Flooding has been a decades-long concern for the residents of the Albany Park neighborhood of Chicago, IL. Following a catastrophic flooding event in April 2013, the city pledged to address the problem affecting the residents’ lives, properties and local businesses. The solution was to construct an approximately 1.6-km (1-mile) long tunnel that diverts more than half the river flow during a flood event from the North Branch of the Chicago River (NBCR) to the North Shore Channel (NSC). When the tunnel is filled with the NBCR overflow, the system will operate as an inverted siphon, bypassing approximately 2,000 cu ft/sec of river water during the 1 percent annual chance design event (100-year storm) and less flow for smaller storm events that exceed the NBCR inlet weir elevation. The diverted flow leaves the tunnel system through the outlet shaft by way of the 20-m (64-ft) long outlet structure located along the east bank of the NSC south of the Foster Avenue bridge at River Park. After the NBCR and NSC river levels have subsided, water remaining in the tunnel system will be pumped out by two dewatering pumps installed within the outfall shaft sump.
It is the first time in Chicago that a tunnel of this magnitude has been used to connect two existing rivers. The completed Albany Park Stormwater Diversion Tunnel Project is considered a major flood-risk reduction measure planned and constructed with the long-term benefit of the community in mind. Because of the project, more than 300 residential structures will be relieved from the 100-year floodplain of the NBCR. Paired with the restoration improvements at the inlet and outlet sites, both within Chicago Park District parks, this project improves neighborhood aesthetics, property values and the overall quality of life for Albany Park residents all within a highly visible public platform spanning three of Chicago’s most affluent aldermanic wards.
The project was funded using a combination of funds from the City of Chicago, Metropolitan Water Reclamation District of Greater Chicago, Illinois Department of Natural Resources and U.S. Department of Housing and Urban Development with Chicago Department of Transportation (CDOT) as the lead city of Chicago agency. Design of the tunnel was performed by MWH Americas Inc. and, overall, eight contractors submitted cost proposals to CDOT. Kenny Construction Co. was awarded the contract and a notice to proceed was issued in April 2016. WSP USA Inc. served as CDOT’s construction manager for the project. Tunneling commenced in late fall 2016 and continued through late 2017. Substantial completion of the project was accomplished in August 2018.
The Albany Park Stormwater Diversion Tunnel consists of a 1,778-m (5,835-ft) long, 5.4-m (18-ft) internal diameter tunnel with a slope of 0.1 percent to the downstream end (Fig. 1). The tunnel depth is approximately 42 m (140 ft) below the ground surface and the tunnel was constructed entirely in rock by a mainbeam tunnel boring machine (TBM) with ground support by a two-pass lining system. The inlet shaft is located at a bend in the NBCR in Eugene Field Park just east of Pulaski Road. The tunnel ends at the outlet shaft at River Park just south of Foster Avenue adjacent to the NSC. Details of the shafts are included in Table 1.
The outlet shaft served as the TBM launch shaft and was the contractor’s main staging area. Up to 2 ha (5 acres) of space were available at the site to utilize as the contractor’s primary work/staging area, positioning a crane, materials storage, laydown area, contractor office trailers, substation, workshop and parking. The outlet structure was constructed with a 20-m (64-ft) long weir set at -0.6 m (-0.2 ft) continous collision detection (CCD), approximately 0.6 m (2 ft) above normal channel flow to prevent inflow into the tunnel under normal flow conditions. The shaft was constructed to an internal diameter of 9 m (30 ft) and is approximately 52 m (170 ft) deep with the upper 21 m (70 ft) constructed through overburden materials and the lower 30 m (100 ft) of the shaft through bedrock. The shaft includes a 4.5-m (15-ft) deep sump located below the invert of the tunnel in which a pump was constructed to dewater the tunnel after flood events.
The inlet shaft, located in Eugene Field Park, was used to retrieve the TBM. The size of the site is approximately 0.4 ha (1 acre), which provides sufficient laydown space for construction activities. The shaft is 43 m (142 ft) deep with depth to bedrock of approximately 21 m (70 ft). The inlet structure was constructed with a 63-m (208- ft) long weir set at 3.1 m (10.1 ft) CCD. The weir spilled to an approximately 6.7-m (22-ft) wide channel which connected to the shaft at -1.7 m (-5.7 ft) CCD. The shaft was constructed to an internal diameter of 5.4 m (18 ft).
Geologic setting and subsurface condtions
Chicago is situated on the eastern flank of the southward-plunging Wisconsin Arch. Silurian rocks thicken eastward into the Michigan Basin and the underlying Cambrian and Ordovician strata thicken southward into the Illinois Basin. The bedrock in Chicago is covered by up to 91 m (300 ft) of unlithified surficial materials consisting of clay, silt, sand and gravel deposited primarily by glacial processes. Silurian dolomites are present at the bedrock surface over nearly the entire city. The Paleozoic-era Silurian system rocks range in thickness from zero in a few areas in the northwestern part of the city to more than 91 m (300 ft) on the far eastern side along the lake shore (Hannes et. al., 2004).
The subsurface investigation program for the project implemented a phased investigation approach (MWH, 2015). The first phase consisted of four borings conducted in May and June 2013. Subsequently, the second phase consisted of eight borings drilled intermittently from July to November 2014. The boring depths ranged from 11 to 52 m (35 to 170 ft) below ground surface. Two of the borings were inclined at 30 degrees from vertical to improve the likelihood of encountering near-vertical joint sets. The remainder of the borings were drilled vertically.
Packer testing was performed in seven borings to evaluate the hydraulic conductivity of joints and bedding planes in the bedrock. Tests were accomplished at intervals ranging from 3 to 12 m (10 to 40 ft) using double packers starting from the bottom of the boring and continuing up hole. Bailout tests were performed in three borings. Results varied from about 10-7 to 10-5 cm/sec in unweathered rock and from 10-6 to 10-3 cm/sec near the top of rock where fractures are prevalent and rock quality is poorer (MWH, 2015). These results from the tests were used to estimate the average groundwater inflow rate into the unlined tunnel which was estimated not to exceed 12.6 L/sec (200 gpm) averaged over the full length of the tunnel (MWH, 2015).
Laboratory testing was performed on samples of soil and rock obtained from the borings. Rock tests included uniaxial compressive strength, specific gravity, density, point load (axial), Brazilian tensile, direct shear, Mohs hardness, Cerchar abrasivity and punch penetration. Soil testing included moisture content, Atterberg Limits and unconfined compression testing.
Based on the results of the investigations, overburden materials were generally found to be about 18 to 21 m (60 to 70 ft) thick and consist predominantly of glacial till material composed of mainly silty clay with trace sand. Below the silty clay materials and overlaying the bedrock is a layer of 1.5 to 3 m (5 to 10 ft) thick till layer primarily composed of very dense sand and gravel. Northern Illinois is underlain by limestones and dolomites of the Silurian system. During the decline of glaciation at the early phase of the Silurian, seas entered the region thereby depositing carbonate sediments. More specifically, bedrock conditions within the project area consist of the Racine Formation underlain by the Joliet Formation. Silurian rocks vary generally in thickness due to erosion effects with maximum thickness in the range of 152 to 182 m (500 to 600 ft) (Mikulic et al., 2010). The region underwent four geologic depositional sequences, the latter of which resulted in the formation of the Racine dolomites.
The tunnel was constructed entirely within the Racine Formation which extends to approximately 61 m (200 ft) below the ground surface in this area. The Racine Formation is an argillaceous dolomite that is often more than 91 m (300 ft) thick and is medium to light gray, dark gray, mottled, gray weathering, medium grained and vesicular to highly vuggy (Willman, 1973). Rock core data within the Racine Formation indicated bedding planes are mostly horizontal to subhorizontal with bedding spacing at the tunnel horizon ranging from 0.06 to 1.2 m (0.2 to 4 ft) (MWH, 2015). Three sets of near vertical joint sets were encountered with spacings described as moderately wide to very wide. Rock quality designation values ranged from 70 to 100 percent within tunnel horizon. Rock mass rating (RMR) classifications were performed based on observation of rock core samples obtained from the borings and subsequent laboratory tests. RMR values presented within the geotechnical baseline report (MWH, 2015) ranged from a worst case of 52 to a best case of 84 with best estimate of 70. The mean uniaxial compressive strength was recorded as 9,340 lbs/sq in.
Launch shaft and assembly chamber construction
Construction commenced with the preparation of the staging area at the outlet shaft location on the banks of the NSC and subsequent excavation of the shaft itself. The shaft served as the TBM launch shaft for tunnel construction. It is approximately 51 m (170 ft) deep with the upper 21 m (70 ft) constructed through overburden materials and the lower 31-m (100 ft) of the shaft through bedrock. Within the overburden, the shaft was excavated to a diameter of 12 m (40 ft) and excavation support was provided by means of a soldier pile and lagging system (Fig. 2). HP 14×73 (Grade 50) piles spaced 1.6 m (6 ft) on center were set in 0.6 m (2 ft) diameter drilled holes from the ground surface to the top bedrock and filled with 200- lbs/sq in. compressive strength grout. Internal bracing of W 12×87 (Grade 50) ring beams were installed at vertical spacings ranging from 1.6 to 2.4 m (6 to 8 ft). At the soilrock interface, a 0.5 by 0.3 m (1.5 ft by 1 ft) cast-in-place concrete ring beam was constructed on the rock ledge around the excavation.
Prior to excavation into bedrock, a pre-excavation grouting program was implemented to limit ground water infiltration near the top of bedrock. Grout holes were installed using rotary drilling equipment at the top of bedrock spaced at 3 m (10-ft) intervals around the perimeter of the shaft. Holes were angled at 20 degrees from the vertical and were 1.5 in. in diameter. Grouting was only performed within the primary grout holes as secondary holes were not required based on the results and grout takes observed in the primary holes.
Excavation within bedrock was performed by controlled drill-and-blast methods. The shaft in the bedrock was excavated to a diameter of 10.6 m (35 ft) with rounds ranging from 1.8 to 3 m (6 to 10 ft) lifts. Rock bolts were installed near the top of the bedrock and just above the tunnel intersection. Rock bolts were No. 8 steel bar elements installed on a 1.5 m by 1.5 m (5 ft by 5 ft) pattern to a length of 3 m (10 ft) and inclined at 10 degrees from the horizontal. Rock dowels were installed to the same pattern and dimensions as the rock bolts throughout the remainder of the shaft excavation.
A 27-m (90-ft) long starter tunnel and 38-m (125-ft) long tail tunnel (Fig. 3) were excavated by controlled drill and blast to provide space for the TBM assembly, trailing gear fitment and operation startup. The starter tunnel was excavated to a 6.7-m (22-ft) span, horseshoe-shaped and was excavated in two stages. The top heading was excavated to a height of 4.2 m (14 ft) and followed with a bench/invert stage of 2.4 m (8 ft). The tail tunnel was excavated to two dimension/sections: a 6 m (20 ft) high, 9 m (30 ft) wide horseshoe-shaped section was excavated directly adjacent to the shaft and a 4.2-m (14-ft) high section for the remainder of the tunnel. The 6-m (20-ft) high section was excavated in two stages with a 4.2 m (14 ft) high top heading and a 1.8-m (6-ft) deep bench/invert. The 4.2 m (14 ft) section was excavated in one stage.
TBM tunnel construction
The tunnel was mined with a remanufactured main beam TBM from Robbins (model MB186-207-3) originally built in 1979 and now named Keri (Fig. 4). Among other updates and repairs, the then 38-year-old TBM was fitted with a larger cutterhead of 6.2 m (20.3 ft) excavation diameter compared to its original configuration of 5.6 m (18.5 ft). It was also fitted by the manufacturer with new mounts for V-mounted cutters, which were the cutter type available in stock by the contractor, as well as with newer technology removable scrappers. The TBM had six drive electric motors of 300 hp each for a total of 1,800 hp. The cutterhead transmission and drive system allowed for a base rotational speed of six revolutions per minute and a total of 714-t (1,576,170 lb)-foot of torque. The TBM’s nominal thrust was 650 t (1,433,500 lbs) with a maximum rating of 1,858,250 pounds. The cutterhead was fitted with 40 cutters. The first four positions in the center are covered by two 43 cm (17 in.), twin V-mount cutter assemblies mounted on a quad saddle. The positions five through 35 are founded on 43-cm (17-in.) V-mount face saddle assemblies while 36 through 40 are fitted with 43 cm (17 in.) V-mount gage saddle assemblies.
Roof support and scaling facilities were installed at two locations on the TBM twin drill decks mounted behind the TBM gripper assembly, each equipped with hydraulic drill packages. This location was used to install any pattern bolting installed for temporary support. Roof support drills can cover from approximately 17 degrees from vertical to 22 degrees from horizontal centerlines. The TBM provided an additional location for installing rock support by means of twin, fixed position drill decks at the front of the TBM, rear of the cutterhead drive motors, and under the extended roof finger shield. The latter was a measure added to the TBM onsite as the original configuration had a solid shield canopy. Use of this location was limited to installation of bolts, dowels and other primary supports for unstable ground with handheld pneumatic equipment where immediate support of the tunnel roof is required before it is exposed from behind the TBM roof supports.
The TBM started mining from the outlet shaft in April 2017 and mined approximately 1,706 m (5,600 ft) of tunnel in a period of nearly five months to reach the target. The TBM excavation was a 24-hour operation, five days per week including time required for any cutter changes. Major repairs and maintenance were reserved for weekends. Daily advance rates ranged from approximately 3 to 40 m/d (10 to 131 ftpd) as indicated on Fig. 5. During the initial period of the learning curve, the TBM encountered a fractured rock zone that necessitated the pause of the TBM advancement for modifications proposed by the contractor and for installation of additional rock-support measures as described in the next section. Once the TBM passed beyond the fracture zone, its productivity rapidly increased and advancement generally remained at an average of approximately 24 m/d (80 ftpd) until reaching the inlet tunnel. Select points showing no progress correspond to cutter changes, holidays or other unforeseen mechanical issues. Minimal seepage and generally good rock mass conditions beyond the fractured rock zone resulted in a good progress and timely completion of the tunnel. The TBM broke through at the inlet shaft on Aug. 30, 2017.
The baseline initial ground support system in the TBM mined tunnel consisted of 2.1 m (7 ft) long, No. 8, Grade 75 steel resin grouted rock dowels installed on a 1.5-m (5-ft) square spacing. Spot dowels, steel-fiber-reinforced shotcrete, wire mesh and rolled steel channels were also installed at selected locations as needed
Fractured rock zone
During the early stages of mining with the TBM at approximately station STA 55+85, a loose rock zone was encountered (Fig. 6). The weak zone originally was encountered between the 8 and 10 o’clock positions facing west (toward the direction of advancement). The TBM managed to advance through the area for approximately 6 m (20 ft) prior to halting due to additional rock loosening experienced around the springline, shoulder and crown areas. This loosening zone corresponded to the existence of a subvertical joint feature, as described in the geotechnical baseline report (MWH, 2015), which intersected the excavation subparallel to the tunnel axis. The report identified such features and the possibility that rock falls may be more frequent within proximity of the two shafts due to tunneling parallel to the predominant joint set striking at N50°W to N70°W.
The material within this zone varied from large rock fragments with widths of up to 0.5 m (1.6 ft) to smaller rock fragments mostly shale-like with occasional clay layers or seams. In areas where the fractured rock zone extended to the springline, the bearing of the gripper pads became inadequate for TBM propulsion. The intersection of these weakness features with the subhorizontal limestone bedding created loosening zones with subsequent overbreaks (Fig. 7). Based on discussions among the contractor, the construction manager and the designer, it was determined to perform onsite modifications to the TBM shield by adding a finger shield and installing a steel set erector to provide for the safe passage of the TBM and the continuation of the excavation.
The contractor also designed and provided 19 W6x20 steel sets that were installed at 1.2 m (4 ft) spacings along with wood lagging. Approximately 30 m (100 ft) of tunnel length were impacted by the presence of this fractured rock zone. The fractured rock zone was no longer observed within the tunnel at approximately STA 54+18 and mining beyond that point generally continued uninterrupted with an average advance rate of approximately 25 m/d (80 ftpd).
The original design of the final liner consisted of 0.3 m (1 ft) thick, unreinforced, cast-in-place concrete. However, installation of the W6x20 steel ribs for additional rock support within the fractured rock zone encroached upon the design line of the final lining. Therefore, a modification was made to the design of the final concrete liner within the fracture zone. Ultimately, a fiber-reinforced, fulldiameter final concrete liner integrated within the steel ribs for the affected section of the tunnel was installed. In addition, a detailed contact grouting plan was implemented to ensure proper contact between the castin- place liner and rock.
Challenging urban environment
In preparation for controlled drill-and-blast and other tunnel mining operations in the dense, urban Chicago neighborhood of Albany Park, a rigorous communication plan was implemented with the various community groups and businesses throughout three aldermanic wards. Regular communications occurred with the local Aldermen, Office of Emergency Management and Communication, the Chicago Transit Authority, numerous utility companies and the North Park University Campus to establish public awareness for the potential impacts of construction. In facilitating the plan, concerns regarding potential conflicts between the outlet shaft blasting activities and the nearby Swedish Covenant Hospital’s facility equipment operations were identified. There was a risk that blast vibrations could affect the operation of the proton accelerator cancer-treatment equipment in the hospital’s facility. In response to the concerns, a communication plan for blast detonations was established that eliminated the risks by coordinating the blasting schedule with the treatment schedule. To provide the hospital with the utmost confidence that all risks were allayed, construction inspection personnel were assigned to be present in the hospital equipment room during detonations and required a hold point from the contractor to confirm no proton accelerator operations were ongoing for each blast detonation. Through this frequent communication, it was possible to coordinate the construction detonations to eliminate any risk to the hospital’s patients while allowing construction to proceed without significant delays.
For controlled blasting operations, the maximum allowable peak particle velocity (ppv) as measured at any adjacent structure or facility per the contract documents was 0.2 in./sec at frequencies of 1 Hertz or less and 0.5 in./sec at frequencies between 2.6 Hertz and 40 Hertz. The maximum allowable air blast overpressure limit was 134 decibels at a 0.1 Hertz frequency, 133 decibels for a 2 Hertz frequency, and 129 decibels for a 5 to 6 Hertz frequency. Four seismographs were installed at both the inlet and outlet shafts to monitor every blast detonation. Locations were determined based on the closest building structure, the nearest bridge structure, closest utility and nearest sensitive community landmark. The seismographs continuously monitored ppv and air blast overpressure. All monitoring devices were programmed to trigger an alert if the thresholds for vibration and overpressure were exceeded. A web-based, mobile-accessible datareporting platform was provided for real-time data and alerts in addition to the required reporting procedures. This provided the project team with instantaneous performance data throughout all controlled blasting operations. During construction, the monitored ppv were below the allowable thresholds for all blasts. The maximum allowable overpressure limit per the contract documents, however, was exceeded once. The contractor was directed to reevaluate his blasting sequence and design to reduce the overpressure levels and after his revisions to its work plan, the remainder of blasts conformed to the contractual requirements.
In addition to blasting, for all excavations the contractor was required to perform a settlement monitoring program. Full topographic data at both the inlet and outlet shafts were obtained prior to construction. In addition, 26 settlement baseline points were established and monitored regularly during all excavation and tunnel mining to confirm a maximum threshold of 0.25-in. settlement was not exceeded. As an added precaution, the contractor was required to offer and perform preconstruction property inspections for building owners within a 152 m (500 ft) radius from the center of both shafts, and 61 m (200 ft) from the centerline of the proposed tunnel alignment, resulting in hundreds of preliminary property inspections.
Lastly, one of the major concerns identified prior to the start of construction was a potential need to operate the tunnel prior to project completion should a flood event occur. Through advanced planning with the project team, an emergency operation plan was developed to allow the rising river waters into the tunnel to help alleviate any flooding. Constant weather forecasts, river-level monitoring, the application of historical empirical data and 24-hour communication afforded CDOT and the project team 12-hour notice to remove the contractor’s 100-year storm flood protection before the river levels compromised access. The first use of the tunnel during construction occurred on May 3, 2018. The temporary steel-plate dam between the weir structure and inlet shaft was removed as the river levels increased, literally opening the flood gates for operation. The tunnel was operated to reduce the risk of local flooding three more times during the next two months prior to the completion of the project. The tunnel operated as planned and the neighborhood did not flood.
The completed Albany Park Diversion Tunnel Project is a great example of a major flood-risk reduction project designed and constructed with the long-term benefit of the community in mind. It provides a high level of flood mitigation to Albany Park Chicago area residents and businesses while enhancing the aesthetics and usefulness of precious open space and recreational areas within a dense urban environment adjacent to the Chicago River corridor.
CDOT made a commitment to provide benefit to the Albany Park neighborhood. In addition to the construction of the tunnel, and as part of the project the outlet shaft site location was restored with a new regulation-size baseball field, a soccer field and several landscaping improvements within River Park. Improvements at Eugene Field Park at the inlet shaft site included landscaping, new trees, a bike path, benches and a water fountain.
The effective techniques and methodology practiced to overcome the construction challenges of controlled blasting in a dense urban environment, encountering a fractured rock mass zone during TBM mining, and multiple tunnel flood operations during construction are prime examples of the numerous accomplishments encountered on the project. But most importantly, the Albany Park Stormwater Diversion Tunnel Project helped improve the overall quality of life for Albany Park residents. The project’s overall success has been recognized for this accomplishment with the 2019 Engineering Excellence Award by The American Council of Engineering Companies, Illinois Chapter and a 2019 National Achievement Award from the Construction Management Association of America.
The authors would like to thank CDOT and Kenny Construction for their continued support and coordination during the project. Special thanks to Dan Burke, Vasile Jurca, Anne Zhang, Conan Chan, JJ Madia and Luis Benitez from CDOT; James Nickerson, Bob Rautenberg, Darrell Vliegenthart, Clay Spellman and Marc Potter from Kenny Construction; and Frank Jaramilla, Michaelangelo Hernandez and Gary Wingfield from the WSP Construction Management Team for without their hard work and dedication to this project, it would not have been the major success story it became.
Hannes, L.E., Sargent, M.L., and Kolata, D.R. 2004. Geologic Atlas of Cook County for Planning Purposes, Illinois State Geological Survey (ISGS). Champaign, IL.
Hazzard, J.F., R.P. Young, and S.C. Maxwell. 2000. Micromechanical modeling of cracking and failure in brittle rocks. J. Geophys. Res. 105: 16,683–16,697.
Mikulic, D.G., J. Kluessendorf and R. D. Norby. 2010. Silurian System and Lower Devonian Series. Geology of Illinois, ed. D.R. Kolata, 158-162.
MWH Americas, Inc. 2015. Geotechnical Baseline Report for Construction of the Albany Park Stormwater Diversion Tunnel. Chicago, IL.
MWH Americas, Inc. 2015. Geotechnical Data Report for Construction of the Albany Park Stormwater Diversion Tunnel. Chicago, IL.
Willman, H.B. 1973. Rock Stratigraphy of the Silurian System in Northeastern and Northwestern Illinois. Illinois State Geological Survey Circular 479. Urbana, IL.