Large diameter bored pile with enlarged base

In stiff cohesive soils, bored piles can possibly be “under-reamed”: the belling technique may prove economical to take advantage of an enlarged pile base with increased end bearing capacity. Large diameter bored piles with an enlarged base or underream are also known as ‘bell out’ piles.

Construction sequence

The shape and the dimension of the base widening depend on the type of belling tool used: standard reamer or bucket reamer ‘bottom hinge’ which has a bell shape (a).
Franki uses the 'top hinge' underreaming model (belling bucket) or standard reamer (b), usually cut at 45 or 60 degree angle, with the maximum diameter of the underream being not more than three times the diameter of the shaft, see figure on the right.

Construction chart

Fields of application

• Foundation for heavily loaded columns 

• Direct contact pile – column 

• Withstand very high loads or horizontal moments

• Resist high traction values 

• The nature of the soil at design depth should allow for the formation of an enlarged pile base

Environmental impact

• Vibration-free installation

• Low noise pollution

• Removal of spoil (pile with soil extraction) and of drilling and/or support fluid from the job site is required

• The water table must be located at least 1 m below the working level

• Workspace surface minimum requirement of 17 m: minimal 15 m from the axis of the wall and 2 m in front of the wall

• A stable, dry and flat work platform is required

Technical specifications

• Shaft diameter from 1.080 to 1.500 mm

• Enlarged base diameter (‘’bell’ diameters) from 1.700 to 2.700 mm

• Concreting is carried out by means of the tremie pipe technique to avoid segregation of the concrete.

• Full length reinforcement of the pile is possible

• Execution possible with injection along the pile shaft

• Installation of the pile up to 40 m deep

• Potential construction of guide walls

• Create piles with a cut off level beneath the working platform level (non-concreted length possible)

Related projects for this technique

Member of logo