I’m building a cement block garage “24 x 32” on a hillside, hard-packed dirt. A foundation of 30 x 10 poured cement. I would like to know would 3-inch crushed rock compress better than 1/2-inch crushed rock for filler under my concrete floor? The cement blocks are for the foundation only, I will stick-build the rest of the structure. The back wall out of the foundation is 10 feet in height, the front wall is 2 feet, so a lot of fill is required.
Edited 12/22/2006 11:21 am ET by ajrich
Replies
I am not an engineer nor an excavator.
However I would look at sand, 3" rock (unless it is 3" minus, meaning it has smaller rock and "fines" with it) rock will not compact very well in my opinion. Perhaps there is an excavator or engineer here who can give all the reasons. My experience (and I have done a lot of jobs requiring large amounts of fill) is that the smaller the particles of the fill the better. Fill is placed in small lifts and compacted as you go. The amount of the lift is determined by the fill and the size of the compaction equiptment.
I hope that you have had that tall wall engineered, that much fill being compacted is going to generate large lateral forces on the walls.
I was pointing out in another thread of his that it also depends what soil and water conditions he is compacting it into.
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I saw that , as you said, not enough information. I was making a lot of assumptions in my reply.
I was Envisioning a daylight cut to bottom of footings and fill all the way up. It is quite possible the has a stepped footing or any one of a number of other variations.
Having seen the consequences of improper fill and compaction both immediatly and yrs. after I am very wary of deep fills without both a soils and structural engineers input . Nothing quite like looking at a 20' tall wall that has cracked , bowed and is tipping over and has to be replaced just 6 months after it was built.
Tearing out and replacing tall walls due to structural failure as a result of wrong materials and technique is a costly way to learn that "just any dirt ain't the right dirt"
What works with one soils type and location can cause failure in other situations.
Short answer: Probably not, but more info would be helpful.
The purpose of compaction is to achieve an appropriate measure of bearing competence, to avoid any lateral displacement, and to avoid settling greater than about 1/650 of the span -- related concepts but not identical.
This is best achieved by use of a well-graded material of suitable composition. That means no organics, minimal expansive component, and a range of particles from silt to about 3/8". The more fractured, generally the better.
The material under your footings to a depth at least equal to the width of the footings, and to a width double the width of the footiongs (e.g. for 16" footings, then 16" deep and 32" wide) is critical.
Now if your footings are going on undisturbed native soil and you are only talking about filling in the area inside the stemwalls and underneath the slab, this is a different story.
You will have some substantial localized loads, but proper reenforcement will go a long way towards taking care of that.
The problem with 3" crushed is you are going to have a tough time with three things; it's hard to handle, it's very hard to hit with enough force to settle, and you are going to have voids unless it has a good mixture of other sizes. If it does have a mixture of other sizes, then the compaction equipment tends to float on the big chunks.
How deep are you needing to fill?