Apr10 Written by:Tom Farin
4/10/2008 6:54 PM
San Anselmo - Track Layout
This area of the NPC trackage was probably my biggest challenge for a variety of reasons.
First, this section of the line came first. So techniques for roadbed construction had to be worked out at the same time the roadbed was being constructed. As late as Friday evening of this Labor day weekend, I was undecided on whether to go with gravel or lay a concrete roadbed.
Second, there is the environment. My layout is on a hill. There is a drop of nearly 40" between Sausalito and the tunnel headed out of San Anselmo toward Cazadero. There are two tunnels under my walkway down to the pond, and a water feature that will run through this area.
Third, it is the most complicated set of NPC track other than in Sausalito. Granted, it is not all that complicated. But as the following track plan shows, the San Anselmo contains a wye. It also contains one of the NPC's largest yards.
The track plan was modeled in mirrored form. I began by running the garden hose along the proposed line. As an earlier photo indicated, the hose was marked off in 10' increments. I used a 10' 2x4 and a level to measure grades between points along the line, continuing to adjust the hose locations until grades fell within my ruling grade limit of 5%.
The hose locations where then staked. The photo that follows is a little confusing, but the main line location was in rough form after the staking.
The following image shows the curve that reverses direction of the main line on the way to Cazadero. The stake in the middle markes the center point of an arc. All curves are at a diameter of 10' or greater.
The next step was to find a beginning grade point for excavation. Maybe the most important elevation is the entry point to the basement from the back yard. Once that elevation was established, all elevations and grades were measured from that point. In this shot, the basement entry grade is set and forms have been laid to allow concrete roadbed to be laid.
From there excavations for roadbed proceeded. The objective was to keep grades between no greater than 5%. Most grades are 4% or less. This photo shows excavation in preparation for form installation prior to pouring concrete. Keep in mind that the forms are 3" high so the actual roadbed location will be considerably higher than the trenches apply.
Once the trenching was complete, form construction proceeded.
At his point, form construction is complete.
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