Pad footing:gravel fill against water table? 4

[IJR]

I have come across an application of pad footing (single footing) where water table is very close to surface. Solution was to excavate to frost depth, place a thick layer of gravel and place the pad on this gravel layer.

What is the rationale?

regards
IJR

 

[ishvaaag]

Probably
1) better quality of concrete over water table
2) further economy (less concrete)
3) acceptable behaviour in case of congelation.
4) maybe a near dry footing (less capilarity problems)

Sincerely I don't think 3 is much to rely upon.
Hence I would have preferably made my footing to -1.2 m or so even if underwater (concreting with a lowered watertable) except if I expect severe capillarity problems.

 

[JAE]

Two thoughts.....

Open, capillary breaking gravel will not heave as much as finer particle soil, silt, clay, etc.

Many times geotechnical engineers will recommend a pad of compacted gravel under footings to increase bearing capacity. The pad usually is fairly thick (24 inches to 48 inches or more) and is usually wider than the footing itself, spreading the load over a larger area so that the underlying soil is placed under smaller compressive stresses to limit settlement or failure.

 

[emmgjld]

I can think of several situations which would suggest your configuration. Based upon your description, the most obvious is the gravel acts as an intermediate 'footing/bearing strata'. This 'footing/bearing strata' is:
usually not subject to excessive frost heave,
is more economical than placing the extra concrete,
can be easily worked into a softer soil surface below the water table, thus increasing foundation soil strength,
limiting settlement and/or foundation soil shear.

 

[Ron]

IJR..
Gravel pad provides better capillarity for free water movement away from footing bottom as JAE described, it provides a buffer against frost heave as emmgjld says....

Further, if you overexcavate the surface soils and backfill with a 4-6 inch gravel layer, you will likely prevent minor, localized "mushy" footing bottom which prevents contamination of the concrete and reduces the potential for small, localized settlement of the soil "mush" if the water table drops even a few inches.

Be careful if you put in a thick gravel pad, as you can get fines to ravel into the gravel voids from the sides, which can create surface subsidence. If you have an overlying slab or other structure, this can create a void under it. If your gravel pad is going to be more than about 12 inches thick, I would line the excavation with geotextile and wrap the top of the pad before concrete placement.

 

do you know web sites dealing with the underwater concreting method used to build the Akashi Kaikyo bridge foundations