S5C7: Roads & Railways

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a) Letting traffic move

The various sections of Chapter 1 on trains and pipelines and Chapter 2 on cars and trucks dealt with land transport technologies and, whilst the topic of the infrastructure making these possible was broached repeatedly, the focus was squarely on the vehicles themselves with the intent to dedicate a full chapter to roads and railways later (there is comparatively less to say about pipelines), which is now. As usual, we will think through both the conceptual aspects, such as form and function, as well as the structural ones. The theoretical, and the experimental if you will.

Unlike maritime and air transport using virtual traffic lanes, land transport cannot abstract the tracks upon which vehicles must travel. This is a consequence of topography, land property rights and considerations relating to stability, traction and friction; more specifically we want surfaces that can support the stress of weight, with low friction and decent traction. Thus, one should think of roads as the infrastructure facilitating car, motorbike and truck traffic. When a road branches off to serve specific addresses, whether it is a shop, a house or a warehouse, this becomes a street. The distinction being that a street is not designed to accommodate high-volume traffic, it is not a main line of communication.

Different configurations can be used to direct and manage traffic safely, starting with a line separation or a physical “reserve” spatially separating two single lanes or two multi-lane sets with traffic moving in opposite directions. The intersections or full-fledge crossings can use hard or soft stops, including traffic lights, or roundabouts can be installed to theoretically avoid the need to stop traffic. Each comes with pros and cons in terms of cost, safety and throughput. When the speed or volume of traffic is too elevated, then the more costly but much more efficient alternative of grade separation is called for; we already saw this principle at play in S5 Section 1.c on urban rail transit and it consists in making use of the vertical axis to channel traffic over two separate horizontal planes. This generally involves ramps, elevated freeways, underpasses or bridges. To cater for all cases in a safe manner and be able to allocate legal responsibilities in case of incidents or accidents, road traffic is subjected to rules and policing.

Railways are significantly different because there are not as many stations, there is less branching off involved, and the traffic is overseen so stops at stations are a known variable and accounted for in the scheduling of other trains. This is even more central to the functioning of urban rail transit where trains can be held up by other trains still on the track ahead. In addition, unlike roads that can connect easily to one another, most railway tracks eventually stop, forming a dead-end. This isn’t really problematic for the reason that railway traffic is regulated and is overseen within and between specific geographic areas. Nor is it a practical challenge; wagons can travel in either direction and the trains can consist of two locomotives, one at each end, or a locomotive can be detached and moved to face another direction using a turntable.

Railways also use gradient separation but mostly to avoid interfering with road traffic and seldom in lieu of a railway junction (those crossings using a bridge are called flying junctions). Most of those crossings involve the splitting or joining of tracks using turnouts (also called switches) relying on the lateral motion of a rail segment to align with one or another track. Less frequently, there are level junction involving an actual track intersection. In those cases, signalling is paramount and is interlocked so that there can never been two greenlighted tracks crossing each other. This type of intersection is nicknamed diamond junction because it involves 2 parallel tracks on one axis and 2 parallel tracks on another, thus forming 4 crossings in a parallelogram shape.

b) Road surface and foundation

Designing roads is not merely about drawing the shortest possible between two points, it involves the careful consideration of physical variables such as the maximum gradient (the degree of incline of a slope) practical for the intended type of traffic – thus it would not be the same for a secondary road used by cars only and for a highway used by a lot of heavy trucks – and the cost of construction which may increase dramatically depending on the topology of the route.

The main ways around steep gradients are cutting through, bridges and tunnels. In the context of earthworks, a cut is the removal of a vertical portion of soil or rock to make way for the road without it having to rise to the level of the surrounding surface. The sides can be nearly vertical in rocky, stable terrain, or may need to be trimmed back and stabilized. Bridges permit the spanning of a chasm or temporary sharp drop in altitude such as that experienced when crossing a valley or river and we will spend more time on their structure in section d) while section e) will be dedicated to tunnels, an underground channel providing a solution to the opposite problem, being the traverse of areas where the land experiences a sharp vertical rise that can’t practically, or economically, be dealt with using an open air road alternative.

As for dealing with horizontality, or lack thereof, the solution lies in the building of embankments, which is the extension or creation of a horizontal surface with a supporting wall or earth bank.

Roads need to be built to sustain wear and tear with the objective of providing a safe and effective surface for road vehicles to drive on. This implies water drainage and preventing organic growth from deforming or weakening the structure but wear and tear will occur over time, hence road maintenance is part of the normal lifecycle and its frequency will be a function of the environment – for example frequent and heavy precipitations or the freezing and thawing of materials – as well as the inherent qualities of the road foundation, sub-surface and surface layers.

Once the topsoil is removed, the native material is compacted to provide structural support and enhanced load-bearing capabilities to the road proper. This is called the subgrade and it acts as the foundation. The type of material it consists of is an important variable in terms of preventing soil shifting and spreading the weight of vehicles. Above that, the first layer to be laid is called the subbase and its primary role is to reinforce the road structure using granular materials such as sand, crushed stones or gravel while including drainage channels. This layer may safely be omitted for pedestrian or bicycle pathways not experiencing heavy loads.

The upper layers of the pavement can technically be further sub-divided but for the sake of simplicity we will only consider the base and surface layers here. The base is commonly made of aggregate consisting of coarse particulate material of various kinds such as basalt, gravel, sand or crushed stone. These can potentially be bound together with some cement or asphalt and, besides load support, the aggregate must minimize the tendency of the surface layer to develop cracks or become uneven. Finally, the surface layer is the one vehicles interface with, therefore it should not be too slippery so wheels can develop traction, and it must be relatively smooth and as even as possible. Because it is the most exposed, the surface layer is also the one requiring the most maintenance.

The most frequent types of road surfaces are as follows:

Dirt roads are the pavement-free version of roads and are essentially exposed subgrade material. They are cheap to build but deform rapidly when experiencing load and precipitations. Thus, they require more frequent maintenance and can’t accommodate heavy traffic in a sustained fashion and the uneven surface doesn’t make speedy driving a safe option.
Gravel roads are cheaper to build than sealed ones, their surface layer is made of crushed stone and sand, possibly directly atop the subgrade materials, and a binder such as silt or clay can be added to improve stability when regular stress is being applied. Gravel is a generic term referring the aggregation of rock fragments between about 2mm and up to 6cm for the coarser elements.
Asphalted roads consist of crushed stone as pavement supplemented with a bituminous binder to keep the material together. Bitumen is the viscous residue of the crude oil distillation process; it is also referred to as asphalt or tar. Because of this, it is not as hard as concrete or stone.
Concrete is made by mixing together cement and aggregate. Cement is typically made with calcium oxide (lime) or calcium silicate and the addition of water makes the substance fluid, and adhesive. As it dries, it binds the aggregate into a hard solid. It is extremely popular in the construction industry.
Before asphalt and concrete, the surface of choice for heavy-duty roads was cobblestones. Those may or may not be quarried in set, regular shapes. Nowadays they are retained, in particular in villages and city centre, for their historical charm.

c) Railway tracks

In the early days of the train, the rails were made of wood and then cast iron. Not so today, the material of choice is steel due to its strength, durability and low friction. Of course, the main exception to this is the maglev technology, a portmanteau of magnetic levitation. It uses powerful electromagnets to counter gravitational forces and avoid the need of having wheel-to-rail contact and the friction that comes with it. So maglev trains can be faster than trains gravitationally bound to steel rails but the technology is energy intensive and requires dedicated infrastructure; I have included a link to the relevant Wikipedia entry at the end of this chapter if you wish to know more about it.

With the relative lack of friction of steel on steel compared to rubber on asphalt comes a lesser ability to handle gradient so, even if many of the construction aspects are similar to those of a road such as the use of various layers ensuring proper load bearing capabilities, water drainage and resistance to terrain deformation, the design of train tracks lacks flexibility and often calls for the adjustment of the topology. Accordingly, one can expect more bridges and tunnels along the route. These aspects, including the maximum degree of curving allowed, is even more paramount for tracks welcoming high-speed trains. Curves should be gentle and the tracks need to be crossing-free and inspected more often for safety reasons – all this increases the construction and operational costs associated with high-speed rail.

The prevalent shape for rails is that of I-beams. Just imagine a bulky slightly rounded top with a narrower vertical portion below that and a spread-out base, significantly wider than the top. This shape is known throughout the construction industry for its strength although, for building, it is typically symmetrical and can look like a short “H” lying on the side. It is also well known that, because metal dilates with temperature, there is some spacing left between rail segments to prevent them from being deflected sideways or upward as they expand and this feature accounts for the regular noise of travelling trains.

What is less known however are the following two characteristics ensuring that trains stay on track, literally. The first is that the wheels of the locomotive and wagons each have a vertical protrusion extending down to line up on the side of the rail top. For trains, these flanges are located on the inside part of the wheel and they counteract the centrifugal force experienced in curves by pressing on the inside of the rail, also causing some metallic grating noise. The second also has to do with turns and maintaining stability; as mathematics would have it, the distance travelled by wheels on the outside track of a curved line is longer than that travelled by the wheels on the inner side. This is problematic because the wheels on either side are locked onto a common axle so there can only be one rate of rotation. This calls for some asymmetry somewhere in the wheel design and indeed wheels are not cylindrically shaped with parallel edges, they are marginally conical with the wider part on the inside of the track – you will have to look carefully but it is the reality. This also means the wheels will automatically centre themselves and generate temporary centripetal force when travelling at speed along a curved track. You may wonder what this means in trajectory; doesn’t this create some kind of wobble? Indeed, it does. The wheelsets of trains tend to shift from side to side. This is called hunting oscillation and I include a link to the Wikipedia entry in section g) if this puzzles you and you wish to know more.

We skipped over this at the outset of this section on the layering of a railroad, so now is a good time to highlight a couple of structural differences distinguishing rail from roads. The trackbed is often made of ballast, a type of very coarse gravel or crushed stone. It drains very well and has excellent load bearing capabilities. In addition, in order to better transfer the weight of the train from the rails and onto the ballast as well as to keep the rails in place and at the same distance from one another, the tracks are tied to concrete slabs or short wooden support beams (sometimes called sleepers) laid perpendicularly to the steel rails and resting on the ballast.

Also, just like some countries drive on the right side of the road and others on the wrong side, so too rail tracks can have different gauges, being the distance separating the two rails, or more exactly the distance between the inner sides of the top parts of the rail beams. Historically, rail networks developed in a fragmented way with no or very little international connectivity. Over time however, there has been a push for standardization and the 1.435m distance has earned the coveted title of “standard gauge” with spacing inferior to this number being called narrow gauge and the wider ones named broad gauge. North America, China and most of the Middle East and Western Europe use standard gauge. Some track sections can cater for two different gauges by adding one or two more rails to avoid transhipment of freight or passengers. Alternatively, trains can be fitted with variable gauge axles and the width is slowly adjusted at the location where the break of gauge occurs.

A final aspect previously mentioned in S5 Section 1.c on urban rail transit is the concept of third rail or live rail generally supplied with direct current to avoid the need for overhead electric lines, a valuable advantage in urban environments for aesthetic and practical reasons. This additional rail can be placed between the two guiding rails or to the side.

d) Bridges and directional forces

We have already seen the purpose of a bridge is to span an obstacle or substantial vertical drop and avoid having to design a much longer road or railroad route, if that is even an option. What I omitted to mention is that in the majority of cases this is done without blocking the potential flow of traffic or water underneath the structure.

Very much like roads, bridges come in different shapes and materials depending on not merely the purpose they serve but also the forces they will bear or may be subjected to. These forces include more than the standard load of passing trains, trucks or pedestrians; to this can be added vibrations, wind gusts, and the weight of the bridge itself. The main forces that need to be resisted are:

Compression, a combination of inward action and reaction with a zero-sum vector. A good example would be the weight carried by columns and the weight of the column itself.
Tension, which is the opposite of compression and is about pulling forces, directed outward. Tensile strength is the ability of a material not to break when being pulled.
Shear is the stress applied perpendicularly to the main axis of a material. Going back to the example of a vertical column, it would be a sideways, horizontal force. The technical definition is “coplanar with a material cross-section” and it is measured in pascal, like pressure.
Torsion shares the same root as torque so one can think about is as a rotational stress or angular deformation. Using the column example once more, imagine the bottom is cemented to the ground and the top is being twisted, meaning torque is being applied; this would be torsion.
Bending is easy to visualize, yet less obvious to explain. Imagine an elongated I-beam like a rail track supported at each of its end and you stand in the middle of the beam. You are applying downward shear on the beam and as the beam sags or bends under this stress, the bottom part will ever so slightly be stretched whereas the upper part will be shortened. This creates a tensile force at the bottom and a compressive one at the top, resulting in a torque-like angular force called the bending moment.

This doesn’t mean materials need to be good at resisting all of these forces, it means that materials need to be selected depending on the bridge design and different ones should be used in different places. Some of the most common materials include ropes and wooden planks for light pedestrian traffic, one can still see many of those in remote mountainous valleys and in wealthier regions the ropes are substituted for steel cables. Early-day bridges used mostly stones and then came cast iron. Nowadays it is mostly steel and reinforced concrete (rebar adds tensile strength).

The simplest of all bridges is without question the plank-like design spanning a rivulet, which finds a grander expression in the form of the single-span beam bridge. Due to the shear and bending stress, the possible span is quite limited and this can be addressed by creating a multi-span passageway with pillars anchored into the ground providing support at each end of a beam. More characteristics of the Roman and medieval period would be the arch bridge where the load is converted into compression forces directed into the pillars. Using cables, it is possible to have the arch structure above the deck and the weight of the spanning section is again translated into compression forces through the arches. Furthermore, if the deck is tied to the arch, then the compression force of the arch creates tension along the deck, helping it to remain horizontal and making the entire structure more integrated, thereby increasing its load-bearing capabilities. This design is called tied-arch bridge or bowstring bridge.

Suspension bridges are in some way the mirror image of arch bridges, the cable between support towers has vertical suspenders tied to the deck and the dead and live loads are thus transferred to the tower as a combination of compressive force (downward) and lateral shear or pulling forces. However, since in this configuration the towers have cables on either side, those sideways forces offset each other. If this is not the case, then the tower should be much sturdier or taller to decrease the shear vector, in either case this proves costlier. A variation of the suspension bridge is the cable-stayed bridge where the cable-suspender combination is replaced by cables anchored directly between the towers and the deck. This means the resulting forces on the structure are compression in the tower and horizontally through the deck and towards the closest tower.

e) Tunnels

During the construction of a tunnel, the exposed material situated above the excavated volume, be it rock or soil, will experience tension created by gravity so it is necessary to use some sort of lining to prevent a collapse. By and large though, the main force to be handled is the sheer weight and resulting compressive force of the material above and around the structure. We have seen in the previous section that arches are a good shape for the purpose of transferring compressive forces, and this explains the shape of tunnels.

The main uses of tunnels are as follows:

Accessing and excavating material such as coal or ore containing precious metals or stones. This is called mining and it can be done at great depth.
Traversing a vertical obstacle, be it a hill or an entire mountain range. The concept can also be carried over for crossing a river and provides an alternative to bridges without the drawback of limited clearance for water traffic. The most extreme example is arguably the 50km-long Channel Tunnel linking the north of France with Southeast England – an extreme feat of engineering.
Tunnels, or in their simplest forms underpasses, provide gradient separation and therefore a mean to increase traffic throughput without creating congestion at an intersection.
Away from the public eye and usage, tunnels are also a favourite of intelligence services and smugglers. Case in points, the secret military tunnels dug by North Korea under the DMZ (demilitarized zone) and into South Korean territory not far from Seoul or the drug smuggling passageways excavated under the border between Mexico and the USA.

Unlike bridges, tunnels don’t need to worry about wind and vibrations is a lesser concern. Nonetheless, there are other safety considerations that need to be factored in, including water leakage and drainage, the exhausting of noxious gases coming from traffic and human exhalation, and the overall stability of the materials and immediate environment, including whether it is located in an earthquake-prone zone. This makes geotechnical studies indispensable at the evaluation stage, ahead of the design phase.

Depending on the lay of the land and the specifics of the tunnels to be built, different types of construction techniques can be leveraged. The simplest of them all is vertical excavation from the surface and, once it is completed, then the top can be covered back with some of the original soil, thereby avoiding interferences with the surface transport tracks once the tunnel has been built – in the jargon this technique is called cut and cover. The other traditional technique is horizontal excavation, known as boring. The equipment for those ranges from shovels all the way to giant machines known as “moles” equipped with a circular cutting wheel. I include a link to the relevant Wikipedia entry in the next section, should you wish to read more about such machines.

The vertical and horizontal excavation principles can also be combined by digging shafts, which may be temporary, before digging horizontally. The key applications of this technique are for personal access to an underground tunnel from the surface and for ventilation, the bringing in of fresh air and exhausting of stale air or noxious gases.

Finally, undersea tunnels can also be realized not by boring through the seabed but by using the cut and cover technique, excavating a trench and then laying out a waterproof tube. Think fibre cable but instead of electromagnetic waves, it is people or vehicles travelling through. This immersed tubed technology allows for most of the manufacturing to be made onshore, which is both faster and cheaper. The flip side is the increased risk of water leakage and the possible exposure to impacts coming from above.

f) Trivia – Aqueducts

Back in the centuries before any type of engine had been invented to deliver the motive power necessary to counter gravity and permit the movement of cargo through any positive gradient, the only method available to the bringing of fresh water to a city, an essential commodity for everyday life, were human or animal power (the carrying of water in amphoras) or the building of an aqueduct all the way from the water source, sometimes tens of kilometres away.

No positive gradient means the water source ought to be located at a higher elevation compared to the final destination and the water had to be channelled as horizontally as possible, including across valleys and hilly terrain. These structures gave rise to some of the most beautiful ruins, especially from the Roman empire, the aqueduct bridges.

Some of them have a multi-tiered multi-arch design; an example of this is the marvellous Pont du Gard carrying water from the Fontaine d’Eure to Nîmes in the south of France – a course of about 50km with an elevation loss of only 17m. The bridge has three tiers and a total span of 275m at the upper level where 40,000 m³ of water transited every day.