UPDATE: A bug fix has corrected the quadrupled the clay depth shown in the map at the end of this post.
For Central Canada’s high-speed rail project, the Montreal to Ottawa segment seems like one of the simplest to design. Multiple existing and historic right-of-ways exist over relatively flat and rural terrain, and it seems to be simply an issue of reusing as much existing infrastructure as reasonable to keeps costs low, limiting the number of new grade separations required, and easing track geometries where the benefits exceed the costs to achieve a maximum speed of around 300 km/h. The most complicated part would be in determining how to address the needs of Canadian Pacific Kansas City freight when and where choosing to displace the company from it’s right-of-way.
There is however a complex set of hidden dangers.
Slide Risk
Growing up in British Columbia, the stories of toil and disaster that characterized the building of the railways through the Fraser Canyon, or the unstable shale roadbed of the Kettle Valley Railroad along the steep slopes of the Coquihalla River brought to mind rough terrain. Not the arrow-straight flats of the Ottawa area.
That’s before I learned about Ottawa and Quebec’s particularly unforgiving local sediment deposit, Leda clay. This is a mix of the ancient silt deposited at the bottom of the Champlain Sea, held together by salt. Once the salt leaches away and the clay is disturbed, it turns to liquid. Large parts of Ottawa and the Saint-Lawrence Valley are built on top of this, and it has wiped out entire villages.
In considering the arrow-straight rail line from the Streetview above, it directly abuts, but isn’t contained in, a known landslide risk area along the Nation River west of Casselman. Given that the line is still 350 m from the river’s edge and much heavier trains than will operate for HSR have operated along this line for a century without causing slides, it seems that this shouldn’t be a problem. However, there is a related problem.
Weak subsurface
Marine clay makes for a poor foundation material. It is expansive, variably hard or soft, brittle, and fatigue prone. The embankments or foundations for rail beds or structures such as overpasses should be expected to settle significantly in soft clay. The brittleness and fatiguing mean that setting can happen unevenly or after many years of service. The clay can also shift in response to changes in ground water, which can be instigated by fractures generated from ground settling.
There are ways to engineer around some of the failings of clay, either by reducing loading with raft foundations or light-weight fill materials, avoiding placing the railway on structures, mixing stabilizers, i.e. concrete, into the soil to make it more firm, or building deep foundations. Highway and rail engineers have learned to deal with this clay, but HSR brings an additional complication.
Supersonic trains
While the speed of sound in air is upwards of 1200 km/h, depending on altitude and humidity, and sound travels even faster longitudinally in dense materials, shear waves can be slower.
This will be familiar to anyone who knows about speed limits on ice roads. If a truck on an ice road approaches the speed of sound for the shear waves, or the system’s critical speed which can be as low as 20 km/h, it can create amplified shock waves that can buckle the surface, with frightening consequences.
In undisturbed Leda clay, this critical speed depends largely on the sound speed, which has been measured near 270 km/h1, such that HSR trains could be expected to produce shock waves. Shallower soft clay deposits may have even lower critical speeds.
There’s a well studied example of these sonic booms affecting a 200m long stretch of railway on a mix of marine clay and organic subsoils in Sweden near Ledsgard, where train speed was designed for 180 km/h. The resulting sonic booms affected both the infrastructure and neighbours. Such shocks could also progressively degrade clay under the railbed, lowering the critical speed and/or damaging nearby structures or banks. Best design practice is still debated, but where linear response of the subsoil can be assumed, a design speed of around 70% of the critical speed is often acceptable. Thus, any HSR line over untreated deposits of Leda clay would be limited to around 190 km/h maximum or less, regardless of track curvature.
While the critical speed in Leda clay may be higher than that found at Ledsgard, Leda clay has less damping than other soft soils. This both allows for more confidence in the linearity of the soil’s dynamics, but also that any vibrations generated will tend to travel farther from the railway, increasing impacts on nearby structures and users.
Extent of clay
Given the multitude of problems with Leda clay, a right-of-way that avoids thick near-surface deposits of clay vastly simplify HSR construction.
Unfortunately, such deposits are largely unavoidable in either the Vaudreuil-Soulanges areas west of Montréal, or the eastern approaches to Ottawa.
Looking at this map, I no longer wonder why Highway 417 deviates so far south from the Ottawa River.
In a later post, I hope to examine some of the consequences of these clay deposits on the design of Central Canada’s high-speed rail.
- At pressures reached at depths below about 10 m, water is force out of the clay, and the critical speed increases slightly, to around 360 km/h. ↩︎
