If the soil nail wall was designed properly with the knowledge that there are buried utilities behind it, then the soil above the top row of anchors should have been modeled as cohesionless. This is because a backfilled trench zone can not take much in the way of tension transversely. Nevertheless, the setback for a new trench should not coincide with the potential failure surface of the minimum safety factor. That is, the trench could be placed well beyond the ends of the anchors or close to the top of the wall. However, I would not put the trench any closer to the wall than a distance equal to the depth of the first row of anchors. Likewise, I would not put the trench past the halfway point of the top anchor. If you can, install the conduit with directional drilling. Pothole for the exact anchor depth first then simply stay above it if you can. Comprende?