Foundation

Any permanent or semi-permanent structure on the surface of Mars will need some type of foundation. A foundation anchors a structure to the ground, preferably to the bedrock. This stabilizes the structure against the affects of gravity, wind, and ground movements due to changes in ground moisture (water and carbon dioxide ice).

Alternatively, many foundations are of the floating type. This means that after testing the soil, the load bearing capacity is determined and the foundation is built in order to spread the load to less than the load bearing capacity of the soil. That is the main purpose of footings under house walls, for example. In a sense, all building that are not supported directly on the bedrock 'float' on the soil.

Unique Martian Considerations

The environment of Mars presents certain unique factors for foundation design.

Reduced Gravity

The lower gravity means that Martian foundations need not be as strong or thick as terrestrial counterparts.

Low Air Pressure

Low air pressure means that the Martian winds have a greatly reduced effect upon structures compared to terrestrial winds of the same speeds.

The martian atmosphere has a density of about 2% the one of Earth. As the energy from wind transmitted to a structure is a equation in the form of E=1/2mV2, it is directly proportional to the mass of the atmosphere. So for the same wind velocity the force, and consequently the pressure, will be 50 times less.

The dust in the atmosphere is a tiny fraction of the mass of the gas and exerts negligible pressure on the buildings even in the strongest winds. However high velocity dust might scratch windows and damages surface finishes.

Extensive Permafrost

It has been suggested that much of the surface is a sort of permafrost. Heat generated by structures could cause the water or carbon dioxide in the permafrost to sublimate, resulting in settling of the ground. Any settlement on this type of ground will need to take measures to protect against this possibility. The loss of CO2 or of water in the soil changes the load bearing capacity of the soil.

Internal pressure

A unique characteristic of martian construction is that the pressure inside the building is very high compared to the pressure outside. The pressure is about 10 tonnes per meter square of wall or floor surface. So a standard 1000 square foot (100 m2) house on Mars would exert 1000 tonnes of force onto its foundations. A large dome, as often illustrated in images of martian settlements will do the same, so a 1000 foot (300m) dome would exert 150² x pi x 10 = 700 000 tonnes of force.

This is why martian structures are more likely to be pressure vessels, spheres and cylinders with rounded ends, rather than domes or rectangular constructions.

Types of Foundations

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Pressurized habitats and unpressurized constructions can have different needs as far as foundations go.

Piles

Piles can operate as friction elements spreading the load vertically through regolith with low load bearing capacity. or they can extend down to stable bedrock.

Footings

A footing is a partial slab under a wall of a column. It reduces the load on the regolith to a safe level bellow the load bearing capacity of the material.

Slab

A slab is a foundation that extends under the entirety of a building, creating a floor and reducing the ground load to the lowest level possible. Slabs on Earth are mostly under compression, however a slab on Mars might serve as part of a pressure vessel and then would be under tension. In such a case it would need to be reinforced with steel bars or other tension resistant materials.

Load bearing capacity of martian surface materials

Large areas of Mars appear to be covered with lava flows and rock. The foundations on such materials would be minimal. As there is no water and no soil on Mars, most soil surface materials should have a high load bearing capacity. Areas with large amounts of water in the regolith might need to be protected from the heat of the colony to maintain their load bearing capacity.

Ice is a plastic material that might flow very slowly under pressure. Tunnels dug in ice in an american base in Greenland, Project Iceworm, closed surprisingly quickly .

Open Issues

• How much on the surface is actually permafrost?
• How deep is the bedrock in likely settlement areas?