Geotechnical Engineering

DHARM

662 GEOTECHNICAL ENGINEERING

ratio of bulb size to the pile shaft size may be 2 to 3; usually a value of 2.5 is used. The bearing

capacity of the pile increases because of the increased base area; the more the number of

under-reams the more the capacity. Field tests indicate that an under-reamed pile is more

economical than a straight bored pile for a given load.

The load capacity of an under-reamed pile may be found in much the same way as for

driven piles [Murthy, 1977 and IS: 2911 (Part I)- 1974]:

Qup = Qeb + Qsf = qb. Ab + fs. As ...(Eq. 16.16)

where qb = unit point-bearing capacity of a bulb

fs = unit skin-friction

Ab = area of section of the bulb

As = surface area of the embedded pile shaft.

This is for a single under-reamed pile.

For a multi-under-reamed pile,

Qup = qb. Ab + fsAs + fAss

where qb = unit point bearing resistance,

fs = unit skin-friction,

fs = unit frictional resistance between soil and soil,

Ab = area of section of the lowest bulb,

As = surface area of the embedded portion of pile above the top bulb, and

As = surface area of a cylinder of diameter bu and height equal to the distance between

the centres of the extreme bulbs.

This is based on the assumption that the soil between the bulbs might move together
with the bulbs at ultimate load. Many factors are involved in the type of failure that may occur
and this is only an intelligent guess.

16.5.2Dynamic Analysis

Dynamic analysis aims at establishing a relationship between pile capacity and the resistance

offered to driving with a hammer. This is appropriate for piles penetrating soils such as sands

and hard clays that will not develop pore water pressures during installation. In saturated

fine-grained soils, high pore pressures develop due to vibration caused by driving; in such

cases, the predicted capacities from dynamic analysis will be different from the value attained

after the dissipation of excess pore pressures.

The loading and failure produced by driving with a hammer occurs in a fraction of a

second, whereas in the structure the load is applied over a fairly long period. A fixed relation

between dynamic and long-term capacity can exist only in a soil for which shear strength is

independent of the rate of loading. This is nearly true in the case of dry sand and also in

medium dense wet sand with coarse grains. In clays and in loose fine-grained saturated soils,

the strength depends upon the rate of shear; in such soils, dynamic analysis of pile capacity

cannot be valid.