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4
DETERMINISTIC METHODS OF ANALYSIS
Deterministic methods, also referred to as traditional methods, calculate a single factor of
safety which is unique to the set of soil properties used to analyse the slope. They do not take
into account uncertainties in the input parameters; it is assumed that the input values are
representative of all soil properties. This chapter will introduce traditional methods of
analysing slope stability including site investigation, stability charts, and limit equilibrium
methods. Chapter 5 applies these methods, along with a comparative (parametric) analysis, to
a baseline case and an industry-specific case study.
4.1
Stability charts
4.1.1
Taylor’s stability charts
Taylor’s stability charts for homogenous undrained (ϕu = 0) and drained (c′ = 0) soil slopes
are a “…simple and straightforward” method of determining factors of safety (Steward et al.,
2011). Taylor’s stability charts are based on three types of failure circles: toe circle, slope
circle and midpoint circle (Taylor, 1937, 1948), as shown in Figure 4.1.
Figure 4.1: Critical slip circles: (a) toe circle (b) slope circle (c) midpoint circle (Steward et al. ., 2011)
Taylor (1937) proposed a stability number (𝑁𝑆 ), to be used in stability charts, as:
𝑁𝑆 =
𝛾𝐻
𝑐𝑢,𝑚𝑜𝑏
4.1
where 𝛾 is the bulk unit weight, H is the height of the slope and 𝑐𝑢,𝑚𝑜𝑏 is the shear strength
mobilised along the failure arc. The interrelationship between 𝑛𝑑 , 𝛽 and 𝑁𝑆 can be identified
in Taylor’s stability charts for undrained clays and drained soils.
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