What a track bed is for ? -
Transfer the load from the track to the
Hold the track in place
Allow for ground movement - stettlement/heave +
Drain away water + stop vegatation growth which can
make the track bed
There is no standard design that fits all situations
so it is worth looking at what the full scale railways do and learn from
Lessons from Standard
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 scales
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"
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 smaller gauges.
What is geotextile fabric
Geotextile fabric is a material used for road bases and has also
found fame for use in weed control in gardening. Basically it is a material
woven from a rot resistant plastic. The weave is fine enough to stop sand and
plant growth going though but coarse enough to let water through. One of the
trade names is Terram but there are several different manufacturers and grades.
Make sure you use a heavy grade.
Do I need it?
Stops ballast sinking in to the soil/clay & stops the
soil/clay ouzing up through the ballast
Stops plant growth coming in to you ballast. Soil and plant growth
in ballast breaks down its bond and so your track formation loses its
Increases the load bearing strength of your track bed
One method to provide a more stable sub base is to lay ballast, to
say a depth of 30mm, on the geotextile fabric and then lay another layer of
geotextile fabric. This forms a sandwich structure which has a greater bearing
A Suggested Track Formation for 5"
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
Depth of ballast will depend on many factors -
Ground conditions - load bearing capacity
Track load - obviously 1ton trains require a stronger designed
Local conditions. e.g. Clay areas where the ground may heave as it
takes in water during the winter and sink in the summer.round can change
dramatically through the seasons.
Heavy vegetation based soil that can sink and compress over
Sandy silt based soil that can wash away with heavy
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.
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
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 20mm.
||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
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.
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 (make holes in it) 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 cover with fabric/polythene and cover with soil. The gaps in
the hardcore act as a store while the water soaks away.
Laying Track On Concrete
To some the ultimate may seem to lay track on concrete to avoid
any future maintenance issues. On paper it may seem a good idea but it can be
costly and still give you major problems.
Concrete track formations require a much deeper footing to remove
soil layers that are prone to movement. One major problem with concrete is it
isn't flexible. Large slabs can often sink or rise and create stepped joints.
When slabs do move the only answer to adjusting the track is to pack it off with
packers or break out the concrete. This operation is labour intensive compared
with adjusting track in ballast.
Where concrete does pay off is in crossings and under track in
station areas where you do not want any settlement.
If you do lay concrete - remember to allow for expansion joints.
Failure to allow for thermal movement can lead to spetacular failures. It is not
unheard of to have a slab suddenly pop up six inches or more.
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