0.5 m. The dense dark brown silty sandy gravel soils are found from depth of 0.5–5m. The strong dark grey fine grained basalt (boulder) covered the depth of 5–5.6m. The dense dark grey fine grained sand found at depth of 5.6–5.8m. Medium strong dark grey fine grained highly fractured and moderately weathered basalt covers depth of 5.8–6.3m. The dense dark brown sandy gravel found from depth of 6.3–9m. Very stiff light orange to whitish brown clayey silt soil are found from depth of 9–13.3m. The weak white limestone from 13.3–13.5m (SABA Engineering, 2015).
The light to orange brown highly weathered marl/shale depth is 13.5–15m and orange to dark brown highly weathered marl/shale depth is 15–18.2m. The medium strong dark grey fine grained slightly weathered and highly fractured basalt rocks are found from depth of 18.2–22.4m (SABA Engineering, 2015).
1.4 Climate
Study area falls under sub-tropical climatic zone with the highest rainfall occur between June and September (Amberber, 2018). Another smaller rainy season is also known between February and May, however, in the study area rainfall amount is generally low. Two rainy seasons are known in the area those are summer and winter (Table 1 and Fig. 3).
Table 1
The mean annual rainfall of the surrounding area (Amberber, 2018)
Month | Jan | Feb | Mar | Apr | May | Jun | July | Aug | Sept | Oct | Nov | Dec |
Addis Ababa | 15.5 | 38 | 66.8 | 89.4 | 83.0 | 130.7 | 259.5 | 276.5 | 170.9 | 36.5 | 7.9 | 9.5 |
Chancho | 28.5 | 24 | 61.6 | 61.0 | 61.0 | 133 | 133.2 | 321.8 | 125.3 | 38.1 | 13.3 | 13.5 |
Derba | 14.2 | 31 | 62.2 | 64.1 | 122 | 119.8 | 307.9 | 320.1 | 159.0 | 27.8 | 9.5 | 16.2 |
Holeta | 23 | 57 | 74 | 95 | 76 | 162 | 287 | 332 | 206 | 22 | 14 | 12 |