Here in no-frost-line Honolulu, I plan to place a heavy utility-use slab beneath my existing termite-eaten shack of a house, which is up on a steep slope 25′ above the tiny road, so it is difficult (at best) to get materials up, or soil out. After I demolish the house, this slab will live on as a parking area.
I’ve long been excavating this space for the slab into the hillside and am now nearly complete, but I abhor the thought of having to bring in any kind of base-course material to prepare the excavation for concrete, not only because this would mean I’d have to excavate that much more soil to provide space for the base, but because it will take many 1.5-ton truck runs to the quarry for me to get the bite-size quantities that this site (and I) can handle.
All of this leads me to question the need for base course (or any other imported subgrade material) to begin with.
My soil is very clayey, so it goes from being dusty and fine when dry, to very sticky and clumpy when wet, but– at what I consider the “optimum moisture content,” it compacts very nicely, finishing off with a hard, dry, “shiny” surface in spots where it has been scraped by machine.
What is the purpose of base-course, really? When applied, it should then be compacted to death, so there is certainly no “cushion” offered through the laborious process of placing and compacting it. And yes, it flattens out nicely, which is great, but not enough justification for the extreme effort involved in getting the stuff in place, in my book. I am not sure if my soil is at all expansive (I think not) but if it were, I would still question the value of a layer of crushed and compacted material placed between concrete and soil: “cushion” would not be the term I would apply to any machine-compacted base-course.
So then: why use the stuff? Why not just compact the native soil and place concrete directly on top of it? If I could pump that base material up here, I might feel differently, but in truth, I would rather add thickness to the slab rather than deal with the base-course problems, if that is indeed a hedge.
Anyone?
Mahalo,
Dave
Replies
Its been a while since I have been to the Island. My first impression is that you do not have highly expansive soil. Highly expansive soil will dry and it will have deep cracks running thru it.
Expansive soil is formed by former lakes of muddy water. Put dirt in a bottle of water, shake it up and then let it settle. The extra fine dirt still floats in the water and then finally settle to make a thin layer of microscopic size particles that are the components of clay.
You are on a hill where expansive soil is not deposited. Just drive your vehicle onto the pad, if the tires do not sink into the dirt or the tires do not compact the dirt, you'll need no gravel under the slab. Just take a look at your neighbors slabs and see if they had any problems.
Your problem will be getting the cement pumped up the hill. Also the hot Pacific Sun might bake the slab unevenly causing a shade crack.
seems like a very regional-specific question...u should inquire with local concrete companies and see what is the norm in hawaii...
i could be wrong but the subgrade materials may be necessary for drainage purposes..
true, regional specs are a good possibility...most of my work has been dealing with plans for federal projects, which are often not well-suited to local conditions and tend to be quite conservative.I guess I need to learn more about soil mechanics. The idea that adding 4" (or whatever, even 4') of compacted material directly and exclusively beneath a slab or other structure in order to mitigate otherwise-poor soil conditions seems flawed, at least in the case when the existing soil can be graded, and doesn't have rock shelves or tips of massive boulders showing, or something like that. But as for sources of local expertise, I am not sure I would consider the local ready-mix suppliers good sources, since though their drivers may see lots of jobsites, the guys in the batch-plant probably don't (maybe an engineer or two I could talk to there, though). If I knew some local concrete contractors, I'd be pestering them for typical prep and their opinions on this. But since I'm doing this myself I hesitate to take up their time...
What is the purpose of base-course, really
Depends.
Under a slab, roadway, or typical inverted T footing?
Slab: On expansive soils, it isolates the surface from movement.
On wet soils it provides a path for water flow and drainage.
On swampy land, it provides an island where trucks can drive and men can walk.
On Radon emissive sites, it provides ventilation.
On mixed soils, rock ledge + sand + clay, it provides a common base type.
On firm, non expansive, non emissive, uniform soil of any type, it's not needed.