Nuclear weapons (like any large ordinance) are intended to be detonated some distance above ground. While they would still have caused widespread devastation even the shallow vault they are in would have cut it to a fraction of their potential. Placing them so close together would likewise cut the damage in half again. Knox clearly would know this and have had a better plan.
The whole sub-plot with the Nigerians serves no purpose. Knox doesn't have any need for them in his plan. He is perfectly capable of deploying and remotely detonating the bombs without them.