According to Mike Otto (one of our old experts), the core trench should be a minimum of 8' wide. As Boondoggle says above, that may be further influenced by the width of your equipment.

Typically, if your core trench is cut significantly below grade, it will be the minimum required width at the bottom - and then actually get a little bit wider over the next several feet of vertical distance. This is because your trench walls will NOT be vertical, but will have to be cut back a little to hold their slope. (Unless of course you are cutting the trench in nearly pure clay that just has sand and gravel streaks.)

The width will then start to taper back closer to the minimum design width of the core trench as you start building material above grade to reach the final dam elevation. For example, with an 8' minimum width, the core trench fill will be 8' wide at the bottom, about 10-14' wide near the original ground level, and then back to 8' wide at the top of the dam.

Also, the core trench does NOT need to be pure clay! Usually, if you have good clay available at the pond location, the good clay is mixed with the material that came out of the pond and core trench excavation, and the resulting mixture is used for the core trench. (No big rocks, organic material, etc. Preferably nothing gravel-sized or larger, but lots of sealed dams have been constructed by good builders where conditions obligated them to include some gravel.)

Pure clay is actually an inferior material. If the water level in your pond goes way down during a drought, there is a chance that a pure clay core trench will dehydrate and large cracks may form.

I believe the minimum clay content is only 20% for a core trench (for well mixed and properly compacted material). The "optimum" content in your area will vary based on clay type, the size distribution of the other particles in your final mixture, etc. I would hope your design engineer has excellent local knowledge in pond construction and is telling you the correct numbers to use.

Good luck on your new pond project!