To recap, the idea of a trench shelter is to get you below surface level to get out of the wind (and all the debris blowing in it), without going too deep that earthquakes could bury you. Okay, so that takes care of wind and earthquakes. The idea of a metal roof is to prevent fire on your roof from incoming flammable gasses that may ignite near you, fireballs, firestorms, etc. Covering the roof in sod gives more protection, as a strong grass root system can keep the roof from blowing away and puts more barrier between you and solar radiation and fire. Be aware, though, that sod/dirt is very heavy and it doesn't take much to weigh a ton. Thus, it is important that you have enough underlying support to keep any sod/dirt covered roof from collapsing. Another thing to consider is pressure. With the passage, it is unknown if pressures will change. The only thing I can relate this to is the pressure created in a nuclear blast. So, what looks like good support under normal conditions might not be under adverse conditions. This is why I urge you to make your roof as strong as you can possibly get it and seek professional help (like an engineer) to make sure it's safe.
We also talk about toileting in a small space for an extended length of time, which is a closely related topic. So, let's get going...
Human bodies tend to get sick if kept cold for too long and there will be a lot of rain during this event, as well as temperatures dropping because the event is expected to happen in the fall to winter time of year. You will end up sick from a respiratory infection from being in the rain for days or weeks. You need to keep warm and dry, as well as prevent debris from hitting you or breathing thick dust.
You also need to prevent scrapes and bruises and broken bones as much as possible, which could happen from being tossed around by the earthquakes. Vehicles are designed for impacts harder than a human body can tolerate and will provide more protection than if you were just rolling around in a shallow trench with no protection at all. In fact, the seat belts will keep you from rolling and will prevent more injuries of that type and would be a reasonable solution to these issues. Also realize that rolling would be caused by slip faults, which create a sideways motion and may not be the only type of earthquake. I lived through the Northridge Earthquake, five miles from epicenter, and it was a thruster, which creates an up and down motion--a more violent earthquake--which causes your body to fly up into the air and then gravity brings you back down, and this would be a repetitive motion, thus more potential for damage. Mountain building is a thruster-type earthquake. The power at play would be like throwing a rag doll into the air and letting it land where it will, but with much more mass and breakable objects in the human body than a rag doll. A better analogy would be throwing a carton of eggs into the air and see how many don't break after it lands a few times.
Another benefit in using a vehicle is that it has a metal roof, which would act as a secondary or back-up to the trench roof, in case the exterior roof gets blown away. Of course, the windows would need to be fortified. In the aftertimes, the vehicle would be a warmer space (earth temperature, but not freezing) to live in until spring, by barricading the open end of the trench with dirt and leaving a smaller opening to go in an out.
My latest idea is to get a very strong car like a Volvo or one of the older vehicles, like a Ford LTD Crown Victoria circa 1984 or earlier, that have steel frames. Another idea is an old Chevy Suburban or a van, the kind with the barn doors in the back which would make for an alternate exit if the side doors become unusable. Or even, as DuaneR suggested, an armored car, if you have the opportunity to score one. Also, have a tool that could cut through the top of the vehicle, just in case. Suburbans can be found with diesel engines, which for the mechanically inclined could be useful in the aftertimes. The vehicle doesn't have to run, you can buy it cheap and have it towed if close enough to your location.
Dig your trench about three to four feet deep, or up to the windows of your vehicle, shallow enough to prevent from being buried by dirt as the earth shifts but deep enough to be out of the wind. Make it wider and longer than your vehicle. Angle the trench so you can drive the vehicle into it on one end but still be at an appropriate depth. It is slightly angled so rainwater can drain out of the trench on the end you drive your vehicle in on, so your trench doesn't flood. You can line the bottom of your trench with rocks so you aren't in a river of mud. Make it longer so you will have room for a pacing trench if you need one (i.e., if you are not tall, you might not need one, the regular trench might work). This would be a trench within the trench, dug lower than the floor of the trench your car is parked on, perhaps behind your car. This will allow the occupants to be able to stand up and pace back and forth. I had to take a bus back from Tuscon after the turbo blew on my Volvo and your legs start to swell when you sit for so long, not to mention you start to get edgy, especially toward the end of the two-day ride. This would be needed while things are building up but when it starts to get bad, you may not be able to get out of the vehicle. This is an argument for a larger vehicle, perhaps an old Chevy Suburban. It has bench seats, a third bench seat. Family members could rotate to be able to stretch out on a bench seat if it is not safe to get out of the vehicle.
Then get one of those metal carports to place over your trench. Dig deep holes for the legs so that the canopy of the carport covers the trench at the surface but have the trench dug deep enough so, if you are unable to get out of the side doors of your car, you will have some space if you have to cut up through the roof to get out, you won't be bumping into the carport canopy right on top of the roof of the car and having to cut through it to get out. You could cement the legs of the canopy if you have the money and time. If there is enough time left, put a layer of sod over the roof with fast growing grass with a strong root system to further protect from the winds which could blow the roof off. If all else fails, get a large piece of metal to use instead of the metal carport and fasten it down as much as you can. Make sure there are no exposed edges for the wind to catch. I saw a storm chaser video where a tornado picked up a firetruck and spun it around, so respect the power of these winds and take extra precautions.
For further protection, tie down the car in case you lose your roof. In a different blog, Doug Copp mentioned a technique of tying down the mini-blast shelter by burying tractor-trailer-sized tires with long lengths of chain to weigh them down. I don't see why it would be any different it there is a car in the trench rather than a mini-blast shelter. In a comment at https://earthchanges.ning.com/profiles/blogs/mini-blast-amp-fallout, Doug says:
"Take steel cables from each corner [of the blast shelter which is round] ... have a scope (nautical term for anchoring a boat) of about 20-30 degrees...and about 100 ft long...bury worn out tractor trailer tires (off the rim) into a 3ft deep pits and fill them back in with the cable attached to it." This should work with a car or RV or any kind of vehicle you might use.
I asked him, "Do you think the cable in the tires should be cemented in before burying them?" and he said, "Not necessary, just bury them 3 to 5 ft deep. They won't come out. Make sure the tires are filled with dirt and not left hollow."
As mentioned, the car has seat belts and will offer protection from the bouncing around from earth shaking. It will keep you warm. It will keep the air cleaner (and by having the trench on a slight incline with one end open there should be adequate air exchange and allow rainwater to run off). It will offer some comfort. It will also protect you from any animals trying to share your space. You should also have a gun and ammo in case you need to chase some snarly critter away so you can get out.
Since it is likely you will have to be in the shelter for many days, you need to provide for waste disposal. The Chevy Suburban or a van would be a good design because they have moveable seats. One seat could be removed to make room for a chemical toilet, or even a geriatric toilet lined with a garbage bag. Another thing to consider is motion sickness. With the earth shifting about, people (especially children) will get sick. It will be very unpleasant to be surrounded by vomit and no way to clean it up. Have plenty of barf bags and liners, and stock up on lots of motion sickness medicine.
Remember also, if the windows are electric you need to provide for a way to roll them down in case there is EMP during this event and it kills your electrical system. You will need to crack the windows for air circulation because the air will be used up quickly in an enclosed space. You don't want airtight or you will suffocate. You can get a camping fan that runs on batteries to keep the circulation moving at Wal-Mart. Another thing to consider is being able to dump your bags of waste outside of the car, unless you have the kind of toilet that uses enzymes and has a large holding tank, or you use Rosemary's suggestion of wood shavings and earthworms that will eat the odor-causing elements.
Also, make sure there is next to no gas in the vehicle because of the risk involved in explosion or fire. I'm not sure about the safety of batteries or oil in such close quarters, like if they give off fumes or the battery could explode, so check with your mechanic.
Do not eat much during this time and make sure what you do eat is not salty, so you won't drink a lot of water. This cuts down on trips to the makeshift toilet. This is where a flushable chemical toilet would be superior to a geriatric bedside toilet lined with a bag, in the short term. The chemicals would keep the odor down. What I don't know is for how long. That is why the geriatric toilet (http://www.emeraldmedicalservice.com/-c-79_54.html?osCsid=ef17031a006d84d6515a7aeab28ae8f1) is probably the better idea, with liners that can be disposed of. Squatting is uncomfortable, especially for older people, so the geriatric toilet is much easier for them to use, as well as younger people too. The older models can sometimes be found at Goodwill and other thrift stores. Disinfect it immediately if you buy used. Rather than an outhouse, this could be used outside the vehicle (but still in the trench) before escalating events force you into the vehicle for good. Always test everything to make sure things fit, work, etc. If the legs are too long for the space, cut them down or buy one with shorter legs. Another idea is the "totable toilet." It would probably work just as well. See here: http://beprepared.com/product.asp_Q_pn_E_CH%20S650_A_name_E_Family%20Sanitation%20Kit. The bucket by itself is $8.95. Maybe the geriatric toilet outside the vehicle and the "totable toilet" inside? Whatever works for you.
If you want gas for the aftertimes, use approved containers and bury them away from your trench.
The weak points are the windows. While the vehicle will be under the canopy, if it blows off there would be no protection. While the roof of the vehicle would still protect you from rain, the windows could be pelted and broken. Tempered glass is stronger than regular glass. Then there is bullet-proof glass, but I assume that would be pricey. You could also use other materials, like scrap sheet metal or plywood put over windows for extra protection, but be sure to affix it securely in case the exterior canopy goes.
Also, think about closing up the open end of the trench. Would it be necessary? (Necessary, if radiation is involved, or the first winds show it would be bad to leave it open. Have a solution at hand if needed.) If so, with what? Sandbags? 55-gallon drums of water? Would the wind just whip them away? Should they be fastened somehow? Considering that a tornado can easily lift a firetruck, I would think, at a minimum, everything in the trench should be secured. And the earthquakes could have things shifting left to right to left and/or bouncing up and down. Factor that in as well. Maybe we should bury ourselves in styrofoam "peanuts"? (Joke.)
The overall appeal of this shelter is that the construction is doable by average people with commonly-available materials and is not out of reach price-wise nor will it take too long to put together. If you can't afford the metal carport ($500-$800 depending on size), then use a big scrap of sheet metal or other metallic object like metal doors from old refrigerators, old car or truck hoods, or the barn doors off the back of old Chevy Suburbans or vans (make sure it's solid metal with no windows in any of them). If what you have doesn't look strong enough, use several, even mix and match metal and lumber, even box springs and mattresses, but put the metal on top.
I would like to hear some feedback. Do you think this would work? Do you see any other weaknesses? If so, how could the weaknesses be remedied?
And, as a reminder, I am not a professional engineer and offer these possible solutions as educational material, food for thought. If you choose to implement this shelter idea, it is your responsibility to ensure it is safe to do. Something like this idea is better than doing nothing at all, and appears to cover many of the risks associated with the increasing earth changes. However, it has not been field tested, so there are no guarantees. In fact, it would probably be impossible to fully test since the upcoming event has never occurred in living memory and factors we could never imagine will probably crop up.
Finally, on another blog, https://earthchanges.ning.com/profiles/blogs/zt-analysis-are-trench, a poleshift.ning moderator came around to discredit this idea of the enhanced trench shelter. Some very good information surfaced, so please read that blog, if you haven't already, especially read the link to the French military operations to see what a trench shelter is like, from those who field tested them: http://www.151ril.com/content/history/french-army/15.
