CIVE 431 Final, Fall 2009

Soil Mechanics Final Exam Time: 120 minutes Fall 2009 Name: Student ID: Page 2 of 7 1. The section through a dam is shown in the figure below: a. Compute the flow under the dam per meter of dam if the coefficient of permeability is 2.4×10-6 m/s. b. Plot the net pressure distribution on the cutoff wall. (20 Points) Page 3 of 7 2. The results of a laboratory consolidation test on a clay specimen are the following. Pressure, σ' (lb/ft2) Total Height of Specimen at End of Consolidation (in.) Δe e 500 0.6947 1,000 0.6850 2,000 0.6705 4,000 0.6520 8,000 0.6358 16,000 0.6252 Given the initial height of specimen = 0.748 in., Gs = 2.68, mass of dry specimen = 95.2 g, and area of specimen = 4.91 in.2: a. Plot the e-logσ' curve b. Determine the preconsolidation pressure c. Calculate the compression index, Cc (20 points) Page 4 of 7 0.4 0.45 0.5 0.55 0.6 1 10 100 1000 10000 Page 5 of 7 3. A clay layer in the field is 15 ft thick and is drained at the top only. Under a given surcharge, the estimated consolidation settlement is 12.21 in. a. What is the average degree of consolidation for the clay layer when the settlement is 3 in.? b. If the average value of Cv for the pressure range is 0.003 cm2/sec, how long will it take for 50% consolidation to occur? c. If the 15 ft clay layer is drained on both sides, how long will it take for 50% consolidation to occur? (20 points) Page 6 of 7 4. The state of stress on a small element is σv = 120 kPa, σh = 40 kPa, and the shear stress on the horizontal plane is 30 kPa. a. Find the magnitude and directions of the major and minor principal stresses. b. If the material is cohesionless sand, can you say whether the element is in a state of failure? Why? (Assume φ = 38.7°). (20 points) Page 7 of 7 5. A sandy soil has a drained angle of friction of 38°. In a drained triaxial test on the same soil, the deviator stress (σ1-σ3) at failure is 1.9 ton/ft2. What is the chamber-confining pressure? (20 points)