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Introduction to Types of Earthwork – Earthwork Forts
Building earth forts dates back to before the Bronze Age, and these were traditionally built using earth as the main component of the material used.
There are still many examples of earth forts around, in all parts of the world, having survived conflict and the different climates for centuries.
This is an article on earthworks, and in particular earthern forts. Here we will examine the methods used in construction along with the different mixtures of earth which made up the all-important defense walls of the earth forts. We begin with a look at the different methods and types of earthworks which could have been used in the building of earth forts.
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Types of Earthworks-Methods used in Building Earth Forts
- Rammed Earth
Some type of foundation was used under the first layer of rammed earth, usually stones and rubble or layers of wooden logs laid lengthways.
Rammed earth was used to build walls, and it consisted of a mixture of clay/earth, sand, ox-blood and straw that is moistened with water into a dough-like substance.
Temporary, movable wooden forms which act as molds are made into which the mix is shoveled to a depth of around 8” before being tamped and compressed to around 4” using a wooden ramming/tamping tool, producing a smooth wall around 24” with apertures being framed with wood left for doors and loopholes.
This is carried on until the level of the layer reaches the top of the forming board.
Once this first layer has set the forms are removed and laid on top of it, lining them up before laying the next layer of the wall.
As the walls progressed apertures would be left and framed with wood to accommodate access doors, and loopholes for air and light.
This is continued until the wall reaches the required height and length, when the surfaces can be briskly brushed to remove any loose earth.
Balustrades and merlons would have required different designs of forming boards, also portable and constructed from wood.
Adobe is another method of building earth forts going back over the centuries.
The adobe mix consists of much the same components as rammed earth except straw is not used and individual bricks made from adobe mix are used to build the walls.
Wooden boxes are fabricated and act as molds for the bricks the sides held together with strips of hide or similar.
These molds can be any reasonable size, but for earth forts, they were probably 24” x 18” x 12” , which is not too big to inhibit drying right through and handling during construction of the walls. They could be laid on their flat giving a width of 24”, and of course if they were two courses that would give 48” wide walls.
The mixture is shoveled into the wooden molds, scraped level across the top, and left to partially solidify, at which stage they are turned out of the mold and left out of the sun to dry out.
Once solidified, they can be used to build walls of around 24” thickness; once again the first layer will be laid on a foundation of rubble or wood logs. The same mix of adobe is used as mortar, with the addition of a little more water to make it spread easier.
1. eartharchitecture- Jahili earth fort in Abu-Dubi
2. scribd -earth forts from rammed earth.
3. penelopeuchicargo – design of earth forts.
4. greenbuilding - rammed earth and adobe techniques
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Design Considerations when Building Earth Forts
- Damage from Water Ingress
The big problem with any earth forts was the ingress of water particularly through the base of the walls which was fatal to the construction. Some type of foundation was required; with drains to take away the water perhaps covered in leather or hide to prevent damp rising up the walls.
The tops of the walls would also have has to be covered if there were no merlons, to prevent water entering the interior of the wall, again probably by hides.
- Access Platforms and Roofs
Platforms for the defenders to stand on were arranged behind the top of the walls and also behind the various loopholes, or meurtriers (apertures left in the wall for observation, to emit light, or firing arrows through) for the same purpose. Often overhanging roofs were built over the walls towards the inside to protect the walls from water ingress and, the soldiers on duty from the sun and rain.
- Drainage and Defense Ditches
A large V- shaped ditch was dug in front of the berms as an additional obstacle against attack, and into which the water drains from the founds were directed.
Access to the earth fort was by numerous doors, up to two at the front and back and two at the sides, being made from several layers of wooden logs.
- Access Doors and Berms
Berms of earth were built up against the outside walls sloping from the bottom towards the top. These were used to protect the walls, through deflecting large boulders or arrows that were hurled by giant stationary Springald (catapults) or a ballista (large bow) towards the earth forts, primarily to breach the walls.
The berms were built at such an angle to inhibit the storming of the walls with ladders (known as escalading; a very popular method of encroachment of the walls), or from a belfry (a high wooden portable tower on wheels).
The fort front gate was built into the berm, made out of logs and double braced, as it was regarded a weak spot.
Reference Web: penelopeuchicargo – design of earth forts