In part 2 of our discussion on evaluating unknown deep foundations, we will focus on the methodology, data collection, benefits and drawbacks, and present project examples for high-strain dynamic pile testing and parallel seismic testing. To read part 1 on this subject, click here.
High-Strain Dynamic Pile Testing
High-strain dynamic pile testing (HSDPT) builds on the same one-dimensional wave theory used with low-strain integrity testing and we can use HSDPT in the same way to determine pile length. However, HSDPT does have the same limitation as low-strain integrity testing to needing to know the wave speed of the material to estimate pile length. Unlike low-stain integrity testing where we impart a relatively small stress wave, with HSDPT the idea is to apply a significantly larger stress to the pile, just like the name suggests. It requires attaching stain and accelerometer gages to the pile. From the measured strain and acceleration, we calculate force and velocity. The force and velocity going down the pile is the energy input and is known at impact. As the wave travels down the pile, reflections occur due to soil resistance and impedance changes.
Figure 1: Data of Pile with Early Unloading
We can use dynamic pile testing to both evaluate a pile’s resistance and integrity. We evaluate the integrity of the pile much the same way as we do with low-strain integrity testing. We look for early returns of the stress wave. While the computer programs we use to collect the data also calculates a beta value, or “damage” indicator, it is up to the engineer to determine if the indicator is truly identifying possible damage. Figure 1 shows a beta value of 71 percent. However, this case is not indicative of a damaged pile but of early unloading due to the pile not being fully mobilized by the impact.
In Figure 2, the data shows a beta value of 29 percent. There is also a clear spike in velocity before the time of the toe response. Therefore, we know there is some form of significant damage to this pile.
Figure 2: Data of Damaged Pile at 55.5 feet
Just like with low-strain integrity testing, we need to know either the wave speed of the pile or the pile length to evaluate a pile’s integrity with high-strain dynamic testing . Unlike low-strain integrity testing, we can use high-strain dynamic testing to evaluate the integrity of practically any pile length and of composite pile sections. The primary advantage of high-strain dynamic testing is its ability to allow us to also evaluate a pile’s static resistance, which is usually something else we do not know for a foundation system of unknown or undocumented length.
Like the other test methods, high-strain dynamic pile testing has some disadvantages. The primary one is that it requires the most equipment to perform, namely a drop-weight or pile-driving system. High-strain dynamic testing also requires access to the actual pile top and carries the greatest risk for damaging the test pile of the three methods we discuss. Lastly, there are numerous challenges to determining the static resistance of a pile using high-strain dynamic testing — but we will save that discussion for another day.
Case Study – Port of Lake Charles
As discussed previously, we first performed low-strain integrity testing on a timber pile that the contractor extracted and was cut to a known length. This allowed us to estimate a wave speed through the timber piles. We then performed the same testing on three additional piles, including the pile we also subjected to high-strain dynamic testing, to evaluate both integrity and estimate pile lengths. With the relatively large variation in possible wave speed for timber piling, performing low-strain integrity testing on a known length and then obtaining clear toe responses on three additional piles with similar wave speeds gave us relatively high confidence in the wave speed we were using. Figure 3 shows the high-strain dynamic testing results with a toe response when using a similar wave speed to the low-strain integrity testing. Therefore, we were able to estimate the pile’s static resistance with a significantly higher degree of confidence.
Figure 3: Timber Pile Dynamic Testing Data
Parallel Seismic Testing
This test method requires drilling and casing a borehole within 5 to 10 feet of the deep foundation element and at least 10 feet below the likely foundation toe depth. A hydrophone or geophone receiver is then lowered to a measured interval and an instrumented hammer is used to impact the superstructure, pile cap, or pile to create a stress wave traveling down through the pile and across the soil to the receiver. The receiver is lowered in the borehole and the process is repeated until the receiver reaches the bottom of the borehole. Figure 4 shows the generalized setup and process of parallel seismic testing.
Figure 5. General Setup for Parallel Seismic Testing
The waveform from each depth interval is analyzed individually to determine when the stress wave first arrives at the receiver. After the analysis of each individual wave, the entire data set is plotted together to analyze the combined arrival times and wave speeds. When plotted together, analysis is performed by observing the changes in the slope of the arrival lines, suggesting a change in wave speed (and material) of the corresponding impact wave.
The advantages of parallel seismic testing include being able to evaluate significantly greater pile lengths than low-strain integrity testing, and not needing to expose the piles to perform the test. We also do not need to know the wave speed of the pile as the analysis is looking for the change in wave speed to identify the pile length. Parallel seismic testing can be used to determine the length of composite piles, although the analysis is significantly more complex than simply looking for three slopes in the plot of arrival times.
Some disadvantages of parallel seismic testing are:
- Does not evaluate the integrity or capacity of the pile.
- Abrupt changes in soil conditions can affect how the wave is transmitted to the sensor and produce inconclusive results.
- Requires constructing a bore hole close to the existing foundation to perform testing. The boring must extend sufficiently below the pile being investigated to identify the change in the slope of arrival times.
- Ambient noise from nearby facilities or traffic can distort data and results.
Case Study – 3rd Street Bridge in Minot, ND
A new flood protection project in Minot will include a protection around the existing 3rd Street Bridge. One of the design options evaluated included placing a new levee through the existing embankment near an abutment and pier. We were asked to evaluate lateral loads, downdrag, and settlement effects of the new levee on the existing bridge foundations. To aid our analysis, we performed parallel seismic testing to confirm the installed pile length of the bridge as no construction records were available. The results of this testing confirmed the design pile length in the plans match those of the tests. Figure 5 shows the parallel seismic results from the pier, with the depth referenced to existing grade. The change in the slope of arrival time with depth at 35 feet matched the design pile length of 30 feet below the pile cap, or 35 feet below grade.
Figure 5. Parallel Seismic Testing Results
Conclusion
Not knowing the length of the existing deep foundation at your project site does not mean you cannot reuse or add load to the existing foundation. Low-strain integrity testing, parallel seismic testing, and high-strain dynamic testing can help determine the length and resistance. Braun Intertec has the tools and experience to help you identify the best method based on the goals and limitations of your project. To learn more about this subject, please join us for our upcoming webinar, To Bridges and Beyond! Deep Foundations and Surface Transportation Projects. You can register using the button below.