By Irwan S. Halim, Vojtech Ernst Gall and Stephane Polycarpe
The Pawtucket Tunnel Project is the first phase of the Narragansett Bay Commission (NBC) Phase III Combined Sewer Overflow (CSO) Program designed to reduce CSOs from the communities of Pawtucket and Central Falls in Rhode Island. Phases I and II of the program were focused on the Providence area and were completed in 2008 and 2015, respectively.
The Pawtucket Tunnel is planned to have a minimum finished inside diameter of 30 ft and a length of approximately 11,700 ft. The tunnel will be a rock tunnel with depth to invert ranging from 115 to 155 ft. The tunnel will be excavated using a TBM and lined concurrently with gasketed precast concrete segments. This project is being implemented using a design-build delivery process. The design-build contractor consists of a joint venture of CBNA and Barletta (CBNA-Barletta JV); also known as CB3A. The prime designer is AECOM. GEI Consultants is assisting AECOM with geotechnical engineering and field support. Design subconsultants include Gall Zeidler Consultants, Mueser Rutledge Consulting Engineers and BETA Group.
The Pawtucket Tunnel Project construction includes a main conveyance and storage tunnel; a tunnel boring machine (TBM) launch shaft and receiving shaft; tunnel pumping station; drop and vent shafts and connecting adits. The tunnel excavation will be with a hybrid TBM, capable of operating in an open or closed, pressurized-face earth pressure balance (EPB) mode if conditions warrant.
As the TBM advances, the tunnel will be lined with precast steel fiber reinforced concrete segments. The project location is shown on Figure 1 below. The TBM Launch Shaft is 60 ft in diameter and approximately 150 ft deep and is located close to the Tunnel Pumping Station (TPS) Shaft in alignment with the tunnel. During the tender design phase as an alternate technical concept (ATC), the TPS and launch shafts were relocated to avoid a known shear zone and brought closer together. The Launch Shaft will be connected to the TPS Shaft with a 10-ft diameter Suction Header Tunnel. A TBM Tail Tunnel is provided as well as a pre-excavated TBM Starter Tunnel. There are four additional drop shaft locations on the tunnel alignment. The Receiving Shaft is 36 ft in diameter and 130 ft deep.
Tunnel Segmental Lining Design
The entire length of the tunnel will be constructed in the siliciclastic bedrock of Rhode Island formation overlain by glacial till deposits and other fill materials and will be located below the groundwater table. A seven-segment universal tapered ring system was adopted as shown in Figure 2. The tunnel lining ring is 14-in. thick, 30-ft, 2-in. in internal diameter, and 6-ft, 7-in. in length, and consists of four regular segments of rectangular geometry, two counter-key segments of right trapezoidal geometry, and one wedge-shaped key segment. All segments will be staggered to avoid creating cruciform joints which could cause leakage and structural distress due to stress concentration. As the key segment cannot always be installed at the tunnel crown, the TBM will need to be able to hold segments in place during ring assembly using an erector and support roller system. The length of the ring was selected by balancing between the constructability factors (ease of transportation, assembly, and ability to negotiate curves) and the utility factors (limiting joint number to reduce leakage and production cost and to increase tunnel advancement rate).
The wedge-shaped key block is often designed as a smaller size piece than other segments because smaller segments are easy to handle. Lately, a large key segment is becoming more accepted in the industry. Our design adopted similar-sized segments. The chord lengths at centerline are about 14.1 ft for the regular segments, 14.0 ft for the counter-key segments, and 14.3 ft for the key segment. It is advantageous to make all the segments as similar size as possible because of structural and constructability reasons. The longitudinal joints evenly spaced within the ring lead to increased load bearing capacity and reduce ring distortions. The larger key segment also can reduce the size of the regular segments (and therefore the number of longitudinal joints). The segments will not be bolted to each other at the longitudinal joints. This will help increase tunnel advancement rate because some bolting is eliminated. The rings will be connected to each other at the ring joints using 14 equally spaced dowels (SOF FAST 110). In addition, 14 pairs of equally spaced shear connectors, i.e. shear bicones with a steel core (Optimas Sofrasar F500), were specified at the ring joints for centering and shear recovery purposes at the tunnel adit openings as described later.
For waterproofing purposes, an all-around ethylene propylene diene monomer (EPDM) compression gasket (Datwyler M389 33 “Doha”) profile was originally specified, however the contractor selected to use an alternative Algaher DV9 IS gasket. The compression gasket is anticipated to resist up to 25 bars of hydrostatic pressure under the design compression and allowable offset scenario, which was sufficient to withstand the maximum anticipated groundwater pressure of 5 bars. The typical segments will be reinforced by steel fibers (no less than 60 pounds per cubic yard of Dramix 4D 80/60). The minimum required strength at 28-day adopted for the design was f_c^’=6,500 psi of the characteristic compressive strength and f_150^’D=700 psi of residual flexural strength at 3.5 mm crack mouth opening displacement (CMOD). The segment thickness was selected to withstand all short-term and long-term loading cases and service conditions. To achieve the desired 100-year service life, the selected segment thickness (14 in.) includes up to 2.35 in. of sacrificial concrete layer to protect the tunnel lining from concrete degradation due to hydrogen sulfide (H2S) gas from the CSO water. The tunnel lining was designed where loss of the sacrificial layer does not impact the 100-year structural integrity of the lining system.
To demonstrate the adequacy of the adopted tunnel lining design, the stress and deformation of the tunnel lining were evaluated using two-dimensional (2-D) numerical analyses as shown in Figure 3. A total of five analysis sections were selected as representative of variations in overburden depths and anticipated ground and groundwater conditions. Among the five analysis sections, the section that was cut at Station 118+00 was found to govern the design due to the presence of intensely fractured soft and weak graphitic shale layer that traversed the tunnel alignment. The graphitic shale layer is an approximately 20-ft thick sub-horizontal layer imbedded within bedrock. During the tender design phase as an ATC, the vertical alignment of the tunnel was raised by 25 ft, which resulted in reduction in lining thickness from 15 in. to 14 in. and the deletion of the hybrid reinforced (fiber and rebar) segments requirement for the graphitic shale zone.
The analysis was done in stages to evaluate the lining forces and the deformations under various loading conditions that are expected during the construction and throughout its design life. These include the variation of the anticipated groundwater levels, dewatering of groundwater during construction, and crossing of intensely fractured fault zone. The analysis showed that the tunnel lining will be able to withstand the anticipated loads and meet the ring distortion requirement for all these conditions. A seismic analysis was performed to evaluate the effects of seismic waves vertically propagating perpendicular to the tunnel axis. A transverse racking analysis was performed to quantify the racking deformations and their effects on the tunnel lining. The analysis was done using the same FLAC model cropped around the tunnel opening to include the rock materials only without top-soil materials.
The segment design was checked against the temporary construction loading scenarios including stripping and handling minimum concrete strength of 1,800 psi, storage and transportation, erection and TBM thrust rams pushing against the segments, during backfill and contact groutings, and for the TBM gantry load and segment feeder immediately behind the erector.
The segment joint design consisted of checking the circumferential dowel system capacity to hold the segments together during erection and maintain gasket compression to adequately seal off the design hydrostatic pressure.
Specialized Hybrid Segment Design
The Pawtucket Tunnel is connected to four drop shafts by tunnel adits along its alignment, three of the adits are shown schematically in the above Figure 4. Two adits are planned to be constructed by sequential excavation methods (SEM) and the third will be constructed by microtunneling.
The SEM tunnel adits will be constructed from the main tunnel by drill and blast as shown in Figure 5 with mining outwards towards the base of the drop shaft structures. The third adit constructed by microtunneling will start from the drop shaft location and be jacked into the main tunnel. The final connection between the main tunnel lining and the adits will be achieved by installing a monolithic cast-in-place (CIP) concrete collar around the segment cuts, as shown in the figure.
At the adit locations in the tunnel, segments will be cut and removed. At these locations the tunnel requires additional bracing to support the temporary forces due to the tunnel wall opening at these locations. Rather than designing the usual external bracing frame at these locations, the opening will be temporarily supported by modified segment rings installed during tunnel construction. These rings have additional reinforcement and shear elements included within the specially designed tunnel segments installed along the four rings immediately adjacent to the opening. While the standard lining design along the alignment is pure steel fiber reinforced concrete (SFRC) with two shear dowels installed at each thrust pad, the specially reinforced segments feature a heavy rebar cage in addition to the SFRC as well as two additional high-capacity shear cones per segment ring joint installed in the middle of the thrust pads between the shear dowels. The geometry of the specially reinforced segments is equivalent with that of the typical segments.
A staged Finite Element (FE) model was developed for the structural analysis of the lining and collar at the TBM tunnel-adit locations that considers all the critical design issues, e.g. the in-situ ground pressures, the temporary and permanent ground water loads, the supporting action (or lack thereof) of friction between rings, the durability of the lining concrete, and the nonlinearities of all the materials involved. An image of the model is shown in Figure 6 below.
The structural analysis indicates that, as expected, the most critical case for design is the temporary loading of the segments immediately following the cutting and removal of the lining and before installation of the CIP concrete collar. Specifically, the analysis indicates that the tensile stresses are too high to be carried by steel fibers alone and this resulted in the requirement for additional circumferential rebar reinforcement and shear cones in these special segments.
TBM Start-up and Launch Sequence
CB3A selected a Herrenknecht hybrid TBM. The project team with Herrenknecht agreed on a Hybrid TBM with open/closed EPB mode capabilities which presents the advantage of controlling muck management, minimized maintenance and optimized production time. The TBM delivery was completed at the end of June 2022.
The 33.8-ft bore diameter TBM has a 46-ft long shield and a 300 ft long back-up, comprised of four gantries to supply power to the shield, and connect the utilities and logistics.
The shield is composed of three main sections with one active articulation. This will enable it to achieve the minimum required curvature of 1,000-ft radius. The structure and seals are designed for 72 psi (5 bar) of pressure.
The 33.8-ft diameter cutter head is designed for hard rock and protected against wear. It is equipped with sixty-four 19-in. disc cutters for high efficiency and reliability in the anticipated conditions.
The machine is fitted with a conveyor belt in open mode and a screw conveyor in EPB mode to move muck from the cutting chamber to the tunnel muck conveyor that will transport the material to the shaft. The face can be closed in less than 10 minutes if required.
The dual mode has a real benefit on the project because it will significantly reduce the need for probe drilling and pre-grouting that a single open mode TBM would require. The selected TBM is shown in Figure 7.
The TBM will be launched and operated from the Launch Shaft located on the main site at the Southern end of the project. It is 62-ft diameter and 155-ft deep, excavated within the soil portion inside an excavation support wall made of secant piles and in the rock using drill-and-blast method. When the shaft excavation reaches the level of the tunnel at a depth of 120 ft, a starter tunnel of 230 ft long and a tail tunnel of 60 ft long were excavated by SEM in two stages, top heading and bench. The remainder of the shaft was then completed after the starter tunnel and tail tunnel were excavated. The bottom of the shaft was backfilled, and a trench was kept below the tunnel invert to install the bottom part of the vertical conveyor as shown in Figure 8.
The TBM was pre-assembled on surface. The 1,665 tons machine was delivered in 82 packages. The pre-assembly on the surface made it possible to lower larger assembled TBM sections into the shaft weighing up to 375 tons as shown in Figure 9.
The main site is set up to provide the necessary logistic support to the TBM operation. A crawler crane of 350 tons capacity handles the segments and the services inside the launch shaft. There is sufficient space on top of the shaft to store up to 60 rings, more than a week of anticipated TBM production.
The TBM muck is transported from the TBM by a chain of conveyors at a rate of 1,250 tons per hour. The muck is lifted in the shaft using a vertical bucket conveyor as depicted in Figure 10.
On the surface the muck is dropped by a radial stacker in a stockpile accumulating 15,000 cubic yards, the anticipated average weekly quantity excavated by the TBM.
The rest of the surface installation comprises a grout plant, electrical distribution, water treatment plant, offices, material storage and workshop. The main launch site is shown in Figure 11.
As of early September 2022, the TBM was set up in the starter tunnel and set for launch in the second half of the month. To minimize the length of the starter tunnel excavation, only three gantries are installed initially, the fourth gantry with non-essential services will be lowered in the tunnel once the TBM has advanced. The transport of segments in the tunnel is by two multi-service (VMS) vehicles shown in Figure 12 that can hold one full ring made of seven segments and one set of temporary services that will be installed in the tunnel to provide fluids, ventilation and power to the TBM.
Tunneling was started in September 2022 and due to be completed by the end of 2023.
Irwan S. Halim, Ph.D., P.E.
VP – Tunnel and Underground Engineering
Dr. Halim is AECOM’s Chief Engineer for water tunnels and has over 31 years of experience in geotechnical and underground projects in the U.S., Canada, and abroad. He has provided engineering and design services for major water/wastewater as well as transit agencies throughout North America. Irwan’s expertise includes design and construction of mined and cut-and-cover tunnels in both soft ground and hard rock; TBM-driven, drill-and-blast, and SEM excavations; Soil/rock-structure interaction analyses; and foundation engineering. Irwan’s past experiences include major SEM, large TBM-driven tunnels, and tunneling through complex and mixed-face ground conditions.
Vojtech Ernst Gall
Senior Tunnel Engineer
Gall Zeidler Consultants
New York, NY
Bio: Dr. Vojtech Ernst Gall has been involved in national and international rail, highway, mining and hydropower tunneling projects ranging from deep tunnels in alpine environments to shallow tunnels in dense urban settings. He has experience with both TBM as well as NATM/SEM construction for both small- and large-scale projects. Dr. Gall’s particular interests lie in the computationally assisted design and planning of TBM driven tunnels, ranging from BIM-technology supported project planning and management to numerical and structural modeling. In addition to his technical expertise, Dr. Gall has been actively involved in the organization of many technical seminars and events. Dr. Gall is Chair of the Underground Construction Association of the Society for Mining, Metallurgy & Exploration (UCA of SME) Young Members group, is the chair of the UCA of SME Working Group ““Information Modelling in Tunneling,” and is an active contributor to the International Tunneling Association (ITA) Working Group 22 “Information Modelling in Tunneling” as well.
Stephane Polycarpe has over 33 years worldwide and multi-disciplinary experience in tunnel construction and major civil works started at the Channel tunnel. His TBMs knowledge is covering the variety of air plenum, earth pressure balance (EPB) and slurry mix shield machines as well as hard rock machines. His soft, hard and mix ground tunneling experience includes canopy vault and grouting, timber headings, ground freezing, drill and blast, SEM excavations in soil/rock and has an experience in tunnel dismantling. In 2020 Stephane joined CBNA. as the Engineering Manager for the Pawtucket Tunnel project.