Tunneling is the process of digging a tunnel under a home foundation. A tunnel usually measures 3 feet by 3 feet in order to provide access for the workers. Tunnels must be dug by hand and is very labor intensive. A key aspect of tunneling is the replacement of the soil after the completed repairs or replacement of the drain lines. This is called BackFill and it is important. Backfill is very labor intensive and must use a different soil that is more easily packed and will support the foundation slab above. An improper backfill, either poor workmanship or the wrong soil, can be problematic in the future.