Foundations: Shallow and Deep

Why it matters

Foundations are critical components of any structure, as they transfer loads from the structure to the ground. Proper design and construction of foundations ensure the stability and safety of buildings, bridges, and other infrastructure. Understanding the differences between shallow and deep foundations helps engineers choose the appropriate type for specific site conditions and load requirements.

Key ideas

• Shallow Foundations: These are used when the soil close to the surface has sufficient bearing capacity. Types include spread footings, mat foundations, and strip footings. They are generally more economical and easier to construct.
• Deep Foundations: Used when surface soils are weak or when loads are very high. Types include piles, drilled shafts, and caissons. They transfer loads to deeper, more stable soil layers or rock.
• Bearing Capacity: The maximum load per unit area that the soil can support without failure. It is crucial for determining the size and type of foundation.
• Settlement: The downward movement of the foundation due to soil compression. It must be within acceptable limits to avoid structural damage.
• IS Codes: Indian standards provide guidelines for foundation design, such as IS 6403 for bearing capacity and IS 2911 for pile foundations.

Formulas

The bearing-capacity expression below is an idealized strip-footing form for vertical concentric loading on level homogeneous soil, using compatible factors from a specified bearing-capacity model. Shape, depth, inclination and groundwater corrections may be needed. It is not the general square-footing equation.

• q_ult = c·Nc + γ·Df·Nq + 0.5·γ·B·Nγ
  • q_ult: Ultimate bearing capacity (kN/m²)
  • c: Cohesion of soil (kN/m²)
  • γ: Unit weight of soil (kN/m³)
  • Df: Depth of foundation (m)
  • B: Width of foundation (m)
  • Nc, Nq, Nγ: Bearing capacity factors (dimensionless)
• s ≈ q·B·(1 - ν²)·I_s / E
  • s: Idealized immediate elastic settlement (m), with q the appropriate net pressure and dimensionless influence factor I_s accounting for geometry and evaluation point; not consolidation settlement
  • q: Applied load per unit area (kN/m²)
  • ν: Poisson's ratio (dimensionless)
  • E: Modulus of elasticity of soil (kN/m²)

Worked example

Given: For a strip-footing arithmetic exercise, a footing of width 2 m is placed at a depth of 1.5 m in a soil with cohesion c = 25 kN/m², unit weight γ = 18 kN/m³, with supplied illustrative bearing capacity factors Nc = 30, Nq = 18, Nγ = 15.

1. Calculate the ultimate bearing capacity

   • Formula: q_ult = c·Nc + γ·Df·Nq + 0.5·γ·B·Nγ
   • Calculation: q_ult = 25·30 + 18·1.5·18 + 0.5·18·2·15
   • q_ult = 750 + 486 + 270
   • q_ult = 1506 kN/m²

2. Apply a supplied safety factor of 3 to net ultimate capacity

   • Overburden at base: q_0 = γD_f = 18 × 1.5 = 27 kPa.
   • Net ultimate capacity: q_net,ult = 1506 − 27 = 1479 kPa.
   • Net shear-based allowable pressure: 1479/3 = 493 kPa.
   • Corresponding gross pressure: 493 + 27 = 520 kPa.

Answer: 493 kPa net or 520 kPa gross for the stated net-capacity safety convention. Settlement may require a lower allowable pressure. Factors here are supplied for calculation practice, not independently selected soil-design parameters.

Common mistakes

• Confusing shallow and deep foundations and their appropriate applications.
• Incorrectly calculating bearing capacity by not considering all components of the formula.
• Ignoring settlement calculations, which can lead to structural issues.
• Misapplying IS codes or using outdated standards.

For GATE CE

Questions often involve calculating the bearing capacity of shallow foundations or the load-carrying capacity of piles. Practice problems on both types of foundations, focusing on understanding the conditions under which each is used and the calculations involved.

Quick check

1. What is the primary difference between shallow and deep foundations?
2. Name two types of shallow foundations.
3. What is the purpose of using a factor of safety in foundation design?

Answers: 1. Depth and load transfer method; 2. Spread footings, mat foundations; 3. To ensure safety by accounting for uncertainties in load and soil properties.