Evaluation of geocell-reinforced backfill for airfield pavement repair

After an airfield has been attacked, temporary airfield pavement repairs should be accomplished quickly to restore flight operations. Often, the repairs are made with inadequate materials and insufficient manpower due to limited available resources. Legacy airfield damage repair (ADR) methods for repairing bomb damage consist of using bomb damage debris to fill the crater, followed by placement of crushed stone or rapid-setting flowable fill backfill with a foreign object debris (FOD) cover. While these backfill methods have provided successful results, they are heavily dependent on specific material and equipment resources that are not always readily available. Under emergency conditions, it is desirable to reduce the logistical burden while providing a suitable repair, especially in areas with weak subgrades. Geocells are cellular confinement systems of interconnected cells that can be used to reinforce geotechnical materials. The primary benefit of geocells is that lower quality backfill materials can be used instead of crushed stone to provide a temporary repair. This report summarizes a series of laboratory and field experiments performed to evaluate different geocell materials and geometries in combinations with a variety of soils to verify their effectiveness at supporting heavy aircraft loads. Results provide specific recommendations for using geocell technology for backfill reinforcement for emergency airfield repairs.

Geotechnical effects on fiber optic distributed acoustic sensing performance

Distributed Acoustic Sensing (DAS) is a fiber optic sensing system that is used for vibration monitoring. At a minimum, DAS is composed of a fiber optic cable and an optic analyzer called an interrogator. The oil and gas industry has used DAS for over a decade to monitor infrastructure such as pipelines for leaks, and in recent years changes in DAS performance over time have been observed for DAS arrays that are buried in the ground. This dissertation investigates the effect that soil type, soil temperature, soil moisture, time in-situ, and vehicle loading have on DAS performance for fiber optic cables buried in soil. This was accomplished through a field testing program involving two newly installed DAS arrays. For the first installation, a new portion of DAS array was added to an existing DAS array installed a decade prior. The new portion of the DAS array was installed in four different soil types: native fill, sand, gravel, and an excavatable flowable fill. Soil moisture and temperature sensors were buried adjacent to the fiber optic cable to monitor seasonal environmental changes over time. Periodic impact testing was performed at set locations along the DAS array for over one year. A second, temporary DAS array was installed to test the effect of vehicle loading on DAS performance. Signal to Noise Ratio (SNR) of the DAS response was used for all the tests to evaluate the system performance. The results of the impact testing program indicated that the portions of the array in gravel performed more consistently over time. Changes in soil moisture or soil temperature did not appear to affect DAS performance. The results also indicated that time DAS performance does change somewhat over time. Performance variance increased in new portions of array in all material types through time. The SNR in portions of the DAS array in native silty sand material dropped slightly, while the SNR in portions of the array in sand fill and flowable fill material decreased significantly over time. This significant change in performance occurred while testing halted from March 2020 to August 2020 due to the Covid-19 pandemic. These significant changes in performance were observed in the new portion of test bed, while the performance of the prior installation remained consistent. It may be that, after some time in-situ, SNR in a DAS array will reach a steady state. Though it is unfortunate that testing was on pause while changes in DAS performance developed, the observed changes emphasize the potential of DAS to be used for infrastructure change-detection monitoring. In the temporary test bed, increasing vehicle loads were observed to increase DAS performance, although there was considerable variability in the measured SNR. The significant variation in DAS response is likely due to various industrial activities on-site and some disturbance to the array while on-boarding and off-boarding vehicles. The results of this experiment indicated that the presence of load on less than 10% of an array channel length may improve DAS performance. Overall, this dissertation provides guidance that can help inform the civil engineering community with respect to installation design recommendations related to DAS used for infrastructure monitoring.

STRENGTH OF CONCRETE MORTAR PREPARED BY PARTIAL SUBSTITUTION OF CEMENT WITH RECYCLED GLASS POWDER

This experiment was conducted to determine the utility of substituting cement with the recycled glass powder (RGP) in mortar mixtures. A total of 21 mortar mixtures were produced using various RGP (FG) ratios (CG), and fly ash (FA) powders. The mortar mixtures were used to prepare cubes which were tested for 7-and 28-day compressive strength. The substitution of cement with FG and CG in mortar resulted in reduced 7-and 28-day compressive strength values. However, the amount and type of RGP substituted for cement plays a crucial role in the determination of mortar strength. Above contraction in compressive strength was observed at an initial maturity than at the final maturity. Further, replacement of cement with Fly Ash showed increase in compressive strength up to certain content. More research and testing for the optimal percentage and size of waste glass powder that can be used is required in flowable fill.

Rapid Airfield Damage Recovery Next Generation Backfill Technologies Comparison Experiment : Technology Comparison Experiment

The Rapid Airfield Damage Recovery (RADR) Next Generation Backfill Technology Comparison Experiment was conducted in July 2017 at the East Campus of the U.S. Army Engineer Research and Development Center (ERDC), located in Vicksburg, MS. The experiment evaluated three different crater backfill technologies to compare their performance and develop a technology trade-off a nalysis. The RADR next generation backfill technologies were compared to the current RADR standard backfill method of flowable fill. Results from this experiment provided useful information on technology rankings and trade-offs. This effort resulted in successful crater backfill solutions that were recommended for further end user evaluation.

Numerical investigation on rigid and flexible pipelines embedded in granular and self-compacting materials

Self-compacting filling material or controlled low-strength material (CLSM) is a cementitious material which is liquid during filling, and it is used primarily as backfill, e.g., in trenches. Several products are currently used as CLSM such as flowable fill, controlled density fill, flowable mortar and low-strength plastic soil–cement. The low-strength requirement is necessary to allow for future excavation of CLSM. A two-dimensional numerical model was developed using the finite element system ABAQUS. In this model, the material behavior of granular soil and CLSM is described using an elasto-plastic constitutive model with Mohr–Coulomb failure criterion. Rigid and flexible pipes were modeled once embedded in sandy soil and once embedded in self-compacting material. The numerical model allows the modeling of the effect of hardening process on the overall behavior of the pipe–soil system. The main objective of this study is to investigate the behavior of rigid and flexible pipelines embedded in CLSM as a filling material numerically and to show advantages and disadvantages in comparison with the presently widely used filling materials like sand.

Full-Scale Laboratory Evaluation of the Effectiveness of Subgrade Soil Stabilization Practices for Portland Cement Concrete Pavements Patching Applications

Adequate quality of subgrade under patched areas can extend the service life of concrete pavements and reduce their maintenance costs. In this study, the performance of various subgrade stabilization scenarios was evaluated and compared with each other in a full-scale laboratory-based setup. For this purpose, a test box with the footprint of 6 × 6 ft and a height of 4 ft was constructed using C steel channels. The test box was used to investigate the effects of various types of soil stabilization methods, such as chemical (cement) stabilization, use of aggregate base course (ABC), geogrid (GG) and geotextile (GT) with ABC, GT with cement-stabilized soil, GT with in-situ compacted soil, flowable fill, and lean concrete. The test results showed that all stabilization techniques successfully improved the performance of the subgrade layer by decreasing the deformation under the fatigue loading representing a single axle load of 9,000 lbf/tire. The use of GT with aggregate-based layers was found to significantly reduce the amount of settlement. Subgrade layers stabilized with GG also experienced lower values of deformations compared with the unmodified (control) section. However, GG was not as effective as GT. The use of cement-treated aggregate and lean concrete reduced the deformations to negligible levels.