Do you not have documentation of the deep foundations supporting the bridge or building at your project site? Have you ever wondered if you can reuse an existing deep foundation for a new structure, or to carry more load from a remodel? There are multiple test methods that can aid in determining several characteristics of undocumented or unknown deep foundations such as: estimating installation lengths, evaluating integrity, and evaluating load-carrying resistance. The method we at Braun Intertec employ depends on the project needs and accessibility to the deep foundation. In this two-part article, we will discuss the methodology of the test methods, our approach in analyzing the collected data, the pros and cons of the test methods, and go through case studies. In Part 1, we will focus on low-strain integrity testing commonly referred to as PIT or SE-IR.
Low-Strain Integrity Testing
Low-strain integrity testing is based on the theory of one-dimensional stress wave propagation. An impact is applied at one end of the deep foundation to impart a stress wave and the response of the stress wave is measured with an accelerometer or geophone. Typically, both the accelerometer and impact point are at or near the top of the deep foundation. As illustrated in Figure 1, the stress wave travels along the deep foundation until meeting a change in the deep foundation’s properties (cross-sectional area, modulus of elasticity, or density) or in the soil resistance. Once it has met a change in property or soil condition, portions or all the stress wave is reflected back to the top of the deep foundation. We can use this test to both determine the relative integrity of the pile and to estimate the deep foundation length.
When evaluating the deep foundation’s integrity based on this method, we look for a reflection or reflections of the stress wave prior to expected length. A deep foundation with good integrity will have a clear reflection from the pile toe with only minor variations to the stress wave between the time of impact and the time of toe reflection. A deep foundation with good integrity might also have negative velocity reflections, often caused by “bulges” from soft soils, auger wobble, or cobble removal resulting in a larger cross-sectional area. On the other hand, a positive velocity reflection indicates an impedance reduction from a void, soil inclusion, necking, or cracked or poor-quality concrete. Although the evaluation can identify significant impedance changes, it is not possible to determine why the impedance change occurs from low-strain integrity testing alone. In addition, what looks like a significant impedance change may not represent a significant change for the deep foundation’s load-carrying capacity.
Since this test method is based on one-dimensional stress wave propagation, estimating an accurate wave speed is critical to estimating the deep foundation length when the length is unknown. We can calculate the wave speed if we know the modulus of elasticity and the density of the material. For steel, we know the wave speed is approximately 16,800 feet per second (fps). However, estimating an accurate wave speed for timber, grout, and concrete can be tricky. The deep foundation’s density and modulus of elasticity varies greatly between different species of wood used for timber piles or mix design and strength of concrete. Typically, good-quality concrete has a wave speed between 11,000 and 15,000 fps. Timber piles have an even larger range of 8,000 to 15,000 fps, and some publications state even higher. We can obtain samples from these deep foundations and run laboratory tests to better estimate density and/or the modulus of elasticity for the given deep foundations. This allows us to better estimate the deep foundation’s wave speed.
A simpler and more accurate way of estimating the wave speed is using a multiple-channel data collection device that uses both an accelerometer and an instrumented impact device, or two accelerometers. For either option, we expose the side of the deep foundation. If we mount the accelerometer a known distance below the deep foundation top and impact the top of the deep foundation with the instrumented hammer, we can measure the time between the impact and the when the stress wave triggers the accelerometer. Then we can calculate the actual wave speed through the deep foundation and use that to estimate the deep foundation length. Similarly, we can mount two accelerometers a known distance apart along the side of the deep foundation, measure the time it takes the stress wave to pass between the accelerometers, and then calculate the wave speed of the stress wave.
Besides not being able to identify exactly why there is an impedance change, the following are some additional considerations of low-strain integrity testing.
- Obtaining usable data at depths where the deep foundation’s length-to-diameter ratio is greater than 30 is unreliable.
- Low-strain integrity testing does not provide information regarding deep foundation resistance.
- A significant impedance change, resulting from a bulge or relatively strong soil/rock, along the length of the deep foundation may obscure the toe reflection or other, more significant changes in impedance.
- Low-strain integrity testing can only determine major defects or integrity concerns.
- When the results do not produce a clear toe response, the test can be inconclusive.
- Evaluation of a composite deep foundation section, such as a pre-cast concrete pile with an H-pile “stinger” or a Monotube pile, is complex and generally unreliable due to the inherent change in impedance properties for these deep foundation types.
Case Study – Red Wing Recycling and RDF Project
After a fire destroyed enough of the existing City of Red Wing’s Recycling and RDF (refuse-derived fuel) facility to require razing the structure down to building’s floor slab in 2017, the City chose to rebuild on the existing site. Before rebuilding, three challenging questions needed to be addressed: what is the condition of the timber piling installed in 1982, can the existing timber piling handle revised design requirements based on current codes, and how to support proposed expansions to the existing facility. We performed low-strain integrity testing to validate that the actual pile lengths matched the pile records we had from the contractor.
To accomplish this, we excavated two test pits adjacent to the exterior foundation lines to expose the timber piles. Once the piles were exposed, we sounded, probed, and visually inspected the piles to confirm they were in good condition. For the low-strain integrity testing on this project, we mounted the accelerometer directly to the exposed timber pile using coupling grease and a machined aluminum block. We struck the top of the foundation directly above each timber pile location with the instrumented hammer at least three times to allow for us to average the collected data for calculations.
As discussed above, striking the top of the foundation a known distance above the accelerometer allowed us to more accurately determine the pile’s wave speed and length, which also improved our evaluation of the pile’s integrity. The low-strain integrity testing data, as shown Figure 3, resulted in calculated pile lengths that were within one foot of the reported pile lengths and indicated the piles were of good integrity. The end results of our investigation and analysis were that no additional piling was need under the existing building, resulting in huge cost savings.
Case Study – Port of Lake Charles
This project included the rehabilitation of an existing berth in the Port of Lake Charles. We were asked to evaluate the length, integrity, and estimate the pile resistance of the existing timber piles at the berth. We will discuss how we evaluated pile resistance in Part 2 of this article. To complete the first two parts of the request, we performed low-strain integrity testing on three piles.
We first performed low-strain integrity testing on a timber pile that the contractor extracted and was cut to a known length. The top graph of Figure 5 shows this data. Testing a pile of known length allowed us to estimate a wave speed for the timber piles. We then performed the same testing on three additional piles to evaluate both integrity and estimate pile lengths. The bottom graph in Figure 5 shows the data for one of the tested pile. 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 pile with similar wave speeds gave us relatively high confidence in the wave speed we were using.
This concludes Part 1 of our discussion of how low-strain integrity testing can be used to help determine pile length and integrity of unknown deep foundation elements. Please look for Part 2 of this series, in which will discuss additional test methods and case studies of unknown or undocumented deep foundations.
To learn about deep foundations for surface transportation projects, you can register for our live webinar, To Bridges and Beyond! Deep Foundations and Surface Transportation Projects, on June 10th. We hope you join us!