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Why Use a Group of Piles Instead of a Single Pile in Foundations: An In-Depth Look

When it comes to constructing robust and reliable foundations for large structures, the decision to use a group of piles instead of a single pile is crucial. Pile foundations are essential in transferring loads from buildings to the ground, especially in areas with weak or unstable soil. This article explores the reasons behind using a group of piles over a single pile, highlighting the advantages and technical considerations that make this approach superior for many construction projects.

Understanding Pile Foundations

Pile foundations consist of long, slender columns made of materials such as concrete, steel, or timber, driven deep into the ground to reach stable soil or rock layers. They are used to support structures with heavy loads or in areas where the surface soil is not strong enough to bear the load on its own.

Advantages of Using a Group of Piles

1. Load Distribution

One of the primary reasons for using a group of piles is the effective distribution of loads. A single pile might not be able to bear the entire weight of a structure, especially if the load is substantial. By using multiple piles, the load is spread across a larger area, reducing the stress on each individual pile and enhancing the overall stability of the foundation.

2. Increased Load Capacity

A group of piles can collectively support much heavier loads than a single pile. This is particularly important for large buildings, bridges, and other structures that exert significant pressure on their foundations. The combined strength of a pile group ensures that the foundation can handle the load without risk of failure.

3. Mitigation of Settlement Issues

Settlement occurs when the ground beneath a foundation compresses under the weight of the structure, potentially leading to uneven or excessive sinking. A group of piles minimizes settlement by distributing the load more evenly and reaching deeper, more stable soil layers. This reduces the likelihood of differential settlement, which can cause structural damage over time.

4. Improved Stability in Lateral Loads

Structures often face lateral loads due to wind, earthquakes, or other forces. A group of piles provides better resistance to these lateral forces compared to a single pile. The collective action of multiple piles enhances the foundation’s ability to withstand horizontal movements, ensuring the structure remains stable and secure.

5. Redundancy and Safety

Using multiple piles introduces redundancy into the foundation design. If one pile fails, the load can be redistributed among the remaining piles, reducing the risk of catastrophic failure. This redundancy is a crucial safety feature, especially for critical infrastructure and high-rise buildings.

Technical Considerations

When designing a group of piles, several technical factors must be considered to ensure optimal performance:

Pile Spacing: Proper spacing between piles is essential to prevent negative interactions such as pile-to-pile load transfer and ensure that each pile can carry its share of the load.
Pile Cap Design: A pile cap is a thick concrete mat that sits on top of the pile group, distributing the load from the structure above to the piles below. The design of the pile cap must accommodate the load distribution and ensure stability.
Soil-Pile Interaction: The interaction between the piles and the surrounding soil plays a critical role in the foundation’s performance. Soil testing and analysis are necessary to determine the appropriate pile length and diameter.
Construction Techniques: The method used to install the piles, such as driven piles or drilled shafts, affects the foundation’s effectiveness. Each technique has its advantages and limitations based on the soil conditions and load requirements.

Conclusion

Using a group of piles instead of a single pile in foundation construction offers numerous benefits, including better load distribution, increased load capacity, reduced settlement, enhanced stability against lateral loads, and added redundancy for safety. These advantages make pile groups an essential component in the design and construction of durable, reliable foundations for various structures. By considering technical factors such as pile spacing, pile cap design, soil-pile interaction, and construction techniques, engineers can optimize the performance of pile group foundations and ensure the longevity and stability of the structures they support.

For more insights on foundation engineering and construction techniques, stay tuned to our blog, where we delve into the latest advancements and best practices in the industry.

If you need a spreadsheet for Pile Group calculation please follow this link

Pile Group Analysis For Rigid Pile Cap Spreadsheet

What are Deep Foundation? The Common Types of Deep Foundation

1. What are Deep Foundations?

A deep foundation is needed to carry loads at depth or for functional reasons from a structure through weak compressible soils or fills on to stronger and less compressible soils or rocks.

Deep foundations under the finished ground surface are founded too deeply for their base bearing ability to be affected by surface conditions, generally at depths > 3 m below the finished ground level.

When unsuitable soils are present near the surface, the deep foundation may be used to transfer the load to a deeper, more capable strata at depth.

2. Types of Deep Foundation

The types of deep foundations in general use are as follows:

Basements
Buoyancy rafts (hollow box foundations)
Caissons
Cylinders
Shaft foundations
Pile foundations

a. Basement foundation

They are hollow substructures built to provide space below ground level for the work or storage. The structural design is driven by its practical needs rather than by considerations of the most effective method of resisting external earth and hydrostatic pressures. In open excavations, they are set up in place.

b.Buoyancy Rafts (Hollow Box Foundations)

Buoyancy rafts or hollow box foundations also known as the floating foundations is a type of deep foundation is used in building construction on soft and weak soils.

They are designed to provide a buoyant or semi-buoyant substructure underneath which reduces net loading to the desired low intensity on the soil. Buoyancy rafts can be constructed to be sunk as caissons, and can also be installed in open excavations.

Buoyancy rafts are more expensive than traditional forms of foundations. For that reason, their use is usually restricted to sites that are on silts, soft sands and other alluvial deposits that are very deep, or where loads can be kept concentric. Schemes requiring underground tanks or where it’s economical to incorporate deep basements into the design are common.

c. Caissons Foundations

A caisson is a sort of foundation of the state of the hollow prismatic box, which is worked over the ground level and afterward sunk to the necessary depth as a solitary unit. It is a watertight chamber utilized for establishing foundations submerged as in rivers, lakes, harbors, etc. The caissons are of three types:

Open Caissons: Open caissons are of hollow chambers, open both at the top and the bottom. The lower part of the caisson has a bleeding edge. The caisson is sunk into place by eliminating the soil from within the shaft until the bearing layer is reached. Well foundations are special type of open caissons used in India.
Pneumatic Caissons: Pneumatic caissons are closed at the top but open at the bottom. A pneumatic caisson has a working camber at its bottom in which compressed air is maintained at the required pressure to prevent entry of water into the chamber. So, these type of excavations are done in dry.
Floating Caissons: Floating caissons are open at the top but closed at the bottom. These caissons are developed ashore and afterward shipped to the site and floated to where these are to be finally installed. These are sunk at that spot by filling them with sand, ballast, water or concrete to an evened out bearing surface.

d. Cylinders

These foundations are placed when there is required to place only a single cylindrical unit.

e. Drilled Shaft foundations

These foundations are constructed by drilling a cylindrical hole within a deep excavation and subsequently placing concrete or another prefabricated load-bearing unit in it.

Their length and size can be easily tailored. Drilled shafts can be constructed near existing structures and under low overhead conditions, making them suitable for use in numerous seismic retrofit projects.

It may, however, be difficult to install them under certain conditions such as soils with boulders, soft soil, loose sand, and sand under water.

e. Pile foundations

Pile foundations are relatively long and slender members designed by driving preformed units to the desired foundation level, or by driving or drilling in tubes to the appropriate depth – tubes filled with concrete before or during withdrawal or by drilling unlined or wholly or partially lined boreholes filled with concrete after that.

Design Of Two-Column Bent Pier On Bored Pile Foundation Spreadsheet

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Classification of piles foundation

Classification of pile with respect to load transmission and functional behaviour

End bearing piles (point bearing piles)
Friction piles (cohesion piles )
Combination of friction and cohesion piles

End bearing piles

These piles transfer their load on to a firm stratum located at a considerable depth below the base of the structure and they derive most of their carrying capacity from the penetration resistance of the soil at the toe of the pile (see figure 1.1).

The pile behaves as an ordinary column and should be designed as such. Even in weak soil a pile will not fail by buckling and this effect need only be considered if part of the pile is unsupported, i.e. if it is in either air or water. Load is transmitted to the soil through friction or cohesion. But sometimes, the soil surrounding the pile may adhere to the surface of the pile and causes “Negative Skin Friction” on the pile.

This, sometimes have considerable effect on the capacity of the pile. Negative skin friction is caused by the drainage of the ground water and consolidation of the soil. The founding depth of the pile is influenced by the results of the site investigate on and soil test.

Fig.1.1 – End Bearing Piles

Friction or cohesion piles

Carrying capacity is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile (see fig 1.2).

Fig.1.2 – Friction Pile

Cohesion piles

These piles transmit most of their load to the soil through skin friction. This process of driving such piles close to each other in groups greatly reduces the porosity and compressibility of the soil within and around the groups. Therefore piles of this category are some times called compaction piles.

During the process of driving the pile into the ground, the soil becomes moulded and, as a result loses some of its strength. Therefore the pile is not able to transfer the exact amount of load which it is intended to immediately after it has been driven. Usually, the soil regains some of its strength three to five months after it has been driven.

Friction piles

These piles also transfer their load to the ground through skin friction. The process of driving such piles does not compact the soil appreciably. These types of pile foundations are commonly known as floating pile foundations.

Combination of friction piles and cohesion piles

An extension of the end bearing pile when the bearing stratum is not hard, such as a firm clay. The pile is driven far enough into the lower material to develop adequate frictional resistance.

A farther variation of the end bearing pile is piles with enlarged bearing areas. This is achieved by forcing a bulb of concrete into the soft stratum immediately above the firm layer to give an enlarged base.

A similar effect is produced with bored piles by forming a large cone or bell at the bottom with a special reaming tool. Bored piles which are provided with a bell have a high tensile strength and can be used as tension piles (see fig.1.3)

Fig 1.3. Under-reamed base enlargement to a bore-and-cast-in-situ pile

Classification of pile with respect to type of material

Timber
Concrete
Steel
Composite piles

Timber piles

Used from earliest record time and still used for permanent works in regions where timber is plentiful. Timber is most suitable for long cohesion piling and piling beneath embankments. The timber should be in a good condition and should not have been attacked by insects.

For timber piles of length less than 14 meters, the diameter of the tip should be greater than 150 mm. If the length is greater than 18 meters a tip with a diameter of 125 mm is acceptable. It is essential that the timber is driven in the right direction and should not be driven into firm ground.

As this can easily damage the pile. Keeping the timber below the ground water level will protect the timber against decay and putrefaction. To protect and strengthen the tip of the pile, timber piles can be provided with toe cover. Pressure creosoting is the usual method of protecting timber piles.

Concrete pile

Precast concrete Piles or Pre fabricated concrete piles : Usually of square (see fig 1.4 b), triangle, circle or octagonal section, they are produced in short length in one metre intervals between 3 and 13 meters. They are pre-caste so that they can be easily connected together in order to reach to the required length (fig 1.4 a) .

This will not decrease the design load capacity. Reinforcement is necessary within the pile to help withstand both handling and driving stresses. Pre stressed concrete piles are also used and are becoming more popular than the ordinary pre cast as less reinforcement is require.

Fig 1.4a – Concrete pile connecting detail

Fig 1.4b – Squared pre-cast concert pile

The Hercules type of pile joint (Figure 1.5) is easily and accurately cast into the pile and is quickly and safely joined on site. They are made to accurate dimensional tolerances from high grade steels.

Fig 1.5 – Hercules type of pile joint

Driven and cast in place Concrete piles

Two of the main types used in the UK are: West’s shell pile : Pre cast, reinforced concrete tubes, about 1 m long, are threaded on to a steel mandrel and driven into the ground after a concrete shoe has been placed at the front of the shells. Once the shells have been driven to specified depth the mandrel is withdrawn and reinforced concrete inserted in the core. Diameters vary from 325 to 600 mm.

Franki Pile: A steel tube is erected vertically over the place where the pile is to be driven, and about a metre depth of gravel is placed at the end of the tube. A drop hammer, 1500 to 4000kg mass, compacts the aggregate into a solid plug which then penetrates the soil and takes the steel tube down with it. When the required depth has been achieved the tube is raised slightly and the aggregate broken out.

Dry concrete is now added and hammered until a bulb is formed. Reinforcement is placed in position and more dry concrete is placed and rammed until the pile top comes up to ground level.

Steel piles

Steel piles: steel/ Iron piles are suitable for handling and driving in long lengths. Their relatively small cross-sectional area combined with their high strength makes penetration easier in firm soil. They can be easily cut off or joined by welding. If the pile is driven into a soil with low pH value, then there is a risk of corrosion, but risk of corrosion is not as great as one might think. Although tar coating or cathodic protection can be employed in permanent works.

Fig 1.6 – Steel piles cross-sections

It is common to allow for an amount of corrosion in design by simply over dimensioning the cross-sectional area of the steel pile. In this way the corrosion process can be prolonged up to 50 years. Normally the speed of corrosion is 0.2-0.5 mm/year and, in design, this value can be taken as 1mm/year.

Composite piles

Combination of different materials in the same of pile. As indicated earlier, part of a timber pile which is installed above ground water could be vulnerable to insect attack and decay. To avoid this, concrete or steel pile is used above the ground water level, whilst wood pile is installed under the ground water level (see figure 1.7).

Fig 1.7 – Protecting timber piles from decay:
a) by pre-cast concrete upper section above water level.
b) by extending pile cap below water level

Classification of pile with respect to effect on the soil

A simplified division into driven or bored piles is often employed.

Driven piles

Driven piles are considered to be displacement piles. In the process of driving the pile into the ground, soil is moved radially as the pile shaft enters the ground. There may also be a component of movement of the soil in the vertical direction.

Fig 1.8 – Driven Piles

Bored piles

Bored piles(Replacement piles) are generally considered to be non-displacement piles a void is formed by boring or excavation before piles is produced. Piles can be produced by casting concrete in the void.

Some soils such as stiff clays are particularly amenable to the formation of piles in this way, since the bore hole walls do not requires temporary support except cloth to the ground surface. In unstable ground, such as gravel the ground requires temporary support from casing or bentonite slurry. Alternatively the casing may be permanent, but driven into a hole which is bored as casing is advanced.

A different technique, which is still essentially non-displacement, is to intrude, a grout or a concrete from an auger which is rotated into the granular soil, and hence produced a grouted column of soil.

There are three non-displacement methods: bored cast- in – place piles, particularly pre-formed piles and grout or concrete intruded piles.

The following are replacement piles:

Augered
Cable percussion drilling
Large-diameter under-reamed
Types incorporating pre caste concrete unite
Drilled-in tubes
Mini piles

History Of Pile Foundation

Pile foundations have been used as load carrying and load transferring systems for many years.

In the early days of civilisation, from the communication, defence or strategic point of view villages and towns were situated near to rivers and lakes. It was therefore important to strengthen the bearing ground with some form of piling.

Timber piles were driven in to the ground by hand or holes were dug and filled with sand and stones.

In 1740 Christoffoer Polhem invented pile driving equipment which resembled to days pile driving mechanism. Steel piles have been used since 1800 and concrete piles since about 1900.

The industrial revolution brought about important changes to pile driving system through the invention of steam and diesel driven machines.

More recently, the growing need for housing and construction has forced authorities and development agencies to exploit lands with poor soil characteristics. This has led to the development and improved piles and pile driving systems. Today there are many advanced techniques of pile installation.