Track Bed Design
Track Bed Design |
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Lessons from Standard Gauge
The above diagram shows a suggested make up for a standard gauge track bed. I have shown it here as it incorporates many features that you have to deal with in miniature railways.
The minimum recommended ballast depth is 150mm. Given a standard sleeper is only 125mm thick ( railway sleeper is 250 x 125 x 2600mm) that could easily equate to 30mm under the sleeper in 5" gauge.
Geotextile fabric has been used to stop the sand blinding sinking in to the soil below. Apart from the sand blinding it is the same for 5" gauge.
One of the most critical things is the different variations for dealing with welded and jointed track and thermal movement. This is often overlooked yet the same expansion rates apply whether it is standard or miniature railway. The sleepers are clearly entrenched in to the ballast to stop them moving. This should also be adopted in 5" gauge.
A Suggested Track Formation
The above diagram is one suggestion for laying 5" gauge track in areas where the ground is fairly dry and stable.
As the ballast is not true scale and the forces being applied are different to full sized track, a ballast shoulder would be pretty in affective. On the above diagram and picture below, it can be seen that the block pavers are the key to stopping sideways movement of the sleepers.
Not only do the block pavers help keep the ballast in they also act as a boundary for the garden and grass. The use of thin timber boards means that the grass grows over and drops in the ballast. The concrete haunching under the blocks helps hold back the garden/grass. Where we laid the blocks straight on to the soil there has been a tendency for them to sink and tip over. The track in the above picture near the camera is slightly too low and in need of lifting.
In an area where the sub base soil is weak or even too strong i.e. clay, it may be necessary to dig down deeper. The deeper you dig the less the ground is susceptible to changes due to season variations.
Examples include -
Clay areas where the ground may heave as it takes in water during the winter and sink in the summer.
Heavy vegetation based soil that can sink and compress over time.
Sandy silt base soil that can wash away with heavy rain.
While you may be inclined to dig down half a metre and bring it up with a compacted hardcore or crushed stone, you need to weigh up and decide if it is worth the effort. A track bed as illustrated above can still accommodate ground movement and if all it takes are a few re-adjustments a year to re settle the track is it worth moving tonnes of soil and other materials with no guarantee that the track still might not move.
Ballast
Is the name of the stone used around and underneath the track. Its main functions are -
Distribute the load from the sleepers to the ground below
Allow drainage of the track
Prevent lateral and longitudinal (sideway and lengthways) movement of the track
Provide an affordable material for packing and levelling track
Your choice of material is usually determined by what you can get hold of at a reasonable price. It wants to be a crushed graded stone and not rounded gravel based stones. The sharp edges of crushed stone interlock to form a better bed for transferring the track loads. We would suggest the stone wants to be graded to a maximum size of no greater than 20mm.
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Left - Monmouth Green stone chippings |
Some of our early Monmouth Green was of a smaller size and included much smaller pieces. While it locked together to form a solid base the smaller size meant that it held a lot more smaller soil particles that had strayed on to the track bed. Soil in the ballast encourages weed growth and reduces drainage which in turn can see sleepers wetter for longer which can lead to rotting.
Living in the suburbs of London our choice of material is limited and expensive. We initially had two 1 ton bags of Monmouth Green, from B&Q, but now buy top up smaller bags as and when. An interesting economic consideration is that a small bag of ballast, from B&Q, is nearly twice the price of the equivilent volume in concrete (without labour considerations).
On standard gauge lines track beds are often lifted around the 20 year mark and the ballast washed and re laid. This is to remove the smaller particles that can affect the ballsts performance. With a garden railway which borders flower beds the intrusion of soil can be more higher, so it may be worth following the fullsized practice. Where we have lifted track we have dug out ballast , washed and sieved it and it is surprising how much muck comes out.
Drainage
If your garden is susceptable to minor flooding or is generally damp all year round, it may be necessary to provide drainage. Introducing a ballast filled trough (the track bed) through damp ground will drain the surrounding ground. If your line is on an incline then you may experience ponding at the low point of the line.
The diagram above shows a track bed with a perforated plastic pipe below the track. The pipe can then be run to a low point such as a soak away or lower part of the garden. It shouldn't be run in to domestic drains as most water boards do not allow this.
To create a simple soakaway simply dig a large hole, line it with a polythene or better still a geotextile fabric and then fill it up with hardcore to around six inches from the groound. Run the pipe in to the hardcore then cove with fabric/polthene and cover with soil. The gaps in the hardcore act as a store while the water soaks away.
Next Stage -Clearances
