Tiltmeters For Slope Monitoring
A tiltmeter is an instrument that measures its own rotation and, therefore, the rotation of the structural element or portion of ground to which it is connected. Nearly all ground movement exhibits some degree of rotational movement. Even landslides that are predominantly translational will produce tilts that are easily detected with conventional electrolytic tiltmeters.
Figure 1. Patterns of tilt behavior. Arrows represent vector rotations (directions and magnitudes of tilt). From Tofani and Horath (1990).
Tiltmeters based on electrolytic level sensors have several important practical advantages. First, they have no mechanical moving parts to drift or wear out. Sensitivity and repeatability are excellent, typically one part in a million (i.e., 1 microinch per inch, 1 micron per meter or 1 millimeter per kilometer). In other words, sensitivity to ground movement exceeds that of the other transducers (servoaccelerometers, vibrating wire and foil strain gauges) commonly used for geotechnical measurements. Because tiltmeters are left in place and continuously recorded, mechanical repositioning errors do not occur.
Slope Monitoring
If a slope is moving, tiltmeter surveying can determine the direction of movement, delimit the areas of deformation and, in many cases, reveal the mechanism of movement (slumping, slope creep, settlement, etc.).
Continuous tiltmeter monitoring insures that events that would go undetected by periodic manual instrument readings are always captured. Tiltmeters developed since 1980 easily fulfill the demand for greater measurement precision and automated surveying. More traditional, manually operated inclinometers do not meet these demands, but still provide one of the few sources of vital subsurface information. Used together, these instruments constitute an effective approach for many slope and structural stability investigations. When used to initially evaluate movement at a site, tiltmeters can indicate the need for inclinometers, the frequency with which inclinometer surveys should be done, and the spatial arrangement of inclinometer holes. Inclinometers, on the other hand, help detect three-dimensional slide geometry.
Photo courtesy of Kane GeoTech, Inc.
Inclinometers and Tiltmeters
When a site shows suspect slope stability, a common response is to immediately install one or more inclinometer holes. If the slope in question is moving rapidly, this approach usually will yield good information about deformation rates and mechanisms after a few weeks or months of diligent inclinometer surveying. More typically, however, movement is slow or episodic. Inclinometer surveying in such cases may not be effective within the time frame or budget allotted to a project. The results are likely to be inconclusive or even erroneous due to aliasing or because the data are too near the limits of system resolution.
To preclude such problems, one or more high-resolution tiltmeters may be deployed in locations originally designated for inclinometer use. If a slope is moving at a slow rate, continuously monitored tiltmeters normally can establish an average movement rate within a few weeks
In-Place Inclinometers and TDR
Time domain reflectometry (TDR) is a relatively new approach to monitoring landslide and embankment stability. Originally developed to locate breaks and faults in communication and power lines, TDR can be used to monitor the movement of earth slopes. Data collection consists of simply attaching a TDR cable tester to a coaxial cable grouted in a borehole, and taking a reading.
TDR is similar to radar: an electrical pulse is sent down a coaxial cable and when it encounters a break or deformation in the cable, it is reflected. The reflection shows as a "spike" on the characteristic cable signature. The relative magnitude and rate of displacement, and the location of the zone of deformation can be determined immediately and accurately.
TDR has some advantages over traditional inclinometers:
Figure 1. Patterns of tilt behavior. Arrows represent vector rotations (directions and magnitudes of tilt). From Tofani and Horath (1990).
Tiltmeters based on electrolytic level sensors have several important practical advantages. First, they have no mechanical moving parts to drift or wear out. Sensitivity and repeatability are excellent, typically one part in a million (i.e., 1 microinch per inch, 1 micron per meter or 1 millimeter per kilometer). In other words, sensitivity to ground movement exceeds that of the other transducers (servoaccelerometers, vibrating wire and foil strain gauges) commonly used for geotechnical measurements. Because tiltmeters are left in place and continuously recorded, mechanical repositioning errors do not occur.
Slope Monitoring
If a slope is moving, tiltmeter surveying can determine the direction of movement, delimit the areas of deformation and, in many cases, reveal the mechanism of movement (slumping, slope creep, settlement, etc.).
Continuous tiltmeter monitoring insures that events that would go undetected by periodic manual instrument readings are always captured. Tiltmeters developed since 1980 easily fulfill the demand for greater measurement precision and automated surveying. More traditional, manually operated inclinometers do not meet these demands, but still provide one of the few sources of vital subsurface information. Used together, these instruments constitute an effective approach for many slope and structural stability investigations. When used to initially evaluate movement at a site, tiltmeters can indicate the need for inclinometers, the frequency with which inclinometer surveys should be done, and the spatial arrangement of inclinometer holes. Inclinometers, on the other hand, help detect three-dimensional slide geometry.
Photo courtesy of Kane GeoTech, Inc.
Inclinometers and Tiltmeters
When a site shows suspect slope stability, a common response is to immediately install one or more inclinometer holes. If the slope in question is moving rapidly, this approach usually will yield good information about deformation rates and mechanisms after a few weeks or months of diligent inclinometer surveying. More typically, however, movement is slow or episodic. Inclinometer surveying in such cases may not be effective within the time frame or budget allotted to a project. The results are likely to be inconclusive or even erroneous due to aliasing or because the data are too near the limits of system resolution.
To preclude such problems, one or more high-resolution tiltmeters may be deployed in locations originally designated for inclinometer use. If a slope is moving at a slow rate, continuously monitored tiltmeters normally can establish an average movement rate within a few weeks
In-Place Inclinometers and TDR
Time domain reflectometry (TDR) is a relatively new approach to monitoring landslide and embankment stability. Originally developed to locate breaks and faults in communication and power lines, TDR can be used to monitor the movement of earth slopes. Data collection consists of simply attaching a TDR cable tester to a coaxial cable grouted in a borehole, and taking a reading.
TDR is similar to radar: an electrical pulse is sent down a coaxial cable and when it encounters a break or deformation in the cable, it is reflected. The reflection shows as a "spike" on the characteristic cable signature. The relative magnitude and rate of displacement, and the location of the zone of deformation can be determined immediately and accurately.
TDR has some advantages over traditional inclinometers:
- lower installation costs
- no limits on hole depth
- immediate determination of movement
- remote data acquisition capability