Continuous Shallow Foundation on a Beach

[swytiz]

Greetings,

I am the construction manager on a 4 story cmu facility being built on a beach in Florida. Not quite a geotech, but I am a civil pe. The facility was designed with a shallow continuous strip footing about 1'4" below grade. The width of the footing is between 3 and 5 foot (ranging in various areas). The design documents say it was designed base off of an allowable soil bearing capacity of 3500lb/ft^2, but when I looked at the boring log I started to feel a little uneasy. Does this sort of structure pass the sanity check? Water table is normally at 6', but I would assume storm surge would change that, and the soil beneath is light brown sand for the first 10', then light gray sand down to 50'. There does not appear to be any real soil improvement in the design documents.

 

[phamENG]

I can't speak to the bearing pressure, but I'd be a little surprised about spread footings that shallow if you're that close to the water. Large scale scour could be a concern - I'd make sure that was investigated.

 

[LittleInch]

"On a beach" is a bit vague. Do you mean literally with waves lapping the edge or 50m back from the edge?

It does seem rather shallow though. Wind erosion alone could get rid of a foot in one gale.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[swytiz]

When I say on a beach I mean it is 30 foot from the berm. You walk over the foot bridge and you are on the beach.

 

[swytiz]

 

[swytiz]

The building shown was a 2 story motel that got demolished a few years back. The new one will be where the red ellipse is.

 

[HTURKAK]

My points ( without knowing the geotechnical reports ),

- 1'4" foundation depth is too shallow . Considering the erosion, scour , a foundation depth around 3 ft would be better..
- Allowable soil bearing capacity of 3500lb/ft^2 seems high..the Allowable soil bearing capacity is defined for 1.0 m below ( at least at my zone).
- Although 3500lb/ft^2 is vague , the bearing stresses with 3 and 5 foot continuous ftg could be lower than this value..

 

[LittleInch]

How far down do you have to go to get below the vegetation layer??

Time to do some bearing tests?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[soilnwavesco]

Hi mate,

If you have any concern about the geotechnical design please check:
1. The minimum scour depth if it is estimated, this is a constraint for the minimum depth of the foundation.
2. The bearing capacity with the Nspt or another in situ testing.
3. An aerial photography (at least 50 years) to check if there is any morphometry change in the landscape (focus on scour)
4. The estimation of the bearing capacity takes into account the tide range because it will affect the effective stress and the bearing capacity of soil.
5. The minimum depth of foundation in your area. I agree with @hturkak.
6. If there is any check regarding liquefaction (Don't know if this apply to you given your location)

If any questions, txt me.

 

[geomane]

I would like to see the calculated scour depth for your project site. If the surficial soils are scoured, which will remove the overburden from above the base of your footings, your bearing capacity will dramatically decrease. A decrease in your depth to groundwater will also dramatically reduce your bearing capacity.

 

[LittleInch]

I would have thought you wanted piles for a 4 story building

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Panars]

Something doesn't seem right. For the conditions you present (Df=1.33 ft, B= 3 ft and depth to water 6 ft), the friction angle would have to be 34 degrees for an allowable bearing capacity of 3500 psf. And if the water comes up so the ground is submerged, the allowable bearing capacity drops to 1800 psf for a sand with a friction angle of 34 degrees. On top of that, beach sand usually has a low friction angle of 30 degrees or so.

 

[dik]

Panars... Can you describe your calculation a little? I'd like to see how you arrive at those numbers. Not so I can play geotekkie, just so I have a feeling how it's done.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[le99]

The berm might have provided protection to the enclosed land. And, according to the Residential Edition of the Florida Building Code, the minimum foundation depth is 12" below the undisturbed ground. Has the design been submitted for the permit yet or not?

 

[geotechguy1]

It's difficult to say without seeing the geotechnical report and investigation logs. I would want to know how they characterized the shear strength of the soil.

Possibly not relevant to Florida but when I practiced in Canada it was common for low and mid tier geotech consultants to just generically give 100 kPa allowable for clay soils and 150 kPa allowable for sands and gravels (the latter of which is roughly 3500lb/ft2).

 

[Ron]

Shallow foundations along the beach are not uncommon, but not desirable for the reasons noted.

@le99....a four story building is not governed by the residential edition of the Florida Building Code. It would come under the primary building code. Multi-family buildings are commercial construction, not residential.

3500 psf is high. Generally we would expect 2500 for the bearing capacity.

30-35 degrees is not unreasonable for coastal sands in Florida.

Storm surge has little to do with groundwater levels. Surge is transient. Has everything to do with scour, however.

I would have put it on piles.

 

[MotorCity]

I could be onboard with it being that far from the beach. I would question how/if the soil was saturated. If the water content of the soil was high, I would think the 3500 psf was a typo or a brave geotech with a lower factor of safety. When you said it was on a beach, I pictured this thing being some sort of building from Baywatch right in the middle of where kids are building sand castles.

 

[le99]

@Ron

Per 2017 FBC,

1809.4 Depth and Width of Footings

The minimum depth of footings below the undisturbed ground surface shall be 12 inches (305 mm). Where applicable, the requirements of Section 1809.5 shall also be satisfied. The minimum width of footings shall be 12 inches (305 mm).

I am not practicing in Florida, so not familiar with their code. The above is just a search result.

 

[le99]

The 3500 psf bearing pressure may be explained by the "presumptive Bearing Values" provided by the 2017 FBC. I guess this site has bedrock in a shallow depth.

TABLE 1806.2 - PRESUMPTIVE LOAD-BEARING VALUES (psf)

1. Crystalline bedrock 12,000
2. Sedimentary and foliated rock 4,000
3. Sandy gravel and/or gravel (GW and GP) 3,000

I don't mean to downplay the oddness of the data/info provided, but there are obviously more things to be looked into before making a call.

 

[Ron]

@le99...

I do practice in Florida. One of the problems with the Florida Building Code is that it uses the International Building Code as a template and some of the stuff does not get edited very well. Table 1806.2 is one of many examples...we have no crystalline bedrock. The best we have is some ratty limestone from near surface down the center of the state to about 40 feet down when you get away from the "Florida Ridge". We have no GW or GP soils...all SP,SM,SC,CL,ML and organic muck.