Soil Cement
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about Soil Cement

We're not talking about rammed earth with huge strong forms nor about  rolling a mix on the roads!

Hey, soil-cement is pretty regular stuff - you just mix your own with regular portland cement and aggregate from your own building site!

Today I cruised the net (again) searching for "soil cement homes" and found very little indeed.  What I did find was many pages describing rammed earth, and many others describing the manufacture of bricks and blocks of soil cement. 

But it would appear that the largest use of soil cement is for bank stabilization in floodways and highway banks, and for roadway bases done by mixing portland cement with the natural roadbed materials, adding water and roller-compacting.

My SolarSense approach to this construction-type is somehow different, and it surprises me that it is not a fairly standard construction process by now.
What is Soil Cement?
Let's define Soil Cement as it applies to this website only.  Soil cement is made by simply mixing the natural earth/rock materials from your building site with sufficient portland cement to make a mix with sufficient strength and plasticity for the construction of floors and walls.  The result is concrete - the quality is dependant on the ratios of the earth, sand, rock, portland cement.
Compared to regular Concrete....
Soil Cement can be very similar to the concrete delivered by the redi-mix trucks at rather high prices, which prices increase if the load is small or if it takes you too long to pour it.  But the materials?....  Redi-mix concrete use sand and gravel - the same stuff you may find on your building site.  It is washed and screened for size, then combined with concrete and water in ratios appropriate for the project.  You'll have problems if it contains too much clay, and certainly you don't need leaves, roots, grass, or other debris - just dig down and get the good stuff.
The chief variable in any soil-cement mix is in the native materials available.  Sand and rock is good; clays give shrinkage problems & increase the percentage of portland cement - plant roots and humus are bad for strength and a problem when finishing.  Every case has its own unique characteristics.  

When one is pouring thick walls of soil cement, the strength of the mix is critical.  Concrete is  tremendously heavy stuff - it weighs 144 pounds per cubic foot, which ironically works out to 1 psi (pounds per square inch) per vertical foot of wall load imposed.   But, considering that the concrete we normally use in footings is 2,500 psi, there's a lot of latitude!  One may wish to consult an engineer about such things.

The loads imposed by walls typically 9 or more feet tall must be considered, both in the design of footings and in the mix for the walls.  For cost-saving, the ratio of portland cement in the mix may be reduced as the wall height increases and the imposed loads are smaller.

Footings:  This house will be HEAVY - you'll need decent bearing - solid ground or rock.  A conservative rule of thumb is that undisturbed soil below frost-line should bear 1000 # / SqFt.   The minimum horizontal rebar would be 2 #4 (1/2") running horizontally; you may need more.

Of course the advantage of all this weight is in its utilization as thermal mass, the essential element in any passive solar home.

Testing:  Where there are concerns about the strength of the mix, pouring test cylinders is simple and inexpensive; testing companies can be found almost everywhere.

Reinforcing with re-bar and/or wire mesh would be required by building codes for most soil-cement wall construction.  It's not especially expensive, and as it greatly enhances the strength and stability of the footings and walls, one would be foolish to ignore it!  Again, consulting an engineer is prudent, and may well save money by avoiding over-reinforcement just to cover our own ignorance!

If this stuff makes some sense to you, you'll be wondering how the heck you're going to handle with all that mixing and pouring.  Time to check out my Pour Easy system!

Updated by RonKZ Sunday, August 15, 2004

copyright 1998 - 07 July 2006 by Ron Klotz-Zellhoefer, SolarSense Designs, Arizona & New Mexico

 Permission is granted and welcomed for personal application only.