SPORE Apollo 12 Regolith and Mega-Regolith Characterization

 

Apollo12 Site Regolith Characterization

The first step in the SPORE project is to gather as much relevant data as possible on the Lunar regolith and mega-regolith at the Apollo 12 site. We’ll need this information so we can figure out how to excavate the material, build the shield wall, and minimize atmosphere loss inside the shield wall. We’ll also need to figure out an acceptable simulant so we can run experiments.

Update to the update:

  • I’ve decided that spending a lot of time trying to characterize the physical properties of Lunar regolith for SPORE experiments isn’t worth the time. Check out the SPORE Pressure Experiment 1 for more information.

UPDATE – This page isn’t finished and I probably won’t touch it again for a while. I feel like I got sucked down the rabbit hole a bit here. It’s easy to get lost collecting all kinds of data, relevant or not. So I’m just going to post it and start experimenting. I’ll come back here and add stuff as I need it. I mean, how well do we need to characterize the regolith or mega-regolith for a shovel to work? Or a pick? Or a bucket?

 

Check out the Lunar Homestead Experimental Location page for why I chose the Apollo 12 site for Lunar Homestead projects. Also, Lunarpedia will host all the information I collect here as well as any regolith (lunarpedia.org/w/Lunar_Regolith) information that isn’t relevant to the SPORE project.

Disclaimer – I’m not a civil engineer and I don’t pretend to be one. So I’m going to have to learn as I go. I’m not sure how much that will help anyway as building on Luna is different than building on Earth. Plus, we don’t know much about the regolith and mega-regolith. Finally, the regolith and mega-regolith are highly heterogeneous so we might be stuck with generalities until we get more data. Please let me know if you see something I’ve gotten wrong.

In reality, very few of our Homesteaders are going to be trained civil engineers either. We need to keep this simple and adaptable. Homesteaders will need to be able to quickly modify equipment and techniques to handle their local conditions. It will be very easy to get bogged down in engineering minutiae.

I’ve read a bunch of papers on excavating Lunar regolith. I haven’t found much that is helpful to SPORE. Some are heavy on theory and general estimates. Others utilize models based on variables used on Earth. Most are focused on robotic mining equipment. Maybe most of our knowledge about excavating Lunar regolith is of little value to the SPORE project since it’s reason for being is to eliminate or change most of the parameters anyway.

 

Relevant parameters

At this point I don’t know which parameters are critical and which are useless. So I’ll just collect everything and figure it all out later.

  • Relative density (takes particle packing into account)
Relative density description

[Lunar Sourcebook (pg 495)]

    • According to the Lunar Sourcebook, the most significant geotechnical variable is the relative density of the regolith , as it controls the other physical properties as well(1, pg 476).
    • The bulk density of any soil sample can vary over a wide range depending on how the particles are packed (1, pg. 494).
    • Two soils with the same bulk densities can have very different relative densities and display different behaviors. The converse (same relative but different bulk) is not true however (1, pg 495).
    • In-situ relative densities for Lunar regolith:
      • Top 15 cm = 65±3% (1, pg 497).
      • 0-30 cm = 74±3% (1, pg 497).
      • 30-60 cm = 92±3% (1, pg 497).
      • Observations based on boulder tracks estimate that the top 400 cm of slopes has a relative density of 61%, indicating that slopes are significantly looser than plains areas (1, pg 499).
      • A relative density of 65-75% is the practical limit for terrestrial soil field compaction. Shock waves from meteoroid impacts has shaken and densified the Lunar regolith (1, pg 495).
  • Particle size distribution
    • In unconsolidated material, such as Lunar regolith, the particle size distribution controls the strength, compressibility, optical, thermal, and seismic properties (1, pg 476).
    • At the surface, the regolith has the following general characteristics:
      • It can be classified as a well-graded/poorly sorted silty sand to sandy silt (SW-SM to ML in the Unified Soil Classification System)(1, pg 478).
        • SW = well-graded sand, fine to coarse sand
        • SM = silty sand
        • ML = silt
      • Median particle size is 40-130 µm. Average of 70 µm (1, pg 478).
      • Approximately half of the soil (by weight) is finer than the human eye can resolve (1, pg 478).
      • 10-20% of the soil is finer than 20 µm (1, pg 478).
  • Particle shapes
    • Individual regolith particles have highly variable shapes, ranging from spherical to highly angular (1, pg 478).
    • Particles are generally elongated, causing them to pack together with a long axes preferred orientation. The physical properties of in-situ regolith are expected to be anisotropic (physical properties have different values when measured in different directions) because of this preferred orientation (1, pg 478).
      • Elongation (the ratio of length to width)
      • Range = 1.31-1.39. Average value = 1.35 (somewhat elongated) (1, pg 479).
    • Roundness (ratio of the average of the radii of the corners of the particle image to the radius of the maximum inscribed circle)(1, pg 479).
      • Silhouette = 0.21 (subangular)
      • Direct light = 0.22 (angular)
  • Specific gravity (ratio of the particle mass to the mass of an equal volume of water at 4º C)
    • Range = 2.3 to >3.2. Recommended value for general scientific and engineering analyses of Lunar soils = 3.1 (1, pg. 481).
      • Agglutinate and glass particles = 1.0 to >3.32 (1, pg. 481).
      • Basalt particles = >3.32 (1, pg. 481).
      • Breccia particles = 2.9 to 3.10 (1, pg. 481).
    • Subgranular voids (see below) within particles decrease the specific gravity. Grinding a sample into fine powder would eliminate the voids and increase the specific gravity of the sample (1, pg. 481).
  • Bulk density (the mass of material contained within a given volume)
    • In-situ bulk density affects bearing capacity, slope stability, seismic velocity, thermal conductivity, electrical resistivity, and penetration depth of ionizing radiation (1, pg. 483).
    • Soil breccia = 2 g/cm3 (2, pg 241)
    • Best estimate for the average bulk density for the top 15 cm of regolith = 1.50±0.05 g/cm3 (1, pg. 483).
    • Best estimate for the average bulk density of the top 60 cm of regolith = 1.66±0.05 g/cm3 (1, pg. 483).
    • The bulk density at 300 centimeters (Apollo 17) is about 1.74 g/cm3 (1, pg. 491).
    • No direct tactile data regarding density is available below 3 meters (the maximum depth of Apollo drive core tubes (1, pg 494).
    • In general, density increases steadily from the surface to about 70 cm (1, pg 492).
    • Below 70 cm the density profile is erratic (1, pg 492).
      • Specific gravity, chemical composition, mineralogy, subgranular porosity, particle shape, particle size distribution, and relative density are all highly variable (1, pg 492).
Regolith bulk density

[Lunar Sourcebook (pg 492)]

  • Porosity (the volume of void space between the particles divided by the total volume)
    • Intergranular (the volume of space between individual particles)
      • Affects both bulk and relative density (1, pg. 481).
    • Intragranular (the volume of reentrant surfaces on the exterior of the particles)
      • Has a strong effect on bulk density (1, pg. 481).
    • Subgranular (the volume of enclosed voids within the interior of the particles)
      • The actual subgranular porosity of individual Lunar regolith particles is poorly understood (1, pg. 481).
    • Void ratio (the volume of void space between the particles divided by the volume of the “solid” particles).
    • Soil breccia = 35% void space (2, pg 241)
    • Best estimates of in-situ Lunar regolith porosity (inter- and intragranular combined)
Regolith porosity

[Lunar Sourcebook (pg 492)]

  • Compressibility (the volume change when a confining stress is applied to soil)
    • Compression index (the decrease in void ratio that occurs when stress is increased by an order of magnitude)
      • Range 0.01 – 0.11 (1, pg 501).
      • Recommended typical value
        • Loose = 0.3 (1, pg 501).
        • Dense = 0.05 (1, pg 501).
      • Agglutinates crush under relatively low confining stress, decreasing the void spaces (1, pg 503).
    • Useful when figuring out structural foundations and footings.
  • Shear strength
    • Governs ultimate bearing capacity, slope stability, and trafficability (1, pg. 506).
    • Cohesion best estimate = 0.1 to 1 kPa (1, pg 506).
    • Friction angle = 30º to 50º (1, pg 506).
    • Values for Peak Shear Stress are all over the place. I’m not sure how they could be used for any sort of planning.
  • Permeability (the quantity of flow of a fluid through a porous  medium in response to a pressure gradient)
    • I only found 2 possible papers on this topic and both were behind paywalls. It doesn’t seem like there has been much research in this area.
    • No direct measurements have been made on Lunar samples (1, pg 517).
    • Test firing of the Surveyor 5 vernier engine on the Lunar surface produced an estimate of 1-7 x 10-12 m2 to a depth of 25 cm (1, pg 517).
    • JSC-1A viscous flow permeability of 1 × 10-12 m2 to 6.1 × 10-12 m2 for bulk densities from 1550 to 2000 kg m-3(3).
  • Diffusivity (defines the molecular flow of a gas through a porous medium in response to a concentration gradient)
    • Diffusivity depends on the gas concentration, the pressure and temperature, and the particle size and shape distributions in the soil (1, pg 517).
    • No direct experiments have been run on Lunar samples. However tests were conducted using a basaltic Lunar soil simulant with a void ratio of 0.6. The tests were run at room temperature under very low vacuum conditions (1, pg 517).
      • Helium = 7,7 cm2/sec
      • Argon = 2.3 cm2/sec
      • Krypton = 1.8 cm2/sec
    • More reactive gases and decreased temperatures will increase “sticking time” (1, pg 517).
  • Bearing capacity (the ability of a soil to support an applied load)
    • Ultimate bearing capacity (the maximum possible load that can be applied without causing gross failure)
      • For a 1 meter footing on the Lunar surface = ≈6000 kPa (1, pg 517)
      • Ultimate bearing capacity increases proportionally with the width of the footing, making it more than sufficient to support any conceivable structure (1, pg. 518).
    • Allowable bearing capacity (load that can be applied without exceeding a given amount of settlement)
      • Significantly less than the ultimate bearing capacity. Allowable bearing capacity controls the design of a foundation (1, pg 519).
      • Need to be calculated based on specific design requirements.
  • Slope stability (the ability of the soil to stand without support)
    • Uncompacted regolith (“dumped”) (1, pg 522).
      • Relative density = 30-40%
      • Safety factor = 1
      • Height = 10 meter
      • Angle = nearly 40º
    • Compacted (once disturbed, the regolith will not return to its original dense state) (1, pg 521).
      • Relative density = 65-75%
      • Safety factor = 1
      • Height = 10 meter
      • Angle = 45º
    • Natural slopes
      • Very little is known (1, pg 522).
  • Trafficability (the capacity of a soil to support a vehicle and provide sufficient traction)
    • Almost any vehicle with round wheels and a maximum ground contact pressure of 7-10 kPa will work (1, pg 522).
  • Soil cohesion
  • Soil friction
  • Tool-soil adhesion
  • Soil weight (lifting/blanket)

 

Apollo 12 regolith

  • Bulk density
    • The shape and thickness of the drive core tubes disturbed the regolith sample, changing the bulk density value (1, pg 485).
    • From core samples (three different estimates) =
      • 1.6-2 g/cm3
      • 1.55-1.9 g/cm3
      • 1.7-1.9 g/cm3
  • Shear strength
    • Cohesion (three estimates) = 0 to 0.7 kPa / 0.1 to 3.1 kPa / 0 to 1 kPa (1, pg 508).
    • Friction angle (three estimates) = 28º to 35º / 13º to 56º / 51º to 59º (1, pg 508).
    • Values for Peak Shear Stress are all over the place. I’m not sure how they could be used for any sort of planning.

 

Apollo 12 mega-regolith

Very little data available. None of the drive cores reached into the mega-regolith.

 

Resources

  1. Lunar Sourcebook (www.lpi.usra.edu/publications/books/lunar_sourcebook)
  2. Lunar Stratigraphy and Sedimentology (lunarhomestead.com/resources/lunar-resources/)
  3. Permeability of JSC-1A: A lunar soil simulant (www.sciencedirect.com/science/article/pii/S0019103510004835)

 

Resources I need to read

 

I also think a detailed spreadsheet for the samples is in order. I’ve started one but I’m not sure how to make it available online. I’m also sure someone, somewhere has already done this but I haven’t been able to find it. Probably just as well. This way I can get a much better idea of exactly what we’re dealing with.

 

Drive Tube Core Samples

The Apollo 12 Drive Core Samples got their own page because I found a lot of information on them. And they’re really important.

4 out of 66 samples.

 

Apollo 12 Basalt Samples

 

43 out of 66 samples.

 

 

Apollo 12 Breccia Samples

 

4 out of 66 samples.

 

 

Apollo 12 Soil Samples

 

15 out of 66 samples.

Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *