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JNTU B.Tech IV FOUNDATION ENGINEERING Previous Question Paper (September - 2022 - R16)
Answer any five questions. All questions carry equal marks.
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Discuss with neat sketch Wash Boring method used in soil exploration.
Answer
The Wash Boring method is a fast and economical soil exploration technique used primarily in soft to stiff cohesive soils and fine-grained sands. It is not suitable for gravelly or bouldery strata.
The process involves the following steps:
- A casing pipe is driven into the ground to prevent the borehole from collapsing.
- A hollow drill rod with a sharp, chisel-shaped chopping bit at the lower end is inserted into the casing.
- Water is pumped under pressure through the drill rod and exits through the holes in the bit, loosening the soil at the bottom of the hole.
- The loosened soil particles are carried up to the surface by the rising water (wash water) in the annular space between the drill rod and the casing.
- The wash water is collected in a settling tank where the soil particles settle, and the water is recirculated.
A neat sketch would show a tripod or derrick supporting a pulley, a water pump connected to a hose leading to the drill rod, the casing pipe, and the settling tank receiving the return flow.
Explain the following terms as related to soil sampling:
i) Disturbed and undisturbed samples
ii) Representative and non-representative samples.
Answer
i) Disturbed and Undisturbed Samples
A disturbed sample is one in which the natural structure of the soil is significantly altered or destroyed during the sampling process, though the mineral constituents remain unchanged. These are suitable for classification tests, grain size analysis, and compaction tests. An undisturbed sample is one where the natural structure, moisture content, and void ratio of the soil are preserved as closely as possible to their in-situ state. These are essential for laboratory tests determining shear strength, compressibility, and permeability.
ii) Representative and Non-representative Samples
A representative sample contains all the soil constituents in the same proportions as they exist in the field. This is vital for accurate index property testing. A non-representative sample does not accurately reflect the field proportions of soil components, often occurring due to the loss of fines during drilling or sampling, making it unsuitable for reliable soil classification or engineering property evaluation.
With the help of neat sketch, explain how Plate Load Test can be performed? How bearing capacity and settlements of foundations are arrived in? Discuss the limitations of this test.
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What are the reasons for slope failures? Differentiate finite and infinite slopes.
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Give the step by step procedure for analyzing the stability of the upstream slope of an earth dam by the Swedish method of slices.
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What is meant by ‘Taylor’s Stability’ number? Where do you make use of it?
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An embankment 12 m high is inclined at an angle of 30° to the horizontal. A stability analysis by the method of slices gave the following forces per running meter: Σ Shearing forces = 500 kN, Σ Normal forces = 1000 kN, Σ Neutral forces = 200 kN. The length of the failure arc is 30 m. Laboratory tests on the soil indicate the effective values c′ and ɸ′ as 20 kPa and 20°. Determine the factor of safety of the slope with respect to (i) shearing strength and (ii) cohesion. The unit weight of a soil of a 30° slope is 17.5 kN/m³.
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Explain the terms with neat sketches: (i) active earth pressure, (ii) passive earth pressure and (iii) at rest earth pressure.
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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 ɸ = 30° and the rest has unit weight of 24 kN/m³ and ɸ = 20°. Determine the active earth pressure force and its position by showing pressure distribution on the wall.
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Illustrate the following with respect to retaining wall: (i) Stability against overturning and (ii) Stability against sliding.
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A gravity retaining wall retains 10 m of a backfill, γ = 17.5 kN/m³, ɸ = 30° with a uniform horizontal surface. Assume the wall interface to be vertical, determine the magnitude and point of application of the total active pressure. If the water table is at a height of 6 m, how far do the magnitude and the point of application of active pressure changed?
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Differentiate between:
i) Shallow foundation and deep foundation
ii) Gross and net bearing capacity
iii) Safe bearing capacity and soil pressure.
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What is the ultimate bearing capacity of a rectangular footing, 1.75m × 3.50 m, at a depth of 1.5 m in a soil for which cohesion 30 kPa, angle of internal friction 15°, and unit weight 18 kN/m³. The bearing capacity factors are Nc = 10.89, Nq = 3.94, and Nγ = 1.42. The water table is deep. If the vertical load on the footing is 1500 kN, determine the factor of safety. Use the IS Code method. Assume shape factor sc = 1.3, sq = sγ = 1; depth factors dc = dq = dγ = 1 and inclination factors ic = iq = iγ = 1.
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Discuss how the shaft friction and end bearing is estimated for a pile embedded in a clay soil.
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A 0.45 m circular pile, 12 m long, is driven in a deposit of medium dense sand (ɸ = 30° and Nq = 9.5). The unit weight of sand is 16 kN/m³. What is the allowable load with a factor of safety of 3? Assume lateral earth pressure coefficient = 1.07. Assume that the pile surface friction angle as two thirds of the angle of internal friction of soil and also water table is at the ground level.
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