Chemical Grout Underpinning

Chemical Grout Underpinning of Uncontrolled Fill Below Floor, Pittsburgh, Pennsylvania

Problem:

Water seeped through cracks in break room, bathroom and maintenance shop inside of metalworking facility. Facility maintenance consultant identified drainage problems and water service leak as sources of water. Investigation holes drilled through floor revealed up to 30 inches of saturated, uncontrolled fill above shale. Voids up to 6 inches were present between the bottom of the floor and the top of the uncontrolled fill, and held standing water during site visit.

The concrete floor was generally intact, but had settled up to 1.3 inches. Buildout of the area, including multiple masonry walls, a kitchen and bathroom had been completed after settlement, and multiple hairline stair step cracks in masonry walls bearing on floor area indicated continued movement. Water coming through joints in floor had caused ceramic floor tiles over approximately 1,000 square feet to delaminate from slab.

The foundation spread footing for the exterior structural wall was showing signs of settlement adjacent to the affected floor area. This included stair step cracking of the exterior wall (masonry block).

Constraints:

Due to the work schedule of the facility, work had to be completed without disrupting use of the facility with no downtime. Ideal time for completion was fewer than two days. Installation had to permit undisturbed use of break room/kitchen, bathroom and maintenance room without removing any fixtures, storage shelves, etc. Stabilization of the exterior wall spread footing had to be able to address instability of very tight (2″ of uncontrolled material) layer of unstable soil between the underlying shale and bottom of the spread footing.

Water and sewer service for the building passed through the exterior wall on top of the spread footing in the area of footing instability.

Solution:

Chemical grout installation to treat band of uncontrolled fill below floor. Due to shallow rock (approximately 36″ below floor elevation), in place treatment of existing soils with chemical grout was determined to be the most cost effective way to transfer the load of the floor to the rock. An alternative repair would have been to install slab piers, however, chemical grouting was determined to be more cost effective, and significantly less disruptive.

Results:

Concrete Jack chemical grouting crew (two people) treated 20 columns of uncontrolled soil in place using soil modification resin. The placement was done through 5/8″ holes drilled through the floor slab using single grout injection tubes at installation pressures of around 300 psi. ¬†Work was done during normal operations of facility, without disruption to operations. Equipment inside facility was limited to an electric drill, small wheeled pump, and a chemical tote.

After the chemical grouting was completed, the area was ready for installation of high density polyurethane to fill the voids between the treated fill material and the floor slab. This foundation repair project took a total of a single day for chemical grout installation and three hours of high density plural component polyurethane grouting work.