Engineering and Construction

The most common type of underpinning involves using the existing foundation and then underpinning the foundation to provide deeper support. This is usually a much less expensive and rigorous method of underpinning than removing the entire foundation. For example, a common type of underpinning is by using continuous strip footings that are connected to the existing foundation. This method of underpinning is often used when the problem soil is near ground surface and the existing foundation is not badly damaged or deteriorated.

Underpinning by using continuous strip footings is a common type of repair for damage caused by expansive soil (Chen, 1988). For example, Fig. 16.43 shows a cross section of a typical design for underpinning using a continuous strip footing. The construction of the footing starts with the excavation of slots in order to install the hydraulic jacks. The hydraulic jacks are used to temporarily support the existing foundation until the new underpinning portion is installed. Steel reinforcement is usually tied to the existing foundation by using dowels. The final step is to fill the excavation with concrete and the jacks are left in place during the placement of the concrete. Figure 16.44 shows the installation of a continuous strip footing that is being used to underpin the existing foundation.

FIGURE 16.43 Underpinning of an existing foundation by using continuous strip footings.

FIGURE 16.44 Underpinning of a structure using continuous strip footings.

An existing foundation can also be underpinned with piles or piers (Brown, 1992). As discussed previously, helical anchors can also be used to underpin existing foundations. These types of deep foundation underpinning are often used when the problem soil is located well below the bottom of the foundation. Greenfield and Shen (1992) present a list of the advantages and disadvantages of pile
and pier underpinning installations.

In addition to underpinning the foundation, repair work may be needed to fix the damaged foundation or strengthen the foundation so that the damage does not reoccur. Figure 16.45 shows the strip replacement method, which is one type of repair for concrete slab-on-grade cracks. The construction of the strip replacement starts by saw-cutting out the area containing the concrete crack. As indicated in Fig. 16.45, the concrete should be saw-cut at a distance of about 1 ft (0.3 m) on both sides of the concrete crack. This is to provide enough working space to install reinforcement and the dowels. After the new reinforcement (No. 3 bars) and dowels are installed, the area is filled with a new portion of concrete.

FIGURE 16.45 Concrete crack repair: (a) strip replacement of floor cracks; (b) strip replacement detail.

Another option is to patch the existing concrete cracks. The objective is to return the concrete slab
to a satisfactory appearance and provide structural strength at the cracked areas. It has been stated
(Transportation Research Board, 1977) that a patching material must meet the following requirements:

1. Be at least as durable as the surrounding concrete.
2. Require a minimum of site preparation.
3. Be tolerant of a wide range of temperature and moisture conditions.
4. Not harm the concrete through chemical incompatibility.
5. Preferably be similar in color and surface texture to the surrounding concrete.

Figure 16.46 shows a typical detail for concrete crack repair. If there is differential movement at the crack, then the concrete may require grinding or chipping to provide a smooth transition across the crack. The material commonly used to fill the concrete crack is epoxy. Epoxy compounds consist of a resin, a curing agent or hardener, and modifiers that make them suitable for specific uses.

FIGURE 16.46 Concrete crack repair: (a) epoxy repair of floor cracks; (b) detail of crack repair with epoxy.

The typical range (500 to 5000 psi, 3400 to 35,000 kPa) in tensile strength of epoxy is similar to its
range in compressive strength (Schutz, 1984). Performance specifications for epoxy have been developed (e.g., ASTM C 881, “Standard Specification for Epoxy-Resin-Base Bonding Systems for
Concrete,” 2004). In order for the epoxy to be effective, it is important that the crack faces be free
of contaminants (such as dirt) that could prevent bonding. In many cases, the epoxy is injected under
pressure so that it can penetrate the full depth of the concrete crack. Figure 16.47 shows the instal-
lation of pressure-injected epoxy into concrete slab cracks.

FIGURE 16.47 Pressure injection of epoxy into concrete slab cracks.