| Thick, black tar is used for damp-proofing the back of the newly-poured concrete backwalls. It is applied ahead of the 'lifts' of backfill used to bring the roadbed back to grade after completion of the backwalls. Lifts are one-foot deep dumps of backfill material, compacted using a mechanical compactor ahead of the next lift (marked by the orange lines on the backwall). |