Shallow foundations are those foundations that transfer the load to the soil at a level close to the lowest floor of the building.
Foundations are generally the lowest part of building walls. They may be made of stones, concrete, or steel.
The foundation aims to support the building, transmitting its weight, both live and dead load, 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 frost damage or soil movement level.
- Be resistant to attacks by sulfate or other deleterious matters found in the soil.
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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 by evenly loading the substrate.
- It provides a level surface for building operation.
The selection of material and type of foundation depends upon the structure type and the underlying soil’s nature.
The 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 from external causes 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 bearing capacity of the base 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 a greater depth from 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 employed when the foundation’s depth is more than 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 benched by chiseling. To anchor the foundation to the rock, several dowel bars of 40mm diameter at 80cm spacing are provided.
On this, a leveling course with lean concrete is laid to serve as a base for the foundation.
With this “unit” we shall deal with shallow foundations because they are common type foundations and can be laid using open excavations.
This type of foundation is practicable for a depth of up to 5m and usually is 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 at least be equal to 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 center 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 is not to be less than 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 footing, they are assumed to be rigid and resting on a homogeneous soil.
The location of the center 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
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 with high bending moments.
The trap serves to transfer column loads onto the soil with equal and 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 activities. To have stability a deeper beam is constructed in between the columns.
Grillage foundation
Grillage foundation is used to transfer the heavy structural loads from steel columns to soil having 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 stanchion are to be carried on a wide slab, and the depth of the foundation is restricted in order to keep it above the subsoil water table.
Raft or Mat Shallow Foundations
A raft (see 1 figure above) is a combined footing that covers the entire area beneath a 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 where the soil mass contains compressible lenses so that the differential settlement would be difficult to control.
Furthermore, the raft eliminates the possibility of differential settlement. It is also used to reduce settlement above highly compressible soils by making the weight of the structure and raft approximately equal to 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 so that extensive movement of the soil, which may have shrunk through drying and swelling due to moisture absorption, can be prevented.
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 save the excessive amount of excavating and materials necessary for the 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 thickness of the concrete or 300mm, whichever is greater.
The steps should not be of greater height than the thickness of the foundation and should be in multiples of brick thickness so that the brickwork will bond without being cut at each of the steps.
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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 for the surface
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