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 degree 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 sets forth the load which a single draft animal can haul, in a vehicle with a particular type of tire, on a level road of a particular surface type, as a multiple of the pull exerted by the animal.
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.
The stones had to be broken so that they were angular; they had to be angular so that they would lock together when compacted. (Smiles, 430; Forbes, 534; 1911 EB). Pebbles rounded by the action of water would not create the desired surface. Likewise, the stones had to be small. If they were much larger than the effective area of contact between the wheel and the road surface—about one inch square—then the stones would not be consolidated by passing traffic. (Reader, 2, 32–3, 37–9).
McAdam was very insistent that no "sand, earth or other matter" be used "on pretense of binding." (Reader, 39; Forbes, However, his road had an "intrinsic" binding agent—the traffic wore down the rocks and the resulting dust acted as a binder. That may explain the modern practice, described by Collier's Encyclopedia, of bonding the modern macadam road "into a solid mass by means of a finely crushed stone rolled into the surface."
The up-timers' sources are not always consistent in the description of macadam roads. For example, the modern EB shows them as having 0.75–1 inch surface layer of gravel or broken stone. However, the 1911 EB says that while "Telford covered the broken stone of new roads with 1/2 in. of gravel to act as a binding material," his rival McAdam "absolutely interdicted the use of any binding material, leaving the broken stone to work in and unite by its own angles under the traffic."
Another problem with the modern EB text is a sin of omission. McAdam cambered, not only the road surface, and the base course, but also the subgrade. While this is depicted in the figure, it is not commented upon. All the encyclopedia says is that the road was "elevated," which was true but not the whole story.
Road Design: Plank Roads
The plank road differs from the corduroy road discussed previously, in that it uses lumber (planks) instead of whole or split logs.
In the period 1835–1855, many plank roads were constructed in New York, Pennsylvania, Ohio, Michigan, Illinois, and other timber-rich states. These roads were typically ten to fifteen miles of length, and fed into canals or railroads. Indeed, they were nicknamed "the Farmer's Railroad."
According to the 1911 EB, "the plank road often used in American forests makes an excellent track for all kinds of traffic." The construction was straightforward. First the road bed was cleared and graded, and drainage ditches dug. Then two or more columns of longitudinal sleepers were put down, and transverse planks were laid (and sometimes nailed or spiked down) on top. The planks were two to four inches thick, eight to sixteen feet long, and made of oak, hemlock or pine. For drainage purposes, the outer sleepers may be set a few inches lower than the inner ones. (Majewki, 9; WHS, ISM, WiscHS, 1911 EB)
These plank roads could be constructed at one-half to two-thirds the price of macadam roads. (Majewski) Naturally, they were cheapest to build on level terrain with forests nearby.
In 1850, Charles E. Clarke told the Prairie State newspaper that the three plank roads near his Illinois farm were "the best roads imaginable—better by far than the best paved or 'macadamized' road—pleasanter for the person riding—easier for the animals, and far less destructive to the carriages that roll upon them." (Clarke) South Carolina manufacturer William Gregg even thought them superior to railroads (Majewski 9).
From the section on "Friction," we know that wood is a "fast" road surface. On a new plank road, stage coaches traveled eight miles per hour (Luedtke; Clarke). Two horses could draw two tons forty miles per day (Clarke). The Watertown, Wisconsin Plank Road reduced the round trip from Milwaukee to Watertown from four to six days, to three, and allowed wagon loads to be increased from 1,500–2,000 pounds, to 3,000; freight rates were reduced by about 25%. (WHS). "Trips which took from four to six days on dirt roads were cut to from ten to fourteen hours over plank roads." (Mason) Unlike unsurfaced roads, plank roads could be used in any season (Majewski, 9).
Not everyone liked plank roads quite so much as Clarke. Asa Stoddard critiqued the Kalamazoo-Grand Rapids highway in verse, asking the reader if he had ever "brave[d] the peril, dare[d] the danger, of a journey on the Plank?"
The reason we hear such inconsistent views is that plank roads were excellent when new, but needed repairs or replacement more frequently than the plank road companies had expected. The boards decayed, warped, or were stolen. (Majewski 2 says that the expected life was 8–12 years, the true one 4–5. WiscHS states a life of 5–6 years, and Clarke says 7–8. Mason says that the roads were in good condition for 3–4 years, then needed constant attention, with maintenance costs running 30–40% of the original construction cost annually.) And toll revenues weren't sufficient to pay for the maintenance. The roads fell into disrepair and became hazardous.
It does not appear that the wood used in the plank roads was treated in any way to make it more weatherproof. It is possible that such treatment, if it could be done economically, would substantially extend the working life of a plank road.
A plank road one mile long, eight feet wide, with three inch thick planks would require 10,560 cubic feet of wood. Then for a mile's worth of two stringers, each three inches wide by three inches thick, add another 3,455 cubic feet. That is a total of about 14,000 cubic feet, or about 1,200 board feet.
Unfortunately, the USE-controlled region of early seventeenth-century Germany is unlikely to be, in th
e near future, the site of a "plank road craze" comparable to the one in nineteenth-century America. That is because there is a relative shortage of wood. (Virginia DeMarce, Charles Prael, Manfred Gross, and Andrew Ramage, private communications.) Wood is the principal fuel, and, by "the early modern period," per capita consumption of wood was about 4–5 cubic meters per year. (Other uses of wood totaled another cubic meter, annually.) (Halstead)
The Black Forest, nonetheless, was a wood exporting region, with pine, fir and spruce being shipped down the Rhine to Mainz, either as timber rafts, or as sawn lumber. (Id.)
Unlike the American wilderness, the forests of Germany—which also include the nearby Thüringerwald —are owned by various nobles, but they are likely to allow plank roads to pass through their territory if it yields a net financial benefit to them. Whether that will prove to be the case is debatable; Virginia DeMarce informs me that the Thüringerwald covers low mountains, and that roads were customarily made simply by taking off the topsoil to expose the bare rock.