Engineers: New La Jolla homes may have destabilized Coast Blvd. and sea cave underneath
Construction of the La Jolla Bay Homes on the former Green Dragon Colony site may have played a role in destabilizing the Koch’s Crack sea cave beneath Coast Boulevard. So states a June 17, 2019 report prepared for the City of San Diego by the TerraCosta Consulting Group.
On March 5, two engineers from TerraCosta (a subcontractor to the Pasadena-based consulting and engineering firm Tetra Tech) entered Koch’s Crack. In the year that lapsed since their previous study, they noticed accelerated erosion “likely resulting in more lost ground supporting the overlying roadway and utilities, as well as the loss of intimate contact between the two primary blocks that support the roof of the cave.”
No one single cause was implicated. However, their report stated: “Recent construction activities and the related heavy traffic may have contributed to the movement of this block and migration of sand into the fracture.”
The Koch’s Crack Sea Cave began forming along a northeast/southwest-trending fault via ocean-wave erosion between 70 to 80 million years ago. According to the report, it now extends eastward about 150 feet under Coast Boulevard, which places at least some of it underneath the La Jolla Bay Homes.
Approved by La Jolla’s community advisory groups in 2011, and then by the City Planning Commission in 2014, the $13 million construction project was designed by Alcorn & Benton Architects and brought to fruition by Turner Construction, the New York-based general contractor that’s also building the new $5 billion L.A. Rams Stadium. Shoring and pipe-setting began in August 2016.
Not only did TerraCosta’s latest report recommend sealing up the cave, so did its 2018 report. In fact, as far back as 1996, when City geologists first entered Koch’s Crack to evaluate its stability, filling it in was recommended. “However,” TerraCosta wrote, “the City elected to not infill the cave due to environmental concerns.”
At the same time, the engineers reported observing no substantial changes in cave stability from 2002 to 2018. Yet, between 2018 and 2019, what they noticed included: “groundwater seepage and widening for the major through-going fracture (fault), localized collapse of a block from the roof, and the apparent migration of sand from the overlying terrace deposits.”
When reached for comment, La Jolla Bay Homes owner Don Allison pointed to another portion in the report blaming only “the existing wave environment” for finally destabilizing the roof of Koch’s Crack “to the point where immediate stabilization measures should be undertaken as soon as permits allow to avoid the consequences of a larger roof rock collapse.”
Allison said he thought that the emergency work was undertaken “out of an abundance of caution” following the deaths of three people when a 30-foot slab of cliff collapsed in Leucadia on Aug. 2.
The Light contacted two independent geologists for their expertise. Both Diane Murbach, co-owner of Murbach Geotech, and San Diego State University professor emeritus of geology Pat Abbott declined to comment about Koch’s Crack for lack of specific knowledge about it. However, Murbach did caution against rushing to judgement.
“To determine the cause, it would have to take an investigation,” she said. “Sea caves are eroded by ocean waves, but adjacent construction has to be considered, and we also look at earthquakes that occurred, climate change and other geologic hazards. It can be one or more causes.”
Abbott said that, whatever the cause, the worst-case scenario appears to have been averted. “The only way to fix a giant hole is to fill it up,” he said. “I don’t know what special blend of fiberglass and concrete they’re using. But if you fill it up, now the road is going on solid material, so it can’t fall in.”
TerraCosta Consulting and the City of San Diego both declined to be interviewed for this story. In an e-mail, a City spokesperson explained: “Our focus is on the current work being done to reinforce the cave underneath Coast Boulevard so that it continues to proceed safely and efficiently.”
(Editor’s note: This story originally misspelled “Koch’s Crack” as “Cook’s Crack,” since it relied on City documents that also misspelled it. Reporter Corey Levitan investigated the origin of the sea cave’s name and made some remarkable discoveries that led to this La Jolla Light story, as well as this San Diego Union-Tribune story about the process.)
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