A sinkhole that trapped a Glendale woman underground at the Los Angeles Equestrian Center was probably caused by cavities created by an outdated drilling technique, according to Department of Water and Power officials.
On Saturday, Carol Wood was pulled unhurt from the 19-foot-deep sinkhole, which was adjacent to an abandoned well, about an hour after she fell in.
She had stood on the wellhead, in the middle of a bridle trail, to allow some horses to pass. When she stepped off, a two-foot-wide crevasse opened, pitching her into an underground cavern.
The well was built in 1977, using a now-defunct technique called cable-tool drilling.
"When you drill, that material can fall in from outside the (well's) casing as it is being drilled," said Laurent McReynolds, operations manager for the DWP.
"When you do that type of drilling, there are always some cavities outside, but how extensive they are is impossible to tell. In 1977, there were probably sizable cavities outside the casing and now they've worked their way to the surface."
The DWP now hires contractors to drill wells with rotary machines that bore with "far less chance of cavities forming outside the casing," McReynolds said.
Earlier in the week, officials speculated that the well's steel casing might have collapsed, but a visual inspection and the well's relatively young age ruled out that possibility.
The last few weeks of stormy weather likely exacerbated the area's instability.
"The rain consolidates material and creates flow channels for water to collect around the concrete and develop a cavity," McReynolds said. "It starts to carry the material away. When you have a heavy rain, these things start to show up more often."
Because the well was inactive, it was not on a regular inspection schedule, but officials said those inspections can only verify the integrity of the well's lining, not the state of the surrounding earth.
"There is no equipment that we know of to determine how many voids there are outside the casing," McReynolds said.
The DWP planned to finish filling in the sinkhole with 75 cubic yards of gravel by the end of the week.
The seven wellheads near the equestrian center have been temporarily fenced off and will be inspected for signs of instability.
"We will fence these off more substantially in the future," added McReynolds. "This is the first we've had a problem of this magnitude. Mostly we're just concerned with people falling and hitting themselves on the wellhead itself."