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Deep Foundation for Multi-Family residential

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Matt Best

Matt Best

Geotechnical
Jul 29, 2022
3
I have a project that is for development of a 13-story multi-family structure in Colorado Springs. The soils beneath the are silty sands with gravel to 70 feet over claystone bedrock. We are considering all options for foundation design, however, most of the options end with large diameter caissons to a depth of approximately 80-90 feet below the ground surface. The total length of the caissons will most likely end up being 50-60 feet in length since there is supposed to be below grade parking. We are also taking into account potential seismic activity. The concern over the caissons is the large diameter, depth and the cost associated with casing the piers due to the potential for the sands to collapse into the piers during drilling. Any alternative ideas would be appreciated.
 
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How coarse are the gravels? Would driven piles be an option? Auger-cast?

What are the foundation reactions?
 
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The gravels are fairly coarse, up to 2 inches were recovered. However, the investigative drilling was performed with a 4-inch diameter, which limited the amount of recovery due to the caving nature of the sands.

We have not received the information from the structural engineers yet, so the foundation reactions are very broad.

We have considered helical piers, however, there are concerns over not reaching the bedrock.
 
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Do you need to reach the bedrock? SPT blow counts?

We build on sand and gravel, in seismic areas, without being able to reach bedrock.
 
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That foundation system seems like overkill. We build more on worse soil conditions in Florida.

 
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Varies with location. Where I am, that would be a precast driven pile job.
 
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A general location would be helpful, due to the characteristic of the various gravelly sand deposits in the area. The Silty Sands with gravel description puzzles me, somewhat, but my experience is over 45 years ago.
If this was 45 years ago, I would say this sounds like downtown area (Ancient Fountain Creek) or Eighth street, top of the hill (Ancient Bear Creek) or Broadmoor Area (Ancient Cheyenne Creek). The bedrock would be the Pierre Formation, sometimes Shale, sometimes a Claystone. There is a difference.
 
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Location shouldn’t matter as much as the structural loads with the density and make-up of the soil.

Since you don’t have the loads, can you give us the general density of the soils and location of the groundwater table?

I know driven piles aren’t common for buildings in Colorado but this may be a job for them. If the structural loads allow, you may be able to get by with shallow foundations or shallow foundations on ground improved soils. Tough to tell with the limited information.
 
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Seismic in CO Springs should be negligible.

I agree that I think you are unlikely to go to bedrock, even at 13 stories -- unless this is a very slender building.

----
just call me Lo.
 
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I have some more information regarding the site. to answer emmgjld, the location is near the downtown area, which is the Ancient Foundatin Creek formation. The density of the sands ranges from 108-134 pcf. All of those test results are in the upper 20 feet, since the caving of the sands limited our ability to collect in-situ samples. Unfortunately, the building is only in concept phase, so we do not have any information for loadings. However, the developer would like to have 2 stories of below grade parking, in addition to the 13 stories above grade. At this point we are considering a mat foundation with ribs for strucdtural loadings or larger diameter piers that do not extend to bedrock.
 
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You need better borings. Get some HSA if above the GWT or cased holes if below. You can't get away with 70' borings using SSA in sands. All of your blow counts will be useless.
 
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I would be talking to the local piling contractors. Driven precast or expanded base case in place (Frankipiles) would be a couple of options, but your local piling contractors would be the best source, constructability wise. They can also advise in relation to the basement retention system.
 
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