CE-802-B Foundation Engineering - Nov 2023 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain Standard Penetration Test and corrections applied to observed N value. Illustrate test with figure.
   How to answer

   • Define SPT purpose and equipment - split spoon, hammer, borehole
   • Procedure - 150mm seating + 300mm test blows, N count
   • Corrections - overburden pressure and dilatancy/submergence, water table
   • Illustrative sketch of setup
2. 7 Marks
   bExplain in detail the rotary drilling technique. State also its advantages over other methods of boring.
   How to answer

   • Principle of rotary drilling - rotating bit and circulating fluid
   • Equipment and procedure steps
   • Sampling in rock and soils
   • Advantages vs auger, wash, percussion boring
3. 7 Marks
   aWhat do you understand by Geophysical method of soil exploration?
   How to answer

   • Define geophysical exploration - non-destructive
   • Seismic refraction and electrical resistivity methods
   • Procedure and interpretation
   • Applications and limitations
4. 7 Marks
   bExplain what is meant by 'safe bearing capacity' of soil. Indicate how the bearing capacity of shallow footing in a given soil can be calculated from the strength characteristics of the soil such as cohesion and angle of internal friction?
   How to answer

   • Define safe bearing capacity - ultimate / FOS + settlement criteria
   • Terzaghi bearing capacity factors Nc, Nq, Ngamma
   • Use of c and phi to compute ultimate capacity
   • Water table and depth/ shape corrections
5. 7 Marks
   aThe width and depth of a footing are 2m and 1.5m respectively. The water table at the site is at a depth of 3m below the ground level. Find the water table correction factor for the calculation of the bearing capacity of soil.
   How to answer

   • Recall water table correction factors Rw and Rw'
   • Locate water table relative to base: Dw, Df, B
   • Compute Rw1, Rw2 by interpolation formula
   • State final correction factor value
6. 7 Marks
   bA footing of $4m \times 4m$ carries a uniform gross pressure of $300\text{ kN/m}^2$ at a depth of $1.5m$ in a sand. The saturated unit weight of the sand is $20\text{ kN/m}^3$ and the unit weight above the water table is $17\text{ kN/m}^3$. The shear strength parameters are $c' = 0$, $\phi' = 32^\circ$. Determine the factor of safety with respect to shear failure for the following cases;

   1. i)The water table is at ground surface
   2. ii)The water table is 1.5m below the surface
   How to answer

   • Write Terzaghi bearing capacity equation for sand c'=0
   • Apply unit weights and water table cases i) and ii)
   • Compute ultimate bearing capacity for each case
   • Factor of safety = q_ult_net / q_applied_net
7. 7 Marks
   aIn a pile group, what are the geometrical properties that are to be considered in bringing out a proper spacing of piles? Explain.
   How to answer

   • Define pile spacing criteria - group efficiency, overlapping stress
   • Geometrical properties - diameter, length, pattern, cap size
   • Minimum spacing codes 2.5-3D
   • Effect on load transfer and settlement
8. 7 Marks
   bDesign a friction pile group to carry a load of $3500\text{ kN}$ including the weight of pile cap, at a site where the soil is uniform clay to a depth of $10\text{ m}$ underlain by rock. The average compressive strength of clay is $50\text{ kN/m}^2$. The clay may be assumed to be of normal sensitivity and normally loaded with a liquid limit of $70\%$. Adopt a factor of safety $2.5$ against shear failure.
   How to answer

   • Estimate undrained cohesion from compressive strength
   • Skin friction and end bearing for friction piles in clay
   • Apply FOS 2.5 to get allowable load per pile
   • Determine number, spacing and group layout for 3500 kN
9. 7 Marks
   aIn a two layered cohesive soil, bored piles of $300mm$ are installed. The top layer has a thickness of $5m$ and the bottom one is of considerable depth. The “c” value of top layer is $40kN/m^2$ and that of the bottom is $100kN/m^2$. Determine the length of the pile required to carry a safe load of $400kN$. Assume a F.O.S. of $3$.
   How to answer

   • Ultimate load = FOS x safe load
   • Skin friction in top 5m with c=40 plus bottom layer
   • End bearing 9*c*Ab
   • Solve for embedded length beyond 5m
10. 7 Marks
    bWhat are the problems and preventive measures of foundations on expansive soils?
    How to answer

    • Define expansive soils - montmorillonite, swell-shrink
    • Problems - heave, cracking, differential movement
    • Preventive measures - under-reamed piles, CNS, drainage
    • Construction precautions
11. 7 Marks
    aExplain in detail about the concept of Under-reamed pile foundation and suitability criteria?
    How to answer

    • Define under-reamed pile - enlarged bulb
    • Construction procedure and types single/multi bulb
    • Load transfer mechanism
    • Suitability for black cotton soil and criteria
12. 7 Marks
    bExplain in detail about the geosynthetics-materials and types.
    How to answer

    • Define geosynthetics and functions
    • Materials - polymer types
    • Types - geotextile, geogrid, geomembrane etc.
    • Applications in foundations
13. 7 Marks
    aA $10m$ high retaining wall with smooth vertical back supports a horizontal backfill ($\phi = 33^\circ, c = 25kPa$, Density above water table $15kN/m^3$ and below water table $18kN/m^3$). The water table is at a depth of $3m$ below the surface of the backfill. The backfill supports a surcharge of $10kPa$. Determine the magnitude and line of action of passive earth pressure.
    How to answer

    • Passive coefficients Kp with c-phi soil
    • Compute stresses above and below water table + surcharge
    • Integrate pressure diagram for total thrust
    • Locate resultant by moment about base
14. 7 Marks
    bA retaining wall $6m$ high with a smooth vertical back retains a clay backfill with $c' = 12kN/m^2$, $\gamma = 18kN/m^3$ and $\phi' = 18^\circ$. Calculate the total active thrust on the wall if tension cracks may develop to the full theoretical depth.
    How to answer

    • Active coefficient Ka for phi'=18 degree
    • Tension crack depth z0 = 2c / gamma sqrt(Ka)
    • Pressure distribution with zero above z0
    • Total thrust as area of net diagram
15. 7 Marks
    aWhat are the merits in Coulomb's earth pressure theory as compared to Rankine's theory? Discuss.
    How to answer

    • Assumptions of Coulomb vs Rankine
    • Inclusion of wall friction, inclination, backfill slope
    • Merits - versatile, realistic for rough walls
    • Limitations discussion
16. 7 Marks
    bWhat are the different modes of failure of retaining walls? Explain with the help of neat sketches.
    How to answer

    • List modes - overturning, sliding, bearing, structural
    • Explain each with stability criteria
    • Neat sketches for each failure mode
    • Preventive design measures