Moving the earth

Take a look at any of the Volume 2 map books in the HS2 Environmental Statement that depict sections of the route that run on the surface, rather than in tunnels, and you will see that the design is dominated by earthworks. These features are a necessary component of any railway, since trains, particularly when running at high speed, require a smooth trackbed that avoids excessive gradients and tight curves in the vertical plane.

Earthworks can either be used to create a “cut”, where the trackbed is below the natural ground level, or for “fill” to raise the trackbed above natural ground level. Where the amount of cut is significant enough the result is a cutting, and significant fill results in an embankment.

Unsurprisingly, I have been taking a close look at Map Book CFA17, which covers Cubbington in Warwickshire, where I live, and the neighbouring parish of Offchurch. My village is fortunate in being built on a ridge that forms the northern boundary of the valley of the River Leam. This ridge is a barrier to HS2, necessitating a cutting up to 12 metres deep to be excavated through it. Although this cutting is now proposed to be shallower than HS2 Ltd originally planned, it should still shield my village from the worst of the HS2 noise pollution, which is why I consider that we are fortunate to be located on the ridge.

However, as the ridge drops away on its southern side to form the valley in which the River Leam meanders, the cutting must give way to embankment to carry the track down to a viaduct that will span the river. Since the trackbed level as it emerges from the southern end of the cutting would be about 25 metres above the level of the riverbed, a substantial embankment is necessary to support the trackbed on a gentle slope down to cross over the river. Also it appears, from information provided by HS2 Ltd, that the viaduct, designed to carry the track a full eleven metres above the riverbed, would be about six metres higher than the minimum required to clear the Leam in full flood.

A similar, but generally lower, embankment is required on the Offchurch (southern) side of the valley. The proposed northern embankment would be about 600 metres long and the southern one would stretch for about 1,100 metres; together, these two embankments would be a very unwelcome intrusion into a beautiful valley. As well as these embankments having a visual impact, the trains running on them will also have the potential to be a significant noise pollution source. Again my village is somewhat fortunate, as higher ground to the north-west of the embankment should shield us from its worst effects. However, our neighbours in Offchurch and Weston-under-Whetherley are not so lucky. Offchurch is particularly badly placed; the village centre is on a hill with an uninterrupted view of the river crossing.

The principal mitigation device proposed by HS2 Ltd for both embankments is what HS2 Ltd engineers have called a “false cutting” when speaking to us, but the Environmental Statement uses the term “raised earthwork” – I prefer the former term, because it describes the purpose of the device exactly, which is to build up earth on both sides of a raised section of track to hide the train as if it were below ground level. The diagram below illustrates, in a very simplified form, the cross-section of such a construction.

Well, I did say that it was simplified! I have assumed straight lines everywhere and that the natural ground is perfectly level. Of course, a real false cutting would have more complex profiles and the ground is likely to be uneven, but the diagram should serve present purposes adequately. For simplicity, I have only shown one half of the structure and assumed that it is symmetrical on the other side of the track; again this is an unlikely assumption, but will serve our purposes.

The gradients of the “internal” and “external” slopes of the shielding embankments in the diagram are not meant to be representative; for convenience I have made them both the same at about a slope ratio of 1:2 (or 1 in 2) – a slope ratio of 1:2 means two metres of horizontal extension for every one metre of height. Volume 1 of the Environmental Statement tells us, in paragraph 5.2.2, that side slopes for cuttings and embankments, which presumably includes the internal slopes of false cuttings, will be 1:2.5, but that this “will depend on local ground conditions and topography, and on the quality of excavated material”. We can expect the gradients of external slopes of false cuttings to vary also with local conditions, which will include whether the slopes will be intended to be used for farming. Paragraph 5.2.3 of Volume 1 advises that “arable slopes will be re-graded to be no steeper than 1:8″.

As I have already stated, the reason for building a false cutting is to shield the train, and possibly the power catenary supports, with a mound of earth either side of the trackway. In the diagram I have called the amount by which the height of the crest of the mound exceeds the trackbed the “shield height”. The height of the crest above the natural ground level is given by adding the shield height to the trackbed height, as shown in the diagram.

I have prepared a simple Excel spreadsheet that allows the effects of changing the basic parameters of a false cutting to be examined. This spreadsheet provides five input fields, indicated by yellow boxes, which I have preloaded with typical values but which you can change.

Unfortunately – or should that be “inexplicably” – the Environmental Statement contains no details of the profile information for any of the earthworks proposed for HS2 Phase 1. Accordingly, it has been necessary for me to make a number of assumptions in order to fill in the input fields of the spreadsheet. For the trackbed height above natural ground level, referred to on the HS2 Ltd drawings as “fill”, I have assumed 7 metres, which is the average trackbed height of the “real” embankment across the northern side of the Leam valley. For the height that the crest of the false cutting extends above the trackbed, which I have called the “shielding height”, I have assumed 8 metres; this should be sufficient to shield the whole of the train and the power gantries. The trackbed width was originally specified as 22 metres by HS2 Ltd, but the parallel access road has now been removed from the design so I have assumed 18 metres. Finally, I have set both the inner and outer embankment slope values at 1:2.5, corresponding to the steepest value that is likely to be used.

For these input values the spreadsheet tells us that the footprint for the false cutting, edge to edge, will be 133 metres. For comparison, the spreadsheet also calculates the width of the footprint for a “simple” embankment; this is assumed to be the trackbed width at the top, with the sides sloping at 1:2.5. Such an embankment has an edge to edge footprint width of only 53 metres, so the land take penalty for using a false cutting in this case is an increase of 151%.

Of course the big advantage of using a spreadsheet is that the impacts of changing input parameter values can be instantly seen and I have to confess that I have been deriving great pleasure from doing just that. I know that it’s a bit sad, but engineers get a kick out of that sort of thing and it’s all fairly harmless.

In the next blog I will explore further what the spreadsheet can tell us.