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Aggregate placement under foundations
2

Aggregate placement under foundations

Aggregate placement under foundations

(OP)
Upon completion of excavation work for building foundations (industrial and low-rise commercial structures), not residential) it is somewhat common in my area for contractors to place dry, un-graded aggregate under the footings prior to forming/rebar/concrete placement, for spread and strip footings, and small 'mats/rafts'.

Seems most of the time the bulk excavation is possibly accidentally over-excavated and aggregate (being less expensive than concrete) is placed to achieve the bottom of footing elevations.

Is it a somewhat universal practice to use aggregate under footings, and if so, is it typically 3/8"/pea-gravel, or larger material? I have seen surge (7" to 1-1/4") material being used in some cases, but most often it is 3/8" aggregate.

Seldom do I see geotechnical recommendations on this subject in the geotech investigation/report for such structures.

RE: Aggregate placement under foundations

When i worked in NZ it was common to recommend GAP65 (well graded granular with max size of 65mm) for general filling, then within 250mm of slab it was GAP40.

In my opinion, it is best to reduce material size as you get closer to footing / slab.

The material directly beneath the slab should have a mosaic look where the gravels create a smooth surface are well knitted / fitted together.

RE: Aggregate placement under foundations

In southern New England installing crushed stone and sometimes even filter fabric for footings and sometimes slabs-on-grade was the standard construction technique. It helps create a nice work space for the form and rebar contractors. If it rains or snows it's easier to get the subgrade back to where it needs to be versus if it was exposed soil. The firm I previously worked for in New England would have a paragraph in their geotechnical report recommending installing a crushed stone leveling pad during wet seasons for this reason.

The earthwork laborers would prefer either 3/4-inch or 3/8-inch stone as it's easier to spread and obtain the correct bottom of footing elevation. The earthwork estimator would prefer 1 1/2-inch stone for its cost. Rebar clearance is also a headache with 1 1/2-inch stone so the inspectors and rebar contractors would complain when it was used.

As for Colorado, I haven't seen it once.

RE: Aggregate placement under foundations

In coastal SC, the fill material used for this purpose is crushed limestone, either FLBC (fossilized limestone base course) or ROC (run of crusher). The only difference in the two materials is documentation (therefore the price) - FLBC has specs for size, etc. ROC is the same thing... minus the specs. Both are the only reliable high quality fill materials locally available.

www.SlideRuleEra.net idea

RE: Aggregate placement under foundations

In some areas where loess soil is present the use of uniformly graded granular fill adjacent to or under footings is highly discouraged.

This is because the fine, silty loess can erode very quickly upon exposure to water - where the loess flows into any voids present in the granular material and create sink-holes or undermine structures or slabs above it.

Now for well-graded granular material, where voids are not significantly present, this isn't a problem.

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RE: Aggregate placement under foundations

In my jurisdiction 6ft frost depth, sump pits are mandatory whether building in a sand esker or a clay crust. It has been common practise to place slag, essentially 3/4" minus from mining waste for years. Now that people have realized copper, nickel, iron ore slag leaches metals they put in crushed aggregate across the entire excavation. The foundation drainage system is built into the aggregate layer perimeter and sump pit is installed. They form the footings and run sewage connections through the aggregate. Some of the homes also have an emergency overflow sump connection into the sewage line.

Slab on grade construction similar concept to put a non frost susceptible layer to run all the electric, gas, water, and sewage lines through.

RE: Aggregate placement under foundations

(OP)
Thank you all, for your regional practices.

The problem I experience with UNIFORMLY graded aggregates placed under footings is that since it cannot be compacted, at the edges of the footing excavation/formwork is that loose aggregate tends to "spill" beyond the footing line, unless the contractor completely fills the entire excavation area with aggregate to approx the same level. If the excavation is significantly over-excavated (in both area and depth), they tend to only provide the aggregate along the footing area plus a small over-width, and hence the 'spilling effect' until back-filling is completed.

RE: Aggregate placement under foundations

This is a common technique, usually with No. 57 stone, in areas of high groundwater to prevent pumping and standing water in the bottom of the footing excavation. Gravel should be compacted with a plate compactor at the least, for uniformity.

RE: Aggregate placement under foundations

IBC 2012 (the year closest to my desk) 1809.2 "Supporting soils" requires that footings be placed on undisturbed soil, compacted fill, or CLSM. This would seem to require that any fill materials be both capable of being compacted and have been compacted after placement. 3/8" pea gravel cannot be compacted.

I have seen a very heavily loaded slab that was placed over uniformly graded 3/8 river gravel sink as the gravel was pushed into the underlying sandy-clayey-silty soil. When we cored through the slab, the gravel near the slab was loose with fines mixed into the gravel below. In our case, while it was apparently a poor decision to use the gravel, it was not strictly prohibited since it was under a soil-supported slab carrying only the contents of the structure.

RE: Aggregate placement under foundations

I haven't used uniformly graded river gravel for supporting loads. I would imagine that it's actually not a great material to use under foundations and slabs. Stacked marbles is what comes to my mind. The only time I've recommended it is was when filling a void between two walls while trying to avoid one wall acting on the other (it's an odd story).

Crushed stone is the only type of uniformly graded gravel that I've used under slabs and footings. You can definitely compact 12-inches of crushed 3/8-inch or 3/4-inch stone and see the lift densify as the angular stones lock into each other. We've used it on parking structures, hospitals, libraries, and other heavily loaded buildings and slabs. If it was a problem using crushed stone, then most of the projects that I worked on in New England would of gotten call backs to look at foundation or slab damage. As far as I know, everything seems ok.

RE: Aggregate placement under foundations

In Japan, it is typical to use a 50 mm thick layer of lean concrete on 150 mm thick layer of granular base aggregate. This allow also for putting markings on the lean concrete to guide rebar and forms installation. See attached photo.


Under slabs, we just use a 150 mm ~ 300 mm granular base aggregate.

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