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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.

Geotechnical instrumentation

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.

Geotechnical Problem

Parameter of Interest

Possible measurement method

Trench stability

Width of trench

 

· Trench width gage

· Inclinometer

Trench verticality

Alignment of sides of trench

· Inclinometer in temporary casing

Guide wall stability

Horizontal and vertical movement of guide wall

· Optical survey

Ground and wall movement

Horizontal movement of ground or wall

 

 

 

 

· Optical survey

· Inclinometer in wall or in soil

· Horizontal multipoint extensometer (HMPBX)

· Tape extensometer across the excavation

Vertical movement of ground or wall

· Optical survey

· Subsurface settlement gage ie. Settlement rods or Multiple Point Borehole Extensometers (MPBX)

Movement of adjacent structures

· Photography

· Optical survey

· Tiltmeter

· Crack gage

Cross-lot bracing

Loads in struts or braces

· Strain gage

Tieback bracing

Tieback load

· Strain gage

· Load gage

Anchor movement

· Telltale

Basal stability

Bottom heave or horizontal ground movement

· Heave gage

· Inclinometer

Groundwater

Groundwater level

· Observation well

Pore pressure

· Piezometer

Advanced information

Wall moments, axial forces, stresses in wall or reinforcement

· Strain gages

Earth stresses

· Earth pressure cells

SiteMaster is our new inclinometer software program. SiteMaster produces professional reports and diagrams for a whole construction site with multiple inclinometers. Please click here to find out...

Geotechnical Problem

Parameter of Interest

Possible measurement methods

Trench stability

Width of trench

 

· Trench width gage

· Inclinometer

Trench verticality

Alignment of sides of trench

· Inclinometer in temporary casing

Guide wall stability

Horizontal and vertical movement of guide wall

· Optical survey

Ground and wall movement

Horizontal movement of ground or wall

 

 

 

 

· Optical survey

· Inclinometer in wall or in soil

· Horizontal multipoint extensometer (HMPBX)

· Tape extensometer across the excavation

Vertical movement of ground or wall

· Optical survey

· Subsurface settlement gage ie. Settlement rods or Multiple Point Borehole Extensometers (MPBX)

Movement of adjacent structures

· Photography

· Optical survey

· Tiltmeter

· Crack gage

Cross-lot bracing

Loads in struts or braces

· Strain gage

Tieback bracing

Tieback load

· Strain gage

· Load gage

Anchor movement

· Telltale

Basal stability

Bottom heave or horizontal ground movement

· Heave gage

· Inclinometer

Groundwater

Groundwater level

· Observation well

Pore pressure

· Piezometer

Advanced information

Wall moments, axial forces, stresses in wall or reinforcement

· Strain gages

Earth stresses

· Earth pressure cells

Slideshow image Slideshow image Slideshow image
SiteMaster: Summary of inclinometer displacements with time SiteMaster: Sample cumulative displacements SiteMaster: Site plan image showing max polar displacements.

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