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Geotechnical instrumentation - monitoring

Geotechnical instrumentation refers to to the instruments used to monitor geotechnical projects or sites requiring such monitoring. Geotechnical instrumentation and monitoring are essential for the successful completion of a geotechnical projects. Limited geotechnical instrumentation may be needed for simple projects but the demands on geotechnical instrumentation and monitoring can be very demaning for critical projects such as tunnels, slopes, and excavations next to sensitive structures.

The principal parameters of interest in geotechnical monitoring are: (1) structurall and soil deformations, (2) stresses acting on structural elements (wall and bracing), and   3) ground water pressures and inflows. Table 1. summarizes the parameters and methods of measurements used for slurry wall excavations.

Inclinometers, piezometers and optical surveys have been used to monitor the majority of the projects compiled in this database. Table 3.2 lists the performance monitoring measured and what archived data could be retrieved for each case study. Generally, the quality of the available instrumentation was dictated by the project requirements. The quality of data in more recent projects tends to be better since computers have facilitated data acquisition and archiving.

In sensitive deep excavations, inclinometers and surface settlement surveys are used in almost all the projects. Thus, inclinometer deflections have been more widely used for comparisons between cases due to their availability in almost all the archives. Water levels and piezometric levels can also be critical.

In a few projects, strain gages or load cells were used to monitor bracing loads but these cases tend to be the exception rather than the rule. In very few projects strain gages and embedment gages were used to deduce moments and axial forces in the slurry wall.

Sometimes, single point extensometers have been used to measure subsurface settlements, but multipoint borehole extensometers have only rarely been used. Earth pressure cells have been used only in test programs.

Monitoring strategies are not significantly different from city to city but the extent of instrumentation does depend on local practices. For example, in Boston, the majority of the inclinometers were installed within the diaphragm walls while in Chicago they have been installed within the retained soil. In Washington, inclinometers have been installed both within the walls and within the surrounding soil.