One of my goals over the past
summer and now into fall, has been to move toward scenery rough-in. As I started down this path, I quickly
realized I needed to prepare for the tunnels on my railroad. My layout features ten of the prototype’s
twenty tunnels from Oakridge up to Cascade Summit. Even down at Oakridge, there is an important
tunnel through the ridge separating Westfir and Oakridge. As seen in the past several posts, I have
been working actively in the Westfir area.
I would like to flesh this out a bit more with Tunnel 22 and the
ridge. After that, there is a steady
progression of tunnels up the mountain grade.
Key elements needed to be in
place for the tunnels include supports at roadbed level around the portals and
initial tunnel liner, tunnel portals, and the initial tunnel liner. This past summer, I installed the roadbed
supports (bracketing the spline roadbed) and placed mock-up tunnel
portals. The portals were cut from
plywood and placed to check geometry. The
mock-ups have been in place for a number of operating sessions and have not
caused any issues with the most demanding rolling stock on my railroad—long wheel-base
locomotives and autoracks.
Tunnel 22 site between
Oakridge and Westfir.
Tunnel 22 did not require
roadbed extensions, as it already had such a plywood base. I already had portals for Tunnel 22. This is the one tunnel on my railroad built
during the Harriman era. As such, I am
able to use commercial tunnel portals offered some years ago by M-Tech, a Sacramento-area
supplier of various useful models for SP modelers. These portals are shown in the photo above
propped up against the backdrop. Quite
by happen-stance, their geometry matches the portal geometry I designed for the
rest of the portals. I did not
investigate these portals until after I had made the initial wave of plywood
mock-ups. They overlay nicely.
Tunnel 21 site, just RR-East
of McCredie Springs on my layout. I had
to place plywood roadbed extensions to provide support for the tunnel portal
and liner.
The remaining key item for
Tunnel 22 will be the tunnel liner. In
common with the rest of the tunnels on the Cascade Line, the initial bit of
tunnel is lined with a cast concrete liner for about the first fifty feet. The remainder of the tunnel interior is lined
with timber that eventually had gunnite (concrete) sprayed on. I will represent the latter with removable foam-core
panels with strip-wood vertical braces representing the original timber
construction. Dark gray paint will
complete the illusion.
The cast concrete tunnel liners
need a more formal modeling approach. I
have been building plastic masters for molds to make plaster castings for the
tunnel liners and a couple of tunnel portal designs. Additional casting masters are planned for
the rock shed portals, portal wing walls and trestle piers. I chose to cast the tunnel liner as two
halves. This saves mold material and
should permit easier removal of the mold from the cast liner.
Tunnel liner master under
construction. The liner is upside down
here.
Tunnel liner master inside
the mold box. The liner surface was
formed with 0.020-inch thick V-groove siding with 0.080 inch spacing. Random “boards had 0.010-inch thick “planks”
attached to help represent the original concrete formers used for the
tunnel. The trapezoidal box fits onto
mold wall shelves to occupy much of the open space above the liner side.
I am using a silicone RTV
(Room Temperature Vulcanizing) product from Tap Plastics:
https://www.tapplastics.com/ Tap Plastics website includes instructional
videos on the mold making process. The
pair of five minute videos on simple mold making plus mixing the silicone are
excellent. Not noted in the video,
though, is just how fast the mixed (two-part) material begins to turn viscous. It looks like the working time for decent flow
of the silicone RTV is about a half hour. I am watching my initial batch slowly drip
from on high (removing air bubbles) and can see I will need to mix a new batch
to complete the first mold. Note to self—make
smaller batches.
Silicone RTV dripping into
the tunnel liner mold box. Tap Plastics
recommends dripping the mixed material from a high location to remove bubbles
from the silicone.
In my first wave of mold
master creation, I built two tunnel portals.
Both represent the cast concrete portals used in the 1926 construction
of the SP Cascade Line (Natron Cutoff).
Fortunately, all tunnels on the line were completed in 1926, so those
date numerals could be used for all of my concrete portals. I used
Grandt Line’s “Building Date Plaques and Number Set” (300-5219) for the
numerals. I built the basic shape with various
bits of thick styrene sheet and strip. I
bored the hole for the tunnel arch using a 2.5-inch hole saw. This provides a hole with at least a 1/8-inch
margin around the NMRA clearance guage.
Boring the tunnel arch. Front and back plates are tack-glued together
and clamped. The 2.5-inch hole saw is
chucked into my variable-speed drill run at a low speed to prevent melting the
plastic.
The tunnel face used Evergreen
V-groove siding, 0.040-inch thick with 0.080-inch spacing, similar to the
tunnel liner. This sized planking is a
bit bigger than what I see in photos of the prototype portals, but helps me
convey the impression of individual boards used for the concrete forms. I attached “planks made of 0.010, 0.015, and
0.020-inch thick by 0.080 styrene strip to represent the form planks.
Basic tunnel portal master under
construction. The thin styrene strips
representing concrete form planks will be trimmed.
Tunnel portal mold
boxes. The portal masters are glued to
the box base. Box sides are attached to
the base using silicone caulk. The side
walls are also held in by base strips.
The cross-walls are held in place by strips on the ends of the side
walls.
As noted, I have built two
tunnel portals. The first, basic portal
will represent about a half dozen of the portals on the line. The second one has extended walls
representing the rest of the portals.
Castings from this master will be trimmed or cut as needed to represent
the top-angled walls used in for several of the portals. I probably could have saved some effort by
just making this larger portal master, but I needed to be sure of my construction
technique with the basic portal.
No comments:
Post a Comment