Dredged Material Management in Immersed Tube Tunnel Construction
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Dredged Material Management in Immersed Tube Tunnel Construction Vahan Tanal, P.E. Senior Vice President, Parsons Brinckerhoff, New York, USA F. Simin Pehlivan Former Deputy Director General, Ministry of Transport & Communication, General Directorate of Railways, Harbours and Airports Construction, Ankara, Turkey Jaw-Nan Wang, Ph.D., P.E. Senior Engineering Manager, Parsons Brinckerhoff, New York, USA Zafer Ozerkan Railway Advisor, Yuksel Proje, Ankara, Turkey Isikhan Guler, Ph.D. Marine Works Division Manager, Yuksel Proje, Ankara, Turkey ABSTRACT: Four case histories are described approximately 10 years apart, where unique solutions were implemented in the disposal of dredged materials for immersed tube tunnel construction. The earliest project in the 1970s used the dredged materials to construct a manmade island for the Second Hampton Roads Cross- ing in Virginia. The second project, the Fort McHenry Tunnel in Baltimore, built in the 1980s, separated clean and contaminated sediments and used the materials to reclaim the nearshore confined disposal facility (CDF) as a modern container terminal. The third, the Ted Williams Tunnel built in the 1990s in Boston Harbor, used large scale solidification/stabilization for contaminated sediments in an upland CDF. The most recent project, the Marmaray Rail Tunnel Crossing under the Bosphorus, will utilize a CDF for contaminated sediments—the first facility of its type in Turkey. The four projects illustrate a dramatic increase in dredging and disposal costs in the past three decades due primarily to continually tightening environmental restrictions. Further- more, the escalating disposal costs threaten the economic feasibility of an otherwise attractive method of subaqueous tunnel construction. 1 INTRODUCTION Tunnel opened, connecting Hampton and Norfolk, Virginia across the James River, a major shipping Up until the 1970s dredged sediments were com- channel. Twelve years later, in 1969, the firm was monly disposed of in specially designated open once again retained to provide design and con- water disposal sites. In immersed tube tunnel con- struction inspection services for a parallel second struction, the material dredged to form the tunnel crossing to meet increasing traffic demands. The trench would be temporarily stored underwater Second Hampton Roads Crossing was opened to near the trench. Once the tubes were sunk, the ma- traffic in 1976. terial would be used as ordinary backfill to cover The second crossing was composed of a 1,930- the immersed tubes. In 1980, during the planning meter southern approach trestle; a 965-meter and design of the Fort McHenry Tunnel in Balti- northern approach trestle; 600 meters of open ap- more that practice was deemed environmentally proaches on the two enlarged portal islands; and a unacceptable. It has since become common prac- 1,920-meter immersed tube tunnel under the chan- tice to plan and engineer confined disposal facili- nel. During this project, dredged material was ties to mitigate negative environmental impacts. beneficially re-used to enlarge a man-made island used for the south tunnel portal (Figure 1), and the process was completed without polluting the water 2 SECOND HAMPTON ROADS CROSSING and with no damage to extensive clam and oyster beds and other nearby marine habitats. Although a In 1954, the Virginia Department of Highways and federal environmental impact statement (EIS) was Transportation retained Parsons Brinckerhoff (PB) not required—as it would be today—all work was to design a crossing of the Hampton Roads ship closely coordinated among various state and fed- channel. In 1957, the first Hampton Roads Bridge- eral agencies and the construction industry to re- duce the risk of disturbing or damaging the envi- tory bodies in the site search was instrumental in ronment. streamlining and expediting the permitting process. The principal environmental requirement govern- ing the design of the CDF stated that any discharge from the disposal site could contain no more than 400 mg/L of suspended solids. Since the size of the site was too small for adequate settling basins, it was necessary to accelerate the settling of the sol- ids from the dredge slurry by flocculation before the effluent could be discharged back into Balti- more Harbor. The facility was designed to receive 2.7 million cubic meters of dredged material, including 460,000 cubic meters of highly contaminated sediments, excavated from the 55-meter trench for Figure 1. Enlarged man-made tunnel portal island. the construction of a twin-tube, eight-lane vehicu- lar immersed tube tunnel. Subsurface profiles de- 3 FORT MCHENRY/SEAGIRT NEARSHORE veloped from borings drilled at the tunnel site indi- CDF cated that the materials to be dredged would consist of very soft harbor bottom deposits As part of the design and construction work for the (460,000 cubic meters), organic clayey and sandy Fort McHenry Tunnel in Baltimore, PB was part of silt deposits (500,000 cubic meters), gravelly sands a joint venture that designed a nearshore CDF 3.2 (920,000 cubic meters), and stiff-to-hard clays and kilometers from the tunnel site. clayey silts (730,000 cubic meters). After a detailed study of alternative upland, near- The requirement to reclaim the CDF for the de- shore and open water disposal site options, the 59- velopment of a marine terminal shortly after fill- hectare Seagirt site in Baltimore Harbor was se- ing, was a major factor in the design of the Seagirt lected as the optimum location which provided suf- site. If all the dredged materials were placed in one ficient capacity and met the design criteria and en- area in an uncontrolled manner, the resulting non- vironmental requirements in regards to dredging, homogenous fill would take many years to con- disposal, and construction. The $60 million dis- solidate at great cost. To provide a design to accel- posal facility, which included a 1,700-meter cellu- erate site reclamation, the softest (and hardest to lar cofferdam offshore containment structure and consolidate) materials which were also the con- 6-meter-high clay-lined upland perimeter dikes, taminated shallow sediments were isolated and was constructed so that it could be converted into a placed in a separate partitioned area. This area marine terminal by the Maryland Port Administra- would require special ground improvement tech- tion. niques and its development into usable land would During the site search, PB evaluated various up- take approximately 10 to 20 years. The better ma- land and harbor locations. Our findings were re- terials, e.g. sands and hard clays, were placed in ported and reviewed in frequent meetings with the the “spoil area” where a stabilization effort would Interstate Division for Baltimore City (IDBC); the commence soon after the completion of disposal Federal Highway Administration (FHWA), which operations. funded 90 percent of the cost of the tunnel, and an The CDF capacity was engineered to accommo- environmental task force composed of representa- date the increased volume (bulking) of the materi- tives of nine different federal, state, and local regu- als after being dredged and redeposited. With al- latory and permitting agencies. Because these lowance for bulking, which varies both with the agencies were regularly informed of our technical type of material being dredged as well as the findings during the study of alternative sites, the dredging method, the required disposal site design required dredging and disposal permit for the se- capacity was estimated at 3.8 million cubic meters. lected site was obtained shortly after completion of the study. The proactive participation of the regula- The materials dredged by a cutter suction dredge side the basin to maintain the required ponding were pumped the 3.2-kilometer distance from the depth. tunnel trench by a 0.7-meter underwater pipeline to To maximize capacity, the dredged material re- the disposal site where the contaminated materials ceived in the disposal area was piled and com- were separated using shut-off valves on the pipe- pacted by bulldozers to build a 3.4-meter-high dike line (Figure 2). The effluent was diverted through 30 meters behind the cells at the elevation of the shaft-type weirs into the treatment and the settling land dikes at +6.4 meters. The land dikes were basins before discharge. Rapid mixers installed in- constructed of compacted granular fill and lined on side the weirs and slow mixers in the treatment ba- the inside face with a clay blanket to prevent seep- sin accelerated the agglomeration of the floccu- age. Dredging and disposal operations were com- lants. pleted on schedule (Figure 4). The site was later consolidated through ground improvement and converted into a container port by the Maryland Port Administration (Figure 5). Figure 2. Y-valves on the pipeline separated the clean and contaminated sediments during disposal operations. The Seagirt nearshore CDF project (Figure 3) was the first major dredging project to be designed for effluent treatment by large-scale flocculation Figure 4. CDF at completion of disposal operations. and sedimentation—a state-of-the-art system in dredged material disposal which has worked re- markably well in settling the finer solids in the ef- fluent and containing them at the disposal site. During disposal operations, the solids that settled inside the settling basin were continuously re- moved by a small hydraulic dredge operating in- Figure 5. Seagirt CDF after conversion into marine terminal. Figure 3. Seagirt nearshore CDF during construction. in the shortest possible period after disposal, as ad- 4 TED WILLIAMS TUNNEL/GOVERNORS ditional land-excavated material from the East ISLAND UPLAND CDF Boston portion of the tunnel construction would be filled over the stabilized sediments. To provide for In 1990, PB in joint venture, designed and man- adequate bearing capacity, the desired shear aged the construction of a solidifica- strength of the compacted solidified sediments was tion/stabilization and upland CDF for the contami- established as 207 kPa in order to accommodate a nated sediments dredged for the construction of the 4.6-meter-high embankment.