Filling Footing Trench With Washed Stone 3

[XR250]

In our area, crappy soil is common so digging down 5 to 6 ft for a strip or spread footing happens alot.
Most geotechs I encounter just tell the builder to fill the hole with washed stone and then pour the concrete on top of that.
How does that actually work? Wouldn't the excavation walls have to have sufficient strength be able to keep the gravel "column" from spreading out? How do they know that?


Thanks

 

[oldestguy]

This isn't quite like using marbles or ball bearings, since there is a degree of internal friction. Surprisingly how even crappy side soil has significant passive pressure resistance. The main reason for the use is that settlement of that "stone" is very minimal. There ares some risks, such as subsequent erosion of soil into the voids due to groundwater flows, etc. leaving settlement from whence that soil came.

 

[EireChch]

Washed stone does not sound like an appropriate structural fill to me. When I hear washed stone i think of a gap graded rounded stone that has no fine particles.

We had a contractor before who tried to backfill beneath footings using a GAP 7/20mm aggregate. It only had particles between 7-20mm in dia. It wasnt compacting to 98% MDD which wasnt a surprise. We made him remove it an replace it with a well graded angular aggregate.

 

[XR250]

Washed stone (in our area anyhow) typically refers to #57 or #67. It is angular but not sure about the grading

 

[Ron]

XR250....the grading is in the number! (#57 and #67 both have specific gradations as defined in ASTM C33).

I will assume you are referring to more of the coastal plains soils than the Piedmont soils in your area. If so, they are not greatly different than those that we deal with here in Florida.

Using washed stone as backfill for any significant depth in a trench is not a great idea, particularly because of the raveling that OldestGuy notes. We generally limit its use to creating a mud mat where the trench bottom is saturated and you don't want to place concrete in standing water. This usually limits its thickness to about 6 inches and it is intermingled with the poorly graded fine sands to stabilize them and to allow concrete placement in the dry.

 

[XR250]

XR250....the grading is in the number!

Whatever , that is why I use Geotechs so I don't have to worry about that minutia.

I am actually dealing with the piedmont plastic clays mostly - not the coastal soils

 

[Ron]

XR250....my comments are applicable to the piedmont soils as well. Raveling and void filling will just take longer.

As for the "minutia".....it is necessary for structural engineers to know basic soil properties and earth pressures (in particular). The more we know about other disciplines with which we interface, the better our understanding of the design and construction as a system. After all, a structure is a combination of discrete elements (soil, concrete, structural steel, rebar, wood, etc.) working together as a system.