Engineer promotes new new Devil's Slide solution

Draft reports floats concept of `dewatering'

Half Moon Bay Review, July 31, 1996

By Eric Rice

A comprehensive study by an East Bay geotechnical engineer claims that a series of measures removing the water under Devil's Slide can permanently stabilize the portion of Highway 1 running over it for less than $10 million.

Dr. H. John Hovland, a member of a panel of engineers that surveyed the land at Devil's Slide last year for San Mateo County Supervisor Ted Lempert, believes that a combination of underground drains and wells, and impervious linings on the surface of the slide plane would be able prevent the slide from moving again.

The slide, a 1,600-foot-long area of rock facing the ocean on San Pedro Mountain, fell about eight feet in the winter of 1995 following heavier-than-normal rainfall. Highway 1, which runs through the slide area, was closed for 160 days before repairs were completed.

According to Hovland's report, which he began in spring 1995, last year's slippage, as well as all other recorded incidences of the slide falling since the late 1930s, can be directly linked to above-normal rainfall. As part of his research Hovland studied rainfall in the Devil's Slide area for every year since 1900.

He found that the slide only begins to collapse when there is between one and four inches of rain above the mean in the month of January or between four and nine inches above the mean in March. For example, in 1995 Devil's Slide had collected 19 inches of rain by Jan. 20, whereas the mean rainfall to that point was only 14 inches.

"I thought that Devil's Slide had been perhaps misinterpreted, or insufficiently studied," particularly with regard to the influence of rainfall, said Hovland in a phone interview on Monday.

Hovland, who studied almost 50 different landslides during 15 years working for PG&E, undertook the study on his own and was not paid for it. He presented preliminary findings of the study to the Mid-Coast Community Council earlier this year.

"Dewatering" slides has been used by both the California Department of Transportation and the federal government for many years, Hovland notes. But he believes his study is precedent-setting in attempting to quantify the amount of rain it takes to activate the local slide. He said he hopes to have it published in a professional journal.

Among the conclusions Hovland reached as a result of his study:

* The road has not collapsed during any year of normal or below normal rainfall.

* All cases of slide movement have started within the two-month period between the beginning of January and early March.

* In the 60 years since Highway 1 was built across the slide, the slide "has disrupted the road and resulted in a serious outage six times, an average of once every 10 years."

* Surplus rainfall from prior years does not appear to reduce the rainfall required to cause sliding during the following year because "the slide effectively drains itself during each dry season."

Hovland's study echoes statements by CalTrans officials in January and February of 1995 when the road was collapsing that it was directly related to heavy rains at the time soaking into the slide plane and weighing the rock down.

According to Hovland, knowing that heavy rain causes the road failures creates an opportunity to prevent the problem from recurring by preventing water from seeping into the slide plane in the first place and by enabling what does get in to drain out more efficiently, thereby reducing the weight on the slide.

"The objective of dewatering at Devil's Slide is to provide enough additional pore-space in the subsurface where the . . . rain above the mean can go without excessively raising the water table," states a draft copy of Hovland's report.

Hovland recommends reducing infiltration into the slide through a series of linings on parts of the surface of the slide.

Deep wells that continuously pump out water from the slide would also be needed to keep groundwater levels below those of an average rainfall year. Hovland calculates that 10 wells operating continuously at two gallons per minute each "would in 1.2 years remove enough water or create enough pore space to prevent the episodes of sliding . . . from once every 10 years to approximately once every 30 years."

The third element of dewatering the slide, Hovland claims, is a series of vertical drainage tunnels throughout the slide plane. As part of last year's repairs, CalTrans had horizontal drains installed, but did not install vertical ones due to technical difficulties, according to Joan Van Velsor, senior engineering geologist for CalTrans' Bay Area office.

On Tuesday, Van Velsor said Hovland's report is being circulated internally and that the agency will respond to it in writing separately from the independent tunnel study the agency is overseeing. The response will come sometime before the tunnel study is completed, she said.

Van Velsor declined to comment specifically on Hovland's study, but said the possibility of dewatering the slide has been studied in the past and was not believed to be a suitable solution.

However, both sides in the tunnel-bypass debate reacted positively to Hovland's study.

John Barbour, spokesperson for the No on T Citizens for Solutions, Not Roadblocks, said he believes it is an alternative that should be more closely scrutinized.

"That is the purpose of the No on T campaign _ to look at all alternatives," he said.

"If he's right, he solved the biggest problem for all of us," Barbour said, adding that Hovland's findings would have to be confirmed by a broad panel of geotechnical experts, however.

Zoe Kersteen-Tucker, spokeswoman for Citizens for the Tunnel Alternative, said that a permanent repair of the slide by dewatering would be allowed under Measure T. Measure T is the November ballot measure asking voters to designate a tunnel as the preferred solution to fix Highway 1.

"It is an interesting question why CalTrans has never studied this alternative," Kersteen-Tucker said.

She said she feels discussion of dewatering would enhance Measure T since it "demonstrates that the tunnel is not the only option."

Copies of the study are available from Montara resident Nancy Maule by calling 728-3388. The Review will also be posting the study on its web site at:

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