ICPI Interlock Design Issue 3 2018

15 InterlockDesign.org 5 SUMMER 2018 ® esign interloc Colma Streets Take the Long Road to Performance B ecause Colma, California sits on roll- ing hills just south of San Francisco, it became the unfortunate recipient of the victims of the devastating 1906 earthquake. There are more buried among sprawling, but elegant cemeteries than those alive in this hamlet. In 1998 the streets and sidewalks within the small resi- dential area supporting about 1,600 citizens was adorned with interlocking concrete pavement. With high-slope streets, some as high as 18%, the impressive project was in- cluded in an August 2006 article on sloped pavement applications in this magazine. According to Brad Donohue, Colma’s Direc- tor of Public works since the late 1990s, the interlocking concrete pavement application was controversial. Some doubted that the pavers would remain stable under traffic on such high slopes. There is no sign of move- ment on the sloped streets and traction loss has never been a problem. Besides consistent widths among the sand- filled joints, surface stability can be likely be attributed to the 45° herringbone pattern. Other factors working in favor of longevity and no surface distresses is mostly car traf- fic, a mild climate, and sandy, well-draining soils with R-values between 65 and 70. The longest street, F Street, used a cement stabilized soil subgrade to compensate for a high-water table that sometimes rises during the rainy, winter season and could potentially weaken or alter the pavement subgrade. Mr. Donohue noted that if the 20-year old streets were paved with asphalt, they would have already applied a slurry or chip seal in the first 8 to 10 years. Within the next 10 years, there would more than likely be asphalt surface milling and fill, or an overlay. This suggests that the life-cycle cost of the Colma’s pavers would likely be lower than asphalt after another 15 or 20 years with another cycle of asphalt milling and overlay. Given the excellent condition of the pavers, this is a reasonable life cycle cost forecast. He noted, regarding costs in long term maintenance and rehabilitation to the resi- dential roadways, Colma is definitely “ahead of the game” on street maintenance costs by using concrete pavers. Mr. Donahue said that municipalities need to take the long view when it comes High-sloped paver of streets entering Colma’s residential neighborhood have performed well for the past 20 years under vehicular traffic.  COVER STORY 