You certainly wouldn't pour dozens of yards of concrete in there. You'd remove the organic soil, fill it with sand, and pour a 4" slab on that after setting up the plumbing.
Alternatively, look at an internal drain tile loop, along with some ventilation. It would be fairly easy to trench in perforated and socked pipe inside the footing. The tough part would be getting through or under the footing.
Here is the solution I implemented to fix this issue and ultimately brought the humidity level to a stable 58%:
-First and foremost the first thing I did was get a Meade Instruments TM005X-M so I could measure the humidity levels in my crawl space and get a baseline. My starting humidity levels were 75%-88%, most likely hitting 100% over the summer.
-I Had a contractor come out and replace gutters, adding two downspouts to the front of the house. In addition I will be digging trenches to send the water at least 10 feet away from the house. This was necessary because there were no downspouts on the front of the house and the water would pour over the sides of the gutters.
-I then laid out 6mil black plastic on about 75% of the dirt in the crawl space. This alone lowered the humidity from 88% to around 75%. I did not remove all of the rocks, and even after crawling on it with kneepads it did not break or tear the plastic. We did remove some of the obnoxiously large Ozark rocks. The 6mil plastic (recycled even) ($75.00) is very strong. I did a 12"-24" overlap and did not seal the seams. We did not go up the wall. We staked it down with garden stakes ($8). It is by no means perfectly sealed, but is covered.
-I sealed up all of the crawl space vents (6) with 1.5" foam, and used expanding foam to make sure the seal was good. ($30.00)
-I installed the humidifier and set it to run in continuous mode. The humidifier model I ended up getting was the FFAD7033R1 which can run in continuous mode by hooking a hose up to it. This hose drains to the sump pump. The sump pump did not have a lid, so I purchased a standard 5gal bucket lid and used waterproof duct tape to seal the lid onto the sump pump, then drilled a hole in it for the hose to come through. This put us around 65% humidity. Later I will set this to a specific humidity to hold at, and it will only run when needed. ($250.00)
-Finally, we covered the final 25% of the crawl space with the same 6mil plastic. During this time we discovered standing water, a couple 1/4" deep small puddles near the front of the house (north, the bad spot). At this time I inspected the area thoroughly and found that the sewer line going through the foundation and out had no sealer around it. This left a 1/2" gap around the entire pipe, going directly to the dirt. Under the pipe there was mud-like residue, so I was now certain this was the root issue. We used expanding foam sealer around the pipe, and laid the 6mil plastic over the puddles. This final step brought us to 58%, and I am confident it will continue to drop.
All in all, I was quoted $10,000.00 to fix this problem with a CleanSpace® system, and ended up spending around $450.00 max plus my time, about 20 hours to fix it myself. Note I don't count the gutters as included in the price since they were old and failing already. Also, the solution from the contractor did not include solving outside water problems, which in my opinion is where you should start.
Additional note about sealed vs unsealed crawl spaces. There is tons of conflicting information on this subject so I decided to test for myself. On a day when the humidity was around 55% outside, and the humidity was around 70% in the crawl space, I setup box fans to suck the 55% humidity air from the outside into the crawl space for around 12 hours straight. Doing this lowered the humidity only by 1-2%, so I knew sealing my vents was the answer.
It may seem counter-intuitive, but when dealing with crawl space issues START OUTSIDE.
Best Answer
It's a "rock pocket". The foundation wall is made of cement (a powder), water and rocks. They are suppose to be mixed throughly before "placing" in the forms. This either didn't get mixed properly or it was the last part of the pour, which often ends up with rocks that didn't get mixed in.
A small rock pocket like this is not a problem. It's more unsightly than a structural defect.
If it shows up on the exterior side of the foundation wall, we'll usually have the contractor apply a "parge coat" (a smooth coat of cement) over the rock pocket in an attempt to make it look like the rest of the wall. Depending on how long it's been there, a parge coat could "stand out" because it will probably be a different color. If you do decide to patch the rock pockets, then I'd test the color and keep the patch as small as possible...less noticeable.
It's somewhat unusual because it's located at a "control joint" in the wall. (Note the "V" groove vertically in the wall.) if the wall is not leaking, I'd leave it alone.