A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
Load tests are conducted to verify the design assumptions and parameters
such as pile friction in soils and sock socket capacity. There are various
failure criteria in current construction industry to determine ultimate load
resistance of piles in pile load test. For instance in 90% criterion of Brinch
Hansen, it is based on the laboratory measured stress-strain relations of
soils and a point is identified in which soil fails. This essentially aims at
looking for the ultimate bearing capacity and hence the ultimate loads. In
fact, this is not intended originally for piles.
For some failure criteria, it does not target at finding out the ultimate pile
capacity. Instead, it looks for the process for onset of soil yielding at the
base of pile toe an allows for controlled displacement. For example, one of
these criteria is reproduced as follows:
Criterion for maximum movement = (PL/AE + d/120 + 4) mm
where P is the load, L is pile length, A is the area of pile and d is pile
diameter.
This failure criterion was developed based on small diameter driven piles.
The term PL/AE refers to elastic shortening of piles. For end-bearing piles,
this term is acceptable for usage. However, for friction piles this may not
truly simulate the actual shortenings of piles because frictional forces along
the pile also come into play. The term (d/120 + 4) represents the amount of
soil movement which triggers the yielding of soil beneath pile toe.
3. Can dynamic pile test (PDA test) be used to replace pile load test
for testing driven pile capacity? (PDA2)
The PDA system consists of two strain transducers and two
accelerometers attached to opposite sides of the pile to measure the strain
and acceleration in the pile. The force is computed by multiplying the
measured signals from a pair of strain transducers attached near the top of
the pile by the pile area and modulus. The velocity measurement is
obtained by integrating signals from a pair of accelerometers also attached
near the top of the pile.
PDA test is commonly used for the following purposes:
(i) Evaluation of driven pile capacity
Soil resistance along the shaft and at the pile toe generates wave
reflection that travel to the top of the driven piles. The time the
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