Design and construction considerations for intersecting tunnels — Charleston, SC
Founded in 1670, Charleston, SC is one of oldest cities in the nation. Since its inception, the city of Charleston has experienced difficulties with stormwater management. With rising tidal waters and stronger storms, stormwater management has only become more difficult. Tunnels and trenchless technology have roots in Charleston dating back to 1928 when a system of water supply tunnels was constructed to bring a new water supply from the Edisto River and Foster Creek to the Hanahan Water Treatment Plant to supplement ground water sources. Today nearly 80 km (50 miles) of tunnels have been constructed or designed in and around Charleston for water, waste water and storm water conveyance.
The city of Charleston first utilized tunnels to address storm water in 1999 for their Meeting Street/Calhoun tunnel. The city selected tunneling to minimize utility impacts and public disruption. Two additional storm water tunnels have been completed since the Meeting Street/Calhoun tunnel. The Market Street Drainage Improvements Project tunnel was completed in 2013 and the final phase of the project is pending construction. The US17 Spring/Fishburne Drainage Improvements Project tunnel system was completed in summer 2020. Additional basins in the Charleston peninsula also have conceptual tunnel systems that may be implemented in the future.
The city of Charleston’s Spring/Fishburne US17 Drainage Improvements Project is a five-phase program designed to alleviate flooding in two drainage basins covering approximately 202 ha (500 acres), or about 20-percent of the Charleston peninsula. This project has several objectives: to improve the mobility, efficiency, emergency preparedness and community livability; and, most importantly, to alleviate many of the flooding problems by reinvesting in the infrastructure.
The transportation advancements (Phases 1 and 2) incorporated safer travel lanes for vehicles; improved intersections for pedestrian safety and vehicle efficiency; Intelligent Transportation Systems (ITS); and new, energyefficient traffic signals. The infrastructure reinvestment (Phases 2 and 3) will consist of constructing improved and additional surface collection systems throughout parts of the basins, drilling and sinking several shafts, boring a deep tunnel system to connect the shafts, (Phase 4) constructing an outfall from the pump station to the Ashley River and (Phase 5) constructing a new pump station adjacent to the Ashley River. Phase 3 was designed by Black & Veatch and Davis & Floyd Inc.; the contractor was Jay Dee Contractors Inc.; and the support of excavation design was completed by FK Engineering.
Geological conditions and concerns
The geology of the Charleston peninsula is that of an estuary, and as such, the shallow deposits (surficial soils) are influenced by a combination of marine and continental processes. The surficial soils were deposited in a range of sedimentary facies including fluvial, overbank, tidal marsh, tidal channel, tidal flat, lagoon, beach, barrier island and shallow marine. Characterized by its extremely low shear strength and high clay content, the surficial soils are susceptible to significant consolidation and settlement over time. As a result, large portions of the peninsula, including city streets, are slowly subsiding. However, 15 to 21 m (50 to 70 ft) below these surficial soils is a geologic formation locally referred to as the Cooper Marl. The Cooper Marl is a relatively thick layer 45 to 60 m (150 to 200 ft) of olive-green, calcareous, medium-to-stiff, sandy/clayey silt. It is an excellent engineering medium used extensively for its load-bearing and self-supporting attributes. Large buildings are almost exclusively founded on pile supports that extend into the Cooper Marl. The Cooper Marl’s strength and standup time can primarily be attributed to the calcareous bonds, as depicted in Fig. 1. However, these bonds are easily broken by traditional and modern mechanical tunneling methods.
Design
The tunnel system was designed to capture the storm water flows from select low points within the Spring and Fishburne basins via drop shafts and transport the storm water through the tunnel system via a pump station and outfall to the Ashley River. The tunnel was designed to a depth of 37 to 46 m (120 to 150 ft) below the ground surface to ensure adequate clearance was maintained from the surficial soils and existing/future building piles, bridge piers and existing waste water tunnels in the project vicinity. The system is comprised of approximately 2,566 m (8,420 ft) of cast-in-place 2.4 to 3.6 m (8 to 12 ft) finished internal diameter (ID) tunnels, four largediameter access shafts 6 to 9 m (20 to 30 ft ID) and eight drop shafts 122 to 137 cm (48 and 54 in. ID).
The system design includes four access shafts and two intersecting tunnels. The access shafts were generally referred to as follows: Lockwood Drive, Harmon Field, Cannon Street and Coming Street. The two tunnels were the Main Line Tunnel and the President Street Branch Tunnel. The Main Line Tunnel extended about 680 m (2,225 LF) from the Lockwood Drive Access Shaft approximately 0.3 percent upgrade to the Intersection, and then 895 m (2,935 LF) approximately 1.0 percent upgrade to the Coming Street Access Shaft. The President Street Branch Tunnel runs about 396 m (1,300 LF) from the Harmon Field Access Shaft 0.3 percent downgrade to the intersection, then about 680 LF 0.3 percent upgrade to the Cannon Street Access Shafts. This “V” shaped alignment serves to ensure that any entrained air can travel back to either the Harmon Field Access Shaft or Cannon Street Access Shaft and that storm water flows to the Lockwood Drive Access Shaft, where a wetwell, pump station and outfall will be constructed in future phases.
The intersection location and geometry was a result of construction easements and hydraulic boundary conditions. Figure 2 provides an aerial view of the surface above the intersection. The President Street Branch Tunnel was designed to run directly beneath President Street in downtown Charleston. The Main Line Tunnel alignment generally follows the Septima P. Clark Parkway (US17), locally referred to as the “Crosstown,” which cuts across the Charleston peninsula. Stakeholders along President Street and the Septima P. Clark Parkway include utilities, schools, homes and local businesses. The resulting Intersection design helped minimize impact to these stakeholders.
Support of excavation for the tunnels was designed for a two-pass installation method. The cast-in-place concrete final lining for the tunnel system used polypropylene fibers to reduce crack frequency and width. Steel reinforcement was only used at tunnel-adit junctions, tunnel-shaft junctions and at the Main Line Tunnel-President Street Branch Tunnel intersection (the Intersection). The typical minimum thickness of the concrete lining was 31 cm (12 in.). The Intersection concrete minimum thickness was 51 cm (20 in.).
Three of the four access shafts (Coming, Cannon and Harmon Field) were constructed using the caisson-sinking method. A cast-in-place circular reinforced concrete caisson could sink under its own weight as the native material in the center of the structure was excavated. Excavation was stopped at regular intervals to allow casting of the subsequent portions of the caisson. The process was repeated until the caisson was extended through the surficial soils and Cooper Marl to the full depth of the caissons. The shafts were excavated within the surficial coils with a clamshell-type bucket mounted on a crane, and within the Cooper Marl using a small mechanical excavator and muck box for spoil removal. This excavation and support methodology allows for the initial support to act as the final shaft lining. Wall thickness design required a minimum of 46 cm (18 in.) of reinforced concrete. This thickness applied for both 6-m and 9-m (20-ft and 30-ft) shafts. An approximately 1.5 m (5 ft) 6 m (20 ft shafts) or 2.5 m (8.5 ft) 9 m (30 ft shafts) thick concrete invert plug was designed to account for buoyancy forces.
The Lockwood Drive Access Shaft design consisted of a two-pass construction method. Initial support through the surficial soils, and a minimum of 3 m (10 ft) into the Cooper Marl, was to be constructed of concrete caisson, sheet piling, secant piles or slurry wall through the surficial soils. Following support 3 m (10 ft) into the Cooper Marl, initial support consisted of ribs and lagging, liner plate or other contractor-selected support through the Cooper Marl. The final lining design consisted of 45- cm (18-in.) minimum reinforced cast-in-place concrete.
Project construction
Lockwood Drive Access Shaft. The first shaft constructed was the Lockwood Drive Access Shaft (LDAS). The LDAS acted as the primary support shaft for the Main Line Tunnel construction and is the future location of the tunnel system pump station. The Main Line Tunnel invert meets the LDAS about 10 m (35 ft) above the base slab invert. This 10 m (35 ft) sump will be used for future tunnel dewatering pump installation and act as a sediment basin for the tunnel system.
The LDAS is situated on a marsh adjacent to the Ashley River. To account for flooding from extreme hightide events and tropical storms, the shaft area was built up from EL 0.5 m (1.5 ft) to about EL 3 m (10 ft) with clean sand, general fill and a stoned surface.
Support of excavation in the surficial soils utilized steel sheet piles supported internally with steel ribs. The sheet piles were socketed approximately 3 m (10 ft) in the Cooper Marl. Excavation was accomplished with a clamshell bucket and a small mechanical excavator. The contractor installed a 1.5-m (5-ft) tall concrete collar which transitioned the support of excavation from the sheet piles to a ribs and lagging system. The ribs and lagging extended through the Cooper Marl to the invert of the shaft excavation.
The LDAS used a 9-m (30-ft) internal diameter castin- place concrete final lining. The concrete final lining was a minimum of 45 cm (18 in.) thick and reinforced with steel rebar. The shaft used a 2.5-m (8.5-ft) thick base slab with the top of slab at approximately EL -52 m (-170 ft). Shaft concrete final lining was installed from EL -52 m (-170 ft) to EL -9 m (-30 ft). The final lining terminates at EL -9 m (-30 ft) where it will connect to future wet well and outfall construction.
Caissons. The Coming Street, Cannon Street and Harmon Field Access Shafts were constructed using the caisson sinking method. In order to facilitate sinking, the contractor used a “cutting shoe.” The cutting shoe was a tapered cylindrical steel structure built with a larger outside diameter than the typical caisson cross-section. The cutting shoe aided sinking by reducing the bearing surface of caisson and creating a shear plane to loosen excavatable material and direct soils to the interior of the caisson as the shaft was sunk. In addition to the cutting shoe, the contractor pumped bentonite slurry into the annulus through a series of 5-cm (2-in.) PVC piping running vertically through the shaft walls, reducing the skin friction that could otherwise cause the caisson to “hang-up” in the Cooper Marl. The pipes expelled the bentonite to the outside of the caisson walls. Following excavation and sinking, the annulus space outside of the caissons was grouted to fill the voids prior to excavating for the tunnel eyes. The contractor and its design engineer elected to increase the caisson wall thickness as an additional measure to facilitate sinking in the Cooper Marl formation. The 6-m (20-ft) ID shafts (Coming Street and Cannon Street) were 0.7 m (2.5 ft) thick. The Harmon Field Access Shaft 9 m (30 ft ID) was 1 m (3 ft) thick. The depths of the shafts ranged from 37 to 46 m (120 to 150 ft).
The contractor was required to prevent uncontrollable sinking, which is more typical and problematic in the weaker surficial soils above the Cooper Marl. In order to prevent uncontrollable sinking, the contractor preloaded the shaft with hydraulic jacks. The hydraulic jacks were anchored to a concrete counterweight with steel threaded rods and applied force to a steel frame that was placed on the shaft. As a result, the hydraulic force was exerted onto the shaft. The pressure applied was generally representative of the next concrete lift, or approximately 145 to 163 t (160 to 180 st). If sinking was noted while the hydraulic jacks were active, force would continue to be applied until no additional sinking was noted. The hydraulic jacks were applied to the shafts in order to prevent uncontrollable sinking during concrete placement. The hydraulic jacks were also utilized during the excavation process in order to encourage incrementally controlled sinking versus unexpected and uncontrollable sinking.
The caissons were constructed in 3.6-m (12-ft) lifts with a modular set of 4.2-m (14-ft) tall internal and external concrete forms. The additional height was used for 0.3 m (1 ft) of overlap at the bottom and 0.3 m (1 ft) of gap at the top for water-stop installation. These forms were only tied together at the top of the forms; wall-penetrating form ties were not used. The shafts were maintained in a flooded state while excavation progressed through the surficial soils. The flooded state prevented the surficial soils from running into the interior of the shaft during excavation and sinking activities. Once the caisson reached 3 to 6 m (10 to 20 ft) of embedment into the Cooper Marl, the interior could be dewatered.
Tunnel and adit initial support. The Main Line Tunnel (approximately 1,650 m (5,400 LF) was excavated upgrade from the Lockwood Drive Access Shaft to the Coming Street Access Shaft. Means of excavation included a refurbished open-face soft-ground tunnel boring machine (TBM). The TBM bore was approximately 4.5 m (15 ft) and utilized a ribs-and-lagging initial support system. The initial support consisted of steel ribs (W4x13) on approximately 1.5-m (5-ft) centers with mixed hardwood lagging between the sets. The ribs were assembled within the tail shield of the TBM and expanded tightly against the ground after the machine had moved forward. Muck was removed from the heading using rail-mounted muck cars.
The first 67 m (220 ft) of the Main Line Tunnel was converted to a 4.5 m by 4.5 m (15 ft by 15 ft) horseshoe. This horseshoe section of the tunnel allowed for the installation of a rail switch for expediting the tunnel excavation. Two muck car trains were utilized during excavation. While one train was being loaded with excavated material, the other train would be unloaded into a muck area on the surface. Each train generally consisted of four 15 cy muck cars, a diesel-powered locomotive, and a flat car for supplying the initial support system to the TBM. A tail tunnel was also utilized to facilitate unloading muck trains. The tail tunnel was constructed with ribs & lagging initial support and ultimately abandoned with flowable fill grout.
The President Street Branch Tunnel (approximately 610 m (2,000 LF)) was excavated downgrade from the Harmon Field Access Shaft to the Main Line Tunnel Intersection, then upgrade to the Cannon Street Access Shaft. Means of excavation included a refurbished openface soft-ground TBM. The TBM bore was approximately 3.3 m (11 ft) and utilized a ribs and lagging initial support system. The initial support consisted of steel ribs (W4x13) on approximately 1.2-m (4-ft) centers with mixed hardwood lagging between the sets. The ribs were assembled within the tail shield of the TBM and expanded tightly against the ground after the machine had moved forward. Muck was removed from the heading utilizing rail mounted muck cars. The President Street Branch Tunnel did not utilize a tail tunnel or switch during the excavation process.
There were eight adit tunnels extending from the primary tunnels to the previously installed drop shafts. Along the Main Line Tunnel were seven adits with the remaining adit extending off the President Street Branch Tunnel. Adit tunnel excavation ranged from approximately 1.5 m (5 LF) to 163 m (535 LF) along the alignment. The longest adit, extending to the Cherry Street Drop Shaft, was excavated with a combination of mechanical (roadheader) and hand-mining techniques. Muck was removed from the Cherry Street Adit with a pair of skid steers traversing through the adit as the excavation progressed. Voids behind the initial support were filled with grout. The remaining adits were excavated via hand-mining techniques.
Excavation advancement in the adit tunnels was generally consistent throughout the project. Following initial support and breakout from either of the primary tunnels, the contractor would “bench” the excavation so that the crown of the excavation was one-to-two initial support sets ahead of the spring-line and invert support. Initial support consisted of rolled steel ribs (W4x13) for the crown and steel beams (W4x13) supporting the ribs. The steel was typically installed on 1.5-m (5-ft) centers with mixed hardwood lagging. Voids behind the initial support were filled with grout.
Tunnel and adit final lining. The tunnel and adits used a cast-in-place concrete final lining. The concrete was reinforced with polypropylene fibers. Steel reinforcement was only used at tunnel-adit junctions, tunnel-shaft junctions, and at the Intersection. The tunnel concrete forms were reusable, collapsible steel forms. A keyway was installed at the face of each concrete segment/ placement. The pumping location moved as needed to facilitate the concrete pump capabilities. Concrete was pumped through a steel slickline with pumping distances reaching 610 m (2,000 ft).
The Main Line Tunnel finished internal diameter was 3.6 m (12 ft). The concrete was placed in a series of segments ranging in length, typically 24 to 37 m (80 to 120 LF). Concrete segment lengths would generally vary to account for tunnel alignment curvature or planning around junctions. Concrete for the Main Line Tunnel was pumped from the surface from the Lockwood Drive Access Shaft the Courtenay Drop Shaft, the Cannon Street Access Shaft and the Ashe Drop Shaft. The formwork was advanced mechanically with an electricpowered hydraulic form carrier. Concrete was placed at an approximate 0.3 percent grade from the Lockwood Drive Access Shaft to the Intersection and approximately 1.0 percent grade from the Intersection to the Coming Street Access Shaft.
The President Street Branch Tunnel finished internal diameter was 2.4 m (8 ft). The concrete was placed in a series of segments ranging in length, typically 27 to 38 m (90 to 125 LF). Concrete segment lengths would generally vary to account for tunnel alignment curvature or planning around junctions. Concrete for the President Street Branch Tunnel was pumped from the surface from the Harmon Field Access Shaft and the Cannon Street Access Shaft. The formwork was advanced mechanically with an electric-powered hydraulic form carrier. The concrete was placed approximately 0.3 percent downgrade from the access shafts to the Intersection, creating a “v” shape from the Intersection up to both access shafts (Harmon Field and Cannon Street).
The finished internal diameter for the eight adits was 2.4 m (8 ft). The concrete was placed in a series of segments ranging in length, typically 3 to 15 m (10 to 50 LF). Seven of the eight adits were completed with one to three segments. The Cherry Street Adit, approximately 163 m (535 LF), required a series of segments to complete. Typical segments length was 15 m (50 LF), varying as needed around curves or at the Main Line Tunnel junction. Concrete final lining for the adits was designed and installed with a flat (0 percent) grade.
Intersection construction
Intersection initial support. The proposed junction design needed to address a series of challenges. One of the primary challenges was maintaining the minimum design wall thickness in the Intersection. The designed wall thickness in the intersection was 20 cm (8 in.) larger than the typical tunnel section (50 cm (20 in.) instead of 31 cm (12 in.)).
The Main Line Tunnel TBM bored an approximate 15-ft (180-in.) diameter tunnel. When accounting for the initial support, approximately four inches thick, or 20 cm (8 in.) impacting the diameter, the resulting initial support diameter was approximately 437 cm (172 in.). The designed final internal diameter was 3.7 m (12 ft) along the Main Line Tunnel 365 cm (144 in.). With the initial support, this resulted in an approximate 36-cm (14-in.) projected wall thickness. In order to achieve the required 50 cm (20 in.) minimum, the contractor proposed reducing the Intersection opening to 3.3 m (11 ft) along the Main Line Tunnel. This design would align inverts at the Intersection, but the crown would drop down about one foot along the Main Line Tunnel.
The primary concern regarding reducing the Main Line Tunnel diameter at the Intersection was potential air entrainment and impact to the hydraulic design of the tunnel system. Addressing the entrained air, the Project team proposed casting pipes in the crown of the Intersection concrete. The pipes would connect the crowns of the two 3.7-m (12-ft) diameter sides of the Main Line Tunnel in order to allow for air to flow upgrade toward the Coming Street Access Shaft, as originally designed. This proposal was ultimately accepted and pursued.
Another challenge regarding initial support of the Intersection was maintaining positive ground support. The contractor constructed two 3.7 x 3.7 m (12 x 12 ft) adits branching out from the Main Line Tunnel. These adits extended out into native ground to receive the President Street Branch Tunnel TBM. The TBM would then “walk” through the intersecting Main Line Tunnel and relaunch through the opposite adit. The completed initial support system is displayed in Fig. 3.
Intersection final lining. The Intersection was the final concrete placed for the tunnel system. The contractor was elected to complete the Main Line Tunnel, President Street Branch Tunnel and adit final lining concrete prior to forming and placing concrete in the intersection. The concrete in the Intersection was formed with a combination of custom-built formwork along with repurposed 2.4-m (8-ft) ID formwork used for the President Street Branch Tunnel final lining, referred by the contractor as “fish mouth” forms.
The concrete along the Main Line Tunnel used custom-built forms constructed out of steel ribs, timber, plywood and mixed hardwood lagging. The steel ribs were rolled to an approximate diameter of 3.3 m (11 ft). Between two steel ribs was plywood reinforced with 1.5-m (5-ft) long two-by timber, resulting in 1.5-m (5-ft) long “quadrant” pieces. Two quadrants bolted together resulted in one 1.5-m (5-ft) long invert section piece. The preassembled quadrant pieces were mobilized to the Intersection via locomotive along the President Street Branch Tunnel. The quadrants were hoisted with a previously installed steel monorail, assembled into 1.5-m (5-ft) invert section pieces, and stored in the Main Line Tunnel for future installation. The concrete along the President Street Branch Tunnel utilized repurposed 2.4-m (8-ft) steel formwork previously used for the President Street Branch Tunnel concrete lining. The forms were torch-cut offsite to configure the unique Intersection geometry. Steel spuds were installed along the invert and spring-line in order to hold the custom forms during install and prevent shifting during concrete placement. Steel bracing was also installed to prevent form movement during placement.
The first concrete placement was about 180 cu yd. Concrete was pumped from the Courtenay Drop Shaft through about 305 m (1,000 LF) of steel slick line. The concrete was distributed throughout the forms using a series of valves and hoses. The valves were connected to additional slick line which extended out from the Main Line Tunnel centerline to the forms. The concrete was adequately vibrated to ensure proper consolidation and flow around the forms. Following completion of the first concrete placement, a PVC waterstop was installed, along with a keyway, at the interface between the first and second placements.
The crown used similar custom formwork as the invert placement. However, the timber-reinforced plywood was replaced with 0.6-cm (0.25-in.) thick by 1.5 m (5 ft) steel plates reinforced with mixed hardwood lagging. Initially the contractor had planned to strip the Main Line Tunnel invert forms and reuse for the second concrete placement for the crown. The contractor elected to construct additional custom forms on top of the installed invert forms. This new approach allowed for the structural benefit of circular forms.
Concrete for the second placement was pumped from the Courtenay Drop Shaft through about 305 m (1,000 LF) of slick line. About 110 cu yd of concrete was needed to complete the second placement. Concrete was pumped through a series of ports installed at the crown in the Main Line Tunnel custom forms and President Street Branch Tunnel fish-mouth forms. Form vibrators were used to properly consolidate the concrete. The contractor installed temporary PVC pipes at both crown ends of the Main Line Tunnel forms to expel air as the forms were filled. The Intersection final lining was successfully completed in March 2020 and can be seen in Fig. 4.
Conclusion
Successful completion of this unique and complex project required coordination and communication between the project team. The US17 Spring/Fishburne Drainage Improvements Project tunnel system was completed in summer 2020. The fourth phase is currently under construction which includes construction of the wet well and outfall. The drainage system will be fully operational after the completion of the final fifth phase.
The authors would like to thank the city of Charleston for their permission to publish this paper and whose work provided the basis for this paper. Charleston is a showcase for the increasing viability of underground construction techniques and other communities can benefit from the city’s foresight and ingenuity. Charleston has demonstrated that determination and strong leadership do eventually pay off.
The authors would also like to thank the project team: Black & Veatch, Davis & Floyd, Jay Dee Contractors Inc., and FK Engineering for their considerable contributions for the completion of this project. Black & Veatch and Davis & Floyd Inc. acted as the owner’s engineer for the project. Jay Dee Contractors Inc. was the contractor for this third phase of the drainage system and provided unyielding expertise and craftmanship. FK Engineering was the design engineer for the contractor. FK Engineering produced initial support of excavation design and contributed to the final support design.
