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Yeager Airport: Deconstruction and reconstruction of the side of a mountain

April 1st, 2019 / By: , , / Feature

FIGURE 1 Aerial view of the 2008 failed EMAS slope

Yeager Airport is located on top of a series of leveled mountains in Charleston, W.Va. Completion of the original airport construction in the 1940s required 9 million cubic yards (6.9 million m3) of soil and rock earthwork (Keller 2017). In 2005, due to Federal Aviation Administration (FAA) safety requirements, it was necessary to construct a safety area at the end of Runway 5 to install a 500-foot (150-m) long engineered material arresting system (EMAS). Approximately 1 million cubic yards (765,000 m3) of fill was placed, in part as a reinforced soil slope (RSS) and a 1.5H:1V unreinforced slope, to build the safety area. The RSS slope zone was 240 feet (73 m) high at a 1H:1V slope. The RSS slope constructed between August 2005 and December 2008 failed in March 2015. The failed slope is shown in Figure 1. The RSS slope failure destroyed multiple homes and a church, blocked Elk Twomile Creek resulting in major flood damage to upstream homes, and disrupted airport operations. The slope failure also damaged power lines, sanitary sewer lines and storm sewer lines, and completely blocked a key corridor road in northeast Charleston.

As part of the restoration plan after the 2015 slope failure, 540,000 cubic yards (413,000 m3) of soil was removed from the end of Runway 5-23 between the summer of 2015 and December 2016. Emergency Runway 5 safety area and slope mitigation work was carried out in three construction phases and was completed in 2017. The deconstruction included a list of up-front challenges and uncertainties that affected safety and mitigation design: materials used, in situ density and strength, remnant grid limits, and the unknown slope failure mechanism. As such, risk was a foremost consideration for the Airport Authority as well as the engineers.

Two of the most critical risk concerns of the authority were personnel safety (workers as well as the public) and the risk of further movements causing a blockage and repeated flooding of Elk Twomile Creek.

FIGURE 2 Removal of vertical soil face at runway level
Slope deconstruction

Phase 1 of the deconstruction was an emergency response focused on stabilizing the upper portion of the slope face. This near-vertical soil face, which was approximately 140 feet (43 m) high, is shown in Figure 2 with the most critical area, referred to as the Chimney, jutting out in front of the main face line. Visibly widening tension cracks on the remaining EMAS fill surface and large portions of the face breaking away reinforced the need to act quickly to limit further damage to the valley below. The observed instability posed safety concerns to workers on the runway trying to deconstruct the slopes. Another concern was the behavior of the debris field below in the event of fallen debris. The sheared geogrid provided some level of reinforcement, though unquantifiable for the deconstruction efforts that would require working from the top down. Given the risk of ongoing collapse, placing personnel and equipment below the vertical face was not acceptable.

The risk of ongoing collapse was confirmed when the failure of 10,000 cubic yards (7,600 m3) of material occurred in late June 2015 just as deconstruction was to begin. The block of soil that collapsed resulted in additional loading on top of the debris pile and movement of the debris mass approximately 50 feet (15 m) toward the creek. Additional movement would potentially cause another blockage of the creek and upstream flooding. The engineering team was given three weeks to complete a design package and solicit construction bids for the deconstruction.

Given the challenges noted and the unusual nature of the remnant slope, it was recognized that traditional geo-technical analysis and detailed slope stability modeling would have to be supplemented by the observational approach. By identifying risks, challenges and alternatives through formal risk register development meetings with stakeholders, the engineering team identified possible solutions and narrowed the focus to developing steps that would improve stability as the work progressed. The key to this was beginning the excavation well behind the face and unloading enough weight of soil as the contractor moved toward the face to offset the loading that would be created by the equipment. Consideration was given to using long-reach equipment; however, industry day meetings (held within this three-week design time) gathered feedback from contractors that the power of such equipment was not likely to be sufficient to excavate the fill mass, particularly with the geogrid present.

FIGURE 3 Exposure and removal of in-place geogrid reinforcement during Phase 1 deconstruction

A key part of the design was specification of daily safety briefings, site log-in sheets, ongoing monitoring of the slope, regular site walks to identify changes in observed cracks, and a buddy system (Figure 3) for anyone approaching the slope face. An instrumentation system consisting of survey targets was installed at the crest of the vertical soil face to monitor possible movements during deconstruction. This was intended to provide early warning of possible movement to the workers.

The removal of 212,000 cubic yards (162,000 m3) of material resulted in a sloped embankment with grades of about 1.5H:1V, with benches at 30 feet (9 m) and 80 feet (24 m) immediately below the runway level, and greatly reduced the risk of further loading being applied to the debris mass below. Below the graded slopes and bench areas, a rock face was encountered and exposed, as shown in Figure 4, and slope configurations were adjusted accordingly. Instrumentation was installed in the first bench constructed 30 feet (9 m) below the runway level. Measurand ShapeArray accelerometers (SAA) and vibrating wire piezometers were placed through the remaining fill and shallow rock both to monitor the stability of the upper areas of the slope while ongoing debris removal continued and to provide monitoring during times when personnel were not on-site. The SAAs were configured such that the engineer would receive emergency notice if slope movements exceeded 0.5 inch (1.25 cm). This magnitude of downhill slope movement was not observed during the remainder of the deconstruction.

FIGURE 4 Phase 3 deconstruction: fill removal over Keystone Drive at the bottom of the cut slope

Phase 2 consisted of the removal of the upper portions of the debris field and resulted in the additional removal of 81,000 cubic yards (62,000 m3) of material. Phase 2 was completed between October and December 2015. Some minor smoothing and grading of the remaining massive debris field were performed at the end of Phase 2 to direct surface runoff away from the creeping slope of the debris. Work was halted at the end of Phase 2 due to weather conditions in the area making work (trucking on the silty soils and steep slopes) difficult.

Phase 3 was initiated in the summer of 2016 to continue removal of a large portion of the debris field and ultimately included reopening Keystone Drive, repairing and restoring the operation of the Charleston Sanitary Authority sewer line and other utilities in the area, and repairing the subsurface stormwater drainage pipes beneath Keystone Drive. Although slopes of about 1.5H:1V with intermittent benches were designed for this work, site conditions, ongoing sloughing of the debris material, and development and eradication of haul roads resulted in removal of additional debris and elimination of several benches where rock surfaces were exposed in the cut. During Phase 3, 250,000 cubic yards (192,000 m3) of material were excavated.

FIGURE 5 Front view of retaining wall prior to winter shutdown

The majority of excavated materials from the three phases was disposed of in the original borrow area to the west of Runway 5 (referred to as the Waste Area). A large flat area and berms were incorporated into the final slope configurations to provide a level of protection against the potential for rock fall debris affecting the safety of the Keystone Drive area below. Phase 3 was completed between July 2016 and April 2017.

Reconstruction

The next phase of the project was a bottom-up constructed retaining wall to restore sufficient runway length to replace the EMAS system and meet FAA safety requirements. The retaining wall is 400 feet (122 m) long and up to 83 feet (25 m) high (Figure 5). The wall extends from a working bench at Elevation 860 feet (262 m) that was a result of Phase 1 deconstruction, to the EMAS at Elevation 945 feet (288 m). The wall was designed and constructed to accommodate the difficult site conditions, taking advantage of the rock exposed near the Elevation 860 bench for wall support. To control load magnitudes as well as ease construction, the wall was designed with two distinct sections. The lower 25-foot (7.6-m) high portion of the wall was designed to retain compacted soil backfill and be secured to the existing sandstone bedrock with rock-socketed steel piles and post-tensioned tieback anchors. The soil backfill used for this portion of the wall was reprocessed borrow material from the RSS deconstruction that was stored in the waste area. This soil backfill also establishes a 2:1 backslope in the area. The upper portion of the wall is to be backfilled with up to 55 feet (17 m) of lightweight geofoam blocks, with a steel post and concrete panel facing for upper portions of the wall.

The wall was designed for a factor of safety of 1.5 for normal conditions and 1.1 for seismic conditions. To maximize extended runway length and wall performance, the wall was designed for a top of wall deflection no greater than 1 inch (2.54 cm). To limit upper wall deflections, a deadman pile system with three rows of tie-rods and lightweight geofoam, as mentioned previously, was used. Plaxis 2D, a finite element modeling software, was used to model the response of the system under staged construction and provide forces and stress development in both the soil and structural elements at critical sections of the wall. Structural components of the wall were designed using American Association of State Highway and Transportation Officials (AASHTO) load combinations and load resistance factors. Further details of this reconstruction are provided by Simon et al. 2019.

Conclusion

In 2015, the RSA and EMAS were destroyed when the 240-foot (73-m) high reinforced soil slope failed at Yeager Airport in Charleston, W.Va. The result of the structural fill failure was a near-vertical soil face in the remnant fill over a massive debris field. Engineered mitigation plans were necessary for the safe removal of 540,000 cubic yards (413,000 m3) of debris and to create a temporarily stable condition in wait of a permanent solution. The cleanup and stabilization of the area resulted in the displacement of the Runway 5 threshold and the shortening of the usable length of Runway 5-23. The deconstruction left a temporary 1.5H:1V slope with working benches at Elevation 860 feet (262 m) and 910 feet (277 m), approximately 17 feet (5 m) wide, and a stockpile of soil to be used as part of the new wall backfill. The retaining wall is 83 feet (25 m) high and extends from the working bench at Elevation 860 feet (262 m) up to Runway 5 elevation at 945 feet (288 m) to restore the EMAS. A key element of achieving this reconstruction alternative was limiting the wall earth pressure loads through the use of geofoam backfill for about 2/3 of the height.

Risk assessments, including a formal risk register, were used throughout this project. Engagement of all stakeholders was a critical element of this project and included airport management, operations, maintenance, contractors; agencies; material suppliers; and others—all contributed to identifying concerns, prioritizations and mitigation option development. Engagement throughout the process is believed to have been a critical component of achieving the rapid response of both the deconstruction and the restoration design and construction.

Acknowledgments

We would like to thank the many others at Schnabel Engineering who were involved in the project as well as the innumerable parties who assisted with the risk register discussed above. The exceptional staff at Yeager Airport provided significant support throughout this effort with the clearly focused goal of restoring the safety of the airport as soon as possible. Airport Design Consultants Inc., S&S Engineering, SWS Engineering and Dr. Arelanno were critical to the design and review of this innovative wall solution.

References

Keller, N. (2017). Yeager Airport and Charleston Aviation, Arcadia, Charleston, S.C.
Simon, J., Cadden, A. W., Shull, P., and Senior, M. (2019). “Restoring RW5 at Yeager Airport: Design and construction of tall retaining wall on the side of a mountain.” Proc., ASCE Geo-Congress 2019, Philadelphia, Pa.

 

Allen Cadden, P.E., D.GE., is a principal at Schnabel Engineering in Chadds Ford, Penn.

Gary Brill, P.E., is the branch leader at Schnabel Engineering in Knoxville, Tenn.

Michael Senior, EIT, is a senior staff engineer at Schnabel Engineering in Chadds Ford, Penn.

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