Raft Foundations and Soil Bearing Capacity Principles

Posted on Mar 15, 2026 in Civil Engineering, Transportation, and Urban Services

Q.2 (a) What is a Raft Foundation and When is it Adopted?

A raft foundation, also known as a mat foundation, is a type of shallow foundation that consists of a large reinforced concrete slab covering the entire area of a building. This slab supports and distributes the load of all columns and walls uniformly over the whole foundation area.

Instead of providing separate footings for each column, a single thick concrete slab (raft) is constructed to support the structure.

Circumstances for Adopting Raft Foundations

A raft foundation is used in the following situations:

1. Low Bearing Capacity of Soil

   When the soil has weak bearing capacity and cannot safely support isolated footings, a raft spreads the load over a larger area.

2. Heavily Loaded Structures

   For buildings with heavy loads (multi-storey buildings, industrial structures), where individual footings may overlap.

3. Closely Spaced Columns

   When columns are very near to each other, separate footings would overlap; hence, a raft foundation is more suitable.

4. To Prevent Differential Settlement

   When soil conditions are uneven and there is a risk of unequal settlement, a raft foundation reduces differential settlement.

5. When More Than 50% of Area is Covered by Footings

   If isolated footings would cover more than half of the building area, it becomes economical to use a raft.

6. Basement Construction

   When a basement is required, the raft slab can also serve as the floor slab of the basement.

Key Advantages of Raft Foundations

• Reduces differential settlement
• Suitable for soft soils
• Economical for large structures
• Provides good load distribution

Summary of Raft Foundations

A raft foundation is a large reinforced concrete slab supporting the entire structure. It is mainly adopted in weak soil conditions, for heavy structures, closely spaced columns, and where uniform settlement control is required.

Q.2 (b) Factors Affecting Soil Bearing Capacity

Bearing capacity of soil is the maximum load per unit area that the soil can safely support without failure or excessive settlement.

The following factors affect the bearing capacity of soils:

1. Type and Nature of Soil

Sandy soils generally have higher bearing capacity than clayey soils.

• Cohesive soils (clay) depend on cohesion.
• Granular soils (sand, gravel) depend on internal friction.

2. Soil Density

Dense soil has higher bearing capacity. Loose soil has lower strength and higher settlement.

3. Moisture Content

An increase in water content reduces shear strength.

• Saturated clay soils have very low bearing capacity.
• A water table near the foundation level reduces effective stress.

4. Depth of Foundation

Greater depth increases bearing capacity due to higher confining pressure. Shallow foundations have lower bearing capacity compared to deep foundations.

5. Size and Shape of Footing

A larger footing area increases total load-carrying capacity. The shape (square, rectangular, circular) affects load distribution. Square footings generally have higher bearing capacity than strip footings.

6. Type of Loading

Vertical loads are safer; however, inclined or eccentric loads reduce bearing capacity. Dynamic loads (machine vibrations, earthquakes) decrease soil strength.

7. Groundwater Table

If the water table is close to the foundation level, bearing capacity decreases. Water reduces effective stress and the shear strength of soil.

8. Soil Stratification

The presence of weak layers below strong layers reduces overall bearing capacity. A uniform soil profile gives better performance.

9. Drainage Conditions

Poor drainage increases pore water pressure. High pore water pressure reduces shear strength.

Bearing Capacity Summary

Bearing capacity of soil depends on soil properties, moisture condition, foundation depth, loading type, and groundwater level. Proper soil investigation is necessary before foundation design.

Q.3(a) Classification of Piles with Sketches

Pile Foundation

A pile is a long, slender structural member made of concrete, steel, or timber, driven or cast into the ground to transfer the load of a structure to deeper, stronger soil or rock strata.

Piles are classified based on:

1. Function (Load Transfer)
2. Material Used
3. Method of Construction
4. Purpose of Use

1. Classification Based on Load Transfer

(a) End Bearing Piles

Transfer load to a hard stratum at the bottom. They act like a column and are suitable when hard soil/rock is available at a reasonable depth.

(b) Friction Piles

Transfer load through skin friction along the surface. These are used when no hard stratum is available at a shallow depth.

(c) Combined End Bearing and Friction Piles

Transfer load by both end bearing and skin friction.

2. Classification Based on Material

(a) Timber Piles

Made of wood. Used for light structures; they are economical but have limited durability.

(b) Concrete Piles

Precast or cast-in-situ. These are the most commonly used, being durable and strong.

(c) Steel Piles

H-section or pipe piles. Used for heavy loads and suitable for deep foundations.

(d) Composite Piles

A combination of two materials (e.g., timber + concrete).

3. Classification Based on Method of Construction

(a) Driven Piles

Driven into soil using a hammer, causing soil displacement.

(b) Bored (Cast-in-Situ) Piles

A hole is drilled and filled with concrete. These are suitable in urban areas due to less vibration.

4. Classification Based on Purpose

(a) Load Bearing Piles

Carry vertical loads.

(b) Compaction Piles

Improve soil density.

(c) Tension (Uplift) Piles

Resist uplift forces (e.g., towers, basements).

(d) Sheet Piles

Used for retaining soil or water.

Pile Classification Summary

Piles are classified based on load transfer mechanism, material, construction method, and purpose. Selection depends on soil condition, load requirement, and site constraints.

Q.4 (a) Terzaghi’s Soil Bearing Capacity Analysis

Karl Terzaghi developed the bearing capacity theory to determine the ultimate and safe bearing capacity of shallow foundations.

1. Assumptions of Terzaghi’s Theory

1. Soil is homogeneous and isotropic.
2. Footing is shallow (depth less than or equal to its width).
3. Load is vertical and centrally applied.
4. Failure surface is well defined (general shear failure).
5. Soil above foundation base provides surcharge only.

2. Terzaghi’s Ultimate Bearing Capacity Equation

For a strip footing, the ultimate bearing capacity q_u is:

q_u = cN_c + γD_fN_q + 0.5γBN_γ

Where:

• c = cohesion of soil
• γ = unit weight of soil
• D_f = depth of foundation
• B = width of footing
• N_c, N_q, N_γ = bearing capacity factors (depend on angle of internal friction ϕ)

3. Explanation of Equation Terms

(a) Cohesion Term (cN_c)

Represents resistance due to cohesion of soil.

(b) Surcharge Term (γD_fN_q)

Represents pressure due to soil above foundation base.

(c) Unit Weight Term (0.5γBN_γ)

Represents resistance due to weight of soil in failure zone.

4. Ultimate Bearing Capacity (q_u)

It is the maximum pressure at which soil fails in shear.

5. Safe Bearing Capacity (q_s)

Safe bearing capacity is obtained by dividing ultimate bearing capacity by the Factor of Safety (FOS).

q_s = q_u / FOS

Generally, FOS = 2.5 to 3.

6. Net Safe Bearing Capacity (q_ns)

q_ns = (q_u – γD_f) / FOS

This excludes the surcharge effect.

7. Types of Shear Failure

1. General Shear Failure – Well-defined failure surface (dense sand/stiff clay).
2. Local Shear Failure – Partial failure surface (medium dense soil).
3. Punching Shear Failure – No clear failure surface (loose sand/soft clay).

Final Thoughts on Terzaghi’s Theory

Terzaghi’s bearing capacity theory provides a formula to calculate ultimate and safe bearing capacity of shallow foundations based on soil properties and footing dimensions. It is widely used in foundation design for safe and economical construction.