Engineering and Construction

Prior to the construction of the project, an instrumentation program may need to be developed. Figure 17.13 shows an example of an instrumentation program for a proposed 75 ft (23 m) deep excavation adjacent to a building that has a shallow foundation system. The subsurface conditions at the site consist of there layers of soil (sand/gravel layer, soft/medium clay layer, and a sand/gravel layer) overlying rock. The groundwater table is located in the upper sand/gravel layer. Prior to starting the excavation, concrete diaphragm walls will be installed from ground surface and anchored in the underlying rock. As shown in Fig. 17.13, the instrumentation program consists of the following:

• Settlement monuments. Seven settlement monuments will be installed along the exterior of the building. The settlement monuments will be used to monitor the settlement of the building.
• Inclinometers. Four inclinometers are proposed and the inclinometers will be installed from ground surface to below the bottom of the proposed excavation. The inclinometers will be used to monitor lateral movement of the concrete diaphragm wall.
• Inclinometer and multi-point subsurface settlement system. One special inclinometer is proposed that can both record lateral movement as well as determine settlement at various depths below ground surface.
• Piezometers. Three sets of piezometers are proposed, with each set of piezometers having one shallow and one deep piezometer to monitor the pore water pressures in the soil layers located above and below the clay layer. If the pore water pressures should drop, then there could be consolidation of the clay layer and settlement of the building.
• Tiltmeters. Two tiltmeters are proposed on the face of the building closest to the excavation to monitor possible tilting of the building facade.
• Optical survey. The purpose of the optical survey will be to monitor horizontal and vertical movements of the diaphragm wall.

FIGURE 17.13 Example of an instrumentation program.

As this example illustrates, the monitoring program could be quite extensive and consist of many different types of devices. An additional part of the monitoring program often consists of documenting the condition of the building prior to the start of the excavation. This is usually accomplished by inspecting the building and taking notes and photographs of observed cracks or other types of damage. If the owner of the building claims damage caused by the excavation, then the pictures can be used to compare the preexisting condition versus the damaged condition of the building.