In last month’s edition, I discussed in situ building materials for short and long-term bases on Mars. I concluded that these building materials would allow for a base design life of up to five years, in part because of the exposure to the extreme Martian environment:
· Near-vacuum conditions.
· Extreme cold.
· Extreme diurnal temperature swings.
· Extreme exposure to cosmic radiation, high energy solar wind, and solar ultraviolet light.
· Exposure to micro- and larger meteorites.
What’s the best way to protect in situ building materials (and their occupants) from being degraded by Mars’ harsh conditions? Build underground.
As a civil engineer working in the Seattle metropolitan area, I had a front row seat to numerous tunneling projects over the course of my career. The I-90 interstate segment between Seattle and Bellevue featured bored tunnels beneath Seattle’s First Hill and cut and cover tunnels on Mercer Island. Later, the Alaskan Way Tunnel was bored beneath the crumbling Alaskan Way Viaduct and downtown Seattle to accommodate the four-lane highway for one-and-three-quarters-miles. Another form of cut and cover tunnel construction I witnessed was the construction of culverts. For intermittent streams or watercourses, a simple pipe sufficed. But over salmon-bearing streams, arch culverts were used. Arch culverts provide a higher clearance than drainage pipes, allowing light in to facilitate fish passage, and a wide base that allowed the continuation of the natural streambed beneath the roadway. Their large cross section spread out floodwaters, slowing downstream velocities within the culvert and immediately downstream.
Boring Many of the largest tunnel projects I witnessed utilized direct boring. Tunnel boring machines can efficiently dig tunnels of immense spans. The aforementioned Alaskan Way Tunnel shaft was fifty-seven-and-a half-feet in diameter—large enough to stack two interior decks to accommodate two traffic lanes in each direction. Moreover, boring machines are maneuverable (within tolerances). The Alaskan Way Tunnel had to dive down eighty feet to clear the pilings supporting the existing viaduct, then swing east and uphill to the north portal above Lake Union. The soil the tunnel machine bored through was largely clay and glacial till - highly consolidated silt, sand and gravel.
The boring machine was 326 feet long and weighed 6,700 tons. It was constructed in pieces in Japan, shipped to Seattle and assembled there. As the machine bored through the soil, spoils were carried to the back via a conveyor belt, then trucked away. As it advanced, precast concrete ring segments were fit into place to prevent soil slippage. The machine featured rooms for operators and mechanics. It ran twenty-four hours a day, seven days a week.
The downsides of tunnel boring machines are that they perform poorly when they encounter solid obstacles such as boulders or bedrock. The Seattle machine encountered an abandoned metal well casing. The shutdown for repairs to the damaged cutter head lasted two years. An excavation pit was dug ahead of the machine, which was advanced to push the cutter head into the pit. The 900-ton cutter head was removed, lifted from the pit, and returned to Japan for repairs. A tunnel boring machine’s enormous size, complexity and susceptibility to damage make it a poor candidate for use on Mars.
These machines also perform poorly in unconsolidated soils like sand and mud here on Earth, or regolith on Mars. Tunneling through loose soils often leads to sinkholes, which can threaten the tunneling operation with collapse or inundation.
Cut and Cover The cut and cover tunnel method utilizes common construction machinery – excavators, dozers and dump trucks. A deep trench is excavated. Parallel concrete footings are placed to support the tunnel walls. The walls are set in place and beams placed over the top. A deck is constructed atop the beams, and soil is backfilled to restore the original topography.
As I noted in The Hydrogen Economy(September ’21 issue), hydrogen fuel cells are already being deployed in mining, construction and transportation equipment. Within the next five to ten years, construction equipment powered by HFCs will be commonplace. The remaining issues for deployment to Mars then become protecting the hydraulic systems from freezing in the extreme cold and reducing overall mass to facilitate transportation to Mars – issues for which existing technologies and substitute materials already exist.
Structural Tunnel Components Last month I noted the likeliest candidate for long-term in situ building materials is a molten sulfur concrete(MSC). What I did not mention was that MSC, like Portland cement concrete, has excellent compressive strength, but poor tensile strength. In layman’s terms that means structural concrete requires reinforcement (think rebar). For the footings to resist uneven settlement, for the walls to resist deformation and for the beams to resist their own mass plus the mass of the soil above it, prodigious amounts of reinforcing material will be required.
There are two downsides to using steel rebar on Mars. The first is its weight. A twenty-foot stick of 3/8” diameter rebar weighs about seven and a half pounds. Construction of a permanent base on Mars would require thousands of sticks of rebar, even after designers account for the reduced loads imposed by Mars’ lower gravity. When providing equipment or materials to Mars, the cost of transportation will far exceed the cost of the materials themselves.
But the other downside may be even more difficult to overcome. Steel is subject to corrosion. While there is little water on Mars to promote corrosion, the sulfur matrix of MSC alone is a highly corrosive environment. If this cannot be addressed, the rebar could swell – causing the concrete to spall away, diminishing the structural strength of the walls, beams and top decking – leading to collapse.
There is a reinforcing material in use today that addresses both concerns—carbon fiber rods. A given length of carbon fiber rod is one-quarter the weight of comparable diameter steel rebar, is nearly twice as strong, and is nearly impervious to corrosion. My money is on the use of carbon fiber rods for structural reinforcement on Mars.
Another building method I referenced earlier in this article could reduce the reinforcement needed for post and beam construction. The use of an arch. Those who have visited Europe may have marveled at elevated Roman aqua ducts still standing after more than two millennia. I have stood in European stone churches dating to 800 AD whose vaulted ceilings relied on the arch. These ancient dry stone masonry structures rely on gravity alone and the ability of the arch shape to transfer loads down the length of the arch to the footings (or supporting vertical columns). It is awe inspiring to stand beneath thousands of pounds of un-mortared stones suspended above one’s head by the genius of Roman engineers.
It may be possible to create stackable MSC units for construction of arch tunnels. Larger units – such as one-third-length arch pieces, could be created with minimal reinforcement. Entire arch segments could also be produced but may require additional reinforcement to facilitate picking them up and setting them into place.
Mars arch tunnel structures as wide as ten meters (thirty-three feet) could be subdivided to create dorm rooms, offices, galleys and shop space, or left open for larger industrial applications. Smaller arches (six meters or less) could serve as connecting tunnels. Even smaller arches could be installed beneath tunnel floors for utility races(water, sewer, power, communications). All constructed on largely in situ materials fabricated on Mars.
The Finishing Touch Finally, arch tunnels made of MSC would need to be sealed internally to prevent release of toxic sulfur compounds and to facilitate an airtight seal to retain the internal atmosphere. Polyethylene (PE) – long polymer molecules composed of simple two-carbon ethylene units – could be readily derived from Mars’ carbon dioxide atmosphere. PE could be heat-applied in sheets or mixed with an adhesive plasticizer and applied like paint.
Depending on the depth of burial, such bases would require little insulation from the cold, be resistant to cosmic and solar radiation, be resistant to all but the largest meteorite strikes, and – barring settlement – be immune to pressure leaks.
For further reading
https://www.structuremag.org/?p=14114https://en.wikipedia.org/wiki/Bertha_(tunnel_boring_machine)https://www.rhinocarbonfiber.com/composite-rebar
