Highway connection through Ouachita Mountains — Arkansas Tunnel study
The Arkansas State Highway and Transportation Department (AHTD) initiated a high-level tunnel feasibility study for the proposed East-West arterial highway under Hot Springs National Park as part of its due-diligence comparison of an open-cut versus a tunnel option to create a bypass to lessen traffic impacts on historic Hot Springs. The new tunnel on the proposed East- West bypass highway would be situated approximately 2 km (1.25 miles) north of a planned intersection with Highway 70, as shown in Fig. 1. The tunnel study addressed feasibility, constructability and conceptual cost and schedule estimates for a new tunnel alignment through elevated ridge terrain featuring an extremely hard novaculite formation. Considering the existing site topography and local geologic conditions, an appropriate tunnel construction strategy, including temporary and permanent tunnel support, was developed. Considering the highly fractured nature of the novaculite, a sequential excavation method (SEM) tunneling approach was developed. The SEM approach was cost estimated and scheduled for comparison with the open-cut alternative.
Existing conditions
Geologic setting. The tunnel alignment passes through addressed feasibility, constructability and conceptual cost and schedule estimates for a new tunnel alignment through elevated ridge terrain featuring an extremely hard novaculite formation. Considering the existing site a prominent ridge of the Ouachita Mountains, which consist of outcrops of the Big Fort Chert, the Arkansas Novaculite and the Stanley Shale. Novaculite is the predominant geologic formation of the ridge, as the less resistant rock has been eroded over time. Novaculite is a dense, hard, fine-grained, siliceous metamorphic rock. It is a subcategory of chert, which has transformed into novaculite after low-grade metamorphism.
Physical properties. Novaculite is a very hard and very dense rock that varies in color from white to grayblack. Novaculite is not only difficult to core with very slow drilling rates but also yields minimal rock recovery due to very brittle properties. Table 1 presents generalized engineering properties of novaculite, which are subject to confirmation based on a site-specific investigation and laboratory testing program including rock mass characterization data.
The difficulties encountered during coring attempts should not be considered as anticipated behavior for the drilling of holes for blasting and/or rock bolting. In fact, percussion drilling can be successfully performed in brittle rock, as brittle rocks do break or spall under concentrated loading. However, increased bit wear should be anticipated due to the relative hardness of the rock. Novaculite’s microcrystalline structure makes it both very hard and very brittle. The formation has also been subjected to tectonically induced folding, which has caused extensive fracture patterns in the rock as shown in Fig. 2.
Subsurface profile and ground water conditions. Difficult drilling access due to remote site location and mountainous terrain limited the available subsurface data to a single test boring shown in Fig. 3 with the approximate tunnel horizon. Ground water was encountered in boring 692 at an approximate elevation of 250 m (817 ft). This indicates a ground-water level approximately 3 m (10 ft) above the tunnel invert. The temperature of the ground water encountered during mining is anticipated to be comparable to the ambient air temperature of the area. The ground water may contain a higher-thannormal concentration of silica due to migration through silica-rich novaculite. Fortunately, silica is not detrimental to either typical waterproofing materials or in-place concrete. The current tunnel design concept considers a watertight (partially tanked) system (Fig. 4) allowing ground water to flow around the tunnel structure and either bypass the tunnel or drain into a subdrain system. The impact of potential localized depression of the ground-water table would be evaluated during subsequent phases of design. A hydrogeologic analysis would also assist in developing drainage design as well as consideration of an alternative, fully enclosed waterproofing concept.
Tunnel design basis
General configuration. Considering a 5.5 percent roadway grade, the proposed two-lane vehicular highway tunnel extension will be approximately 365 m (1,200 ft) long. The tunnel study is based on a straight horizontal alignment crossing through the Hot Springs National Park Mountain in Arkansas accommodating a vertical curve with a high point midway between the portals. The relatively short length of tunnel and the existing rock formation favor mining by sequential excavation method. The cross section provided in Fig. 4 had been developed in conformance with the most suitable shapes for this type of tunnel and the specific space requirements of a two-lane highway tunnel.
Rock loading. The study considered a permanent reinforced concrete tunnel lining to be constructed after initial support installed over the entire tunnel length. The tunnel final lining is subject to rock loads imposed by the overburden, the surrounding ground and hydrostatic pressure from ground water. The lining will be subject to minimal hydrostatic pressure due to the location of the ground water, which is only 3 m (10 ft) above the tunnel invert. The hydrostatic pressure is anticipated to decrease over time due to the tunnel’s pressure relief and drainage system.
A top heading and bench sequential excavation scheme assuming 1.2-m (4-ft) round lengths was used in the tunnel study. An initial rock support system consisting of spiles, lattice girders, shotcrete and rock dowels was considered installed immediately after the full cross section was excavated over the length of the round. Upon completion of the excavation and installation of initial support, drainage and waterproofing, the final liner will be constructed. Rock loads on the completed tunnel primarily impart compressive stresses in the final liner. However, shear and bending stresses will be exerted on various locations in the liner due to the nonuniform curvature of the tunnel cross section and uneven distribution of rock loads. The final liner design thickness and reinforcement will be governed by the combination of these parameters of compressive, shear and moments. Rock-loading estimates were performed using applicable methods, including empirical (Terzaghi, 1946), Norwegian Geological Institute’s (NGI) Q-system and, for comparison, a numerical finite-element method (FEM) analysis. Specifically, based on the lone nearby boring log, the rock core within the tunnel excavation zone of influence was reported as hard, slightly weathered novaculite with a variable degree of fracturing and included localized, more closely fractured and ferrous stained zones. Site-visit observation notes and photographs confirmed the presence of moderately to highly fractured rock conditions as well as steep cuts with minimal erosion. Hence, the rock-mass condition was characterized as moderately blocky and seamy (MBS) to very blocky and seamy (VBS) under the Terzaghi rock classification system (Terzaghi, 1946) and as corresponding rock mass quality Q = 1 to 5, representing poor to fair conditions, by NGI’s Q-system. Table 2 and Fig. 5 summarize the vertical rock support load and equivalent height of rock using these systems.
The FEM analysis (Table 3 and Fig. 6) was based on a ground-relaxation scheme, conservatively assuming a 30 percent ground relaxation value, in order to evaluate previously determined values as well as internal member forces for final lining design.
Drained versus undrained. Reduction of external forces acting on the final liner is promoted by enveloping the waterproof liner with a geotextile material that will facilitate the draining of ground water, thereby reducing hydrostatic pressure. Water collected at the base of the liner will flow through perforated pipes installed on each side of the tunnel to discharge at portals. The drained condition was considered representative of the tunnel conditions featuring variable ground-water levels reflecting the local topography, indicating limited (< 1.2-m or 10-ft) hydrostatic head.
Fire load. Fire effects on the final liner include: (a) additional bending forces due to temperature gradient occurring between the hot interior face and the cool exterior face of the liner, (b) local cracking and explosive spalling of concrete along the face directly exposed to fire and (c) reduction of tensile strength of reinforcing steel. The fire load resulting from combustion of a vehicle is generally assumed to be equivalent to the energy of 50 MW over a two-hour period. Mitigation measures include: (a) incorporating additional bending stresses due to temperature gradient into the design analysis as an extreme loading case to verify integrity of the liner, (b) using polypropylene fibers in the concrete mix to minimize explosive spalling, which help to create voids in the concrete to allow expansion of water vapor and limit buildup of internal pressure, and (c) increasing concrete cover or installing a passive fire protection system to protect reinforcement from being excessively heated (i.e., reduce the thermal exposure of the steel).
Construction considerations
The tunnel study included the following construction considerations, which informed the development of the cost estimate.
Excavation and support sequence. The predominant rock formation is very hard. However, mechanized excavation methods, including roadheader and backhoe excavator, appear feasible due to the moderately to highly fractured condition of the rock mass. Drill-andblast excavation does not appear to be necessary based on existing rock mass conditions. Figure 7 shows a feasible excavation sequence totaling five drifts under the top heading and bench arrangement. The proposed initial support system for this type of rock condition includes spiling presupport measures and steel-fiberreinforced shotcrete (SRFS) with lattice girder and rock dowels on 1.2 m (4 ft)-round length.
Rock drillability. As described in the physicalproperties section, the host rock possesses a Moh’s relative hardness scale value of 7, corresponding to the upper end of the hardness scale, and hence would require specialty percussion button bits for drilling holes for presupport spiling and rock-support dowels.
Portals. Portal development commences with precut construction via top-down excavation of headwall for the launch portal. The portal construction begins using mechanized rock excavation equipment to build a headwall battered at the steepest stable slope possible based on actual rock-mass orientation, which extends to the proposed portal invert elevation. Anticipated support includes pattern rock dowels combined with structural shotcrete layer. Per Federal Highway Administration (FHWA) recommendations, a portal canopy structure projecting approximately 15 m (50 ft) from the headwall will be constructed as fall protection. Additional fall protection over the portal features a catch-fence device at the base of the slope, which would be subject to periodic maintenance and cleaning. Specifically, the portal canopy section will be constructed of expanded metal liner covered by a nominal 10 cm (4 in.)-thick shotcrete layer externally applied.
Lattice girder encased in nominal 20-cm (8-in.) shotcrete will line the interior side of the metal liner. Waterproofing layer will be installed between the inner shotcrete face and the final cast-in-place concrete lining. Collection of tunnel drainage will be provided by sedimentation (holding) tanks at each portal.
Construction cost estimate
A rough-order-of-magnitude (ROM) construction cost estimate for the 365 m (1,200 ft)-long roadway tunnel based on anticipated material quantities, equipment and crew sizes came to $68 million, including an approximate 10 percent contingency over a 16-month construction duration.
Conclusions
Lessons learned. The results of the tunnel feasibility study indicate that: (a) the tunnel option is constructible through the prevailing geological formation using mining methods that require commonly available equipment and local workforce, (b) the tunnel option presents an environmentally favorable solution, with minimal surface impacts and (c) an economically sound tunnel solution providing safe highway connection through mountainous terrain is possible. (References available from the authors.)
