Designing Gravel sub-base

[RememberTheAlamo]

Building 50 x 100 house on non-expansive sandy-clay sub-surface. Depth to foundation is nine feet. Perimeter footing is 12" x 36", with 12" wide poured walls.

For the sub-base, intend to use 4" depth of 2" stone (#3), covered with 4" of 1" stone (#57).

House has 80,000 lbs of steel concentrating the bulk of the upper-storyweight on 14 points along the foundation footings.

The combined 8" of gravel seems like surely enough for stability and load distribution.

Is there a structural risk associated with mixing the two gravel sizes and using a total of 8" of gravel?

Believe this would help distribute the high load points better . . . but have located no experts to confirm or challenge the design.

Please contribute thoughts and advice!

Larry (in Virginia)

 

[civilperson]

A savings in effort/cost could be made if the gravel were all one type, the 57 seems appropriate, (passing a 1" sieve). Use of a geofabric can lessen the thickness requirement.

 

[RememberTheAlamo]

Hi CivilPerson!

Appreciate the reply . . . you are a star!

. . . and yes, was intending to use a geofabric.

. . . so, I gather, using two different gravel sizes would not be a problem. The additional gravel is only about $1,600. The additional stability and ability to distribute the loads seems to make economic sense.

. . . The original drawings say a "minimum" of 4" of gravel, but we could not determine the "desirable" depth.

Does the use of the larger rock, on the bottom layer, add to stability and load distribution . . . or is this just something I picked up in a text book back in school?

Appreciate the tips!

Yours,

Larry

 

[civilperson]

Well graded rock has a variety of sizes to match the associated voids between the larger pieces. Change of size in the gradation at various levels can be good when used for filter purposes. The difference between the two stated maximums is negligible and the smaller size will handle easier for building a subbase for a concrete floor. Use either one or both, no structural advantage for either that I know.