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Underwater Soil Mechanics Problem

dphull's picture

I am an undergraduate student conducting a research project involving the design of a tool to aid astronauts to perform experiments on lunar surfaces and would like to request some direction about which analytical tools would be best suited for this application (i.e. FEA or DEM software/techniques). The specific scenario of interest is to design a tool that can act as an anchor in a sand-like medium (regolith) on the surface of the moon. Assuming the design implements a method to bury the tool in the sand, the principle goal is to maximize the resistance to a tensile axial load (initial concepts include a sort of helical drill) while buried under the soil. The design will be modelled with the expectation that a future protoype will be tested in an experimental setup consisting of a quantity of lunar regolith simulant submersed in an underwater neutral-buoyancy tank. An advisor has recommended use of EDEM software, and a fellow student is learning about Ansys. Which software would be be suited to underwater soil mechanics problems? Can you recommend introductory learning resources for DEM, soil mechanics, or related concepts? I have consulted standard civil engineering soil mechanics textbooks (i.e. books by Braja M. Das), but the unique underwater scenario makes many sources to be of little use for this project. This is my first post to iMechanica; thank you for your time and attention.




It's a good write-up but I think you need to first bring clarity to your goals. In the main, consider the following question: What is the focus of your research project on?

(A) Do you want to design the overall tool itself? If so, does it include designing for its individual components such as the anchoring part, the hellical drill, the motor, the drive-train? Does it include designing for their mechanisms? their assembly? Or does it mean only the body of the tool which would hold these other components?


(B) Do you want to model the process of drilling in order to push the base of this tool into the soil, and then, the process of securing the base in such a way that the base, thus anchored, could then support the design levels of the expected tensile loading induced in the tool during service?


(C) Do you want to model the stress/strains produced in the supporting foundation rocks so that the design tensile loads could be supported by the foundation?


(D) Do you want to model the process of drilling into regolith itself? That is to say, do you wish to model fragmentation of rocks into small pieces, the processes of the drilling/grinding/whatever else? (If so, does the project include the process and mechanisms for transporting the soil samples upwards through the body of the tool? Or does the project involve creating a cavity in the rocks, removing the soil and throwing it away, and then lowering some instrument into that cavity?)

Broadly speaking, if (A) and (C), IMO, using FEM would be good enough. If (B) and (D), I think using only general purpose FEM would fall short, and special techniques for handling modeling of drilling/soil transport would be required. Here, techniques like DEM could be helpful. (I don't myself know DEM except for some broad outlines.)

Other iMechanicians could pitch in with suggestions, of course.

But in any case, at the UG level, you don't have a lot of time in hand---at the most a semester (or a year, but on part-time basis). So, you need to focus, and cut down on the scope, so that the project is at all doable in the time available.

2. One more comment:

Since the lunar surface has no water, regolith does not contain water (or so I think).

My guess therefore is that the experimental testing of the actually fabricated tool would be conducted in the underwater testing facility only in order to simulate the lower gravity-load levels.

Thus, the particular mechanics of the wet soils (such as those at river/sea beds) should be of no relevance to your design, neither should the attendent phenomena such as corrosion and wetting of the tool parts (i.e. assuming that the drilling process itself does not involve use of a potentially corrosive fluid).

However, it does mean that designing for the movements of the part, and even for stresses/strains/displacements, would now have to take into account not only the lower levels of the gravity but also the near-vacuum loading conditions existing on the outside (there being no air on the lunar surface).

Also, the existence of sharp temperature gradients could be significant because of thermal expansions/contracions and stresses.

Finally, I am not sure if greater irradiation would play a role.

Hope this helps.



dphull's picture


Thank you for the thourough response, I did find it helpful. These are questions I will need to consider more deeply before moving forward, but I can say that the ultimate goal is to design the entire tool, however, the drill geometry is of the greatest focus. This project is to be entered as an entry for a student competition, therefore, the experimental simulation environment will drive the design to a greater extent than than the theoretical conditions of space (I acknowledge, in example, that lunar soils indeed would not likely contain water). Can anyone provide suggestions as to what extent I should focus my design approach to the the unique underwater experimental conditions as opposed to fundamental soil dynamics and/or the actual conditions of space? Furthermore, whether the design process should be guided by a static stress/strain analysis or an analysis on drilling dynamics? For example, assuming the drill is to be mechanised with vibratory action, would it not be reasonable to expect that the displacement of each stroke of the drill would affect the momentum of surrounding water as in the experiemental setup differently than the surrounding atmosphere in space?

Some clarifications: 

  • the act of collecting soil samples is not related to this project; remaining stationary in soil is the only goal
  • the regolith simulant to be used will be a bucket of common sand (to be submerged in a sort of swimming pool)

I appreciate the assistance; please correct me if these general questions are not suited to the discussion forum. 


1. Ummm... The questions are not ``general;'' they in fact crucially involve mechanics, and thus, IMO, they do fit in here perfectly well. (In fact this remark by become redundant by now; you can see that your post has already been promoted to the main page by the moderators.)

2. I think you need to ask the organizers what exactly the design objectives are like.

From an engineering design point of view, there are too many differences in the functional specs involving the lunar ambient environment vs the physical simulation environment existing underwater on earth. The mechanical chracteristics of sand (its flow, stress/strains developed in it) would change significantly with the absence/presence of water. But it doesn't stop there. Presence of vacuum vs. air vs. water has other implications too. Just as an example, think about the necessity of having to cover the exposed moving machine parts and electrical connections/circuit boards etc. when they would be exposed to water vs. to vacuum. And that is apart from the purely mechanical loading considerations (of water vs. vacuum).

3. As an off-hand guess, I am not sure if the waves induced in the water would be a significant issue; the forces due to the resistance put up by the soil could be several orders of magnitude greater. But then, I haven't considered shock-waves here. Further, the mechanics involved here is entirely different from what I have studied, and so I can't tell (and in fact could very well be wrong).

4. As to design being driven by (computer) simulation, I think it is a very welcome approach. For designing the anchoring mechanism, granular materials would behave differently from ordinary soil, and very differently from rocks. For this purpose, guess you will have to look into the mechanics of granular materials. I have no idea about what software they use in this field.

5. An important part of such projects is the creativity of possible solutions and the fun in thinking about them.

Just as an example: Think of creatively using the pressured air existing inside the lunar module, the fact that the tool would initially be inside the (air-pressurized) chamber before an astronaut brings it out, and the existence of the ambient vacuum once it is brought on the outside. While designing the tool, can you minimize on the moving parts by creatively using the pressure difference existing between the vacuum and the initial air pressure it would have? ... That was just an example.

Since such activities are so much fun, I would rather stop here and let you have all of it :)

6. But of course, do feel free to raise questions or seek guidance about broader things, e.g., information about different analysis/simulation techniques, softwares, experimental results, prior design case studies, etc.

7. Personally, topics like granular mechanics, DEM, etc. are off the limits of what I know, so let me sign off (at least for now), but others could surely help you.



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