tv [untitled] October 2, 2011 3:00pm-3:30pm PDT
>> welcome to the department of building inspection brown bag lunch. you are always invited to join us on the third thursday. today, we have a special program about san francisco's neighborhoods geology. we have frank, the geotechnical engineer who will walk us through a lot of this. we also have an architect who knows a lot about the history of the city. he keeps his eyes open and has a lot of information to share. we also have the chief building inspector.
we are going to go through this by having frank give us a brief overview of the geology of sentences go. then we're going to look at a series of slides around the city. and see how the geology of the city affects the environment. their special problems and issues that arise we will try to answer questions as we go, particularly related to how the environment release to the underlying geology of the city. those are questions that rarely get asked. this is a chance for you to join us and ask your questions as well. welcome, frank. i see that you brought a big aerial photograph with overly geology. >> it is a big google map with overly geology. the different colors depict the different formations or deposits beneath san francisco.
san francisco is a young environment. it is a relatively young environment. the basement rock beneath san francisco is known as the franciscan complex or formation. it is throughout the city, most notably twin peaks, edge hill, telegraph hill. every once in awhile, you hear about those who make the news with a rock fall or landslide. usually occur in the telegraph and twin peaks. . above the rock are the soil deposits.
the most common is dune sand. it is nothing but rocks that has been worn down from the sierras and deposited along the beaches. the wind blew that dune sand over most of the city. it is this mustard color. on the avenues, it is very thick. it can be up to 400 feet thick. as you moved south across slope boulevard, that is the tolar foundation. it was named after the first to score every in -- after the first discovery in the cemetery. the man was out there and noticed this material was unique. he started mapping it. he traced it all the way up to
slope. the dune sand generally has the consistency of sugar. it is fine grained sand. it is usually loose. it is poorly graded. the colma foundation can be the same stand but it is denser. it may have to play in it. it is much stronger. it is an excellent supporting material. it is up to 10,000 years old on the dunes and. the colma is 40,000 to 60,000 years old. in some areas, it may be as old as 130,000 years. as you move across the city and get to the border, you get to ground that was originally under water. that sits on top of bay mud.
wea is ak ve-- that is a very y with the consistency of jello that has been left in the refrigerator for several days. the others are residuals soil. they are either a product of the weathering of the underlying law. peluvium is soil that has been deposited by gravity. san francisco, dune sand, colma, and the others. very simple. because san francisco is so young. -- because san francisco is so young, it is riddled by faults.
i have a map that shows the faults across the bay area and san francisco. the granddaddy of all of these is the san andreas fault. it is the longest. it can generate the greatest earthquakes. >> it is just south of san francisco. it goes offshore down here. >> from my house to the fault is about 3 miles on average. >> the closest area in san francisco to the fault is this area here in the southwest part of the city. people said that the closer you are, there are conditions that exacerbate issues. >> the fault's effect on development depends on the fault type and soil component. the san andreas is the main fault. there is the hayward fault.
the difference between the two is that the san andreas is predominantly offshore. the hayward fault actually crosses beneath populated areas and poses a larger threat. as we speak, there is a 67% probability of a major earthquake on one of these faults in the next 30 years. it is of major -- high probability. it is a magnitude 6.5. the 1906 was 7.9. the 1957 earthquake was 5.7 on the san andreas. the loma prieta was 7.1.
it was so far with it we got a little effects of it. that is the overview of the geology. >> i would like to setup a wonderful old chart of san francisco from the 1850's and 1860's. let's look at this northeast corner of the city. it is really interesting. i am sure that frank has a lot to say about it. the thing i find absolutely surprising about this is that there is this large area called mission bay. out in the middle of the bay is mission rock. many of you have been to mission rock resort. this is part of the shore line now. look how far off shore it was in the 1850's. this has all been filled. many of these areas are now filled. let's talk about that. >> this is montgomery street.
e have thembaradero -- now we have the embarcadero and marina. this map is the infamous lipofaction map. the green areas are the areas that will have potential lip ofaction. that is a loss of strength of material when it is subjected to vibration. it can occur in sand and silt. those have to be saturated or below the ground water level. you can almost type dismount tie
t --tie this map -- this area right here is the same as this one. it is all silt and will liquefy. the marina is in this area here. you can correlate the advancement of the shoreline of san francisco with the potential for liquefaction. >> it is a perfect example. it is all sold and liquefaction. >> treasure island is unique. it was originally developed to be the location of the world fair with the understanding that once it was finished, that would become the location of san francisco international airport. it is 11,000 feet long. in those days, the thought that was more than enough to land any
conceivable airplane. they thought it was ideal. they said that we have all kinds of sand out in the bay from the sacramento river. we will take the stand out of the bay and create this island. the sand is loose and below the water. it liquefied in 1957. it liquefied in 1989 during the loma prieta. it was similar to what happened in the marina. >> as they move along and developed treasure island, the developer also has to provide stabilization programs. >> the sand has to be strengthened so that it will not liquefy. the buildings have to gain support below the sand in more confident material. >> we were talking about the waterfront.
here's the very building. here is the intersection of the water and the land. the area where the ferry building is now was what? >> it was silt. to create the embarcadero, they had all of this exposed rock. they decided to mine telegraph hill for the rock and build a levee and bill in behind the levee. they blasted the rock. it was illegal but they did it anyway. they took the rock and built a rock dike all along here. beneath the fill the latem are
of this mud with the consistency of jello. that layer may be as thick as 110 feet. >> is bad dyke founded on the mud -- is the dike founded on the mud? >> it is. when they dumped the rock into the bay mud, it was like dumping rock into jello. it went down into the mud so that they could build off of that. it then took the angle of repose. it is stable. san francisco came and built piers on piles over the top into
the water. >> the piers are built up over the water. >> refusal is when you cannot advance the pile anymore. this is under the bay mud over there. they meet refusal. they gain support and bearing on top. saltwater attacks concrete. in the old days, they did not understand the effects of salt water, chloride on concrete and steel. a lot of these piles may have started out 24 inches in diameter. they may only be eight or 10 inches in diameter now. whatever steel was in there is long gone all along the waterfront.
>> wonderful buildings. at the double dog and a beer. >> this is wonderful. it is right on the waterfront. it is due piles. >> that is the location of what is now the san francisco port authority? >> yeah, port offices. and there is a new office there and slip. this is one of the few places where they recently provided a small boat docking on the waterfront. there are very few places where you can stop on the waterfront, and have built a brand new public dock. so we're down near the waterfront. what do we have here? >> in many areas, it is shallow
enough that working with the owners, we can convince the owner that maybe he could use some below grade parking, and we make the decision to go deep below the surface and put the building on a foundation. let me back up. there are several foundation types in san francisco. the one that was used in the 1800's was called a wood grillage. they would chop down trees and it would lay lincoln logs, same principle, make the excavation, and they would lay lincoln logs and build what is called a redwood grillage. fortunately, the termites cannot attack at and it lasts hundreds
of years. >> is resistant to decay. >> the second most common foundation, as we move forward, the cement can along. >> all the homes, all the buildings rest on just a typical spread type footing. the spread type flooding takes the load, comes down, and spreads it out over a larger area. that is why it is called spread footing. as the soil types get more and more difficult, we start looking at social foundations that may still be spread. a spread type foundation such as
a mac. -- such as a mat. it is a blob of concrete that is the enough so that if it tries to move, it moves as a unit as opposed to individual footings. eventually, as you move closer and closer to the shoreline, where you cannot get economically justified basement, we use piles. those are the most expensive, where we drive -- the old days, again, we used to take trees, turn them upside down and drive them until they stop. with concrete, we went to circular piles, then square piles. now we have all overcast and micro piles, numerous file types that can be tailored through the actual conditions that exist at the site. this particular case, they're
making an excavation in the rock. there are some parts of san francisco where the rock is shallow. they go down and grab the rocked, and they put them at foundation on the rock and they can build 60-story tower. perfect example, as you approach the bay bridge, on the left there is the building called 1 rincon hill. that is 63 stories tall with a basement, and it sits on a foundation in rock. if you look to the left, you see the infinity. it has the shaped. that is the rock site. they dug down, put it on rock. as you move north, you come to the southern pacific building or the embarcadero centers or the
trans america towers. there, the rock is 40, 60 feet deep. but the decision was made that we need parking. so they elected to dig down to the dents supporting soil and build a foundation on the dance material below the bay mud. so that is a rock site, rock expedite -- rock excavation. this is part of the temporary shoring system that retains the walls of the excavation and prevents the building, other buildings from falling into the hole. >> where were we? >> we were driving around in the marina. we were looking over the perfectly flat marina green, at
the whole marine area, which is also perfectly flat because it was artificial filled from the 1915 exposition. >> that is correct. it was hydraulics and pumped in from the bay. same principle, they built a dike on the outside it, and then pumped in the sand. and as we all know, in 1989, a lot of damage. lawrence can speak to the concept of soft story problems with the buildings, and the look of viable soil. in perfect concert with each other, you get major damage, sometimes collapsed. that is the subject of -- i guess we did a video of that about a year ago? >> would try to pick up all the hazards.
>> this is recreation of the natural landscape? >> yes, but the four points set on a rock. there is a natural rock outcrop their. the natural material was reclaimed land, because it was the original airport for san francisco. it was about 4,000 feet long. in those days, they had small propeller planes and it was adequate. the bridge itself is supported on case sounds that go to rock. -- is supported on casings that go to rock. it is predominantly sandstone and shale. it was the shape of a dome. it was dome shaped. because san francisco was
growing, because they needed the dike, that allowed miners to come in and open quarries. and they blasted. they had no concern about environmental issues, no concern about how they would leave the quality of the rock. as a result, they opened up fractures in the rocke. even today, we are paying for their mining negligence. we read it just last year where 455 vallejo, there was a rockslide that affected properties on broadway and montgomery. at rock slide occurred because of blasting that occurred back in the turn-of-the-century that left the rock week. 100 years later,