Underpinning with a New Foundation.

Underpinning is often required to support a structure that is sinking or tilting due to ground subsidence or instability of the superstructure. The most expensive and rigorous method of underpinning would be to entirely remove the existing foundation and install a new foundation. This method of repair is usually only reserved for projects where there is a large magnitude of soil movement or when the foundation is so badly damaged or deteriorated that it cannot be saved.

Figure 16.38 shows the manometer survey (i.e., a floor level survey) of a building containing two condominium units at a project called Timberlane in Scripps Ranch, California. The building shown in Fig. 16.38 was constructed in 1977 and was underlain by poorly compacted fill that increased in depth toward the front of the building. In 1987, the amount of fill settlement was estimated to be 4 in. (100 mm) at the rear of the building and 8 in. (200 mm) at the front of the building. As shown in Fig. 16.38, the fill settlement caused 3.2 in. (80 mm) of differential settlement for the conventional slab-on-grade and 3.9 in. (99 mm) for the second floor. The reason the second floor had more differential settlement was because it extended out over the garage. Note in Fig. 16.38 that the foundation tilts downward from the rear to the front of the building, or in the direction of deepening fill.

Manometer survey: (a) first floor; (b) second floor.
FIGURE 16.38 Manometer survey: (a) first floor; (b) second floor.

Typical damage consisted of cracks in the slab-on-grade, exterior stucco cracks, interior wallboard damage, ceiling cracks, and racked door frames. Using Table 7.3, the damage was classified as severe. Due to on-going fill settlement, the future (additional) differential settlement of the foundation was estimated to be 4 in. (100 mm).

In order to reduce the potential for future damage due to the anticipated fill settlement, it was decided to underpin the building by installing a new foundation. The type of new foundation for the building was a reinforced mat, 15 in. (380 mm) thick, and reinforced with number 7 bars, 12 in. (305 mm) on center, each way, top and bottom. In order to install the reinforced mat, the connections between the building and the existing slab-on-grade were severed and the entire building was raised about 8 ft (2.4 m). Figure 16.39 shows the building in its raised condition. Steel beams, passing through the entire building, were used to lift the building during the jacking process. After the building was raised, the existing slab-on-grade foundation was demolished. The formwork for the construction of the reinforced mat is shown in Fig. 16.39. The mat was designed and constructed so that it sloped 2 in. (50 mm) upward from the back to the front of the building. It was anticipated that with future settlement, the front of the building would settle 4 in. (100 mm) such that the mat would eventually slope 2 in. (50 mm) downward from the back to the front of the building.

Raised building.
FIGURE 16.39 Raised building.

After placement and hardening of the new concrete for the mat, the building was lowered onto its new foundation. The building was then attached to the mat and the interior and exterior damages were repaired. Flexible utility connections were used to accommodate the difference in movement between the building and settling fill.

Another underpinning option is to remove the existing foundation and install a mat supported by piers. The mat transfers building loads to the piers that are embedded in a firm bearing material.

For a condition of soil settlement, the piers will usually be subjected to downdrag loads from the settling soil.
The piers are usually at least 2 ft (0.6 m) in diameter to enable downhole logging to confirm endbearing conditions. The piers can either be built within the building or the piers can be constructed outside the building with grade beams used to transfer loads to the piers. Given the height of a drillrig, it is usually difficult to drill within the building (unless it is raised). The advantages of constructing the piers outside the building are that the height restriction is no longer a concern and a powerful drill rig can be used to quickly and economically drill the holes for the piers.

Figure 16.40 shows a photograph of the conditions at an adjacent building at Timberlane. Given the very large magnitude of the estimated future differential settlement for this building, it was decided to remove the existing foundation and then construct a mat supported by 2.5 ft (0.76 m) diameter piers. The arrow in Fig. 16.41 points to one of the piers.

In order to construct the mat supported by piers, the building was raised and then the slab-on-grade was demolished. With the building in a raised condition, a drill rig was used to excavate the piers. The piers were drilled through the poorly compacted fill and into the underlying bedrock. The piers were belled at the bottom in order to develop additional end-bearing resistance. After drilling and installation of the steel reinforcement consisting of eight No. 6 bars with No. 4 ties at 1 ft (0.3 m) spacing, the piers were filled with concrete to near ground surface. Figure 16.42 shows a close-up of the pier indicated in Fig. 16.41. To transfer loads from the mat to the piers, the steel reinforcement (No. 6 bars) at the top of the pier is connected to the steel reinforcement in the mat.

After placement and hardening of the new concrete for the mat, the building was lowered onto its new foundation. The building was then attached to the mat and the interior and exterior damages were repaired. Flexible utility connections were used to accommodate the difference in movement between the building and settling fill.

Another underpinning option is to remove the existing foundation and install a mat supported by piers. The mat transfers building loads to the piers that are embedded in a firm bearing material.

For a condition of soil settlement, the piers will usually be subjected to downdrag loads from the settling soil.
The piers are usually at least 2 ft (0.6 m) in diameter to enable downhole logging to confirm end-bearing conditions. The piers can either be built within the building or the piers can be constructed outside the building with grade beams used to transfer loads to the piers. Given the height of a drill rig, it is usually difficult to drill within the building (unless it is raised). The advantages of constructing the piers outside the building are that the height restriction is no longer a concern and a powerful drill rig can be used to quickly and economically drill the holes for the piers.

Figure 16.40 shows a photograph of the conditions at an adjacent building at Timberlane. Given the very large magnitude of the estimated future differential settlement for this building, it was decided to remove the existing foundation and then construct a mat supported by 2.5 ft (0.76 m) diameter piers. The arrow in Fig. 16.41 points to one of the piers.

In order to construct the mat supported by piers, the building was raised and then the slab-on-grade was demolished. With the building in a raised condition, a drill rig was used to excavate the piers. The piers were drilled through the poorly compacted fill and into the underlying bedrock. The piers were belled at the bottom in order to develop additional end-bearing resistance. After drilling and installation of the steel reinforcement consisting of eight No. 6 bars with No. 4 ties at 1 ft (0.3 m) spacing, the piers were filled with concrete to near ground surface. Figure 16.42 shows a close-up of the pier indicated in Fig. 16.41. To transfer loads from the mat to the piers, the steel reinforcement (No. 6 bars) at the top of the pier is connected to the steel reinforcement in the mat.

Construction of the mat foundation.
FIGURE 16.40 Construction of the mat foundation.

Construction of a mat supported by piers. The arrow points to one of the piers.
FIGURE 16.41 Construction of a mat supported by piers. The arrow points to one of the piers.

FIGURE 16.42 Close-up view of Fig. 16.41.

 TABLE 7.3 Severity of Cracking Damage
 Severity of Cracking Damage

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