This is not my field, so my post may be out of line,
Have you had excessive rains? Is your water table usually that high?
My point is: maybe pour this basement when the water table is down or when rains have not been so heavy in the near past. Consider the summer.
My point is that how will the concrete thoroughly cure out when there is so much moisture in the ground below?
Rocks would cause the water to drain out better, but look at all the work in removing the sand, digging down more, installin the rock, then pouring with the idea that it might not "properly" cure out.
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