An earth structure is a building or other structure made mostly from soil. Because soil is easy to find, people have used it for building since ancient times. It can be mixed with other materials, pressed together, or baked to make it stronger.
Soil is still a cost-effective material for many uses. It can also be kind to the environment during and after construction.
Earth structure materials can be as simple as mud or mud mixed with straw to make cob. Strong homes can also be built from sod or turf. Soil can be made stronger by adding lime or cement, or by pressing it into rammed earth. Building is faster with pre-made materials like adobe bricks, compressed earth blocks, earthbags, or fired clay bricks.
Types of earth structures include earth shelters, where homes are partly or fully covered with soil. Native American earth lodges are examples. Wattle and daub houses use a framework of woven sticks and poles to support mud walls. Sod houses were built along the northwest coast of Europe and later by settlers in North America. Adobe or mud-brick buildings are found worldwide and include homes, apartments, mosques, and churches. Fujian Tulous are large, strong rammed earth buildings in southeastern China that house up to 80 families. Other types of earth structures include mounds and pyramids used for religious purposes, levees, mechanically stabilized earth walls, forts, trenches, and embankment dams.
Soil
Soil forms when rock is broken down by chemical or physical processes, moved, placed, and settled. Soil contains particles such as sand, silt, and clay. Sand particles are the largest, ranging from 2 to 0.05 millimeters (0.0787 to 0.0020 inches) in size, while clay particles are the smallest, measuring less than 0.002 millimeters (7.9 × 10⁻⁵ inches) in diameter. Sand and silt are mostly made of inert rock materials, including quartz, calcite, feldspar, and mica.
Clay is often made of phyllosilicate minerals, which have a flat, sheet-like structure. Because clay particles are so small, they interact with each other through physical and chemical processes. Even a small amount of clay can greatly change the physical properties of soil. For example, kaolinite clay does not expand or shrink much when wet or dry, making it useful for brick-making. In contrast, smectite clays expand or shrink significantly when wet or dry, making them unsuitable for building.
Loam is a mixture of sand, silt, and clay, with none of the components being the most common. Soils are named based on the amounts of sand, silt, and clay they contain, such as "Silt Loam," "Clay Loam," and "Silty Clay." Loam construction, which includes adobe construction using unfired clay bricks, is an ancient building method. It was used in early civilizations in the Mediterranean, Egypt, Mesopotamia, the Indus, Ganges, and Yellow river valleys, and in Central and South America. By 2005, about 1.5 billion people lived in homes built with loam.
In recent years, loam construction has become popular again in developed countries. It is valued for reducing fossil fuel use and pollution, especially carbon dioxide, during production. It also creates comfortable living spaces because the material has high mass and absorbs heat well. The two main techniques are rammed earth or clay, known as pise de terre in French, and adobe, which typically uses sun-dried bricks made from a mixture of mud and straw.
Materials
Earth is often processed before being used in construction. It can be mixed with water to create mud, straw can be added, and materials like lime or cement may be used to strengthen the earth. Compacting the earth can also increase its strength.
Coursed mud construction is one of the oldest methods for building walls. Wet mud is shaped by hand to form the base of a wall and allowed to dry. Additional layers of mud are added one at a time, each drying before the next is placed, until the wall is complete. In puddled mud construction, wet mud is placed into a handmade form and left to dry. In Iran, this method is known as chine construction. Each layer of a wall is about 18 to 24 inches (460 to 610 mm) thick and high. This technique is often used for garden walls, not for homes, likely because of concerns about wall strength. A drawback is the long time needed for each layer to dry. In areas where wood is available, wood-frame houses may be built with mud used as insulation. A similar method, using cob, was used in parts of England.
Cob, sometimes called "monolithic adobe," is a natural building material made from soil containing clay, sand, small stones, and organic materials like straw. Cob walls are built in layers without mortar and require at least 30% clay in the soil. Cob can be used as insulation in post-and-beam buildings or as load-bearing walls that support up to two stories. Cob walls should be at least 16 inches (410 mm) thick, with a width-to-height ratio of no more than 1:10. Both inside and outside, cob walls are usually covered with a mix of lime, soil, and sand. Cob is fireproof and helps keep indoor temperatures steady. Tests show cob can resist some earthquake activity. However, building codes in developed countries may not approve of cob as a construction material.
Cut sod bricks, known as terrone in Spanish, can create strong walls. The sod is taken from soil with a dense layer of grass roots, often found in river valleys. It is dried upright before use. European settlers in North America found that sod from dried sloughs was least likely to break from freezing or rain. In Ireland, Scotland, and Iceland, turf was once used for house walls, and some turf houses still exist today. A well-maintained turf house in a cold climate can last fifty years or more. In Iceland, the best turf is called Strengur, the top 5 centimeters (2.0 inches) of grass.
Clay is hard and strong when dry but becomes soft when wet. Dry clay helps hold earth walls together, but walls exposed to rain or water from a roof may become soaked. Earth can be stabilized by adding materials like burnt lime to make it more weather-resistant. This practice has been used for centuries. Portland cement or bitumen may also be added to strengthen earth for construction, though 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.
Soil used for stabilized earth should ideally have 65% to 75% sand. Soils with low clay content or less than 15% non-expansive clay are suitable. Clay content can be reduced by adding sand if available. If clay makes up more than 15%, more than 10% cement may be needed to stabilize the soil, increasing costs. If soil has little clay and includes 10% or more cement, it becomes concrete. Cement production releases large amounts of carbon dioxide, making it less environmentally friendly. Low-density stabilized earth is porous and weak, so the earth must be compacted using machines that make blocks or the "rammed earth" technique.
Rammed earth is a method of building walls using natural materials like earth, chalk, lime, or gravel. A rammed earth wall is made by placing damp soil into a temporary form, which is then manually or mechanically compacted. The form is removed after compaction. Rammed earth is made with little water, so it does not require long drying times. However, it is vulnerable to moisture, so it must be placed on a base that prevents rising damp, covered to keep water out, and protected with plaster, paint, or sheathing.
In China, the Longshan people built rammed earth walls around 2600 to 1900 BC, during the time cities first appeared in the region. Thick sloping rammed earth walls became a feature of traditional Buddhist monasteries in the Himalayas and were common in northern India, such as in Sikkim. The technique spread to the Middle East and North Africa, and the city of Carthage was built with rammed earth. The Romans later brought the technology to Europe. Rammed earth structures can last for a long time. Most of the Great Wall of China and the Alhambra in Granada were built with rammed earth. In Northern Europe, some rammed earth buildings are up to seven stories tall and over 200 years old.
The Romans created durable concrete strong enough for load-bearing walls. Roman concrete used broken bricks and rocks mixed with mortar. The mortar included lime and pozzolana, a volcanic material that added strength. Structures like the Colosseum, completed in 80 AD, still stand today. Their long-lasting nature may be due to the dry mix of mortar and aggregate, which was compacted to remove air pockets. Though made from earth-based materials, concrete structures are not typically considered earth structures.
Building units
Mudbricks or Adobe bricks are shaped bricks made from sun-dried mud. People in many parts of the world started using them as civilizations grew. Building with bricks saves time because each layer of mud does not need to dry one by one. Ancient Egyptian wall paintings show that adobe-making was very advanced by 2500 BC. Today, adobe is still used in many African countries. Adobe bricks are usually made by mixing sand, clay, and water until it is sticky, then adding straw or grass to hold the mixture together. The mud is placed in wooden molds, pressed flat, and then removed to dry for several days. After drying, the bricks are stood upright to cure for a month or longer.
In the southwestern United States and Mexico, adobe buildings had thick walls and were rarely more than two stories tall. Adobe churches in these areas were no taller than about 35 feet (11 meters). Because adobe surfaces are fragile, they are often covered with materials like mud plaster, lime plaster, whitewash, or stucco. These coatings are reapplied regularly. Historically, adobe walls were built by placing bricks together with mud mortar, which expands and contracts at the same rate as the bricks when wet, dry, hot, or cold. Modern adobe can be strengthened with cement, but using cement mortar can cause older adobe bricks to break down because cement and adobe react differently to temperature changes.
Compressed earth blocks (CEB) were once made by pressing soil into wooden molds with a stick. Today, they are made by compressing soil in machines that are either hand-operated or powered. In areas with limited resources, hand-operated machines are often used because they are affordable and can make uniform blocks. These machines may create blocks with interlocking shapes to reduce the need for mortar. Some blocks have holes or grooves where materials like bamboo can be inserted to help buildings resist earthquakes.
To make CEB, the soil must have enough clay to hold the block together but not too much clay that causes the block to crack. When soil is mixed with cement to stabilize it, the blocks need about three weeks to cure. During this time, the blocks should be stacked closely and watered regularly to prevent them from drying out. This can be difficult in hot, dry areas where water is scarce. Covering the blocks with plastic sheets may help reduce water loss.
Earthbag construction is a building method inspired by old military bunker designs. Almost any local soil can be used, though a mix similar to adobe is best. The soil is wetted and packed into woven bags or tubes made of polypropylene or burlap. Plastic mesh is sometimes added for extra strength. Polypropylene bags are most common because they are strong, inexpensive, and widely available. Bags are placed in layers, with barbed wire between each layer to prevent slipping. Each layer is pressed flat after being placed. Structures made with polypropylene bags are flexible like adobe, while those using mesh tubes are more like rammed earth. Earthbags can be used to build domes or tall walls. When soil is stabilized with cement, they can also be used for retaining walls.
Firing clay bricks in a kiln began around 3500 BC. By 1200 BC, fired bricks were used to build strong structures across Europe, Asia, and North Africa, and they remain important today. Modern fired bricks are made from clay or shale, shaped, and then heated in a kiln for 8–12 hours at temperatures between 900–1150 °C. The result is a ceramic made mostly of silica and alumina, with other materials like quartz sand. The amount of air inside the brick depends on the materials and the heat used. The color of the brick depends on the iron and calcium carbonate in the soil and the oxygen in the kiln. Bricks can break down over time due to salt buildup, freezing, or exposure to acidic gases.
Bricks are placed in rows and bonded together with mortar, which is a mix of cement, lime, and sand. A wall made of a single layer of bricks includes stretcher bricks, which are laid with their long side facing out, and header bricks, which cross the wall from one side to the other. Different patterns of stretchers and headers are used in brickwork, such as English, Dutch, and Flemish bonds.
Examples
Earth sheltering has been used for thousands of years to build homes that save energy. There are many different ways to design these homes. One type is completely underground, with an open space above to let in air and light. Another type is built into a hillside, with windows or doors on one or more sides. Some homes are built on the ground with earth piled against the walls, or with a roof covered in earth.
Pit houses built by Hohokam farmers between 100 and 900 AD in what is now the southwest United States were partially buried in slopes facing south. These homes were used for hundreds of years. In Matmata, Tunisia, many ancient homes were built 12 meters (39 feet) below ground level, with courtyards about 12 meters (39 feet) wide. People entered these homes through tunnels. Other examples of underground or cliff-based homes exist in Turkey, northern China, the Himalayas, and the southwest United States. Some Buddhist monasteries in Tibet, Bhutan, Nepal, and northern India were built into cliffs or caves. Interest in these building methods grew again in the 1970s. Building a home into the ground helps keep it cooler in hot weather and warmer in cold weather.
An earth lodge is a round building made by some Native American groups in North America. These homes have wooden posts and beams, forming a dome shape. A typical structure has four or more central posts in the ground connected by cross beams. A hole in the center allows smoke to escape. Around the central posts, shorter posts form a larger ring, also connected by cross beams. Rafters extend from the central beams to the outer ones, and slanted or vertical walls are made from split planks or beams. The structure is covered with sticks, brush, or grass, then layered with earth or sod. Some groups covered the entire structure with mud, which hardened into a shell.
Wattle and daub is an old building method where sticks or vines are woven between upright poles, and then mud mixed with straw and grass is plastered over the walls. This technique was used worldwide, from the Nile Delta to Japan, where bamboo was used for the wattle. In Cahokia, now in Illinois, USA, homes were built with floors lowered 1 to 3 feet (0.30 to 0.91 meters) below ground level. A version of this method is called bajareque in Colombia. In prehistoric Britain, circular wattle and daub shelters were built where clay was available. Today, this technique is still used in timber-framed buildings, but walls are no longer structural in modern buildings.
European settlers in the prairies of North America, where wood was scarce, often built their first homes in dug-out caves on hills or ravines, with a covering over the entrance. Later, they built sod houses. Farmers used a plow to cut sod into bricks 1 by 2 feet (0.30 by 0.61 meters) and stacked them to form walls. Sod strips were placed grass-side down, arranged like bricks in three rows, creating walls over 3 feet (0.91 meters) thick. Walls were built around door and window frames, and corners were secured with rods. Roofs were made of poles or brush, covered with grass, and sealed with sod. Sod houses were strong but often damp and dirty unless walls were plastered. Roofs sometimes leaked or collapsed in heavy rain.
Many buildings around the world are made of mud bricks or adobe. The city of Shibam in Yemen, a World Heritage Site since 1982, is famous for its ten-story mud-brick buildings. The Djinguereber Mosque in Timbuktu, Mali, was first built in the 14th century using round mud bricks and a stone-mud mixture. The Great Mosque of Djenné in Mali, built in 1907, is based on an earlier design from 1280. Mudbrick buildings need regular maintenance, and some were damaged by rulers who let them collapse.
The Casa Grande Ruins in Arizona, now a national monument, is a four-story adobe structure built by the Hohokam people between 1200 and 1450 AD. A Jesuit priest named Father Eusebio Kino first recorded the building in 1694, but it had already been abandoned. A temporary roof was added in 1903, after the building had stood empty for centuries.
Huaca de la Luna in northern Peru is a large adobe temple built by the Moche people. It was built in stages and reached a height of about 32 meters (105 feet), with three main platforms, four plazas, and many smaller rooms. The walls were decorated with colorful murals and friezes, some from 400–610 AD.
Examples of earth-based buildings include:
– High-rise mud brick buildings in Shibam
– Mud wall and mosque in Timbuktu
– Old mud dwellings and modern mud mosque in Mali
– Great Mosque of Djenné, Mali, in 1972
– Casa Grande Ruins National Monument in Arizona
– San Francisco de Asis Mission Church in New Mexico
– Interior of Huaca de la Luna in Peru
– Art on an adobe building at Shantiniketan University in India
A Fujian Tulou is a type of rural home built by the Hakka people in southeastern China between the 13th and 20th centuries. These homes are large, enclosed, and fortified, with thick rammed earth walls up to five stories high. A tulou might house up to 80 families. Smaller buildings inside are surrounded by thick walls that include halls, storage areas, wells, and living spaces. The walls are made by compacting earth mixed with stone, bamboo, and wood, and are up to 6 feet (1.8 meters) thick. These homes are well-lit, ventilated, and resistant to wind, earthquakes, and extreme temperatures.
Ziggurats were tall temples built by the Sumerians between the end of the 4th millennium BC and the 2nd millennium BC. They rose in steps to a temple up to 200 feet (61 meters) high. The Ziggurat of Ur used about three million bricks, each no longer than 15 inches (380 mm). The largest ziggurat was in Babylon, possibly the Tower of Babel mentioned in the Bible. It was destroyed by Alexander the Great, and only its foundation remains. The structure originally stood 300 feet (91 meters) high on a base 660 feet (200 meters) wide. Sun-dried bricks were used inside, and kiln-fired bricks were used on the outside, held together with clay
Structural issues
Regions with low earthquake risk are safe for most earth buildings. However, historic building methods often fail to withstand even moderate earthquakes because earthen materials have three problems: they are weak, heavy, and break easily. Despite this, earthen buildings can be designed to resist earthquake forces.
To improve earthquake safety, builders should focus on four factors: soil strength, construction quality, building layout, and reinforcement. Stronger soil helps create stronger walls. Adobe builders can test block strength by dropping them from a set height or using a lever to break them. Builders using methods like earthbags, cob, or rammed earth may test smaller samples by crushing them after drying them in an oven.
Construction must be consistent and well-understood to ensure strong buildings. A robust layout includes buildings that are more square than long, symmetrical (not L-shaped), and avoid "soft" first floors (floors with large windows or unsupported columns). Guidelines in New Zealand require enough bracing walls in each direction, based on wall thickness, floor height, spacing between bracing walls, and the weight of upper floors or roofs.
Building techniques that are flexible, such as contained earthbags or tire walls used in earthships, may reduce collapse risks better than brittle, unreinforced earth. Adding gravel bases can help buildings absorb earthquake energy. Walls can be reinforced by containing them, such as with adobe, to prevent material loss during shaking. Confined masonry is effective for adobe buildings to resist earthquake forces up to 0.3 g.
Reinforcement methods like plastic or wire mesh, steel, fiberglass, or bamboo rods can increase wall strength. Earth resists pressure well but is weak when twisted. Reinforcement must cover areas likely to break and be securely attached to improve stability. Bond beams at the top of walls are essential and must connect firmly to walls.
Builders should know that organic materials inside walls, like wood, may decay before the building is no longer used. Reinforcement must be attached carefully to handle stronger forces. The best adobe walls with shear strength used horizontal reinforcement directly connected to vertical steel bars that span from the foundation to the bond beam.
Interlaced wood in earthen walls can reduce earthquake damage if the wood is not damaged by rot or insects. Examples include finely woven wood patterns like Dhajji or similar methods.