Hint: Use the Impulse-Momentum theorem:

where is the average force you are looking for. You can easily compute the in ft/s. Now imagine how much mass is hitting the wall in 1 second of time. This will give you your average force. And notice that we don't care about how high the dirt is for the first part, but that we need to know it for the second part. So if the dirt is higher than the 12" object then only that fraction of the dirt that is up to 12" high will impact the object on the wall.

-Dan