The On-Line Magazine of Rideable Model Railroading
 NUMBER EIGHTY EIGHT

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JULY 22, 2007  

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  Building the North Pacific Coast Railroad 

Timber Culvert Construction
 

David crosses a "timber" culvert on his Willamette
 


Written by Ken Stanfield

One of the interesting things about building outdoor railroads is dealing with the rain and the resulting runoff.  Most of the time a simple piece of pipe under the track is all that is necessary to insure good drainage.  However, I chose to make some of my drainage culverts more interesting and more prototypical by building “timber” culverts.  They are easy to build, robust and have a realistic appearance.  These culverts could have been constructed of treated wood, but I used the same composite material that I used for my ties – Trex.   Trex is a trademark of the Trex Company, Inc. Once installed, they are virtually maintenance free. This is how I did it – keep the ideas you like, discard the rest.

The basic design consists of 2x6s stacked to the desired height in 1 ˝ inch increments.  Measure or estimate the height to the bottom of the ties and divide by 1.5; round up to the next whole number.  A 10 inch culvert will have  7 layers of material with the top layer made with 2x2 Trex stringers to support the ties. A word of caution.  Trex is not recommended for structural members, so I limited my spans to 24 inches.  The
strength of the stringer plus the strength of the rail supports the load without deflection on my railroad.

Below the stringers, the layers are built from 2x6s that have been notched on each end.  My top 2x6 layer is 6 inches longer than my ties.  Each successive layer is 3 inches longer then the one above.    The result is an end support with a 45 degree slope.  (1 ˝ inches out; 1 ˝ inches down on each side)  By notching the ends, the bulk of the 2x6 is hidden by ballast, leaving only the square 1 ˝  inch ends exposed.  Thus the wall on each side supports the track and retains the roadbed.


The basic pieces of a culvert: unassembled; assembled and installed.

The only “trick” in the construction  of the end walls is that the Trex tends to be convex on one side.  I ran each piece on edge through my table saw to remove this crown.  With 2x6 material, I had to run it twice, once on each edge, to provide a flatter surface.  

Once the major pieces were cut, I used quality coated deck screws to secure each piece to the next.  Start with the bottom two layers and work your way up.  I also used some sealant between each layer to help prevent water infiltration.  The deck screws are the weak link in this construction method.   They will rust off before the Trex deteriorates.  Remember to leave space for a ˝ inch hole on each side for a piece of rebar to anchor the supports to the ground.   Drill a hole in the end of the top 2x6 on each side wall.  These need to be loose holes so that the rebar does not disturb the end walls during installation.   Once the two end walls are assembled,  turn them on their backs and attach the decorative trestle bent to the face of each one.  Either 2x2s or tie material is cut to length and each piece secured with two deck screws.  The outside bent legs are sloped out at about 5 degrees.  I used a cross brace on the taller culvert.  While having sides of equal height is convenient, there are times when unequal heights will be necessary.


Water drains through a culvert built six layers high.

Next, locate the stringers on the top layer and attach with deck screws.  For a 2 foot span, I use two 35 inch stringers (5 ˝ inches on overlap on each side). The outside measurement should be about 9 ˝  inches.  The stringers are the top layer of the side walls.  Cut additional 2x2 material to fill in between the stringers and along each side 1 ˝ inches wider than the ties.  These pieces will retain the ballast.  This completes the illusion that the side walls are thin while giving ample support to the stringers and track.  From the photos you can see that once the culvert is installed  the side walls appear to be 1 ˝ inches thick.

Installation can be handled in one of two ways – I have used both.  First, preinstall the culvert and build the roadbed up on each side. This method requires an estimate of  how high the culvert should be.  This works best on straight level sections of track where a slight height miscalculation is not too important. The second method is to  install the roadbed then dig out for the culvert.    This method works best on curves and/or grades.  You have actual measurements to work from.  When I install another culvert, this second method is  the method I will use.  


Method 1: Culvert before roadbed.                                                                   Method 2: Roadbed before culvert.

Carefully center both end walls on the right-of-way and check the distance between them.  Mark the footprint and estimate the vertical distance that each side must be lowered.  Next, excavate a shallow (about 2–3 inches deep ) trench below each wall.  (Check again to make sure the footprint is right.)  Fill each trench with dry concrete. (Be sure your concrete has small rocks – big ones get in the way.)  Place the first wall and level  as necessary.  Next, position the second wall and level as necessary.  Cross check the distance between the walls, the square of the wall with the center line and the level of the walls with each other and the roadbed.  Take your time – this step requires a lot of fuss.  You may have to take the walls out and adjust the dry concrete several times.  Be patient and do a good job.  Once you are satisfied with the result,  carefully insert the four rebars and hammer them flush with the top 2x6 to pin the end walls in position.  Be careful not to disturb the end walls during this process.  When the end walls have been installed to your satisfaction,  gently water the dry concrete.  The concrete will set up in time and provide a firm footing for your culvert walls.

With the walls and stringers in place, it is time to finish-grade the roadbed and install track.  Again, I have done this two different way.  Method one, I prebuilt the deck with the ties attached to the stringers.  Then I installed the stringers and deck as a unit.  The ties are an integral part of the culvert structure.  Method two, I installed the bare stringers then placed the track with ties installed on the rails.  This is now my preferred method.  Long term maintenance is simpler and installation of the rail to the ties occurs in the shop instead of the field.  Once the track is installed, a few deck screws through the ties and into the stringers holds the track in place.  If fine adjustments are needed later, use metal shims under the stringers to level the track.


Stringers with ties attached                                                                                 Stringers without ties

A few notes about design variations.  The double culvert pictured above has standard end walls as previously described.  The center section is a very solid support also made from 2x6s.  The ends of each layer are U shaped.  Ballast is used to fill the ends and give the appearance of a double wall.  The two foot span limitation could be extended if steel stringers were substituted for the 2x2 Trex.  Attaching the ties to the stringers would be a challenge but the result would be a deck girder bridge with timber end walls.


Troy takes his home built switcher over the new double culvert.
(The “wood” guard rails are not yet installed.)

 

Because the culverts are resting on concrete, over time the track will tend to settle a little on each side.  On-going maintenance requires the addition of ballast and a little track leveling.  The only other maintenance is weed control under the culverts.  These “timber” culverts are a great addition to the North Pacific Coast Railroad.

Be Safe

If you attempt a project like this, you are responsible for calculating the load handling ability of your culvert.  Failure of a culvert can result in injury or death. The author and this website are not responsible for checking your calculations or workmanship.

 

Written by Ken Stanfield

 

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