Yes a certified arborist would be a good idea. Go to https://www.treesaregood.org/findanarborist" Otherwise interview one or more to see who has the most experience with this kind of situation. 
Are you saying that your tree trunk is 8 feet 9 inches wide, or it that the circumference? If so, it must be a very large and old tree.  If you had entered your city, I might have been able to go to Google Earth and look at your address from the "sky view." 
Based on your information, you have a decision based on the possibility of Emerald Ash Borer attacking your tree in the future and saving an existing tree. How close will the trench be to the tree trunk? If it is 26 feet, you should be okay. Also is there plenty of space for the tree roots to have expanded around the other sides of the tree. Tree roots anchor the tree into the ground. Please provide a drawing, picture or additional details or consult a local arborist.  

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The picture clarifies the problem. If the people who are digging the trench love trees, you might be able to convince them to hand dig and to go around the big roots leaving them in place and installing the water and sewer lines around them. Cutting the smaller circumference roots would not be as damaging as cutting the big roots. If the trench goes from the street to the house wall, possibly one quarter of the roots that support the tree would be cut on that side. 
And from the picture it looks like the street side of the tree roots may have been or will be cut also. I am a Tree Care Advisor, not a certified arborist and cannot give you a definite answer. Since you have what looks like a spruce started near the ash, you will have a front yard tree. I am concerned that if the ash tree is kept, it will be stressed from having some of its roots being severed. That will make it succeptible to going over in a wind storm or being attacked by Emerald Ash Borer. Sorry you have to make such a decision.  

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