City begins emergency work on unstable cave at La Jolla Cove

Geology experts in June pointed to a weakness zone in Cook’s Crack, a cave in La Jolla underneath Coast Boulevard.


An unstable cave on La Jolla’s coast prompted emergency work starting Friday, Aug. 9 to reinforce the cave and the roadway above it, San Diego officials said. Due to the stabilization work required, portions of Coast Boulevard adjacent The Cove will be closed for the next six weeks. The sidewalk alongside the work remains open for pedestrian access.

According to a City statement, geologists discovered a weakness zone in the sea cave known as Cook’s Crack, underneath Coast Boulevard, and recommended immediate action be taken to fix the problem.

Kris McFadden, director of the City’s Transportation & Storm Water Department, explained that the City has been inspecting the area for more than 20 years, and only recently identified the weakness.

“We were looking at repairing the road, but before we do that, we had to make sure the sub-surface is stable,” he said. “This area has been inspected since the 1990s, and the most recent report did not indicate there was an immediate issue. But we initiated another investigation, and findings just recently that came back to indicate the supporting structure under the cave was starting to erode. The good news is, it is only under the street and limited to about a 50- or 60-foot section of Coast Boulevard.”

In an “abundance of caution,” and at the direction of the Mayor (Thursday) night, they started work immediately. “We didn’t want to waste any time,” McFadden said, “so we decided to proceed. The decision to close the street was not taken lightly.”

The City closed the sections of Coast Boulevard Thursday night and initiated an emergency contract to reinforce the cave, according to the statement.

City engineer and Public Works Department director James Nagelvoort told La Jolla Light that the stabilization project has two phases. The first involves injecting a glue-like material into the weaker layer of rock to increase its density and help bind the fractures. For the second phase, contractors will reinforce the top of the cave with concrete.

Jim Quinn, senior engineering geologist with the San Diego Development Services Department elaborated: “The City is going to inject a grout into the soil material at the top of sea cave to stiffen it up, make it stronger and less susceptible to collapse. Then crews are going to put in a small dam at the mouth of the cave and incrementally fill the cave with a slurry. Then crews will remove the dam and all the temporary structure, and texture the cement-ous material, so it blends in with the bluff face.”

Are there other worries?

As to whether there were any other sea caves or areas on the La Jolla coastline of concern, Quinn said this was the only one that is of concern right now.

Getting into the technical nitty-gritty, Quinn said: “Cook’s Crack is a sea cave that has developed in the Point Loma formation, and is probably 70 to 80 million years old. Now it has been uplifted and is exposed to the surf. There are a number of fractures in the Point Loma formation that we call faults. As the waves attack the bluffs, these areas with faults are eroded.”

Under Coast Boulevard, he added: “There is a soil-like unit, less than 100,000 years old, that is of lower strength. The cave, over time, tends to grow, and as it grows, the roof rock tends to fall; you get these big, square-shaped blocks falling from the roof. When some of the roof rock fails and gets to the terrace, then we are extremely concerned that we could have a collapsed sea cave.”

There are currently collapsed sea caves at Sunset Cliffs near Monaco Street in Point Loma, Quinn said. “This is a normal process of coastal erosion in the rocky intertidal zones.”

Not visible from The Cove or Scripps Park, he said the area currently looks like a “fissure in the rock” that opens up into the cave.

Nagelvoort added that there is no hazard to those who recreate in The Cove waters.

“It is not advised that people go into that cave, but there is no threat to those who swim,” he said. “Other than when we build the dam, all other activities will be done from the street, so you won’t really notice the work from inside the water. The thing to remember is that this is being done for health and safety purposes.”

The construction comes one week after an oceanfront bluff in Leucadia collapsed and killed three women on the beach below.

— Lauryn Schroeder of The San Diego Union-Tribune contributed to this report.


12:49 p.m. Aug. 9, 2019: This story has been updated with additional information about the geologist’s report.