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Q001

Is there a reference to testing pile capacity by re-driving / re-striking after it has "set up" in an American Society for Testing and Materials (ASTM) or other standard?

 ASTM D4945-08 is a standard for high strain dynamic testing, either during driving or on restrike. Even though the expression “set-up” is not used, it is implied in the reasons for restrike testing:

1.    Section 4.2 mentions that one of the factors affecting the foundation capacity from a dynamic test is “elapsed time since initial installation” (for driven piles this implies a restrike).
2.    Note 1 mentions “dynamic restrike tests that account for soil strength changes with time”.
3.    Section 6.1 directs “Allow sufficient time (...) prior to testing” and continues “Determine the pile response to high strain dynamic test 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 a restrike.”
4.    Section 6.8 states that “ If the test results are used for static capacity computations, then dynamic measurements should (also) be performed during restrikes of the deep foundation, after waiting a period of time following the initial installation sufficient to allow pore water pressure and soil strength changes to occur.
5.    Section 7.6.2 prescribes that the Test Report contain Date of test(s), sequence of testing (e.g. “end of driving” or “beginning of restrike”), and elapsed time since end of initial driving for restrikes,
The Pile Driving Contractors Association (PDCA) Installation Specification (intended for private sector work but very similar to the American Association of State Highway and Transportation Officials (AASHTO) installation specification for transportation projects), refers to “restrike” in Item 4.4.3:
“Dynamic testing shall be conducted in accordance with ASTM D4945 (...) If the required ultimate pile capacity is not achieved at the end of driving, the Contractor shall restrike the dynamic test pile following a waiting period specified in the contract documents or as directed by the Engineer...” The Commentary to this section adds: “Because the ultimate pile capacity of a pile may change substantially during and after pile driving, waiting after driving for additional testing may be beneficial for a safe and economical pile foundation. If possible, the dynamic test should be performed as a restrike test if the Engineer anticipates significant time dependent increases in nominal strength called setup, or reductions called relaxation.”
 
A copy of American Society for Testing and Materials (ASTM) D4945 may be purchased from http://www.astm.org/Standards/D4945.htm
 
A copy of the PDCA Installation Specification can be downloaded from www.piledrivers.org

Q002

Is there a relation between concrete strength and wave speed? How can I determine the minimum acceptable wave speed?

Based on information reported in American Concrete Institute (ACI) 228.1 R-95 and some other codes and specifications, the concrete compressive strength is approximately proportional to the compression wave velocity to the 4th power. This means that, if σ is the concrete strength, W is the wave speed, and k1 and k2 are constants:

σ = k1 × W4 or W = k2× σ1/4

Those relationships can be used to compare the strength and wave speed of concretes with similar mix. Suppose the minimum acceptable strength for the concrete of a drilled shaft project is σA. Also suppose that on a given shaft on that project you determine a wave speed WM (by performing a Pile Integrity Test or by Cross Hole Sonic Logging) and measure a strength σM (by extracting a cylinder, for example). A relationship may be established for the minimum acceptable wave speed on other drilled shafts on the same project, WA, as follows:

WA = WM ( σA / σM ) 1/4

 

Q003

How are the applicable energy quantities calculated?

1.    For Pile Driving Analyzer® (PDA) (EMX, EFV, ETR, {ETH - diesel hammers only}),

2.    For SPT (EMX, EFV, ETR, E2E, E2F, EF0, EF2, EV2)

EMX is calculated as the maximum of the integral of force times velocity, over the whole record. It is the best theoretical method for calculating the maximum energy transferred to the foundation or SPT rod, and is the only one approved by American Society for Testing and Materials (ASTM) 4633-05 and European Standard ISO 22476-3:2005 for SPT energy calculations. Because it searches the whole record, it is independent of the length (LE) setting. The EMX value is compared with the rated energy of the hammer to calculate ETR, that is, ETR (%) = 100 * EMX/PE, where PE is the maximum rated potential energy of the hammer. The PDA also calculates ETH, which can be used only with open end diesel hammers. It is defined as ETH(%) = 100 * EMX/(Wr * STK), where Wr is the weight of the ram, and STK is the open end Diesel stroke calculated by the PDA based on the time between blows. EFV is exactly the same as EMX, and is provided for consistency with ASTM D4633-05 terminology.
 
The E2E method does the same computation as EMX, but it stops the integration at (2 * LE)/c. It can be used to stop the integration at a time corresponding to a given LE setting. It should be noted that ending the integration before the end of the record is not the procedure recommended by ASTM 4633-05 and European Standard ISO 22476-3:2005 for SPT energy testing, and that changing LE will also change the capacity calculations on PDA testing.
 
EF2 was used exclusively for SPT energy measurements on previous versions of ASTM D4633, at a time when a reliable method for measuring the velocity on SPT rods was not available. It is the maximum of the integral of the square of the force divided by the impedance, over the whole record, and is based on the theoretical proportionality between force and velocity along most of the first 2L/c period on SPT energy tests. In real practice this proportionality is hard to achieve due to non-uniformities along the rod, imperfect joints, etc. Other factors also affect the accuracy of this method, so several correction multipliers had to be used. It was later recognized that these correction methods did not yield reliable results (please refer to the Appendix on ASTM D4633-05 for further information). EF2 is therefore generally inaccurate and obsolete, and should not be used . EF0 is the same as EF2. The E2F method does the same computation as EF2, but stops the integration at 2L/c. It has the same lack of accuracy as EF2. EV2 is the maximum of the integral of the square of the velocity multiplied by the impedance, over the whole record. It is based on the same proportionality principle as EF2, so it suffers from the same lack of accuracy.
 
We would like to stress that EF2, EF0, E2F and EV2 should not be used to determine the energy transferred to an SPT rod or to a foundation, and are provided for research purposes only.

Q004

What is the difference between the efficiency of Hydraulic Hammers and that of Diesel Hammers?

The term "efficiency" can have several interpretations. In wave equation analysis (GRLWEAP) it is the ratio of the potential energy to the kinetic energy of the ram just prior to impact. A wave equation efficiency of 80% best matches the normal performance of most diesel hammers, therefore that is the value recommended by GRLWEAP. The wave equation efficiency of hydraulic hammers has variations among the manufacturers; the 95% recommended by GRLWEAP is a good assumption for those with built in energy monitors. In PDA testing, the term "Energy Transfer Ratio" (ETR) is the ratio of the energy transferred into the pile to the maximum rated energy of the hammer. This value depends not only on the hammer but also on the pile type, since typical plywood cushions used with concrete piles absorb some energy.

Diesel hammers have a median energy transfer ratio of 37% on steel piles (less on concrete piles), if you compare the energy transferred to the pile to the maximum rated energy of the hammer, calculated at the rated stroke. The stroke of a diesel hammer, however, is also a function of the soil resistance and of the elasticity of the pile, so frequently the diesel hammer does not achieve the full rated stroke. Consequently, the actual potential energy of diesel hammers is often lower than its maximum rated energy, which results in lower values of ETR. The PDA also calculates ETH, which is the ratio of the energy transferred to the pile to the actual potential energy of diesel hammers, that is, weight of ram times measured stroke (STK).

For hydraulic hammers on steel piles, the ETR may be up to 95% of the reading of the “energy monitor” of the hammer. ETR will be lower for concrete piles due to the energy stored in the pile top cushion. In addition, it should be noted that hydraulic hammers can be, and usually are, operated at lesser strokes than their maximum (for example with concrete piles to avoid high tension stresses in easy driving). In those cases the ETR values calculated by the PDA based on the maximum rated energy of the hammer will result in lower values.

It should be noted that energy rating, or efficiency, or even energy transfer is not necessarily the best measure for selection of a hammer. The capacity of the pile in installation, together with the ability of the hammer to impart force (which also depends on the pile strength or stiffness), may be a better way to assess the suitability of one hammer for a given project site. A wave equation analysis is always recommended to help select the hammer type for any particular application.

 

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