The test site was a rural location just outside of West Frankfort, Illinois. The test foundation consisted of ordinary concrete block stem walls resting on 8” x 16” concrete strip footings. These walls were not grouted or reinforced in any fashion.
The walls were four courses high and 40 ft. long. The walls ran side by side and were paired with one another and supported soil-filled wooden bins which were used to stimulate the load of a house (see Figure 1).
The walls were erected above two future longwall mine panels. Figure 2 shows the location of the north-south test walls in relation to the relevant longwall panels. The walls were built before any subsidence took place. Significant damage in the walls was induced by the subsidence movement from the northern longwall panel that was beneath the test site. After extracting the first panel, the walls were repaired using the new strap technique. The southern panel which was adjacent to the test foundations then passed the test site. Consequently, the steel retrofitted test foundation experienced additional mine subsidence movements.
The strap retrofit system used in the field test is illustrated in Figure 3. There were five main steps taken in retrofitting the damaged test foundations. First, the steel required for the repair was determined.
Second, the first stage of repair was done and the walls were cleaned. This included filling open wall cracks in the expected wall of compression with mortar. By doing this, the large wall deflections due to crack closure were prevented. Shear strength was restored to the walls by grouting reinforcement into the cells of the concrete blocks in areas exhibiting diagonal shear cracks. Cleaning the walls was then completed, thus enabling the epoxy and fiber-cement to adhere to it. Third, the steel reinforcing straps were prepared for attachment. Fourth, the external reinforcing straps were attached to the walls to increase wall bending capacity. Finally a surface finish of fiber-cement was applied to the walls to improve retrofit appearance.
Based on the field measurements taken, the steel strapped walls experienced only about one-half the bending of the ground. At the most severe levels, the ground angular distortion was equal to 1/1200. Through straining and bending of the test wall, the surface coat of fiber-cement maintained their aesthetic quality (see Figure 4).
This method of foundation repair:
- Gives an aesthetically pleasing finish
- Costs less than equivalent alternatives
- Needs no special equipment or hard to find materials
- Can be easily performed by local contractors
Although the procedure was developed for mine subsidence, it could also be applied to other foundation damage scenarios. This repair method and field test were funded by the Illinois Mine Subsidence Insurance Fund.