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.
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
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 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.