P.S.
Is this a cut situation or a fill situation? Are you constructing the wall from the top down? Are you installing the lagging in lifts working downward or are you open cutting 12 feet deep and then stacking the precast lagging from the bottom up?
Precast lagging is not recommended for cut situations because it is too hard to work with and to hold in place when lower cuts of excavation are made. Precast lagging usually needs to be installed by stacking and backfilling it starting at subgrade and then working upward. Steeply sloped open cuts can make an unsafe or unstable situation for taller walls.
Also, are you going to stress the tieback anchors? If so, you will probably damage the precast panels when you jack the tiebacks.
I would not backfill the drill holes with stone. I'd use lean mix concrete or flowable fill.
Tiedback walls with precast lagging were not a good idea in the 1980's and are an even worse idea today.