Controlling slide danger

Limiting amount of moisture in soil is key to averting landslides, geologist tells Laguna residents.Rain is the enemy of hillside communities.

A strong connection exists between above-average rainfall and landslides, according to city consulting geotechnical engineer Hannes Richter.

“The average rainfall in Laguna is just under 13 inches a year, and every time it exceeds the average we get a landslide somewhere in town,” Richter said. “Rain soaks into the ground, and you’ll have problems.”

Richter was the guest speaker Tuesday at public forum sponsored by Village Laguna to help residents better understand the city’s geology and how to protect their property and themselves in the event of a landslide.

Laguna is pockmarked with potential -- but not confirmed -- slide zones, mapped by the state.

“If you are in one of the areas and it concerns you, or you see telltale signs, the question you have to ask is, What do I do about it?” Richter said

However, a slide in one area does not automatically portend a slide in an adjacent area. Each area must be specifically investigated, he said.

The most thorough process begins with aerial photographs of the site being analyzed by a geologist. The next step is drilling deep into the earth, perhaps 100 feet, and lowering down a geologist to examine the strata.

Movement-measuring devises called inclinometers are installed, followed by laboratory testing.

“Think of the geologist as a doctor doing exploratory surgery,” Richter said.

Solutions are designed based on information gleaned during the investigation.

Among the options to stabilize a slide: digging up the soil and recompacting it, installing caissons -- pillars sunk into the soil -- and pinning the slope with tiebacks.

Hundreds of tiebacks are being installed in Bluebird Canyon, each capable of withstanding 300,000 pounds of force.

Dewatering is usually the most cost-effective option, and that was been begun in Bluebird in August and still continues.

“It takes a long time for that much water to get in, and it takes a long time to get it out,” Richter said.

Drain pipes are essential.

“We are using the whole bag of tricks in Bluebird Canyon,” Richter said.

Once completed, perhaps by next fall, the hillside will be one of the safest locations in town, according to Richter.

“But how do we pay for all this?” Richter asked.

The city was originally denied federal funding, but due to the efforts of Sen. Dianne Feinstein, that decision was reversed.

Another funding mechanism is to form a neighborhood hazard abatement district, paid through property tax bills.

“And of course, vote for Measure A,” Richter said.

Measure A will fund programs and projects that would be delayed or abandoned to pay for the Bluebird Canyon repairs, which will be made whether or not the measure passes in the Dec. 13 election.

The measure also will create a disaster emergency fund of 10% of the city’s general fund, about $3.5 million.

“It’s a terrific idea,” Richter said. “We have problems with rainfall and we will always have problems with rainfall.”

Individual property owners can help safeguard their property by landscaping with drought-tolerant plants, controlling burrowing rodents that let water seep into the ground, installing proper irrigation and controlling drainage.

More sophisticated options probably need to involve a neighborhood or at least several adjacent properties, Richter said.

“Information may not be sufficient from the investigation of one lot,” Richter said.