Methodology Dynamic Load Test On Pre Cast Pile October, 2019
METHODOLOGY FOR
DYNAMIC LOAD TEST ON R.C.C. CAST-IN-SITU BORED PILES
Contractor: Dipon Gas Co. Ltd.
AFRA CORPORATION LTD.
Flat-2/A, House-05, Road-2/3,
Banani, Dhaka-1213, Bangladesh
Tel: +880-2-55040066, +8801955-520820, +8801722-166848, +8801955-520800
Web: afracorporation.com, Email: afracorporation@gmail.com
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Methodology Dynamic Load Test On Pre Cast Pile October, 2019
1. Introduction
High Strain Dynamic Pile Testing is commonly performed on driven pile, micro pile, cast in place pile,
steel pile and other pile type. It has been proven to be cost effective and reliable. It involves attaching
strain sensor and accelerometer to the pile by bolts, to measure and determine the effect of impacts on a
pile. The impact is often applied by the pile driving hammer on a driven pile, but may also due to the
impact of a large drop weight applied to a bored or augured cast pile, or pile driven in by hydraulic
injection. High Strain Dynamic Pile Testing is guided by ASTM D4945-2008.
2. Preparation of DLT Test
Dynamic load testing is conducted to determine the static capacity of a pile either by a drop hammer or
by a re-strike from a pile driving hammer. Measurements are made for a number of blows, the quality of
which is judged by the test engineer who indicates when the test should be stopped.
2.1 Preparing the pile
If permanent steel casing is not used during the pile construction, then a pile top extension or build up,
consisting of a thin walled casing or equivalent will be used to extend the pile head by about 1.0 m to
1.5 m. The use of casing is to prevent damage to the pile head resulting from the hammer impact. In most
cases, the concrete used for the buildup should be equivalent to the pile. The concrete must cast to the
same level at the top of casing, and to a good even and level finishing.
The pile head must be readily accessible either by extending the buildup above ground, or by excavation
surrounding the pile to provide working area. The depth of excavation should be enough for the
attachment of sensors at least 1 pile diameter from the pile top.
The sensors are attached to the pile diametrically opposite to each other (if 2 sensors are used or at 90 degrees
apart if 4 sensors to be used). The sensors attachment should be onto a smooth concrete surface to prevent the
sensors from warping or bending during attachment. This can be done either by cut open
2 square outlet on the casing which most of the time render smooth surface, or by grinding or chipping to
smoothen the surface. Schematic diagram of the sensors attachment is included in Figure 2. In permanent steel
cased pile, the sensors can be attached to the steel casing.
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Methodology Dynamic Load Test On Pre Cast Pile October, 2019
2.2 Transducer Mounting Methods
Transducers can be attached anywhere on the pile, though it should be noted that transferred energy decreases
down the pile and blow eccentricity increases closer to the pile top. Typically, the transducers are mounted at
a distance of 2–3 pile diameters from the pile top such that a uniform stress wave is recorded with sufficient
energy and minimal blow eccentricity. As well, transducers should be attached far enough from the pile top
to avoid end defects but close enough to avoid damage from the ground at final penetration or the hammer
sleeve. Local contact stresses are reduced with cushioning.
Note:
For regularly reinforced (not pre-stressed) concrete piles, the transducers should be attached
nearer the pile top where pile tension is closer to zero and the pile top has not experienced
normal cracking.
For re-strikes, the pile should be excavated 2–3 pile diameters.
During high water level, standard PDA transducers must remain above sea level.
For concrete piles, the transducers are mounted on the pile using anchor bolts (e.g. Hilti HKD-
S bolts) and a mounting template.
2.3 Testing Equipment:
Sr. No. Equipment List Quantity
01. Pile Driving Analyzer (PDA – Model PAX-4) 01 No.
02. USB Conditioner 01 No.
03. Combined Strain & Accelerator Sensor 02 No.
04. Caliber & drilling tool PDA.31200 01 No.
05. Data Connection Cable 40 m 01 No.
06. Articulated variable weight Drop Hammer 01 no.
07. Misc. Drilling Tools & Equipment as required 01 set
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Methodology Dynamic Load Test On Pre Cast Pile October, 2019
2.4 Test Methodology:
Dynamic pile test will be conducted after the pile has acquired sufficient concrete strength or at a time
to be agreed by the Engineer. Dynamic measurement of force and velocity will be collected by the strain
sensors and accelerometers attached to the pile. The PDA monitors the strain and acceleration caused by
the hammer impact, and processes these signals after each hammer blow to give immediate visual. The
signals are digitized by PDA, results are computed, and the data array of the signals for a blow is stored.
The data may be interpreted for pile bearing capacity, compression stresses induced at top and bottom,
tension stresses along the shaft, energy transferred to the pile or shaft, and pile integrity. It should be
noted that High Strain Dynamic Pile Testing estimates for the pile capacity indicate the mobilized pile
capacity at the time of testing. Increases and decreases in pile capacity with time typically occur (soil
setup / relaxation).
2.5 Testing Procedure:
i. High Strain Dynamic Pile Testing is performed by an experienced engineer who is familiar with all
aspects of data acquisition and analysis to assure that correct results and interpretation be made.
ii. PDA testing involves the transducers attached to the selected test pile. Two strain transducers and two
accelerometers will be attached diametrically opposite to each other at a recommended minimum
distance of 1.0 pile Ø or 1.0 W. Please refer to Figure 2 for layout of typical strain sensor and
accelerometer attachment to test pile and setup of equipment at site.
iii. The PDA will be connected to the transducers via the main computer cable or wireless transmitters.
iv. The testing hammer will then be set-up to the test pile and impacts applied to the pile top. The records
will be examined to assess data quality, the driving system and pile stresses.
v. Each blow monitored by the PDA will be displayed for immediate data quality control and are stored
digitally on the hard disk for later replay and further analysis if required.
vi. Typically, for most bored pile testing, a minimum of 3-5 impacts will be required, starting from a low
drop height and increasing gradually until the required pile test capacity has been reached or the pile had
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Methodology Dynamic Load Test On Pre Cast Pile October, 2019
set per blow of 4-5 mm. If the set is small (less than 3mm per blow) and the activated pile resistance is
less than the required test capacity, a higher drop height will be used to activate more pile capacity,
subject to review of the pile stresses for the previous blow. If the set is large (5mm per blow), a further
impact is usually not required as the pile is deemed to have achieved its ultimate capacity.
vii. Surveying instrument should be provided on site by Contractor (or others) so as to record the
settlement of the pile head after each hammer drop to ascertain the set of the pile.
viii. At the completion of testing, selected individual pile blow data will be transferred for further analysis
using CAPWAP program.
ix. All information pertaining to the selected test pile(s) including pile construction records must be
furnished to the engineer for testing and analysis purposes.
Drop Hammer
Guiding frame will be used to ensure the drop hammer does not fall eccentrically that may cause damage
to the pile or equipment, and also as site safety measure. By ensuring the drop hammer fall centrally and
creating a full surface impact also ensure the quality of the data.
Generally the drop height of drop hammer falls within the range of 1% to 2.0% of the test load divide by
the hammer weight. For restrike test, it is usually started with a relatively low drop height. The uniformity
of stresses can be checked, and adjustments to cushion thickness or ram/shaft alignment made before
application of subsequent blows. Subsequent blows can be made from a higher drop height
subjecting to the following conditions:
1. If the data during testing shows the pile has yet to be mobilized,
2. Pile integrity is still intact, and
3. The compression stress at the top is still within the limit. Generally the compression stress at the top should
not exceed 0.85fcu.
Plywood cushion should be placed on the pile top as to absorb excessive vibration created during
the hammer impact.
2.6 Field data:
The following pile properties will be recorded in our field sheet after the test from a selected blow:
RMX - Maximum ‘Case Method’ capacity
EMX - Energy transferred to pile
FMX - Maximum Compressive Force at sensor
CSX - Maximum Compressive Stress
RSU - Original ‘Case Method’ capacity for high friction case
BTA - Integrity Factor
The PDA automatically processes each blow recorded during the testing. Over 30 variables can be computed
by the analyzer. These will include the total pile resistance, static pile capacity, hammer energy, pile integrity
factor, pile compression and tension stresses.
During the testing, the PDA automatically checks for pile integrity and ‘out-of-range’ stresses. In the event of
any damage being detected, the PDA will indicate the location of the damage and provide an estimate of the
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Methodology Dynamic Load Test On Pre Cast Pile October, 2019
severity of the damage. Once the stresses exceed that of specified values, PDA automatically provides a warning
of any high stresses situation. Pile integrity and driving stresses along the whole pile will be checked
continuously during the testing and an immediate indication will be given onsite.
3.0 CAPWAP ANALYSIS:
In all cases we highly recommend CAPWAP ‘signal matching’ analysis of the data as a better way to estimate
pile capacity. CAPWAP is a rigorous numerical analysis which models the pile and soil behavior.
Selected representative blows will be transferred for further analysis using the CAPWAP suite of software. CAPWAP
analysis involves applying the measured pile top force and velocity-time as a boundary condition to a wave equation
model of the pile. The pile-soil model will be continuously adjusted until a close agreement is obtained between
the measured and computed pile top data.
This will allow for the elimination of the dynamic response and static load distribution along the pile shaft (skin
friction) and the pile end bearing can be computed. The elimination of the dynamic response also allows the
program to calculate the load-settlement response of the pile.
The PDA and subsequent CAPWAP analysis will effectively give a more accurate estimation of static capacity,
skin friction, end bearing and the pile head settlements at any given loads.
4.0 REPORTING OF RESULTS
The final report will be furnished with the following:
- pile details and working load
- date of installation and test
- pile identification according to the construction drawings
- length of pile below grade
- total length; length below instrumentation
- hammer details
- pile total capacity, skin friction and end bearing
- computed load-settlement curves
- force and velocity matched curves
-
5.0 List of Personnel available for the Test.
Analyzing and Preparation of Test Report:
1. Engr. Md. Altaf Hossain Bsc.Engg (civil),FIEB
2. Engr.Md. Nuruddin,Bsc.Engg(civil),MIEB
3. Md.Mohiuddin Hons & Masters, EDU (Dhaka University.)
4. Engr. Maruf Ahmed, B.sc. Engr.(civil)
5. Engr. Ariful Islam, B.sc. Engr.(civil)
6. Engr. Salim Ahmed, B.sc. Engr.(civil)
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Field Data Modeling and Maintenance of Test Archive:
7. Engr. Maruf Ahmed, Bsc.Engg(civil)
8. Engr. Ariful Islam, B.sc. Engr.(civil)
9. Engr. Salim Ahmed, B.sc. Engr.(civil)
Conducting of Field Test:
10. Engr. Maruf Ahmed, Bsc.Engg(civil)
11. Engr. Ariful Islam, Bsc.Engg(civil)
12. Engr. Salim Ahmed, Bsc.Engg(civil)
13. Engr. Mahfujur Rahman, B.sc. Engr.(civil)
14. Engr. Aslam Hossain, Dip. Engr.(civil)
15. Engr. Golam Rabbanee, Dip. Engr.(civil)
Skilled Technicians for assisting Testing Engineers at Site: 2 persons.
6.0 General Safety and Precaution:
All operation DLT shall be carried out in such a manner so as to minimize, avoid or eliminate the exposure
of people to hazards. Following examples of safety rules shall be followed in addition to general safety
requirements applicable to construction operation.
a. Keep all work areas, walkways and platforms clear of scrap, debris and small tools, mud, oil or other
slippery substances.
b. All struts and devices used for the test system of the test pile shall be free from any susceptible
attachments..
c. Attachments to the test pile(s), pile cap, drop hammer, frame, locking and lock releasing system shall be
designed and installed to transmit the required loads with an adequate factor of safety.
d. Loads shall not be hoisted, swung, or suspended over anyone and shall be controlled by tag lines.
e. All personnel shall stand clear of the drop hammer and other attachments.
f. Only authorized personnel shall be permitted within the immediate test area..
g. All reaction system shall be designed and constructed to have a reactive capacity sufficient to resist a load
20% greater than the maximum test load to be applied.
h. All struts, blocking, bearing plates, and testing equipment shall be accurately aligned to minimize
eccentric striking of the drop hammer and true verticality of the hammer drop shall be controlled so as
not to affect the test results.
i. Attachments to the test pile(s), pile cap shall be designed and installed to transmit the required loads with
an adequate factor of safety.
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7.0 Calibration Certificate:
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Methodology Dynamic Load Test On Pre Cast Pile October, 2019
8.0 Photographs:
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