<great willowgarth civil

Great Willowgarth Railway – Civil engineering 2017

Construction work on the GWR started in 2006, with the building of a station and return loop in front of the garage, together with the first stage of the line down the garden. The track base mainly comprised a concrete slab laid behind, and level with the top of, an existing retaining wall.

The concrete slab was 4” thick laid over well compacted hardcore. In hindsight, this was maybe too massive since, over the years, the concrete began to settle, perhaps because its weight crushed the underlying earth and clay. Initially, exterior grade self-levelling compound or exterior tile cement was laid on top of the concrete, to bring the slab level with the wall, again. But this just added to the weight and, in the end, caused part of the retaining wall to burst, which in turn accelerated the rate of settlement.

So, it was time for a radical solution – demolish the failed retaining wall and re-build to an even MORE massive specification. On the evening after returning home from the Elsecar show, before the light faded, a start was made to remove all the trackwork from the concrete slab and failed retaining wall in front of the garage. The job was finally completed by floodlight and torchlight, to reveal the sorry state of the failed retaining wall and cracked concrete slab, shown in the photograph.
By Friday in the week after the Elsecar show, all the failed retaining wall had been taken down and most of the space behind was dug out. At this stage, one full skip had been taken away, another was full and ready to be removed and there was a pile of earth to put into the third skip. Also completed was most of the excavation for the footings for some new walls. With such good progress, Saturday saw a trip to Marcway to buy some new pointwork, to have ready to lay once rebuilding was complete. Some old pointwork can be salvaged, but this will take time.
After footings for two new walls had been laid, a new rear wall was built from concrete blocks. A problem emerged at this point, when it became clear the old retaining wall was far from level. On measuring, it was found to have had a fall of about 5cm. Whilst this gradient has been comfortably within the power of a typical 16mm scale locomotive, there was a problem in that the new concrete block wall had been laid level. Oh s**t. Ignoring this issue for the time being, a 9” front wall was then constructed, with an inner leaf of concrete blocks and outer leaf to match the existing brickwork of the retaining wall. This was laid on the correct slope (!) and was built on a new alignment, part in front of the old wall (where it was in a reasonable state) and part at an angle to the previous wall.
Finally, the space between the two walls has been filled with rubble from the previous construction, topped off with well-tamped crusher and a new concrete slab. The slab is laid on top of the rear block wall, creating the gradient that should have been built into that wall. Fingers crossed that it corrects the oh s**t problem! Now, time to start relaying some track. All being well, it will be done in time for the 2017 “Steam by Starlight”.

Phil Thompson goes in for proper engineering!!

The (very) long term development plans for the Great Willowgarth Railway were set out in the November 2008 issue of ‘16mm Today’.

The target for 2009 is to complete Phase 2 of the development, which includes Lovetot station, with its triangular junctions at both ends, together with the triangular junction at Busley station (you’ll need to look at 16mm Today for the plans).

The challenge is that all of this has to be built across a steep slope, where the railway track on one side is at ground level and the track at the other is about 90 cms above ground level.

Cue some real civil engineering!! The intention is to lay a number of concrete footings in steps down the slope, with the space between each step being the depth of two or three concrete blocks plus the related mortar layer. A concrete block wall will then be constructed on these footings to achieve the required height. This wall will be faced with real sandstone to tone in with an existing dry stone wall. Part of the block wall will have two rows of blocks, with a gap between, to provide sufficient width for Lovetot station. Once the mortar is sufficiently “off”, the gap between the two block walls will be filled with crusher run and a concrete slab laid on top.

The diagram (not to scale)

shows the proposed concrete slabs, which will vary between around 1m and 8m in length, with slabs 2 to 5 (and part of 6) being wide enough for the station, 1, 16 and 7 to 12 being single track width and the rest being double track width. To complicate matters further, there are gradients to deal with:

Slab 1 (which will carry trackwork at ground level) is at 1 in 60.
Slabs 2 to 5, 8, 9 and 16 (and their block walls) are level.
Slabs 6 and 13 to 15 (and their block walls) are at 1 in 56.
Slabs 12 to 10 (and their block walls) are at 1 in 45.

The photos show the work as it is progressing.

Firstly, the position of slabs 1 to 6 were marked out “on the ground” by digging down 50mm – 100mm. String and pegs were used to mark out the straight lines and radius templates (cut from hardboard) were used to mark out the curves. These templates can be seen in the photos – my set comprises all radii from 4’6” to 7’6”, at 3” intervals. In use, I rest the template on the ground and draw a metal tent-peg on one side to score the ground, then cut along the marked line with a spade.

Each slab is then dug out to subsoil level, to a depth sufficient to provide around 5” of hardcore plus 5” of concrete. There is an element of “best guess by eye” in this process.

Wooden pegs are then hammered into the trench, with their tops set at the surface level of the intended slab. The pegs are marked with a line 5” from the top, so that hardcore (crusher run) can be rammed in place up to the line.

Slab 1 is being dug out and the first pegs inserted. These are rising left to right at 1 in 60. There is a 2-block step down just before the two arms of the Y join up.

Step 1: the positions of slabs 1 to 6 are marked out on the ground and digging out commences.

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The trench is dug down in steps, the pegs ensuring that each step is level. An extra-long peg is hammered in place in the middle of each step and set so that its top is level with the pegs in the step above. Another peg is then hammered alongside, so that its top is two or three blocks (plus mortar thickness) lower than the long peg. The long peg can then be removed and the remaining pegs on that step can be levelled from the first one.

My aim was to start with slab 5, then lay two rows of blocks on it, before casting slab 4. Then lay three more rows of blocks, to reach the level of slab 3 – and so on. Unfortunately, there was a slight ****-up with the pegs and slabs 5 and 4 were laid 1 cm too high. To correct this, the mortar between the bottom five courses is a little thinner than it should be.

Also, some small alterations have been made as the job has progressed:

Slabs 9, 10 and 11 were laid as a single slab.
Slab 6 now includes a large part of the intended slabs 7 and 13. To compensate, slabs 7, 8 and 13 to 15 will be a little smaller.

The double-thickness block wall for Lovetot station starts to take shape.

This wall slopes down at 1 in 45. The trackwork will run underneath Lovetot station, as the branch line to Fosters End.

Slabs 7 and 8 will be cast here – being short and narrow, they should be the two easiest slabs to prepare.

The massive slab 6 took a day and a half to dig out. Over 8m long, it needs to fall at 1 in 56. Measuring this gradient by eye and guesswork, I managed to dig it an inch or so deeper than necessary.

The pegs in slab 6 are 67cms apart. A piece of 12mm ply is placed on the top of the lower peg and then set level with the upper peg, to give the 1 in 56 slope.