Pier Recommendation for Residential Structure located on a Slope

[GranitebayGeo]

I am working on a soil report for a residential project in the Bay Area, CA, and I was wondering how you would determine and recommend the minimum length of the piers and min embedded depth in the bedrock. -Thanks

 

[TigerGuy]

That's a pretty wide open question. Deep foundations rely on end bearing and side friction to hold up (or down) the load. Lateral resistance characteristics keep the elements vertical (or near the original installation angle).

What load combination are you trying to resist? How much movement can the structure tolerate? What foundation system are you wanting to use?

 

[GranitebayGeo]

It is a typical 1,500 sqft residential structure, which is located close to the crest of a slop, in a landslide zone. The log shows the bedrock at 22 ft, and on top of it all fat Clay.
I need to determine the minimum length of piers and min bedrock penetration.

 

[MTNClimber]

The vertical load from the structure is not the concern, it’s the lateral loading on your piles from a landslide which can snap your piles like a twig.

But to answer your question about embedment into bedrock, we would need to know more about the type of bedrock, strength, RQD, etc. Do you have a boring log to share? A wild guess right now would be 5-10’ of embedment into bedrock.

Also, I presume the owner understands that they are building on top of a landslide, and don’t care if they lose their house or their life.

 

[GranitebayGeo]

Thanks, MTNClimber. Yes, they are aware of the risk. The upper soil layer is all stiff "CH". The bedrock is Claystone. We don't get rock coring for residential projects, but they are all Claystone or Sandstone, and we assume a typical Qu. I have done many projects in CO, where we determine the length of the piers and min bedrock penetration based on the uplift pressure from the expansive soil. But My project here is pier footing, on top of a slope in a landslide zone.

 

[theCorkster]

From an engineering perspective, seems that the controlling depth and pier size factors will be the lateral load from soil creep and lateral load from landslide movement and then figuring out the resistance of the bedrock beneath the soil/landslide. The bedrock orientation relative to the slope is very important (in slope, out slope, cross slope). Down slope dipping sandstone/claystone/mudstone can be problematic as the pie auger can deflect due to hard sandstone. You should be working with a good engineering geologist to get that information, and check with the local building department on geotech design reports on adjacent locations.

If located within a larger, older landslide mass, be careful about what exists down slope. I've seen what was considered to be "ancient" landslides creep and move due to downs slope projects. And neither you or you client will have control of that.

What ever you do, you need to make sure that the current client knows that they will have to disclose everything that you have done, and their acceptance of "risk." To accomplish this, there has to be some form of document that will be attached to the property title.

If any distress occurs to the property and/or structure, your seal will found. And if there are any problems, it is my experience that the folks that build homes on or adjacent to landslides in the Bay area because the views are "killer" are usually very well off and have resources (attorneys) to help them into you and your insurer's pocket book.