Friday, May 20, 2011

A Trip Down Macadam Lane

Figure 1: Asphalt paths in a Southern California cemetery circa 1920. Note the surface layer of asphalt is crowned over a foundation layer of sub-grade material.

Following up on the Trench 1 discussion. The possible macadam road coming to light is of interest for a couple of reasons: a.) it provides a date range, and b.) it is an unusual feature not often seen or discussed in historical archaeology.

Tar Macadam Pavements

A tar macadam road consists of a basic macadam road with a tar-bound surface. It appears that the first tar macadam pavement was placed outside of Nottingham (Lincoln Road) in 1848 (Hubbard, 1910; Collins and Hart, 1936). At that time, such pavements were considered suitable only for light traffic (i.e., not for urban streets). Coal tar, the binder, had been available in the U.K. from about 1800 as a residue from coal-gas lighting. Possibly this was one of the earlier efforts to recycle waste materials into a pavement!

Soon after the Nottingham project, tar macadam projects were built in Paris (1854) and Knoxville, Tennessee (1866) (Hubbard, 1910). In 1871 Washington, D.C. extensively used a "tar concrete" for road construction. Sulfuric acid was used as a hardening agent and various materials such as sawdust, ashes, etc. were used in the mixture (Hubbard, 1910). Over a seven-year period, 156 acres (630,000 square meters) were placed. In part, due to lack of attention in specifying the tar, most of these streets failed within a few years of construction. This resulted in tar being discredited, thereby boosting the asphalt industry (Hubbard, 1910). However, some of these tar-bound surface courses in Washington, D.C., survived substantially longer - about 30 years. For these mixes, the tar binder constituted about 6 percent by weight of the total mix (air voids of about 17 percent). Further, the aggregate was crushed with about 20 percent passing the 2.00 mm (No. 10) sieve. The wearing course was about 2 inches (50 mm) thick. Hot tar paving products have not been used in the U.S. for many years. (source: Pavement Interactive)

There is a voluminous amount of engineering technical literature discussing macadam road systems and, after spending much of the week perusing the various incarnations such roads could take, this particular variant deserves mention:

Double Pitch Grouted Macadam. Road Board Specification No.4, to Be Laid and Consolidated in Two Layers. – Section 4. The finished thickness of this section will be 4 ½ inches. The material used for the lower layer will consist of broken-up calcerous sandstone (Kentish ragstone), graded from 3 ins. Down to 2 in. gage. The lower layer is consolodiated by rolling , and the pitch binder, the same as in Section 3, is poured in but not brought up to the surface of the stones forming the lower layer, the pitch lying ½ in. below such surface with the object of providing a key for the upper layer.

For the upper layer will be composed of hornfels or elvan from the Penlee (Cornwall), broken to 1 ½ -in gage, costing 12s. per ton delivered on the road with 5 per cent of chippings of the same stone graded from ½-in. down to ¼ -in., costing 7s. 6d. per ton delivered on the road to be added during the process of final rolling so as to form the finished surface. The quantity of pitch required for the double grouting for the two layers is approximately 3 to 3 ½ gals. Per yard super.
The surface is to be finished by the chippings added during the rolling as in the case of single pitch grouting. The Kent County Council will construct this section. (Source: Engineering & contracting, Devoted to the Economics of Civil Engineering Design and to Methods and Costs of Construction, Volume 36, July-December,(Myron C. Clark Publishing Company, 1911), p. 228)

About eleven years later, the construction standards are materially the same; however, there is distinction made between heavy and light trafficked roads:
The thickness of the pitch-grouted macadam coat is is usually 2 in. on lightly trafficed road, and from 2 ½ to 3 in. on other for single pitch-grouting, and from 4 to 4 ½ in. for double pitch-grouting.

The material for pitch-grouting is broken to 1 ½ in. standard gage. In addition to this, 10 per cent of chippings of the same stone, varying from 3/8 to ¾ in . is used for closing after grouting with the melted pitch. The pitch employed usually complies with Roads Department specifications as follow, the viscosity being modified to suit climatic and local conditions by varying the quantity of tar oils. (Source: Engineering News Record,Vol. 89, July 1st-December 31st, (McGraw-Hill Company: New YorK, 1922) p. 565)

In the late 19th and early 20th century, a wide variety of road systems were developed and patented. Among them were systems utilizing bituminous binding agents for rock aggregates. Figure 1 above discusses some possible interpretations of DSCQHR T1 feature.

The possible DSCQHR macadam lane appears composed of a two layer construction similar to the double pitch-grouted macadam specifications. There appears to be a foundation layer(Red A) and a macadam road surface (red B). The foundation layer appears constituted of consolidated calcerous sandstone of which a fair amount has been recovered at the site (see image below).

The macadam layer appears to contain a considerable amount of hornfels aggregate.

Given the slope of the surrounding topography, the DSQHR macadam lane may have incorporated terracotta drainage tiles at regular intervals to handle wet season sheet-wash (see red C).

Given that we have the engineering specifications for construction thicknesses and material size standards, it is possible to determine if the DSCQHR feature fits such an identification.

Lastly, it is possible to assign a general date range between 1877 and the early 1920s for construction of this particular feature.

As a historical aside, tar macadam and asphalt paving systems were favored near schools on the premise that such pavements were more quiet and therefore less disturbing to students. Here is an excerpt from the San Francisco Call to this effect:

Want Noiseless Pavement in Front of Schoolhouse – Supervisors Order Asphalt to Be Laid on Harrison Street Between Fourth and Fifth.

The Supervisors’ Street Committee yesterday reported in favor of paving with asphalt the roadway of Harrison street between Fourth and Fifth. The Board of Works had recommended that the block be paved with basalt blocks, excepting the portion thereof in front of the Whittier School, which was to be paved with either asphalt or bitumen, the entire work to cost $13,500. The committee decided that the entire block should have a noiseless pavement for the good of the school. (San Francisco Call, Volume 98, Number 64, 3 August 1905)

How marvelously considerate!


  1. Hello David,

    Thanks for the link about the road construction, history, and composition. Yesterday during the dig I noticed that there is a layer of what seems to appear to be tar macadam into the large trench that you where digging, was this layer a perfect sign for a possible service road? I am not sure if that layer was true, but it shows how strata can tell us a glimpse of history. I have found other deposits of what seems to be part of tar macadam or asphalt in other locations across the site. When we look and create data charts for the material found about road material, I wonder if this will lead to a better foundation onto how it changed the natural landscape.

  2. Hello Stephen: thank you for the comments. I agree that the road system feature is an important stratigraphic reference point and likely datable.

    It would appear that there are two probable fill events, one related to the original construction related to the road and a second fill layer that covered the road at a later date. The two fill layers appear to contain a wide date range of artifacts. Perhaps the later fill layer relates to grading of the subdivision to the east of the DSCQHR.

    As some of the team have noted, all of the bioturbation is working havoc on our ability to sort out a text-book stratigraphic sequence. This makes the stratigraphic record of the units furthest west of the utmost importance as they may not be subject to the fill activity near the road feature. When we compare the data sets, we may get a better picture of what is going on. Cheers!


Feel free to comment on what you see & read here!