Rammed Earth Homebuilding

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

Foundations

One of the beauties of rammed earth is that as soon as the walls are set, the roof, whether manufactured trusses, hand-cut timbers, or round vigas, can be set immediately. Overnight the building begins to take on a sense of completion, and work on surfacing the roof can begin immediately.

The simplest and least expensive of foundation systems involves nothing more than locking the base

of the wall into the earth, essentially beginning the construction of the wall a certain distance below grade. This approach works only in cases where either the climate is very dry or the wall material is not subject to deterioration from contact with moisture or soil organisms. Also, the ground under the wall must be firm enough to resist differential settling.

A more traditional and permanent system involves the excavation of a trench somewhat wider than the wall and deep enough to extend below the depth of winter freezing. The trench is then filled with a matrix of large and small stones, to achieve good weight distribution, and the wall built on top.

In masonry construction, whether block or mortared stone, the irregular surface on top of the stone foundation results in a strong mechanical bond with the wall, an important consideration in seismic areas. A variation on the rock foundation, which has been used effectively in many applications, is the "rubble-filled" trench with a poured-in-place concrete grade beam on top. The "rubble," normally gravel, road base, or crushed stone, uniformly distributes the weight of the walls over the full width of the trench. The concrete grade beam ties the wall system together at its base and makes the required strong connection between the walls and the footing. Where groundwater is significant or winter freezes severe, drain lines should be installed at the bottom of the trench and the water diverted either to daylight or to dry wells.

The concrete beam should be poured high enough to lift the base of the walls above the surrounding ground to protect them against saturation. In areas where the ground freezes during the winter, the rock-filled trench type of foundation can yield major savings in both labor and materials. When wet soil freezes, it expands with enough force to lift even a heavy building. If moisture is allowed to accumulate underneath a foundation, and that saturated soil then freezes, serious structural damage can result. The greater the frost depth, the deeper the foundation must extend into the ground to be certain no freezing occurs underneath it.

In some parts of the world, the frost depth is eight feet or more. Compare the cost of filling a nine-foot trench with cracked rock to the expense of pouring concrete footings nine feet into the ground and then constructing an eight-foot-high water-impervious wall merely to bring the building to the point of starting the actual house walls! The most common foundations today are built of reinforced, poured-in-place concrete. A concrete foundation typically is comprised of two parts: the spread footing and the stem wall. The footing distributes the weight of the building onto the earth below grade. The stem connects the walls of the building to the footing. The width of the footing is a factor of the loadbearing capacity of the soil on which the building is anchored.

RSS | Comments | E-mail | Print