CE 343 Slope Stability

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Accreditation Process in North American universities 1

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Stability of Slopes

Prof. Dr. A.M.M. Safiullah

Ahsanullah University of Science & Technology

October , 2011

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Geotechnical Engineering – II

CE 3433.00 Credits, 3 Hrs/WeekSyllebus

Soil Investigation Techniques

Settlement Computations Types of Foundations

Bearing Capacity of shallow and deepfoundations

Settlement and distortion of foundations

Design and construction of shallowfoundations, raft and piles

Stability of slopes

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With low ground water, failure occurs on shallow, straight, or slightly curved surface. Presence of

tension crack at the top of the slope influences failure location. With high ground water, failure occurson the relatively shallow toe circle whose position is determined primarily by ground elevation.

Analyze with effective stresses using strength c’ and f’ from consolidated drained (CD) tests. Porepressure is governed by seepage condition. Internal pore pressures and external water pressuresmust be included.

Natural Slope in coarse grained soil with some cohesion

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Stability depends primarily on ground water conditions. With low ground water, failures occur as

surface sloughing until slope angle flattens to friction angle. With high ground water, stable slope isapproximately ½ friction angle.

Analyze with effective stresses using strength f’ . Slight cohesion appearing in test envelope isignored. Special consideration must be given to possible flow slides in loose, saturated fine sands.

Natural Slope in coarse grained cohesionless soil

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Failure occurs on circular arcs whose position is governed by theory. Position of ground water table

does not influence stability unless its fluctuation changes strength of the clay or acts in tensioncracks.

Analyze with total stresses, zoning cross section for different values of shear strengths. Determineshear strength from unconfined compression test, unconsolidated undrained triaxial test or vane sheartest.

Natural Slope in normally consolidated or slightlypre-consolidated clay

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Location of failure plane is controlled by relative strength and orientation of strata. Failure surface is

combination of active and passive wedges with central sliding block chosen to conform tostratification.

Analyze with effective stress using c’ and f’ for fine grained strata and f’ for cohesionless material.

Natural Slope in stratified soil

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Strength of old slide mass decreases with magnitude of movement that has occurredpreviously. Most dangerous situation is in stiff clay which is softened, fractured, orslickensided in the failure zone.

Creep movements in old slide mass

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Usually minimum stability occurs during placing of fill. If rate of construction is controlled,allow for gain in strength with consolidation from drainage.

Analyze with effective stress using c’ and f’ from CU test with pore pressure measurement. Apply estimated pore pressures or piezometric pressures. Analyze with total stress forrapid construction without observation of pore pressures, use shear strength fromunconfined compression or unconsolidated undrained triaxial.

Failure of fill on soft cohesive foundation with sand drains

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Usually minimum stability obtained at end of construction. Failure may be in the form of rotation or translation, and both should be considered.

For rapid construction ignore consolidation from drainage and utilize strengths determinedfrom U or UU tests or vane shear in total stress analysis. If failure strain of fill andfoundation materials differ greatly, safety factor should exceed one, ignoring shearstrength of fill.

Analyze long-term stability using c and f from CU tests with effective stress analysis,applying pore pressures of ground water only.

Failure of stiff compacted fill on soft cohesive foundation

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Method of slices

''tan'

N LacF

)cos('tan'

ulW Lac

Fellenius:

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Simplified Bishop Method

Simplified Bishop Method:

Shear force at the base of the slice :

Resolving forces in vertical direction:

Substituting after rearrangement,

021 X X

)'tan''(1

sin'tan'

coscos'F

lcul N W

sin'tancos

cossin'

]'tantan

sec}'tan)('[{

ubW bcW

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Slope stability for an embankment

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buW bc

]'tan)('[

)'tantan

1(cosF

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buW bc

]'tan)('[

)'tantan

1(cosF

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Meghna-Donagoda Flood Embankments

CE 343 Slope Stability

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