Pile Cap in a bridge – Definition – Types and Construction Procedures

 Pile cap is a solid foundation that carries the load from the pier and transfers equally on the underneath group of piles. A pile cap is just like a thick concrete mat and the rebar dowels are embedded into it along with the dowels of the pier.

Definition

A thick concrete mat that is constructed above group of piles that carries the load from the pier and transfer it to the underneath group of piles is called a pile cap.

Need of a Pile Cap - Reason

As you’re aware of the fact that the pile foundation is a type of deep foundation that is used in areas where the bearing capacity of soil is not sufficient or the superimposed load is greater. Pile cap in most of the cases is covered/buried under the soil and thus in most cases it is applied with bitumen to protect the concrete.

Whether you’re constructing a pile foundation for a building or for a bridge, you have to provide a pile cap so that the load can be equally distributed among the underneath piles. This pile cap is the only thing that joins together group of piles and make them one monolithic structure.

Did you know ? A deep foundation is necessary in areas of low bearing capacity because the top soil doesn’t have adequate strength to bear the load. The mostly used deep foundation is the pile foundation.

Did you know? Another area where you can find a pile cap sort of arrangement is that of piled raft foundation. That’s a foundation when a concrete slab or raft of a house rests on a group of piles.

Some important Points for a Pile Cap

·         A pile Cap should be at a depth that can provide sufficient resistance against flexural and shear stresses. A pile cap with greater depth has a greater rigidity that helps in its even spreading of load.

·         Sometimes during casting or boring of a pile, a deviation is there from the verticality of the pile and in that case the pile cap design must have to accommodate such deviation.

·         A pile cap must overhang the outer piles by sufficient distance i.e. approximately 100 to 150 mm.

·         A pile cap design must also consider the underground water table, frost attack, and shrinkage or swelling of the ground (in case of clayey soils).

·         It has to be rigid so as to transmit the load from the wall or columns equally to the group of piles.

Types of Pile Cap

 Have you ever heard of terms like a rigid pile cap or a flexible pile cap? Yes these are actually the two basic types of pile cap depending on its rigidity in behaviour.  

In case of rigid pile cap, the piles at the edges are subjected to higher axial and bending loads i.e. sometimes the piles on edges may have to bear two or three times more of the load at the central pile. Keeping in view the individual behaviour of the pile you will know that the pile at the centre has to bear greater axial load as compared to the edges. But due to rigid or stiff behaviour there’s no local settlement of the centre pile and instead the piles at the edges will take up higher fraction of the total load.

But in case of flexible pile cap, individual piles bear load unequally and thus there is non-uniform distribution of loads among piles. The pile near to load will take up more load as compared to the one that is located far away.

Design of Pile Cap

For design purpose, the pile cap is considered using a truss system principle i.e. strut and tie model. The design and type of shape of pile cap depends on the number of piles underneath. The detail of the pile cap design includes the size, depth, and spacing of pile cap. The geometry also depends on the number of pile caps.    

It can be designed just like a normal footing on the soil but instead of uniform reaction from the soil, the reactions are concentrated either point loads or distributed.

The shape of the pile cap can be square in case of 4 piles but for 3 piles you can make use a trapezoidal shape of the pile cap. The thickness of the pile cap has to be decided so that it can match the anchorage-bond length requirements of pier and pile dowels and their punching shear requirements.

Assumptions

·         Pile cap is perfectly rigid.

·         Pile heads are hinged to the pile cap and hence no bending moment is transmitted to piles from pile caps.

·         Since the piles are short and elastic columns, the deformations and stress distribution are planer

Reinforcement Details

The main reinforcement of the pile cap is designed form bending moment and it is provided at the base or the bottom. Sometimes negative reinforcement at the top is also provided when the depth of the pile cap is not enough. In case the conditions require, the pile cap needs to have a cage of reinforcement to avoid bursting due to high principal tension. A minimum clear cover of 75 mm is required for the pile cap that has to get contact with earth.

Construction Procedure for Pile Cap

Excavation

The first step in the construction procedure for pile cap is to excavate the soil upto the depth and dimensions required as per design. By using excavators the pile cap is excavated but only after the concrete of pile has gained enough strength minimum of 7 days. In case the pile cap design depth is more than 1.5 meters intermediary benches or steps shall be provided to avoid collapse and failure of slope.

Pile chipping

After excavating the pile cap upto desired depth, the pile caps or head are visible. The next step is pile chipping upto the pile cut off level. The depth of pile cut off level is specified in the drawings. However, it must be ensured that there is no unsound or cracked concrete of the pile in the pile cap. In such cases the concrete shall be removed and repaired afterwards. 

This process is called concrete cutting or concrete chipping. The purpose of cutting pile head is to avoid unsound and slushy concrete that has been pumped out during tremie method of concrete piling.

Preparation for Blinding Concrete

The next step is to provide 75 mm blinding or levelling concrete. The purpose of this concrete is to achieve a levelled surface and avoid undulations. Another purpose is to make a sound ground for the rebar to be fixed.

But before pouring blinding concrete, there are some preparations that has to be made. The ground has to be levelled and compacted by vibratory hand compactor. In order to evaluate the field compaction, field density test has to be performed in comparison to the maximum dry density. Afterwards, a polythene sheet is laid over the formed layer and the blinding concrete is poured.

After pouring blinding concrete, water proofing treatment is carried out by first painting the surface of blinding concrete with primer. After primer, the area is covered with adhesive water proofing membrane. If the membrane is not available a bitumen coat can do the purpose.

Reinforcement for Pile Cap

The next step is to fix rebar of pile cap. This has to be done as per the approved shop drawings and beforehand a bar-bending has to be prepared showing the cut off length of rebar and shape. Only approved reinforcement bar is to be used and clear cover must be ensured to avoid any corrosion. In case of mesh reinforcement, rebar chair and rebar spacers have to be used to ensure proper shape of the pile cap.  

Formwork

After detailed inspection of pilecap, formwork will be fixed as per of approved quality. Usually in areas where pile cap will be backfilled, smooth finish is provided to close the air gaps in the concrete. The formwork has to be cleaned and de-shuttering oil is also applied on the formwork to facilitate removal.

Concreting & Curing

After all the arrangements have been in place the concrete pouring will commence. Internal vibrators are used for compaction of concrete and it must follow the minimum height of fall requirement.
Water Proofing of Pile Cap Concrete
The next step is water proofing of pile cap and then backfilling at the end. 
 
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