Types of Foundations and the Importance of Soil Assessment in Construction Industry

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A foundation is the structural element that transmits building loads safely to the ground. The selection of the appropriate type of foundation involves both an understanding of the structural requirements and the nature of the soil on site. Different soils require different foundation solutions, so the investigation of soil conditions is a critical activity prior to the commencement of any construction. This article describes the major foundation types and identifies fundamental soil considerations, which influence foundation design.

1. Shallow Foundations​

Shallow foundations are used where adequate load-bearing soil exists near the ground surface, generally within a depth of about 3 meters. These foundations distribute structural loads over a broad area and are commonly used for low- to medium-rise construction where the near-surface soils can safely support the load.

a. Strip Foundation​

A strip foundation is a continuous band of concrete placed beneath load-bearing walls.
It is usually chosen in cases:

• The soil conditions are firm and uniform.
• The load from the structure is distributed linearly.
• Settlement risks are minimal.

Strip foundations are easy to construct and work well where the soil is stable, and the load is roughly even along the wall length.

b. Pad / Isolated Footing​

Pad or isolated footings are individual concrete blocks supporting single columns.

They are characterized by:

• Shapes are usually square, rectangle, or circular.
• Relatively low to moderate loads.
• Flexibility in size, depending on column load and soil bearing capacity.

These are cost-effective foundations for structures with columns that are sufficiently spaced and whose loads are reasonably predictable.

c. Combined Footing​

A combined footing is used to carry two or more columns, especially those that are close together, or when one column is at an edge where a separate footing cannot be centered.

This type of footing:

• Allows even distribution of loads from adjacent columns.
• Prevents eccentric loading issues.
• Provides greater area of support when space is at a premium.

In constrained layouts, or wherever the variation in load intensity necessitates integrated footing designs, combined footings can provide structural stability.

d. Raft / Mat Foundation​

A raft or mat foundation covers a large area, essentially covering the full footprint of a structure.
It is usually used when:

• The soil has a low bearing capacity.
• The loads from columns or walls are closely spaced.
• Differential settlement should be minimized.

Raft foundations reduce settlement risk by spreading the load across a wide slab and can be used to provide a solid base even on weak or compressible soils.

2. Deep Foundations​

Deep foundations transfer loads through the weak surface soil layers to deeper, competent soil or rock strata. They become necessary in cases where the upper soil cannot safely support structural loads, especially for high-rise buildings and industrial structures, or in areas with highly variable ground conditions.

a. Pile Foundation​

Pile foundations are lengthy, slender members, either driven, drilled, or bored into the ground to reach stronger layers.

Key features of Pile foundations include:

• Ability to transfer loads through either end-bearing or skin friction.
• Suitability for sites with loose sand, soft clay, or water-logged conditions.
• Compatibility with heavy loads and deep soil deposits.

Common types of piles are :

• End-bearing piles: These transfer loads directly to hard layers or rock.
• Friction piles rely on friction between soil and the pile surface.
• Steel piles: Used in marine and industrial situations.

Concrete piles are also durable and widely used for structural buildings.
Pile foundations provide strength and stability when shallow options are not feasible.

b. Pier / Caisson Foundation​

Pier or caisson foundations are those for which large-diameter shafts are drilled into the ground until competent soil or rock is reached.

They are selected when:

• Very heavy loads must be supported.
• Control of the groundwater is required.
• Deep, stable strata can be accessed by drilling.

Applications where these foundations are commonly used include bridges, heavy columns, waterfront structures, and deep excavations.

3. Special Foundations​

In such soil conditions, there are special designs for foundations to counteract soil movements and low strength or to avoid excessive settlement. Special foundations are specifically designed for unique ground behaviors that typical shallow or deep systems cannot manage effectively.

a. Under-Reamed Piles​

Under-reamed piles are piles with bulb-shaped enlargements near the base.
They are particularly effective in:

• Expansive soils, like black cotton soil.
• Areas experiencing soil swelling and shrinkage.
• Circumstances that require the resisting of uplift forces.

It increases the bearing capacity and stability of the bulb, which would ensure reliable support even against soil movement due to moisture variations.

b. Floating Foundation​

The concept of a floating foundation also relies on the key idea of balancing the weight of the structure with the weight of excavated soil.
This foundation is used when:

• The soil is very weak or compressible.
• Settlement control is critical.
• Traditional foundation types are not practical.

Floating foundations balance the forces to limit settlement and, hence, provide a safer support in difficult environments with soft clay or silt layers.

Understanding the types of foundations and their appropriate applications is paramount for ensuring structural stability and long-term performance. Shallow foundations can be used for firm, near-surface soils, while deep foundations are required if loads need to reach stronger materials deeper within the ground.
In cases where soils behave unusually or present a problem, special foundations offer solutions.
The selection of a type of foundation essentially calls for the evaluation of certain properties of the soil, including bearing capacity, compressibility, and moisture characteristics.
Accurate soil evaluation and proper selection of foundations can provide structures with the necessary support, safety, and longevity in a wide range of ground conditions.

Soil Type to Foundation Type (Quick Guide)​

Soil Type	Recommended Foundation
Dense Sand / Gravel	Shallow foundations (strip, pad, raft)
Soft Clay	Raft or pile foundation
Loose Sand	Pile foundation
Expansive Soil (Black Cotton Soil)	Under-reamed piles
Very Weak Soil	Pile or Floating foundation
Rock	Shallow stepping or pad foundation