Page 39 - Unit2.docx

methods are presented. These two methods are refereed as geotechnical and
dynamic methods. This section too has worked examples showing the
application of the formulae used in predicting the bearing capacity of piles made
of different types of materials.

Learning outcome

When students complete this section, they will be able to

• understand the theoretical back ground of the formulae used in pile
design
• carry out calculation and be able to predict design bearing
capacity of single piles
• appreciate results calculated by means of different formulae

4.2 The behaviour of piles under load

Piles are designed that calculations and prediction of carrying capacity is based
on the application of ultimate axial load in the particular soil conditions at the site
at relatively short time after installation.

This ultimate load capacity can be determined by either:

• the use of empirical formula to predict capacity from soil
properties determined by testing, or
© It's Engg. Life
• load test on piles at the site

Fig.4-2, When pile is subjected to gradually increasing compressive load in
maintained load stages, initially the pile-soil system behaves in a linear-elastic
manner up to point A on the settlement-load diagram and if the load is realised at
any stage up to this point the pile head rebound to its original level. When the
♦
load is increase beyond point A there is yielding at, or close to, the pile-soil
♥
interface and slippage occurs until point B is reached, when the maximum skin
friction on the pile shaft will have been mobilised. If the load is realised at this
♣
stage the pile head will rebound to point C , the amount of permanent settlement
being the distance OC. When the stage of full mobilisation of the base resistance
is reached ( point D), the pile plunges downwards with out any farther increase of
load, or small increases in load producing large settlements.

♦
No end-bearing is mobilised up to this point. The whole of the load is carried by the skin

friction on the pile shaft see figure 4-2 I)

♥
The pile shaft is carrying its maximum skin friction and the pile toe will be carrying some load

♣ At this point there is no further increase in the load transferred in skin friction but the base
load will have reached its maximum value.