Building on a slope site

[deli001]

As shown below, I have this structure on a slope site (up to 6m height at the front). The cyan ones are the piers as columns to support concrete slab (shown as red) above.
My question is what is the effective length of piers/columns (6m or more than 6m to consider the restraints from soil)? I am going to design this structure as normal suspended structure using piers/columns to RC beams/slab on top as moment frames. If there any issues that I need to consider, like soil & water loss etc? Do I need to spray the slope site (shown as blue line) or anything? Do you have better options instead of using piers as columns?
Thank you.

 

[STrctPono]

The answer really depends on what kind of structure this is, the site conditions, and what exists at the lower toe of the slope? What you have depicted looks very similar to some marine piers/wharfs that I have designed prior, so in that sense, this could work well.

The probability of soil loss also depends on the slope of the soil and whether or not there is revetment or vegetation growing on the slope to counteract erosion. You will need guidance from a qualified geotechnical engineer to help guide you on embedment depth of your piers. If lateral seismic analysis is a concern, due to the variation of flexibility in the structure along the different pier lines, I would suggest running a response spectrum analysis while modeling soil springs (derived from p-y curves) along the height of the embedded pier.

Are your piers going to be precast concrete piles, steel piles, or drilled shafts?

As far as a preliminary point of fixity.... that completely depends on the soil type. It will be deeper if soft soils are present and shallower in stiff soils but is typically a function of the pier diameter...

 

[SlideRuleEra]

My question is what is the effective length of piers/columns (6m or more than 6m to consider the restraints from soil)?

Do you have better options instead of using piers as columns?

If the pier/columns are supported by footings, I would consider effective length to be full length of the pier/column (including all of the portion underground). Excavation for the footings will guarantee that pier/columns are surrounded by back fill. Back fill, no matter how carefully selected and compacted, is not undisturbed soil. Ignore it for lateral support.

Frankly, if the pier/columns are so closely designed that effective length makes a difference is their acceptability... they are too small.

IMHO, driven piling are better for this application than footing/pier/columns, but may not be cost effective unless there are a large number.

http://www.SlideRuleEra.net

 

[deli001]

Thank you.

It is a two storey industrial structural. Looking at the geo report, there are around 1-2m fill and then clay underneath.
I am going to use drilled rc piles (not footings as SlideRuleEra mentioned) and rely on its bearing and skin friction to resist gravity loads. In this case how do I design this piles as columns especially when it comes to effective length?
For soil loss issue I cannot find more information from geo report. Lets assume it will happen now before I get further info from the geo consultant, what can I do here?

 

[HouseBoy]

Gravity loads are straightforward (I assume)
Wind and seismic loads (base shear) might be a bit of a trick but I have no idea of the scale of things.

Stability of the slope will affect the lateral load on the piers (for bending).
Resistance to bending will be provided but the soil socket.

Length of columns (for design) will be dependent on the ability of the clay to provide lateral resistance. Typically the top few feet of the resisting soil are ignored and resistance is provided over the remaining depth of the socket with the top 2/3 providing passive resistance and the bottom 1/3 providing active resistance (I think I have those terms right).

Overall stability looks like it can be provided by the fact that the slab is running over to the shallow depth foundations but IF the is lateral pressure from the slope, there will be bending in the piers.
I'd expect the length of the columns should be assumed to be at least 1/3 of the soil socket depth (maybe half the depth to be conservative.

Seems like the soils engineer should be giving you more info about the soil capacity. Maybe if you can provide them with information about the different loading conditions, they can answer better. That's where I'd start (by discussing all of the sources of loading with them).

 

[deli001]

Thank you for your help.
I am wondering what about the stability of the soil itself (ie the soil lost) refer to the failure plane below.

 

[dhengr]

Deli001:
I would get the GeoTech people reinvolved for these kinds of questions, over and above the incomplete report you already have. Particularly, overall slope stability w.r.t. the entire bldg. footprint, bigger than, but much like the green line you show in your last sketch. Not just the little erosion you are showing. Then, you might look at making the bldg. 3 or 4 stories tall on the right side, and stepping the whole found. back into the slope at some reasonable structural spacing and floor height change intervals. This would give the client more enclosed space and the three enclosure walls would probably cost less than the piled foundation and heavy structural floor you show in red.

 

[deli001]

Thank you.
Can you elaborate on "three enclosure walls"?
I will be asking the geo consultant for more information. But now I still have some general questions.

1. Because the structure is at the middle of the slope (the blue line will continuous down at around 20 degree, so the lowest point will be 20m lower then FFL. Does that mean it is likely the piles (understand that I need further info from geo report) need to embed start from 20m below FFL?

2. For the left hand side, I was trying to have slab on grade but I am concerned that since it will add more loads to the soil, increasing the chance of the soil erosion to happen? How do I analysis this even if I have the detailed geo report?

3. Do you have other footing system that you recommend?