Grant

The first thing you do is calculate the average width of the dam. You add the top and bottom widths and divide by 2. In your case this comes to 23 ft. (I'd actually recommend going with a 10-12 foot wide dam top if you plan on driving over it very often).

From your description it sounds like you have a 150 ft stretch that will be 8 ft tall. Thus you multiply 150 x 8 x 23 to get 27,600 cubic feet, or 1,022 cubic yards.

For the tapering end you divide the length of 150 by 2 to make this a rectangular box. Then also multiply by 8 and 23 to get 13,800 cubic feet or 511 cubic yards.

Thus your total fill would be 1,533 yds, but you should probably add in 5-10% for shrinkage.

You don't mention anything about the core trench and the fill for that, but in the dams I've built we've figured 10-12 ft wide, probably an average of 8-10 ft deep (this will depend on your substructure) and running the full length of your dam. Just estimating in your case, if you take 300 x 12 ft wide by 8 ft deep would be 28,800 cu ft, or 1,066 cubic yards. In my experience, contractors will charge 1.5 to 2 times the rate for the core trench excavation and fill, as they will for the general fill.

Do you have an engineered design of the dam showing the elevations, slopes, and calculations for the core trench and fill? If you do, then the contractor bids should be fairly easy to compare, since they should be able to show the rates for the various activities. Also the design should include the principal and secondary spillway structures, and how the inpoundment is to be protected from wave action.

On the other hand, if you've just got some stakes set out and it's up to the contractor to esimate all the yardages and determine what they want to do for the spillways, it would be understandable that you've got a range of bids. If the latter is the case, I'd sure recommend you to get your USDA NRCS folks out there to give you some advise and provide engineering. They used to do this for free. However if you need to pay a private engineer to do this, it will sure be worth the money.