How to Use Thermal Integrity Profiling to Assess Soil Nail Integrity
Thermal Integrity Profiling is a relatively new nondestructive method that uses the temperature generated by curing cement (hydration energy) to access the integrity of the elements. Initially developed at the University of South Florida to evaluate the homogeneity and integrity of the concrete mass within drilled shafts, as well as the alignment of the reinforcement cage, the method was eventually incorporated in instrumentation developed for commercial use by PDI, from Cleveland, and FGE from Plant City, Fla. The testing method has since been part of several drilled shafts quality control programs, has been employed to test augered cast-in-place (ACIP) piles, and has been expanded and adapted to investigate diaphragm or retaining walls, jet grouting columns and micropiles. The case study described herewith is the first application of this testing method to soil nails. A full description of the demonstration project is available in Piscsalko et al.(2014).
Thermal Integrity Profiling is most frequently performed by affixing Thermal Wire cables to the longitudinal bars of the reinforcing cage of a shaft or, in the case of ACIP piles, to the reinforcing center bar (it may also be performed by lowering thermal probes in access tubes pre-installed in the foundation element). The soil nails tested during this demonstration were instrumented along the tensioned center bar, by attachment of a Thermal Wire cable with digital temperature sensors spaced every 6 inches. Once the instrumented tension bar was set and grout was injected, a Thermal Acquisition Port (TAP, Figure 1) was attached to each cable and data collection began. Every 15 minutes, the TAPs recorded and stored measured temperature at each sensor location, making it possible to generate profiles of temperature versus depth. After a sufficient amount of time had elapsed, each TAP was connected to the main unit of the Thermal Integrity Profiler (TIP, Figure 2), so data could be downloaded for further analysis.
The TIP results allowed for the evaluation of soil nail shape, integrity and grout quality, and of the location of the center bar. The overall average temperature for all Thermal Wire readings over the embedded depths can be directly related to the overall volume of grout installed. Soil nail integrity may be assessed by averaging the temperature measurements from each cable at each depth increment. If the measured average temperature versus depth is approximately constant, the soil nail is considered to be of uniform shape and quality. Bulges can be identified as localized increases in average temperature, while insufficient grout quality or section reductions can be identified as localized decreases in average temperature. Because soil and slurry pockets produce no heat, areas of soil intrusion or inclusion are indicated by lower local temperatures.
The soil nails tested were 6 inches in diameter and contained a #6 epoxy coated center bar. The Thermal Wire cables, measuring 20 feet and fitted with 40 digital temperature sensors, were affixed to the center bars (Figure 3) prior to placement in the drilled hole. Once the center bar was in position in the drill hole, a tremie tube was inserted and pressure grouting performed. Three or four centralizers were located along the center bar.
Test Soil Nails 1 and 2 had no planned manufactured defects, while Test Soil Nail 3 was sacrificial, having been installed with two planned manufactured defects (Figure 4) consisting of sand bags taped to the tension rod (center bar). Defect 1 measured 3 inches by 4.5 inches by 15 inches and was located 13.5 feet from the end of the center bar nearest the wall face. Defect 2 measured 2 inches by 3 inches by 15 inches and was located 17.9 feet from the end of the center bar nearest the wall face.
Test Soil Nails 1 and 2: The profiles of temperature (Fahrenheit degrees) versus depth (feet) throughout the length of the nail are mostly constant and thus there is no indication of major structural integrity problems (Figure 5). The roll-off (reduction) in temperature at the top is caused by extra heat losses due to the grout/air interface at the ground surface. There is generally a roll-off in temperature near the bottom of a grouted element as well, caused by extra heat loss due to the grout/soil interface at the base. That phenomenon is not observed because the Thermal Wire stopped short of the bottom of the drilled holes of these soil nails. Minor changes in temperature are noticeable, most likely due to the center bar shifting slightly within the nail. The upper portions of the nails recorded higher than average temperature values, probably an indication of an increase in cross sectional area. Temperatures measured farther down the nails do not exhibit much variation from the average.
Test Soil Nail 3: Temperature reductions at the locations of installed defects are observed (Figure 6). At 13.5 feet, the location of Defect 1, a temperature reduction of 7 degrees is evidenced. The sandbag had an approximately cross sectional area of 13.5 inches squared, theoretically reducing the cross sectional area of the soil nail (28.3 inches squared) by 48 percent. At 17.9 feet a temperature decrease of roughly 4 degrees is present. This corresponds to the location of Defect 2, which was built with a cross sectional area of 6 inches squared for a theoretical reduction of soil nail cross sectional area of 21 percent.
The demonstration project showed that temperature decreases due to planned soil inclusions are identified via Thermal Integrity Profiling, while intact soil nails are also recognizable. The anomalies may be quantified through data analysis, and an approximate effective cross sectional area reduction predicted as a percentage decrease from the cross sectional area of design. The estimated reductions based on the interpretation of Thermal Profiling were 37 percent and 17 percent, respectively, relatively close to the theoretical reductions caused by installed manufactured defects (48 and 21 percent respectively).
Reductions of effective cross sectional area below 10 to 15 percent, although measurable, will likely not be predicted by Thermal Integrity Profiling due to the effects of normal shifting of the center bar as well as minor changes in drill hole diameter. All soil nails evidenced expected slight temperature decreases at spans between the centralizer locations, consistent with center bar shifting. Temperatures near the centralizers were closer to the average nail temperature.
The demonstration project was successful in demonstrating the use of Thermal Integrity Profiling for assessing the integrity of soil nails.
Piscsalko, G., Alvarez, C., Belardo, D., Galvan, M., October 2014. Using Thermal Integrity Profiling to Evaluate the Structural Integrity of Soil Nails. Deep Foundations Institute 39th Annual Conference on Deep Foundations: Atlanta, Georgia; 195-202.