The SPORE Design Version 2 uses tunneling methods instead of trench digging.
Humans have been digging tunnels by hand for thousands of years so this shouldn’t be anything we can’t figure out.
Current preferred method
Linear Plate (aka Liner Plate) method
This has over taken the Tunneling Shield as my preferred method for SPORE. It’s really simple and low tech. I’ll need to modify it to fit our Homesteader’s needs though.
- Holes are cut into the face where the ceiling will be.
- Steel plates (or in our case, iron or cast basalt) are inserted into the holes, joined together, and supported.
- The area under the roof plates is excavated and the walls are installed.
- Repeat step 1 with new plates.
The advantages of the Linear Plate method:
- The excavation face can be controlled using the Top Heading and Bench method. This helps prevent large excavation faces from caving-in.
- Does not require the Full-Face method of excavation. This eliminates the need for bulkheads to compartmentalize the excavation face.
- It doesn’t require a tunneling shield. We’re basically talking about pressure hull plates and temporary supports. Everything could be manufactured on Luna.
- Homesteaders can remove large rocks (cobbles) that are in the way. Harder to do that with a tunneling shield.
- The excavated area can be any size or shape. It’s not constrained by the size and shape of the tunneling shield.
The disadvantages of the Linear Plate method:
- There’s a lot of exposed regolith/mega-regolith. This could cause significant O2 and pressure loss.
- A lot more dust exposure because of the exposed regolith/mega-regolith.
The disadvantages can probably be mitigated. I just need to figure out how.
- We’re not going to know the ground conditions in advance. (no advanced exploration)
- Homesteaders won’t have the time or resources to do a detailed characterization of the regolith before tunneling begins.
- The regolith (and most likely the mega-regolith) is highly heterogeneous and highly variable. Samples from the same site can be significantly different.
- We’ll have to deal with the ground conditions as we find them where we find them. Homesteaders need a system that can handle everything they encounter.
- Due to the nature of the regolith, and suspected nature of the mega-regolith, we probably won’t encounter many (if any) hard-rock conditions. At least until we start tunneling much deeper.
- The regolith is composed of gravel sized and smaller rocks and rock fragments. (need verification)
- The mega-regolith is thought to be similar to the regolith but with less smaller particles and more larger particles (need verification)
- Homesteaders will probably encounter large rocks fairly often. We’ll need a way to either remove them or break them in place.
- It I unknown how compact the regolith is. Voids, fractures, large boulders, and other features could cause significant pressure loss.
- Stand-up time (how long the ground will safely stand by itself at the point of excavation before collapsing) is a critical concern. Cave-ins are one of the biggest threats Homesteaders will face using SPORE. We need to assume that stand-up time will be minimal because we won’t know what we’re digging into until we’re digging into it. It is better to assume we need to instantly and constantly support the ceiling and walls even if that’s not the case.
- The pressure from the regolith above and around the habitat needs to be considered. The pressure will be much less than on Earth due to the 1/6 Earth gravity. The air pressure inside the both the incomplete and completed pressure hulls should offset the regolith static pressure.
- We need to protect the Homesteaders as they tunnel.
- Ceiling and wall collapse
- Explosion/Fire in pure oxygen atmosphere
- Toxic atmosphere (pure O2)
- Abrasive regolith
- We need to minimize oxygen and pressure loss (not something tunnels on Earth need to deal with) through exposed regolith.
- We need to anticipate 1/6 G excavating problems. It might be difficult to use conventional tools.
- Groundwater is one of the biggest problems for tunneling on Earth. Luckily (?) it’s not a concern for Homesteaders. Although access to water would be a huge help in other areas.
- Homesteaders are not trained tunneling engineers. They should have some basic knowledge and training in using SPORE however. SPORE needs to be simple and adaptable enough for minimally trained people to safely use it in a wide variety of situations.
There are many techniques for creating a tunnel. Some can be discarded due to environmental conditions (Luna has no groundwater), soil conditions (Luna has no clay), and complexity/expense. Below are the most relevant that I’ve found.
The tunneling shield is the precursor to the boring machines. Its primary functions are to protect the miners and prevent cave-in’s. It’s more complicated than the Linear Plate method and requires more equipment. The real problem is that it was designed to move through soft and wet soil. The Lunar regolith and mega-regolith probably won’t behave that way.
This is an image of the tunneling shield used to construct the Thames Tunnel. It was probably from a contemporary newspaper but I got it from Wikipedia (https://en.wikipedia.org/wiki/Tunnelling_shield). Wikipedia has a lot of good info on tunneling shields but much of it isn’t really relevant to what we need.
Key features of a tunneling shield:
- The tunneling shield is a single movable frame. It moves forward as a unit and its dimensions are the dimensions of the tunnel.
- Jacks (usually hydraulic) are used to push the cutting edge of the shield into the excavation face. The jacks (called propelling jacks) push against the unfinished edge of the lining (or pressure hull in our case). Face jacks support the excavation face. Lunar Homesteaders might be able to use heavy manual screws instead of hydraulics.
- The rear top part of the shield includes an overhanging component that protects the site where the lining (or pressure hull) is constructed.
- The Tunneling Shield method requires the use of the Full-Face excavation method. The entire excavation face needs to be removed before the shield move forward. This excavation method increases the risk of a face collapse, especially if the face is large. Horizontal and vertical bulkheads are used to divide the shield into work compartments as a way to reduce the risk of an excavation face collapse.
- Because the entire face is exposed, it’s easier to identify unexpected obstacles and remove them. It’s also easier to identify and document the composition of the soil (regolith) being excavated.
Here’s how a SPORE tunneling shield might work.
- Behind the tunneling shield would be the unfinished habitat pressure hull. It wouldn’t be pressurized with breathable gas yet but the structure and pure oxygen atmosphere would be sufficient to support the regolith/mega-regolith overburden.
- The tunneling shield is located at the excavation face (the exposed regolith and mega-regolith that the Homesteaders are cutting into). The shield protects the miners as they excavate the excavation face.
- The jacks push the tunneling shield into the excavation face. The cutting edges penetrate the excavation face.
- Homesteaders remove regolith/mega-regolith from each compartment of the tunneling shield.
- When enough excavation face has been excavated, the tunneling shield is moved up to the new excavation face.
- The jacks are removed. New pressure hull is constructed in the gap (leaving room to re-install the collapsed jacks) between the shield and the finished pressure hull. The, now compressed, jacks are replaced.
- Repeat steps 3 and 6 until the habitat pressure hull is large enough.
- Disassemble the tunneling shield and cap off the habitat pressure hull.
At least that’s how it goes in my head at this time. Of course, there’s still a lot of research and experimentation that needs to be done.
The other key function of a Lunar tunneling shield, that isn’t really needed on Earth, is to reduce the loss of oxygen through the exposed regolith. The primary location of oxygen loss will be the excavation face. A way to counter this might be to have removable panels cover the exposed excavation face except for the area being excavated. This might be easy to do since we have to compartmentalize the shield and face anyway.
Use removable panels at the excavation face may:
- Reduce oxygen loss.
- Allow access to the face so it can be excavated.
- Allow the Homesteader to be protected while installing ceiling supports at the face.
I honestly don’t know if removable panels will make a difference. It’s just an idea now.
Another solution could be to include a temporary, movable “wall” behind the tunneling shield. This would minimize the volume of atmosphere exposed to the excavation face and may reduce them amount of oxygen lost. It also seems like it would be a pain to get the excavated material through the wall to where it needs to go.
The Tunneling Shield method was my top choice until I started learning about the Linear Plate method. I think some version of the Tunneling Shield could be useful for more established Homesteads and settlements. Especially once we learn a lot more about the physical composition of the Lunar regolith and mega-regolith.
Classical methods (for soft soil)
Many of the world’s great tunnels were created using these methods. They are slow and tedious but they work. Unfortunately, all of them use a lot of supports and shoring. That’s fine if you have a lot of cheap wood on hand. Not so good for Lunar Homesteaders. Sure, you can remove the supports once the final ceiling/walls are in place but you still need a lot of them to start.
Additionally, these techniques don’t help minimize the amount of exposed regolith. Each square meter of exposed regolith increases the amount of oxygen (and atmospheric pressure) lost through it. I had originally envisioned using temporary, thin cast Lunar iron plates to cover the exposed regolith until the permanent pressure hull could be built. That seems like a lot of unnecessary work now.
These methods could be used, once Homesteaders have a more substantial resource base, to create very large spaces. Multiple tunnels (aka “drifts”) are excavated near each other. The separating material is then excavated, leaving a large open space.
- Belgian, English, American, German, Australian, and Italian methods – They’re all pretty similar. Just the geometry of the supporting structure and how the tunnel is excavated changes.
- Fore-poling and Needle beams – Basically, poles (usually wood) are driven into the excavation face (where the excavation is taking place) to form a temporary ceiling the material under the poles is then excavated while supports for the poles are put in place. The poles are a permanent part of the ceiling.
- Army/Case method – Simple but only good for short tunnels.
This is kind-of what I had originally planned for SPORE. Basically, you dig a trench and put a roof over it that will support the material that the tunnel will be buried in. Then you fill in your trench or cover over the roof. SPORE was going to build the roof, cover it, and then start digging down.
- Top-down method – You make a shallow excavation and build support walls. The roof is then built and covered. The rest of the excavation and construction takes place under the roof.
- Bottom-up method –A trench is excavated that is as deep as your tunnel (or habitat) will be. You then build the structure in the trench and backfill the hole.
The major problem with this technique is that you have to do a lot of work on the surface. The bottom-up method is the worst as almost all the time spent is on the surface. The top-down method is better but your work crew (human and/or robotic) are still going to be exposed and at risk until the roof is finished and covered.
Even the SPORE version requires too much surface exposure. Time to construct the Shield Wall (the roof) and time to move enough regolith onto the Shield Wall. We’ll either need to risk our Homesteaders or design robots that can do the job. Unfortunately, we’ve been working on the robot angle for decades and still aren’t there yet.
Time for another plan.
Tunnel boring machines (TBM)
Just like it sounds. Using big machines to bore holes through the ground. These may be useful in the future but they are impractical for Homesteads.
- Boring machines are complex devices. Many of the components would have to made on Earth and transported to Luna. There would have to be a constant supply of replacement parts as well.
- They would have to be rather big to excavate spaces the size we need. Current diameters would be limited by the diameter of rockets transporting the machines. We would have to ship them in pieces from Earth and assemble them on Luna. And since we need the machines to make the livable spaces underground, we’ll have to assemble them on the surface. And, now we’re back to that problem.
- Combine these two problems with the expense of transporting stuff to Luna and you have a deal-breaker. It might be possible when Homesteads can create most of the components from Lunar materials. Earth can supply the electronics.
- Boring machines will also require a lot of energy (electrical, fuel, etc.) to do their thing. Our power-strapped Homesteads would be hard pressed to power one.
- Like all big industrial machines, they produce a lot of hazards (noise, vibration, dust, heat, etc.). Boring machine operations would probably make living conditions in the Homestead highly uncomfortable or downright dangerous. They will certainly be dangerous for the people operating them.
- Building a Homestead isn’t the same a creating the Channel Tunnel. This would be a high-tech large-scale solution to a low-tech small-scale problem. That’s the opposite of Lunar Homesteading.
Tunnel boring machines are out for now.