JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III FOUNDATION ENGINEERING Previous Question Paper (August/September - 2021 - R18)
Answer any five questions. All questions carry equal marks.
Main
What is meant by N value? Why should we apply correction for the N value obtained from the field? Briefly explain the corrections.
Answer
The N-value, or Standard Penetration Resistance, is defined as the number of blows required to drive a standard split-spoon sampler 300 mm into the soil after an initial seating drive of 150 mm, using a 63.5 kg hammer falling from a height of 750 mm. It is a primary indicator of the relative density of cohesionless soils and the consistency of cohesive soils.
Corrections are necessary because the raw field N-value is influenced by equipment efficiency, borehole conditions, and the effective overburden pressure. Without these corrections, the N-value does not represent the true engineering properties of the soil, leading to inaccurate foundation design parameters.
The primary corrections are:
- Overburden Pressure Correction: As the effective overburden pressure increases with depth, the soil becomes denser, artificially inflating the N-value. The corrected value N' is calculated as N' = N × (350 / (σ'₀ + 70)), where σ'₀ is the effective overburden pressure in kPa. This correction is typically applied only for granular soils.
- Dilatancy (Water Table) Correction: In fine sands and silts below the water table, the development of pore water pressure during rapid driving prevents full dissipation, resulting in an artificially high N-value. If N' > 15, the corrected value N'' is calculated as N'' = 15 + 0.5 × (N' - 15).
- Energy Correction: Standardized to 60% of the theoretical free-fall energy (N₆₀), this accounts for variations in hammer type, hammer release mechanism, and rod length. It is expressed as N₆₀ = N × (η_H × η_B × η_S × η_R) / 0.60, where η represents the efficiency factors for hammer, borehole, sampler, and rod length respectively.
What is the significance of soil exploration? Explain.
Answer
Soil exploration is the systematic process of investigating the subsurface conditions of a site to obtain essential data for the design and construction of safe, economical foundations. Its primary significance lies in mitigating the risk of structural failure and ensuring the long-term stability of the project.
Key objectives and significance include:
- Determination of Soil Profile: It identifies the sequence, thickness, and lateral extent of various soil and rock strata, which is critical for selecting the appropriate foundation type (e.g., shallow vs. deep).
- Assessment of Engineering Properties: It provides samples for laboratory testing to determine shear strength, compressibility, permeability, and bearing capacity, which are direct inputs for structural design.
- Groundwater Conditions: It identifies the position of the water table and its seasonal fluctuations, which significantly influence soil strength, excavation stability, and potential uplift pressures.
- Selection of Foundation Depth: It helps in identifying competent bearing strata, allowing engineers to determine the optimal depth to avoid settlement-prone or expansive soils.
- Construction Planning: It informs the choice of construction equipment and methods, such as the need for dewatering, shoring, or ground improvement techniques.
- Economic Feasibility: By providing accurate data, it prevents over-designing (which increases costs) and under-designing (which leads to safety hazards and costly remedial measures).
Discuss the following:
- Undisturbed soil
- Disturbed soil
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Why earth slopes fail? Discuss.
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An infinitely long slope is made up of a c-φ soil having the properties cohesion (c) = 20 kPa and dry unit weight of soil (γd) = 16 kN/m3. The angle of internal friction and critical height of slope are 400 and 5 m respectively. What is the angle of internal friction of the soil to maintain the limiting equilibrium.
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What is Taylor’s stability number? Discuss its use.
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A cutting of depth 10 m is to be made in soil which has c = 30 kN/m2, γ = 19 kN/m3 and φ = 0. There is a hard stratum below the original soil surface at a depth of 12 m. Find the safe slope of cutting if the factor of safety is 1.5. For Df = 1.20; Sn = 0.143 for i = 300; Sn= 0.101 for i = 150.
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Write the assumptions of Rankine’s theory.
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A retaining wall of height 5 m retains a sand fill, whose angle of internal friction, φ = 300 and unit weight, γ = 18 kN/m3. Estimate the active earth pressure force and its position from bottom of wall.
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List the various types of retaining walls and discuss the gravity retaining wall and cantilever retaining wall.
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Discuss how factor of safety against sliding is estimated for a gravity retaining wall.
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A square footing rests on a uniform thick deposit of stiff clay with an unconfined compressive strength of 120 kPa. The footing is located at 1.2 m below the ground surface and is to carry a total load of 1000 kN. The unit weight of clay soil 20 kN/m3 and ground water level is at a great depth and its effect can be ignored. Determine the dimension of square footing required for the loading and soil conditions given with a factor of safety of 3.
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A bored concrete pile 30 cm diameter and 6.50 m length passes through stiff clay. The average undrained shear strength of clay is 50 kPa upto pile tip and 100 kPa below the pile tip. Find the ultimate load carrying capacity assuming adhesion factor of 0.3.
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