[MountainShe]

Large established evergreen trees can be fatally damaged by root injury. Trenching through the feeder roots in the first 18 to 24" of soil is a really bad idea. These trees put out a root system that can extend three times the height of the tree.
Sounds as though you have already done damage by previous work around them. unless you want to loose them, I suggest you keep your trenches and lawn away from the trees outside canopy at the very least 25'. You may not see the trees start to decline for a a year or so after the damage is done, but decline and weaken they will and you are taking a big chance on killing them.
Also remember this, trees raise property value a lot more than a lawn and planting beds do. You loose those trees you loose value in your property plus have to pay thousands of dollars to have then taken down once they die. It is a no-brainer to me.l I also have fully mature huge Cedars in my yard, I do not have a lawn and my planting beds are raised in planters to protect my trees roots and to protect the plants as well.
If you plant in the ground you will find the tree roots strangling your plantings in a few years anyway.