Under-Slab Plumbing Repairs — Tunneling

Versus Cutting Through The Slab

First, a description of the different DFW Slab Foundations — to assist in understanding under-slab repairs.

Most DFW homes have a slab designed to ‘float’ upon the Texas clay soils.  When the expansive clay soil in DFW gets wet, it soaks up moisture and gets larger.  The slab rises in the areas where the soil is wet.  As this soil dries, the clay can shrink to the point that becomes dust.  Where the soil dries out, the slab lowers when the soil shrinks.

Your DFW slab foundation type is largely determined by the home’s age.  Note: Property Tax Records show year of construction.

TYPE 1 SLAB: If your home was built in the 1950’s, it’s typical the slab foundation is a 4″ thick concrete-pad with no reinforcing steel. There is typically no perimeter (outside edge) beam.

TYPE 2 SLAB: In the later 1950’s and early 1960’s, reinforcing steel was added to the 4″ slab area before the cement was poured.  The reinforcing steel rebar was welded together and in a square pattern.  Type 2 slabs also have perimeter beams 16″deep X 8″ wide.

slab foundation rebar

TYPE 3 SLAB: Homes built between the mid 1960’s and mid 1990’s are Type 3.   This slab has rebar reinforcement in BOTH the four-inch slab AND all beams.  Type 3 slabs have Perimeter Beams Plus Internal Beams every 15 feet.  Beams are usually around 20″ deep X 8″ wide.

POST-TENSION SLAB:  Since the mid 1990’s a newer slab version, Post-Tension Slabs, are used where slab foundations must be more substantial to accommodate changing soil-conditions (like those in DFW).  Post-Tensioned slabs typically have perimeter & internal  beams that are 24″ deep X 12″ wide.

Post-Tension Slabs have cables installed inside the slab before cement is poured.  The steel strands run through the concrete in a grid pattern. going through both the slab and all beams.  Each metal cable is inside a plastic tube that protects the steel from corrosion, plus allows it to move during the tensioning (tightening) operation.

diagram of post-tension cable used in slab foundations

Post-Tension cables compress concrete, enabling it to better resist cracks caused by shrinkage or soil conditions.  Once the concreted has cured (dried) for 5-7 days, the cables are tightened so each cable exerts around 28,500 pounds of inward pressure toward the center of the slab.  Once tensioned, the cables can never be loosened.

slab foundation post-tension cables   slab foundation post-tension cables before tensioning cable tensioning machine for post tension slab

Cables installed before cement is poured.   Untensioned cables exiting slab.   A Machine tensioning (tightening) the cables.

This Video Shows Post-Tension Cables as they are being tightened.   Video Showing Post-Tension Cable Tightening

Can A Post-Tension Slab Be Cut For Under-Slab Repairs?

cutting through post tension slab

Yes, provided the contractor has the skills to determine where the cables are, and the skill required to cut around them. This photo above shows an example of a Post-Tension Slab that has been cut open while properly avoiding the cables.  After the under-slab repair is complete, the holes in the ground are refilled and the cement is patched.

What Happens If A Post-Tensioned Cable Is Cut During Under-Slab Repairs?

The cables installed are tightened to around 28,500 pounds of tension. If severed, each cable finds the path of least resistance:

  • Some cables retract back into the slab.
  • Other cables retract, then pop out of the slab on the outside edge of the foundation.
  • Other times, the cables bust through the top of the concrete creating a ripple-effect of damage along the cable’s location toward the outside edge of the home.  This destroys the concrete and flooring and can be quite expensive to repair. Injury can occur if someone is standing on top of the cable when it releases.

SOURCE: http://www.posttensioncablelocators.com/about_post_tension_cables.html

How To Know If Your Home Has A Post-Tensioned Slab

1. If your DFW home was built during mid 1990’s or later, it likely has a Post-Tension Slab.

post tension slab stamp do not cut or drill

2. A stamp like this likely appears somewhere (often in the garage floor).

patch covering post tension cable end

3. Once cables are tightened, a small concrete patch covers the end of each cable.  This is usually visible when looking at the foundation, just above the soil.

Al’s Plumbing, Heating and Air Conditioning Tunnels Under The Slab For Repairs.

Benefits Of Tunneling To Make Under-Slab Repairs:

  • The Slab remains in one piece, just like it was designed and constructed to be.
  • Your home does not become a construction-zone while repairs are made.
  • You don’t have to move out while repairs are being made.

photo of tunnel under slab  tunnel under slab to sewer pipes

Realizing the many benefits, Al’s began tunneling for under-slab repairs several years ago.   Due to the risk of personal injury, or possibly severe damage to a Post-Tension Slab foundation if a cable is cut, Al’s does not offer that service.

Al’s Plumbing, Heating and Air Conditioning is a full-service Plumbing & HVAC provider since 1989.  We service Denton & Collin counties, northern Dallas County and northeast Terrant County.   We have up to a 12 Service-Truck Fleet to serve you promptly.  We offer 24/7 Emergency Service for both HVAC and Plumbing

Al’s provides Licensed, Full-Service Plumbing Repairs or Replacements.  We employ Nate Certified Heating & A/C  Service Technicians who maintain, repair or replace any brand of Furnace, Air Conditioner or Heat Pump.

Call Us Today, or Schedule On-Line, for an appointment to resolve any plumbing, heating and a/c problems you have.