Underpinning And Shoring

In the urban environment, there is a lack of available land on which to construct new structures. Thus, older buildings are torn down to make way for new structures. When this occurs, older buildings surround the new building's site. The new building's footings and basement excavation may extend below the footings and perimeter walls of the older buildings. When this occurs, the CM/GC must analyze the conditions that exist and make sure that the older surrounding buildings will remain stable during the excavation of the new building. This may require the temporary support of the older buildings' footings or the placement of a wall around the new site that will support any load emanating from the older buildings' footings. Whichever stabilization process is selected, the CM/GC needs to review the foundation plans of the surrounding buildings. If the plans are not available, then other sources must be sought out. The public library, local building department, and historical societies are good sources of information.

If no documents are available, then bore scopes or test pits may have to be dug around where the existing footings are located. This must be done with care so that the existing footings are not disturbed. The owner's design team and the CM/GC and a specialist subcontractor should be brought in to review the conditions. Recommendations will then be made as to which method should be employed with associated cost and schedule. In some cases, a balance may have to be made between the cost to stabilize the existing structures and the proposed schedule. Several underpinning and sheeting methods can be used to stabilize adjacent buildings and the surrounding soil of the new building's excavation. In addition, a surveyor should be retained to monitor the condition of the existing building to make sure no movement is occurring.

Underpinning

Jet grouting—A series of pipes are run in the ground under the footings of the existing buildings. Grout under high velocity is then sent through the pipes. The grout breaks up the soil structure and creates a solid grout-soil mass under the footings. Work could then continue with the excavation of the new building.

Freezing of the soil—A closed pipe system surrounds the footings and refrigerant then runs through the pipes. The soil then becomes frozen under the existing footings and the new excavation work could continue. In addition, this may be a temporary method until a more permanent underpinning method is employed as will be described further. This method can also be used to temporarily stop water flow that may be affecting the site construction.

Minipiles—Small diameter (4 to 10 in.) piles are driven through the foundation of the existing building (and the new building's partly excavated area). These piles can be made of steel, steel pipe (filled with concrete), or concrete and would then be driven into the soil surrounding the existing footings. If vibration is a problem, then pre-bored concrete or pipe piles would be placed in the ground and then grouted in place. The minipiles could be used as end bearing or friction piles (if sufficient pile surface area can develop the required frictional stresses). See Exhibit 6-32 for a typical minipile installation.

Straddling footings with needle beams—New footings can be installed on either side of the existing footings. Then steel beams (called needle beams) would be pushed through the soil to be supported by the new footings. The needle beams would then support the old footing.

Wall replacement—Walls can be replaced or extended by digging under the existing wall and placing a new wall footing foundation. This would be accomplished by removing small sections of the wall (to be confirmed by the engineer) and then placing new walls on top of the newly created wall footing. See Exhibit 6-34A for details on wall replacement methods.

Prior to starting any work, the checklist shown in Exhibit 6-33 should be reviewed.

Shoring and Sheeting

Soldier beams and lagging—Steel H piles are driven into the soil to a depth below the excavation level. As the earth is removed for excavation, wooden slots (lagging) are placed between the H piles (see Exhibit 6-34). Depending on the stability of the soil, it may also be necessary to anchor the H piles. This is accomplished by placing rods (or

Exhibit 6-32

Minipiles detail.

Existing Building

Exhibit 6-33

Underpinning and shoring checklist.

1. Obtain drawings of the existing building's foundation plan.

2. If required, dig small test pits around the foundation without disturbing the structure.

3. Prepare a plan of the new excavation and foundation in relationship to the existing building's foundation.

4. Show a section at the elevation of the new foundations in relation to the old foundation and the distance between the two. This has to be accomplished at all impacted foundation locations.

Indicate the type of existing foundation (spread footing, piles, mat, etc.). Indicate the material of the existing foundation (stone, concrete, steel). Indicate type of soil or rock encountered under the old foundation. Determine estimated load on the existing foundation. Locate any water and determine the elevation.

Review all information with the soils engineer and underpinning specialty contractor.

Exhibit 6-34

Soldier beams details.

cables) into the soil or rock behind the H piles and lagging and then anchoring them in place by grouting the rods or cables. If the soil is not capable of taking the load of the anchors, then rackers are placed to brace the soldier beams and lagging from inside the excavation. This method is problematic because it restricts working around the rackers within the excavated area.

Steel sheeting—Four feet length (or longer) of steel sheeting is "piled" into place to the depth of the excavation. The sheeting sections are attached to one another by interlocking with the next section. As the earth is excavated, the sheeting is anchored back to the soil or rock as described in the previous paragraph.

Slurry wall—A clam bucket excavates a trench around the site. As soon as the trench reaches a depth where the earthen walls are no longer stable (and will collapse) a slurry mixture (bentonite) is placed in the trench to stabilize the walls. Once the trench and slurry reaches a depth below the excavation elevation, then a reinforcing steel cage is placed in the trench. Once this process is completed, concrete is poured into the trench, replacing the slurry mixture (which is reused). Once the concrete hardens, the concrete wall is then anchored to the soil or rock as stated previously. In some cases, precast pre-stressed concrete panels are used in lieu of pouring the concrete (with the reinforcing steel cage). See Exhibit 6-35 for a slurry wall section and Exhibit 6-36 for a photograph of a slurry wall with tiebacks.

Exhibit 6-34A

Alternative supporting walls.

Above grade

Below grade

Existing foundation wall

Building's basement floor slab

Original footing

New foundation wall

(pour in alternate 4.0' sections)

New footing

Exhibit 6-35

Slurry wall detail.

Clamshell digger

Slurry

Reinforcing cage

Concrete poured

Concrete wall

Tie backs

Clamshell digger

Slurry

Reinforcing cage

Concrete poured

Concrete wall

Tie backs

Tie Back Anchor Installation Procedures
New excavation

Exhibit 6-36

Slurry wall with anchors.

Exhibit 6-36

Slurry wall with anchors.

Soil mixing—This procedure mixes grout with the soil (by rotating paddles) to create small long cylinders (that reach the bottom of the excavation). The continuous linking of these cylinders would provide a wall around the site.

Soil nailing—Reinforcing steel bars of approximately V2 in. to 2 in. are placed horizontally into the soil with plates on the outside of the soil. Then grout is injected along the length of the reinforcing bars. This nailing creates a solid block of soil that can sustain the earth's pressures. This method can increase the shear strength of the soil.

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