A study claiming that the existing stretch of Highway 1 through Devil's Slide could be made safe by "dewatering" San Pedro Mountain is too simplistic, the California Department of Transportation has concluded.
Nevertheless, Berkeley geotechnical engineer H. John Hovland said he stands by his report. He suggested that CalTrans has prematurely dismissed a fast and inexpensive repair of the Devil's Slide problem without giving it the same consideration as it has the tunnel.
According to CalTrans' geotechnical staff, the only way to permanently and reliably resolve the long-running problems associated with Devil's Slide is to avoid the unstable area altogether.
"Avoidance is by far the most sure solution for accomplishing this goal, and is achieved by the proposed tunnel," CalTrans stated in its response to Hovland. "With the ever-present threat of irreparable damage to the existing roadway, we do not believe it would be responsible to consider any solution which cannot offer similar certainty."
Hovland, in turn, said Monday that he he is conducting additional analysis for an updated version of the study, but he stands by the findings of his study.
"CalTrans has spent more than $2 million studying the feasibility of a tunnel," Hovland responded, suggesting that the same level of commitment toward his dewatering proposal would prove it to be equally acceptable.
Hovland's study examined rainfall on the slide and its relation to triggering movement of Highway 1. If heavy rain causes road failures, future slippage of the road can be headed off by preventing water from seeping into the slide plane, Hovland believes. The water that does seep in can also be drained more efficiently, he contends.
Hovland concluded that a combination of drainage wells and tunnels would remove water from the ground. An impervious lining on the mountainside would also be used to keep more from seeping in. He estimated that such a repair could be done for less than $10 million.
In 1995 the Highway 1 roadbed fell about eight feet forcing the closure of road between Pacifica and Montara for 160 days. Hovland was a member of a panel of engineers who informally studied Devil's Slide during the closure at the request of then-county Supervisor Ted Lempert. Hovland undertook the dewatering study on his own and was not paid for it.
CalTrans does not dispute Hovland's primary assertion that excessive rainfall can result in high groundwater levels in the mountain, which, in turn, can cause the road to fall. However, it disputes that rainfall is necessarily the cause of all slide activity.
Hovland's report is flawed, CalTrans contends, because it does not consider other characteristics of the slide area, including its depth, the eroding toe of the slide offshore, and the flow of groundwater from other parts of the mountain into the slide zone. Nor does the report include "important, site specific" information on groundwater levels collected by CalTrans from piezometers.
According to CalTrans, the slide's groundwater table is multi-layered and does not drain in one continuous path. As a result, the underground wells and tunnels proposed by Hovland would not adequately drain the mountain, the agency concluded.
Construction of two dewatering tunnels is also problematic, CalTrans asserted. One tunnel would be dug out of a vertical slope 100 feet below the roadway, raising questions about how equipment and workers would get to it. The second tunnel proposed would closer to the road but would be in an area of unstable rock, raising concern for the safety of construction and maintenance workers.
Also, the large blanket Hovland proposes to prevent water from seeping into the mountain would require extensive environmental study, if it were allowed at all.
In response, Hovland said he is currently conducting additional analysis of the depth of the water table and other disputed aspects of his report using computer models. That analysis, as well as consideration of CalTrans data gleaned from instruments installed in the slide that Hovland was unable to review for the initial report, will be presented in an updated version this spring.
But he said he has found nothing to change his belief that dewatering can stabilize the mountain. He noted that it has been used with success in California and other countries around the world.
Despite the large margin of victory for Measure T and the tunnel in November, Hovland's dewatering proposal has picked up some local voices of support.
Montaran Nancy Maule, who has been fighting for a permanent repair of the existing alignment for 25 years, said she believes CalTrans dismissed Hovland's report out of hand.
"They don't really want to save that road," she said. "To them, it doesn't mean anything."
Montaran Vic Abadie, a geologist himself, also believes in Hovland's findings.
"It's clear in their response (CalTrans) really didn't devote much time to understanding what John said," Abadie said.
Abadie noted with irony the parallel between CalTrans' "ignorance of dewatering" and its steadfast denial that a tunnel was a workable solution prior to conducting last year's tunnel study.