I would suggest that you talk to a local plumber who is familiar with digging soakaways in the area where you live. Be prepared for bad news, though.

On the assumption the holes did not fill with water when you first planted the trees, this could just be a result of all the torrential rain some areas of the country have experienced recently - don't know where you are, so can't say how likely that is, but a clay subsoil is quite normal in places like London. If the area you're planting is lower than surrounding land, then it might need drainage works to cope during winter when its wet, hard to say without having more information.

I wonder as to the size of the trees that you planted. If they were quite small, then they were unable to cope with the high volume of water that filled the planting holes and starving them of oxygen. Before going to the expense of digging a large soakaway, you might experiment with trying just one or two more mature trees and digging a deeper planting hole with plenty of crushed grit mixed with you soil in the bottom of the holes. As they start to grow away they will then cope with access water. If all fails and I guess that you are trying to create a tall screen then you might have to resort to something like Crataegus (Hawthorn) to do the job.

If it looks like a permanent problem there are trees and shrubs that will tolerate wet ground. Willow, dogwod would be OK and even cherry laurel which I have seen growing in very boggy land. Alder too, but its not very decorative.