Foundation, Concrete and Earthquake Engineering

What are the Spacing Requirements of Under Reamed Piles?

Under reamed piles are cast-in-situ piles of concrete which have enlargement like bulb near base of pile. These piles are the safest solution for soils that are susceptible to suffer differential settlement or for buildings those are on filler-up soils or for expansive soils like black cotton soil.

The enlarged bulb not only increases the bearing but also provide anchorage of the foundation to eliminate the harm from expansion of soil. Many researches are carried out in India and all over the world to invent the effective foundation solution for black cotton soil. These researches show that the under-reamed piles are the safest and ideal solution for this type of soil.

Under reaming operation for under reamed pilesIn this post we will discuss the spacing of piles that are ideal to have maximum efficiency from under-reamed piles. In this regard, this has to conclude that spacing of piles depends on:

1.Type of piles

2.Ground nature

3.The way in which loads are supposed to transfer from piles to ground

Normally spacing between under-reamed piles do not kept closer than 2 times the diameter of piles (Du). The spacing is measured form centers of piles. Where different pile diameter is used in construction Du is calculated from the average diameter of adjacent piles. In especial conditions where the spacing have to be closer than the above defined value, the spacing of 1.25X Du can be used with a 10% reduction in load bearing capacity of pile. The maximum limit is provided by economy i.e. if spacing becomes 2.5X Du the resulting capping beam is very heavy.


  1. The correct terminology for the cast-in-place or (cast in-situ) deep foundation system discussed here is "Caissons" or "Drilled Piers". Not Piles. Bell-shaped caissons are rarely used anymore, but some engieers (the older ones, typically) still specify them.

    Piles are mechanically driven supports, for which several different material types can be used, depending on location and labor force. Piles are typically constructed from either of these materials, Regularly Reinf. Concrete, Steel (W & S-Sections or H-Piles and Timber.

    The Soil strata and magnitude of the anticipated loads (Gravity or Uplift) dictate the spacing for either of these types of deep foundation systems.

    SEoT Engineering, PLLC

  2. It is a way to calculate the resistance of pile group. That sumarize the influence of pile spacing, reduction in the capacity per a pile due to lesser spacing, various types or pile diameters. It is generaly an option to drive longer piles to achieve more of capacity. it is also wise to arrange piles according to the above building columns location and distribution of loads (if possible because of many load combinations). Generaly pile locations need to match columns to avoid punching. Pile cap dimensions are selected to be able to satisfy shear requirements for a single pile of for a group of piles, as well. It is wise to provide sufficiently thick cap beam-rigid to make even distribution. Embedment of piles in pile cap may be fixity but not always. Piles could be driven on an angle to save spacing and contribute to resist horizontal loads.

  3. Actually spacing of the cast in-situ pile depends on nature of soil . However for Clayey soil Spacing should not less than 3 times diameter of pile and for Sandy soil 2 times diameter of pile