1. a) Define effective stress. (2Ã10=20). b) Write Coulomb's shear strength equation. c) Define critical voids ratio. d
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V Semester B.E. (Civil) Degree Examination, December 2014/January 2015 (2K11 Scheme) CE – 506 : EARTH AND EARTH RETAINING STRUCTURES Time : 3 Hours
Max. Marks : 100
Instructions : i) Question No. 1 is compulsory. ii) Answer any other 4 full questions selecting atleast one from each Part. iii) Missing data, if any, may be assumed suitably. 1. a) Define effective stress.
(2×10=20)
b) Write Coulomb’s shear strength equation. c) Define critical voids ratio. d) Name the shear strength parameters. e) Write Laplace equation for 2-D flow. f) Define earth-pressure at rest. g) Write an equation for Taylor’s stability number. h) Define phreatic line. i) Name Skempton’s pore pressure parameters. j) Name two theories for the calculation of earth pressures. PART – A 2. a) The D10 size of a soil is 0.01 mm. Assuming ( 15 ) D10 as the pore size. estimate the height of capillary rise assuming surface tension of water as 75 dynes/cm.
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b) Explain the concept of effective stress with the help of spring analogy.
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c) Determine the neutral and effective stress at a depth of 16 m below the ground level for the following conditions : water table is 3 m below ground level, G = 2.68, C = 0.72, average water content of the soil above water table is 8%.
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P.T.O.
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3. a) Explain the principle of the direct shear test. What are the advantages of this test ? What are its limitations ? b) Explain the classification of shear tests based on Drainage conditions.
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c) Following are the results of a Direct shear test Normal load
(N)
100
200
300
Peak shear load
(N)
90
180
270
Ultimate shear load (N)
75
150
225
Determine the angle of shear resistance in i) dense state ii) loose state. 4. a) Explain Mohr-Coulomb strength theory.
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b) What are the advantages and disadvantages of triaxial shear test ? Explain.
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c) A series of shear tests were conducted on a soil sample and each sample was sheared until failure. The following are the principal stresses obtained. Test No.
(kN/m2)
(kW/m 2)
1
200
600
2
300
900
3
400
1200
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Plot the Mohr’ circles and hence determine the strength envelope and angle of internal friction. PART – B 5. a) Differentiate critically between Rankine and Coulomb theories of earth pressure.
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b) Determine the active and passive earth pressure, given the following data : Height of Retaining wall = 10 m, φ = 25°, rd = 17 kN/m3. Ground water table is at the top of the retaining wall. Draw the stress distribution diagrams.
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c) A retaining wall, 7.5 m high, retains a cohesionless backfill. The top 3 m of the fill has a unit weight of 18 kN/m3 and φ = 30° and the rest has a unit weight of 24 kN/m3 and φ = 20°. Determine the pressure distribution on the wall.
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PART – C 6. a) Explain step by step procedure for the determination of active earth pressure using Culmann’s graphical procedure. 10 b) A retaining wall with a vertical back 5 m high supports a cohesionless backfill of unit weight 19 kN/m3. The upper surface of the backfill rises at an angle of 10° with the horizontal from the crest of the wall. The angle of internal friction for the soil is 30° and the angle of wall friction is 20°. Determine the total active earth pressure per metre of the wall and mark the direction and point of application of the thrust. Use Rebhan’s graphical method. 10 7. a) Write brief critical notes on ‘Taylor’s Stability Number’.
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b) Explain ‘Critical height’ and ‘Depth of tension crack’. What is the maximum depth to which a trench of vertical sides can be excavated in a clay stratum with C = 50 KPa and r = 16 kN/m3 ? Assume the clay to be saturated.
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c) A 5.5 m deep canal with a side slope of 1 : 1 is made in a soil strata having C0 = 20 KPa, φ 0 =12°, c = 0.8 and G = 2.64. If Taylor’s Stability Number is 0.108, determine the factor of safety with respect to cohesion when the canal water is full upto the top of the bank. Also determine the factor of safety for the case of sudden draw down of the Taylor’s stability number for this condition in 0.139.
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8. a) Define “Flow Net”. What are the salient characteristics of a flownet ? b) Briefly discuss : i) exit gradient ii) seepage force iii) piping.
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c) A homogeneous earth dam has the following properties. Height of Dam = 40 m Top width = 6 m Free board = 3 m U/s and D/s slopes = 2 : 1 Horizontal toe drain = 40 m Permeability of soul = 3 × 10– 6 m/s. Construct the ‘Flow net’ graphically and estimate the seepage loss through the dam section per metre length of the dam. ———————
