embedded turnouts for the hiawatha lrt project · and hong kong. these “flexive tongue” designs...
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Embedded Turnouts for the Hiawatha LRT Project
Author: David F. Peterson, PE
Parsons Corporation
Design Manager, Hiawatha Light Rail Transit Project
September 30, 2003
ABSTRACT
Installation and maintenance of turnouts in paved areas have always been
something to avoid for both railroads and transit agencies. For the development
of the Hiawatha Light Rail Transit Project, a key element is at-grade rail access
to downtown Minneapolis. The present north end in downtown requires
crossovers in Fifth Street and Nicollet Mall, a confined area congested with
utilities and vehicular traffic. Embedded turnouts – turnouts installed in paving –
are required in this and other locations along the Hiawatha corridor.
Minnesota Transit Constructors (MnTC), the design-build contractor for the
Hiawatha Project Office, elected to use #6 flexive double tongue switches for
paved areas. This European Edgar Allen switch design differs from AREMA
standard double tongue turnouts by having a flexible switch point set in a fixed
casting, a transit – type design also being used in Tampa’s new LRT system.
For containment of stray electrical current to prevent corrosion of nearby
infrastructure, the embedded track is electrically isolated. The turnout is
constructed in a concrete “tub” comprised of concrete and insulating membrane
and sealed with elastomeric grout. The turnout and tub are constructed with
three concrete pours, with the insulating membrane installed between the first
and second pours.
In most cases, the turnouts are power operated with switch machines between
the switch point castings, and heated with calrod-type electrical resistance
heaters.
The turnouts were fabricated by Progress Rail Services and will be installed by
MnTC’s subcontractor Railworks Track Systems in early to mid 2003.
1. INTRODUCTION
Construction and operation of turnouts in paved areas have always been a less-
than-desired alternative to ballasted designs. These “embedded” turnouts
require closer construction tolerances and higher maintenance and are avoided
when possible.
The Hiawatha Light Rail Transit project in Minneapolis and Bloomington,
Minnesota is currently under construction. Hiawatha is a new system linking
downtown Minneapolis with the Minneapolis – St. Paul International Airport and
the Mall of America in Bloomington. The line is approximately 12 miles in length
and includes seventeen stations located in commercial and residential areas.
Low floor Bombardier light rail vehicles will operate at up to 55 mph at 7 ½
minute headways at peak travel periods. The system requires six embedded
crossovers, primarily at the end points of the system to cross trains over for
return movements.
The design-build contractor, Minnesota Transit Constructors (MnTC), worked
closely with the representatives of the Hiawatha Project Office (HPO) to identify
the most appropriate embedded track and turnout system for the Hiawatha LRT
system.
2. TURNOUT EVALUATION
The HPO’s two agencies – the Minnesota Department of Transportation and
Metro Transit, decided during the design development process to use tee rail in
lieu of girder rail for embedded track applications. This in turn led to discussions
of alternative types of turnouts.
AREMA’s Portfolio has two embedded turnout types that in the past have been
used both for railroad and light rail transit applications. (See Portfolio Plan No.
980-60 and other referenced plans). These turnouts, using tongue and mate
and double tongue switches, would have performed satisfactorily. However,
these turnout designs are traditionally intended for railroad use, and considering
the lower axle loads of Light Rail Transit (LRT) applications, other options were
considered.
During the evaluation of turnout options during design development, the HPO
and MnTC considered the embedded switches manufactured by Edgar Allen
Limited in the United Kingdom. Edgar Allen’s switches are currently in use by
Tampa, Florida’s Hartline system and are being installed in New Orleans’
streetcar system. They are also in use in Sheffield and Manchester, England
and Hong Kong. These “flexive tongue” designs are ideal for paved track
applications and can provide maintenance advantages. Flexive tongue turnouts
involve a solid manganese steel body with a flexible manganese tongue, acting
as the switch point, fastened to the body by two screws. The tongue is easily
removed for maintenance or replacement without removing the entire unit.
Electrical resistance heaters (calrods) can be placed along the sides or
underneath the manganese castings.
3. METALLURGY
Edgar Allen’s flexive tongue turnouts were introduced in 1992 for the Sheffield
Supertram system in the UK. The Supertram system opened in 1995, and
Edgar Allen supplied turnout points and castings for six 25m and three 100m
radius turnouts. Both the manganese points and bodies are rated at a Brinell
Hardness of approximately 250 at manufacture – Edgar Allen reports that in the
five years after opening of the Supertram system the bodies and points have
work hardened to approximately 400 BHN. The castings and points are
manufactured using austenitic manganese steel.
4. TURNOUT DESIGN CONSIDERATIONS
The Hiawatha designers evaluated other aspects of the turnout design that would
ultimately impact the performance of the turnouts. These factors should be
considered for any transit project that requires turnouts in paved areas.
a. Paving
Depending upon whether vehicles will actually drive upon the turnout, careful
consideration should be taken regarding the type of paving to be used around the
rails and other turnout components. For Hiawatha, it was decided to use
portland cement concrete “islands” between the rails, with asphalt cement
concrete infill to approximately six inches from the rail head. This would give
better support to the traffic that could potentially cross the turnout and was
considered more aesthetically attractive. Based upon discussions with other
transit properties, it was decided that using portland cement concrete against the
rail would not perform well due to movement of the rail against the concrete,
potentially breaking the concrete. It could also present problems with
replacement of rail and turnout components and might not provide satisfactory
rail to rail insulating properties.
b. Electrical Isolation
The embedded track for the Hiawatha system was insulated from the ground to
protect adjacent utilities and structures from electrical corrosion by using the
rubber boot system manufactured by the Iron Horse Corporation cast in portland
cement concrete. The turnouts, however, could not use the boot system due to
the irregular size and shape of frogs, switch castings, and plates. As a result, the
embedded turnouts were constructed in insulated concrete “tubs”, which are
comprised of reinforced concrete with a rubberized isolation liner with protection
board. The rails as they enter the tubs are themselves isolated with bulkheads
perpendicular to the track. The turnout materials, including rail, plates, frogs,
switch machines, switch castings, and the concrete islands discussed above are
installed in the tub. Rail and switch components are mounted on elastomeric
grout pads that provide additional insulation and shock resistance.
c. Switch Machines
The Hiawatha LRT system, as currently designed, has six crossovers in
embedded track. Four crossovers are powered using Hanning & Kahl (H&K)
switch machines, and the other two crossovers use hand throw turnouts. Spring
switches, although common on other systems, were not utilized for operational
reasons.
The power switches are powered by H&K Model HW(E)61V. The switch
machines are located between the switch casting and each switch machine box
is drained with non-conducting PVC pipe to the storm drainage system. Power
and signal conduit to the switch machine and turnouts run through the isolation
membrane and are insulated. Both bonded and “poly bar” insulated joints are
used and are identical to those in standard track applications. Hand throw switch
boxes are also located between the switch castings.
d. Switch Heaters
Since Minnesota winters are long and cold with plenty of snow, the embedded
turnouts will be heated using electrical resistance heaters (calrods). There was
considerable discussion within the design team regarding whether to install the
heaters along the side of the switch castings or underneath. There is no ideal
way to heat the switches, since the heaters will be in contact with other materials
such as asphaltic concrete or elastomeric grout. Discussions with other transit
properties revealed that both methods of heating are used. The Hiawatha
designers decided to place two heater rods in metal conduits under each casting,
above the concrete and far enough away from the elastomeric grout pads to
avoid overheating of the pads. The heater rods can be pulled out of the conduit
and replaced without removing the switch casting or conduit. By installing the
heater rods underneath, it avoids the inevitable melting of asphalt concrete along
the side of the casting, and the dual heater rods provide redundancy.
e. Drainage
The paved areas in downtown Minneapolis and suburban Bloomington are
generally flat and drainage is a challenge. In warm climates, poor drainage in
embedded turnouts can be a nuisance – in Minnesota, with long periods of sub-
freezing weather, frozen switch mechanisms could put a turnout out of service for
months. The embedded turnout tubs and embedded trackbed are provided
surface drainage and the switch machine boxes are also drained. Switch heater
rods will be placed in the switch drains to keep them clear during the winter
season.
5. CONSTRUCTION METHODS
At the time of this writing, the embedded turnouts for the Hiawatha LRT project
are under construction. The construction of the turnouts is very time consuming
compared to construction of ballasted turnouts and takes several months from
start to finish. The construction of the turnouts can be discussed in terms of the
three concrete pours, as follows:
a. First Pour – The Isolation Tub
The reinforced concrete isolation tub is constructed on select base material, with
conduits in place for drainage and systems (traction power and signal) facilities.
The first concrete pour is eight inches thick, with walls up to top of rail
longitudinally on the field side of the rails, and with the concrete bulkheads to top
of rail between the rails at the ends of the tubs.
After placement of the concrete, the isolation liner is placed on the bottom and
sides of the tub. The rubberized liner is heat welded and the welds electrically
tested. The liner is installed with one-eighth inch thick protection board on both
sides of the liner.
The following exhibits show the details of the turnout design. The three stages of
construction, labeled “First Pour,” “Second Pour,” and “Final Pour” are included.
The concrete pour for that phase is cross-hatched in the cross sections.
Hiawatha LRT Design-BuildPlan ViewIsolation Tub for Crossover
BITUMINOUSPAVEMENT INFILL
TRANSVERSEC OF CROSSINGL
B
B
CONCRETE(TYP.)
C TRACKL
1’ MIN (TYP.)
FLOW 2’-0”MIN. 9º31’38” PITO
NO. 6 TURNOUT WITHDOUBLE TONGUE SWITCHPER SPEC. 17031 (TYP.)
17’-1” 4’ MIN.
A
A
C
C
PS
SWITCH AND COVER PLATE
SWITCH DRAIN(AT EACH SWITCH)
DETAIL “A” (TYP.)SWITCHBLOCK OUTCONCRETE (TYP.)
2’-0” (MIN.)TUB DRAIN(AT LOW END)
CONCRETE (TYP.)
FLOW
BITUMINOUSPAVEMENT INFILL
C TRACKL
EMBEDDED TRACK SLAB
38’ MAX.POURED POLYMETRICJOINT SEALFOR LESS THAN 2”GAP IN RAIL HEAD(TYP.)
12’-3
”4’-0”
MIN.
7’-6”
MAX.
Exhibit I, Plan View, Isolation Tub
Hiawatha LRT Design-Build
Isolation TubFirst Pour
#4 REBAR@ 18’ ON CENTERALONG C TRACKC TRACKL C TRACKLL
CURB AND GUTTERAND SIDEWALK
ELASTOMER GROUT (TYP.)
6” AGGREGATE BASE, CLASS 5 (02211)6” NON-FROST SUSCEPTIBLE SOIL
ISOLATION TUB
1
3
1
6
PROTECTIONBOARD
ISOLATIONMEMBRANE2
2
4” x 4”CHAMPHER
BITUMINOUSPAVEMENT(TYP.)
3-0”6”
MIN.1% 1%
4’ VARIES 4’3-0”
6”MIN.
1%
4’-8 ½” GAUGE 4’-8 ½” GAUGELEVEL(TYP.)3” CLEAR
(TYP.)
BITUMINOUSCONCRETEIN FILL (TYP.)#4 @ 18”
(TYP.)2 1/2” CLEAR(TYP.)
PCC
CONST.JOINT(TYP.) INSET
(TYP.)
Exhibit II, Isolation Tub, First Pour
Hiawatha LRT Design-Build
TUB EMBEDDED TRACK SLABSection D-DEnd of Isolation Slab
TOP OF RAIL
6” 2”
ELASTOMER GROUT SEALFOR MEMBRANE EDGE
1” EXPANSION JOINTMATERIAL AND SEALER
#6 BETWEEN RAILSTUBREINFORCEMENT
6”6”
EPOXY COATED#9 DOWEL @ 12”
2’-0”1’-0”#6 TOTAL 3
9”
SPECIAL TRACKWORKPLATE WITHE.G. PAD 1
#5 BETWEENRAILS
115RE RAIL
BEND UP #6 BARHORIZ. REINF.
4” x 4”CHAMFERISOLATION SLAB
ISOLATION MEMBRANETUB (WORKING SLAB)
PROTECTIONBOARD
Exhibit III, End of Isolation Slab
b. Second Pour – Support for the Rail
A strongback system and jacks are used to support the assembled turnout in the
proper line and grade prior to the second concrete pour. Anchor bolts and
inserts for the turnout plates are placed in the plates in position for embedment in
the concrete. Plastic caps are placed over each of the e-clips to prevent fouling
during the concrete pour and for protection afterward when asphalt is placed
against the rail.
Hiawatha LRT Design-Build
Isolation TubSecond Pour
#4 REBAR@ 18’ ON CENTERALONG C TRACKC TRACKL C TRACKLL
CURB AND GUTTERAND SIDEWALK
ELASTOMER GROUT (TYP.)
6” AGGREGATE BASE, CLASS 5 (02211)6” NON-FROST SUSCEPTIBLE SOIL
ISOLATION TUB
1
3
1
6
PROTECTIONBOARD
ISOLATIONMEMBRANE2
2
4” x 4”CHAMPHER
BITUMINOUSPAVEMENT(TYP.)
3-0”6”
MIN.1% 1%
4’ VARIES 4’3-0”
1%
4’-8 ½” GAUGE 4’-8 ½” GAUGELEVEL(TYP.)3” CLEAR
(TYP.)
BITUMINOUSCONCRETEIN FILL (TYP.)#4 @ 18”
(TYP.)2 1/2” CLEAR(TYP.)
PCC
CONST.JOINT(TYP.)
6”MIN.
INSET(TYP.)
Exhibit IV, Isolation Tub, Second Pour
The concrete is placed within the tub to a level one inch below the turnout plates.
The one inch vertical gap will be filled with elastomeric grout, which provides
electrical isolation and support for the plates, after the second pour concrete is
cured. The reinforcement in the second pour is of course separate of the
reinforcement in the first pour due to the design of the isolation liner, but hoops of
rebar are included in second pour to secure the “islands” above, which are part of
the third pour.
c. Third Pour – Concrete Islands
While elastomeric grout is being poured under the turnout plates, systems work
can be done, including bond wires to rails for power feeds and signals, setting
and adjusting the switch machines, and installing and testing the insulated joints.
The third and last concrete pour brings the concrete between the rails and on the
field side of the rail against the isolation liner to top of rail.
Hiawatha LRT Design-Build
Isolation TubFinal Pour
#4 REBAR@ 18’ ON CENTERALONG C TRACKC TRACKL C TRACKLL
CURB AND GUTTERAND SIDEWALK
ELASTOMER GROUT (TYP.)
6” AGGREGATE BASE, CLASS 5 (02211)6” NON-FROST SUSCEPTIBLE SOIL
ISOLATION TUB
1
3
1
6
PROTECTIONBOARD
ISOLATIONMEMBRANE2
2
4” x 4”CHAMPHER
BITUMINOUSPAVEMENT(TYP.)
3-0”6”
MIN.1% 1%
4’ VARIES 4’3-0”
1%
4’-8 ½” GAUGE 4’-8 ½” GAUGELEVEL(TYP.)3” CLEAR
(TYP.)
BITUMINOUSCONCRETEIN FILL (TYP.)#4 @ 18”
(TYP.)2 1/2” CLEAR(TYP.)
PCC
CONST.JOINT(TYP.)
6”MIN.
INSET(TYP.)
Exhibit VI, Isolation Tub, Final Pour
The approximately six inch gap between the ball of the rail and the island
concrete will be filled in later to top of rail, along with a flangeway, with an asphalt
concrete infill to finish the work.
6. SUMMARY
The embedded turnouts for the Hiawatha project will be operated in a harsh
weather environment and will be critical to the successful operation of the
system. The Edgar Allen turnouts, while fairly new to North American transit
systems, have been proven to be reliable in Europe and its design should
perform well in street conditions in Minneapolis and Bloomington.