Project OverviewProject History

Project Overview

Light Rail at MSP Today

The Minneapolis-St. Paul Metropolitan Airports Commission (MAC) has been implementing a $3 billion Minneapolis-St. Paul International Airport (MSP) airportexpansion plan at the nation's eighth busiest airport since 1996.

In the late 1990s, the first LRT system to be developed in the state of Minnesota was being planned by the Minnesota Department of Transportation (Mn/DOT). Its completion links three of the region’s most popular destinations: Downtown Minneapolis, MSP and the Mall of America, with high-quality and efficient LRT service. However, to make the crucial link to the airport MAC had to look underground for solutions.

The MSP LRT Tunnel and Lindbergh Station project consists of approximately 7,400 feet of twin bored tunnel boring machine (TBM) single-pass mined construction and mined cavern LRT station construction and 920 feet of cut-and-cover tunnel sections at the north and south tunnel approaches. The alignment begins to the north of MSP at the FortSnelling Military Reservation, where the LRT tracks run underground, passing under the North Parallel Runway (Runway 12L-30R), Concourse C, and the Lindbergh Terminal's inbound roadway, and connects to the LRT station beneath the parking toll plaza. The system continues underground from the underground station, passing under the Lindbergh Terminal'soutbound roadway, Concourse G, and South Parallel Runway (Runway 12R-30L), and comes up to street level adjacent to 34th Street and the new Hubert H. Humphrey Terminal.

The LRT platform at the Lindbergh Station is approximately 65 feet below the level of the below grade Transit Center. A series of escalators, stairsand elevators are used to move passengers up from the LRT platform to the automatic people mover Transit Center levels and the concourse connector. Ancillary spaces for mechanical and electrical operation of the LRT system are located at the north and south ends of the platform.

The opening of the Lindberg Station today connects directly to the Hiawatha LRT line providing high quality transportation access to the airport and key Twin Cities economic centers. As Minnesota's gateway to the world, the success of the Hiawatha LRT line is enhancedby the MSP LRT tunnel and Lindbergh Station project, and the seamless collaboration between MAC, the MinnesotaDepartment of Transportation, MetCouncil, Metro Transit and the engineering and design consultants responsiblefor its successful delivery.

Fast FactsThe HNTB Design Team Tunnel Characteristics:

Completion Date:

TUNNELS FACTS

+ Two side-by-side tunnels, each 1.8 miles long (longest! American Engineering and Testingin Minnesota)(Geotechnical Exploration)

+ Bored tunnel 1.4 miles long! Charles Nelson and Associates

+ 920 feet of cut-and-cover sections at the north(Geotechnical Engineering) and south approaches

+ Tunnel 65-70 feet below surface! Hatch Mott MacDonald+ Herrenknecht 250-foot-long, 500 ton Earth Pressure (Bored Tunnels and Tunnel Ventilation)

Balanced Tunnel Boring Machine utilizing a pre-cast ! Hammel Green & Abrahamson (Architecture) bolted lining, with a 21-foot bore diameter

+ Each push by the borer excavates about 100 tons of earth to ! Liesch and Associates (Environmental Compliance)advance 5 feet. The Tunnel Boring Machine (TBM) advances

! LTK Engineering Services (Light Rail Transit Systems)on average 80-100 feet per day.

+ Tunnel precast lining system begins manufacturing. From September 2001 through September 2002, more than 21,000 precast tunnel segments are manufactured. More than 873,900

+ Tunnels: August 2004 square feet of precast concrete line the twin tunnels. + Prominently featured in major trade publications, including + Light rail transit stations: December 2004

Engineering News-Record, Tunnel Business Magazineand Tunnels and Tunneling.

+ Innovative light rail tunnel precast tunnel lining systems wins the coveted Henry H. Edwards Industry Advancement Award Hole-Through Dates:by the Precast/Prestressed Concrete Institute (HNTB in

+ Southbound Tunnel: April 25, 2002 conjunction with County Materials Corporation)+ Each tunnel is capable of supporting future wide-bodied + Northbound Tunnel: October 29, 2002

aircraft, weighing up to 650 tons

2001 2002

Minneapolis-St. Paul International Airport Light Rail Tunnel & Station Project Timeline2003 2004

August 21, 2001 Tunnel boring machine makestrip from Duluth to the Minneapolis-St. PaulInternational Airport.

September 2001Tunnel precast lining system begins manufacturing. FromSeptember 2001 through September 2002, more than 21,000 precast tunnel segments are manufactured.

November2001First bore on Southbound tunnel begins.

June 2002 TBM moved from one site to opposite to start the Northbound tunnel.

April 2002 Southbound tunnel TBM hole-through(4/29/02)

July 2002First bore on the Northbound tunnel begins.

August 2002 Cover story of national tunneling trade publication Tunnel Business Magazine.

October 2002 Northbound tunnel TBM hole-through(10/29/02),signifying the completion of the tunneling process. More than 873,900 square feet of precast concrete line the twin tunnels.

November2002TBMdisassembled,prepared for shippingoverseas.

December 2002 Crossover connection between the Lindbergh Station and tunnels is made.

November2002Cover story of nationalengineeringandconstructiontradepublicationEngineeringNews-Record.

December 4, 2004Grand opening! Last segment of the Hiawatha light rail transit line is officially open.

November 2004Signage and wayfinding system installed, final station clean-up complete.

July 2004Final electrical installed, lighting complete.

April 2004Decorative ceiling panels installed. Designed tomirror the shape of an airplane wing.

August 17, 2001 Tunnel boring machine (TBM) arrives at port in Duluth from Holland.

October 2003Innovative light railtunnel precast tunnel lining systems wins thecoveted Henry H. Edwards Industry AdvancementAward by the Precast/PrestressedConcrete Institute (HNTB in conjunction with County Materials Corporation)

January 2003Precast LindberghStation wall placement begins.

September2003Stationplatformbeginsconstruction.

October 2003Rail connection to the Hiawatha line begins at the airport. February

2004Station rail connection to the Lindbergh Station is made.

Fact Sheet

Category and codeCustom Solutions — ZZ

Primary use of structureLight rail transit passage

Number and dimensions of pre-cast concrete componentsPrecast tunnel lining system:

Southbound tunnel: 1,474 rings (10,318 segments)

Northbound tunnel: 1,480 rings (10,360 segments)

Total: 2,954 rings (20,678 segments)

Approximate dimensions of rings: u Outside diameter of completed ring is 20'-6"

u Inside diameter of completed ring is 18'-10"

u Average width of the segment is 5'-0"

u Thickness of the segment is 10"

One ring is composed of: u Four standard segments each being 60° of total 360° ring

u One counter key segment that is 60° of total 360° ring

u One counter key segment that is 45° of total 360° ring

u One key segment that is 15° of total 360° ring

Multiple rings form the desired tunnellength using types of rings:u Type RS or Right Straight

u Type RT or Right Taper

u Type LS or Left Straight

u Type LT or Left Taper

Size and total square footageSouthbound tunnel: 7,370 lf (436,060 sf)

Northbound tunnel: 7,400 lf (437,840 sf)

Total: 14,770 lf (873,900 sf)

Structural system usedPrecast concrete tunnel lining system

Detailed project cost dataPrecast concrete tunnel lining system: $19,523,880

Total project cost: $109,890,000

Project timeline from design tooccupancyDesign: Fall 1999 to Summer 2000

Contract Award/NTP: January 19, 2001

Completion of TBM tunnels with precast concrete tunnel liningsystem: October 2002

Fact SheetPrecast/Prestressed Concrete Institute Award

1 INTRODUCTION

MSP is the nation's eighth busiest airport pernumber of passengers. The Minneapolis-St.Paul Metropolitan Airports Commission(MAC) owns and operates MSP and sixreliever airports throughout the Twin Citiesmetropolitan area. The MSP facilityaccommodates more than 30 millionpassengers annually as a major hub for theairport's largest tenant, Northwest Airlines.

Since 1996, The MAC has beenimplementing a $3.5 billion MSP 2010airport expansion plan. This expansionproject includes:

• An expanded parking facility and newinbound roadways at the airport'sLindbergh Terminal

• A new transit center, providing access tomass transit buses, proposed light railand shuttles to off-airport rental facilities

• A new airport runway and reconfiguredtaxiways

• A new, ten gate Hubert H. Humphrey(HHH) Terminal

• A new Automated People Mover (APM)system at the Lindbergh Terminalbetween the transit center, parkingfacility and the terminal, and a separateAPM linking the main terminal with thenew concourse and regional terminal

Tunneling Under an Airport – The MSP Light Rail TransitTunnel and Station Project

Brian T. Hamilton, P.G.MSP LRT Deputy Program Manager and Project Office EngineerHNTB Corporation, Minneapolis, Minnesota, USA

ABSTRACT: A new light rail transit (LRT) tunnel and station is being constructed under theMinneapolis-St. Paul International Airport's (MSP) main terminal and two main parallel run-ways. Constructing a LRT tunnel under the heart of one of the nation’s busiest airports presentednew dimensions to project risk beyond controlling project cost and achieving constructionschedule goals. With the economic consequences of any disruption to airport operations beingunthinkable, ground disturbances and movements below runways and disruption of the airportslandside and airside operations needed to be minimized.

In this paper, the design and the construction management teams present a discussion of the de-sign methodologies and risk management strategies that were used to control costs, minimizeproject delays and minimize impacts to airport operations at the surface. This paper will be ofinterest to owners and engineers planning underground projects, especially those that presentunique risks during construction.

• A skyway connector linking concoursesC and G, parking, auto rental and thetransit center

In the late 1990s, the first LRT system to bedeveloped in the State of Minnesota – theHiawatha Transit Line – was being plannedby the Minnesota Department ofTransportation (Mn/DOT). Uponcompletion, the 11.6-mile light rail linewould link three of the region's most populardestinations, Downtown Minneapolis'Nicollet Shopping Mall, MSP, and the Mallof America. Revenue service fromDowntown Minneapolis' Nicollet Mall toFort Snelling would begin in late 2003 andservice to the airport and the Mall ofAmerica would follow by December 2004.In 2002 dollars, the overall cost of theHiawatha Transit Line is $675.4 million.

2 PRELIMINARY ENGINEERING

The airport portion of the Hiawatha TransitLine includes a tunnel underneath the heartof MSP, as shown on Figure 1. In thepreliminary engineering stage, the tunnel ranunderground starting at the north side of theairport, from the Fort Snelling site inMinnesota Air National Guard property,under Runway 12L-30R, under theLindbergh Terminal's parking revenue plazaadjacent to a proposed transit center, underRunway 12R-30L, daylighting along East70th Street, south of Signature FlightSupport. LRT trains would stop at twostations on the airport campus. One located70 feet below the Lindbergh Terminal andanother at the airport's south side near a newHHH Terminal. When completed, LRTwould serve as the primary transportationlink between the two airport terminals(Minnesota Department of Transportation,1999).

Figure 1

At this stage, the entire line being proposedwas to be constructed using a design/buildcontract model.

2.1 Design and Construction Planning atthe Airport

In the fall of 1998, the MAC formed apreliminary design and engineering team toinvestigate the proposed Hiawatha LRTTunnel and Stations at MSP. The team, ledby HNTB Corporation, includes HammelGreen and Abrahamson, Inc. (architects ofthe Lindbergh Station), CNA ConsultingEngineers (mining and geotechnicalengineering), and Hatch Mott MacDonald(tunneling and ventilation). The MAC teaminvestigated the design requirements for anunderground Lindbergh Terminal stationthat would connect to the APM level andTransit Center at the east end of the newparking decks. The airport's initial interestwas to preserve ground surface areas andidentify zones for cut-and-coverconstruction for future LRT ventilation andexit shafts, as well as a future stair andescalator connection for public access fromthe station platform to the APM and TransitCenter levels above.

Based on the team's preliminary finding, theMAC created “placeholders” for the shaftsand a future Transit Center connection. Ar-eas were identified and reserved at groundlevel around the Lindbergh Terminal'sRevenue Plaza, Transit Center and the out-bound roadways to be maintained “utilityfree” to preserve space for future shaft andcut and cover construction. The MAC teamalso organized a series of preliminary coor-dination meetings with the Mn/DOT designteam and the Hiawatha Project Office(HPO), to discuss tunneling techniques, con-struction phasing and access restriction onMAC property. These discussions identifieda number of conceptual issues of specialconcern to the MAC. Those issues included:

• MAC Lindbergh Terminal Station’sidentity and finish quality

• Limited ground level spaces for cut andcover construction along East 70 Streetand the Lindbergh Terminal

• Limited construction access and phasingopportunities due to adjacent MAC air-port development projects and roadwaycompletion schedules

• Concerns for utility relocation costs andrestricted access on the East 70th StreetLRT right-of-way due to concerns overimpacts to airport tenants and FAA Part77 airfield safety clearance restrictions

• Restricted automobile access posed byat-grade LRT crossings surrounding thenew HHH Terminal

• Construction risks of disruption to air-port operations from geologic featuresbelow the airport's South Parallel Run-way (Runway 12R-30L)

These issues were discussed in detail be-tween the MAC and the HPO throughoutspring and summer of 1999. In the latesummer of 1999, the MAC authorized theformal creation of a MAC design team.

The MAC design team prepared a cost up-date and review of the HPO’s preliminaryengineering documents. The team also heldseries of bi-weekly work sessions starting inthe fall to explore alternative design and en-gineering solutions that addressed MAC’sconcerns. Two alternative delivery methodswere examined and compared for theMAC’s evaluation. Those included a de-sign/build scenario where the MAC designteam would prepare design/build documentsto be incorporated in the overall HPO de-sign/build project. A second alternativelooked at a more traditional design/bid/buildscenario where complete documents wereprepared and bid separately by the MAC.The options were compared in detail forschedule implication, risk assessment, costeffectiveness, and MAC design control.

The design team worked to further definethe MAC’s design expectations and criteriafor the LRT project. These included:

• Near zero subsidence under active taxi-ways and runways

• Restricted site and construction access atthe Lindbergh Terminal area, due to sev-eral years of ongoing construction andrecently completed landside projects

• Roadway and access complications atthe HHH Terminal on East 70th Streetand 34th Avenue

The underlying goal of the MAC was toproduce a cost-effective LRT route and sta-tions through the MAC property to serve thetravelling public in a safe, high quality envi-ronment corresponding with other MACterminal facilities. The MAC's senior staffalso outlined a number of specific goals in-cluding:

• Developing a cost-effective design andengineering solution that is consistentwith the funds allocated by HPO for theproject

• Working collaboratively and construc-tively with the HPO team to develop asolution that best serves the needs of thetravelling public and respects the MAC’soperational and safety standards

• Creating a signature station at the Lind-bergh Terminal with an identity andquality level that is appropriate to andcommensurate with adjacent MAC ter-minal facilities

• Minimizing airside and landside disrup-tion to the MAC during the LRT tunneland station construction

• Creating performance and engineeringstandards and documents that guaranteeto minimize disruption to MAC airsideoperations that could result from groundsubsidence due to underground tunneling

• Identifying early construction needs forsurface shafts in order to minimize fu-ture disruption or reconstruction of re-cently completed projects

• Identifying clear, obvious circulationpaths that enhances traveler wayfindingbetween the proposed LRT station andother terminal facilities

• Minimizing long-term impacts to 34thAvenue South and East 70th Street

In early winter of 1999, MAC authorized thedesign team to proceed with the de-sign/bid/build delivery model. Bids wereopened for the MSP LRT Tunnel and Sta-tion project just 10 months later in August2000. Excavation of the Hiawatha LRTTunnel began in October 2001 by a jointventure between Obayashi Corporation andJohnson Brothers Corporation. Constructionof the rail line and the Lindbergh and HHHStations is expected to continue until the fallof 2004.

3 PROJECT DESCRIPTION

In its final design format, the MSP LRTTunnel and Station project consists of ap-proximately 8,100 feet of cut-and-cover,twin TBM or single mined tunnel construc-tion with a single mined station shown onFigure 2. The project consists of approxi-mately 7,400 feet of twin TBM or singlemined tunnel construction and mined stationconstruction. The alignment begins to thenorth of the MSP at the Fort Snelling Mili-tary Reservation, where the LRT tracks rununderground, passing under the North Par-allel Runway (Runway 12L-30R), Con-course C, and the Lindbergh Terminal’s in-bound roadway, where it connects to theLRT station beneath the parking toll plaza.The system continues underground from theunderground station, passing under theLindbergh Terminal’s outbound roadway,Concourse G, South Parallel Runway (Run-

way 12R-30L) and comes up to street leveladjacent to 34th Street and the new HHHTerminal. Associated open cut and cut-and-cover construction will be completed at ei-ther end of the tunnel.

The LRT platform at the Lindbergh Stationwill be a below grade facility, approximately65 feet below the level of the below gradeTransit Center. A series of escalators, stairs,and elevators will be used to move passen-gers up from the LRT platform to the APM,Transit Center levels and the concourse con-nector. Ancillary spaces for mechanical andelectrical operation of the LRT system aredistributed at the two ends of the platform.

The LRT platform at the HHH Terminal willbe an open at-grade facility. The LRT routenear the terminal will be associated with theplanned parking facility serving the termi-nal.

The project includes construction of a twinbarrel tunnel, sections of cut-and-cover tun-nel construction, open cut boat sections, andan underground cavern rapid transit station(Lindbergh Station) on or immediately adja-cent to MSP property. Construction of thetunnel shall be by one of the two methods ofconstruction as identified in the contractdocuments. Alternative 1 is construction ofthe tunnels using only a tunnel boring ma-chine the entire length of the tunnels andAlternative 2 is the construction of the tun-nels south of Lindbergh Station using a tun-nel boring machine and mining the tunnel

segment north of Lindbergh Station usingconventional mining equipment.

3.1 Alignment Selection Process

The horizontal and vertical geometry of thetunnel is based upon the Hiawatha CorridorLight Rail Transit Design Criteria preparedby the HPO (Minnesota Department ofTransportation, 1999). The track plan andprofile from the preliminary engineeringsubmittal formed the basis for further studyand refinement as design controls becamemore defined. Numerous alignment alterna-tives were developed with the MAC's con-cerns in mind. Those alignment alternativeswere developed subject to certain horizontaland vertical controls and design criteria.Cost comparisons were also prepared for thevarious alignment alternatives and associ-ated construction configurations. Eachalignment was ranked according to potentialimpact on airport operations, constructabil-ity concerns and estimated construction cost.

3.2 Horizontal Alignment

The horizontal controls included: the pro-posed HHH Terminal buildings, the locationand orientation of the HHH Terminal Sta-tion, the configuration of access and egressroadways at the HHH Terminal, the locationof a proposed Signature Flight Supportbuilding just to the north of an existing han-gar on the northeast corner of the intersec-tion of East 70th Street and 34th AvenueSouth, the location of the buried alluvialvalley, the location and orientation of Lind-bergh Station, and tunnel construction meth-ods.

The MAC design team considered the geol-ogy at the airport to be one of the most im-portant controls on the horizontal alignment.The geology beneath the airport campusconsists of a layer of limestone, over a thinshale layer over a deep layer of sandstone.Approximately halfway between 34th Ave-

Figure 2

APM platform Skyway/Concourseconnector

LRT station

nue South and the Fort Snelling MilitaryReservation, alluvial deposits break thelimestone formation. The width of thisbreak increases from north to south. Theteam concluded that the length of the tunnelthrough the alluvial valley should be kept toa minimum. The tunnel alignment wasmoved to the north to minimize the tunnelboring within the buried alluvial valley.This plan would also have the advantage ofeliminating the tunnel boring directly underthe south parallel runway.

The preliminary design submission alsoproposed that the transit line share the ex-isting corridor between East 70th Street andPost Road – a distance of approximately ahalf-mile. This work would require thecomplete reconstruction of East 70th Street.The resulting street layout would provideone lane in each direction and could not bewidened in the future without the removal oftwo buildings occupied by Signature FlightSupport. The MAC design team also identi-fied construction phase constraints related toconstruction phase violations of FAA Part77 airfield clearance restrictions at the proj-ect's southern portal and impacts on the FortSnelling National Cemetery just to thesouth. After reviewing the airfield clearancerestrictions, the MAC team decided to limitopen excavation to outside of these clear-ance limits. Other concerns were identifiedinvolving disruptions to Signature FlightsSupport’s operations during construction.

The track alignment that eliminated any re-construction on East 70th Street was pro-posed that would allow funds designated forthe roadway reconstruction to be spentsolely on the transit line. This plan wouldalso provide schedule benefits and eliminatedetours and other disruptions to normal traf-fic flow. As a result of this decision, thealignment was moved off East 70th Street tothe airfield to the north. The southern portalwas also moved to the west to remove FAAPart 77 airfield clearance violations thatwould have required the closure of a majortaxiway for the duration of construction ac-tivities. The tunnel would surface just to thewest of 34th Avenue South.

3.3 Portal Locations

The preferred portal locations were subjectto FAA airfield clearance restrictions andairfield development plans and constructa-bility concerns. The south portal was lo-cated in a manner that would minimize dis-ruption of traffic on 34th Avenue South andEast 70th Street during tunnel construction.The north portal was located at a pointwhere cut-and-cover tunnel construction canbe completed without violating the safetyarea as defined by the FAA.

The MAC wanted to preserve the limestonecap with the FAA airfield safety areas of therunways and associated taxiways. These re-quirements would not allow excavation oflimestone within 250-feet of the North Par-allel Runway and 507 feet south of theSouth Parallel Runway. Preservation of thelimestone cap is required due to live loadconsiderations. The construction must resultin a surface capable of supporting Class Vaircraft (Boeing 747 or equivalent size)within the safety areas.

The FAA Part 77 airfield clearance restric-tions are intended to ensure an obstacle freevolume of space surrounding the runwaysand taxiways. As such, the portal locationsmust respect these restrictions. The con-struction contractor would not be able to useequipment such as cranes without restric-tions within these limits. In most cases, anyconstruction within the Runway Safety Area(RSA) would require a closure of the run-way or taxiway, and relaxation of the RSAby the FAA was not possible.

These criteria suggests that the interface ofthe cut-and-cover tunnel and themined/bored tunnel at the northern end ofthe alignment would need to be constructedno closer to the north parallel runway than250-feet plus some buffer zone to allow forconstruction operations. At the south side,the cut-and-cover to retained cut interfacewould need to be no closer than 560 feetfrom the centerline of the South ParallelRunway (Runway 12R-30L).

3.4 Vertical Alignment

The vertical alignment is mainly a functionof the required at-grade elevations, the un-derground elevation required at LindberghStation, the water table, the extent of thelimestone formation near 34th Street Southand the location of Runway 12L-30R at thenorth side of the airport.

The vertical controls included: the elevationat the HHH Terminal Station, the elevationof 34th Avenue South, the horizontal extentand elevation at the bottom of the limestoneformation just east of 34th Avenue South,the water table, the elevation at the northernlimit of the airport property, FAA Part 77airfield clearance restrictions, the tunnelconstruction method, and the creation of alow point in the tunnel just south of theLindbergh Station platform – was consid-ered the optimal location to collect and re-move track bed drainage.

4 GEOLOGICAL CONDITIONS

The majority of the tunnel will be con-structed in the St. Peter sandstone below thePlatteville limestone. However, the align-ment crosses glacial soil filled buried valleysin three locations.

The Twin Cities metropolitan area, includ-ing the area of direct interest to the LRTproject, is underlain by nearly 1,000 feet of

sedimentary rocks of early Paleozoic age.These gently dipping to near-horizontalrocks form the Twin Cities structural andhydrologic basin. A mantle of glacial andpost-glacial deposits covers the area. Ingeneral, the overburden materials along theLRT alignment consist of fills, alluvial, ortill materials.

The stratigraphic section underlying theproject area consists of approximately 1,000feet of sandstone, shale, and carbonate rocksof Cambrian and Ordovician age. Theyoungest formation is the Platteville forma-tion, an approximately 30-foot thick forma-tion, which is divided into several dolomiticlimestone and dolomite members. At thebase of the Platteville limestone is the thin(2-feet to 5-feet thick) Glenwood shale thatoverlies the St. Peter sandstone. The St.Peter is a 150-foot thick massive sandstoneunit composed of fine-grained to medium-grained, well-rounded and uniformly gradedquartz sand. The majority of the LRT tunnelwill be constructed in the St. Peter sandstone(Minnesota Geological Survey, 1972).

In the Platteville limestone, steep to verticaljoints is common at a typical spacing of 20feet to 40 feet. Many are tight, allowing lit-tle water movement, whereas others transmitwater readily. Open bedding planes in thelimestone are water bearing. The uncon-fined compressive strength of unweatheredrock in the Platteville formation typicallyranges from 9,000 psi to 35,000 psi (Metro-politan Airports Commission, 2000).

flake off when exposed by excavation. Itdoes form a relatively impervious layer sothat water in the overlying limestone iscommonly perched above it. The uncon-fined compressive strength of Glenwoodshale has been found to be as high as 7,200psi in samples taken for the construction ofthe Minneapolis East Interceptor in Minnea-polis (Metropolitan Waste Control Commis-sion, 1985). However, considerably weakershale occurs in layers in the formation.

The St. Peter sandstone is low-strengthquartz sandstone. Most of the formation is

Glacial drift

Platteville limestone

Glenwood shale (2-3 ft.)

St. Peter sandstone > 60 ft.

uniform, white, friable, and contains morethan 98 percent silica. Closely spaced weakbedding and cross bedding planes can resultin very weak sandstone. Irregularly spaced,steeply dipping (more than 70 degrees)joints are present in the sandstone. Watermoving along a joint can erode the friablesandstone, and, where the loosened sand canmigrate, voids may develop. Voids havebeen known to form near or next to linedtunnels and within hundreds of feet of riverbluffs and buried valleys. The unconfinedcompressive strength of the St. Peter sand-stone typically ranges from 0 psi to 500 psi.However, nodules or concretions have beenfound with compressive strengths as high as14,600 psi (Metropolitan Waste ControlCommission, 1985). These nodules arescattered, but can create problems duringhydraulic mining and hand excavation andcan damage cutter teeth on tunnel excava-tion machines and road headers. Nodulesizes range from a few cubic inches to sev-eral cubic feet.

As shown in Figure 3, the tunnel alignmentlies in buried valleys in three areas. In theseareas, glacial soils now exist in areas previ-ously occupied by bedrock.

5 CONSTRUCTION CONSIDERATIONS

5.1 Geological Impacts on TunnelingMethods

The MSP LRT project has a unique geologicsetting in that the ground conditions alongthe tunnel length comprise a limestone cap,underlain by soft sandstone with unusualproperties above the water table, both inter-sected by buried valley containing glacialdeposits below the water table. Above theburied valley lie taxiways, utilities andstructures for MSP, which require protectionduring tunneling works, to avoid disruptionto airport operations due to ground move-ments. Traditionally, in the Minneapolisarea, the sandstone would be excavated eas-ily beneath the limestone cap with a minimalneed for support and final lining. However,the presence of the buried valley and theneed to protect the operations of the airportdictate that special measures be applied tosuccessfully construct the tunnel in that sec-tion.

There are various construction methodsavailable to achieve the MAC's objectives.The option of using a closed face TBM withgasketed segmental lining with the TBMdriven in EPB mode was selected by the de-sign team. This method was chosen becauseit provides continuous face support andground support, hence lower risk. A closedface TBM more easily copes with boulders.The EPB option was chosen despite a higherinitial cost, skilled workforce requirementsand intensive mechanical/electrical supportrequirements.

In general, the tunneling methods that didnot provide continuous face support in com-bination with a watertight lining, were con-sidered higher risk in terms of potential dis-ruption to the airport operations. Many ofthe construction methods do not provide ef-fective control over ground movements.The selection of a closed face TBM utilizing

Figure 3

Buried valley

a gasketed segmental lining offers the bestcombination of lower risk and quality con-struction required by the MAC design ob-jectives.

5.2 Tunnel Construction Alternatives

Construction of the tunnel will be accom-plished by one of the two methods. Alter-native 1 is construction of the tunnels usingonly a tunnel boring machine the entirelength of the tunnels and Alternative 2 is theconstruction of the tunnels south of Lind-bergh Station using a tunnel boring machineand mining the tunnel segment north ofLindbergh Station using conventional min-ing equipment. The prospective bidders hadto identify their choice at the time of bid-ding.

The mined tunnel option consists of a singletunnel bore containing both the northboundand southbound tracks. This option occursbetween the north end of the Lindbergh Sta-tion and the start of the North Portal cut-andcover-tunnel. Similar construction methodshave been used in the Twin Cities for under-ground space development and sanitarysewer meter chambers.

The mined tunnel option requires excavationof greater sandstone volumes than the TBMoption. This option is economically viablebecause of the low unit excavation costsachievable with large excavation equipment.A contractor that chooses the mined tunneloption will use large loaders and backhoesfor excavation, trucks for haulage, and alarge backhoe mounted roadheader for lime-stone roof and sandstone wall trimming.Much of the tunnel may be excavated in onepass. Near the station, where the profile isdeeper, the contractor may need to excavateusing a top heading, followed a short dis-tance back by bench excavation. Wall panelinstallation will be done later with minimumimpact on more critical activities.

5.3 Lindbergh Station

The MAC's primary objectives for the de-velopment of the Lindbergh Terminal Sta-tion was to create a signature station with anidentity and quality that is commensuratewith adjacent MAC terminal facilities. Theywanted to identify clear and obvious circu-lation paths that enhances traveler wayfind-ing between the proposed LRT station andother terminal facilities. The LindberghTerminal construction would need to be ac-complished in a manner that minimizes dis-ruption to the airport operations during theconstruction phase.

Areas around the Lindbergh Terminal hadbeen under construction continuously sincethe MSP 2010 airport development programstarted in 1996. Most of this constructionwork was due to be completed about thesame time that the LRT construction wouldbe starting. The MAC team decided that itwas not desirable to disturb these new fa-cilities just as their construction was beingcompleted or to subject the traveling publicto several additional years of construction.Mn/DOT's concept would have requiredextensive cut and cover construction opera-tions with the transit plaza area. The MAC'sconcept of a mined station would minimizethe cut-and-cover operations. A mined sta-tion would mitigate concerns that relate toeconomics, utility relocation, space alloca-tion and schedule.

Due to the schedule constrains, the MACdecided to move forward with the design

Future Lindbergh LRT Station

and construction of the portion of the con-nection structure required to be constructedwith cut-and-cover construction methods.This construction would be accomplishedcoincident with the Transit Center construc-tion in the spring and summer of 2000.

The MAC team also modified the LindberghStation design from a binocular station con-figuration with a 300-foot-long platform to amonocular station with a platform length of280 feet. A monocular station arrangementwould further achieve the MAC's objectiveof creating a signature architectural station,while a shorter platform length would enablethe project to reap a substantial cost savings.A 280-foot platform would still be able toaccommodate three car trains. The mo-nocular arrangement also resolved space al-location concerns over the space dedicatedfor mechanical/electrical equipment. Theplatform length would still be able accom-modate three 94-foot-long rail cars.

The Lindbergh Station will be constructedwithin a mined cavern in sandstone below alayer of limestone below the airport parkingtoll plaza. The 60- by 30- by 500-foot-longcavern will be lined with precast wall andceiling panels. The middle 280 feet of thecavern will be a two-story high public plat-form area. Each end of the cavern will in-clude two levels of ancillary spaces for me-chanical and electrical systems. The cavernfloor will consist of slabs-on-grade for apassenger platform and track support struc-ture. A signal and communications vaultand a sump pump room are located belowthe platform level slab. Airshaft structureswith emergency exit stairs extend verticallyfrom each of the ancillary spaces to the sur-face. Within the platform area and betweentracks, escalators, an elevator and stairs con-nect the platform with a mezzanine level.The mezzanine spans the tracks to connectto an upper level tunnel leading to the base-ment of the Transit Center building. Theconnection to the Transit Center buildingwill be constructed by excavating from thesurface and covering the excavation with astructure to support the grade level.

5.4 Settlement Control

Given the economic consequences of anydisruption to airport operations, ground set-tlement controls and mitigation demandsspecial attention. To minimize settlementrisk on airside operations, candidate tunnelalignments under consideration that pre-served limestone cap within the safety areaof the runways and their associated taxiwayswere favored. Minimum turning radii alongtunnel alignments were maintained at no lessthan 1,000 feet.

The MAC team specified the continuous useof pressure balanced TBM buried valley ar-eas for the project. The correct use of aclosed face TBM would provide an effectivemeans of controlling the short-term immedi-ate settlement due to tunneling. The contin-ual operation of the TBM in closed modemaintaining enough face pressure, to mini-mize ground movements and water inflowsat the tunnel face was considered importantto settlement control.

Procedures are being utilized to monitorground movements, interpret the results, andidentify trends in a prompt manner as theTBM progresses under key areas. This en-ables adjustments to be made to the operat-ing parameters of the TBM or any plannedmitigation measures to be implementedearly, before potential problems and seriousimplications develop. Technical specifica-tions also established trigger values and set-tlement limits for the various structures un-der which the tunnel pass.

Southbound Tunnel Moveable Work Zoneover buried valley

In the buried areas where the TBM wouldprogress under operating area within the air-ports, like aprons or taxiways, a plan wasdeveloped to keep aircraft out of the area. Insome cases, aircraft gates would need beclosed to aircraft. Impact to airport opera-tions is being kept to a minimum through theuse of moving work zones where the areaclosed progresses along with the mining op-erations.

6 VALUE ENGINEERING AND RISKEVALUATION

The move to make the tunnel constructionunder airport control was despite theHiawatha Light Rail Transit's ambitious costand schedule objectives. A difficult politicalsituation in Minnesota led to the promisethat the Hiawatha LRT system would haveto be constructed within budget and thesystem would need to be operational by theend of December 2004. Hiawatha LRT offi-cials set the maximum budget of the MSPLRT Tunnel and Station early in this pre-liminary design stage to $117 million dollarsand set the award date / notice-to-proceeddate of the contract to September 18, 2000.That meant that the bid opening for a tunnelcontract would need to be no later thanAugust 2000.

In this working environment, every designdecision was made with cost, schedule andrisk impacts in mind. Value engineeringstudies and cost comparisons of alternativeswere made at each stage of the project.These studies included reviews of alignmentalternatives, evaluating underground spaceneeds and civil design concepts. Verticalalignment alternatives compared varyinggrades (4 or 5 percent grades) were used. Areview of horizontal alignment alternativesidentified a need to move the alignment offEast 70th Street to save the costs of recon-structing the street. Another study identifiedan opportunity to reduce the length of theLindbergh Station by 20 feet.

Risk evaluations identified a need to move aportion of the horizontal alignment to thenorth to keep the tunneling activities withina buried alluvial valley away from under-neath runways and also led to the selectionof the specified tunneling methods and set-tlement reduction measures.

7 CONCLUSION

The Hiawatha Transit Line is the first LRTsystem to be developed in the State ofMinnesota. The 11.6-mile light rail linewould link three of the regions most populardestinations, Downtown Minneapolis'sNicollet Shopping Mall, the MSP, and theMall of America. An integral portion of thiseffort is the construction of a 1.8-mile tunnelunderneath the MSP, the nation's eighthbusiest airport, and adjacent to an ambitious$3.5 billion dollar airport expansion project.

The project is located in a crowded,unforgiving airport environment. Theconstruction is being subjected tochallenging ground conditions, stringentperformance criteria, a tight constructionschedule and budget, and close publicscrutiny. This project is requiring aconcentrated effort from the MAC the HPO,the consultants and responsible contractorsto ensure the successful completion of thework and its completion by the mandateddeadlines and funding limits.

REFERENCES

• Minnesota Department of Transporta-tion. Hiawatha Corridor Light Rail Tran-sit Preliminary Engineering Submittal.1999.

• Minnesota Airports Commission, MSPLRT Tunnel and Station Project Geo-technical Design Report (prepared byCNA Consulting Engineers). 2000.

• Metropolitan Waste Control Commis-sion. Contract Documents for the Min-neapolis East Interceptor, Volumes 4aand 4b - Geotechnical Report. 1985

• Minnesota Geological Survey, P.K. Simsand G. B. Morey, eds. Geology of Min-nesota: A Centennial Volume. 1972

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DNR Asset Preservation$4 Million Bonding RequestInfrastructure: Roads & Sewer

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Upper MississippiAcademy Building

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Education Center

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Fort Snelling Upper Post

Hennepin County Public Works

Ramsey

Dakota

BLOOMINGTON

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RICHFIELD MSP INTL.AIRPORT

FT.SNELLING

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Produced by Hennepin County Housing, Community Works & Transit

E x i s t i n g c o n d i t i o n s a n d f u t u r e o p p o r t u n i t i e s

Fort Snelling Upper Post

Publication date: 5/6/2014

This map has been created for informational purposes only and is not considered a legallyrecorded map or document. Hennepin County makes no warranty, representation, orguarantee as to the content, accuracy, timeliness, or completeness of any of the informationprovided herein.

About this map

National historic designation combined with its location within unincorporated Hennepin County and complexjurisdictional authorities have hampered redevelopment opportunities for Fort Snelling Upper Post. Thenearby Fort Snelling LRT station has great potential to support current recreational and future housing,educational and redevelopment uses. Hennepin County and the National Parks Service, Veterans Affairs, MNDept. of Natural Resources, MN Historical Society, and Minneapolis Park and Recreation Board in 2012 formedthe Joint Powers Agreement that clarifies the roles and responsibilities of the partners and lays the groundworkfor future investment. This map captures the wide range of activities that already animate the Fort SnellingUpper Post area and indicates some of the redevelopment opportunities just over the horizon.

Building 55, 65, 67State Bonding STSHomes Stabilization2008 $500,0002010 $1.2 Million

Upper Mississippi DevelopmentMortenson Development and ConstructionUpper Mississippi Academy Charter School

Bishop Henry WhippleGSA Building Restoration

Veterans AffairsHomeless Veterans Housing

Veterans AffairsHomeless Veterans Housing

58 Housing Units

Minnesota Historical SocietyBuilding RestorationVisitor Services Enhancements

Minneapolis-St.Paul International Airport

MnDOT

/ 0 500 1,000Feet

Joint Powers Agreement Partnership

Joint Powers Agreement Partnership