Construction Techniques for Cast-in-Situ Reinforced Concrete

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A pile foundation is considered for construction whenever the load of a structure is large, and a thick and weak soil layer is present below. It is a type of deep foundation that transfers the load of the superstructure to a stiff layer through the weak soil strata. 

Piles can either be constructed at the project site or pre-casted and transferred to the construction site. Cast-in-situ pile construction involves drilling a borehole, placement of reinforcement cage, and placement of concrete.

Several pile construction techniques, like rotary drilling with kelly (a type of drilling pipe), twin rotary drilling, and auger cast-in-situ are available for the construction of cast-in-situ concrete piles. These piling techniques are different in the drilling process that is followed, the method of creating supporting excavation, and the method of concrete placement. 

The selection of the piling technique is dependent on soil conditions, availability of equipment and skilled labor, technical specification relating to the site, and the overall cost.

Construction Techniques for Cast-in Situ Reinforced Concrete Pile

Rotary Drilling with Kelly, Borehole Supported by Casing

This method uses rotary drilling to excavate a borehole, and a casing supports its sides. 

The casing is a steel tube with a diameter slightly greater than the pile diameter, driven into the ground using a rotary drive before the drilling operation begins. A casing oscillator can be used for larger pile diameters and greater depths. 

This technique is suitable for all types of soils and constricted construction sites. One of the outstanding advantages is that it doesn’t produce vibrations during drilling. 

The diameter of the pile ranges from 60 cm to 180 cm. The depth of the pile is 40 m, with the possibility of reaching greater depths.

Rotary Drilling with Kelly, Borehole Supported by Hydrostatic Pressure

In this technique, rotary drilling is used to excavate the borehole, but bentonite solution or polymer slurry is employed to support the wall of the borehole. 

A short-length casing is used at the beginning of the work to guide the excavator, and a bucket extracts the excavation spoil.

This construction method is suitable for all types of soils, large pile diameters and depths, and vertical piles. 

The drilling operation, again, is free from vibrations, and the depth of the pile could be around 40 m or deeper, and its diameter ranges from 40 cm to 240 cm. Figure-2 shows the details of the construction procedure.

Rotary Drilling Using Twin Rotary Head

Counter-rotating twin drives enable contractors to carry out continuous flight auger drilling and install casing simultaneously. It is appropriate for all types of soil on constricted sites and can be installed against existing walls.

The construction machinery generates no vibration, which is a desirable feature, especially in urban areas. The depth of the pile reaches around 15 m, and its diameter is between 63 cm to 88 cm. 

In this technique, the concrete pouring process is carried out while the auger is lifted. So, the reinforcement cage is placed after the completion of concrete placement. That is why a vibration or driven force is required to place the reinforcement cage.

Auger Cast-In-Situ Pile

As seen from the figure below, continuous auger is used to drill the borehole, lift up drilled soil, and pour concrete while the auger is moved up. As a result, the borehole does not need casing or hydrostatic pressure to prevent damage to the borehole wall.

Auger cast-in-situ pile is suitable for all kinds of soil and construction sites with limited space. It is vibration free, and a depth of around 18 m can is easily accessible. The diameter of the auger cast-in-situ pile ranges from 40 cm to 100 cm.

Displacement Pile Construction

Displacement pile is one of the unique pile construction methods, which is suitable for contaminated soil to prevent disposal of drill spoil. It is also suitable for soft soil and constricted construction sites.

Special features of displacement piles are high bearing capacity due to compaction of skin soil, avoiding drilling spoil, high performance, and reduced concrete utilization compared to auger cast-in-situ piles.

The drilling diameter is 42 cm, 51 cm, and 61 cm, and the pile depth is around 34 m.  

Grab Construction, Borehole Supported by Casing

A crawler crane and casing oscillator are used in grab pile construction. Grab construction is appropriate for certain types of soil, like sand and gravel, with high demand for casing technology. It is also suitable for locations where it is economically feasible to use a chisel to break bedrock and boulders.

Grab construction needs a minimum distance from existing structures since it does not affect its surrounding. The diameter of the pile ranges from 62 cm to 200 cm, and its depth is around 50 m.  

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