BUILDING SITE ANALYSIS
Good site drainage is very important in preserving the stability of tire walls – especially where earthcliffs are used. Controlled drainage both surface and subsurface must be provided to insure that the foundation area remains dry throughout the life of the building. There shall be no standing water within 50 feet of building. Landscaping of adjacent grounds shall be designed to detour all surface water runoff around the building. it is also necessary to determine if any underground springs exist on or near the site.
In the same tradition as the thick adobe walls southwestern Indians built centuries ago, the walls of this massive solar home are designed to keep the interior cool during the day by absorbing the sun’s rays until nightfall when the energy begins to come out of the walls and heat the home. non-traditional, however, is the use of tires to support the adobe walls. These tires are packed with adobe earth and stacked flat in a brick-like manner. A sand, straw, and adobe mix is then used to pack the spaces between the tires and as a plaster to cover the wall surface. The walls serve an ecological purpose by providing a use for otherwise discarded tires and an economic purpose in creating the needed mass for passive solar heating and energy conservation.
Based on the tests conducted for this analysis and the calculations performed, it is felt that the rammed earth-filled tire walls used in Earthship construction will provide safe and dependable support for the structure. The laterally braced walls have sufficient strength from their massive structure and internal tire – to – tire friction to resist the sliding and overturning pressures imparted to the walls from the outside.
Concrete footings below the tire walls are not necessary because the typical width of the tire wall, 2’-4”, disperses the weight in such a way that the pressure under the walls is so low that settlement should not occur. (See Section IV. “Soil Loads” and Evaluation Letters from Delapp Engineering in Part I).
The tire walls are capable of resisting an equivalent fluid pressure of 36 pounds per cubic foot (pcf) in a free-standing condition. However, there should be no lateral pressures exerted against the tire walls until they are restrained via the U configuration or battered tire wall at the closed ends and the wall returns at the open ends. With these restrains in place and the roof diaphragm completed, the tire walls will be capable of resisting lateral earth pressures as great as 128 pcf.
Frost protection must be provided for the bottom courses of tires at the short wall supporting the inclined windows and for the tires atop the earth cliff. This can be achieved either through berming soil against the tire walls of through burial of the bottom course. The required depth of bury or berming can be obtained from your building official.
For more thorough analysis of tire wall construction, see Buckhorn Geotech’s engineering evaluation of Weaver residence included in this report and analysis by DeLapp Engineering (see evaluation letters).
BATTERED NORTH TIRE WALL
After analysis of the new wall configuration shown in sk-1 and sk-2, Buckhorn Geotech concluded that straightening the formerly curved back wall will not adversely affect the tire wall strength nor its ability to withstand lateral earth pressures so long as the top of the wall is laterally braced by the roof system. This bracing can be achieved through either the use of a beam constructed against or on top of the tire wall, a soil anchor or tieback into the fill embankment, or through a direct connection between the tire wall and the roof system. in the latter case, the roof must be constructed with sufficient strength and rigidity to transfer the lateral load to the north-south shear walls.
Battering of the tire wall will increase the stability of earth cliff by moving the wall load away from the cliff face. It will also enhance the wall’s ability to resist overturning forces applied through the retained soil. However, the height of the battered wall should not exceed 6 feet without special provisions for backfilling. Where a height greater than 6 feet is desired, the fill should consist of granular material compacted to 90 to 93 percent of its Modified Proctor Density. The fill should be placed in concert with the wall construction that each overhanging tire rests both on the tires below and on the compacted fill. It should be noted that over-compacting of the fill adjacent to the tires could damage the wall. Care must also be taken to insure that the battered wall interlocks at each course where it adjoins the north-sough walls. Blocks should not be used in the interlocking area. If blocks are needed, they should be placed in the north-south walls at least two tires away from the joint. Also, the blocks should be staggered so that they are not placed one atop the other nor should they connect on the diagonal.
It is recommended that you have your building site investigated by a soils engineer to insure that it is suitable for construction of an Earthship. The worst case soil loads for various snow loads are shown at the end of this section of the generic Earthship calculations and can be determined using the chart below.
Use local snow load requirements to calculate minimum Soil Bearing Capacity (Soil Load) required for your building site. Have soils engineer verify that adequate bearing capacity is provided at site.
The bottom tires of a tire wall should be seated on dry, firm, undisturbed soil with a bearing capacity meeting the requirements shown above. The soil should not contain organic debris or any pockets or lenses of soft yielding material.
The attachment of the roof to the top of the tire wall is important to provide the lateral bracing needed to stabilize the tire wall. To insure an adequate connection, the anchor bolts tying the tires to the concrete grade beam should extend at least 7 inches intot he concrete cavity carved into the top row of tires. There should be at least 3 inches of concrete covering the anchor bolts on all sides. Also, the cavity should extend at least 2 inches beyond the inside lip of the tires as shown on the design drawings. Anchor bolts from the grade beam to the sill plate should conform with Uniform Building Code requirement for conventional construction.
LATERAL LOADS AT TOP OF TIRE WALLS
The tire walls will impart small lateral loads to the roof diaphragm that must be resisted to assure the wall stability. These loads are calculated by a soils engineer based on the soil pressure of each particular site. These loads can be determined from the graphs shown below and on calculation sheets 39 and 40. The first graph is for the walls on the east and west sides of the structure and the second graph is for the north walls. note that the plywood diaphragms will have to be blocked or some other form of additional lateral bracing provided if the lateral loads exceed 285 plf along the walls at least and west ends, or 215 plf along the wall at the north end of the structure.