Engineers remained at a loss to explain how an enormous sinkhole opened in a Richmond street last week, as most evidence remains buried in muck.
Nor could they offer a firm time estimate for repairs, or whether they likely will encounter another.
The 40-by-60-foot crater about 20 to 30 feet deep formed swiftly last week, engulfing cars, sidewalk and part of Via Verdi. A corrugated steel culvert pipe beneath the road may have played a role in the collapse, but it's unclear to what extent.
"We're still investigating why it happened," interim City Engineer Edric Kwan said. "This is the first time we've ever seen a sinkhole in Richmond."
Sinkholes rarely stir up dust in California, at least compared with limestone-filled Gulf of Mexico states such as Texas and Florida. Town-swallowing subterranean caverns generally form because groundwater erodes limestone bedrock with spectacular efficiency.
"There are different kinds of sinkholes, and they are caused by different things," said Leslie Gordon, spokeswoman for the U.S. Geological Service. "Those caused by limestone erosion do not occur around here."
Forgotten mine-shaft collapses also happen in coal country, not so much in California, Gordon added. But plenty of physical forces exist to cause unexpected elevation dips.
"Whatever was underneath it is no longer there, whether through erosion or chemical reactions or some mechanical change, and so the surface layer gave way," Gordon said. "That's the most simple way to think about a sinkhole."
Civic engineers, particularly in the East, also see a nexus between sinkage and moldering infrastructure beneath the streets. A 2008 report by the New Jersey Department of Transportation, for example, found that corrugated steel culvert pipes begin to deteriorate after about 30 years.
"A pipe collapse may result in the above roadway settling, or itself collapsing, which would prove very costly in terms of traffic delays and roadway repair," notes the abstract. New Jersey is developing a plan for maintenance and replacement of its venerable pipes.
A 22-by-16-foot corrugated steel culvert pipe runs beneath Via Verdi, Kwan said, allowing San Pablo Creek to cross below the road. Kwan estimated its age to be about 30 years.
The sinkhole definitely crushed the pipe, but investigators will not know whether its failure caused the sinkhole, or vise versa, until thoroughly clearing out the pit, which still contained a car Thursday.
Work continues, weather permitting. City crews and a private contractor hired to excavate and build a temporary road for residents living farther up Via Verdi contend with sporadic rain. Plastic and fabric sheathed the site Thursday morning.
Residents now use a temporary, one-lane gravel path built on the shoulder of the road. Crews hooked up portable streetlights to direct traffic. Several blocks of suburban, single-family housing and two townhouse complexes sit beyond the sinkhole on streets adjoining Via Verdi.
The city contacted neighboring Rolling Hills Memorial Park about building a longer-term, two-lane road through part of the cemetery's property, Kwan said, but parties haven't reached an agreement.
Contact Karl Fischer at 510-262-2728. Follow him at Twitter.com/kfischer510.