Stop and call the engineer

Do not deepen on a hunch. The engineer who issued the design needs to assess the change and reissue a depth or a different solution.

Common changes

Made ground deeper than expected, a soft seam, an unrecorded drain, an old garden pond fill, or roots beyond the predicted zone.

Cost and timing

Honest extras at this stage are cheap compared to remedial work later. Get the new design in writing, agree the cost in writing, and only then continue.