Pile Foundation - I

PILE FOUNDATION

Introduction

A pile is basically a long cylinder of a strong material such as concrete that is pushed into the ground to act as a steady support for structures built on top of it.

Pile foundations are used in the following situations:

1. 1   When there is a layer of weak soil at the surface. This layer cannot support the weight of the building, so the loads of the building have to bypass this layer and be transferred to the layer of stronger soil or rock that is below the weak layer.
2.      When a building has very heavy, concentrated loads, such as in a high rise structure, bridge, or water tank.

Pile foundation is required when the soil bearing capacity is not sufficient for the structure to withstand. Pile foundations are capable of taking higher loads than spread footings.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 15 m.

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 developed 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 multistory 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.

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 multistory 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.

There are two types of pile foundations, each of which works in its own way.

End Bearing Piles

In end bearing piles, the bottom end of the pile rests on a layer of especially strong soil or rock. The load of the building is transferred through the pile onto the strong layer. In a sense, this pile acts like a column. The key principle is that the bottom end rests on the surface which is the intersection of a weak and strong layer. The load therefore bypasses the weak layer and is safely transferred to the strong layer.

Friction Piles

Friction piles work on a different principle. The pile transfers the load of the building to the soil across the full height of the pile, by friction. In other words, the entire surface of the pile, which is cylindrical in shape, works to transfer the forces to the soil.

To visualize how this works, imagine you are pushing a solid metal rod of say 4 mm diameter into a tub of frozen ice cream. Once you have pushed it in, it is strong enough to support some load. The greater the embedment depth in the ice cream, the more load it can support. This is very similar to how a friction pile works. In a friction pile, the amount of load a pile can support is directly proportionate to its length.

What are piles made of?

Piles can be made of wood, concrete, or steel. 

In traditional construction, wooden piles were used to support buildings in areas with weak soil. Wood piles are still used to make jetties. For this one needs trees with exceptionally straight trunks. The pile length is limited to the length of a single tree, about 20 m, since one cannot join together two tree trunks. The entire city of Venice in Italy is famous for being built on wooden piles over the sea water.

Cross sections of various pile foundations

Concrete piles are precast, that is, made at ground level, and then driven into the ground by hammering - more on that later. Steel H-piles can also be driven into the ground. These can take very heavy loads, and save time during construction, as the pile casting process is eliminated. No protective coating is given to the steel, as during driving, this would be scraped away by the soil. In areas with corrosive soil, concrete piles should be used.

How piles are used

As pile foundations carry a lot of load, they must be designed very carefully. A good engineer will study the soil the piles are placed in to ensure that the soil is not overloaded beyond its bearing capacity.

Every pile has a zone of influence on the soil around it. Care must be taken to space the piles far enough apart so that loads are distributed evenly over the entire bulb of soil that carries them, and not concentrated into a few areas.

Engineers will usually group a few piles together, and top them with a pile cap. A pile cap is a very thick cap of concrete that extends over a small group of piles, and serves as a base on which a column can be constructed. The load of this column is then distributed to all the piles in the group.

How piles are constructed

Engineers will usually group a few piles together, and top them with a pile cap. A pile cap is a very thick cap of concrete that extends over a small group of piles, and serves as a base on which a column can be constructed. The load of this column is then distributed to all the piles in the group.

Piles are first cast at ground level and then hammered or driven into the ground using a pile driver. This is a machine that holds the pile perfectly vertical, and then hammers it into the ground blow by blow. Each blow is is struck by lifting a heavy weight and dropping it on the top of the pile - the pile is temporarily covered with a steel cap to prevent it from disintegrating. The pile driver thus performs two functions - first, it acts as a crane, and lifts the pile from a horizontal position on the ground and rotates it into the correct vertical position, and second, it hammers the pile down into the ground.

Piles should be hammered into the ground till refusal, at which point they cannot be driven any further into the soil.

Special Piles

Pile driving is very noisy and causes massive vibrations through the soil. For this reason, it is sometimes difficult to use them in sensitive locations. For example, if an operational hospital or science lab is to be extended, driving piles would cause unwanted disturbance. Their use is also restricted in residential areas in many countries. The vibrations could also cause structural damage to older buildings that are close by. In such situations it is possible to use micro piling or helical piling, neither of which relies on hammering.

Micro piles or mini piles are small piles that are constructed in the following way:
Step 1: a hole a little larger than the pile diameter and the full length of the pile is dug into the ground using an apparatus like a soil boring machine.
Step 2: a precast concrete pile is lowered or pushed into the hole.
Step 3: a concrete grout is poured into the gap between the pile and the earth.

Helical piles are steel tubes that have helical (spiral) blades attached to them. These can be drilled into the ground, meaning that the pile acts as a giant drill bit, and is rotated and pushed into the ground from above, much like a screw drills into wood. Once the steel pile is driven into the ground, a pile cap is poured on top of the pile to prepare it for the construction above.

Pile foundation - Static Analysis

HOW TO CALCULATE PILE LOAD CAPACITY? (STATIC ANALYSIS)

The ultimate bearing capacity of a pile is the maximum load which it can carry without failure or excessive settlement of the ground.

The bearing capacity of a pile depends primarily on 3 factors as given below,

1. Type of soil through which pile is embedded
2. Method of pile installation
3. Pile dimension (cross section & length of pile)

While calculating pile load capacity for cast in situ concrete piles, using static analysis, we need to use soil shear strength parameter and dimension of pile.

LOAD CARRYING CAPACITY OF PILE USING STATIC ANALYSIS

The pile transfers the load into the soil in two ways. Firstly, through the tip-in compression, termed as “end-bearing” or “point-bearing”; secondly, by shear along the surface termed as “skin friction”.

LOAD CARRYING CAPACITY OF CAST IN-SITU PILES IN COHESIVE SOIL

The ultimate load carrying capacity (Q_u) of pile in cohesive soils is given by the formula given below, where the first term represents the end bearing resistance (Q_b) and the second term gives the skin friction resistance (Q_s).

Where,

Q_u = Ultimate load capacity, kN

A_p = Cross-sectional area of pile tip, in m²

N_c = Bearing capacity factor, may be taken as 9

α_i = Adhesion factor for the ith layer depending on the consistency of soil. It depends upon the undrained shear strength of soil and may be obtained from the figure given below.

Variation of alpha with cohesion

c_i = Average cohesion for the ith layer, in kN/m²

A_si = Surface area of pile shaft in the ith layer, in m²

A minimum factor of safety of 2.5 is used to arrive at the safe pile load capacity (Q_safe) from ultimate load capacity (Q_u).

Q_safe = Q_u/2.5

LOAD CARRYING CAPACITY OF CAST IN-SITU PILES IN COHESION LESS SOIL

The ultimate load carrying capacity of pile, “Q_u”, consists of two parts. One part is due to friction, called skin friction or shaft friction or side shear denoted as “Q_s” and the other is due to end bearing at the base or tip of the pile toe, “Q_b”.

The equation given below is used to calculate the ultimate load carrying capacity of pile.

Where,

A_p = cross-sectional area of pile base, m²

D = diameter of pile shaft, m

γ = effective unit weight of the soil at pile tip, kN/m³

N_γ= bearing capacity factor

N_q = bearing capacity factor

Φ = Angle of internal friction at pile tip

P_D = Effective overburden pressure at pile tip, in kN/m²

K_i  = Coefficient of earth pressure applicable for the ith layer

P_Di = Effective overburden pressure for the ith layer, in kN/m²

δ_i = Angle of wall friction between pile and soil for the ith layer

A_si = Surface area of pile shaft in the ith layer, in m²

The first term is the expression for the end bearing capacity of pile (Q_b) and the second term is the expression for the skin friction capacity of pile (Q_s).

A minimum factor of safety of 2.5 is used to arrive at the safe pile capacity (Q_safe) from ultimate load capacity (Q_u).

Q_safe = Q_u / 2.5

IMPORTANT NOTES TO REMEMBER

• The value of bearing capacity factor N_q is obtained from the figure given below.

bearing factor value

• The value of bearing capacity factor N_γ is computed using the equation given below.

• For driven piles in loose to dense sand with φ varying between 30⁰to 40⁰ , k_i values in the range of 1 to 1.5 may be used.
• δ, the angle of wall friction may be taken equal to the friction angle of the soil around the pile stem.
• The maximum effective overburden at the pile base should correspond to the critical depth, which may be taken as 15 times the diameter of the pile shaft for φ ≤ 30⁰and increasing to 20 times for φ ≥ 40⁰
• For piles passing through cohesive strata and terminating in a granular stratum, a penetration of at least twice the diameter of the pile shaft should be given into the granular stratum.

Pile foundation - Classification

HOW TO CLASSIFY PILES BASED ON CONSTRUCTION MATERIALS?

A pile is a slender, structural member installed in the ground to transfer the structural loads to soils at some significant depth below the base of the structure. Structural loads include axial loads, lateral loads, and moments. Another term commonly used in practice for pile foundations is deep foundations. Structures that cannot be supported economically on shallow foundations are normally supported by pile foundations.

On the basis of materials used for the construction of pile foundation, it can be classified in to following 5 types.

1. Concrete piles
2. Steel piles
3. Timber piles
4. Plastic piles
5. Composite piles

1. CONCRETE PILES

There are several types of concrete piles that are commonly used. These include cast-in-place concrete piles, precast concrete piles, drilled shafts, and barrette piles. Cast-in-place concrete piles are formed by driving a cylindrical steel shell into the ground to the desired depth and then filling the cavity of the shell with fluid concrete. They are called displacement piles. The steel shell is for construction convenience and does not contribute to the load transfer capacity of the pile. Its purpose is to open a hole in the ground and keep it open to facilitate the construction of the concrete pile. Plain concrete is used when the structural load is only compressive. If moments and lateral loads are to be transferred, then a steel reinforcement cage is used in the upper part of the pile.

Precast concrete piles usually have square or circular or octagonal cross sections and are fabricated in a construction yard or a factory from reinforced or prestressed concrete. They are preferred when the pile length is known in advance. The disadvantages of precast piles are problems in transporting long piles, cutting, and lengthening. A very popular type of precast concrete pile is the Raymond cylindrical prestressed pile. This pile comes in sections, and lengths up to 70 m can be obtained by stacking the sections.

Typical design loads are greater than 2 MN.

Micropiles (also called minipiles, pin piles, needle piles, or root piles) are small-diameter (50 mm to 340 mm) pipe piles (pushed or driven) or grouted (jet or post or pressure) piles.

They are particularly useful for

• Sites with low headroom,
• Congested areas,
• Sites with restricted access, and
• Foundation repair or strengthening.

2. STEEL PILES

Steel piles come in various shapes and sizes and include cylindrical, tapered, and H-piles. Steel H-piles are rolled steel sections. They are non-displacement piles. Steel pipe piles are seamless pipes that can be welded to yield lengths up to 70 m. They are usually driven with open ends into the soil. A conical tip is used where the piles have to penetrate boulders and rocks. To increase the load capacity of steel pipe piles, the soil plug is excavated and replaced by concrete. These piles are called concrete-filled steel piles. The soil plug may adhere to the pile surface and moves down with it during driving. This is called plugging.

3. TIMBER PILES

Timber piles have been used since ancient times. The lengths of timber piles depend on the types of trees used to harvest the piles, but common lengths are about 12 m. Longer lengths can be obtained by splicing several piles. Timber piles are susceptible to termites, marine organisms, and rot within zones exposed to seasonal changes. Timber piles are displacement piles.

4. PLASTIC PILES

Plastic piles comprise a variety of composite materials that include polymer composites, PVC, and recycled materials. These piles are used in special applications such as in marine environments and within soil zones exposed to seasonal changes.

5. COMPOSITES

Concrete, steel, and timber can be combined to form a composite pile. For example, the portion of a timber pile above groundwater level that is likely to suffer from decay due to termites or rot may be replaced by concrete. Similarly, the portion of a steel pile within a corrosive environment can be covered with concrete or other protective materials.