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I Semester M.E. (Civil) (Geotechnical Engg.) Degree Examination, January 2015 2K8GT 105 : DESIGN OF SHALLOW FOUNDATIONS Time : 3 Hours

Max. Marks : 100

Instructions : 1) Answer any five full questions. 2) Missing data may be assumed suitably. 1. a) Explain the differences between Terzaghi’s and Meyerhof’s theories for bearing capacity.

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b) A 2m × 2m footing has the geometry and loading as shown in Figure 1. The properties of the foundation soil are γ = 18.2 kN/m3, φ = 20° and C = 30 KPa. Is the footing adequate against shear failure with a factor of safety of 3. Use Hansen’s recommendations. Refer tables for bearing capacity, shape, depth, inclination, ground and base factors.

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Figure 1

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2. a) Explain with neat sketches, Meyerhof’s method of computing ultimate bearing capacity of foundations on slopes. 10 b) A square footing 1.2 m × 1.2 m rests at a depth of 1 m in saturated clay layer 4 m deep. The clay is normally consolidated having an U.C.C. strength of 40 kN/m2. The soil has a liquid limit of 30% , γ sat = 17.8 kN/m3, W = 28% and G = 2.68. Determine the load which the footing can carry safely with a factor of safety of 3 against shear. Also determine the settlement if the footing is loaded with this safe load. Use Terzaghi’s analysis for bearing capacity. 10 3. a) Explain the various methods of determining the bearing capacity of rock.

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b) A drilled pier with diameter 1.0 m founded at 3.5 m in a rock mass has to carry 25000 kN/m2 pressure. The average RQD of the core below the pier point is 50%. Estimate the allowable bearing capacity for the pier base. 10 Take γrock = 24.5 kN/m 3. 4. a) Explain the step by step procedure of proportioning of footings.

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b) A total load of 1000 kN is uniformly distributed over a rectangular footing of size 2m × 3m shown in Figure - 2. Find the vertical stress at a depth of 3.0 m below the footing at point C, under one corner and D, under the centre. If another footing of size 1 m × 3 m with a total load of 500 kN is constructed adjoining the previous footing, what is the additional vertical stress at the point ‘C’ at the same depth due to the construction of the second footing ? Refer Figure 2 for influence value. 10

Figure 2

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5. a) Explain briefly reliability of settlement calculations. b) Explain the structural tolerance to settlement and differential settlement.

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c) Proportion a square footing to carry a load of 1200 kN from a column. The depth of foundation is to be kept at 2 m below the ground surface. Maximum permissible settlement of the footing is 40 mm and a factor of safety of 3 is required against shear failure. The subsoil is sand with an average corrected ‘N’ value of 18 as established from borings. Water table is at a large depth. Use Teng’s correlations.

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6. a) Explain with neat sketches, the determination of ultimate bearing capacity of footing resting on stratified deposit.

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b) What is contact pressure ? Explain with neat sketches, the contact pressure distribution in case of flexible and rigid footing on i) Cohesionless soils ii) Cohesive soils.

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7. a) Explain briefly the procedure for calculating bearing capacity of mat foundations.

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b) Explain the procedure to be adopted for the control of mat foundation settlements.

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8. Write short notes on the following : a) Effect of size of footing on settlement. b) Structures on fills. c) Footing depth and spacing. d) Isobars.

(4×5=20)

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