Part One
Static Pile Compression Load Test
1 Introduction
Maintained load (ML) test is one of the principal types of compressive loading test on piles, in which the load is increased in stages to some multiple, say 1.5 times or twice the working load with the time-settlement curve recorded at each stage of loading and unloading. The ML test may also be continued to failure by progressively increasing the load in stages. The ML test is best suited for contract work, particularly for proof loading tests on working piles. It is also suitable for use where empirical methods are employed to predict the ultimate load from the measurement of residual deflections after returning the test load to zero at four or five stages up to the maximum. [1]
2 Instrumentation
2-1 Loading Frame The loading frame shown in (Fig. 1) is designed to apply a maximum axial compressive load of more than (200 tons). The clear distance between the test pile and the kentledge base is not less than 2m. [2]
(Fig. 1)
2-2 Hydraulic Jack
One hydraulic jack, with relevant capacity, is provided on the top of the pile's head to transfer the load increments and decrements during the loading and unloading stages of the test. [2, clause 3.2.1]
2-3 Dial Gauges
Two dial gauges, which are Sweden-made, are provided to monitor the pile's settlements at different two points at the pile's perimeter. The dial gauges are mounted between the pile's head and two reference beams. The capacity of each dial gauge is (100mm) with an accuracy of (0.01 mm). [2, clause 4.1.2]
2-4 Reference Beams
Two reference beams, which are made of steel (I–sec.), are supported by the natural ground and used in measuring the settlement of pile by the dial gauges. Each reference beam works as simply a supported beam, which is pinned at one end and a roller at the other end. Supports of the reference beam shall be firmly embedded in the soil at least a clear distance of not less than (2m) from the edge of the tested pile or three tested pile diameters from the center of the pile, whichever is greater. Figure (2) shows the possible arrangement of instrumentation for measuring the vertical movements of the pile. [2, clause 4.1.1], [3, clause B15.11.2].
2-5 Test Plate
Steel test plates of sufficient thickness ( but not less than (50 mm)) to prevent it from bending under the applied loads, must be centered on the pile and set perpendicular to the longitudinal axis of the pile. For tests on individual piles, the size of the test plate shall be not less than the size of the pile butt nor less than the area covered by the base of the hydraulic jack, the size of the test plate shall be not less than twice the area covered by the base of the hydraulic jack. For tests on precast or cast-in-place concrete piles or on pile groups, the test plate when used shall be set in high-strength quick-setting grout. [2, clauses 3.1.4, and 3.1.5]
2-6 Bearing Plate
A steel bearing plate of adequate thickness must be set between the top of the jack ram and the bottom of the test beam. The bearing plate must be of sufficient size to accommodate the jack ram and properly bear against the bottom of the test beam. The bearing plate must have sufficient thickness to prevent it from bending under the loads involved, but not less than (50 mm). [2, clauses 3.1.4, and 3.1.6
2-7 Protection of Testing Equipment
Throughout the test period, all equipment for measuring load and movement must be appropriately protected from adverse effects of sun, wind, and precipitation. Temperature readings must be taken at the start, end, and at maximum load of each loading cycle. [3, clause B15.12.1]
2-8 Prevention of Disturbance
Construction activity and persons who are not involved in the testing process must be kept at a sufficient distance from the test to avoid disturbance to the measuring apparatus. [3, clause B15.12.2]
3 Test Procedure
The test procedure which is specified by the Institution of Civil Engineers (ICE) can be summarized as follows in Table (1); [2]
The load must be maintained at the specified value for not less than the period shown in Table (1) and until one of the following rates of settlement criteria are satisfied:
- For pile head settlement of less than 10mm, each load increment must be maintained until the rate of settlement is reducing and is ≤0.1mm/hour.
- For pile head settlement between 10mm and 24mm, each load increment must be maintained until the rate of settlement is reducing and is ≤ 0.01 × pile head displacement/hour.
- For pile head settlement of greater than 24mm,
each load increment must be maintained until the rate of settlement is reducing and is ≤0.24mm/hour. The rate of the settlement must be measured over a minimum period of 30minutes. For any period when the load is constant, time and settlement must be recorded immediately on reaching the load, and at least at the following rates:
For the first 15 minutes every 5 minutes
For the first 1 hour every 15 minutes
For the first 4 hours every 30 minutes Thereafter every 1 hour
4 Criteria to Stop the Test
Generally, the test must be stopped if one of the following cases occurred;
- The load reaches its maximum specified value.[2]
- 2- The load at which settlement continues to increase without any further increase of load. In this case, the jacking of the pile must continue until the settlement equals 15% of the pile diameter or diagonal dimension. [1, 2]
- 3- Failure of the pile materials (due to defects in the pile). [4]
- 4- Failure of the concrete to reach the design compressive strength. [4]
- 5 Discussion and Conclusions The results of the test will be reported including the following items
- Time recording, load increments and decrements, and pile settlements.
- Load-settlement curve.
- Time-settlement curve.
- Time-load curve.
The test results are discussed according to the general requirement to stop the test. The behavior of the pile is studying by using the above-mentioned curves to know if there is any defect in the pile shaft or not. At the same time, the test results are compared with the requirements of the settlement at the allowable and ultimate loads adopted by the client.
5 References
1-M. Tomlinson, J. Woodward, “Pile Design and Construction Practice”, 1994.
2- ASTM Standard D 1143, “Standard Test Method for Piles Under Static Axial Compression Load”, 2004.
3- Institution of Civil Engineers, “ICE Specifications for Piling and Embedded Retaining Walls”, 2 nd edition, 2007.
4- Shell Iraq Petroleum Development B. V., “Pile Testing”, Doc. No.: GEN-00000-0000-4100-CG7880-0000
Part Tow
Dynamic Pile Load Test
1 Introduction
The dynamic test of piles PDA, also called high strain dynamic test of piles, is used for testing vertical or batter piles individually to determine the force and velocity response of the pile to an impact force applied axially by a pile driving hammer or similar device that will cause a large strain impact to the top of the pile [1]. In addition, this test is used for both bored and driven piles. Frequently the purpose of this test is to determine pile bearing capacity because there are increases and decreases in the pile capacity with time typically occur after a pile is driven (soil setup/ rexlation). Therefore, dynamic testing during restrike usually yields better indication of the long-term pile capacity than a test at the end of pile driving [2]. The PDA test is best suited for contract work, particularly vary from project to project and the its specifications for one site may not applicable to another such as pile integrity, damage investigation, establish driving stresses, pile shaft and end bearing capacity, inspecting hammer performance, etc. [2].
2 Instrumentation
2-1 Impact Force Application
Any conventional pile driving hammer or similar device is acceptable for applying the impact force provided it is capable of generating a net measurable pile penetration, or an estimated mobilized static resistance in the bearing strata which, for a minimum period of 3m, exceeds a sufficient degree the working load assigned to the pile, as judged by the engineer in charge. The device must be positioned so that the impact is applied axially to the head of the pile and concentric with the pile. A typical device is shown in (Fig. 1). [1]
2-2 Apparatus for Obtaining Dynamic Measurements [1]
The apparatus must include transducers, which are capable of independently measuring strain and acceleration versus time at a specific location along the pile axis during the impact event. Each two of these devices must fixed on opposite sides of the pile, must be securely attached so that they do not slip. Bolt-on, glue-on, or Weldon transducers are acceptable. The apparatus must include the following:
- Force or Strain Transducers.
The strain transducers shall have a linear output over the entire range of possible strains. When attached to the pile, their natural frequency shall be in excess of 2000 Hz. The measured strain shall be converted to force using the pile cross-section area and dynamic modulus of elasticity at the measured location.
- Acceleration, Velocity or Displacement Transducers.
Velocity data shall be obtained with accelerometers, provided the signal is capable of being processed by integration in the apparatus for reducing data. A minimum of two accelerometers with a resonant frequency above 2500 Hz shall be at equal radial distances on diametrically opposite sides of the pile.
- Placement of Transducers.
The transducers must be placed, diametrically opposed and on equal radial distances, at the same axial distance from the bottom of the pile so that the measurements compensate for bending of the pile. When near the upper end, they must be attached at least 1.5 pile diameters from the pile head. This is illustrated in (Fig. 2) and additional details is provided in references [1] and [3].
- Apparatus for Recording, Reducing and Displaying Data.
The signals from the transducers [1 clause 5.2] during the impact event shall be transmitted to an apparatus for recording, reducing, and displaying data to allow determination of the force and velocity versus time. Figure (3) shows a typical device which may be used in the high strain pile test
3 Test Procedure [1,4]
The test procedure which specified by ASTM 4945 could be summarized as following steps:
- General:
Record applicable project information. Attach the transducers to the pile, perform the internal calibration check, and take the dynamic measurements for the impacts during the interval to be monitored together with routine observations of penetration resistance. Determine properties from a minimum of ten impact records during initial driving and, when used for soil resistance computations, normally from one or two representative blows at the beginning of re-striking.
B-For determination of strain wave speed for concrete, the wave speed should be determined from the impact event if a tensile reflection wave from the pile toe is clearly identified. Alternatively, place the pile on supports or level ground free and clear from neighboring piles and obstructions. Attach accelerometer to one end of the pile and strike the other end of the pile with a sledge hammer of suitable weight.
C-Preparation: Mark the piles clearly at appropriate intervals. Attach the transducers securely to the piles by bolting, gluing, or welding. For pile materials other than steel, determine the wave speed. Position the apparatus for applying the impact force so that the force is applied axially and concentrically with the pile. Set up the apparatus for recording, reducing, and displaying data so that it is operational and the force and velocity signals are zeroed.
D-Taking Measurements: Record the number of impacts for a specific penetration. For drop hammers and single acting diesel and air/steam/hydraulic hammers, record the drop of the ram or ram travel length. For double acting diesel hammers, measure the bounce pressure, and for double acting steam or compressed air hammers, measure the steam or air pressure in the pressure line to the hammer. For hydraulic hammers, record the kinetic energy from the hammer readout when available. Record the number of blows per minute delivered by the hammer. Take, record, and display a series of force and velocity measurements. Compare the force and the product of velocity and impedance at the moment of impact.
4 Test Results
The test results which recorded from measurements of field are transformed to the computer to generate the required figures, and tables for displaying and assessment of the test result quality. This is achieved by using PDA-W [5] software in addition to both CAPWAP and CRAWEAP programs which are provided by PDI company. Figure (4) shows the obtained force and velocity graph for typical test. Also, the PDA-W provide the following curves of the results
[F, V]: Force, Velocity
[WD, WU]: Wave Down, Wave Up
[RS, RT]: Resistance Static (includes Case Damping Constant, JC), Resistance Total (JC=0)
[E, D]: Energy, Displacement
[F1, F2]: Force 1, Force 2
(i.e., individual Force curves) [V1, V2]: Velocity 1, Velocity 2 (i.e., individual Velocity curves)
Results of curves and tables can be summarized in the following list:
FMX: Maximum force.
VMX: Maximum velocity.
DMX: Maximum displacement.
DFN: Final displacement.
EMX: Maximum energy.
CSX: Maximum compression stress.
CSI: Maximum individual compression stress.
CSB: Maximum Toe compression stress.
TSX: Maximum tension stresses.
TSN: Maximum tension stress.
BTA: Integrity factor.
LTD: Length to damage.
RMX: Maximum capacity estimate based on JC damping factor.
RA2: Maximum capacity estimate (independent of damping assumptions).
SFR: Shaft resistance.
EBR: End Bearing Resistance.
5 Data Interpretation
[5] Result interpretation should be by a professional engineer who knows wave theory and pile driving. The PDA investigates driving stresses, pile integrity (damage), hammer performance, and bearing capacity. For best results in capacity, test the pile during restrike with 7 days wait to allow for strength changes with time. The Geotechnical engineer should review capacity results for uplift, settlement, and PDA assumes pile is uniform; if non-uniform, it must be use CAPWAP. Geotechnical engineer should review results, and recommendations considering uplift, settlement, and negative friction.
Stresses: PDA finds average Max Compression Stress (CSX) at sensors (using assumed modulus) and Max Compression Stress for any strain sensor (CSI).
Damage:
PDA inspects Wave Up for damage (should increase monotonically during first 2L/c). Damage is usually a Sharp Reduction in Wave Up. For non-uniform pile, PDA may find "damage". Generally, when the Integrity Factor (BTA), which is one of the test results is greater (90), the pile is OK; while when BTA is less than (60) the pile is rejected. For other values of (BTA), Table (2) is used as a judgment noting that not give capacity of damaged piles. [5]
Capacity:
This is often the most difficult result to interpret. PDA estimates the capacity at the time of testing only; often it is a reduced strength during driving. We suggest testing during restrike for best long-term capacity and comparison with static tests. Ideally the restrike wait period is similar to wait for static test; often 7 to 14 days is suggested. Set per blow should be minimum 3 mm (less than 100 bl/ft) to assure full capacity mobilization.
6 Discussion and Conclusions
This clause includes a comparison between the computed maximum stresses by
the test and the specified values by the designer. Also, BTA value is compared with specified values in Table (2) to estimate if there is any damage in the pile shaft. While the maximum capacity of the pile (shaft and end bearing capacity), which obtained from the test, will compare with specified value by designer. Accordingly, it can be concluded if the tested pile could be accepted or rejected.
7 References
1- ASTM Standard D 4945, “Standard Test method for Piles High-Strain Dynamic Testing of Piles”, 2004.
2-George Goble, and Garland Likins on The Application of PDA Dynamic Pile Testing, Online Publisher.
3- Pile Driving Analyzer model PAX PDAL User’s Manual November 2008 from PDI.
4-CIRIA Report No 144, “Integrity Testing in Piling Practice,” CIRIA, London, 1997.
5-PDA-W Manual of Operation February 2009 from PDI.