Two stretches of Sonoma Mountain Road off of Adobe Road near Petaluma are dotted
with potholes after being turned into dirt last summer. Road Warrior photo

Sonoma County’s test of turning paved roads into dirt is earning an A and a F for durability.

Last summer, the county Transportation and Public Works Department took three pothole-patched stretches of Sonoma Mountain Road and after grinding, pulverizing, injecting a hardening enzyme and compacting, crews left a hard-surfaced, dirt road to see if it could improve poor roads without having to completely rebuild them. If the test is successful, department officials say, more of the county’s rural roads will be turned into dirt.

A 600-foot segment between Bennett Valley and Kenwood appears to have survived the winter storms so far largely unscathed, with only a handful of tiny potholes. Definitely a success.

But to the south two other test stretches of Sonoma Mountain Road off Adobe Road near Petaluma are a mess, dotted here and there with significant potholes. Definitely a failure.

Rob Houweling, operations coordinator for the roads department, said the difference in how well the test sites are performing boils down to that asphalt grindings were incorporated into the Petaluma-area segments but not in the Kenwood-area one.

He said the enzyme, biodegradable Perma-Zyme11x, didn’t bond well with the asphalt grindings, leaving pockets that fell victim to the weather. He said that after potholes started appearing crews reworked and smoothed the southern test sites, but it rained shortly afterward, preventing the enzyme from drying and hardening. Crews also have spread base rock on the southern sites to try to smooth out the road.

The next major step is this summer to again rework the southern sites but add finer material for the enzyme to bond with. Until then, he said, he may send out a road grader to again smooth out the road.

Howeling said also to blame for the potholes are motorists who drive too fast, which causes the road to wear out.

Signs at the test sites set a 15-mph speed limit, but few drivers seem to be following them on the lightly traveled sections. As the Road Warrior last week bounced through the Petaluma-area potholes at 15 mph, a luxury SUV passed going twice as fast or faster.

Howeling said the county has received some complaints about the test sites, including dust during the dry season, but he’s had just as many calls about problems on paved sections of Sonoma Mountain Road.

David Brown of sonic.net last week had to drive down the southern part of Sonoma Mountain Road to visit a client and wasn’t thrilled by the potholes.

He said he’s driven the road two or three times before and had considered driving his Jeep but went with his Audi TT sports car for its comfort. He complained that the road, like many in the county, “tears up” cars.

Also not a fan of the dirt test is Nicolas Ceja, who works at a ranch along Sonoma Mountain Road near Petaluma. He said he’s come across accidents that he said were caused when the drivers lost control on the dirt.

To see a Perma-Zyme11x video of how the road improvement process works, CLICK HERE.

To read last summer’s story on the roads being pulverized, CLICK HERE.