Grantville Gazette, Volume I
Page 41
McAdam abandoned the Telford base, and indeed all reliance on set stone, and instead relied exclusively on eight inches or more of broken stone. He allowed the rocks to be compacted by traffic.
McAdam's methods were so successful that the compacted broken stone road is known as "macadam." Macadam is a great road surface for horse-drawn traffic, but it is not well suited, without modification, to automobiles. We will consider the design of macadam roads in more detail in a later section.
The first European asphalt and concrete roads appeared during the end of the century in question, but they did not come into prominence until automotive traffic forced their adoption.
Road Design: Route
Ideally, roads would be nearly straight and nearly flat, while quick and cheap to construct. Unfortunately, the landscape usually doesn't cooperate. If the straight line path encounters a hill, the builder has three choices: ascend and descend it, curve around it, or cut (or even tunnel) through it. Departures from linearity may also be desirable in order to avoid a stream or marsh, or to follow a coastline, or to cross a river at a more favorable point for fording or bridging it.
Sometimes there was both a "high road" and a "low road" connecting two points, the high road being used when the lower one was too soggy to be traversed (Hulbert, 44–45).
Roman road engineers showed a predilection for the "military crest": a road just below the crest of the hill, on the slope facing away from the frontier, so as to conceal troop movements from the enemy. (Chevallier 89).
Road Design: Drainage
Highway engineers say that the three most important aspects of road design are drainage, drainage and drainage. (U. Texas, I:45). Standing water turns earth into mud, of course.
Drainage typically involves such expedients as raising the road, road grading and camber (see below), longitudinal ditches (or gutters), culverts (so water runs beneath the road rather than over it), and subsurface transverse drainage pipes. (The latter were used by Telford, see Smiles 429.)
The drainage ditches should themselves be graded, so they are self-cleaning (U. Texas, 7), and it may be necessary to have them feed into a containment pond of some kind if the road is subject to heavy rainfall.
Roadbuilding Methods: Crossing Marshy Ground
Hilaire Belloc opines that an extensive marsh is actually a much greater obstacle to overland movement, unaided by roadwork, than are forests, hills or even rivers. (Belloc, 14).
In Belgium, Holland, and Lower Germany, log roads have been used in swampy areas since 2500 B.C. (Von Hagen 178). American pioneers cut down trees of similar length and laid them in the direction of travel. The logs could be used whole, or split in half. (Hulbert 48–51, Luedtke)
The 1911 Encyclopedia Britannica comments drily, "this is ridiculed as a 'corduroy road,' but it is better than the swamp." (A suitable saying would have been, "better logs than bogs.")
Instead of laying just one set of logs, the corduroy road can have two layers, for example, transverse logs over longitudinal stringers. (Hindley, 11–12; Von Hagen, 178, Modern EB). The modern American military has also built heavy corduroy roads, with three layers of crossed logs. (FM 5-436, Chap. 14). Pegs can be used, at intervals, to connect the layers. The purpose of the additional layers is not to increase the load rating, but to make sure that the surface doesn't sink below the mud.
The logs can be placed on loose branches, or on fascines (bundles of brushwood), rather than directly on the marshy soil. If timber is not available, one can use fascines by themselves, or together with sapling sleepers and binders. (Id.)
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In the 1632 Universe, corduroy roads may be laid as access roads for logging operations in heavily forested regions, such as the Thüringerwald . Obviously, the logs are readily available, and the road needs to be maintained only so long as there are still trees left to cut.
The other major use of corduroy roads will be by the military. Corduroy roads were used extensively in the American Civil War. Writing about the siege of Richmond, Joel Cook said, "Corduroy roads ran in all directions through the swamps, and every general had his roads leading wherever he wished." (Cook 273)
Likewise, a study of the Eastern Front in World War II said that "war could never have been waged in the vast swamp regions of Russia had they not been made accessible by improvised corduroy roads." (CMH)
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There are other ways of crossing swamps. Blind Jack Metcalf built roads over bogs by laying down gorse and heather in a criss-cross fashion, then spreading gravel over the bundles. This has aptly been termed "floating a road." (Albert, 137; Borth, 85).
Besides using simple corduroy roads, the Romans created elaborate swamp-spanning causeways, called pontes longi (long bridges). The via Mansuerisca in Belgium was structured, from bottom to top, as follows: pilings with crossbeams, longitudinal joists, transverse logs, limestone paving cemented with clay, and finally gravel. (Chevallier 89–90).
Road Design: Width
Traffic moves on what is technically termed "the traveled way" or "carriageway," and which may be divided into one or more lanes. The roadway is the entire width of surface on which a vehicle may stand or move, and thus includes both the traveled way and the shoulders (and any median strip). The road is the entire right of way, and thus consists of the roadway and the roadsides, from fence to fence.
Nonetheless, in this section, I will use the term "road" to mean the "traveled way."
The necessary width depends on what traffic the road will bear. The Roman roads were ten to thirty feet wide, with the norm being in the fifteen to eighteen feet range. (Hindley 42) Tresaguet and Telford both favored an eighteen foot wide carriage way, but the Cumberland Road
in the USA had a twenty foot breadth. (EB)
The 1911 Encyclopedia states that fifteen feet is wide enough to allow the "easy passage of two vehicles;" plainly they are thinking of wagons rather than motor cars. According to the AASHTO "Green Book," the standard lane width for modern automotive traffic is 3.6 meters (twelve feet). However, rural roads can have widths as small as 2.7 meters (nine feet).
"Plank roads" (see below) were often constructed with a single lane, eight feet wide. One lane roads will need to have occasional turn outs to allow vehicles to pass each other.
The USE's roads need to be wide enough to allow the passage of its armored personnel carriers (APCs), which are converted coal trucks.
Road Design: The Ruling Gradient
The ruling gradient is the average vertical grade as one travels along the centerline of the road. The grade is usually expressed, not as so many degrees of slope, but as a ratio of the vertical change to the horizontal one. For example, a grade of 1 in 40, which corresponds to a slope of 1.4 degrees, means that there is a change of one vertical foot as you travel 40 horizontal feet. Prior to 1800, steep grades of 1 in 12 were common on English turnpikes (Reader, 17).
Keeping gradients small makes it easier for draft animals to haul a load, and hence reduces the fuel consumption by automobiles and trucks. It also minimizes brake and tire wear.
If the traffic is moving uphill, then the steeper the gradient, the greater the degree to which the force of gravity is directed in opposition to the uphill movement. In other words, the horse or motor vehicle must lift more of its own weight in order to proceed. If the load is one long ton (2240 pounds), then the "grade resistance" is 22 pounds for a gradient of 1 in 100, 45 for 1 in 50, and 112 for 1 in 20 (Gregory 127).
Downhill movement is of course easier, since gravity is then on your side, but only if the gradient is not so great that a braking force must be exerted to keep control. And, of course, if you are zipping downhill in one direction, that means you will be trudging uphill when you return.
Gradient is an issue for motor traffic, not just horse-drawn wagons. Steep uphill grades reduce speeds, while precipitous downhill ones increase brake wear. Grades also affect tire wear and fuel consumption.
The effect is dependent to some degre
e on the weight of the vehicle. The Encyclopedia Americana says that "a grade of 6% or 7% has little effect on passenger-car speeds but greatly slows truck traffic."
It may seem as though the road, ideally, should be perfectly level, but this is not the case. A level road doesn't drain well. The 1911 Encyclopedia says that the minimum ruling gradient should be 1 in 150, and the master road builders of the nineteenth century typically preferred gradients of 1 in 30 or 1 in 40. Their roads rise and fall gradually, rather than remaining level.
The Encyclopedia Americana notes that the crests of hills should be flattened to increase visibility.
Road Design: Elevation and Camber
Elevating the road bed above the ambient ground level helps to reduce the influx of groundwater. This tactic, which dates back to ancient times, is why major roads are called highways.
Again to ease drainage, roads have a convex cross-section, known as "camber." While used by Roman engineers, it was not a universal practice in the seventeenth century.
In 1607, Thomas Procter pointed out that standing water was the bane of roads, and urged general adoption of a convex road surface. Nonetheless, until the mid-nineteenth century, there were experiments with other approaches. The "Ploughman's Road" was horizontal, but elevated and flanked with deep ditches. The "Angular Road
" was slanted to one side only. In 1736, R. Phillips urged the merits of a concave road. His theory was that the water would run down the center and carry away loose material. (Albert, 135–8). In 1810, McAdam warned against a road which was "hollow in the middle," but seemed to think that a level road was just fine since "water cannot stand on a level surface." (Reader, 37). Unfortunately, it can.
On the other hand, a steep camber is also undesirable. It makes fast-moving vehicles prone to overturn (Gregory, 131; Forbes, 528, 531), especially as they negotiate curves, and the traffic tends to crowd onto the central portion of the road, causing it to form ruts. (U. Texas, I:6; 1911 EB).
1911 EB generalizes that the usual rise in the center is one-fortieth to one-sixtieth of the width. It can be shallower if the surface is waterproof; Gregory (131) teaches 1 in 48 for macadam, 1 in 60 for tar macadam, 1 in 72 to 1 in 96 for asphalt, and 1 in 80 to 1 in 132 for concrete.
Road Design: Friction
Friction is both bane and boon for traffic. Up to a point, the lower the friction the better; the greater a load that a draft horse can pull, the less fuel an automobile must consume to cover a particular distance. However, on a frictionless surface, an object at rest would remain so, its wheels spinning uselessly, and one in motion could not stop.
Table 2.2.3 in the Transportation Cost FAQ on www.1632.org
Road Design: Unsurfaced Roads
Construction of a primitive road (WVDOT type A) just means clearing a path: cutting back bushes; felling trees and removing their stumps; taking out boulders which block the way.
The next step up (WVDOT type B) is to grade and drain the road.
What the WVDOT calls a type C "soil-surfaced road" is more aptly termed a "stabilized soil road." The native earth can be strong or weak, and more or less susceptible to rainfall and temperature changes. In a stabilized road, this is altered by chemical or physical means.
The 1911 EB says that "in carrying traffic over a clay soil a covering of 3 or 4 in. of coarse sand will entirely prevent the formation of the ruts which would otherwise be cut by the wheels; and if the ground has, already been deeply cut up, a dressing of sand will so alter the condition of the clay that the ridges will be reduced by the traffic, and the ruts filled in." Collier's Encyclopedia notes, more generally, that sand can be added to clay, clay to sand, cement to soil, and oil to soil, all to create a more weather-tolerant road surface. Such hybrid soil roads are very cheap to construct (Oglesby 633; Gillette).
The civil engineers of Grantville may be aware of other stabilization techniques. For example, calcium, magnesium and sodium chloride can be added to soil to make the particles adhere better. (Id.). The modern EB suggests addition of small amounts of lime, portland cement, pozzolana, or bitumen to the top eight to twenty inches of the ground.
Road Design: Surfaced Roads, Generally
Surfaced roads provide a "wearing surface" (also known as the "pavement," the "road metal," the "carpet," and the "surface course") which is in actual contact with the traffic, and provides enough friction for the vehicles to make headway, but not so much as to unduly slow movement.
Pavements are usually classified as rigid (like concrete, mortared brick or fitted stone), flexible (like asphalt, wood, and compacted stone) or granular (like gravel and sand).
Each surface has its unique characteristics in terms of strength, water resistance, friction, and so forth. For example, the modulus of elasticity, a measure of the extent to which a material deflects in response to stress, ranges from 280–300 for asphaltic concrete, to 30–40 for coarse sand. (Kezdi, 255)
The combined rolling and air resistance (the two are hard to separate) experienced by a one ton vehicle traveling 25 mph on pneumatic tires is, on average, 32 pounds for concrete, 35 for sheet asphalt, 38 for grout filled bricks, 34 for wood blocks, 40 for graded and maintained soil, 50 for gravel or firm natural soil, 70 for well packed snow, and 75 for soft natural soil. (Agg, 13).
Road Design: Lanes, Trackways and Road Rails
Sometimes, a road carries both heavy and light traffic. The former may need a pavement which is "overkill" for the latter. One expedient is to have lanes with different road surfaces. For example, American plank roads were sometimes built with just one eight foot wide lane of planks, flanked by a dirt lane. If the traffic justified it, the company built a second plank lane.
It is conceivable that we will build hybrid roads, with both a hard concrete lane for military vehicles, and an asphalt, wood, macadam or stabilized earth lane for horses. Napoleon reportedly favored a "tripartite road" with cobbles for the artillery, a macadam-like surface for the infantry, and an earth road for the cavalry. (Forbes, 536).
The second approach is the trackway; that is, two longitudinal bands of stone, or even steel plate, separated so as to match the wheel spacing on the heavy vehicles intended to use it. Creating the trackway was less expensive than covering the entire width of the road with metal. A double wheel trackway, on which four horses could pull a load of seventeen tons, was in use on the Albany-Schenectady road from 1834 to 1901. (Gregory, 141–2).
The most extreme form of the trackway is the road rail, used typically in mines, which evolved eventually into the modern rail track.
Road Design: Pavement Structure
The native soil and rock underlying a roadbed were once called the "foundation" or "basement," but it is now customary to refer to them as the "subgrade." The term "subgrade" is also used to refer to imported soil (you might use this in building a road across a swamp).
The subgrade needs to be able to support the load. Peat bears a mere 56 pounds per square foot. You can put one to four tons on a square foot of chalk, two to five on one of fine sand, three to seven on clay, four to eight on gravel, and up to eighteen tons on ordinary rock. Clay and chalk are better when dry than when wet, and rock is unpredictable because it can have soft spots and even cracks. (Gregory 129–30). A native foundation which is unreliable will be removed and replaced with an alternative subgrade.
For drainage purposes, the subgrade is usually raised above the original ground level. Before one can build the pavement structure over it, one must be sure that it is stable. Early builders simply allowed the material to settle. However, in modern times, the subgrade is compacted by rollers.
There may be one or more layers separating the surface from the subgrade. These may be called, simply, the "base course" or "the sole." When there are two distinct layers, these may be identified
as an upper "base course" and a lower "subbase course," and there can actually be more than two distinct layers. Also, with asphalt surfaces, there can be a thin "binder course" between the asphalt and the base course.
Road Design: Gravel- and Loose Stone-Surfaced Roads
A simple improvement on the basic dirt road is to cover it with gravel. The 1911 EB says, "Smooth rounded gravel is unsuitable for roads unless a large proportion of it is broken, and about an eighth part of ferruginous clay added for binding. Rough pit gravel that will consolidate under the roller may be applied in two or more layers, but each must be of similar composition, or the smaller stuff will work downwards." The recommended foundation is "rough chalk sufficiently rolled to stop the gravel while draining off the surface water."
The Lancaster (Pennsylvania) Turnpike (1794) was hard-surfaced with gravel and broken stone. (WBE)
Road Design: Macadam Roads
Macadam roads are still in use today, especially in areas such as New England, where rock can be collected readily. However, rather than letting the broken stone surface be compacted by traffic, one layer at a time, as taught by McAdam, modern builders use heavy rollers. (Collier's)
The use of steam rollers for this purpose was first introduced around 1876, and by the end of the nineteenth century perhaps 90% of the major roads of Europe had been "macadamized." (Forbes, 535). Nowadays, the principal highways employ asphalt or concrete surfaces, and only secondary or tertiary roads are of the macadam type.
The modern EB states correctly that McAdam taught use of "small, single-sized, angular pieces of broken stone." However, what is lacking there is the explanation of just how critical several of these features were.