WHAT IS RAMMED EARTH?

• Rammed earth is a simple construction technique based on compacting earth between formwork to make a homogeneous mass wall.

• Rammed earth construction is a process of compressing a damp mixture of earth that has suitable proportions of sand, gravel,and clay into an external supported frame that molds the shape of a wall section creating a solid wall of earth.

• Traditional stabilizers such as limeor animal blood were used to stabilize the material, but cement has been the stabilizer of choice for modern times.

• After compressing the earth the wall frames can be immediately removed and require an extent of warm dry days after construction to dry and harden.

• The structure can take up to two years to completely cure, and the more it cures the stronger the structure becomes.

• When the process is complete it is much like constructing a hand made wall of solid rock.

HISTORY & ORIGINS

• Rammed earth technology has been around for thousands of years, as an ancient sustainable construction material, used to form monolithic walls.

• Long sections of the Great Wall of China, and famous buildings such as the Alhambra in Granada, and the Potala Palace in Lhasa are built in rammed earth.

• Many European castles and the cores of pyramids were made of rammed earth, then faced with stone.

RAMMED EARTH SPECIFICATIONS

• Properly constructed rammed earth walls are unaffected by rain, wind, fire or termites.

• Load Bearing Rammed earth walls are generally 18 to 24 inches thick.

• Non-Load Bearing Rammed earth walls are generally 10-14 inches thick.

• Rammed earth compressive strength is approximately 300psi.

• Rammed earth walls can be stucco-ed, plastered, or left bare to expose the soft, rhythmic layers of earth, like sediment lines in sandstone.

• Rammed earth can be used in any style building with any kind of roof.

• Properly designed, a rammed earth structure will store the sun’s heat each winter and block it each summer, yielding energy savingsyear after year.

• Rammed earth will begin to cure immediately, and can take from several months to several years, depending on weather and humidity to complete the process.

SOILS

• Rammed earth may have originally been developed in climates where humidity and rainfall did not permit the production of soil block.

• For soil block to cure uncovered, there must be at least 10 rain-free days.

• Soils with high clay contentmay be more suitable for ramming, as they tend to crack in blocks when curing.

• Rammed earth soil mixes must be carefully prepared by screening, pulverizing, and mixing to ensure a uniform mix and to break up any clumps.

• The soil mix needs to be carefully balanced between clay, sand, and aggregate.

• A wider range of soils are suitable when a small amount of cement (6% of mixture) is added.

• ‘Stabilized Rammed Earth’ is a strong masonry product which provides excellent thermal mass.

• Earth is a widely available building material with virtually no side effects associated with harvesting for use in construction. The earth used is typically subsoil, leaving topsoilreadily available for agricultural uses.

• Often the soil can be used on the site where the construction takes place reducing cost and energy used for transportation.

• Transporting the soil mix to the forms is a demanding tasks.

• Large quantities of soil must be moved and transported vertically for placement in the forms. This process is not the same as pouring concrete, because the material is not liquid.

• Traditionally, workers passed baskets or buckets of earth up to where it was needed.

• Hoists can also be used effectively for this task.

• Soil is taken from the ground and if necessary sieved to remove larger particles.

• Additives such as straw or limemay be mixed into the soil, which is then dropped into the formwork in layers, usually around 15 cm high.

• Additives such as Kaolin and Bentonite clays are typically used to improve the strength and mechanical characteristics of the soil. 

• Chemical stabilization using lime, Portland cement, and industrial waste products, rich in cementing agents, have been used in an attempt to improve the strength of the rammed earth.

SOIL TESTING

• The tensile strengthof rammed earth samples has been investigated using standard geotechnical and rock mechanics testing methods.

• Shear box testingof rammed earth samples has compared the strength of jointed rammed earth with that of homogeneous specimens. Further testing investigated the strength of  the rammed earth parallel and perpendicular to the compaction plane.

• Reinforcement of rammed earthusing specialist geo-gridshas been tested using the shear boxes, with some surprising results. Investigations on the strength of the bond of steel reinforcing bar embedded in rammed earth were have been undertaken.

FORMWORK

• Form work for rammed earth must be stableand well-builtin order to resist pressure and vibrationresulting from ramming.

• Small, simply designed forms that are easy to manageare most effective.

• Ease of assemblyand dismantlingshould be considered when designing forms.

• A variety of materials can be used, including wood, aluminum, steel, or glass fiber.

• To construct a rammed earth wall, a formwork box is constructed using two parallel timber sides, supported on two or more horizontal timbers.

• Vertical timbers are then placed through holes in the ends of the horizontal timbers thereby restraining the formwork.

• These vertical timbers are connected at the top using rope, forming an open box.

•With traditional formworks, the boards on both sides are held apart and kept together by spacers, which pierce the wall causing openings that must be filled in after removal of formwork.

•With a special formwork, rounded corners and curved walls can also be formed, by varying the size of the boards and spacers.

•With formwork so intricate, the boards must be stiff so that they do not bend outwards while ramming is underway.

•The formwork should be easy to adjust in both vertical and horizontal directions.

• Variations in the thickness of the walls must also be controllable within a specified tolerance.

• A rammed earth wall can thus be constructed using very little manpower and crucially without recourse to temporary works such as external scaffolding.

• Systems for keeping form work in position vary.

• Small clamps adapted from concrete form work techniques work well, although small holes are left when the clamps are removed. Other methods include locking hydraulic jacks, or form work built on posts.

TOOLS

• Once a soil "lift"of 6 to 8 inchesin thickness is in place, the soil is rammed.

• Ramming can be accomplished manually or mechanically.

• Manual ramming is an ancient technique using a large, specially shaped tool with a long handle called a rammer.

• Rammers weigh around 18 pounds, and have heads of woodor metal.

• Differently shaped heads are designed to perform ramming for various form shapes, especially for corners.

• Pneumatic or electric rammers are also used today is speed up the ramming process generating jackhammer-like actions, strengths, and results.

• A pneumatically powered backfill tamper is then used to compact the material to around 50%of its original height.

• Mechanical impact ramming uses pneumatic ramming machines. Only rammers specifically designed for soil are effective (rammers which are too powerful or too heavy will not work).

• Such equipment is quite expensive, but impact ramming is highly effective, and if the soil mixture is good, creates high quality rammed earth.

CONSTRUCTION PROCESS

• The formwork is constantly removedand placed on the next set of horizontal timbers where the process is repeated. Once the formwork has been moved on, the horizontal timbers are removed from the wall leaving characteristic holes. Upon completion of one horizontal level the formwork is moved vertically, the mass of standing wall being known as a lift.

• The wall is so solid that, if desired, the forms can be removed immediately. This is necessary if wire brushingto reveal textureis desired otherwise walls become too hard to brush after around 60 minutes.

• Walls take some time to dry out completely, but this does not prevent further work on the project.

DESIGN CONSIDERATIONS

• Rammed earth walls have low tensile strength, and should be reinforced by providing a bond or collar beam.

• Beams can be constructed of concrete, wood or steel.

• Vertical reinforcingmay also be done, and may be required by some building officials.

• All openings in rammed earth walls, such as windows and doors, must have lintels to span the opening width. 

• Water flow and moisture controlis critical to protect structural walls. Special detailing to accommodate manufactured windows may be necessary to accommodate wall thickness.

REINFORCEMENT PROPORTIONS

• All openings for doors and windows will require aframe. Wood, as opposed to metal, is recommended due to the corrosive action of moisture from the soil material.

• Careful attention to both roof and opening details is necessary to protect the structure from water damage.

• Foundations required by most codes are concrete reinforced with steel.

• Soil block material may be used as a filler material between piers of a reinforced concrete pier and beam foundation.

• Historically, many structures built with earth materials had no foundation, or used sand and gravel foundations. The latter are excavated trenches filled with two parts sand to three parts gravel. Trench bottoms were graded to provide positive drainage.

• Soil material block should not be used in below grade walls unless supported on both sides. Natural moisture from the ground may infiltrate the block, resulting in reduced compressive strength.

BAMBOO REINFORCED RAMMED EARTH

• A Rammed earth wall is erected over a bamboo reinforced monolithic stone foundation, using a special T-shaped steel slip form.

• The 31 inch wide wall sectionsare rammed continuously until the final wall height is reached so that no horizontal shrinkage cracks can occur.

• Each section is separated by a ¾ inch gapto provide for independent movement during seismic activity.

• Four vertical bamboo rodsact as reinforcement for the wall sections, and together with the gap give sufficient stability against horizontal loads created by seismic activity.

• The bamboo is interconnected with the reinforcement of the foundation as well as with the ring beamthat connects the tops of all the wall sections.

• Using rammed earth as a construction material greatly reduces the amount of CO2 released during the construction of the building.

• The manufacture of cement requires the burning of limestone and produces CO2 as a waste product.

• Around 10% of global CO2 emissions are from the cement industry.

• In using soil taken directly from the site, there is no transport requirement, directly reducing CO2 emissions and taking vehicles off the road.

THERMAL PERFORMANCE

• The high thermal mass of rammed earth walls mean they act to naturally regulate the internal temperatureof a building.

• If a rammed earth wall is designed into the heating system of a structure, the energy required to both heat and cool the building can be greatly reduced, which further reduces the CO2 emissions of the building.

• Rammed earth naturally regulates the internal relative humidity of the building, producing an improved air quality.

• This contrasts greatly with air conditioning systems, which act to dry the air in a building and contribute to harsh working and living environment.

STRUCTURAL PERFORMANCE

• Rammed earth is perfectly able to act as load bearingmembers within a structural system.

• The strength of dry, un-stabilized rammed earthis close to 1MPa.

•The strength and stiffnessof the material reduce greatlywith increasing water content, and thus the building must be well designed in order to maximize the structural potentialof the rammed earth.

FEASIBILITY

• Rammed earth complies with Building Regulations for insulation, strength, water resistance, fire and acoustics.

• Not all soils are suitable for use as rammed earth. The soil must be compacted at a particular water content to ensure optimum compaction, and this water content should be predetermined before construction begins.

• Structural design of Rammed earth buildings is usually very simple, with walls acting in compression, but detailing is important to ensure the rammed earth is used to its maximum potential.

EMBODIED ENERGY

• The inherent recycle-ablilityof rammed earth, and the reduction in CO2 emissions during the lifetime of the structure, the high thermal mass and thus low operating costs all mean that rammed earth has a much lower embodied energy than comparable building materials.