Building Walls with Rammed Earth
Rammed earth walls are made from earth and water. The ideal soil to use for these structures is a blend of small gravel, coarse or fine sand, and clay. The best ratio for these materials is seventy percent sand mixed with thirty percent clay. The soil in the area of your root cellar can be tested to see if it is suitable for this use; if not, you can have better soil brought in. Cement can also be added to the soil to increase its strength as well as help to keep moisture out of the walls. Steel reinforcing bars are often used in the foundation and walls for added structural support.
The basic procedure for building a rammed earth structure has not changed in the past two millennia. Suitable moist soil is compacted one layer at a time into a form that is constructed where the wall is to be. The form can be made out of two pieces of plywood with wood connecting them. The form is constructed the width and height of the wall you are building. Once the form is full and the last layer is compacted, the form is taken down and the process repeated for all remaining walls. The earth is compacted by hand or by machines.
The benefit to making rammed earth walls is that earth is always readily available. In addition, it is unprocessed, low-cost, heat-storing, load-bearing, recyclable, and durable. A properly built rammed earth root cellar gains free heat from the winter sun and stores that heat in its walls, preventing your food from freezing. During summer months, the walls absorb excess heat from the inside if the structure is properly shaded and ventilated, helping to keep the interior cool and allowing your food to stay fresh longer.
To test your soil to determine if it will work well for building a rammed earth structure, construct a small sample block so it can be checked for strength and weather resistance. Spray the block with water from your garden hose at full force for an hour to see how it holds up. If necessary, modify your earth mix and make another test block.
There are concerns in many government building departments regarding the safety of rammed earth structures, and this may make it more difficult to get a permit to build a larger structure. However, your root cellar can easily be built small enough to bypass any need for a permit. Historically, rammed earth structures have stayed standing for hundreds of years. They are not dirty, do not wash away in the rain, and don't crumble in an earthquake.