7. Slope Stability Analysis 161
7.2 Factors and Engineering Measures that Influence
Instability of soil slope is a feature that often occurs in actual engineering. The soil slope will slide if it is not correctly controlled, and it will have a significant impact on the progress of engineering.
The slope failure will also cause accidents, thus threatening human lives and properties. The main influencing factors and the commonly faced problems in slope stability in engineering will be described briefly.
7.2.1 Essential characteristics of slopes
Slopes refer to rock and soil mass with an inclined surface. The slopes which are formed by natural geological processes, such as hill slopes or cliffs, are called natural slopes. The slopes formed by artificial excavation or backfilling, such as the slope of channel, foundation pit, embankment, are called artificial slopes. The components of the slope are shown in Fig. 7.2.
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Fig. 7.2. Components of slope.
7.2.2 Factors influencing for slope stability
The factors that mainly influence slope stability are as follows:
(1) Slope angle, β: In general, slope stability increases with a decrease in the slope angle. It would be uneconomic in foundation pit excavation. A steep slope is economical for excavation but is not safe enough.
(2) Slope height, H: Similar to the slope angle β, the slope will have low safety with a large slope height.
(3) The physical and mechanical properties of the soil (e.g.
unit weightγ, shear strength parameters c andϕ): The slope is unstable due to the increase of the unit weight and shear stress of soil. The stable slope can transform to an unstable state due to several reasons, e.g. external loading, shear strength reduction, and pore water pressure generation, as a result of earthquakes, rainfall infiltration, and fluctuation of underground water level, respectively.
(4) The infiltration of rainfall and seepage of underground water: Rainfall infiltration could increase the water content of the soil, and the pore water in the soil plays a lubricating role in the weak layers of the slope. When the groundwater seepage is flowing in the slope, it could be safe for the slope to slide in a direction opposite to the direction of the seepage flow force;
however, it is dangerous when both of them move along the same direction. The extensive practical engineering experience has also proved that landslides and slope failures often occur during the rainy season or are caused by heavy rain.
(5) The influence of vibration: The strength of soil can be decreased by the vibration load yield due to the pile compaction,
dynamic compaction, engineering blasting, vehicle movement, etc. On the contrary, soil strength also can be reduced as a result of earthquakes, e.g. the liquefaction of the soil in slope can be induced by the seismic load. The saturated, loose, and fine sand is more accessible to liquefaction due to the shock of the earthquake.
(6) The impact of human activities and the ecological environment: Slope stability could be significantly influenced by human activities, such as preloading on the slope crest, excavation, or river erosion at the toe of the slope. Climatic changes can also influence the slope stability, e.g. change the soil from dry to wet, shrink to expand, and freeze to melt. Ultimately, the soil will be softened and its strength will be reduced.
In conclusion, slope instability is usually triggered by the external influencing factors, e.g. the shear strength of soil can be reduced by the external influencing factors. Thus, additional attention should be paid to the impact of external influencing factors to enhance the stability and safety of the slope.
7.2.3 Engineering design and measures
In engineering practice, the stability and safety of the slope are evaluated by the factor of safety K, which indicates the safety of the slope under the most dangerous conditions. The value of K is usually defined as the ratio of the slip resistant force (moment) to the sliding force (moment) on the failure surface. When the failure surface is fixed, the value of K can be obtained via the method of slope stability analysis to determinate the slope stability.
In order to enhance the stability of the hill slope, cutting slope, and embankment, the factor of safety K should be greater than 1.0. Taking into account the class of buildings, the condition of the foundation, the strength parameter of the soil, the average consolidation degree of the soil, the calculation method, and local experience, K is determined to be 1.1–1.5. Some situations that are prone to slope instability in engineering are as follows:
(1) Foundation pit excavation:In general, the foundation depth of shallow clay foundation d = 1–2 m. Vertical excavation can be used to save the earthwork, and the fast mechanized
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construction can also be used. However, when the foundation depth is greater than 5 m, the foundation pit will collapse due to vertical excavation. If the slope of the foundation pit is too gentle, it will lead to an increase in the workload and the have an effect on the adjacent buildings. Thus, taking into account the above reasons, it is necessary to design a safe and economic slope of foundation pit through the slope stability analysis.
(2) Loading on the slope crest: Constructing the building or stacking heavy objects on the top of the slope may render the stable slope unstable and lead to sliding. If the building is far away from the slope, there is no influence on the slope. Therefore, a safe distance should be determined according to the engineering requirements.
(3) The artificial river embankment, earth-filling earth dam, road embankment, and cutting slope: A lot of time and work could be saved with an appropriate slope design for these engineering projects where the slope is at a very long distance.
7.3 Stability Analysis of Cohesionless Soil Slope