I have had only five or six (now we have four or five--one sadly died recently, after coming up from molt shell-less for some reason; Major Tom the jumbo has been down for about a year now, so I'll know the exact number if and when he resurfaces) in the 90g; they churn it together over time. The idea is to get the EE to a good "sand castle-y" consistency (usually it's about right, from expanding the bricks, if that is how you get your EE), then lay a layer of EE, then sprinkle some sand over it. If you are trying for a 5 to 1 mix, you can use a big cup and put in five cups of EE, then sprinkle one cup of dry sand over that, then repeat. Every few inches, take a stick (or your finger--I keep a little dowel near the tank that is about the width of my ring finger; I need the longer stick because the substrate is much deeper than I can "poke" with just my finger, once it is at the depth necessary) and poke it down in the substrate; if the hole stays well-formed, you've got good consistency. If it seems too crumbly, just spritz a little water over it to add a bit of moisture. Less is better, since you can always add more water, but removing it is more problematic.hprmom wrote:@Dragonsfly, that's brilliant! I only have 5 crabs, will they really get it all mixed up fairly quickly? I'm almost halfway up now, doing it like @Jennifer711983. Well, actually we're trying to do a little sand art (kind of a parfait, but with everything already mixed) so we're doing a few batches of 5:1 and then a couple of 1:1 for a dark "stripe" (I think it will be fun to watch the crabs re-arrange it). I have a 7-year old helper so there's a little free labor, but you know, he gets distracted actually building sandcastles in the bucket and then the job goes really s-l-o-w-l-y!
If just 5 crabs really will mix it up, we might switch over to your parfait method to finish filling the tank tomorrow. I'm just, you know, new-crabber paranoid about not personally testing the mixture for consistency as I go!
I also think I'm going to try a small ee "pit" on the forest side of the tank, I'm thinking just dig a pit a little deeper than a water pool and fill it with ee (I realize the crabs will mix it in, just curious to see if they prefer it). When using straight ee, does it need to be at any particular consistency, or just the moisture from expanding the brick is fine?
Between tomorrow and the Monday holiday, hoping to have this build done in a couple of days!
I like to give them an all-EE "jungle end" as well as an all-sand "beach end"--after two years, they have mixed them somewhat, and one in particular likes to dig down for the day in the beach end, but I'm pretty sure they all molt in the all-EE end or the mostly-EE middle of the tank. So at least mine do seem to prefer the EE to the sand, at least in terms of molting substrate.
When I stirred the jungle end recently, I did add a little bit of sand to even the formerly all-EE end; mostly because I don't know why the one crab came up shell-less (I found his shell in one of the all-EE corners), so I'm trying adding sand to see if that helps with whatever mysterious issue caused him to abandon his shell underground. So now, there's at least some sand in all areas of the tank; but that end is still pretty much "all EE."
Make sure you allow a good amount of time for the whole tank (substrate included, which takes the longest) to balance out for temp and humidity before you add any crabs). Best wishes!
EDIT: hang on, I think you must be going for a 5 part SAND to 1 part EE mix? That's different, then. Then I would use the sand-layering method I mentioned for sand-only parts of the tank--layer in your "5 parts" of sand dry, spritz it with some water, wait a bit to let that settle in to sand castle consistency, then layer in your "1 part" of EE (already moistened), then repeat. Works the same way, but you do have to spritz and wait and then check the sand, whereas if you have mostly moist EE and only some sand, the moistened EE will usually "share" its moisture with the sand, and you can just check as you go and the process goes more quickly. Same "parfait" idea though; just a little fussier time-wise with mostly (or all) sand. But yeah, the crabs will mix it up for you eventually.
I can't really say whether that "eventually" will be "fairly quickly," because that depends how active and "diggy" your bunch are, but one thing about crabs is, "quickly" doesn't really apply to them. These are animals that live in a VERY different notion of time than we do--they regularly spend months in pitch darkness not moving at all--so a few days, or weeks, or even months simply do not mean to them what they mean to us, at all. One thing you can say about crabs and crabbing--they are great teachers of the nearly-lost virtue of patience, if we are willing to learn.

Then again, if your kid is having fun mixing the water in with the sand and making sand castles, then let HIM do it! His springy tendons are unlikely to be endangered by playing in the sand. And again, time with your kid is too precious to rush anything. Trust me. Before you know it, the only thing he'll want from you is the car keys. Enjoy his "dawdling" with you while it lasts! Best wishes!