TUNNEL PREPARATION -- 1 -- MOLD MAKING

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.