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Best to use fill or excavate on basalt rock for residential foundation

1ownerbuilder (Agricultural) (OP)
20 May 10 13:24
I am in the process of getting excavation bids to lay the foundation of a residential home situated in central Washington on basalt rock.  The garage (4 car) will be slab on grade and the remaining 1 story home on crawl space.
The slop has a 4.5' grade, from north to south, and I don't want the foundation to go over 4' if possible due to county engineering requirements.
A septic system also needs to be installed.

I also don't want to go dig too low and lose the view (lot sits on a cliff).

My question is which is the best way to go?  Should I excavate, which has many unknown factors due to the rock, or go with fill.  
I am a first time General Contractor/owner/builder.  This first step could be potentially one of the most expensive of the project.

Any thoughts, ideas, suggestions would be extremely helpful
beej67 (Civil/Environmental)
20 May 10 23:25
I'm interested to hear how this septic system on a basalt rock cliff face works out.  My admittedly limited experience designing septic fields makes what you're describing seem rather difficult, seeing how the whole point of septic systems is to distribute liquid waste into the soil substructure, and it sounds like you have very little of that to work with.  Most septic field regulations I've seen on the opposite end of the country (Georgia) demand that septic fields are always in cut, never in fill, and they have a tendency to get really damn big if you're building them in rock prone areas.  

By GC/owner/builder, does that mean you're going to own the thing when you're done?  If I were you, I'd start with a pretty detailed geotechnical investigation, including soil borings, to determine exactly where the rock is.  You should also get perk tests where you think you're going to put your septic field, and hire someone to design it before you get too happy with your lot layout.  If the infiltration rate is too low, that septic field could get a lot bigger than you think.

Outside of the septic issues, your question becomes one of cost.  Price trucking fill dirt to the site, price blasting rock, then raise/lower your building pad on your grading plan until it's the cheapest.  Pretty typical grading VE exercise.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

Zambo (Civil/Environmental)
21 May 10 0:32
Depending on the level of the rock I would as much as possible cut and fill to mimimise the movement of material onto or off the site.

Is it a septic tank that you're considering? You'll need to get a copy of the local regulations and plan this to be near the road for occasional emptying.
Dozerman56 (Civil/Environmental)
21 May 10 19:32
I strongly suggest that if you haven't already gotten approvals for your septic field, do it before going any further. In many jurisdictions they'll tell you exactly where you're going to put it and exactly where your repair area will be. Most will also have setback requirements from your foundation to your septic field, and your well also, if applicable. You kind of need to put the 2 dimensional puzzle together first before working on the third dimension.
1ownerbuilder (Agricultural) (OP)
21 May 10 20:21
Septic is not an issue.  The drain field has already been placed in an area and permitted with the appropriate soil profile.
beej67 (Civil/Environmental)
22 May 10 14:07
Do you know how deep the soil strata goes before you hit rock?  Is the rock rippable or would you have to blast it?


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

1ownerbuilder (Agricultural) (OP)
22 May 10 20:42
The soil is 24" in the shallowest area.  We don't want to blast due to close proximity of other homes and well.  We would need to rip or hammer it.  After meeting with a couple more excavators, I am leaning towards fill.  Ripping/hammering through rock has so many unknown variables.   Turns out the rock is andesite, not basalt.
Using fill only, gets the house higher than I originally wanted, but it also improves our already spectacular view.  We would do decking instead of patio.  
Grouser (Civil/Environmental)
28 Jun 10 15:03
What is the frost depth at your site?  Are there high winds that require uplift resistance?  Fill should provide a better "cushion" if seismic activity is a concern.  Will high rainfall amounts be an erosion concern?


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