Double layer mat foundation with infill between layers - What is the purpose? 1

[StLHokie]

I'm in the process of conducting a structural seismic evaluation in accordance with ASCE 41-17 for an existing hospital ICU in a high seismic zone and came across the foundation sections that can be seen in the image below:

As you can see, there is essentially two foundation layers that sandwich an infill layer. The building is a 1-story concrete shear wall structure with lightly reinforced walls surrounding the majority of the building. The bottom foundation is a mat foundation, with the footprint matching the footprint of the entire building. The top foundation slab ties directly into the walls with rebar being developed, so there appears to be some sort of load transfer at the interface.(So it wouldn't just behave as a slab-on-grade)

The building is in a warm climate (Puerto Rico) so frost depth isn't a concern, and as far as I know, there aren't any vibration isolation requirements of the floor. The only thing that I could think of would be that the foundation is acting as a ballast to resist uplift loading from wind or sliding from seismic loads, but even then, this seems excessive.

Has anyone seen a design like this before? Is there a particular reason why the foundation was designed as such? I'd like to make sure I know the reason for the design before getting into modeling to make sure my bases are covered.

 

[KootK]

I would expect that it truly was intended to be just a slab on grade.

 

[HTURKAK]

Yes...I have seen...indeed i designed.. The competent layer 2 ft below the grade ..the designer founded the mat two feet below and has one foot thk ..the second slab , slab on grade having thk. 4" and top level 1 ft above the grade due to architectural.. .The compacted fill also used for the utilities..plumbing and sanitary installations..

 

[slickdeals]

I have seen this detail used when you're dealing with hydrostatic head. The soil helps to act as a ballast and also allows to act as a layer for utilities.

 

[JAE]

Perhaps also to get below frost level?

 

[r13]

Down to competent soil, and fill the void with excavated soil?

 

[TheDW]

Do you have access to the geotech report referenced on the details?

Maybe its something to do with liquefaction

 

[StLHokie]

@KootK
That was my initial thought, but it doesn't make sense that they would use a mat foundation below the slab on grade. Typically if you've got a slab on grade, you've got spread/strip footings for building loads. Because it's only a 1-story building, and allowable bearing capacity for the soil is 2000psf, you could definitely get by with isolated footings. The mat portion doesn't make sense unless the intention was to have a complete seal of the foundation. That's why I'm hesitant to just classify it as a slab on grade.

@HTURKAK
Not sure I understand what you're trying to say

@slickdeals
This makes the most sense thus far. The site isn't too far from the ocean, so I imagine the water table could be relatively high. There are also some additional detaisl that show utilities running through the infill layer.

@JAE
As mentioned previously, the site is in Puerto Rico, where temperatures rarely fall below 75 degrees!

@retired 13
That would be fine, but the fact that the mat foundation and the floor slab seem to be continuously connected to seal the infill into the space is what is strange.

@TheDaywalker
Unfortunately I don't have access to the geotech report, but as part of our contract requirement we will need to have a site specific hazard report done with field shear wave velocity measurements. If it comes back that the soils are susceptible to liquefaction, it's definitely a possibility, but something I have very little experience with.

 

[r13]

the floor slab seem to be continuously connected to seal the infill into the space is what is strange.

A box needs to be formed to hold the contained soil, so continuously connected all around. Note there is 4' elevation differential between the base of mat foundation and the finished grade. If neither of these elevations can be changed, this is one of the cheapest solution, I think.

 

[KootK]

. The mat portion doesn't make sense unless the intention was to have a complete seal of the foundation.

That's just not the case. There can be tons of practical reason why the structural raft slab might need to be placed below the SOG without getting fancy:

- Utility runs as mentioned, especially in a mech intensive environment like a hospital.

- Frost as mentioned where there is frost.

- Sometimes contractors prefer a second pour to finish nicely.

- Sometimes that's just the elevation where the good soil's at.

 

[SlideRuleEra]

...allowable bearing capacity for the soil is 2000psf, you could definitely get by with isolated footings. The mat portion doesn't make sense...

The site isn't too far from the ocean, so I imagine the water table could be relatively high. There are also some additional details that show utilities running through the infill layer.

Makes a lot of sense... poor soil with large settlement likely (despite low bearing pressure)... add utilities to the infill:

As settlement occurs, everything (including the mat, infill, utilities and the building) settle uniformly. This design may be a cost effective way to avoid having to use a driven pile foundation.

Building settlement "crushing" utilities happens, just takes the right (or maybe should say the wrong conditions).

http://www.SlideRuleEra.net

 

[JLNJ]

With the column loads out at the edge, maybe the foundation system needed the overburden to keep the raft soil bearing stresses at the edge from being too high.

 

[wannabeSE]

This is common with mat foundations. The top slab is not structural (typical SOG) Like Hturkak said, the soil area between the mat and the SOG is for running utilities.

 

[MOJOJOHN]

Economy. Filling to required floor elevation is much cheaper than concrete, then slab on grade.