General Use and Applicability of Slope Stability Charts

7/31/2019 General Use and Applicability of Slope Stability Charts

1/6

General Use and Applicability of Slope Stability Charts

a. Slope stability charts provide a means for rapid analysis of slope stability. They can be used forpreliminary analyses, for checking detailed analyses, or for complete analyses. They are

especially useful for making comparisons between design alternatives, because they provide

answers so quickly. The accuracy of slope stability charts is usually as good as the accuracy

with which shear strengths can be evaluated.

b.

In this appendix, chart solutions are presented for four types of slopes:

(1) Slopes in soils with = 0 and uniform strength throughout the depth of the soil layer.

(2) Slopes in soils with > 0 and c > 0 and uniform strength throughout the depth of the soil layer.(3) Infinite slopes in soils with > 0 and c = 0 and soils with and > 0 and c > 0.

(4) Slopes in soils with = 0 and strength increasing linearly with depth.

Using approximations in slope geometry and carefully selected soil properties, these chart solutions can

be applied to a wide range of non homogenous slopes.

Averaging Slope Inclinations, Unit Weights and Shear Strengthsa. For simplicity, charts are developed for simple homogenous soil conditions. To apply them to

non homogeneous conditions, it is necessary to approximate the real conditions with an equivalent

homogenous slope. The most effective method of developing a simple slope profile for chart analysis is

to begin with a cross section of the slope drawn to scale. On this cross section, using judgment, draw a

geometrically simple slope that approximates the real slope as closely as possible.

Fundamentals of Slope Stability Analysis

a. Conventional approach. Conventional slope stability analyses investigate the equilibrium of a mass

of soil bounded below by an assumed potential slip surface and above by the surface of the slope.

Forces and moments tending to cause instability of the mass are compared to those tending to resist

instability. Most procedures assume a two-dimensional (2-D) cross section and plane strain conditionsfor analysis.

Successive assumptions are made regarding the potential slip surface until the most critical surface

(lowest factor of safety) is found.

The stability or instability of the mass depends on its weight, the external forces acting on it (such as

surcharges or accelerations caused by dynamic loads), the shear strengths and pore water pressures

along the slip surface, and the strength of any internal reinforcement crossing potential slip surfaces.

The factor of safety. Conventional analysis procedures characterize the stability of a slope bycalculating a factor of safety. The factor of safety is defined with respect to the shear strength of the

soil as the ratio of the available shear strength (s) to the shear strength required for equilibrium (), that

is:

Bishop Method limitations assumptions:_


2/6

b. Limitations. Horizontal equilibrium of forces is not satisfied by the Simplified Bishop Method.

Because horizontal force equilibrium is not completely satisfied, the suitability of the Simplified

Bishop


3/6

Method for pseudo-static earthquake analyses where an additional horizontal force is applied is

questionable.

The method is also restricted to analyses with circular shear surfaces.

c. Recommendation for use. It has been shown by a number of investigators (Whitman and Bailey

1967; Fredlund and Krahn 1977) that the factors of safety calculated by the Simplified Bishop Method

compare well with factors of safety calculated using rigorous methods, usually within 5 percent.

Furthermore, the procedure is relatively simple compared to more rigorous solutions, computer

solutions execute rapidly, and hand calculations are not very time-consuming. The method is widely

used throughout the world, and thus, a strong record of experience with the method exists. The

Simplified Bishop Method is an acceptable method of calculating factors of safety for circular slip

surfaces. It is recommended that, where major structures are designed using the Simplified Bishop

Method, the final design should be checked using Spencers Method.

The factor of safety is the same for all slices.

3. The factors of safety with respect to cohesion (c) and friction (tan ) are equal.

4. Shear and normal forces on the sides of each slice are ignored.

5. The water pressure (u) is taken into account by reducing the total weight ofthe slice by the water uplift force acting at the base of the slice.

Modified Swedish Method

d. Limitations. The principal limitation of the Modified Swedish Method is that calculated factors of

safety are sensitive to the assumed interslice force inclination. Depending on the inclination assumed

for the interslice forces, the factor of safety may be either underestimated or overestimated compared to

the value calculated by more rigorous methods that fully satisfy static equilibrium. The sensitivity of

the method appears to be due in large part to the fact that moment equilibrium is not satisfied.

e. Recommendations for use. The force equilibrium procedure is the only method considered to this

point that can be utilized for analyses with general shaped, noncircular slip surfaces. Although the force

equilibrium method is not as accurate as Spencers Method (described next) for analyses of general-

shaped noncircular slip surfaces, the force equilibrium method is much simpler and is therefore suitable

for hand calculations, whereas Spencers Method is too lengthy for hand calculations.

Accordingly, the force equilibrium method is recommended for use in hand calculations where

noncircular slip surfaces are being analyzed. If the force equilibrium method is being used to check

calculations that were performed using Spencers Method, the side force inclination used for the hand

calculations should be the one calculated by Spencers Method (Section C-5).

Spencers Method and the force equilibrium procedure should produce identical results when the sameside force inclination is used in both methods. The Modified Swedish Method is useful where existing

slopes have been designed using the method and are being analyzed for new conditions, such as

updated pore pressure information, or where alterations are to be made. Using the same method will

allow meaningful comparison of results to those from previous analyses. For all new designs,

preference should be given to the Simplified Bishop (circular slip surfaces) and Spencer (noncircular

slip surfaces) Methods.


4/6

Taylor`s chart


5/6


6/6

General Use and Applicability of Slope Stability Charts

Documents

Transcript of General Use and Applicability of Slope Stability Charts