SiAlON co- dopados con Si, Al

transferred to the pile ‘EMX’ w ill be relatively low and it will ta ke longer to drive the pile, so productivity suffers. Also, if the hammer is not performing well and a blow count or set per blow is used as the pile acceptance criteria, then the pile could be accepted prematurely at an actual pile capacity which is dangerously low, and foundation failures could result. It is generally to everyone's advantage to have a well performing hammer so the contractor gets completed as efficiently and quickly as possible, and just as importantly the engineer is assured that the pile embedment is sufficient and the capacity is adequate.When a blow count (or set/blow) is part of the driving criteria, as is almost always the case, it is also important that the hammer perform consistently during the course of any project, so periodic testing for hammer performance is recommended.

13.2 Energy Measurements 13.2 Energy Measurements

The 8G computes energy transferred from the integral over time of the product of force times velocity (equivalent to the work done on the pile). The maximum energy ‘EMX’

can b e comp ared with the ham mer' s ra ted or potentia l ener gy ( ) to deter mine an energy transfer ratio ‘ETR’, which is an indication of the overall efficiency of hammer driving system. ETR compares EMX with the manufacturer’s rating, while ‘ETH’ compares with the potential energy from the computed hammer stroke for open end diesels. The maximum transferred energy EMX) is typically 20 to 60 percent of the manufacturer's rated energy, ER, depending on pile and hammer type. Lower energy transfer ratios usually indicate a hammer in need of repair or a driving system in need of modification.

Typical hammer performance can be assessed in Appendix D, showing statistical histograms of transfer ratios for different pile type/hammer combinations. These figures

E _p= W _r h

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The hammer operating rate in blows per minute ‘BPM’ can be determined by the PDA, up to the speed limit of the PDA. If the 8G misses a blow during acquisition, the BPM value of the PDA will be half the actual BPM; this can be corrected by the ‘Blow Number Filter’ function ( Sec tion 8.5. 2.1 on p age 107 ).

A n ew ou tp ut quanti ty is the time from ri se to peak ‘T RP’ which is a meas ure of rise time (in milliseconds) which in turn depends on hammer cushion, helmet weight and pile cushion when applicable.

The effectiveness of different hammers can be compared. Hammers with similar rated energies but of different types, or hammers of the same model, can be compared for their performance by the energy transfer, input forces, and overall effect on the blow count. In general hammers with higher stroke (e.g. diesels) work best when high capacity or deeper embedments are required and hard driving is anticipated. Heavier rams with shorter strokes (typical of air or hydraulic hammers) are very effective in softer soils or in cases where much of the capacity comes from set-up.

13.3 Hammer Stroke (Open-Ended Diesel Hammers)

The potential energy for open end diesel hammerspotential energy for open end diesel hammers can be computed from this stroke (STK or h) times the hammer’s ram weight, W. This calculated potential energy can then be compared to the maximum energy transferred to the pile, EMX, and reported as ETH, the hammer transfer efficiency ration, normalized for the computed hammer stroke.

13.4 Calculations for External Combustion Hammers 13.4 Calculations for External Combustion Hammers

Further analysis of dynamic test data is possible by looking at the momentum calculations (MF0 from force, or MW0 from wave-down). If the ram weight ‘WR’ is input, the maximum ram velocity at impact ‘VRI’, (for ECH hammers only) can be calculated

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losses are occurring (e.g. primarily in the hammer or the lower driving assembly). These momentum equations can sometimes lead to unusual answers and results should be viewed with judgment and accepted only if reasonable.

For external combustion (air/steam/hydraulic or drop) hammers on steel piles, the force in the hammer cushion, ‘FCP’, and the hammer cushion stiffness, ‘KCP’, can be computed. These computations require input of the helmet weight ‘WH’, and weight of the ram ‘WR’.

13.5 SPT Energy Measurements 13.5 SPT Energy Measurements

The 8G energy measurements of EMX are also applicable to SPT soil samplers and dynamic penetrometers according to the ASTM D4633 Standard (and also as mentioned in ASTM D6066) where the term EFV is used (EMX and EFV give exactly the same result).

Due to high accelerations, SPT subsections are instrumented with glued-on foil sensors rather than using bolt-on strain transducers (contact PDI for details). SPT energy measurement allows evaluation of a normalized N-value (called “N ₆₀“) to compensate for vari at ions in SPT device efficiencies to improv e up on soil streng th estima tes fr om

SPT-N values. N₆₀ can be computed from the measured N, the measured energy transfer (EMX), and 60% of the theoretical potential energy Wh for the SPT ram (Wh) from the expression

The 8G has an optional software add-on program (SPT ANALYZER) that specifically meets the sampling rate and filtering requirements of ASTM D4633 and the European norm. It provides for higher sampling rates and also has a higher analog filter cutoff (less analog fil teri ng of th e signals. Furt her deta ils regard in g the op eratio n of the PDA-S softwa re in

SPT mode are covered in “SPT Dat a Collec tion” on p age 175 .

13.5.1 Historical note on SPT Energy Calculation 13.5.1 Historical note on SPT Energy Calculation

It should be noted that a previous obsolete version of ASTM D4633 was a measurement standard that had been withdrawn. It considered the normal proportionality of uniform rods between force and velocity and therefore required only measurement of force and obtained energy from the integral of the force squared (divided by impedance). The result of this computation are given by the 8G in the quantity ‘EF2’. The method also required several “correction factors”, particularly for short rods. These correction factors are NOT contained in EF2 but must be applied separately. However, when rods are non-uniform, this proportionality assumption is in error and the results are also misleading.

Errors were also potentially serious for the joint masses, and particularly if the joints were loose causing early tension reflections. The time ratio of first tension return compared with the theoretical 2L/c is shown by the quantity ‘RAT’, which was specified to be between 90 and 120% for a valid test. To avoid the complexity, and possible errors from this EF2 method, and considering that the correct method of energy evaluation integrating the product of force times velocity is contained in the EMX method, t he EMX method is now the de facto standard in ASTM D4633 and in use by many test agencies today and is the PDI endorsed method of SPT evaluation.

N ₆₀  N EM X   0.6W  h

 

---=

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