Why tunnel at all?
The question of tunneling typically comes up under one of two situations:
The perimeter is to be lifted more than normal and to reduce lift-induced stress interior support is deemed to be prudent.
After a normal perimeter lift, we eventually have another drought and the interior area of the slab follows the shrinking soil downward while the perimeter piers and pilings prevent the perimeter from settling as much as the interior.
Tunneling is expensive. For that reason alone most few people opt to tunnel under the foundation. They decide to live with the foundation in a bowl shape or they opt to let the contractor jackhammer holes in the interior of the slab to allow access for installing interior piers or piles.
Is there an alternative to tunneling?
Interior piers and piles can be installed without tunneling through access provided by opening holes in the slab. This is the major advantage to tunneling: without tunneling, the only way to underpin the interior area of the slab would be to install the piers or piles from inside the house. The house would be torn up to some degree; it would be unlivable.
How tunnels are constructed
Tunnels are created by digging the tunnels out by hand. It is labor intensive and that is why it is expensive.
What could go wrong?
- When the tunnels are constructed, the foundation will not be continuous supported so it may sag to some extent.
- After the foundation repair work is complete, the soil that was removed to create the tunnels has to be replaced and compacted. This should be done under the direct supervision of a Texas Professional Engineer, but this is rarely done in my experience. The ones I see most often state that the contractor has assured the engineer that the compaction was done properly.