Geologist Tells How to Determine Stability of Hillsides

Laguna Beach engineering geologist Fred Pratley has been studying hillsides in the county for 30 years and has watched how heavy rains can cause mudslides and landslides that undermine buildings. After this weekend’s storms, Pratley surveyed the damage to a group of houses on a Laguna Beach hillside where one home burned and another family was evacuated. Pratley answers often-asked questions about the stability of local hills and canyons.

Question: What makes a hillside stable?

Answer: A mass of hard rock that is not exposed to high ground-water infiltration. Massive bedrock, anything from a half-dome to massive sandstone that has sound drainage. It appears the bedrock moved in Monday’s Laguna Beach slide because it probably had a clay seam in it that got wet and failed.

Q: What makes a hillside unstable?

A: The presence of clay and high ground-water levels. That’s oversimplified, but those two guys are dangerous. If you have clay units interspersed with bedrock, you have an unstable potential. But the water is really the culprit. Basically, it is water that causes these slides. That’s the bottom line.

Q: Are other homes in similar danger, even without more rain?

A: No, not in that area. A few of us walked around on Monday and did not see signs of distress or any homes that are in immediate danger.

Q: What are the warning signs homeowners should look for?

A: Look at openings in the walls, such as doors and windows. They should be symmetrical. If they start to stick, it might be swelling from the rain, but it could be the shape of the wall. Check to see if the frames are symmetrical and if there are cracks that weren’t there two weeks ago. If so, that might imply that you have movement in the foundation or movement in the material on which the foundation rests. Other clues are tension cracks in wet soils. This means the soil has moved, and it may reflect (that) the bedrock underneath is also creeping.

Q: Is it easy to tell what kinds of hills or ridges are prone to instability?

A: The obvious thing to look for on a hillside is a slide that has already happened. It looks like a bowl-shaped depression with drainages developed along the sides. Classically, you can see these slides bordered by these drainages. Nowadays, people should be able to rely on the local agency, the city or the county, to identify the areas of potential instability like Laguna Beach has. The slide area is shown on city maps as a potential area of instability.

Q: What should a buyer do before purchasing property on a hillside?

A: Borings are vital, but clients hesitate to pay for them. Sometimes you need only one or two, but we certainly can’t just look at the rock exposed on the surface and be able to tell what is underneath. Borings can reveal a clay seam that could prove to be unstable. But they are expensive. One boring could run $3,000 to $4,000 just to drill the hole. The hole has to be large enough for a geologist to climb down and analyze the material, nose to nose, so to speak.