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 reflections arrive at the pile top is a function of their locations along the pile length. The measured force and velocity at the pile top thus provide necessary information to estimate soil resistance and its
distribution.

(ii) Measurement of pile stress during driving
Compressive stress at pile top is measured directly from strain transducers.

(iii) Measurement of hammer energy delivered to the piles
The hammer energy delivered to the pile is directly computed as the work done on the pile from the integral of force times displacement( ?Fdu ) and this can be calculated as force times velocity integrated over time (?Fvdt ).

(iv) Determine if pile damage has occurred.
Pile integrity can be checked by inspecting the measurements for early tension returns (caused by pile damage) before the reflection from the pile toe. The lack of such reflections assures a pile with no defects.

There is a growing trend of using PDA test to check driven pile capacity instead of traditional static load test which is more expensive and time consuming. Some engineers may have reservation on the use of dynamic formula to evaluate driven pile capacity because of the fear of improper selection of quake values and damping factor.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

High Strain Dynamic Testing consists of two main types, namely Dynamic Pile Monitoring and Dynamic Load Testing. Dynamic Pile Monitoring involves the use of PDA to perform real-time evaluation of Case Method pile capacity, hammer energy transfer, driving stresses and pile integrity for every blow count. On the other hand, Dynamic Load Testing is another technique that is evolved from wave equation analysis. CAPWAP Analysis makes use of field measurements obtained by PDA and wave-equation type analytical method to predict pile performance such as static load capacity, pile-soil load transfer characteristics, soil resistance distribution, soil damping and quake values. CAPWAP Analysis is carried out on the PDA data after the test is complete.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

In some design codes, it allows the simultaneous usage of shaft and base resistance of rock sockets to resist loading. Past experience showed that the shaft resistance could be mobilized in rock socket provided that the length of rock socket is less than three times the pile diameter. Such provision eliminates the need for bell-out because higher pile capacity could be derived from the simultaneous usage of shaft and base resistance of rock sockets. Bell-outs are commonly used in piling industry but its effectiveness is doubtful because of some practical issues like the inability of thorough cleaning of pile base.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

The possible methods to reduce downdrag on piles are mentioned below:-

(A) Coating piles with friction reducer such as bitumen.

(B) Predrill a hole firstly, followed by putting in the pile and subsequent filling of annulus with bentonite slurry.

(C) Drive the piles with an oversize shoe and fill the annulus with bentonite slurry.

(D) Adopt double tube method in which the inner pile takes up the structural load and the outer pile carries the downdrag load.

(E) Preload the soils to accelerate settlement.

(F) Use electro-osmosis to enhance water content around the piles and reduce the friction between pile and soils.

(G) Increase the pile capacity by using larger diameter or longer length so that the effect of downdrag is thereby reduced.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

Coating driven piles with bitumen serves the following purposes:

(i) It acts as friction reducer and could effective reduce the effect of downdrag.

(ii) It offers protection of steel piles against acid attack from soils.

(iii) It helps to prevent pile corrosion.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.