METHOD STATEMENT FOR DYNAMIC LOAD TEST AND PILE
DRIVING ANALYSIS TEST
DYNAMIC PILE TESTING EQUIPMENT
Dynamic pile testing
and instrumentation will be performed using the PDA/CAPWAP system on the tested
pile.
Measurements of
the force and velocity waves induced in the pile during pile testing are
collected by force and acceleration transducers, which are fixed near the pile
head.
TEST PROCEDURE
Dynamic pile
testing can be conducted on bored piles by means of a heavy impact at the pile
head. The impact is often provided by a suitable drop weight and the response
is measured in terms of force and acceleration close to the pile head.
For testing, the
pile may require some excavation prior to the field-testing. The exposed length
of pile have to be at least 1.5 diameters of the pile or 1.0m whichever is
greater above the penetration level for pile instrumentation.
The pile head
will be protected for the impact by means of steel casing, which is used to
build up the pile.
A guide frame
will be positioned onto the pile. The frame is used to guide the drop weight
under the fall.
A steel plate of
about 50mm is placed on top of the pile head so as to protect the pile head
from damage under the hammer fall.
3 nos. of 6mm
diameter holes are drilled on two opposite sides of the pile for attachment of
gauges at 1.5 diameters below the pile top. Two pairs of force transducer and
accelerometer are bolted onto the pile using wall plugs.
During testing,
the transducers will be connected to the Pile Driving Analyzer and its
associated storage computer equipment via a main 19-wire cable (up to 50m
long).
The client will
supply and operate the piling hammer to induce a driving force onto the pile.
The drop weight is positioned and drop onto the pile under controlled height.
Dynamic measurement of force and velocity will be collected by gauges attached
to the pile. This data will be processed by the PDA to give immediate visual
and permanent record onsite.
Each blow
monitored by the PDA will be displayed on the built-in LCD screen/ oscilloscope
in the form of force and velocity traces for immediate data quality control and
computation. The PDA will also provide onsite results such as: -
a.
Mobilized static load capacity
based on the CASE method
b.
Pile Integrity – location and
extent of damage
c.
Pile Stresses – maximum
compression forces at pile top/ toe
d.
Hammer Performance – maximum
energy transferred to the pile
The transducers and accelerometers will be dismantled at the end of
the testing. PDA data will be transferred and stored in a disk for subsequent
report and analysis.
The above procedure will be repeated for the next pile test.
A survey of the pile top is taken before and after the testing to
obtain the permanent set per blow. The client is also to provide the bore log
information and pile construction record for the test pile.
DATA ASSESSMENT AND CAPWAP ANALYSIS
The pile top force and velocity signals recorded in the field will
be processed and a representative blow will be selected for further analysis
using the CAPWAP suite of computer software.
CAPWAP analysis involves applying the measured pile top force/
velocity time record as a boundary condition to a wave equation model of the
pile comprising of continuous segments.
The soil model is continuously adjusted in an iterative procedure
until the computed pile top force time record is in close agreement to the
measured pile top force time record. When good agreement is obtained between
measured and computed pile top data, the soil resistance parameters are assumed
to provide the best accurate model of the actual soil behavior.
A typical presentation of the CAPWAP results will be as follows: -
- CAPWAP model a match curve of computed pile top force to the measured pile top force time record
- Total computed soil capacity – sum of Skin Friction and Toe Bearing
- Computing Load against Settlement curve
REPORTING
Initial results are normally available within 24 hours or two
working days of the completion of each test. These shall include the following:
-
a)
the assumed damping factor
b)
the assumed wavespeed
c)
the maximum force applied to
the pile head
d)
the maximum pile head velocity
e)
the maximum energy imparted to
the pile
f)
the maximum tension force
experience by the pile
g)
the field estimated static
resistance
h)
the maximum driving stresses
A full report shall be submitted within 10 days of the completion of
the testing. This report shall include the follwing: -
a)
report all information given in
preliminary report
b)
information of pile size and
working load
c)
date of pile installation
d)
date of test
e)
pile identification and
location
f)
length of pile below existing
surface
g)
total pile length, including
projection above existing surface at time of testing
h)
length of pile from
instrumentation position to pile toe
i)
hammer type
j)
best estimated static capacity
k)
pile integrity
l)
force/ velocity versus time
trace
m)
computed load versus settlement
QUALITY ASSURANCE
Quality control is the single most important element in a successful
dynamic test. Every stage in setting up and operation of the testing is subject
to thorough quality assurance checking procedures. The equipment has many
inbuilt checking routines and signals warning display.
Quality assurance procedures are also followed in data processing
and CAPWAP analysis. Check reviews and re-analysis will be carried out prior to
final reporting.
The international acceptance of PDA/CAPWAP system and the proposal
to use Geotechnical engineers trained and experienced in the application of the
system are the key issues in providing the necessary quality assurance.
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