Page 198 - Urban Construction Project Management

Logistics 153
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 condi-
tion 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 remov-
ing 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