JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech III Year FOUNDATION ENGINEERING Previous Question Paper (August - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Main
Describe open excavation methods of exploration. What are their advantages and disadvantages?
Answer
Open excavation methods are direct subsurface exploration techniques used to inspect soil and rock strata in their natural state. These methods involve the physical removal of soil to create an opening, allowing for a detailed visual examination of the soil profile.
Common Methods:
- Test Pits: These are shallow excavations, typically square or rectangular, dug manually or using mechanical equipment. They are ideal for exploring soil up to a depth of 3 to 4 meters.
- Trenches: These are long, narrow excavations used to study the lateral continuity of soil strata and to locate geological features like faults or buried channels.
- Shafts: These are deep vertical excavations used when the required depth exceeds the capacity of test pits. They are often lined with timber or concrete to prevent collapse.
Advantages:
- Provides the most accurate and reliable information regarding the sequence, thickness, and nature of soil strata.
- Allows for the collection of high-quality undisturbed samples for laboratory testing.
- Enables direct observation of the groundwater table and the structural arrangement of soil layers.
- Permits in-situ testing, such as plate load tests or vane shear tests, at the bottom of the excavation.
Disadvantages:
- These methods are highly labor-intensive and time-consuming.
- They are generally restricted to shallow depths due to safety concerns and increasing costs as depth increases.
- Excavation below the groundwater table is extremely difficult and often requires expensive dewatering systems.
- Significant site disturbance occurs, which may not be desirable in urban or sensitive environments.
What is a bore log? Give a typical example.
Answer
A bore log (or borehole log) is a comprehensive technical report that documents the subsurface conditions encountered during a drilling or excavation operation. It serves as a permanent record of the soil and rock profile at a specific location, detailing the sequence of strata, their physical properties, and the depth of the water table.
A typical bore log includes the project name, location, borehole number, ground elevation, drilling method, date of execution, and a detailed table containing:
- Depth: The vertical distance from the ground surface.
- Soil Description: Classification of soil (e.g., Silty Sand, Stiff Clay) based on visual inspection and testing.
- Sample Type: Indication of whether the sample is disturbed or undisturbed.
- N-value: Standard Penetration Test (SPT) blow counts for each interval.
- Water Table: The depth at which groundwater was encountered.
0-2m: Topsoil (Dark brown)
2-6m: Silty Sand (Medium dense, N=15)
6-10m: Stiff Clay (Grey, N=22)
What is standard penetration test? In what type of soils is the test is preferred? What are its important specifications? Discuss why overburden correction is applied for measured SPT N.
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Distinguish between disturbed and undisturbed samples.
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Explain different modes of slope failures with suitable reasons.
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Find the critical height of a slope of infinite extent having a slope of 30°. The slope is made of stiff clay having cohesion of 20kPa, angle of internal friction of 20°, void ratio of 0.7 and specific gravity of 2.7. Consider the following cases for the analysis: i) The soil is dry ii) The slope is submerged.
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What is stability number? What is its utility in the analysis of slopes?
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An embankment 10m high is inclined at 35° to the horizontal. A stability analysis by the method of slices gave the following forces. Total normal force = 900 kN, Tangential force = 420 kN, Pore water pressure force = 200 kN. If the length of failure is 25m, find the factor of safety. The soil has cohesion of 30 kN/m² and angle of internal friction 15°.
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Discuss the stability of cantilever retaining wall against overturning and sliding failures.
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A 10m high retaining wall with smooth vertical back supports a horizontal backfill (φ = 33°, c = 25kPa, Density above water table 15kN/m³ and below water table 18kN/m³). 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.
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A 5m high rigid retaining wall has to retain a backfill of dry, cohesionless soil having the following properties: Angle of internal friction = 30°, void ratio = 0.74, specific gravity = 2.68. Plot the distribution of Rankine active earth pressure on the wall and determine the magnitude and point of application of the resultant thrust.
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What are the merits in Coulomb’s earth pressure theory as compared to Rankine’s theory? Discuss.
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Explain 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.
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Determine net ultimate bearing capacity of the circular footing of size 2.5m located at 1.5m depth below ground level in pure clay soil and for the following cases: When water table is at (i) far below from foundation base and (ii) Ground level. Consider the following soil characteristics: cohesion = 50 kPa and unit weight of soil is 18 kN/m³. Use Terzaghi’s theory.
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In a pile group, what are the geometrical properties that are to be considered in bringing out a proper spacing of piles? Explain.
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What are the various approaches available to estimate the load carrying capacity of a single pile? Discuss the static formulae for pile capacity.
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