Shallow and Deep Foundations: Which and when?

The foundations are broadly categorized into deep foundations and shallow foundations. Both have their types of footing, design factors, merits and drawbacks. The ability to determine the time when to incorporate shallow and deep foundation is an important factor that engineers, architects and construction professionals must know.

One of the most important decisions that must be made in the building design and construction is the selection of the correct type of foundation. One of the elements that hold the superstructure (which is the visible portion of a building) to the ground is the load-transferring element known as the foundation. Failure to select the foundation properly even the most well thought of structure may suffer settlement, even cracks or even failure.

In this article we will discuss the various types of foundations like deep and shallow, the types of foundations including isolated footing, combined footing, strip footing, raft foundation and pile foundation, and present the advice on which type to use under different circumstances.

What is a Foundation?

A foundation is an element of structure that supports the weight of building to the underlying soil or rock. Its primary functions are:

• Load transfer- It moves vertical and lateral loads in a safe manner to the ground.
• Settlement control It reduces differentiation settlement of the building.
• Stability – It stops the overturning, slipping or tilting of the building.
• Durability – It refers to its ability to resist all ground effects such as soil moisture, frost action and others.

The decision of shallow and deep foundation is based on the bearing capacity of the soil, type of structure, magnitude of loads, the water table level and the cost of construction.

Shallow foundation vs Deep Foundation: The Fundamental Difference.

Shallow Foundation: This is a foundation which passes the load to the ground at a comparatively shallow depth (typically within 3 meters of ground level). It is dependent upon bearing capacity of soil at the surface.

Deep Foundation: A foundation that shifts the pressure to further and more robust soil layers, or rock levels that can be 10 50 meters deep or further. It does not depend on weak surface soils, spreads the load using end-bearing or skin friction.

Rule of thumb:

When the depth of the foundation is less than or equal to the width of foundation, it is regarded as a shallow foundation.

In case the depth is a lot larger than its width, it is known as a deep foundation.

A. Shallow Foundations:

There are two types of shallow foundations.

Shallow foundations are cost-effective and are often applied in low- to medium-rise buildings in situations where a load may be supported by soil close to the ground.

1. Isolated Footing Pad Footing.

Description: The type of support is a single column. In shape square, rectangular or circular.

Application: In cases where the columns are not closely spaced and where the loads are not heavy.

Suitability:

• Good soil bearing capacity (> 200 kN/m 2).
• Medium and light structural loads (residential houses, small commercial buildings).
• Example: Two storey house with RCC framed structure.

2. Combined Footing

Description: Two or more columns would be supported when they are near each other and the individual footings would overlap. It may be trapezoidal or rectangular.

Application: It is used when there is uneven loading or where the columns are located close to property boundaries.

Suitability:

• Used when space is limited.
• Cost-efficient instead of several independent footings.

3. Strip Footing (Wall Footing)

Description: A sustained footing which carries a load bearing wall.

Application: To be used in masonry walls or columns that are close to one another.

Suitability:

• Brick or stone wall structures that are low rise.

4. Mat Foundation is also known as Raft Foundation.

Description: This is a thick reinforced concrete slab that is over the whole building footprint, and upon which several columns and walls are founded.

Application: Applicable in the case of a low-bearing soil with which there is a uniform distribution of loads.

Suitability:

• Major, massive constructions (apartment buildings, warehouses).
• Soil with low SBC (~75–150 kN/m²).
• Where there is a requirement to minimize the differential settlement.

Types of Deep Foundations

Deep foundations are necessitated where the surface soils are weak, compressible or in cases where the loads are very substantial.

1. Pile Foundation

Description: Long and slender columns composed of concrete, steel or timber, which are pushed or cast into the ground.

Load Transfer Mechanism:

• End-bearing piles- Load onto hard strata or rock at huge depth.
• Friction piles Transfer load frictionally on the shaft of the piles.

Use:

• Tall buildings, bridges, marine structures..
• In case the groundwater table is high.
• In loose or compressible soils.

2. Pier Foundation

Description: Huge pillars cylindrical in shape that are built on site to withstand great weight.

Application: In situations where there is a concentration of load and the soil in the surface is weak.

3. Caisson Foundation (Well Foundation)

Definition: This is a waterproof box that has been buried in the ground, most often in the rivers or along beaches.

Application: To be used in bridge piers, docks in the marine and offshore structures.

Factors that influence the shallow foundation vs deep foundation

Basic Considerations that Influence the decision: Shallow versus Deep Foundation.

In making decision on any type of shallow and deep foundations, engineers put into consideration the following:

Soil Bearing Capacity (SBC)

• High SBC ( > 200 kN/m 2): Shallow foundations are adequate.
• Low SBC (Less than 100 kN/m 2): Deep foundations such as piles are necessary.

Load of the Structure

• Light civilizations (residences, single storey shops): Isolated, strip underpinnings.
• Massive (high-rise, industrial facilities) Raft or pile foundations.

Groundwater Table

• Shallow footings can be caused by high water table. Deep foundations are safer.

Depth of Hard Strata

• In case of firm strata, depth of 2-3 m becomes available, then the foundation is shallow.
• When the firm strata is at 10-20m and above → Deep foundation.

Differential Settlement

• In soils that are not compressible evenly, raft or pile foundations can be used to reduce the settlement.

Shallow foundation vs Deep foundation : Cost Considerations

The shallow foundations are less expensive and quicker to build.

Deep foundations involve specialized equipment and they are more costly.
Guidelines in Practice: When and Which to Use.

Use Isolated Footing when:

• The bearing capacity of soil is great.
• Loads are light to moderate.
• Columns are widely spaced.

Use Combined Footing when:

• There are close or near boundaries which are two columns.
• Loads are unequal.
• Economy is required instead of independent isolated footings.

Use Strip Footing when:

• Stabilizing a load bearing wall.
• Masonry or RCC buildings of small scale construction.

Use Raft Foundation when:

• Soil is low SBC but is equally compressible.
• The loads of heavy weight are distributed in a vast area.
• There must be control over settlements.

Use Pile Foundation when:

• The soil is so weak that it reaches far down.
• Massive concentrated loads should be moved.
• Building is close to water bodies or in the city with deep foundations.

Apply Caisson or Pier Foundations in cases during which:

Constructing bridge piers or docks on the sea.

Working in rivers or coastal contracts.

Case Examples:

Case 1: Residential Building on Soil of A Strong Soil.

SBC 250 kN /M 2 on a sandy gravel soils G +2 house on sandy gravel soils Isolated footings only.

Case 2: Soft Clay Apartment block.

In an apartment where SBC is 100 kN/m 2 only, a 10-storey structure allows loads to settle through the foundation and reduce settlement.

Case 3: High-Rise in Coastal City.

A 40 storey tower in Mumbai with marine clay soils, and high water table have Pile foundation to ensure the load was transferred to lower rock strata.

Case 4: Bridge Across a River

Piers in bridges should be stable against flowing water and Well (Caisson) foundation is adopted.

Benefits of Shallow foundation vs deep foundation

Shallow Foundations:

Advantages:

• Simple and cost-efficient in its building.
• None of the special equipment is needed.
• Quick execution.

Disadvantages:

• Not suitable for weak soils.
• Affected by ground water changes.
• Shallowness of transfer of load.

Deep Foundations:

Advantages

• Can support very heavy loads.
• Applicable on weak or compressible soil.
• Is able to penetrate problematic upper layers of soil.

Disadvantages:

• Costly and time-consuming.
• It needs special equipment and skills.
• Construction control on quality is essential.

Conclusion:

The decision of shallow vs deep foundation lies in a compromise between the soil conditions, structural loads, safety requirement and cost.

Isolated, combined, strip, or raft footing are all the most practical foundations to use in small medium projects on firm soils.

Deep foundations which may include pile foundations, caissons or piers are essential to large projects that require a foundation with high strength, like weak or compressible soil or an exceptionally high load.

The knowledge of the types of footing and when to apply pile foundation or raft foundation is a guarantee of stability, durability, and economy in construction.