Interlocking & Permeable Interlocking Concrete Pavements Address Functional and Visual Concerns
Interlocking concrete pavement improved the redesign of Third Street in Davis, California (pop. 70,000) by clearer visual and functional connections with the century-old UC Davis campus’ main entrance. Directly adjacent to the university campus, downtown Davis projects a classic “college town” feel; compact, vibrant, and bicycle-friendly. The place is so simpatico with bicyclists that it was voted the top California city for cycling. Given this reputation, it’s no surprise that Davis supports the U.S. Bicycle Hall of Fame and the California Bicycle Museum.
Third Street is best known locally as the site for the year-round farmer’s market. Long envisioned as a gateway between UC Davis and downtown, plans to create a signature bicycle and pedestrian corridor have been planned for decades with public and incremental private improvements occurring throughout the years. “The Third Street segment has always been on our list as an important bridge between the university and our downtown area,” says Brian Abbanat, Senior Transportation Planner for the City of Davis.
Prior to the redesign, the two-block area of Third Street leading to the campus entrance served as a pass-through for vehicles, pedestrians, and bicyclists. There wasn’t much character and sense of place, he explains. The combination of frayed concrete sidewalks and roadways plus flooding along the flat street due to poor drainage and tree roots blocking gutters and drains led to the city’s decision to reconstruct the area. The flatness of Davis’ terrain is a blessing and curse: a blessing to cyclists and a curse on stormwater drainage.
Plans called for a comprehensive streetscape improvement addressing drainage problems while creating a gateway street between the city’s downtown and the university entrance, points out Nathan Lozier, vice president of RHAA Landscape Architecture and Planning in San Francisco. “Davis is a bicycle town. Over 4,000 bicycles travel through the Third Street area between downtown and UC Davis daily. Diversion bollards to control automobile traffic and deep cross-street drainage swales were safety hazards to bicyclists,” he says. There is also much pedestrian traffic, so it was important to address narrow sidewalks, deteriorating pavement, and inadequate lighting for pedestrians and bicyclists, he adds.
Visually, the plan also recommended better connecting downtown to the campus, says Mr. Lozier. “Prior to improvements, you could stand at the downtown end of the two-block area and not even know that the university was there,” he says. “The existing street profile was also steeply crowned to divert water into deep gutters along the roadway.” The 850 foot (245 m) street length includes 675 ft (206 m) of permeable interlocking concrete pavement (PICP). The two-block area previously had four-foot wide sidewalks next to four-foot wide tree lawns with on-street parking and bollards divert pass-through traffic at an intersection. The new design eliminates on-street parking in exchange for wider sidewalks, flattens the street to make it safer for pedestrians and bicycles, installs more bicycle racks throughout the area, and uses custom art—an obelisk of bicycle parts—in the center of a newly rebuilt intersection circle to control traffic flow.
In addition to the streetscape improvements, the project includes rebuilding three intersections, two of which with interlocking pavers and replacement of concrete entryways and patio areas with pavers for adjacent business owners. “Several business owners welcomed the expansion of pavers onto their properties, which expanded the sense-of-place and seamlessly tied their businesses to the new streetscape,” explains John Martin, senior associate, RHAA Landscape Architecture and Planning.
A combination of PICP and (non-permeable) interlocking concrete pavement was selected for the project, with non-permeable pavers used in intersections and entryways into service alleys, says Martin. The concrete pavers were supplied and installed by ICPI members. “City representatives requested pavers over asphalt anywhere that vehicles turned to reduce any risk of pavers shifting,” he explains.
PICP was chosen for the roadway to reduce runoff and flooding while improving quality of the stormwater. “In the roadway, the permeable and non-permeable pavers are 80 mm thick,” says Mr. Martin. Three colors of pavers are used in the herringbone and circular patterns in the project. “The design originally specified a 7/8 inch (22 mm) bed of ASTM No. 8 stone, over 4 inches (100 mm) of ASTM No. 57 on 6 inches (150 mm) of ASTM No. 2. We reviewed more readily available local aggregates and substituted them when we had difficulty finding the originally specified aggregates,” he adds.
“Our stakeholders recognized that permeable pavers are best practice for this use and the subgrade was fairly permeable because it was a former creek bed with gravelly, sandy soils,” says Mr. Lozier. “Pavers were also attractive because the project included some reconstruction of underground utilities, and city representatives liked the idea that future maintenance could be easier because pavers can be removed and replaced more easily for repair of utilities than concrete or asphalt.”
Residents expressed concern about the pavers in public meetings held to allow residents to comment on proposed designs, says Lozier. “They were worried that pavers would not provide a smooth surface that was comfortable and safe for pedestrians and bicyclists,” he explains. “We were able to point out other areas in the city where pavers are used, and everyone agreed that there were no problems.” The original design proposed a fairly intricate paving pattern. “We talked to contractors during the development of the plan to ensure that the design was achievable during installation,” says Mr. Lozier. “The layout uses a herringbone pattern with selected pavers laid out with different colors to suggest a circular design.”
In addition to changing specifications for aggregate used in the project to address difficulty obtaining material, there was also a need to develop an engineering detail at the intersection corner, says Mr. Martin. “Some of the intersection corners are flush with the sidewalks to allow delivery trucks to drive onto them, so the paving profile had to be adjusted to transition from a pedestrian-only profile to a vehicular traffic profile at these corners,” he explains. This required additional 6 inches of ASTM No. 2 stone to the base and a reduction of the ASTM No. 57 stone, he adds.
“We have never used interlocking pavers on a city street before this project, only on private parking lots, but we were comfortable with the subgrade engineering,” says Mr. Abbanat. “The area is not heavily traveled but there are some heavy trucks delivering to local businesses, so the pavement needed to handle that weight.”
Feedback on the project has been positive, and interlocking pavers could be considered for future projects if appropriate, says Mr. Abbanat. “Pavers are an important factor when creating a pedestrian-friendly feel for an area that also includes vehicle and bicycle traffic.”