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Building on a mountain side with fault lines

Building on a mountain side with fault lines

Building on a mountain side with fault lines

There is a .25 acre residential plot on the side of a mountains in Utah. There is houses all around it, but they were all built 30 years ago when the building code was not as strict. The property is on a pretty good slope. After the survey was done, it was found that the property had 3 fault lines cutting the property into about 3 equal sections. Because of this, the property was not able to be built on. The question is if you put in reinforced cement pillars on each of the outside sections deep into the ground and then connect them with steel beams, so bypassing the middle section all together, wouldn't that meet the building code?
I have attached a quick picture of what I am talking about. The blue circles would be where the cement pillars would be install. Or can you never cross any fault line when building a house? Some reason in my mind I would think the 2 outside sections would have the same stability and the middle one would be the one that would flow the other direction in the case of a earth quake. I could be completely wrong since I study Mechanical not civil. Thinking about it more, I think i am trying to apply tectonic plate movement to simple faults on a mountain side, as if the outside sections would move the same, or shift the same since they both are up against the middle section. The code probably has more to do deal with mountain soil movement and stability then anything to do with earth quake stability. If this is right, then it wouldn't matter if you connect the outside 2 sections, the only option would be to put the cement pillars in deep enough to overcome the issue of the soil movement. Again, I am a mechanical not civil so these are all guesses.



RE: Building on a mountain side with fault lines

I think I would lean to fixing to the center one and erecting outriggers of sorts to the other two allowing the movement to occur rather than trying to restrain it, which will not work with nature involved.

Mike McCann, PE, SE (WA)

RE: Building on a mountain side with fault lines

I would have to ask too, are these truly fault lines or just local fracture zones in the rock?

Mike McCann, PE, SE (WA)

RE: Building on a mountain side with fault lines

Yes I think msquared is probably on the money. The sketch actually shows only two "faults" providing the three sections alluded to. If these truly are faults then they almost certainly travel onto other lots where houses have been constructed. If zero or only minimal movement has occured over 30 years, I'd be quite comfortable at least proposing construction. Mind you if there is another fault or fracture zone in the third dimension , then yes there is definitely the potential for instability. Cries ut for a quick review by a competent geotech engineer.

RE: Building on a mountain side with fault lines

From what I heard the city denied building on it because of fault lines. I guess builing on the middle ones and like a tree foundation might work. I am sure it would be a lot safer then the building across zones. It is a local soil fault line. I am not sure the exact name, Msquared48 called it local fracture zones, but I thought fractured zones is where the 2 sections are separating creating different levels or a divide between them?

When you look at utah mountians and see the lines in them, they call them fault lines, so I am not sure why these wouldn't be called fault lines?


RE: Building on a mountain side with fault lines

Major and some minor fault lines are normally found on geologic maps, WHEN KNOWN, where documented and/or recognizable vertical and/or horizontal movement has occurred due to seismic events.

Fracture zones can be smaller faults, per se, not shown on geologic maps, but they can also be areas where the rock has fractured or weathered due to freeze thaw action. It depends on your local area.

I would really be surprised for three fault lines, where documented seismic movement has occurred, to be located on one building lot. How big is the lot?

Mike McCann, PE, SE (WA)

RE: Building on a mountain side with fault lines

As far as I know, building codes prohibit building on active faults, that is faults which displayed movement in the latest 10000 years (according to USGS). Some other agencies define a fault active if movement can be proved in the latest 40000 years. Bottom line is that there is a non negligible likelyhood of present occurrence. Fault are defined active by governement agencies or by peer-reviewed reliable scientific articles.
If those faults are active, I wonder about the wisdom of erecting a new building amongst them, no matter the countermeasures and no matter the existance of other buildings. Unless it's a depot without people living inside. Also, maybe I missed something, usually a minimum safety distance is required from active official fault lines like in California codes.
If they are not active, then the issue is just that of a mechanical dishomogeneity or fractured area.


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