Read Design and Construction of the South Cobb Tunnel text version


Mike Robison Jacobs Engineering David Rendini Parsons Ted DePooter Jacobs Associates Judy B. Jones Cobb County Water System ABSTRACT In 2008, the Cobb County Water System began construction of the South Cobb Tunnel, its second wastewater tunnel. The South Cobb Tunnel is an 8.9-km (5.5-mile) long, 8.2-m (27-ft) diameter, hard rock tunnel. At its maximum depth of 122 m (400 ft), it is the deepest tunnel constructed in the Atlanta area. This paper describes ground and groundwater conditions encountered along the tunnel alignment, excavation using the refurbished Herrenknecht S-288 tunnel boring machine (TBM), excavation and concrete lining of the 34.7-m (114-ft) diameter pump station shaft and two 13.4-m (44-ft) diameter construction shafts, and construction of six tangential vortex intake structures that transfer flow from interceptor sewers to the tunnel. INTRODUCTION Cobb County Water System (CCWS) provides for the collection and treatment of wastewater for most areas of Cobb County, a metropolitan county located northwest of Atlanta, Georgia. In 1993, CCWS adopted a Sewer System Master Plan, which identified anticipated wastewater service requirements based on population projections developed by the Atlanta Regional Commission. The service requirements identified in the plan provided Cobb County with a framework to plan infrastructure improvements. In 1998, following recommendations of the plan, CCWS began design of its first tunnel, the Chattahoochee Tunnel, to serve the eastern part of the county. The 5.5-m (18-ft) excavated diameter, 4.9-m (16-ft) finished diameter Chattahoochee Tunnel runs 15.1 km (9.4 miles), extending from the Indian Hills subdivision to the R.L. Sutton Water Reclamation Facility. The tunnel provides conveyance and wet weather storage of the sewer system flows. Along its length, flow is transferred to the tunnel through a series of four intake structures, designed as tangential vortex drop shafts. Construction of the Chattahoochee Tunnel began in mid-2000 and was completed in late 2004, when flows were diverted from the intakes to the tunnel. In 2006, considering the success of the Chattahoochee Tunnel, CCWS decided to begin design of its second tunnel, the South Cobb Tunnel. The South Cobb Tunnel consists of an 8.2-m (27-ft) excavated diameter, 7.3-m (24-ft) finished diameter main tunnel that runs 8.8 km (5.5 miles), extending from the Sweetwater construction shaft, located near the Douglas County line, to the South Cobb construction shaft adjacent to the South Cobb Water Reclamation Facility. Along its length, flow is transferred to the main tunnel through five intakes, designed as tangential vortex drop shafts. The Sweetwater intake connects to the main tunnel through a 3-m (10-ft) horseshoe-shaped, 418-m 146



Figure 1. South Cobb Tunnel plan and profile

(1,369-ft) long drill/blast tunnel at the Sweetwater construction shaft. Three other intakes, located at Silver Creek, Carroll Creek, and Interstate 20 (I-20), connect to the main tunnel along its length. The project also includes the 2.6-m (8.5-ft) excavated diameter Nickajack tunnel, which extends 982 m (3,223 ft) from the existing Nickajack pump station to the South Cobb construction shaft. The fifth intake on this project is located at the South Cobb construction shaft. The TBM tunnel and the Nickajack tunnel converge at the South Cobb construction shaft, and from there a 183 m (600 ft) long drill/blast tunnel goes to the South Cobb pump station. The pump station is a 31.7-m (104-ft) finished diameter, wet pit/dry pit arrangement with a pumping capacity of 490,000 cubic meters per day (130 million gallons per day [mgd]). The plan and profile of the South Cobb Tunnel are shown in Figure 1.



DESIGN At the time of design, the South Cobb Tunnel was the sixth major hard rock tunnel in the Atlanta area. In addition to CCWS's Chattahoochee Tunnel, the City of Atlanta had previously constructed the Intrenchment Creek Tunnel and Three Rivers Tunnel in the late 1970s and 1980s. The City also had recently constructed the Nancy Creek Tunnel, and construction of the West Area CSO Tunnel was underway. The Atlanta area is quite good for both drill/blast and TBM tunneling. Most of the ground can be supported by rockbolts, although steel sets are sometimes needed in less competent ground. Groundwater inflow rates are relatively low. Concrete lining is used as needed to support areas of fractured rock and areas of groundwater inflow. With this knowledge, design of the South Cobb Tunnel began. Risk Management At the beginning of the design process, CCWS decided that the contract for the South Cobb Tunnel would include many of the risk management tools currently in use on tunneling projects. Knowing the importance of understanding ground and groundwater conditions, CCWS determined that an extensive subsurface investigation would be performed. In addition, the contract would include both a geotechnical data report (GDR) and a geotechnical baseline report (GBR). CCWS also decided to include a differing site conditions clause, use escrow bid documents, and establish a dispute review board (DRB). Other risk management measures were used, including early selection and engagement of the construction manager, and prequalification of contractors for both tunnel construction and pump station construction. Enlisting the services of the construction manager during the design process enabled an additional review of the design, and performance of constructability and biddability reviews. Prequalification of contractors is discussed later in this paper. CCWS also set aside allowance items in the bid schedule to enable payment for unforeseen work elements, specialty consultants to the owner, and regulatory compliance work. Additionally, the bid schedule included an item for critical path time extension, in which contractors bid the daily cost of extending the project in the event that additional tunnel concrete lining or modified contact grouting beyond the quantities shown in the bid was required. Regional Geology The South Cobb Tunnel is located in the Piedmont region of the southeastern United States. In general, the geology of the Piedmont in the greater Atlanta area consists of medium-grade metamorphic rocks that have been intruded by granitic rocks in some places. A key characteristic of the Piedmont region is the thick mantle of residual soil that grades downward into the underlying bedrock. At the South Cobb Tunnel, ground conditions range sequentially with depth from the soil zone, to the transition zone, to the bedrock zone. The soil zone includes all residual soil and any overlying fill or alluvial soil down to the point where N values from Standard Penetration Tests (SPT) are consistently greater than 100 blows per foot. The transition zone lies below the soil zone and consists of partially weathered and fractured rock that is typically degraded to some extent by weathering, at least along the fractures. The transition zone begins where SPT N values are consistently greater than 100 blows per foot and extends down through the zone of core stones. The bedrock zone lies below the transition zone and generally consists of faintly weathered to fresh rock.



Groundwater in the Piedmont occurs in all three zones. The water table generally occurs within the soil zone at depths of around 3 m (10 ft) or less in lowland areas to depths of around 12 to 15 m (40 to 50 ft) in upland areas. Geologic Mapping The first step in the subsurface investigation was geologic mapping, which consisted of outcrop mapping, joint measurements, and lineament analysis. Outcrop mapping and joint measurements were used to identify general trends and types of rocks. Lineament analysis provides an indication of underlying geologic structure. In the Atlanta area, lineaments tend to form linear topographic draws and valleys along weaker, more easily eroded places in the rock mass. The information obtained from geologic mapping was used to design the subsurface investigation and to aid in evaluation of conditions between borings. Subsurface Investigation An extensive subsurface investigation was performed to determine anticipated ground and groundwater conditions along the tunnel alignment and at the shafts. The program consisted of 72 deep rock core borings and shallow soil and transition zone borings, including 25 deep borings and 23 shallow borings along the main TBM tunnel alignment, 10 borings along the Nickajack tunnel alignment, 6 borings along the Sweetwater tunnel alignment, and 8 borings in the pump station and along the Pump Station tunnel alignment. Six of the deep rock core borings were inclined, designed to intersect the foliation of the rock. Soil and transition zone borings were drilled with either hollow stem auger or rotary wash methods. Samples were collected during drilling using split spoon samplers to obtain SPT N values. Rock core borings were performed using the Longyear HQ tripletube, wire-line system. The HQ triple tube system produces an approximately 97-mm (3.8-in.) diameter hole and a 61-mm (2.4-in.) diameter core. The advantage of the triple-tube coring system is that it subjects the rock core to less damage during extraction from the core barrel, resulting in better recovery of shattered and weathered rock. Core recovery and RQD were measured and recorded in the field. Rock descriptions consisting of name, color, texture, and mineralogy were made at the core storage facility, along with core photographs. Other characteristics, including joint count and weathering index, were taken at the storage facility. Packer permeability tests were performed at 6.1-m (20-ft) intervals in the coreholes. In general, eight tests were performed over the bottom 49 m (160 ft) using a straddle packer assembly. The tests were conducted by measuring the volumetric rate at which water could be injected into the formation under constant pressure. Permeability calculations were made based on the results. Borehole geophysics was performed in 28 of the coreholes following drilling. Borehole geophysics included an acoustic televiewer log, a combination suite, and a full wave sonic log. The geophysical testing provided data used in determining joint orientation, identifying water-bearing zones in the rock, and determining matrix porosity. A variety of laboratory tests were performed on rock samples obtained from the drilling operation. Point load tests were performed to determine the point-load index strength. Tests were performed both parallel and perpendicular to foliation in order to evaluate the anisotropy of the rock. Unconfined compressive strength, Brazilian tensile strength, Cerchar Abrasivity Index, acoustic velocity, and direct shear tests were performed on selected rock samples. SINTEF Drillability Index tests were performed to aid in evaluating TBM boreability. X-ray diffraction tests were performed in order to quantify the mineralogy of the different rock types along the tunnel alignment.



Table 1. Ground types in TBM tunnel

TBM Tunnel East of Sta. 55+00 Ground Type A B C Expected Length, m (ft) 0 732 (2,400) 914 (3,000) Approximate Percentage 0% 40% 60% TBM Tunnel West of Sta. 55+00 Ground Type A B C Expected Length, m (ft) 4,551 (14,930) 2,134 (7,000) 427 (1,400) Approximate Percentage 64% 30% 6%

Ground Conditions The ground along the project is divided into the eastern region and the western region. The boundary between the two regions is Sta. 55+00 of the main TBM tunnel. The tunnels in the eastern region, which include the Nickajack tunnel and the Pump Station tunnel, are shallower and the rock is much more weathered and fractured. The ground in the eastern region is dominated by wet, blocky, and seamy conditions. In contrast, the tunnels in the western region, including the Sweetwater tunnel and I-20 tunnel, are considerably deeper, the ground is much less weathered, and the fractures are more commonly tight. Foliation over most of the tunnel alignment has an average strike of about N66° E and dip of about 36° SE. However, foliation undulates, causing both the strike and dip to vary within about ±20° of the average. More extreme variations occur locally along the alignment. At the Sweetwater construction shaft, the dip is about 70°SE. The rock types along the alignment consist of mylonite and granitic gneiss in the eastern region and schist, granitic gneiss, quartzite, and amphibolite in the western region. The unconfined compressive strength of the rock varies depending upon rock type, ranging from 220 MPa (32,000 psi) for the granitic gneisses and quartzites to about 34.5 MPa (5,000 psi) for the chlorite schists. The average unconfined compressive strength for the 101 samples of rock tested was 162 MPa (23,500 psi). Three ground types were recognized along the alignment. They were designated Type A, Type B, and Type C, and were defined on the basis of their Rock Mass Rating (RMR) scores, as follows: Type A ground has an RMR greater than 60. Type A ground was expected to require light to minimal support. Typical characteristics of Type A ground are fresh or faintly weathered rock, excellent to good RQD, moderate to wide joint spacing, and two sets of joints or fewer. Type A ground could produce isolated wedges. Type B ground has an RMR of between 60 and 41. Type B ground was expected to be blocky and to require moderate support. Typical characteristics of Type B ground are joints with significant weathering, fair to poor RQD, close joint spacing, and three or more joint sets. Type B ground typically produces numerous wedges and can ravel. Type C ground has an RMR of 40 or less. It was expected to be difficult ground requiring substantial support. Type C ground is likely to be blocky and seamy, shattered, and/or raveling. Type C ground typically contains thick intervals of moderately to thoroughly weathered rock. The amount of each of the expected ground types in the TBM tunnel was baselined as indicated in Table 1. Expected groundwater inflow was baselined at 4,542 L/m (1,200 gpm) at the downstream end of the tunnel, with half the inflow expected to come from east of Sta. 55+00 and half from west of Sta. 55+00.



The GBR also described three areas along the alignment where ground conditions were expected to be difficult. They included the Nickajack area characterized by blocky and seamy ground with a steady supply of groundwater from the Chattahoochee River, the Pump Station area (which contains a large dipping foliation shear extending from the South Cobb construction shaft to the Pump Station shaft), and the Oak Ridge area (which contains a series of major shear zones with highly fractured and thoroughly to moderately weathered ground). TBM Tunnel Initial Support Three types of initial support were designed to support the ground following excavation by the TBM. The initial support types, corresponding to the three ground types recognized along the alignment, were described as follows: Class A support consists of a row of four grouted bolts in the crown and upper sidewalls, plus welded wire fabric. The bolts are spaced 2.1 m (7 ft) apart radially and 1.5 m (5 ft) apart along the tunnel axis. The bolts have a minimum embedment length of 3 m (10 ft). Class B support consists of a row of eight bolts or dowels in the crown and sidewalls, along with welded wire fabric, mine straps, and channels as needed. The bolts or dowels are spaced 1.8 m (6 ft) apart radially and 1.5 m (5 ft) apart along the tunnel axis and have a minimum embedment length of 3 m (10 ft). Class C support consists of full-circle expanded steel ribs. The ribs are spaced 1.2 m (4 ft) apart along the tunnel axis. The GBR also states that in specific situations it will be necessary to supplement these three support types with spot bolts, mine straps, or other means. TBM Tunnel Final Lining The TBM tunnel was designed to be partially lined with cast-in-place concrete, where needed for structural support and/or groundwater control. The design anticipated the entire length of the main tunnel east of Sta. 55+00 would require lining, as would 63 percent of the main tunnel west of Sta. 55+00. The concrete lining was designed to support full hydrostatic load. East of Sta. 55+00, 300 mm (12 in.) of lining were required. West of Sta. 55+00, with the exception of between Sta. 94+00 and Sta. 173+00, 400 mm (16 inch) of lining were required. To account for 50 mm (2 in.) of corrosion, and because it is impractical to change the excavated diameter of the tunnel, 450 mm (18 in.) of final lining were specified. Between Sta. 94+00 and Sta. 173+00, the hydrostatic head is greater than 90 m (300 ft). In this section, reinforcement was added to the lining. Reinforcement is also required in any areas where Class C support (steel ribs) are installed. Modified contact grouting is required behind the cast-in-place concrete lining to complete the ring. Modified contact grouting uses high pressures both to fill the annulus between the concrete and the rock wall and force grout outward into open rock fractures against a head of water. The allowable groundwater inflow for the completed project is 1,140 L/m (250 gpm), based on a criterion of 200 gallons per inch diameter per mile per day (O'Rourke, 1984). Connecting Tunnels and Chambers Three connecting tunnels--the Pump Station tunnel, the Sweetwater tunnel, and the I-20 tunnel--transfer sewer flow from their respective intake structures to the chambers below. The western drill/blast tunnels and chambers were expected to encounter ground and groundwater conditions similar to those baselined for the western region



of the main TBM tunnel. Similarly, the eastern drill/blast tunnels and chambers were expected to encounter conditions similar to those baselined for the eastern region. The connecting tunnels were designed to be lined with fiberglass-reinforced pipe (FRP) and their chambers lined with reinforced, cast-in-place concrete. Nickajack Tunnel The Nickajack tunnel lies in the eastern region, with the intake chamber located in the Nickajack area. Of particular concern was that the chamber and eastern end of the tunnel were located about 20 m (65 ft) from the bank of the Chattahoochee River. In this area, the cover over the crown of the tunnel was as shallow as 27 m (90 ft), and the surface was covered by a shallow pond, wetlands, and the floodplain of the river. About eight percent of the tunnel was expected to encounter Type C ground, and cumulative groundwater inflow rates were baselined at 2,270 L/m (500 gpm). Pump Station Shaft The pump station shaft is approximately 65 m (212 ft) deep with an excavated diameter of about 37 m (120 ft). The soil zone was expected to extend 8 to 20 m (25 to 65 ft) deep, underlain by 6 to 9 m (20 to 30 ft) of transition zone, before reaching the bedrock zone. Groundwater was expected at a depth of about 7.5 m (25 ft). A large foliation shear crossing the pump station was identified in the subsurface investigation. The shear zone consisted of undulating, shattered rock dipping from northwest to southeast with a vertical thickness of about 7.5 m (25 ft). Joint surfaces within the zone contained clay, micaceous gouge, and talc with slickensides. The shear entered the shaft at about 27 m (90 ft) below ground surface and exited the shaft at about 60 m (200 ft) below ground surface. Considering the many possibilities for initial support of the soil zone, this design was left to the contractor. It was expected to consist of sheet piles or secant piles with ring beams. The engineer designed the initial support system for the foliation shear consisting of tensioned rock anchors extending across the shear. The engineer also designed an initial support system consisting of rockbolts and shotcrete for the bedrock. Construction Shafts The South Cobb and Sweetwater construction shafts both had design diameters of 13.4 m (44 ft). The South Cobb shaft extended to a depth of 58 m (190 ft), while the Sweetwater shaft extended to a depth of 88 m (290 ft). Each shaft would encounter varying thicknesses of residual soil, grading down through the transition zone to the bedrock zone. Initial support of the soil zone in both shafts was left to the contractor. Systematic initial support of the bedrock zone was not required; rather, cast-in-place concrete lining in 3-m (10-ft) lifts from the top down was specified. Intakes The intakes were designed as tangential vortex drop structures. Each consisted of a concrete structure constructed approximately 6 to 9 m (20 to 30 ft) below grade, a deaeration chamber at tunnel depth, and a drop shaft and a vent shaft. The surface structures would be excavated by open-cut excavation methods. The drop shafts and vent shafts would be drilled using either raise boring or blind boring. The drop shaft excavations were expected to range in diameter from 1.5 to 2.0 m (5.0 to 6.5 ft), while the vent shaft excavations were expected to range from 0.6 to 1.2 m (2 to 4 ft).



BID PHASE As the design phase was coming to an end in mid-2007, the project moved on to the bid phase. The bid phase began with prequalification of tunnel prime contractors and pump station subcontractors during the summer of 2007. Four contractors submitted tunnel prequalification packages, two contractors submitted pump station subcontractor prequalification packages, and one contractor submitted tunnel and self-performing pump station prequalification. All were determined to meet the requirements, thus making them eligible to bid. A mandatory prebid conference was held in September 2007 and bids were received in December 2007. Four tunnel contractors formed two-way joint ventures and submitted bids as prime contractors. A third bid was received from the contractor prequalified for both the tunnel and pump station work. Bid prices ranged from $305,000,000 to $376,258,000. The project was awarded to the low bidder, SheaTraylor Joint Venture, on March 26, 2008. Their subcontractor for the pump station was Archer Western. The time period needed from bid opening to project award was primarily a result of the bid amounts being in excess of the owner's anticipated amount. The owner investigated alternatives for project funding and evaluated the construction scope, timing, and procurement during this period. The performance time allowed by contract was a period of 2,132 calendar days from notice-to-proceed to substantial completion, plus 60 calendar days from substantial completion to final completion. Liquidated damages were set at $11,000 per calendar day of delay until substantial completion and $4,000 per calendar day of delay from substantial completion to final completion. CONSTRUCTION Schedule Notice-to-proceed was given to the contractor on July 14, 2008. The project's scheduled six-year duration resulted in a project completion date of July 14, 2014. As of December 31, 2010, the project is 181 days ahead of schedule. Thus far, two weather-related events have impacted the project schedule. In September 2008, Hurricane Ike knocked out power at the steel ring beam supplier's production facility, resulting in a five-day delay in sinking the Sweetwater construction shaft. In September 2009, a 500-year storm event swept through the Atlanta area, leaving the construction site isolated and without access for a three day period. Early Changes A cooperative spirit was adopted at the onset of the project by the owner, contractor, construction manager, and engineer. Several early changes to the contract were implemented, resulting in cost savings and schedule improvement. The contract envisioned TBM tunneling beginning from the South Cobb shaft. The contractor was given an alternative in the contract allowing TBM tunneling from the Sweetwater shaft. The contractor elected to drive the tunnel downgrade from the Sweetwater shaft to the South Cobb shaft, citing concerns for operating construction cranes within the flight path of Fulton County Airport. The contract stated the maximum height restriction for construction equipment mandated by the Federal Aviation Administration. Accordingly, the contractor elected to establish its site offices at the Sweetwater shaft site. An acoustic barrier wall system was required at the Sweetwater site, due to the close proximity of neighboring residential properties. The contractor developed, with



the assistance of a sound engineering consultant, an alternative to the specified commercially manufactured system. The contractor's proposed system promised superior performance to the specified system and was adopted for use. The construction documents required the construction shaft to be concrete lined from the top down as excavation progressed. The contractor designed an initial support system consisting of steel ring beams with concrete lagging to advance through the soil and transition zones, and rockbolts with mesh in the bedrock zone. These support methods made it possible to sink the construction shafts to full depth and then concrete line from the bottom up. This improved the quality of the concrete lining at the horizontal construction joints, and required fewer changes of activities for the contractor. The contract documents allow for value engineering change proposals (VECPs) from the contractor. Net cost savings are shared by the owner and contractor for accepted proposals. Two VECPs have been implemented on the project. The first VECP put forth by the contractor involved a slight reduction in the diameter of the TBM tunnel. Specifications required an 8.3-m (27.3-ft) diameter TBM and the contractor proposed significant cost savings by using an 8.2-m (27.0-ft) diameter TBM, as a used machine in this size was readily available. Negotiations ensued, and the engineer and owner agreed to accept the reduced diameter, while maintaining the specified 7.3-m (24.0-ft) finished diameter in the concrete lined sections of the tunnel. The second VECP put forth by the contractor involved a reduction in the amount of wire mesh in the Class A initial tunnel support. The design required wire mesh in the tunnel arch section extending from the 10:00 to 2:00 o'clock positions. The contractor proposed to install the center section from about 11:00 to 1:00 o'clock in areas that did not warrant more support, as determined by examination of the rock condition at the heading. All parties agreed to the proposal and a deductive change order was issued. Probing and pre-excavation grouting was not required by the contract. However, the contract did require that the TBM be configured to allow for probe drilling and preexcavation grouting should the owner determine its necessity. Prior to the start of TBM mining, the contractor proposed probing the entire length of the tunnel at the contractor's expense and requested compensation if pre-excavation grouting was performed, reasoning that grouting would reduce groundwater inflows, improve the ground, and reduce modified contact grouting costs. Negotiations ensued, resulting in an agreement that the contractor would probe in the areas most likely to have problematic ground or groundwater. In the event that groundwater flow through a probe hole exceeded 90 L/m (20 gpm), then grouting would be performed and paid for by the owner. If groundwater inflows were less than 90 L/m (20 gpm), any grouting performed would be at no cost to the owner. Construction Shafts, Starter and Tail Tunnels The contractor commenced work at the Sweetwater shaft site on July 14, 2008, upon receipt of notice-to-proceed. An acoustic barrier wall system was erected following site clearing. Initial excavation of the Sweetwater construction shaft was accomplished with a track excavator and transitioned to conventional drill/blast methods as the excavation progressed. As previously discussed, the initial support system consisted of steel ring beams with concrete lagging to advance through the soil and transition zones. Rock bolts and wire mesh were installed for ground support in the rock zone. The shaft was concrete lined from the bottom up using a 4.5 m (15 ft) tall steel jump form. After the shaft reached bottom, the starter and tail tunnels were excavated. The tail tunnel was 14 m (47 ft) long, and the starter tunnel was 115 m (376 ft) long. These tunnels were horseshoe-shaped, 9 m × 9 m (29 ft by 29 ft) sections, driven utilizing



Photo 1. Drill jumbo in Sweetwater construction shaft drill/blast and supported with rockbolts and steel fiber reinforced shotcrete. Within a 10-month period, the shaft was sunk and the starter and tail tunnels driven. Similar construction methods were used on the South Cobb construction shaft, where excavation started on October 17, 2008. The first blast was required after sinking 14.3 m (47 ft) through the soil and transition zones. Blasting continued for seven months until the shaft was deep enough for TBM removal. Concrete lining was installed in this shaft progressing from bottom up, similar to the Sweetwater construction shaft. Both shafts will need one or more blasts to sink them to design elevation to allow the reinforced concrete shaft bottom to be placed. Photo 1 shows the contractor's threeboom jumbo, which was used to drill blast holes for advancing the construction shafts through rock. The steel ribs and concrete lagging system used to support the soil and transition zones is also seen. TBM Tunnel The TBM for the project is the refurbished Herrenknecht S-288, which had previously been used to drive the Clear Creek Tunnel on the City of Atlanta's West Area CSO Tunnel project. The machine was refurbished and modified over an approximate oneyear period at a facility located approximately 40 km (25 miles) from the Sweetwater launch shaft. Photo 2 shows the refurbished TBM, just prior to transport to the jobsite. Concrete invert, gripper walls, a rail system, and a sump were constructed in the starter and tail tunnels at the Sweetwater shaft in preparation of launching the TBM. Upon readying the starter and tail tunnels, delivery of TBM components and on-site assembly of the TBM began in May 2009. The TBM launched on September 1, 2009, with the cutting of the first rock. From the TBM, muck is conveyed by a continuous conveyer belt system to the surface and then loaded into trucks for transport to various locations. In accordance with the contract, ownership of the muck resides with the contractor, who assumes all responsibility for its disposal. Initial ground support along most of the TBM tunnel alignment is provided by bolts and wire mesh, with additional spot bolts, wire mesh, mine straps, and rolled



Photo 2. Refurbished Herrenknecht S-288 TBM channel installed as needed. Steel rib support was required in one area of the tunnel to December 31, 2010, with 18 steel ribs (W6×25) installed through a 21 m (68 ft) section. TBM excavation has progressed at a faster pace than anticipated in the baseline schedule. As of December 31, 2010, a total of 7,688 m (25,222 ft) or approximately 88 percent of the main tunnel has been excavated. In the western region, 94 percent of the excavated ground was Type A, 6 percent was Type B, and only 5 m (17 ft) was Type C. In the eastern region, as of December 31, 2010, approximately 70 percent of the ground was Type A, 26 percent was Type B, and 4 percent was Type C. Pump Station Shaft The pump station shaft was constructed using a contractor-designed secant pile wall support system in the soil and transition zones, and was sunk through rock using drill/blast with shotcrete and bolts for support. Construction of the pump station shaft is not discussed in detail in this paper, as two other papers relating to construction of the pump station shaft are expected to be published in the proceedings for this 2011 RETC. Nickajack Tunnel and Chamber The Nickajack tunnel was driven by subcontractor, W.L. Hailey, using a 2.6-m (8.5-ft) diameter refurbished Jarva TBM. The contract allowed the contractor the choice of driving this connector tunnel using either drill/blast or a TBM. A probing and preexcavation grouting program beginning at Sta. 25+00 and continuing to the end of the drive was required by contract because of concerns with groundwater and ground conditions adjacent to the Chattahoochee River. Initially, the subcontractor decided to drive the Jarva TBM to Sta. 25+00, at which point it planned to withdraw the TBM and finish mining with drill/blast techniques. Of primary concern with the TBM were the tight confines of the small diameter bore and the anticipated difficulty with probing and grouting. As TBM excavation progressed, geologic conditions were found to be rather favorable. In addition, the surface grouting program performed over the Nickajack chamber indicated favorable ground conditions. As a result, a re-examination of the grouting program was made by the subcontractor, and ultimately the subcontractor decided to install a probe drill on the TBM. Fortunately, insufficient groundwater was encountered



Photo 3. Refurbished Jarva TBM used in Nickajack tunnel to trigger pre-excavation grouting, and the TBM was able to complete the entire drive to the back end of the Nickajack chamber. Photo 3 shows the refurbished Jarva machine. Initial ground support consisted of rockbolts, wire mesh, mine straps, and some cribbing in one soft zone. If it had been necessary to install steel ribs, the TBM would have been backed out and the remaining tunneling would have been by drill/blast techniques. The chamber was enlarged from the TBM opening using drill/blast. The contract initial support consisted of shotcrete and lattice girders, but the condition of the rock was good enough to support with rockbolts. Following excavation, the chamber was lined with cast-in-place silica fume concrete for durability. As of December 31, 2010, cleanup of the tunnel is ongoing and preparation is underway to install FRP pipe through this tunnel section. Connecting Tunnels and Chambers The tunnels and chambers to connect to the intake sites are being driven by drill/ blast techniques, with rockbolts for initial ground support. The Sweetwater connecting tunnel was driven at grade from the bottom of the Sweetwater construction shaft. For the other connecting tunnels, elevated work decks were constructed over the rail in the main TBM tunnel, and connecting tunnel work was staged from the decks. Equipment was transported from the shaft on a large platform lift by rail to the decks. The platform lift would then raise the equipment to the deck elevation, where the equipment was unloaded. Chambers will be lined with cast-in-place silica fume concrete. Connections from the chambers to the main tunnel will be lined with FRP pipe. Intakes and Drop Pipes At the intake sites, intake structures will connect the interceptor sewers to drop pipes that transmit flow to chambers at tunnel level. The intake structures will be constructed of cast-in-place concrete, and will contain stainless steel tangential vortex inducers mounted on flanged ductile iron pipe (DIP). A separate vent pipe connecting to the chamber is also constructed of flanged DIP. These DIP pipes are installed in an oversized casing socketed into bedrock, and a raise bored hole, which connects to the chamber below. The DIP is concreted in place utilizing a drain pipe and concrete placed by tremie pipe. The drain pipe is grouted up after the concrete installation around the DIP is complete.



As of December 31, 2010, support-of-excavation (SOE) is underway for the intake structure at Nickajack. The subcontractor, Carter Concrete Structures, has chosen to support this site with sheet piles and walers. Dewatering is necessary due to the high groundwater elevation. The subcontractor's SOE design is underway for the other intake sites. The intake structures consist of a box built around the existing interceptor sewer, a channel to lead the flows to the tangential vortex inducer, and the drop pipe. At the time of activation, the section of the existing interceptor sewer passing through the structure will be sawn out, allowing sewer flow into the channel leading to the drop shaft. SAFETY Safety is a prime concern for the project. The contract documents require the contractor to have a full-time safety manager and a project-specific safety plan. The contractor uses a variety of safety training--tool box meetings, hazard analysis, near miss reporting, incident investigation, substance abuse testing, environmental testing, and a system employing both rewards and disciplinary action. As of December 31, 2010, approximately 900,000 man hours have been worked on the project with a recordable incident rate that is approximately 50 percent lower than the construction industry average. CLOSING The South Cobb Tunnel is Cobb County Water System's second successful tunneling project. As of the writing of this paper, December 31, 2010, the six-year project is ahead of schedule by one-half year. Success is attributed to the cooperative relationship enjoyed by the owner, contractor, construction manager, and engineer. Like most tunnel projects, the South Cobb Tunnel has not been without its difficulties. However, the cooperative atmosphere has resulted in a timely resolution of issues, in cost savings and schedule improvements. ACKNOWLEDGMENTS The authors wish to thank the men and women involved in the building of this project. Further, we thank the contractor, Shea-Traylor Joint Venture, their main subcontractors, Archer Western, W.L. Hailey, and Carter Concrete Structures, and their suppliers, Herrenknecht and American Commercial, for their cooperation on this project. The authors also credit the photographs to Mr. Dwayne Easterling of Jacobs Associates. REFERENCE O'Rourke, T.D. 1984. Guidelines for Tunnel Lining Design. American Society of Civil Engineers.


Design and Construction of the South Cobb Tunnel

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Design and Construction of the South Cobb Tunnel