Introduction
Foundations are among the most important structural components of any building or civil engineering structure. They form the lowest part of a structure and transfer loads from columns, walls, beams, slabs, and other structural elements safely to the ground. A properly designed foundation prevents excessive settlement, tilting, cracking, and structural failure while maintaining the overall stability of the building.
The choice of foundation depends on several factors, including the magnitude of structural loads, type of soil, groundwater level, depth of competent strata, nearby buildings, construction methods, and economic considerations. Foundations are broadly divided into two categories: shallow foundations and deep foundations.
An important factor in selecting and designing foundations is soil bearing capacity, which represents the ability of the ground to safely support structural loads without shear failure or excessive settlement.
Shallow Foundations
Shallow foundations are those in which loads are transferred to soil located relatively close to the ground surface. They are generally used when suitable load-bearing soil is available at a small depth.
A foundation is typically considered shallow when its depth is approximately equal to or less than its width.
Shallow foundations are commonly used for houses, low-rise buildings, schools, commercial buildings, and other structures with moderate loads.
Isolated Footing
An isolated footing, also called a pad footing, supports a single column. It is one of the most common types of shallow foundation.
The footing may be square, rectangular, circular, or stepped depending on the load and structural requirements.
Its purpose is to spread the concentrated load from the column over a larger area of soil so that the pressure remains within safe limits.
The approximate footing area can be calculated using:
Where:
= required footing area
= structural load
= allowable soil bearing pressure
For example, if a column load is 600 kN and the allowable soil bearing capacity is 150 kN/m²:
A footing of approximately 2 m × 2 m may therefore be considered initially, subject to detailed structural design.
Combined Footing
A combined footing supports two or more columns.
It is commonly used when columns are placed close to each other or when an exterior column is located near a property boundary and an isolated footing cannot be centered beneath it.
Combined footings may be rectangular or trapezoidal depending on the magnitude and position of column loads.
They help distribute loads efficiently and reduce uneven settlement.
Strip or Continuous Footing
A strip footing consists of a continuous strip of concrete constructed below a load-bearing wall or a closely spaced row of columns.
It is widely used in masonry buildings and low-rise construction.
Strip foundations distribute wall loads over a larger area and are relatively economical where soil has sufficient bearing capacity close to the surface.
Raft or Mat Foundation
A raft foundation is a large reinforced concrete slab that supports several columns and walls simultaneously.
It is used when:
Soil bearing capacity is relatively low
Columns are closely spaced
Separate footings would overlap
Differential settlement must be reduced
The building includes a basement
Structural loads are relatively high
A raft distributes the load over almost the entire building footprint, thereby reducing pressure on the underlying soil.
Advantages of Shallow Foundations
Shallow foundations are usually economical, easy to construct, and simple to inspect.
They require less excavation than deep foundations and can often be constructed using conventional equipment.
They are particularly suitable when good soil is available near the ground surface.
However, shallow foundations may not be suitable where upper soil layers are weak, highly compressible, expansive, filled, or prone to excessive settlement.
Deep Foundations
Deep foundations transfer structural loads to stronger soil or rock located at greater depths.
They are generally used when surface soil is unable to safely support the structure.
Deep foundations are common in high-rise buildings, bridges, industrial structures, transmission towers, marine structures, and buildings constructed on weak or highly compressible soils.
Pile Foundations
Pile foundations are among the most widely used types of deep foundations.
A pile is a long, slender structural element driven, bored, screwed, or cast into the ground.
Piles may be made from:
Reinforced concrete
Prestressed concrete
Steel
Timber
Composite materials
Pile foundations transfer loads through one or both of the following mechanisms.
End-Bearing Piles
End-bearing piles transfer structural loads through the pile tip to a strong layer of soil or rock located below weaker surface strata.
They behave somewhat like columns extending through soft soil until they reach a competent bearing layer.
Friction Piles
Friction piles transfer loads primarily through friction developed between the pile surface and surrounding soil.
They are useful where no strong bearing layer is available at a practical depth.
Combined Action
In many practical situations, piles transfer loads through both end-bearing and skin friction.
Pile Groups
Structural loads are often too large to be supported by a single pile. Therefore, piles are arranged in groups.
The tops of the piles are connected through a reinforced concrete element known as a pile cap.
The pile cap receives the load from the column and distributes it among the individual piles.
Pile-group behavior must be carefully assessed because the combined performance of several piles can differ from the simple sum of their individual capacities.
Pier Foundations
Pier foundations are large-diameter vertical foundation elements constructed by boring or excavating deep holes and filling them with reinforced concrete.
They are often used when competent soil or rock is available at moderate depth and the structural loads are substantial.
Piers usually have larger diameters than conventional piles.
Caisson and Well Foundations
Caissons are large deep-foundation units commonly used in bridge construction, waterfront structures, and other projects near or within water bodies.
They may be constructed as:
Open caissons
Box caissons
Pneumatic caissons
In India, well foundations are commonly used for bridge piers and abutments.
They are designed to resist vertical loads, lateral forces, overturning moments, and the effects of scour.
Soil Bearing Capacity
Soil bearing capacity refers to the ability of the soil to support structural loads transmitted through a foundation.
If foundation pressure exceeds the capacity of the soil, the soil may fail in shear or undergo excessive settlement.
Both conditions can damage the structure.
Ultimate Bearing Capacity
Ultimate bearing capacity is the maximum pressure that soil can withstand before experiencing shear failure.
It represents a theoretical failure condition and is generally not used directly for normal design.
Safe Bearing Capacity
Safe bearing capacity is obtained by dividing the ultimate bearing capacity by an appropriate factor of safety.
For example, if:
and the factor of safety is 3:
The design pressure should therefore normally remain within this safe limit, subject also to settlement requirements.
Allowable Bearing Pressure
Allowable bearing pressure considers both soil shear strength and acceptable settlement.
In practice, settlement often governs foundation design even when the soil has adequate shear strength.
Therefore, the allowable pressure should be selected such that neither bearing failure nor excessive settlement occurs.
Factors Affecting Soil Bearing Capacity
Several factors influence soil bearing capacity.
Soil type: Dense sand, gravel, stiff clay, and rock generally have greater bearing capacity than loose sand, soft clay, organic soil, or uncontrolled fill.
Foundation depth: Greater foundation depth may increase bearing capacity because of higher confinement and overburden pressure.
Foundation width: The size and shape of the footing influence the stress distribution beneath the foundation.
Groundwater level: A high water table can reduce the effective strength of soil, particularly in granular soils.
Soil density and consistency: Dense sands and stiff clays generally perform better than loose sands and soft clays.
Load characteristics: Eccentric, inclined, dynamic, and cyclic loads can reduce foundation performance.
Settlement characteristics: Compressible soils may experience excessive deformation even when they do not fail in shear.
Approximate Safe Bearing Capacities
The following values are indicative only and should not replace a proper geotechnical investigation.
| Soil Type | Approximate Safe Bearing Capacity |
|---|---|
| Soft clay | 50–100 kN/m² |
| Medium clay | 100–200 kN/m² |
| Stiff clay | 200–300 kN/m² |
| Loose sand | 50–100 kN/m² |
| Medium dense sand | 100–250 kN/m² |
| Dense sand | 250–450 kN/m² |
| Gravel | 300–600 kN/m² |
| Weathered rock | 450–1000 kN/m² |
| Sound rock | Above 1000 kN/m² |
Actual values vary considerably depending on field conditions.
Soil Investigation Methods
Proper foundation design requires geotechnical investigation.
Standard Penetration Test
The Standard Penetration Test, or SPT, is commonly used to estimate soil resistance and density.
A standard sampler is driven into the soil using repeated hammer blows. The number of blows required for a specified penetration provides the SPT N-value.
Cone Penetration Test
The Cone Penetration Test, or CPT, involves pushing a cone-shaped probe into the ground and continuously measuring resistance.
It is useful for identifying soil layers and estimating engineering properties.
Plate Load Test
A plate load test involves applying increasing loads to a steel plate placed at foundation level.
The corresponding settlement is measured to determine load-settlement behavior and estimate bearing capacity.
Laboratory Tests
Common laboratory tests include:
Grain-size analysis
Atterberg limits
Moisture-content tests
Direct shear tests
Triaxial tests
Consolidation tests
Unconfined compression tests
These tests provide information about soil strength, compressibility, and settlement characteristics.
Settlement of Foundations
Settlement is an important aspect of foundation design.
It may occur as:
Immediate settlement
Consolidation settlement
Secondary settlement
Uniform settlement is generally less harmful than differential settlement.
Differential settlement occurs when one portion of the building settles more than another. It can lead to cracked walls, distorted doors and windows, tilted columns, uneven floors, and structural damage.
Foundation design must therefore control both total and differential settlement.
Selection Between Shallow and Deep Foundations
Shallow foundations are preferred when competent soil occurs near the ground surface and predicted settlement is acceptable.
Deep foundations are used when upper soil strata are weak or when the structure carries very large loads.
For example, a small residential building constructed on dense soil may use isolated or strip footings. A high-rise building on the same site may require a raft or pile system because its structural loads are much greater.
The final decision should consider safety, construction feasibility, groundwater conditions, adjacent structures, cost, and long-term performance.
Conclusion
Deep and shallow foundations are fundamental elements of structural and geotechnical engineering. Shallow foundations such as isolated footings, combined footings, strip footings, and raft foundations are suitable where adequate soil strength exists close to the surface. Deep foundations such as piles, piers, and caissons are required when weak surface layers must be bypassed or when structures impose very large loads.
Soil bearing capacity plays a critical role in deciding foundation dimensions, depth, and type. However, bearing capacity alone is not sufficient. Settlement, groundwater, soil profile, structural loading, construction conditions, and long-term stability must also be evaluated.
A properly investigated and designed foundation provides the essential link between a building and the ground, ensuring that structural loads are transferred safely while maintaining stability, durability, and serviceability throughout the life of the structure.
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