Pile Foundations Requirements and Functions in the Construction Industry

Pile Foundations Requirements and Functions in the Construction Industry

Pile foundation is required when the soil bearing capacity is not sufficient for the structure to withstand. This is due to the soil condition or the order of bottom layers, type of loads on foundations, conditions at site and operational conditions.

Many factors prevent the selection of surface foundation as a suitable foundation such as the nature of soil and intensity of loads, we use the piles when the soil have low bearing capacity or in building in water like bridges and dams

A pile foundation consists of two components: Pile cap and single or group of piles. Piles transfers the loads from structures to the hard strata, rocks or soil with high bearing capacity. These are long and slender members whose length can be more than 15m.

Piles can be made from concrete, wood or steel depending on the requirements. These piles are then driven, drilled or jacked into the ground and connected to pile caps. Pile foundation are classified based on material of pile construction, type of soil, and load transmitting characteristic of piles.

The use of pile foundations as load carrying and load transferring systems has been for many years. Timber piles were used in early days, driven in to the ground by hand or holes were dug and filled with sand and stones. The use of steel pile started since 19th century and concrete piles since 20th century.

With the change in technology and industrial revolution, many advance systems have been devloped for pile driving from the invention of steam and diesel pile driving machines.

The use of pile foundations is increasing day by day due to non-availability of land for construction. Heavy multi-storyed building are being constructed, and load from these structures can not be directly transferred to ground due to low bearing capacity issue and stability issues of building during lateral load application. So, demand for use of pile foundations are increasing day by day. Due to this demand for piles, there have been many improvements in piles and pile driving technology and systems. Today there are many advanced techniques of pile installation.

Pile Foundation Functions

Function of Pile Foundation:

As other types of foundations, the purpose of pile foundations is:

• To transmit the buildings loads to the foundations and the ground soil layers whether these loads vertical or inclined
• To install loose cohesion less soil through displacement and vibration.
• To control the settlements; which can be accompanied by surface foundations.
• To increase the factor of safety for heavy loads buildings

The selection of type of pile foundation is based on site investigation report. Site investigation report suggests the need of pile foundation, type of pile foundation to be used, depth of pile foundation to be provided. The cost analysis of various options for use of pile foundation should be carried out before selection of pile foundation types.

Unless the ground condition is rocks, for heavy construction and multi-storied buildings, the bearing capacity of soil at shallow depth may not be satisfactory for the loads on the foundation. In such cases, pile foundation has to be provided. The number of piles in a pile groups required is calculate from the pile capacity of single pile and the loads on the foundation. Piles are a convenient method of foundation for works over water, such as jetties or bridge piers.

Different Types of Foundations and Its Uses - Concrete RCC Foundation

There are different foundation types and uses of these depends on the soil condition and loads from the structure. It is advisable to know suitability of each foundation types before making any decision for their selection in any construction project.

Following are the foundation types and their uses:

1. Spread footings and wall footings:

Spread footings are those whose base is more wider than a typical load bearing wall foundations. This is used in case of buildings. The wider base of this footing type spreads the weight from the building structure over more area and provides better stability.

Spread Footing

Spread Footing

Spread footings and wall footings are generally used for individual columns. walls and bridge piers. These footings are used where the bearing soil layer is within 3m (10 feet) from the ground surface. The soil bearing capacity must be sufficient to support the weight of the structure over the base area of the structure.

These foundations should not be used on soils where there is any possibility of ground flow of water above bearing layer of soil which may result in scour or liquefaction.

2. Mat Foundations:

Mat foundations are those which are spread across the entire area of the building to support heavy structural loads from columns and walls.

Mat Foundation

Mat Foundation

The use of mat foundation is for columns and walls foundations where the loads from the structure on columns and walls are very high. This type of foundation is used to prevent differential settlement of individual footings, thus designed as a single mat (or combined footing) of all the load bearing elements of the structure.

This type of foundation is suitable for expansive soils whose bearing capacity is less for suitability of spread footings and wall footings. This type of footing is economical generally when one-half area of the structure is covered with individual footings and wall footings is provided.

These foundations should not be used where the ground water table is above the bearing surface of the soil. Use of foundation in such conditions may lead to scour and liquefaction.

3. Pile Foundations:

Pile foundation is a type of deep foundation which is used to transfer heavy loads from the structure to a hard rock strata much deep below the ground level.

Pile Foundation

Pile Foundation

Pile foundations are used to transfer heavy loads of structures through columns to hard soil strata which is much below ground level and where shallow foundations such as spread footings and mat footings cannot be used. This type of foundation is also used to prevent uplift of structure due to lateral loads such as earthquake and wind forces.

Generally used for soils where soil conditions near the ground surface is not suitable for heavy loads. The depth of hard rock strata may be 5m to 50m (15 feet to 150 feet) deep from the ground surface. The pile foundations resists the loads from structure by skin friction and by end bearing. Use of pile foundations also prevents differential settlement of foundations.

4. Drilled Shafts:

Drilled shafts is also a type of deep foundation and has action similar to pile foundations discussed above, but are high capacity cast-in-situ foundations. It is also called as caissons. It resists loads from structure through shaft resistance, toe resistance and / or combination of both of these. The construction of drilled shafts or caissons are done using an auger.

Drilled Shafts or Caisson Foundation

Drilled Shafts or Caisson Foundation

This foundation can transfer column loads larger than pile foundations. It is used where depth of hard strata below ground level is location within 10m to 100m (25 feet to 300 feet). This foundation is not suitable when deep deposits of soft clays and loose, water-bearing granular soils exists. It is also not suitable for soils where caving formations are difficult to stabilize, soils made up of boulders, artesian aquifer exists.

How to determine the minimum depth of foundation required - Foundation Depth Calculator

Many factors affects the determination of depth of foundation. Calculation for foundation depth is done based on type of soil, ground water table, loads from structure, bearing capacity of soil and other factors.

• General factors to be considered for determining depth of foundation are:
• Load applied from structure to the foundation
• Bearing capacity of soil
• Depth of water level below the ground surface
• Types of soil and depth of layers in case of layered soil
• Depth of adjacent foundation

The minimum depth of foundation should be considered to ensure that the soil is having the required safe bearing capacity as assumed in the design. However, it is advised to carry out soil investigation before deciding on depth of foundation. Soil investigation report will suggest the foundation depth based on the type of structure, soil properties, depth of water table, and all other variable that should be considered. Soil investigation report provides bearing capacity of soil at different levels and at different locations.

Determine Minimum Required Depth of Foundation

When the soil investigation report is not available, the depth of foundation should be selected such that it is not affected by swelling and shrinking of soil due to seasonal changes. Depth of foundation should also consider the depth of water table to prevent and scour below the ground.

For foundation near existing foundation, It must be ensured that pressure bulbs of foundations do not coincide if the depth of new foundation has to be taken below the depth of existing foundation.

The foundation should not be contracted at shallow depth considering the frost action in cold countries.

Rankine’s formula provides the guidance on minimum depth of foundation based on bearing capacity of soil.

Where, h = minimum depth of foundation

p = gross bearing capacity

 = density of soil

  = angle of repose or internal friction of soil.

The above formula does not consider the factors discussed above and just provides the guidance on minimum foundation depth, assuming that the foundations are not affected by factors such as water table, frost action, types and properties of soil etc. as discussed above. This formula does not consider the loads from the structure on the foundation.

In the Rankine’s formula, it can be seen that foundation depth depends on the bearing capacity of soil, so, if the bearing capacity of soil increases, the depth of foundation also increases.