JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year II Semester FOUNDATION ENGINEERING Previous Question Paper (March - 2024 - R18)

A soil profile is a vertical section of the soil extending from the ground surface down to the underlying bedrock or the limit of engineering interest. It provides a visual and data-driven representation of the various soil horizons or layers that have developed over time due to weathering, deposition, and biological activity.

• Each layer, or horizon, is distinguished by its unique physical, chemical, and mechanical properties, such as grain size distribution, density, moisture content, and shear strength.
• In geotechnical engineering, the soil profile is essential for determining the stratigraphy of a site, which dictates the selection of foundation type, the estimation of settlement, and the assessment of bearing capacity.
• A typical engineering soil profile includes the topsoil, organic layers, various strata of clay, silt, sand, or gravel, and the depth to the groundwater table.

The depth of soil exploration for a building is determined based on the significant depth, which is the depth up to which the stress increment from the building load is likely to cause shear failure or significant settlement. For a 15-storey heavy steel structure, the following criteria apply:

• Significant Depth Rule: The depth of exploration should generally extend to the depth where the vertical stress increment due to the building load is approximately 10% of the initial effective overburden pressure.
• Empirical Rule (Building Height): For multi-storey buildings, a common rule of thumb is to explore to a depth equal to 1.5 times the width of the foundation or until a firm, incompressible stratum (such as rock or dense gravel) is encountered.
• Specific Estimation: For a 15-storey building, assuming an average storey height of 3.5 meters, the total height is approximately 52.5 meters. According to standard geotechnical practice (IS 1892), for heavy structures, the depth of exploration ($D$) is often estimated using the formula:
  
  
  D = 0.5 × S^0.7
  
  where S is the number of storeys. Substituting S = 15:
  
  D = 0.5 × (15)^0.7 ≈ 0.5 × 6.45 ≈ 3.22 meters (This is a minimum; however, for heavy steel structures, local building codes typically mandate exploration to a depth of 20 to 30 meters or until refusal, to account for deep foundation requirements like piles).
• Conclusion: Given the heavy nature of a 15-storey steel building, a depth of 25-30 meters is recommended to ensure the soil profile is adequately characterized for deep foundation design.