Consideration of Tunnel Alignment Alternatives

Technical Memorandum
Considerat ion of Tunnel

Alignment Alt ernat iv es
Green Line Extension Project
Prepared for Executive Office of Transportation & Public Works

Boston, Massachusetts
Title : vhb log oc.ep s -VHB L og o Ge n era l Pu rp o se E PS file .
Prepared by /Vanasse Hangen Brustlin, Inc.

Cre a to r: D .Bu ccel la
Cre a tion Da te :
PB Americas, Inc.
N ovember 2008
Table of Contents
Table of Contents .................................................................................................................................... 1

List of Figures.......................................................................................................................................... 2
List of Tables ........................................................................................................................................... 2
Introduction................................................................................................................................ 3
1.1 Overview.................................................................................................................... 3
1.2 Overview of Green Line Extension Proposal ............................................................. 3
1.3 Tunnel Alignment Proposals...................................................................................... 4
1.4 Tunnel Construction Technologies ............................................................................ 5
1.4.1 Cut and cover tunneling ................................................................................ 5
1.4.2 Depressed section with decking.................................................................... 5
1.4.3 Deep bore tunneling...................................................................................... 6
T-1 – Clarendon Hill Tunnel: Tunnel from Ball Square to Alewife Station via Powder House
Square and Clarendon Hill..................................................................................................................... 8
2.1 Description of Proposal .............................................................................................. 8
2.1.1 Stations Locations and Spacing.................................................................... 9
2.1.2 Advantages of the Alignment ...................................................................... 12
2.2 Consideration of Tunnel Methodology ..................................................................... 12
2.2.1 Cut and Cover Tunnel ................................................................................. 13
2.2.2 Deep Bore Tunnel ....................................................................................... 13
2.2.3 Conclusions................................................................................................. 13
2.3 Order of Magnitude Additional Cost......................................................................... 14
T-2 – Medford Hillside Tunnel: Tunnel along Lowell Line from College Avenue to Mystic Valley
Parkway/Route 16.................................................................................................................................. 15
3.1 Description of Proposal ............................................................................................ 15
3.2 Review of Proposal .................................................................................................. 16
3.2.1 Existing Conditions...................................................................................... 17
3.2.2 Proposed Extension in Open Cut................................................................ 17
3.2.3 Proposed Deep Bore Tunnel under Right-of-Way ...................................... 17
3.2.4 Proposed Deep Bore Tunnel under Boston Avenue................................... 18
3.2.5 Alternative Tunnel: Cut & Deck Section..................................................... 19
T-3: Tunnel under Union Square......................................................................................................... 22
4.1 Description of Proposal ............................................................................................ 22
4.2 Evaluation of Options............................................................................................... 23
4.2.1 Tunnel from Fitchburg Alignment ................................................................ 23
4.2.2 Tunnel under Somerville Avenue................................................................ 24
4.2.3 Evaluation of Station Options...................................................................... 24
T-4: Mainline Tunnel from Lechmere to Mystic Valley Parkway/Route 16................................... 26
5.1 Description............................................................................................................... 26
5.2 Review of Proposal .................................................................................................. 26
5.2.1 Horizontal Alignment ................................................................................... 26
5.2.2 Vertical Alignment ....................................................................................... 26
5.3 Order of Magnitude Cost.......................................................................................... 27
Conclusions........................................................................................................................................... 29
6.1 Comparison of Tunnel Alternatives.......................................................................... 29
6.2 Technical Issues ...................................................................................................... 30
6.2.1 General ....................................................................................................... 30
6.3 Conclusion ............................................................................................................... 30
\\Mabos\projects\10303.00\reports\Tunnel_Me
morandum\Draft Document- 1 Alternatives Analysis for Tu nnel Alignm ent Prop osals
TunnelAlts_11252008 for posting.doc
List of Figures
Figure 1-1 Existing Green Line with Proposed Extension to Somerville and Medford ...................... 4
Figure 1-2 Cut and Cover Tunneling ................................................................................................. 5
Figure 1-3 Depressed Section with Decking – Green Line and Commuter Rail................................ 6
Figure 1-4 Deep Bore Section Below City Street (Twin Bore Configuration)t ................................... 7
Figure 1-5 Single Bore Tunnel for Green Line................................................................................... 7
Figure 2-1 T-1 Clarendon Hill Tunnel: Green Line Extension with Tunnel from
Ball Square to Alewife.............................................................................. 9
Figure 2-2 Tunnel T-1 at Ball Square .............................................................................................. 10
Figure 3-1 T-2 Medford Hillside Tunnel: College Avenue to Mystic Valley Parkway/Route 16 ...... 16
Figure 3-2 Existing Section Looking North ...................................................................................... 17
Figure 3-3 Open Cut – Green Line and Commuter Rail .................................................................. 17
Figure 3-4 Green Line in Deep Bore Tunnel in Right of Way.......................................................... 18
Figure 3-5 Green Line in Deep Bore Tunnel under College Avenue............................................... 19
Figure 3-6 Decking over Tracks-Green Line and Commuter Rail ................................................... 19
Figure 4.1 Options for Union Square Tunnel Alignment and Station Locations .............................. 23
List of Tables
Table 2-1 Station Spacing between Ball Square and Alewife ......................................................... 10
Table 2-2 T-1 – Clarendon Hill Tunnel Additional Construction Costs ............................................ 14
Table 3-1 T-2 – Medford Hillside Tunnel Additional Construction Costs......................................... 21
Table 4-1 T-3a – Union Square via Fitchburg Line Tunnel Additional Construction Costs ............. 25
Table 4-2 T-3b – Union Square via Somerville Ave. Tunnel Additional Construction Costs ........... 25
Table 5-1 T-4 – Mainline Tunnel Additional Construction Costs ..................................................... 28
Table 6-1 Comparison of Tunnel Alternatives ................................................................................. 30
1
Introduction
This chap ter w ill p rovid e an overview of the intent of this d ocu m ent, as w ell as an
overview of the variou s tu nnel alignm ents p rop osed .
1.1 Overview
A nu m ber of su ggestions for incorp orating tu nnel alignm ents into the p rop osed
Green Line extension have been su ggested by m em bers of the Ad visory Com m ittee
and the general p u blic. The p rop osals offer the p otential of m itigating som e im p acts
of the extension (e.g., noise and vibration d u ring and after constru ction) as w ell as
allow ing for som e variations in the alignm ent that w ou ld be p ossible w ith a tu nnel
alignm ent.
This m em orand u m w ill su m m arize the p rop osals received , consid er alternative
tu nnel constru ction m ethod ologies, exam ine the feasibility of the tu nnel alignm ent
alternatives, and p resent ord er-of-m agnitu d e constru ction cost com p arisons.
1.2 Overview of Green Line Extension

Proposal
The Green Line extension is p rop osed to inclu d e the follow ing elem ents:
 Relocated Lechm ere Station

 Mainline extension along the Low ell Line to College Avenu e or Mystic Valley
Parkw ay
 Sp u r to Union Squ are
See Figu re 1-1.
Figure 1-1: Existing Green Line with Proposed Extension to Somerville and
Medford
1.3 Tunnel Alignment Proposals

The follow ing tu nnel alignm ent alternatives have been p rop osed :
 Proposal T-1 – Clarendon Hill Tunnel: Tu nnel from Ball Squ are to Alew ife
Station via Pow d er H ou se Squ are and Clarend on H ill (GLAM tu nnel p rop osal)
 Proposal T-2 – Medford Hillside Tunnel: Tu nnel from College Avenu e to
Mystic Valley Parkw ay/ Rou te 16
 Proposal T-3 – Union Square Tunnel: Tu nnel from Prosp ect Street u nd er Union
Squ are
 Proposal T-4 – Mainline Tunnel: Com p lete tu nnel alignm ent from Lechm ere to
Mystic Valley Parkw ay/ Rou te 16 w ith branch to Union Squ are
Consid eration of each of these p rop osals is p resented in Sections 2 throu gh 5 of this
rep ort.
1.4 Tunnel Construction Technologies
This section p resents an overview of typ ical tu nnel constru ction technologies that are
u sed in u rban environm ents:
 Cu t and cover tu nneling

 Dep ressed section w ith d ecking Figure 1-2: Cut and Cover Tunneling
 Deep bore tu nneling
1.4.1 Cut and cover tunneling

Cu t and cover tu nneling involves
excavating the tu nnel in an op en cu t.
Excavation begins at the su rface and
extend s d ow nw ard . The sid es of the
excavation m u st be p rotected by
retaining w alls, w hich m ay be either
tem p orary (e.g., sheeting) or p erm anent
(e.g., slu rry w alls or trad itional
reinforced concrete w alls). In
p rep aration for excavation, u tilities m u st
be relocated or su p p orted in p lace. Also,
traffic m u st be d etou red to create a w ork
zone to p erform the excavation.
After the excavation is com p leted and

the p erm anent w all constru cted , the
tu nnel roof (typ ically, reinforced
concrete) is installed . Figu re 1-2
illu strates the step s in constru cting a cu t
and cover tu nnel.
1.4.2 Depressed section with decking

Mu ch of the Green Line extension along the Low ell Line is in a d ep ression or cu t
section, w here the trackbed is abou t 15 to 20 feet below stre et level. This p rovid es the
op p ortu nity for d ecking over the tracks, creating sections of tu nnel. This ap p roach is
u sed throu ghou t the MBTA’s Sou thw est Corrid or.
For the section north of College Avenu e, the op tion of d ecking over the tracks creates
a tu nnel section at a low er increm ental cost than other m ethod s. Typ ically, the w alls
w ou ld be requ ired to retain the em bankm ent and w id en the trackbed from the
existing tw o tracks to fou r tracks. Thu s, the increm ental cost is the d ecking 1.
Figure 1-3: Depressed Section with Decking – Green Line and Commuter Rail
The d ecking can p rovid e a visu al and noise bu ffer betw een the tracks and ad jacent
resid ences. The d ecking can also be land scap ed to restore lost vegetative cover d u e
to w id ening of the trackbed .
1.4.3 Deep bore tunneling

Deep bore tu nneling is a m ining techniqu e that u ses a tu nnel boring m achine (TBM)
op erating w ell below the grou nd su rface. The TBM bores horizontally , creating a
circu lar tu nnel. The TBM can d rill throu gh rock or any kind of soil. H ow ever, the
typ e of TBM u sed for rock d iffers from the typ e u sed for soil.
The d iam eter of a tu nnel boring can vary from as little as 3 feet to as m u ch as 60 feet.
In earth, tu nnels are typ ically abou t 2 d iam eters below the su rface. For a 20-foot
bore, the top of the tu nnel w ou ld be abou t 40 feet d ow n or d eep er.
Su bw ay d eep bore tu nnels are often accom p lished as tw in bores – one tu nnel for
each track. This m ethod w as u sed for the Red Line from H arvard Squ are to Davis
Squ are. See Figu re 1-4. For the Green Line, each bore w ou ld be abou t 24 feet,
ou tsid e d iam eter.
An alternative ap p roach w ou ld be a single larger bore w ith both tracks in one tu nnel.
See Figu re 1-5. The larger cross section w ou ld cost consid erably m ore d u e to the size
of the boring m achine requ ired .

1
The decking also braces the tops of the retaining walls, thus reducing the moment load in the walls. Therefore, even
though the walls must carry the additional load of the deck itself, the net benefit of horizontal bracing creates a more
efficient wall section.
Figure 1-4: Deep Bore Section below City Street (Twin Bore Configuration)
Figure 1-5: Single Bore Tunnel for Green Line
Source: EOTPW Draft Environmental Impact Report for Urban Ring (Figure 2-20)
2
T-1 – Clarendon Hill Tunnel:
Tunnel from Ball Square to Alew ife Station
via Pow der House Square
and Clarendon Hill
At the October 25, 2007 Ad visory Grou p m eeting, the Green Line Ad visory Grou p
for Med ford (GLAM) officially su bm itted a p rop osal for an Alternative St u d y for the
Green Line Extension p roject. The requ est is to p erform an ad d itional analysis on this
alternative rou te as an ad d end u m to the cu rrent Green Line Extension Draft
Environm ental Im p act Rep ort (DEIR) stu d y.
2.1 Description of Proposal

The alignm ent p rop osed by GLAM in an October 25, 2007 letter w ou ld red irect the
cu rrent p rop osed Green Line alignm ent beginning at Ball Squ are. Rather than follow
the cu rrently p rop osed alignm ent along the Low ell Line, the GLAM p rop osal
p rop oses an alternative align m ent that w ou ld have it generally follow Broad w ay in
Som erville throu gh Teele Squ are/ Clarend on H ills u ntil reaching Rou te 16/ Alew ife
Brook Parkw ay. At this p oint, the p rop osal su ggest the alignm ent shou ld follow
Rou te 16/ Alew ife Brook Parkw ay u ntil end ing at the MBTA’s Alew ife Station, w here
it w ou ld be co-term inating w ith the existing Red Line and share the existing p arking
garage located at the ju nction of Rou te 2.
Du e to the lack of an existing su rface right-of-w ay (ROW), GLAM has p rop osed
p u tting the service u nd ergrou nd for the m ajority of its d eviation from the cu rrently
p rop osed Green Line alignm ent. The GLAM p rop osal su ggests stations at Ball
Squ are, Pow d er H ou se Bou levard , and Clarend on H ill, as w ell as at the existing
Alew ife Station. See Figu re 2-1 for a m ap of this p rop osed alignm ent.
Ad d itional cap acity im p rovem ents ad vocated in the GLAM alternative inclu d e
exp ansion of the garage at Alew ife to accom m od ate m ore rid ers and the
d evelop m ent of a com m u ter rail/ Green Line interm od al station at Ball Squ are.
Figure 2-1: T-1 Clarendon Hill Tunnel: Green Line Extension with Tunnel from
Ball Square to Alewife
Note: Tunnel segment is from Ball Square to Alewife
2.1.1 Stations Locations and Spacing

Beginning w ith the d iversion from the p resently p rop osed rou te at Ball Squ are,
stations w ou ld be located at intervals of ap p roxim ately 0.4 to 1.1 m iles, as ind icated
in Table 2-1.
Table 2-1 Station Spacing between Ball Square and Alewife
Segment Length (Miles) Route

Ball Sq. to Powder House Sq. 0.4 Broadway
Powder House Sq. to Clarendon Hill 0.8 Broadway
Clarendon Hill to Alewife 1.1 Rt. 16/Alewife Brook Parkway
Tail tracks (600 feet includes switches) 0.1
TOTAL LENGTH 2.4
Ball Square Station
The alignm ent ap p roaching Ball Squ are is along the w est sid e of the Low ell Line
right-of-w ay. At Ball Squ are, the alignm ent w ou ld cu rve w estw ard to follow
Broad w ay head ing tow ard s Pow d er H ou se Squ are.
To transition vertically to the d ep th of a tu nnel, the tracks w ou ld need to d escend

p rior to Ball Squ are, thu s m aking Ball Squ are an u nd ergrou nd station. A likely
location for the station m ight be u nd er Broad w ay, ju st w est of Boston Avenu e. See
Figu re 2-2.
Figure 2-2: Tunnel T-1 at Ball Square
Powder House Station
The alignm ent w ou ld follow Broad w ay from Ball Squ are to Pow d er H ou se Squ are in
Som erville.
The p rop osal envisions an u nd ergrou nd station at Pow d er H ou se Squ are. This
station w ou ld accom m od ate Tu fts University and the su rrou nd ing Som erville
neighborhood s. Portions of Med ford w ou ld also be w ithin w alking d istance.
The Pow d er H ou se Station w ou ld be abou t 0.5 m iles from the existing Davis Squ are
station, p rovid ing som e overlap of the catchm ent areas, bu t also p rovid ing tw o
transit line alternatives for rid ers in neighborhood s betw een the stations.
Clarendon Hill Station
The alignm ent w ou ld continu e to follow Broad w ay from Pow d er H ou se Squ are p ast
Teele Squ are to Clarend on H ill.
The p rop osed station w ou ld be located near the MBTA Clarend on H ill bu s tu rn -
arou nd . This location is the site of the form er streetcar yard . Cu rrently the MBTA
ow ns a sm all p arcel of land that serves as the off-street term inu s of bu s rou tes 87 and
88. The p rop osed station w ou ld serve as an interm od al transfer p oint w ith these bu s
lines.
This station w ou ld serve the im m ed iate neighborhood and w ou ld be w ithin w alking

d istance of Teele Squ are. It is also w ithin w alking d istances of resid ential
neighborhood s in Arlington, Som erville and Med ford . The site is ad jacent to existing
high-d ensity hou sing, an eld erly hou sing p roject, and low incom e hou sing located
betw een Broad w ay and Pow d er H ou se Bou levard . This station w ou ld also be abou t
0.5 m iles from the existing Davis Squ are station, p rovid ing som e overlap coverage
and tw o op tions for rid ers w ho live in betw een.
Alewife Station
From Clarend on H ill, the alignm ent w ou ld continu e w est along Broad w ay to
Alew ife Brook Parkw ay. The alignm ent w ou ld tu rn sou th and follow the Parkw ay to
the existing Alew ife station.
This p rop osal su ggests that the Green Line be co-term inal w ith the Red Line at the
existing Alew ife station com p lex. This w ou ld p rovid e a connection betw een the Red
and Green Lines ou tsid e the d ow ntow n core of the su bw ay system . The Green and
Red Lines w ou ld share a com m on garage. The p rop osal envisions an exp ansion of
the cap acity of the garage.
Located at the eastern end of the Rou te 2 exp ressw ay, the station’s cap tu re area
inclu d es Arlington, Lexington, and other com m u nities in the Rou te 2 corrid or as w ell
as tow ns along the I-95/ Rou te 128 corrid or.
The Green Line tu nnel w ou ld need to p ass u nd er the Red Line tu nnel, w hich is fairly
shallow . A series of interconnecting stairs, escalators and elevators w ou ld p rov id e
access betw een the tw o station p latform s and the station fare collection and entrance
areas.
Tail Tracks
Typ ical of all term inal stations on the MBTA su bw ay system , tail tracks w ou ld be
p rovid ed after the p latform s at Alew ife Station. This allow s for tem p orary storage of
an extra train d u ring op erating hou rs as w ell as a p lace to tem p orarily p ark a
d isabled train u ntil it can be m oved to the m aintenance facility. A typ ical
configu ration w ou ld be 2 or 3 tracks, w ith tu rnou ts (sw itches) w hich connect ea ch
tail track w ith each station track.
2.1.2 Advantages of the Alignment

Accord ing to GLAM, benefits of this u nd ergrou nd Green Line extension w ou ld
inclu d e:
 The alignm ent w ou ld serve an Environm ental Ju stice (EJ) com m u nity near
Clarend on H ill/ Teele Squ are in Som erville
 The u nd ergrou nd constru ction w ou ld lessen airborne contam inants for the local
com m u nity com p ared to an above grou nd constru ction and op eration
 There are no environm entally hazard ou s sites along the p rop osed alignm ent
w hereas there are som e sites ad jacent to Low ell Line
 Und ergrou nd constru ction “lessens the risk of em inent d om ain” takings
2.2 Consideration of Tunnel Methodology

This alignm ent d eviates from the existing MBTA com m u ter rail right-of-w ay and
ru ns u nd er city streets and other road w ays. Tw o tu nnels m ethod ologies cou ld be
em p loyed :
 Cu t and cover
 Deep bore
2.2.1 Cut and Cover Tunnel
Section 1.4.1 and Figu re 1-2 p resent an overview of the cu t and cover tu nnel
constru ction m ethod ology.
For this m ethod ology to be feasible, the alignm ent m u st follow either existing
road w ays or other u nd evelop ed land w here su rface constru ction is p ossible. As this
alignm ent generally follow s both Broad w ay and Alew ife Brook Parkw ay, it w ou ld be
p ossible to constru ct a cu t and cover tu nnel.
A second im p ortant consid eration for this m ethod ology is traffic interru p tion. As
Figu re 1-2 illu strates, d u ring the tu nnel constru ction, the road w ay w ou ld be greatly
red u ced in w id th or p ossibly closed to traffic. This w ou ld be d ifficu lt, d u e to the
high traffic volu m es along both Broad w ay and esp ecially Alew ife Brook Parkw ay.
The p arkw ay corrid or also inclu d es both w etland s associated w ith Alew ife Brook
itself and the DCR p ark reservation. Constru ction w ou ld also im p act both these
w etland resou rce areas an d the p arkland s along the corrid or.
Other im p acts d u ring constru ction w ou ld inclu d e noise, vibrations, d u st,

constru ction vehicle traffic, and tem p orary d isru p tions/ inconveniences to access to
bu sinesses and resid ences.
2.2.2 Deep Bore Tunnel

Section 1.4.3 and Figu res 1-4/ 1-5 p resent an overview of the d eep bore tu nnel
constru ction m ethod ology.
While the tu nnel cou ld be bored from either end , for the p u rp oses of this concep tu al
analysis, it is assu m ed that it w ou ld be bored from the Ball Squ are end . A m ining
shaft w ou ld be bu ilt near Ball Squ are to the d ep th of the tu nnel. The tu nnel boring
m achine (TBM) w ou ld be low ered in the shaft and tu nnel boring w ou ld com m ence.
Thou gh the m ain tu nnel w ou ld be bored from u nd ergrou nd , su rface constru ction
w ou ld be requ ired at stations, ventilation shafts and access shafts. For safety d u ring
constru ction, p eriod ic access shafts (abou t 5,000 feet ap art) w ou ld be need ed for
em ergency access and evacu ation. At these su rface constru ction locations, the
im p acts w ou ld be sim ilar to that of cu t-and -cover tu nneling.
2.2.3 Conclusions
Based on a p relim inary consid eration of each m ethod , the u se of d eep bore tu nneling
w ou ld have less su rface im p acts along a corrid or that inclu d es d ense u rban
d evelop m ent as w ell as p arkland s and w etland s.
2.3 Order of Magnitude Additional Cost
An ord er-of-m agnitu d e estim ate w as m ad e of the ad d itional cost of this tu nnel
alternative. The “ad d itional cost” rep resents the d ifference betw een the p referred at -
grad e alignm ent along the Low ell Lin e right-of-w ay and this tu nnel alignm ent. As
su ch, the costs com m on to any alternative (e.g., track, p ow er and signals) are not
inclu d ed . Also, no estim ate w as m ad e of the real estate costs for p rop erty takings
and easem ents for the tu nnel, stations, sh afts and other su p p ort facilities inclu d ing
p ow er su bstations.
Included Not Included

Tunnel construction Trackwork
Additional cost of underground stations Traction power
Tunnel ventilation including ventilation shafts Signals and communications
Access shafts
Tunnel lighting Real estate costs
Tunnel fire protection (dry standpipes)
Tunnel drainage
Table 2-2 inclu d es the estim ated qu antities of w ork and the ad d itional cost for either
a single or d ou ble bore tu nnel.
Table 2-2 T-1 – Clarendon Hill Tunnel Additional Construction Costs
Item Quantity Double Bore Single Bore

Tunnel Construction 2.4 miles
Number of Underground 4
Stations
Ventilation Shafts 7
Access Shafts 3
Tail Tracks 600 feet
Tunnel Lighting 2.4 miles
Tunnel fire protection 2.4 miles
Tunnel drainage 2.4 miles
TOTAL LENGTH 2.4 miles $1.54 billion $2.13 billion
3
T-2 – Medford Hillside Tunnel:
Tunnel along Lowell Line from College
Avenue to Mystic Valley Parkway/Route 16
At variou s Ad visory Grou p m eetings, p rop osals to consid er a tu nnel alignm ent
w ithin Med ford w ere p u t forth. The overall alignm ent w ou ld be the sam e, w ith
stations at College Avenu e and Mystic Valley Parkw ay/ Rou te 16. H ow ever, instead
of w id ening the track bed and relocating the com m u ter rail tracks, this p rop osal
w ou ld have the Green Line ru n below the grou nd , thu s, m inim izing the tem p orary
constru ction and p erm anent im p acts on the com m u nity.

The p rop osal is to su bstitu te a tu nnel alignm ent in p lace o f the p rop osed alignm ent
that w ou ld locate the Green Line and com m u ter rail tracks sid e -by-sid e in the
existing railroad right-of–w ay. See Figu re 3-1.
Figure 3-1: T-2 Medford Hillside Tunnel: College Avenue to Mystic Valley
Parkway/Route 16
This p rop osal w as p u t forth becau se of the p otential benefits of red u ced im p acts both
d u ring constru ction and thereafter:
 Less im p acts d u ring constru ction:

 N o constru ction in right-of-w ay (no rem oval of trees, no earthw ork and
constru ction of retaining w alls
 N o relocation of com m u ter rail tracks to the east sid e of the right -of-w ay
 Less im p acts after constru ction:
 Little to no noise or vibrations from Green Line op erations
 Retains visu al bu ffer of existing vegetation along right -of-w ay
3.2 Review of Proposal

While the initial p rop osal w ou ld be for a d eep bore tu nnel u nd er the right -of-w ay,
there are a nu m ber of alternative ap p roaches to constru cting this segm ent of the
Green Line extension in a tu nnel. These op tions inclu d e:
 Deep bore tu nnel u nd er MBTA right-of-w ay

 Deep bore tu nnel u nd er Boston Avenu e.
 Decking over tracks in MBTA right-of-w ay
Figu res 3-2 throu gh 3-6 p rovid e a com p arison of these alternatives w ith existing
cond itions and w ith the p rop osed Green Line extension next to the com m u ter rail
tracks in an op en cu t.
3.2.1 Existing Conditions

Figu re 3-2 show s the existing tw o com m u ter rail tracks in a cu t section w ith
vegetated slop es on either sid e and resid ential stru ctu res near the right -of-w ay.
Figure 3-2: Existing Section Looking North
3.2.2 Proposed Extension in Open Cut

Figu re 3-3 show s the w id ening of the cu t by installing retaining w a lls on either sid e
and excavating the slop es, resu lting in a loss of the existing vegetation on the slop es.
The com m u ter rail tracks are shifted to the east sid e of the w id ened cu t. The new
Green Line tracks are bu ilt along the w est sid e.
Figure 3-3: Open Cut – Green Line and Commuter Rail
3.2.3 Proposed Deep Bore Tunnel under Right-of-Way

Figu re 3-4 show s the Green Line extension in a d eep bore tw in -tu nnel u nd er the
right-of-w ay. To p rovid e at least 2 d iam eters of cover on the tu nnels, the Green Lin e
tracks w ou ld be in the ord er of 60 feet below the existing track bed , or abou t 80 to 90
feet below street level.
Figure 3-4: Green Line in Deep Bore Tunnel in Right of Way
3.2.4 Proposed Deep Bore Tunnel under Boston

Avenue
Since a d eep bore alignm ent d irectly the Low ell Line right-of-w ay w ou ld resu lt in
very d eep stations (80 to 90 feet below street level), consid eration w as given to a d eep
bore alignm ent u nd er Boston Avenu e. With d eep bore tu nneling, the alignm ent d oes
not have to follow the MBTA right-of-w ay exactly. The tu nnel w ou ld be w ell below
u tilities and bu ild ing fou nd ations, so there are m ore op tions w ith regard to the
horizontal alignm ent.
By follow ing Boston Avenu e, the tu nnels cou ld theoretically be shallow er. In
general, Boston Aven u e is abou t 20 feet higher than the existing Low ell Line
trackbed , so the tu nnel cou ld also be abou t 20 feet shallow er. See Figu re 3-5.
Figure 3-5: Green Line in Deep Bore Tunnel under College Avenue
In ord er to m ake a final d eterm ination, it w ou ld be necessary to have d etailed

su bsu rface inform ation as to soils typ es and the d ep th of bed rock. Cu rrent
inform ation is lim ited to som e borings at the existing brid ges over the tracks at
College and Winthrop Avenu es.
3.2.5 Alternative Tunnel: Cut & Deck Section

An alternative to d eep bore tu nneling w ou ld be d ecking over the tracks in the Low ell
Line right-of-w ay. See Figu re 3-6.
This alternative w ou ld be constru cted m u ch in the sam e w ay as the p rop osed

extension. The existing cu t is w id ened , the slop es are excavated , retaining w alls are
installed and the fou r tracks constru cted . The d ifference is that a d eck (or roof) is
constru cted over the tracks, thu s, creating a tu nnel.
Figure 3-6: Decking over Tracks – Green Line and Commuter Rail
As Figu re 3-6 illu strates, the d eck p rovid es a nu m ber of ad vantages to the abu tters:
 Rem oves the visu al im p act of the tracks

 Red u ces noise
 Allow s for land scap ing over the d eck
This alternative w ou ld be sim ilar to the Sou thw est Corrid or, w here both transit and
com m u ter rail tracks are in a d ep ressed cu t w ith sections of the cu t covered in
d ecking.
With this alternative, a key issu e is ventilation for the com m u ter rail tracks, as the
p assenger and freight trains u se d iesel locom otives. One ap p roach is to p rovid e
sections of d ecking alternating w ith sections that are op en to the air. The op en air
sections p rovid e the necessary ventilation.

grad e alignm ent along the Low ell Line right-of-w ay and this tu nnel alignm ent. As
and easem ents for the tu nnel, stations, shafts and other su p p ort facilities inclu d ing

Access shafts
Tunnel drainage
Table 3-1 inclu d es the estim ated qu antities of w ork and the ad d itional cost for the
single or d ou ble bore tu nnel, as w ell as for the d ecking op tion. There is no
significant cost d ifference w hether the d eep bore tu nnel is u nd er the Low ell Line or
u nd er Boston Avenu e.
Table 3-1 T-2– Medford Hillside Tunnel Additional Constructions Costs
Item Quantity Double Bore Single Bore Decking

Number of 2
Underground Stations
Access Shafts 1
TOTAL LENGTH 1.2 miles $1.02 billion $0.76 billion $0.40 billion
4
T-3: Tunnel under Union Square
A nu m ber of com m ents w ere received regard ing the u se of a tu nnel for p art or the
entire Union Squ are sp u r. A tu nnel cou ld allow the Union Squ are station to be
located closer to the heart of the squ are, w hich is a com m ent m entioned d u ring the
p u blic m eetings.
These p rop osals d iffer from the Beyond Lechm ere Stu d y alternative that w ou ld have
d iverted the m ainline throu gh Union Squ are via the Fitchbu rg Line right -of-w ay
w ith a tu nnel u nd er Union Squ are and Prosp ect H ill and connecting w ith the Low ell
Line right-of-w ay sou th of Med ford Street. This alternative w as elim ina ted d u ring
the Beyond Lechm ere Stu d y.

While a sp ecific alignm ent w as not p rop osed , the objective of the p rop osal w ou ld be
to locate the new station u nd er the heart of Union Squ are. To achieve this, variou s
alignm ent op tions are p ossible. For the sake of this evalu ation, the op tions inclu d e:
 Tu nnel u nd er Som erville Avenu e w ith p ortal in Brickbottom area

 Tu nnel u nd er Prosp ect and Washington Streets w ith p ortal along the Fitchbu rg
Line
Also, for the sake of this op tion, three station sites w ere consid ered (see Figu re 4-1):
 1. Vicinity of intersection of Prosp ect Street and Som erville Avenu e (Fitchbu rg
alignm ent, only)
 2. Som erville Avenu e w est of Prosp ect Street (Som erville Ave. alignm ent, only)
 3. Vicinity of intersection of Som erville Avenu e, Washington Street and Webster
Street (either alignm ent)
Figure 4-1 Options for Union Square Tunnel Alignment and Station Locations
4.2 Evaluation of Options
4.2.1 Tunnel from Fitchburg Alignment

This op tion w ou ld inclu d e an at grad e alignm ent along the Fitchbu rg Line right-of-
w ay. West of Med ford Street, the tracks w ou ld d escend to a p ortal ju st east of
Prosp ect Street. The objective w ou ld be to m inim ize the tu nnel length and cost by
m axim izing the at-grad e p ortion of the alignm ent.
After the p ortal, the tracks w ou ld cu rve to the north u nd er p rivate p rop erty and then
u nd er Prosp ect Street. Op tion 1 for the station w ou ld be ju st sou th of the intersection
of Prosp ect Street and Som erville Avenu e. If the station is not at the Op tion 1 site,
the tracks w ou ld cu rve w est u nd er Som erville Avenu e and then u nd er Washington
Street. Op tion 3 for the station w ou ld be ju st beyond the intersection of Washington
Street, Webster Avenu e and Som erville Avenu e. Tail tracks w ou ld be inclu d ed ju st
beyond the station.
Tunnel Method
Du e to the relative short length of this tu nnel (abou t 1,500 feet), cu t and cover
m ethod s w ere consid ered . H ow ever the d ow n sid e of cu t and cover w ou ld be the
extensive im p act on Union Squ are, com ing after the cu rrent d isr u p tion for the m ajor
sew er and d rain constru ction along Som erville Avenu e.
4.2.2 Tunnel under Somerville Avenue

This op tion w ou ld leave the Fitchbu rg right-of-w ay ju st w est of the Brickbottom
resid ential bu ild ing. Ru nning p arallel to McGrath H ighw ay, it w ou ld d escend into a
tu nnel p ortal before crossing u nd er McGrath H ighw ay and follow ing the alignm ent
of Som erville Avenu e into Union Squ are. For illu strative p u rp oses, Figu re 4-1 show s
the alignm ent tu rning u nd er Washington Street ju st w est of Union Squ a re.
H ow ever, an alignm ent continu ing w est u nd er Som erville Avenu e cou ld be
consid ered .
The station op tions inclu d e Site 2 ju st w est of Prosp ect Street and Site 3 ju st w est of
Union Squ are itself.
Tunnel Method
Du e to the congestion and heavy existing u tility infrastru ctu re, d eep bore tu nneling
is assu m ed for this op tion.
4.2.3 Evaluation of Station Options

Figu re 4-1 illu strates 3 p ossible locations for a Union Squ are su bw ay station. Since it
is a stated objective of this tu nnel op tion to have the statio n in Union Squ are, only
station op tion 3 p rovid es for that. Therefore, station op tion 3 w ill be consid ered the
p referred station location.

Access Shafts
Tunnel drainage
Tables 4-1 and 4-2 inclu d e the estim ated qu antities of w ork and the ad d itional cost
for the single or d ou ble bore tu nnel for the Fitchbu rg and Som erville Avenu e
alignm ent op tions.
Table 4-1 T-3a– Union Square via Fitchburg Line Tunnel

Additional Construction Costs

Number of 1
Access Shafts 0
TOTAL LENGTH 0.3 miles $500 million $600 million
Table 4-2 T-3b– Union Square via Somerville Ave. Tunnel

Additional Construction Costs

Number of 1
Access Shafts 0
TOTAL LENGTH 0.8 miles $620 million $820 million
5
T-4: Mainline Tunnel
from Lechmere to
Mystic Valley Parkw ay/Route 16
5.1 Description
This p rop osal is sim p ly to constru ct the m ainline extension by d eep bore tu nnel
instead of an at-grad e alignm ent along the Low ell Line right-of-w ay.
5.2 Review of Proposal
5.2.1 Horizontal Alignment

It is assu m ed that the alignm ent w ou ld generally follow the Low ell Line and that
stations w ou ld be at the sam e locations as the at -grad e p rop osal. Althou gh the d eep
bore m ethod d oes not requ ire the alignm ent to follow the Low ell Line right-of-w ay,
for evalu ation p u rp oses the alignm ent w ill be assu m ed to generally follow the rail
line.
5.2.2 Vertical Alignment

Consid eration w as given to w here the extension w ou ld transition into the tu nnel .
The relocated Lechm ere station is p rop osed to be an elevated station. Proceed ing
ou tbou nd , a transition cou ld be m ad e p rior to the Fitchbu rg Line crossing. The
d ow nsid es of this ap p roach w ou ld be the steep slop e requ ired to p ass u nd er the
Fitchbu rg Line and the com p lexity of the ju nction w ith the Union Squ are sp u r.
A transition near Yard 8 p rovid es for a gentler transition slop e into the tu nnel and
retains the ju nction w ith the Union Squ are branch as d esigned .
The six m ainline stations from Washington Street north w ou ld all be in the tu nnel.
Tail tracks w ou ld be p rovid ed after Mystic Valley station.
5.3 Order of Magnitude Cost


Access Shafts
Tunnel drainage
Table 5-1 inclu d es the estim ated qu antities of w ork and the ad d itional cost for the
single or d ou ble bore tu nnel.
Table 5-1 T-4 – Mainline Tunnel Additional Construction Costs

Number of Underground 6
Stations
Access Shafts 4
TOTAL LENGTH 3.7 miles $2.1 billion $3.0 billion
6
Conclusions
6.1 Comparison of Tunnel Alternatives

This technical m em orand u m consid ered 4 tu nnel alternatives:
 Proposal T-1 – Clarendon Hill Tunnel: Tu nnel from Ball Squ are to Alew ife
Station via Pow d er H ou se Squ are and Clarend on H ill (GLAM tu nnel p rop osal)
 Proposal T-2 – Medford Hillside Tunnel: Tu nnel from College Avenu e to
Mystic Valley Parkw ay/ Rou te 16
 Proposal T-3 – Union Square Tunnel: Tu nnel from Prosp ect Street u nd er Union
Squ are
 Proposal T-4 – Mainline Tunnel: Com p lete tu nnel alignm ent from Lechm ere to
Mystic Valley Parkw ay/ Rou te 16 w ith branch to Union Squ are
Table 6-1 p resents a su m m ary of the alternatives and resp ective ad d itional costs.
N ote that the ord er-of-m agnitu d e constru ction cost estim ates rep resent only the
ad d itional cost of each tu nnel alter native. The “ad d itional cost” rep resents the
d ifference betw een the p referred at-grad e alignm ent along the Low ell Line right-of-
w ay and the tu nnel alignm ent. As su ch, the costs com m on to any alternative (e.g.,
track, p ow er and signals) are not inclu d ed . Also, no estim ate w as m ad e of the real
estate costs for p rop erty takings and easem ents for the tu nnel, stations, shafts and
other su p p ort facilities inclu d ing p ow er su bstations.
Table 6-1 Comparison of Tunnel Alternatives
Item
T-1: Clarendon T-2: Medford T-3a: Union T-3b: Union Sq. T-4: Mainline
Hill Tunnel Hillside Tunnel Sq. Tunnel via Tunnel via Tunnel
Fitchburg Line Somerville Ave.
Ball Sq. to Ball Sq. to Mystic Prospect St. to Poplar St. to Brickbottom to
Alewife Station Valley Parkway Union Sq. Union Sq. Mystic Valley Pkwy.
Length of Tunnel 2.4 miles 1.2 miles 0.3 miles 0.8 miles 3.7 miles
Number of 4 2 1 1 6
Ventilation Shafts 7 3 2 3 11
Access Shafts 3 1 0 0 4
Estimated Additional 1.54 billion $1.02 billion $500 million $620 million $2.1 billion
Construction Cost
(Twin Bores)
Estimated Additional $2.13 billion $760 million $600 million $820 million $3.0 billion
Construction Cost
(Single Bore)
Estimated Additional N/A $400 million N/A N/A N/A
Construction Cost
(Decking)
6.2 Technical Issues

6.2.1 General
All tu nnel alternatives are technically feasible, in that a tu nnel cou ld be constru cted
along any of the alignm ents p rop osed . In all cases, the existing su bsu rface soils
inform ation cu rrently available is su fficient to d eterm ine that tu nneling is feasible,
bu t not su fficient to d eterm ine other d etails, su ch as w hether a bored tu nnel w ou ld
be in rock, soil, or m ixed rock and soil cond itions.
6.3 Conclusion
While the tu nnel alternatives w ou ld red u ce su rface d isru p tion d u ring constru ction as
w ell as p ost-constru ction noise and visu al im p acts, these im p acts, u nd er the su rface
alternative, are not exp ected to be significant and can be m itigated cost effectively .
Therefore, the ad d itional costs of tu nnel constru ction are not w arranted .

Consideration of Tunnel Alignment Alternatives

Uploaded by

Copyright:

Available Formats

Consideration of Tunnel Alignment Alternatives

Uploaded by

Document Information

Original Description:

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Consideration of Tunnel Alignment Alternatives

Uploaded by

Copyright:

Available Formats

Technical Memorandum

Considerat ion of Tunnel

Green Line Extension Project

Prepared for Executive Office of Transportation & Public Works

Title : vhb log oc.ep s -VHB L og o Ge n era l Pu rp o se E PS file .

Prepared by /Vanasse Hangen Brustlin, Inc.

Table of Contents .................................................................................................................................... 1

1.2 Overview of Green Line Extension

 Relocated Lechm ere Station

See Figu re 1-1.

1.3 Tunnel Alignment Proposals

 Cu t and cover tu nneling

1.4.1 Cut and cover tunneling

After the excavation is com p leted and

1.4.2 Depressed section with decking

1.4.3 Deep bore tunneling

Figure 1-5: Single Bore Tunnel for Green Line

2.1 Description of Proposal

Note: Tunnel segment is from Ball Square to Alewife

2.1.1 Stations Locations and Spacing

Segment Length (Miles) Route

Ball Square Station

To transition vertically to the d ep th of a tu nnel, the tracks w ou ld need to d escend

Figure 2-2: Tunnel T-1 at Ball Square

Clarendon Hill Station

This station w ou ld serve the im m ed iate neighborhood and w ou ld be w ithin w alking

2.1.2 Advantages of the Alignment

2.2 Consideration of Tunnel Methodology

Other im p acts d u ring constru ction w ou ld inclu d e noise, vibrations, d u st,

2.2.2 Deep Bore Tunnel

Included Not Included

Table 2-2 T-1 – Clarendon Hill Tunnel Additional Construction Costs

Item Quantity Double Bore Single Bore

3.1 Description of Proposal

 Less im p acts d u ring constru ction:

3.2 Review of Proposal

 Deep bore tu nnel u nd er MBTA right-of-w ay

3.2.1 Existing Conditions

3.2.2 Proposed Extension in Open Cut

3.2.3 Proposed Deep Bore Tunnel under Right-of-Way

3.2.4 Proposed Deep Bore Tunnel under Boston

In ord er to m ake a final d eterm ination, it w ou ld be necessary to have d etailed

3.2.5 Alternative Tunnel: Cut & Deck Section

This alternative w ou ld be constru cted m u ch in the sam e w ay as the p rop osed

 Rem oves the visu al im p act of the tracks

3.3 Order of Magnitude Additional Cost

Included Not Included

Item Quantity Double Bore Single Bore Decking

4.1 Description of Proposal

 Tu nnel u nd er Som erville Avenu e w ith p ortal in Brickbottom area

4.2 Evaluation of Options

4.2.1 Tunnel from Fitchburg Alignment

4.2.2 Tunnel under Somerville Avenue

4.2.3 Evaluation of Station Options

4.3 Order of Magnitude Additional Cost

Table 4-1 T-3a– Union Square via Fitchburg Line Tunnel

Item Quantity Double Bore Single Bore

Table 4-2 T-3b– Union Square via Somerville Ave. Tunnel

Item Quantity Double Bore Single Bore