Excavation

Excavation is a method for removing earth and rock from a site in order to get to a certain elevation, usually below street level. The reasons for getting to a lower elevation could be for:

1. Basement storage area

2. Mechanical equipment room

3. Electrical rooms, especially for entering electric service that would emanate from a transformer that is installed below the street level

4. Utilities coming into the site from those that are buried in the street

5. Spread footings for the foundation

6. Parking garage

7. Special functions for the building (meeting rooms, gym, etc.)

Initial Layout

The site engineer indicates on the site plan the boundary of the area to be excavated. The subcontractor responsible for the excavation checks the layout that was initially created by the CM/GC's surveyor. Once the layout is confirmed, a fence is established around the site (with access gates for equipment), so the public is protected from the excavation. In addition, the CM/GC is protecting the site for security reasons and for possible intrusion from unauthorized personnel. The CM/GC has to be concerned about injury to unauthorized people who may enter the excavation for whatever reason (vandalism, burglary, or just for fun).

The Equipment

Heavy equipment is brought in to start the removal work. This can consist of bulldozers, rock crushers, trucks, backhoes, compressors, tunnel drills (for any required tiebacks), or even a crane with a scope bucket attached. Trucks will need access to the site for removal of the dirt. In order to protect the public, when equipment enters or leaves the site a flag person is stationed at the access gates stopping pedestrians from crossing in front of the equipment that may be arriving or leaving the site. When the excavation is completed, the equipment is removed from the bottom of the excavation by the use of a crane.

Excavation Stability

When the excavation reaches a certain elevation below the initial grade level and the point at which the sides of the excavation may become unstable, then some retaining system is required. The side of the excavation may become unstable because the soil cannot maintain a 90° cut or the pressure being exerted by the soil from street traffic loads or other conditions becomes too great. When this condition exists then the methods outlined in the section on sheeting will have to be implemented. If an existing building is close to the excavation, then shoring and underpinning may be required. Exhibit 6-37 is a photograph of a building being braced due to adjacent excavation. Exhibit 6-38 is a photograph of a typical urban site where contiguous buildings are very close together to the proposed building site. See the section on shoring and underpinning for the methods used to stabilize the existing structure. In addition, a ramp has to be created so that the trucks are able to climb out of the excavation. If water is found when excavation continues, then some form of dewatering process will be required. See the section on dewatering for the methods used to contain or divert water.

Rock Excavation

At one time, rock was removed by blasting with dynamite. In order to protect the public, metal mats were placed so that the rock pieces would not fly into the air and injure someone. Now, in most urban situations, blasting is not allowed due to the potential dangers, dust, vibration, and noise. Rock crushing and drilling machines are now used to break up the rock. Exhibits 6-39 and 6-39A are photographs of a drilling machine and rock crusher. After the rock is crushed, the heavy equipment is brought in for placing the rock onto the trucks. This excess excavation material is sold to a third party for use in another project or process (sea walls, road and railway beds, etc.).

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Exhibit 6-37

Bracing of wall.

Exhibit 6-38

Buildings surrounding proposed site.

Exhibit 6-39A

Rock crusher.

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