The building’s foundations transfer the structure’s weight to the ground. Although the term ‘foundation’ is a broad term, each building type has various distinct types of foundations. The majority of buildings have a form of foundation structure immediately underneath each column to move column loads to the ground. It is necessary to decide the suitability of each type of foundation before implementing them in a building project. Broadly, there are two types of foundations: shallow and deep foundations. The types of foundations used under shallow and deep foundations for building construction are discussed and their applications.

TYPES OF FOUNDATIONS

The different types of foundations used in Building construction or you can say, types of foundations for homes are as follows

Shallow foundation

Additionally, shallow foundations are referred to as spread footings or open footings. The term ‘open’ refers to the fact that foundations are constructed by excavating all of the earth down to the bottom of the footing and then installing the footing. During the initial stages of construction, the entire footing is clear, so it is an open foundation. The theory is that each footing distributes the column’s accumulated load over a wide area, ensuring that the total weight on the soil does not surpass the soil’s safe bearing capacity.

1. Individual/Isolated footing
2. Combined footing
3. Strip foundation
4. Raft or mat foundation

Deep Foundation

1. Pile foundation
2. Drilled Shafts/Caissons

DETAIL DESCRIPTION

Individual Footing or Isolated Footing

Individual footings, also known as isolated footings, are the most frequently used foundation in building construction. This foundation is used with a single column and is often referred to as a pad foundation.

Pic: Isolated Footing Design Details with Site Photo (Credit: CTCIVIL.IR)

Individual footings are square or rectangular in form and are used while the columns carry the structure’s loads. The column’s size is determined by the load on it and the soil’s safe bearing capacity. Rectangular isolated footings are used where the base is subjected to moments caused by eccentric loads or horizontal forces.

For example,

Consider a column with:

Vertical load = 400 kN

Safe Bearing Capacity = 150 kN/m2

Then,

Area of the footing required = 400/150 = 2.67m2

So, for a square footing, the length and width of the footing will be 1.63 m x 1.63 m.

Combined Footing

When two or more columns are near enough, their isolated footings overlap, and a combined footing is created. Since it is a composite of isolated footings, their structural architecture is distinct. This footing is rectangular in form and is used while the columns carry the structure’s loads.

Pic: Combined Footing Under Construction (Google)

Spread/Strip footings and Wall footings

Spread footings have a broader base than regular load-bearing wall foundations. The wider foundation of this style of footing distributes the weight of the building structure over a larger area, increasing stability.

Individual columns, walls, and bridge piers are supported on spread footings or wall footings where the bearing soil layer is within 3m (10 feet) of the ground surface. The soil bearing capacity of the structure must be adequate to withstand the structure’s weight over the structure’s base area. These should not be used on soils with the risk of groundwater flowing above the bearing layer, resulting in scour or liquefaction.

Raft/Mat Foundations

Raft or mat foundations are varieties of foundations that span the entire structure’s total size to support the structural loads imposed by columns and walls. Mat foundations are used for column and wall foundations where the structure’s loads on the columns and walls are weighty. This is used to avoid unequal settlement of individual footings, and therefore all load-bearing components of the construction are constructed as a single mat (or joint footing).

Pic: Mat or Raft Foundation Design Drawings and Site Photo (Source: Google)

It is appropriate for expansive soils with a lower bearing capability for spread footings and wall footings. When individual footings and wall footings support one-half of the building, a raft base is cost-effective. These foundations should not be used in areas where the groundwater table is higher than the soil’s bearing surface. In such conditions, the use of foundation can result in scour and liquefaction.

Pile Foundations

Pile foundations are used to move heavy loads from buildings to hard soil strata located far below ground level and inaccessible to shallow foundations such as spread footings and mat footings. This is often used to keep the foundation from being uplifted due to horizontal loads such as earthquakes and wind forces.

Pic: Pile Foundation Details (Google)

Pile foundations are typically used in soils where the ground surface conditions are insufficient to support large loads. Strong rock strata can be found at depths ranging from 5m to 50m (15 ft to 150 feet) below the ground level. The skin friction and end bearing of the pile base provide resistance to the loads imposed by the frame. Additionally, the use of pile foundations avoids unequal base settlement.

Drilled Shafts/Caisson Foundation

Drilled shafts, also known as caissons, form a deep foundation that operates similarly to pile foundations mentioned previously but are cast-in-situ foundations with a high volume. It resists loads imposed by the structure through shaft resistance, toe resistance, or a combination of the two. An auger is used to build drilled shafts or caissons.

Drilled shafts are capable of transferring column loads greater than those supported by pile foundations. It is used in areas where the depth of hard strata underground level is between 10m and 100m (25 feet to 300 feet). Where deep layers of soft clays and loose, water-bearing granular soils occur, drilled shafts or caisson foundations are not suitable. Additionally, it is not ideal for soils with difficult-to-stabilize caving deposits, boulder-filled soils, or soils with an artesian aquifer.

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BASIC REQUIREMENTS OF FOUNDATION

Talking about the basic requirements of foundation, for satisfactory performance, all types of foundations should satisfy the following general criteria:

Lateral displacement

If the foundation of the structure is embedded in the soil, lateral earth pressure can result in shear and moment loads. Latitudinal displacement must be restricted. Bridge constructions have a maximum tolerance of 2.5 cm.

Settlement

Settlement is known as the vertical movement of the ground as a result of changes in stress. This is the most critical requirement of all since it happens when a vertical load is added to the base. Although some degree of settlement is expected in new constructions, it must be kept within the appropriate limits established by regulatory standards. The most critical factor affecting the extent of damage to a structure is whether or not the settlement is standardized. As a structure sinks uniformly, there are no cracks or minor damage. However, if one side of the building settles more than the other (differential settlement), significant damage will occur to the structure.

Ground heave

Ground heave is the vertical upward displacement of a structure’s base caused by the expansion of the soil. It occurs as a result of the expansion of clay soils, which expand when wet. Due to the soil’s inherent inability to extend downward or sideways, the exposed upper surface of the soil increases. This phenomenon must be understood during the structure’s design and construction phases.

Durability

A foundation must be functional for the life span of a project. Being placed underground, it needs to address severe issues like chemical, physical, and biological processes. Some methods to make the foundation durable include utilizing chemical resistant concrete, implementing a thicker layer of concrete to protect steel reinforcement or using insulating materials that will decelerate the weathering processes.

Tilt

When heave or settlement do not occur uniformly around the foundation, the structure tilts. Since it may cause serious damage to the structure of a building, it must be prohibited. In tall buildings, the angle of tilt does not exceed 0.12 degrees from horizontal.

FOUNDATION SELECTION CRITERIA FOR BUILDINGS

For all types of foundations, there are some particular selection criteria. The followings are detailed explanations for the selection of types of foundations.

Loads of Structure

The choice of building materials such as bricks, stone, steel, and concrete impacts the choice of base. Calculating the foundation’s settlement is another aspect related to the structure’s loads and influences the foundation selection. Shallow foundations are favoured for low-rise buildings. However, for high-rise structures, a substantial foundation is needed. A deep foundation is necessary due to the highly compacted nature of the ground at a greater depth.

Soil Bearing Capacity

The decision to use a shallow or deep foundation can be made depending on the soil bearing pressure. For shallow foundations up to four stories, a permissible bearing pressure of at least 100kN/m2 or greater is effective. However, higher structures may consider a raft base as long as the measured modulus of subgrade reaction is not exceeded.

Adjoining Structure/Water Bodies/Slopes

When the adjacent structure’s foundation is very similar to the foundation to be constructed, it will affect the choice and protection of the adjacent structure. If the foundations of an adjacent structure are incredibly close to the target base, this will influence the decision, as the adjoining structure’s protection is paramount. The selection of an appropriate foundation form would be more complicated if the neighbouring structure is a high rise or an older structure. Counting should consider factors such as proximity to a river, lake, natural drain, or slope. For buildings on sloping terrain, isolated RCC pad foundations supported by stilts are usually favoured. No residential structure should be built on a slope greater than 25 degrees.

Soil Types

When the soil has a low bearing capacity, a more robust and suitable base should be chosen than when the soil has a high bearing capacity. The bearing capacity of soil is described as its ability to support structural loads without shear failure or intolerable settlement safely.

Water Table Level

If the groundwater table is below the foundation’s formation level, a shallow footing such as isolated or combined footings should be used. Additionally, raft/mat footing should be considered for areas with a higher water level. This is to combat uplift pressure and the impact of water on the structure during the early stages of construction to prevent any overturning moments. If this alternative is unavailable, deep foundations such as micro piles or bored piles should be considered to provide the necessary uplift resistance.

Ease of Construction

The foundation should be chosen with consideration for the ease with which it can be executed/constructed on-site. The building of various types of foundations involves labour with a diverse skillset and a range of abilities.

Natural Disaster and Extreme Weather

If the region has a history of serious natural disasters or extreme weather, these criteria should be chosen accordingly. The historical record or data on natural disasters and extreme weather can guide the selection of foundations.

Economic Design

Economic considerations can influence the foundation selection when there are several viable foundation options for a given project site. Nevertheless, selecting an economic base does not jeopardize the foundation’s protection, workmanship, strength, or durability.

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Selection of Type of Foundation

The selection of a particular type of foundation is often based on a number of factors, such as:

1.

Adequate depth

The foundation must have an adequate depth to prevent frost damage. For such foundations as bridge piers, the depth of the foundation must be sufficient to prevent undermining by scour.

2.

Bearing capacity failure

The foundation must be safe against a bearing capacity failure.

3.

Settlement

The foundation must not settle to such an extent that it damages the structure.

4.

Quality

The foundation must be of adequate quality so that it is not subjected to deterioration, such as from sulfate attack.

5.

Adequate strength

The foundation must be designed with sufficient strength that it does not fracture or break apart under the applied superstructure loads. The foundation must also be properly constructed in conformance with the design specifications.

6.

Adverse soil changes

The foundation must be able to resist long-term adverse soil changes. An example is expansive soil, which could expand or shrink causing movement of the foundation and damage to the structure.

7.

Seismic forces

The foundation must be able to support the structure during an earthquake without excessive settlement or lateral movement.

Based on an analysis of all of the factors listed above, a specific type of foundation (i.e., shallow versus deep) would be recommended by the geotechnical engineer.

The image given below can be used as a guide for selection of an appropriate type of foundation based on different soil conditions.

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