CE En 544 - Exam #2 KEY

Winter Semester 2018

Essay Questions

You may refer to the equation sheet.

13. (4.5 pts) (slope-limit equil) What is the fundamental equation used by the limit equilibrium procedure? In terms of this equation, describe the two basic ways that a slope may fail.

The equation is: F = s/t where F = the factor of safety s = shear strength of the soil along the failure surface t = the shear stress needed along the failure surface to hold the slope in static equilibrium. The slope fails when F<1. This can happen if the shear strength is too low or if the shear stress is too large.

14. (4 pts) (slope-limit equil) List the two main conditions under which the infinite slope equations are applicable.

1. Limiting strata at a shallow depth parallel to the slope 2. Cohesionless materials (c=0).

15. (5 pts) (slope-limit equil) List the steps involved in a limit equilibrium stability analysis

1. Candidate failure surface is selected 2. Stresses along failure surface are calculated assuming soil is at limit of static equilibrium (t) 3. Total available strength along surface is calculated (s) 4. Factor of safety is computed as: FS=s/t  5. Process is repeated until critical failure surface with minimum factor of safety is found

16. (4 pts) (slope-imp details) If c = 250 psf, phi = 30 deg, and unit wt = 128 pcf, compute the depth of a tension crack that would eliminate tension in a method of slices slope stability analysis. Assume that FS=1.2.

cd = c/F = 250/1.2 = 208.3 phid = atan(tan(phi)/F) = atan(tan(30)/1.2)) = 25.7 deg dcrack = 2Cd/(gam*tan(45-phid/2)) = 2*208.3/(128*tan(45-25.7/2)) = 5.2

17. (4 pts) (slope-imp details) Describe two approaches for eliminating tension in the active zone for a stability analysis.

1. Add a tension crack where the crack depth is sufficient to remove negative lateral stresses. 2. Use a modified MC failure envelope where the left side has been modified to ensure that the shear strength is zero when the normal stress becomes negative.

18. (5 pts) (slope-causes) List five ways in which the shear strength can decrease in a slope.

Any five of the following: Increased pore pressure (reduced effective stress) Cracking Swelling (increase in void ratio) Development of slickensides Decomposition of clayey rock fills Creep under sustained loads Leaching (quick clays) Strain softening (peak vs residual strength) Weathering Cyclic loading

19. (3 pts) (slope-causes) List three ways in which the shear stress can increase in a slope.

Any three of the following: Loads at the top of the slope Water pressure in cracks at the top of the slope. Increase in soil weight due to increased water content Excavation at the bottom of the slope. Drop in water level at the base of a slope. Earthquake shaking