Santa Cruz engineers hope an innovative approach will stop Westside sinkholes
For years, Santa Cruz has defended itself from the ocean’s power with shields of armored rock, mending holes in the collapsing bedrock with riprap—boulders of varying size. But in the ceaseless battle between the city and the sea, Mother Nature has gained some more ground—or more accurately, taken it away.
Our most recent problem is a sinkhole, 15 feet wide and 28 feet deep, at the rear of an 88-foot-long cave that runs parallel to West Cliff Drive. Located between Woodrow and Columbia streets, the sinkhole is the fourth in a series of problems that has plagued the location for decades. The most recent crumble happened on March 1, just in front of the last repair—a 2009 fill-in of the third sinkhole in the last 27 years.
On March 1, a combination of high tide and high surf surged through the sea cave and up the ramp of riprap left from previous fill-ins, causing the bedrock to collapse along with the soil above it, according to Rodney Cahill of Mesiti-Miller Engineering.
“The high-energy wave environment scoured the last piece of critical material from the roof of the sea cave, causing a collapse of the cave,” says Cahill. “And then subsequent wave activity removed the loose soils and smaller rock from previous repairs.”
This particular ocean cave near Woodrow also happens to lie on a fault line—another factor in its continual collapse over the years.
Abandoning the conventional riprap countermeasures, city staff, with the help of Cahill and other engineers and geologists, have devised a new plan utilizing concrete to plug the sinkhole, along with a curved wall within the cave that will minimize wave damage. Workers will work on the hole from above and below, resulting in some street closures on West Cliff Drive.
“This is a different approach, a more long-term fix for this specific location, so that it doesn’t continue under the road, and weaken the road,” said Mark Dettle, director of Santa Cruz Public Works, at a recent city council meeting. “We’re trying to work with Mother Nature. You’re not going to be able to win. She wins in the long-term, but we’re trying to slow it down.”
Vice Mayor Don Lane approves of the innovative solution, but fears that the possibility of rising sea levels as a result of climate change could lead to more collapses in the coming years.
“I hope that we’re just lucky enough to keep West Cliff Drive as it is for the foreseeable future,” says Lane, “but the reality is, in the decades out, I think West Cliff will be vulnerable to losing some of its footprint.”
Cahill and others at Mesiti-Miller surveyed the sea cave with consultants from Zinn Geology. Cahill, who also worked on the seawall for Jack O’Neill’s East Cliff Drive home, created the specifications for a curved wall—a unique design that has never been used before.
At a total cost of $250,000, the city contracted Monterey Peninsula Engineering to fill the sinkhole and build the curved wall, which serves as a shield for the concrete plug behind it. But the engineers found that the specifications they had outlined in March were obsolete before construction began.
“We actually found that the sinkhole had enlarged during the period of time from the initial survey in March to the current construction,” says Cahill.
“We had to take new measurements and update the map so that we could modify the repair to complete the construction.”
Once construction did begin on June 3, the first step was to remove the sand that had washed up inside of the cave. Workers then placed sandbags at the 20-foot tall mouth of the cave, creating a berm to block waves from entering the construction site.
The crew will pump concrete grout inside the cave, solidifying the existing boulders from previous repairs. Next, working from the surface of West Cliff Drive, they will fill the sinkhole with concrete up to about 10 feet below street level. Workers will also place soil over the concrete plug for the remaining 10 feet, to match the terrace deposits that surround it.
To build the new curved wall, the construction crew will first use dry concrete to create a flat wall approximately 35 feet into the cave, and then disperse concrete from a high-pressure hose to shape the ocean-facing curve. The wall will come inward at the center and curve outward at its top and bottom, to repel the waves back toward the ocean.
While construction is under way, West Cliff Drive will be closed from Columbia Street to Woodrow Avenue, but will be open in the evenings and on weekends when no construction is being done. If all goes according to plan, meaning no large storms or unintentional collapses occur during the sinkhole repair, the process will be completed before Fourth of July weekend, according to project manager Steve Wolfman, associate civil engineer for the city’s Public Works Department.
As for the future of the plug and the curved wall, Wolfman and the other engineers involved in the repair foresee the concrete lasting far beyond the lifespan of previous riprap fills, and it may in fact hold up longer than the surrounding bluffs, which according to Zinn Geology are eroding at a rate of about 1 foot per year. This means that the sea cave may no longer be a cave in the future, and the artificial curved wall, which will essentially be hidden inside the hollow once it is completed, may one day be exposed.
Vice Mayor Lane hopes this project is not just a temporary fix and feels the city needs to develop a long-term plan for the eventual fate of West Cliff Drive.
“I hope that it works in a more lasting way,” says Lane. “I can’t say that I’m confident in the long, long-term because I think that it’s just such a vulnerable place where the power of the ocean usually wins.”