Frequently Asked Questions
1) Why GPC and N_GAPA reports are images (not editable)?
GPC and N_GAPA reports are presented in images (not text). Therefore, when viewers zoom large on a computer screen, the report's "font" does not look sharp (i.e., low resolution and appears to be blurry). When printing the report to A4 (letter) paper size, the image resolution is of good quality.
The advantage of image (instead of text) in report is that cheating (i.e., Adobe Pro text editing) is not possible. This feature will earn the owner's trust.
2) Can Dynamic Load Tests (DLT) detect pile length?
Answer: Yes - most of the time.
No - sometimes.
In these two pictures, L/D = 150. Each pile has 4 segments (i.e., 3 welded splices, or joints).
In the top picture (Fig. 1.a), the skin friction (side resistance) is significant. Toe tension reflection is NOT visible to detect pile length. This pile has very good resistance and the input pile length can be trusted.
In the 2nd picture (Fig. 1.b), the skin friction is small. More hammer force was transferred to the toe of the pile, and activated more tension reflection from the pile toe impedance change. Thus, it's easy to detect pile length.
Fig. 1.a and 1.b
3) How to evaluate concrete wave speed if toe tension reflection is not clear?
Answer: Use a combination of 2 approaches:
a) Based on concrete strength: WS (ft/s) = 391 (r * f’c)0.25
r = density in pcf; f’c = concrete strength in psi
or WS (m/s) = 1162.6 (r t/m3 * f’c MPa)0.25.
Example, for regular concrete: r = 2.5 and f’c = 50 MPa, thus WS = 1162.6 * (2.5 * 50)0.25 = 3887 m/s = 12753 ft/s
for centrifugal concrete (PHC): r = 2.6 and f’c = 80 MPa, thus WS = 1162.6 * (2.6 * 80)0.25 = 4415 m/s = 14485 ft/s
b) Based on proportionality of the WU curve (if you judge that f’c may vary from pile to pile, or from lab condition to field condition).
When changing WS, the WU may become non-proportional (for example, it may have a dip near the T0 time if WS is too low, or may show large skin friction near the top of the pile if WS is too high, when in reality, no soil exists near the top of the pile in this example).
4) What is proportionality?
Traditionally, proportionality was defined as F / VZ (or FVP) ratio of 1 at the FMX (peak force). This is generally true for:
a) Non-diesel hammer (i.e., any hammer that is not diesel)
b) Pile with no skin friction in the top portion of the pile (for example, off shore piles where a significant portion of the pile is in water - i.e., no skin friction)
However:
*diesel hammer: In easy driving, FVP can be less than 1.0, and data is still proportional.
*any hammer, if there is significant skin friction in the top portion of the pile, FVP can be more than 1.0, and data is still proportional.
Summary: Proportionality means that WU is smooth from 0 to 2L/c. It does not matter what FVP value is.
Details: Section 8.1 of GPC Review User Manual;
Slide 8, Slides 11-13 of GPC Training at FDOT (See Links from Top Menu);
which is at 12:40 and 15:10 to 21:30 of Video 14 on YouTube (See List of Videos from Links from Top Menu).
5) Is lump CASE damping factor JC a pure soil parameter?
No.
It was a universal misunderstanding that JC is a pure soil factor. Because of this misunderstanding, many users would try to distort signal matching analysis to force the JC value to be within 0.4-0.9 range.
JC, first and foremost, is influenced by the size of the hammer energy (compared to pile resistance and pile impedance). (Larger hammer produces higher toe velocity)
Then, with an average hammer and an average pile size (says, concrete with L/D =30 to 50), JC is a soil factor.
Extreme examples:
A 1-kg hammer to strike 100-ton resistance pile, the pile toe never moves. JC =0 (similarly, for Drilled Shafts or Bored Piles, most testing hammers are not able to move the pile toe to a significant displacement, and the resulted JC is typically low).
A 10-ton hammer to strike the same pile (obviously same soil), the pile toe just plunges. The resulted JC is very high - typically much larger than 0.9.
Because of this, normally we see that steel piles will encounter higher JC values than concrete piles (same soils). This is because steel piles generally have less impedance (much smaller cross sectional area). Furthermore, the shorter the pile, the higher the JC (and vice versa) because toe velocities of short piles are generally larger. Here are CAPWAP examples where JC is much larger than 0.9.
Conclusion: JC is not always within the typical range of 0.4 to 0.9.
Also note that the lump CASE damping JC has no relationship to the Smith damping values (Smith damping values are always soil-dependent).
6) Why for a same hammer, at the same job site, falling at the same drop height (i.e., stroke height), but EMX values are so different?
EMX = maximum value of the integration of F and V.
Therefore, for a pile with mostly positive velocity (positive when the pile is going down, and negative when the pile is rebounding), then EMX can be high.
In the 1st example (of a pile with not much resistance), EMX will occur at 10.5L/c, where F*V starts to become negative. Since V is high, despite a low FMX of 120 kip, EMX is quite large at 8 k-ft (and ETH is 50%)
In the 2nd example (of an end-bearing pile) EMX will occur at 9.5L/c. However, V is close to 0 from 5L/c to 9.5L/c, as such the integration of F*V is low (despite a high FMX of 163 kips), and EMX = 4.2 k-ft (ETH is only 26%).
In the 3rd example, the 3rd pile even has more end bearing, and velocity is less. Comparing pile 2 to pile 3, we will see that the positive portion of pile 3's velocity is below that of Pile 2 (orangish red hatch in Fig. 4.)
Thus, despite both piles having similar FMX of 163 kips, pile 3 has smaller EMX of 3.4 k-ft (ETH=16%).
Fig. 2. Example 1.
Fig. 3. Example 2 - End-bearing pile (EMX = 4.2 k-ft)
Fig. 4. Example 3 - Two end-bearing piles (EMX = 4.2 & 3.4 k-ft)
7) Can CAPWAP run on GPC file?
No.
Tdms files can not be exported to CAPWAP format. However, N_GAPA is extremely easy to use, the parameters have the same names (SS, ST, QS, QT, SO, OP, TG, UN, etc.) as CAPWAP. Once users use N_GAPA more than a few times, they will love it .
8) Can N_GAPA run on PDA file?
Yes, if the PDA blow is copied to Clip Board (Ctrl C), then N_GAPA can read it (see Example folder in Links )
9) Why maximum displacement in the Simulated Static Load Test graph (in CAPWAP or in N_GAPA) is typically higher than DMX?
When DMX occurs during dynamic event, not all resistances (skin friction or end bearing) reaches the maximum values of Ri shown in "Node Table". For example, DMX = 10 mm occurred at time of 15ms.
At 7ms, element i=1 reached the maximum value R1, while elements i=2 to i=N-1 have not reached the corresponding maximum values of Ri.
At 8ms, element i=2 reached the maximum value R2, but element 1 displacement has now rebounded back, thus skin friction at element i=1 was no longer at its peak of R1.
Eventually at 15ms (at the time of DMX), skin friction at few of the top elements have come down (from their corresponding peaks), skin friction at few middle elements reach their peak values, while skin friction at few of the bottom elements may have not reached the peak values yet.
Static signal matching capacity RU is the sum of all the peaked Ri values.
During a Static Load Test, since the top load is held for a long time (i.e., "Static" mode), all of the maximum Ri values are kept at the same time (not at different times as the wave propagates down in the Dynamic event). Therefore, the maximum displacement in the Simulated Static Load Test graph is typically higher than the DMX value.
Additionally, for concrete material, during static mode, it is assumed that E(static) = 0.8 * E(dynamic).
10) DLT is carried out to failure (Ultimate Load). Can the pile still be used at full capacity as production piles?
a. For driven piles: Yes.
b. For bored piles (drilled shafts): No
For driven piles, the pile perimeter surface is smooth. The skin resistance (known as skin friction) will regain back its strength per conventional soil mechanics (considering setup or relaxation as well).
For bored piles (drilled shafts), the concrete surface is rough. Additionally, there was chemical bonding between the freshly cured concrete and the soil. The DLT test failure has not only broken this chemical bonding, but also created a slightly smoother sliding surface.
11) Can GPC use non-GPC gauges?
Yes. For example:
12) Soil Setup or Relaxation and the Need to Test after Pile Installation:
a) Soil Setup:
Resistance: ri = c + (sh – uhydrostatic) *tan(fi) sh is horizontal stress
During driving: ri = cremolded + (sh – uhydrostatic – uexcess) *tan(fi)
•in clay, ri during driving is low, but blow counts are high.
reason: permeability is extremely low, water is trapped; thus, pile driving is on top of an elastic water balloon -> high blow counts (but low static resistance as water has no static resistance).
- overtime, cremolded may gradually regain strength (toward the initial c)
- overtime, excess pore pressure dissipates (back to hydrostatic condition)
Thus, resistance would increase overtime. This is called Setup.
b) Soil relaxation in saturated i) very dense silty fine sands; or ii) shales.
driving process causes new void (yes!, new void, instead of soil densification in most other soils): i) very dense soils dilate; ii) shales crack.
Low enough permeability prevents water to fill in the new void immediately.
Thus, uexcess is negative => ri is currently artificially inflated (not the true long-term resistance).
- overtime, excess pore pressure dissipates (back to hydrostatic condition)
Thus, resistance would decrease overtime.
Conclusion: for some types of soils, Long Term Capacity (testing after a long wait time after installation) is much different than the Current Capacity.
13) Plugging or Coring of Open-Ended Pipe Piles and H Piles
to be continue
14) I am accidently on Standby mode, but I noticed that GPC is still recording data blow. Is it normal?
There are 2 modes (Standby as orange, and Drive as green color):
1. Standby. This mode can be displayed as i) Click to Start Drive or ii) in Standby, Click to Resume
"Standby" is when you wait for the Contractor on the same pile (i.e., you haven't finished testing, there are still future blows, but either you or Contractor is not ready to test yet). We strongly recommend the users to display Sensor Check Balance while waiting.
2. Drive. This mode is displayed as In Driving. Click to Pause
In Standby mode, GPC will still intermittently collect some hammer blow (just in case user is on Standby by accident, when pile driving is actually happening)
In Driving mode, GPC will collect all hammer blows that have sufficient energy.