... >> i am the chief building inspector. we are here at the base of telegraph hill to talk about a subject of great interest for the people of san francisco which is rockslide, slope stability, which caused a dramatic front-page news. i am here with two knowledgeable and wonderful guests.

i am here with a geotechnical engineer and a geologist. we are here to talk about rockslide, rockfalls, and related issues. what is the difference between a geologist and a geotechnical engineer? >> and engineering geologist deals with identifying site characteristics, mapping, the ground's surface, collecting all of this data. the engineer can come up with medications and designs. >> the structural engineer might be working on a building if there is a building involved. what is your role? >> i am interested in the physical properties of the earth and how the earth will behave:

subject to different load changes. when you are building a building, you are applying a load and you are wondering if the earth will be able to supply that load. we work closely together and we come up with practical engineering solutions. >> this is when someone wants to build something. also, we have a rock piece of land. we have to have a resolution. >> in the u.s., about 2/3 of the population lives in areas that are prone to landslides. about $2 billion of damage occurs annually from landslides. unfortunately, 20-25 million people die as a result of landslides o.

>> much of the coastline is either a bright red or a beige print th. >> here we are at the base of telegraph hill on lombard street. this is owned by the city. behind you is a large piece of something exposed. you are looking at a large class that was xextricated in a quarry about hundred years ago. this is a secretive sandstones, shales, accumulated debris. essentially it ended up piled up here. the quarry activity was so intense and they used some much at dynamite that the kind of over blasted. 10 feet of the face was left

shattered. you can see the fresh colors and a pile of debris which is precariously perched on the edge of a cliff up there. it is more fresh and more recent than the rest. it stands out because there's no vegetation. there is no weathering of material. those are the kinds of things you look for. >> there are many types of slides. there is the bedrock slides which we are looking at in is a combination of a block slide and where the material is just out of balance and is falling off. rainfall pushes from the ground water and out of the cliff. as you can see, the slides artistic one of many types of landslides or slope failures. >> do we have all of these kind of failures in san francisco? >> unfortunately, we have them

all. the rock from here is used from the sea wall. it came from this hill. it was a itgnoll that was present. because of this blasting that occurred, the rock is weak. while most of the time, there are times of heavy rain. sometimes, this has nothing to do with the rainfall. iraq is 50 to an old and then it just falls. in this particular area where we're standing, we have seen the wall at the base. >> the big steel beams with the fence. this is an effective way of

preventing the rock that does fall from causing damage. that might protect the bottom of the hill but it doesn't do anything over the hill. >> it has long been my understanding that the city of san francisco says and the building department takes this position that there is no one buildable lot -- unbuildable lot. i wonder if this is realistic. >> if money is an issue, we can develop solutions that will mitigate the impact. that a solution can be eliminating the hill, that can eliminate the problem. or build a structure right into the hill so there is no longer an exposed face or bold enough

of iraq together so that it acts as a big buttress to prevent further movement. -- or hold enough of the rock together so that it acts as a big buttress. we are allowing the wall of the building to act as a retaining structure. there was a time when we did not get any. there was an epic one before in 1982. we brought with us and rainfall charge. >> you were saying that this is somehow correlated to rainfall. this goes from 1914 until 2006. it is the red line in the backgrounds which are the annual rainfall from each year is plotted on here. you can see it varies all over the place. if you take an average, the

average is a thin gray line. that does not give you a feel for what the rainfall really is. that is an average over four years. you can see that these build up until we had the big events before. >> you expect to see them during the apocrine fall. in fact, they might occur at that time but they may occur the year after and two years after because it has taken awhile for it to accumulate. this triggered some major landslides that we saw. these are portions of the cliff rotates out to sea. >> when we look at this cliff,

can you tell us what of the features that we can see? >> we are lucky to have a very fresh exposure. there is a large overhanging block. at the very edge, you can see the roots coming out. they are part of the problem. in addition, these are sandstone and shale. they have been uplifted and faulted and put into the position they are in today. on the cliff face, you can see the bedding of the shale. there are vertical cracks that filled the water and plans is growing. -- and plants are growing. the cumulative effect of the fresh cut with the debris sitting on the ledge, luckily we have a chance there to protect us from rocks going on the

street. >> this is one type of rock salt and we will look at a few others. what will we see? >> i think it would be appropriate. we have been talking about rock salt. there is instability that occurs at telegraph hill. this was more in the earth flow and debris flow kind of movement. this moves downhill as a result of the rations from the earthquake. that is where we are headed next. >> here we are on the next stop of our landslide for parent though this is the lagoon ouna a

area. this is different from where we all were just add on telegraph hill. this is a whole different concept. tell us what we have to. we have the big hill behind us, what kind of formations do we have? >> this part of san francisco, we are out in the dune sands which makes up the hill to my right. a crusty old drainage that we are standing is bedrock, 160 million year-old sandstone. the project right in the hillside over there and then farther down along the path that goes to the reservoir. >> there are rock slides, there is unconsolidated material.

we have an earth float type slide and a debris flows which are common on the these hillsides where you start to see the evidence of creaks when the trees start to lean over. >> this is a slow movement of the slope. >> yes. you can start to get moving. that will push the trees over a little bit. this had a special history. we had a lot of damage here. i guess the water from leaking laguna runs underground out to golden gate park. it covers about two or three blocks along side to seventh avenue. there was a lot of water damage we might have even had some liquefaction soil failure.

we had a major failure. >> that's right. this tribute goes all the way to golden gate. this is in a mountain lake and beyond ththat, it was a surface flow. this is a reservoir. on my right is this defensive the positive dune sand. the wind blows, it comes from the ocean beach. because the sand is moving, you can see evidence along this side of the hill in 1989. the vibrations in accelerated. you have a significant amount of movement to the point that the homes that uc had very severe damage for two blocks. we recognized what the problem

was at the time, it was a matter of loose sand on a very steep slope. thank you for the effort. we were able to get federal funding and design which you see in the upper walls. the lower walls was actually billed as part of the wpa. the upper wall was built in 1991-'92. this is a concrete wall and it has rocked going through the wall that extends the distance of 80 feet beyond the wall. the next earthquake occurs, the wall is designed to resist and the movement of the sand and allowed it to move down hill. this is a classic example of the creek and degree flow and exists all the way the area. we might see the other evidence. we have done nothing to improve

the quality and the strength of the dam. >> one of the things that has come up in the policy issue over and over is that people are concerned that construction of a new building or the upper story might exacerbate these kinds of soil problems. we have extra review where we might have extra problems. it does that construction have an impact? it is a good idea to come to a peer review or a geotechnical review process as part of determining. >> in january of this year, the california building code requires almost all of san francisco that we get geologist

and a geotechnical engineer. it was a state building code rather than just the local requirements. >> as you recall, this was called the edge hill ordinance. >> is this the same kind of condition? >> this is more of the rock, all on one side and on the other side -- >> this is a combination. >> a lot of sand hills were scrapes down and deposited to other places without being engineered or consolidated and people build on top of that. we have a whole change