Sorry to hear about the trouble you have had.  Unfortunately it sounds like there's not a whole lot you can do about what has already happened in your case.  You could maybe try going to small claims court without an attorney, and hope that the judge believes your word against theirs.  Try contacting Maine ROADWays at roadways@juno.com for further suggestions.

Going forward, you could amend your bylaws as you suggested, to require a bond.  We've thought about doing that for our road association.  What we do have already is a clause that says if any member damages the road, that member is responsible for the cost of repairs.  We've used that clause successfully once, but we were fortunate in that the member admitted the damage and paid what he owed.  We did have pictures of the damage.  So I would recommend both amendments to your bylaws - one requiring a bond and one requiring reimbursement for damage.

That still leaves you with the problem of proving the extent of the damage.  My recommendation in that regard going forward is that as soon as you see someone getting ready to do construction, go out and take a lot of pictures showing the condition of the road before construction begins, if possible, or at least as soon as possible after it begins.  If you can also get pictures or even videos of the trucks coming in, so much the better.  Then get more pictures when the construction is complete, preferably including some from the same angle as your original pictures, and some zooming in on the damage. 

Tip - if you can get pictures early or late in the day when the sunlight is at a low angle, the shadows will do a better job of showing the ruts.  Or you can set a ruler or small object in the picture to help show the depth of the ruts.  Also, if you have repairs done, make sure to keep records of how much it cost to actually repair that damage, rather than having it be an undeterminable part of the bill for maintenance of the whole road.