City engineers will be drilling into the hillside and extracting and testing soil samples on the scene of the catastrophic Mount Soledad landslide over the next week to determine what exactly went wrong, and how best to fix it in the long-term.
Rob Hawk, the city’s senior engineering geologist, described the status of the geological investigation of the landslide which occurred just before 9 a.m. Wednesday, Oct. 3 at a noon press conference on Friday, Oct. 5.
“The landslide is quasi-stable,” said Hawk. “It was very dynamic on Wednesday. It’s slowed down and come to, basically, a stop.”
But Hawk added the slide may not be completely over. “It will continue to adjust over time,” he said. “It’s not a stable situation.”
Asked what else could go wrong, Hawk replied: “If you added water to it, things would change drastically. But it’s dry (right now) and could give us some time to work on it.”
With construction crews working and rigs banging in the background, Hawk said geologists are gathering as much information as possible from beneath the surface of the landslide. “We’re trying to get an idea of what’s below us, evaluate the geology of the area, which is critical because you can’t do anything to fix this slide until we know what we’re doing to fix it won’t aggravate or incite some other movement adjacent to it. Once you know that, then you can start making plans about where you want to go, ultimately, to repair the damage.”
Hawk explained the soil underlying the landslide area is sedimentary rock, meaning it is layered. He likened the soil strata underneath the landslide to a layer cake. “On the outside, when you’re standing on top of the cake,” he said, “you can’t really see what the layers are.”
But when geologists drill holes down through the various soil layers, and geologists go down to inspect them, they can map those layers, describe what they see, and measure their orientation. Said Hawk: ‘This layer cake has been tilted, and that’s one of the problems: It looks like the way it was tilted added to this problem of (landslide) deposit.”
Temporary repairs are being undertaken to stabilize the landslide area so it won’t move any more. Hawk said temporary drainage barriers, piping and small pumps are being inserted in the landslide area to direct drainage away from the site.
Hawk added geological informaton gathered on-site is critical because the more engineers know about the conditions that caused the landslide, the better -and quicker - they’ll be able to arrive at a solution as to how to fix it. Information gathering will take the form of computer modeling and laboratory testing of soil samples taken. “In a week,” he said, “we’ll have an idea of areas of concern, where you need more information and more testing and more modeling.”
Asked what the possiblity is of red-tagged homes, those severely damaged, being saved, Hawk replied: “I think we can write off the ones in the (sink) hole pretty safely, because they’ll be in the way of any repair and they’ve been badly damaged. The ones on the sides that are going to be in the way during reconstruction, we’re going to have to look at whether those can be reoccupied, repaired and restored to a safe and stable condition. There might be something about the site repair that requires that they be removed. We don’t know that yet.”
When queried as to whether the existing geology on Mount Soledad Road made a landslide an accident waiting to happen, Hawk said was a loaded question. “It’s an area that was developed during a time (1960s) when the (building) standards were much less than they are today,” he said. “it’s just like an automobile. If you’ve ever driven a 1960s car, they handle very differntley from a 2007 car because there’s a lot more engineering that goes into newer cars. The same thing is true of the geotechnical world. The techniques they employed in 1960, we wouldn’t allow to be done today.”
The possibility of rain, or other water intrusion, is something that could complicate the landslide investigation. “Water makes it (soil) heavier,” Hawk said. “Gravity is what drives a landslide. Water can soften things up. And we worry about erosion. This is not a good place for a pond. We don’t want a lot of water pouring into the hole. We are very concerned about water.”
Repairing Mount Soledad Road, indefinitely closed to through-traffic, will not be easy, or quick. “We’re going to be looking for the most long-term solution to reopen the road and the alley, and keep it stable and maintain it,” said Hawk. “In a few weeks, we’ll have a better answer about how we’re going to go about it (repairs). But we can’t do a timeline because we don’t know all the things we have to do yet.”
Hawk noted everything about the situation must be carefully analyzed before any repairs can be done to avoid the possibility of missteps that could prove tragic. “You don’t want to cause more problems than you’re solving,” concluded Hawk.