Shallow foundations are those that transfer load to the soil at a depth close to the lowest floor of the building.
Foundations are generally the lowest part of building walls. They may be made of stone, concrete, or steel.
The foundation aims to support the building and transmit its weight, both live and dead, to the subsoil without failing as long as the building lasts.
A good foundation should not:
- break under a load
- be destroyed by the roots of trees
- be affected by the presence of water
The building regulation states that the foundation must:
- Safely transfer all dead, imposed, and wind loads to the ground without settlement or other movement that would impact the stability or cause damage to the building itself or any adjoining building.
- Be taken down below the frost-damage or soil-movement level.
- Be resistant to attacks by sulfate or other deleterious matters found in the soil.
Designing foundations for medium-sized structures—such as 3- to 5-story apartment buildings, small warehouses, or commercial offices—requires a balance between cost efficiency and structural safety.
Shallow foundations are the preferred choice when the soil at a shallow depth has sufficient bearing capacity to support the load without excessive settlement.
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Table of Contents
Objectives of Foundations
The main objectives of providing foundations may be as follows:
- To provide stability to the structures against many disturbing forces such as wind, rain, earthquake, etc
- Distribute and transmit the load on the structure or building to a larger area of underlying support.
- To prevent excessive and differential settlement of the structure, evenly distribute the load on the substrate.
- It provides a level surface for building operations.
The selection of material and foundation type depends on the structure type and the underlying soil.
Both Shallow Foundations and foundations should be designed to be constructed economically.
So, you can see that foundations must be durable to resist harmful elements such as salt.
Furthermore, they must be designed to resist movement caused by external factors such as seasonal moisture changes in the soil, frost heave, erosion and seepage, landslides, earthquakes, and mining subsidence.
Before deciding upon foundation size, Engineers must ensure that:
- The base’s bearing capacity doesn’t exceed the allowable soil pressure.
- The settlement of the foundation is within reasonable limits.
- Differential settlement is so limited as not to cause any damage to the structure.
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Classification of Foundations
Foundations are classified under two categories;
- Shallow foundations
- Deep foundations
Shallow Foundations
As discussed above, shallow foundations transfer the load to the soil at a level close to the lowest floor of the building.
Deep Foundations
These foundations transfer the load to the subsoil to greater depths than the ground surface and are generally employed only when the conditions of the substrate, the loading of the structure, or both dictate.
These include piles and various types of pier foundations. They are used when the foundation’s depth exceeds 5 meters.
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Selection of the Foundations
Selection of the type for a particular site depends on the following considerations:
- Nature of the subsoil
- Water table level
- Nature and extent of difficulties, e.g., the presence of boulders, buried tree trucks, etc, are likely to be met with
- Availability of expertise and equipment
- Location of the building
- Types and the use of the building
For foundations laid on a rock, the rock should be chiselled. To anchor the foundation to the rock, several 40mm diameter dowel bars are provided at 80cm spacing.
For this, a levelling course of lean concrete is laid as a base for the foundation.
With this “unit”, we shall deal with shallow foundations, as they are a common type and can be laid using open excavations.
This type of foundation is practicable to depths of up to 5m and is usually convenient above the water table.
The base of the structure is enlarged or spread to provide individual support. This type of foundation is provided for structures of moderate height built on sufficiently firm, dry ground.
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Types of Shallow Foundations
- Wall footing foundation
- Isolated or column footing foundation
- Combined footing foundation
- Strap or Cantilever footing foundation
- Continuous footing foundation
- Grillage foundation
- Raft or mat foundation
- Stepped foundation
In Figure 1 below, you can see the Wall, Strap, Matt, Isolated, and Combined Footing Foundations as mentioned above:
Wall footing foundation
The footing can be either simple or stepped. The base foot can be of concrete or entirely of one material.
Simple footing is used for light structures. They have only one projection, beyond the width of the wall on either side.
The width of the concrete base should be at least twice the width of the wall.
The depth of the concrete bed is at least equal to the projection because the angle of the spread of the load from the wall base to the outer edge of the foundation is at least 45 degrees, which results in the thickness being not less than the projection of the base beyond the face of the wall so that the bearing area at the base is not affected.
Very wide strips are reinforced to keep their depth within economic limits. Generally, the projection provided in the footing is kept at 150mm on either side, and the concrete mix comprises cement, sand, and aggregate in the proportion of 1:3:6 or 1:4:8. Figure 1 above.
Isolated or Column Footing Foundation
Isolated columns (figure 1 above) are normally carried on an independent slab of concrete, commonly called a pad foundation, with the column bearing on the centre point of the slab.
The area of the foundation is determined by dividing the column load by the safe bearing capacity of the soil, and its shape is usually square.
The thickness is not less than the projection of the slab beyond the face of the column. The thickness must be at least 150mm.
Reduction in thickness can be effected by reinforcing the slab.
Combined Footing Foundation
Combined footing (see Figure 1 above) supports two or more columns in a row. The combined footing can be rectangular if both columns carry equal loads or trapezoidal if there are space limitations and they carry unequal loads.
Generally, they are constructed with reinforced concrete. In the design of the footing, they are assumed to be rigid and to rest on homogeneous soil.
The location of the centre of gravity of column loads and the centroid of the footing should coincide.
The maximum bending moment is the design value for the reinforced concrete footing.
Strap or Cantilever Footing
As Shallow Foundations, Strap footing (figure 1 above) consists of two or more individual footings connected by a beam called a strap. It is used where the distance between the columns is so great that a combined trapezoidal footing becomes quite narrow, resulting in high bending moments.
The trap transfers column loads to the soil, producing equal, uniform soil pressure under both footings.
Continuous Footing Foundation
In this type, a single continuous R.C. slab is provided as the foundation of two or more columns in a row.
This type of footing is suitable at locations liable to earthquake activity. To have stability, a deeper beam is constructed between the columns.
Grillage foundation
Among shallow foundations, the grillage foundation is used to transfer the heavy structural loads from steel columns to soil with low bearing capacity.
It is light and economical. The required base area to reduce the pressure intensity is obtained at a shallow depth, and hence, deep excavation is not essential.
It consists of rolled steel joists placed in single or double tiers. In double-tier grillage, the top tier is placed at a right angle to the bottom tier.
The tiers of rolled steel are completely embedded in concrete to protect the steel from corrosion. The concrete fill is not supposed to take any load.
Web stiffeners to resist buckling may be required, particularly in the top layer under the stanchion.
Grillage foundation is expensive and, therefore, is only used when very heavy loads from steel stanchions are to be carried on a wide slab, and the depth of the foundation is restricted to keep it above the subsoil water table.
Raft or Mat Shallow Foundations
As a Shallow Foundation, the raft (see 1 figure above) is a combined footing that covers the entire area beneath the structure and supports all the columns.
A raft foundation is used where the allowable soil pressure is low or the structure loads are heavy.
Generally, a raft is used when the soil mass contains compressible lenses, making differential settlement difficult to control.
Furthermore, the raft eliminates the possibility of differential settlement. It is also used to reduce settlement over highly compressible soils by making the weight of the structure and raft approximately equal to the weight of the excavated soil.
Usually, when hard soil is not available within 1.5m to 2.5m, a raft foundation is adopted.
It is advisable to extend the raft or provide a protective apron beyond the effective ground-bearing area to prevent extensive soil movement, which may have shrunk due to drying and swelling from moisture absorption.
Holes for pipes and services should not be cut into the raft as they may cause loss of strength; provision of holes should be made when the raft is being concreted, and the design should make allowance for them.
Stepped Shallow Foundations
These are Shallow foundations used where a structure is built on a sloping site and are intended to reduce the excessive excavation and materials required for a foundation if it were kept at one level.
The minimum lap of one layer of concrete over the lower one must be equal to the concrete thickness or 300mm, whichever is greater.
The steps should not exceed the foundation thickness and should be multiples of the brick thickness so that the brickwork will bond without being cut at each step.
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Key Points From the Post Shallow Foundations
- Foundation soils are usually softer than structural materials. A foundation is therefore required to spread the load from a structure so that it is transmitted safely into the earth’s crust.
- A site investigation must always be conducted to determine the nature of the soil beneath a structure. Trial pits and boreholes are excavated and samples sent to the laboratory for testing.
- A foundation must not cause shear failure in the soil nor be subject to excessive settlement. The base of a foundation must not be too close to the ground surface.
- Most foundations are concrete, and as the foundation load increases, they change from being simple unreinforced pads and strips to reinforced pads, strips or rafts. In problem soils or for heavily loaded structures, piled foundations may be required.
- The depth of an unreinforced concrete foundation must be at least equal to the maximum projection of the foundation from the loaded column or wall.
- Reinforced concrete foundations must be designed to resist bending failure and both simple shear and punching shear.
- For gravity retaining walls, the resultant vertical force must remain within the middle-third of the base to ensure that the entire base remains in compression.
- In addition, the wall must not slide forward, and the soil must not be subject to a deep-seated failure.
Shallow Foundations: Wrapping Up
The foundation is that part of the structure in direct contact with the ground.
Some of the objectives of providing foundations may be:
- To provide stability to the structures
- Distribute and transmit the total load coming on the structure to the ground
- To prevent the excessive settlement
- To provide a level surface
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