Slope Stability Problems.

Excavation slope instability may result from failure to control seepage forces in and at the toe of the slope, too steep slopes for the shear strength of the material being excavated, and insufficient shear strength of subgrade soils.  Slope instability may occur suddenly, as the slope is being excavated, or after the slope has been standing for some time.  Slope stability analyses are useful in sands, silts, and normally consolidated and overconsolidated clays, but care must be taken to select the correct strength parameter.  Failure surfaces are shallow in cohesionless materials and have an approximately circular or sliding wedge shape in clays.

      a. Cohesionless slopes resting on firm soil or rock.  The stability of slopes consisting of cohesionless soils depends on the angle of internal friction  φ’, the slope angle, the unit weight of soil, and pore pressures.
Generally, a slope of 1 vertical (V) on 1 1/2 horizontal (H) is adequate; but if the slope is subjected to seepage or sudden drawdown, a slope of 1V on 3H.is commonly employed.  Failure normally occurs by surface raveling or shallow sliding.  Where consequences of failure may be important, required slopes can be determined using simple infinite slope analysis.  Values of  φ’ for stability analyses are determined from laboratory tests or estimated from correlations (para 3-6).  Pore pressure due to seepage reduces slope stability, but static water pressure, with the same water level inside and outside the slopes, has no effect.  Benches, paved ditches, and planting on slopes can be used to reduce runoff velocities and to retard erosion.  Saturated slopes in cohesionless materials may be susceptible to liquefaction and flow slides during earthquakes, while dry slopes are subject to settlement and raveling.  Relative densities of 75 percent or larger are required to ensure seismic stability, as discussed in later.

      b. Cohesive slopes resting on firm soil or rock.
The stability of slopes consisting of cohesive soils depends on the strength of soil, its unit weight, the slope
height, the slope angle, and pore pressures.  Failure usually occurs by sliding on a deep surface tangent to
the top of firm materials.  For relatively high slopes that drain slowly, it may be necessary to analyze the stability for three limiting conditions:

          (1) Short-term or end-of-construction condition.   Analyze this condition using total stress methods, with shear strengths determined from Q tests on undisturbed specimens.  Shear strengths from unconfirmed compression tests may be used but generally may show more scatter.  This case is often the only one analyzed for stability of excavated slopes.  The possibility of progressive failure or large creep deformations exists for safety factors less than about 1.25 to 1.50.

          (2) Long-term condition.   If the excavation is open for several years,  it may be necessary to analyze this condition using effective stress methods, with strength parameters determined from S tests or R
tests on undisturbed specimens.  Pore pressures are governed by seepage conditions and can be determined using flow nets or other types of seepage analysis.  Both internal pore pressures and external water pressures
should be included in the analyses.  This case generally does not have to be analyzed.

          (3) Sudden drawdown condition,  or other conditions where the slope is consolidated under one loading condition and is then subjected to a rapid change in loading, with insufficient time for drainage.   Analyze this condition using total stress methods,  with shear strengths measured in R and S tests.  Shear strength shall be based on the minimum of the combined R and S envelopes.  This.case is not normally encountered in excavation slope stability.

      c. Effect of soft foundation strata.  The critical failure mechanism is usually sliding on a deep surface tangent to the top of an underlying firm layer.  Short-term stability is usually more critical than long-term stability.

The strength of soft clay foundation strata should be expressed in terms of total stresses and determined using Q triaxial compression tests on undisturbed specimens or other methods described previously.

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