Historical street paving

September 2012 » Features » PROJECT CASE STUDY
Wood paving techniques modified for the 21st century.
Michael Lev
Research into methods used in the early 1900s to pave this Chicago alley with wood blocks led to updated techniques and materials to meet modern environmental, health, safety, stormwater, and traffic requirements while retaining historic elements

As one of only two remaining wood-paved areas in the City of Chicago, the Historic Wood Paver Restoration project rehabilitated a deteriorating alley in the Gold Coast neighborhood of Chicago and returned a national treasure to the community. Originally constructed in 1909 of wooden paver blocks, the alley was in great need of repair. Wagon wheels, horse and automobile traffic, harsh weather conditions, and winter salt treatments had taken their toll, causing drainage deficiencies, uneven surfaces, and ADA compliance issues.

Project
Historic Wood Paver Restoration, Chicago.

Participants

Chicago Department of Transportation TranSystems
Knight Engineers & Architects Inc.
Infrastructure Engineering Inc.
S.T.A.T.E. Testing, LLC
MQ Sewer & Water Contractors Inc.
C.R. Schmidt

Product application

Rehabilitate a deteriorating wood block-paved alley while maintaining its historical integrity and meeting modern environmental, health, safety, and traffic requirements.

Because of the neighborhood residents' interest in maintaining the historical integrity of the alley, restoring it became a priority for the Chicago Department of Transportation (CDOT). The alley is listed on the National Register of Historic Places and as a City of Chicago Historic Preservation Division Landmark.

The project involved extensive research into wood block street construction best practices from the late 1800s and early 1900s, only to find that what worked in the past was not suitable for a modern-day installation. While paving streets and alleys with wood blocks was common from the 1840s through the 1920s, it could certainly be called a lost art today. The landmark designations given to this historic wood paver pavement strongly guided the team's work. Only two sections of wood block pavement remain within the City of Chicago, and the alley just south of North Boulevard between State Parkway and Astor Street represents the more complete and larger piece.

Paving thoroughfares and alleys with wood blocks was state-of-the-art in the early 1900s. The process consisted of first preparing a compacted subgrade of stone, sand, wood planks, or concrete; placing wood blocks derived from first growth forests that had been treated onsite in boiling creosote; and then spreading a final layer of coal tar, pitch, asphalt, or some other sealer as a finishing touch. The result was a reasonably sturdy, cost-conscious, and attractive surface that called upon abundant natural resources, cheap and ready preservative methods, inexpensive labor, and construction technologies that made the best of cheap petroleum byproducts. Wood block paver pavements filled an important gap at the time.

They were economical for one thing. Also, among the competing technologies, asphalt and concrete had not yet advanced to the point of cost effectiveness or widespread availability, and brick and cobblestone were both more expensive and quite a bit noisier.

As it turns out, each of the steps – the virgin forest, the dipping in boiling creosote, the sealing with coal tar – was overtaken by improving technologies and then, over time, outlawed altogether. This process made very poor use of non-renewable resources such as virgin forests, and employed a timber preservation technique (creosote) that brought with it many environmental and health issues, which were only bested by the same health and environmental issues that accompanied the spreading of coal tar. Nonetheless, the paving technology was an important part of the country's roadway and urban byway heritage, and worth replicating in the modern day.

Wood and preservative selection
Extensive coordination and consultation occurred between CDOT, various consultants, potential paver manufacturers, lumber suppliers, wood salvagers, paver installation contractors, and material testing firms to carefully select the species of wood from currently available and renewable sources as well as determine a sustainable preservative application.

At first, the design team considered using pavers cut from used railroad ties. A few benefits of this material included a preexisting creosote treatment, salvage of existing materials, broad availability, and dense wood (most railroad ties available in the area are oak).

Unfortunately, while initial testing leaned positive, later investigations showed that the railroad tie wood did not possess sufficient strength after weathering. In addition, it was noted that creosote treatment did not penetrate entirely through the existing ties to the heartwood, so much of the wood surface placed in the field would be untreated oak, which is prone to rot. Finally, samples emitted offensive odors even months after fabrication. Therefore, community and engineering reasons eliminated ties from consideration. Various salvaged or repurposed wood pavers from factory floors also were considered at early stages, but testing showed they could not maintain their shape when exposed to weathering.

Modern means of wood preservation such as pressure or AC 2 treatment were considered next. Several issues became apparent as a proximate manufacturer could not be readily located, and concerns arose about how to effectively treat such small pieces of lumber thoroughly. This finding led to an additional concern of possible contamination of underlying soils and people/pets tracking chemicals into residences. The city disliked the use of any petroleum-based materials, sealers, or treatments within the alley because of smells and the potential to contaminate nearby or underlying soils, so none were used.

Options then turned to woods that are naturally durable. Ipe wood was considered, but eliminated because of a lack of domestic sources, over-logging concerns, and rainforest deforestation associated with the species. Finally, black locust emerged as the wood species of choice. Black locust lumber is renowned for its durability and insect/rot resistance. Use of black locust allowed for creosoting or other chemical preservative applications to be avoided, and the wood is dense enough to alleviate any concerns of buoyancy. The untreated wood has a visually pleasing appearance and does not emit offensive odors like a creosoted, tarred, or sealed wood or surface.

Drainage design
Another important issue surrounded drainage and its environmental impact. The existing installation primarily drained through sheet flow over the surface of the pavers, sealer, and debris that had collected between the pavers over the years. Mortaring-in of the pavers was considered to replicate existing imperviousness, but ruled out because changes in humidity and the resulting expansion of the paver blocks would cause the mortar to crumble and disintegrate.

Without installation of a sealer, much of the water in the new installation would percolate through the sand between the paver blocks and into the sand bedding. TranSystems embraced the city's goal to keep runoff out of the sewer system by coring weep holes into the concrete base at strategic locations to allow for infiltration through the paver base and sand system and percolation into the underlying soils. The varying depth of bedding sand allowed for minor re-profiling of the alley to facilitate proper drainage flow while still keeping the existing concrete base.

Base preparation
The decision to retain and reuse the existing concrete base not only limited the volume of material removed from the site and quantity of fill/concrete placed, but also allowed the city not to disturb any contaminants that may have leeched into the soil under this alley during the last 100 years. In the majority of city construction projects, soil immediately beneath concrete bases generally contains various carcinogens and hydrocarbons, necessitating special disposal procedures. Furthermore, the removal of this soil often inadvertently allows some of these contaminants to become airborne. The design of this project limited waste, precluded the disturbance of potentially contaminated soil, and reduced construction costs in the process.

New black locust wood blocks of the sizes originally used in 1909 were placed on a sand bed overlying a concrete base with cored weep holes to allow stormwater infiltration into underlying soils.

Design and construction
Public and Aldermanic input strongly guided the design and construction, and CDOT worked with the community for more than 10 years to determine the preferred restoration parameters. Community stakeholders included the Catholic Archdiocese of Chicago Archbishop's residence, built in 1885, which adjoins to the alley, and the numerous influential residents who live in the vicinity. Consensus strongly supported retaining wood pavers as the pavement surface to preserve the historic integrity of the area, even though this alternative required more complicated design and research than a standard concrete alley replacement.

Tours of historic sites in Chicago commonly stop at this pavement installation to discuss its history and share its significance in the development of early street systems. Therefore, the City of Chicago's Historic Preservation Commission, which subsequently approved the installation, directed that suitable existing wood blocks be re-laid in one area of the alley so that the public can continue to view the pavement as it would have appeared in the early 1900s. New wood block pavers of the sizes originally used in 1909 comprise the remaining portion of the pavement surface.

All project participants respected the fixed construction budget. The team even devised a way to reuse the existing concrete base with some minor repair, and other quality-respectful value engineering modifications were made during construction. These efforts permitted the alley to be opened to traffic ahead of schedule and with strong public approval.

The result is a beautiful architectural tribute to the integral role wooden paver blocks played in the development of the street systems in Chicago and around the country, facilitating the transportation of people and goods essential to the entire nation's growth and connectivity.

Project highlights

  • Conducted expansive research of wood block paving methods used in the 1890-1920 era to determine best practices that could be applied to this project.
  • Evaluated various wood types, sources, and preparation as well as methods of wood preservation, treatment, and weathering to ensure paver block longevity.
  • Re-laid suitable existing pavers in one area of the alley to replicate as closely as possible the original appearance of the alley.
  • Specified, fabricated, and installed new wood block pavers.
  • Reutilized a subsurface concrete layer with minimal patching.
  • Designed and constructed ADA-compliant alley aprons and sidewalk transitions.
  • Improved drainage pattern and reduced stormwater runoff from the alley.

Michael Lev is vice president of Kansas City, Mo.-based TranSystems (www.transystems.com), a professional services consulting firm.


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