Earth structure
An earth structure is a building or other structure made largely from soil. Since soil is a widely available material, it has been used in construction since prehistory. It may be combined with other materials, compressed and/or baked to add strength.
Soil is still an economical material for many applications, and may have low environmental impact both during and after construction.
Earth structure materials may be as simple as mud, or mud mixed with straw to make cob. Sturdy dwellings may be also built from sod or turf. Soil may be stabilized by the addition of lime or cement, and may be compacted into rammed earth. Construction is faster with pre-formed adobe or mudbricks, compressed earth blocks, earthbags or fired clay bricks.
Types of earth structure include earth shelters, where a dwelling is wholly or partly embedded in the ground or encased in soil. Native American earth lodges are examples. Wattle and daub houses use a "wattle" of poles interwoven with sticks to provide stability for mud walls. Sod houses were built on the northwest coast of Europe, and later by European settlers on the North American prairies. Adobe or mud-brick buildings are built around the world and include houses, apartment buildings, mosques and churches. Fujian Tulous are large fortified rammed earth buildings in southeastern China that shelter as many as 80 families. Other types of earth structure include mounds and pyramids used for religious purposes, levees, mechanically stabilized earth retaining walls, forts, trenches and embankment dams.
Soil
Soil is created from rock that has been chemically or physically weathered, transported, deposited and precipitated.Soil particles include sand, silt and clay. Sand particles are the largest at in diameter and clay the smallest at less than in diameter.
Both sand and silt are mostly inert rock particles, including quartz, calcite, feldspar and mica.
Clays typically are phyllosilicate minerals with a sheet-like structure.
The very small clay particles interact with each other physically and chemically.
Even a small proportion of clay affects the physical properties of the soil much more than might be expected.
Clays such as kaolinite do not expand or contract when wetted or dried, and are useful for brick-making.
Others, such as smectites, expand or contract considerably when wet or dry, and are not suitable for building.
Loam is a mix of sand, silt and clay in which none predominates.
Soils are given different names depending on the relative proportions of sand, silt and clay such as "Silt Loam", "Clay Loam" and "Silty Clay".
Loam construction, the subject of this article, referred to as adobe construction when it uses unfired clay bricks, is an ancient building technology. It was used in the early civilizations of the Mediterranean, Egypt and Mesopotamia, in the Indus, Ganges and Yellow river valleys, in Central and South America.
As of 2005 about 1.5 billion people lived in houses built of loam.
In recent years, interest in loam construction has revived in the developed world. It is seen as a way to minimize use of fossil fuels and pollution, particularly carbon dioxide, during manufacture, and to create a comfortable living environment through the high mass and high absorption of the material.
The two main technologies are stamped or rammed earth, clay or loam, called pise de terre in French, and adobe, typically using sun-dried bricks made of a mud and straw mixture.
Materials
Earth usually requires some sort of processing for use in construction. It may be combined with water to make mud, straw may be added, some form of stabilizing material such as lime or cement may be used to harden the earth, and the earth may be compacted to increase strength.Mud
Coursed mud construction is one of the oldest approaches to building walls. Moist mud is formed by hand to make the base of a wall, and allowed to dry. More mud is added and allowed to dry to form successive courses until the wall is complete. With puddled mud, a hand-made mud form is filled with wetter mud and allowed to dry.In Iran, puddled mud walls are called chine construction. Each course is about thick, and about high. Typically the technique is used for garden walls but not for house construction, presumably because of concern about the strength of walls made in this way.
A disadvantage to the approach is that a lot of time can be spent waiting for each course to dry.
Another technique, used in areas where wood is plentiful, is to build a wood-frame house and to infill it with mud, primarily to provide insulation. In parts of England a similar technique was used with cob.
Cob
, sometimes referred to as "monolithic adobe", is a natural building material made from soil that includes clay, sand or small stones and an organic material such as straw. Cob walls are usually built up in courses, have no mortar joints and need 30% or more clay in the soil. Cob can be used as in-fill in post-and-beam buildings, but is often used for load bearing walls, and can bear up to two stories. A cob wall should be at least thick, and the ratio of width to height should be no more than one to ten. It will typically be plastered inside and out with a mix of lime, soil and sand. Cob is fireproof, and its thermal mass helps stabilize indoor temperatures.Tests have shown that cob has some resistance to seismic activity. However, building codes in the developed world may not recognize cob as an approved material.
Sod or turf
Cut sod bricks, called terrone in Spanish, can be used to make tough and durable walls. The sod is cut from soil that has a heavy mat of grass roots, which may be found in river bottom lands. It is stood on edge to dry before being used in construction.European settlers on the North American Prairies found that the sod least likely to deteriorate due to freezing or rain came from dried sloughs. Turf was once extensively used for the walls of houses in Ireland, Scotland and Iceland, where some turf houses may still be found. A turf house may last fifty years or longer if well-maintained in a cold climate.
The Icelanders find that the best quality turf is the Strengur, the top of the grass turf.
Stabilized earth
Clay is usually hard and strong when dry, but becomes very soft when it absorbs water. The dry clay helps hold an earth wall together, but if the wall is directly exposed to rain, or to water leaking down from the roof, it may become saturated.Earth may be "stabilized" to make it more weather resistant. The practice of stabilizing earth by adding burnt lime is centuries old.
Portland cement or bitumen may also be added to earth intended for construction which adds strength, although the stabilized earth is not as strong as fired clay or concrete. Mixtures of cement and lime, or pozzolana and lime, may also be used for stabilization.
Preferably the sand content of the soil will be 65% – 75%. Soils with low clay content, or with no more than 15% non-expansive clay, are suitable for stabilized earth. The clay percentage may be reduced by adding sand, if available.
If there is more than 15% clay it may take more than 10% cement to stabilize the soil, which adds to the cost.
If earth contains little clay and holds 10% or more cement, it is in effect concrete.
Cement is not particularly environmentally friendly, since the manufacturing process generates large amounts of carbon dioxide.
Low-density stabilized earth will be porous and weak. The earth must therefore be compacted either by a machine that makes blocks or within the wall using the "rammed earth" technique.
Rammed earth
Rammed earth is a technique for building walls using natural raw materials such as earth, chalk, lime or gravel.A rammed earth wall is built by placing damp soil in a temporary form. The soil is manually or mechanically compacted and then the form is removed.
Rammed earth is generally made without much water, and so does not need much time to dry as the building rises. It is susceptible to moisture, so must be laid on a course that stops rising dampness, must be roofed or covered to keep out water from above, and may need protection through some sort of plaster, paint or sheathing.
In China, rammed earth walls were built by the Longshan people in 2600–1900 BC, during the period when cities first appeared in the region. Thick sloping walls made of rammed earth became a characteristic of traditional Buddhist monasteries throughout the Himalayas and became very common in northern Indian areas such as Sikkim. The technique spread to the Middle East, and to North Africa, and the city of Carthage was built of rammed earth. From there the technology was brought to Europe by the Romans.
Rammed earth structures may be long lasting. Most of the Great Wall of China was made from rammed earth, as was the Alhambra in the Kingdom of Granada. In Northern Europe there are rammed earth buildings up to seven stories high and two hundred years old.
Concrete
The Romans made durable concrete strong enough for load-bearing walls. Roman concrete contains a rubble of broken bricks and rocks set in mortar. The mortar included lime and pozzolana, a volcanic material that contributed significantly to its strength. Roman concrete structures such as the Colosseum, completed in 80 AD, still stand.Their longevity may be explained by the fact that the builders used a relatively dry mix of mortar and aggregate and compacted it by pounding it down to eliminate air pockets. Although derived from earth products, concrete structures would not usually be considered earth structures.
Building units
Mud brick or adobe brick
Mudbricks or Adobe bricks are preformed modular masonry units of sun-dried mud that were invented at different times in different parts of the world as civilization developed. Construction with bricks avoids the delays while each course of puddled mud dries. Wall murals show that adobe production techniques were highly advanced in Egypt by 2500 BC.Adobe construction is common throughout much of Africa today.
Adobe bricks are traditionally made from sand and clay mixed with water to a plastic consistency, with straw or grass as a binder.
The mud is prepared, placed in wooden forms, tamped and leveled, and then turned out of the mold to dry for several days.
The bricks are then stood on end to air-cure for a month or more.
In the southwest United States and Mexico adobe buildings had massive walls and were rarely more than two stories high.
Adobe mission churches were never more than about.
Since adobe surfaces are fragile, coatings are used to protect them.
These coatings, periodically renewed, have included mud plaster, lime plaster, whitewash or stucco.
Adobe walls were historically made by laying the bricks with mud mortar, which swells and shrinks at the same rate as the bricks when wetted or dried, heated or cooled. Modern adobe may be stabilized with cement and bonded with cement mortars, but cement mortars will cause unstabilized adobe bricks to deteriorate due to the different rates of thermal expansion and contraction.