3D Monitoring

TerraDat is now offering a new service involving the installation and management of a novel ground displacement and structural monitoring system. Using a highly accurate rope-like array of accelerometers (gravity sensors), 3-D plots of x-y-z movements over time can be measured to accuracies of down to ± 0.5mm at regular intervals along the length.

In addition, our system also measures vibration in three axes, and temperature. The duration of a monitoring project can vary from seconds (e.g. deformations of railway track bed) to months or years (e.g. slope stability analysis, earthquake monitoring). click to view realtime tree movement using array

Applications include:

– In-situ monitoring of unstable slope
– Monitoring of civil structures such as bridges, tunnels or retaining walls
– Road and Railway settlement / displacement
– Volcanic eruption prediction
– Slope stability related to permafrost
– Monitoring of mines and excavations
– Measuring drill-hole or pipeline shape
– Borehole or blasthole orientation measurement
– Laboratory research (shaker tables)
– Earthquakes

There are many more uses in the geotechnical and other industries.

Unlike conventional ground monitoring systems such as collections of tilt sensors or arrays of sensors that bend in a single direction, our method uses a flexible, calibrated 3D measuring system requiring no other guides or fixturing. It bends freely, without a preferred axis and can be installed vertically or horizontally. It rolls up for easy shipping and storage and has the additional advantage of being reusable.

Wireless Control with Event Alarms

Versions are available for wireless solar powered field installation with an enhanced acquisition bandwidth for laboratory research installations. If the installation detects a vibration or movement that is over a preset threshold, an alarm notification can be sent by way of warning of for examples: ground movement signalling imminent slope failure, volcanic eruption or seismic event.

3d displacement measurement

Advantages

The shaped accelerometer array has the following advantages over manual or in-place inclinometers:

SAA construction
  • Higher spatial resolution
  • Larger tolerance to shearing
  • Autonomous data collection
  • Ease of calibration
  • Vibration data provided
  • Reusable
  • Inexpensive casing
  • Low installation costs
  • Low maintenance costs
  • Higher spatial resolution
  • Fully automated/ fewer
    errors
  • No drift

Installation and Operation

array installation schemeTerraDat provides a full service for design, installation and management of a monitoring system. The sensor array will slide directly into 27 mm (1.049″) ID PVC casing. It may then be rotated to align an X mark at the top with a geographical feature (e.g. “North”, or “down-slope”). The casing may be grouted or supported by sand in a larger hole (cased or not). Sand allows for extraction even after large shears; grout is often used in construction sites where deformations are expected to be minimal.

The length of an installation can vary from a few metres to hundreds of metres depending on the size and data point density required.

Alternatively, the array may be buried horizontally either in a road cut / trench or fixed directly onto a structure via retaining clips. The Data Concentrator (DC) is typically solar powered and has a capacity to store up to 3 years of data.

The DC automatically connects to internet using GPRS or other wireless means where data are served automatically to the customer’s internet connection. Alternatively the stored data can be downloaded via USB connection to a laptop.

Once downloaded, the data can be analysed and exported as text files using the bespoke software.

An installation can be remotely accessed at any time to configure the DC to change alarm levels, acquisition times, connection times, etc.

horizontal installation to measure ground deformation
vertical installation in borehole
Horizontal installation in trench with plastic protective conduit
Vertical installation in cased borehole

Determination of 3D position:

The shaped array comprises segments of fixed length (305 or 500mm). Gravity measurements by triaxial accelerometers allow the 3D shape to be determined if segments are within ~60° of vertical. Otherwise if the installation is near-horizontal, 2D shape (e.g. settlement) is measured.

Measurements are relative to stable ground (e.g. stable soil at end of borehole), or to a survey reference point which we can obtain using high precision GPS surveys.

3D software  monitoring example

(above left) Magnitude of deformation in a vertical installation. Horizontal axis represents time intervals over monitoring period of 4 months. (Top left) XY plot shows view of deformation over time from active soil layer viewed down Z axis.

Vibration Monitoring

The analysis software includes 3D real-time vibration capture, display, and export, using FFT algorithms. Field installations can be equipped to trigger on earthquakes or other vibration events and record 3D vibration at selected depths.

vibration monitoring

(above) graph of vibration acceleration vs. Frequency (Hz) along measuring array

Geophysics and Slope Monitoring

In addition to providing an accurate and cost effective method for detecting settlement or displacement, we can combine our considerable expertise in mapping the subsurface to identify the cause of movement. Examples of this include: profiling the depth to soil and bedrock interfaces representing potential slip planes; identifying water saturated zones more prone to movement; measuring ground stiffness and shear properties using surface wave analysis; mapping voids above which subsidence could occur; mapping zones of poorly compacted ground.

Geophysical survey techniques are non-invasive and therefore ideal for extrapolating an increased understanding of the subsurface away from a monitoring array where displacement has been detected. TerraDat has supervised postgraduate research projects on landslide characterisation and participated as an industrial partner in a Europe-wide project researching methods for characterising permafrost in relation to slope stability.

Accuracy

Angular range of sensors: +/-360 degrees (software selection required for 2D/3D modes).
Range of software 3D mode: +/- 60 deg. of vertical
Range of 2D SAARecorder mode: 360 deg. in vertical plane. Rotations must be about Y axis.
Range of Horizontal mode in SAA3D: +/- 60 deg. of horizontal.
Long-term accuracy relative to starting shape: +/-1.5 mm for 32 m SAA (notes 1, 4, 5
Long-term accuracy of “absolute” center-line shape relative to gravity vector: +/-1.5 mm for 32 m SAA (notes 2, 4,5).
Short-term resolution relative to starting shape: +/-0.5 mm for 32 m SAA (notes 3, 4,5)
Long-term accuracy of tilt/segment within 20 deg of vertical: +/- 0.0005 rad = 0.029 deg (notes 1, 4, 5).
Azimuth error of joints: < +/- 0.25 deg.
Orthogonality within segments: +/- 0.1 deg.

(note 1: based on field measurements of vertical arrays for 1.5 years of operation).
(note 2: Absolute shape is the polyline describing the center-lines of the segments, without regard for variations in the dimensions of the SAA coverings).
(note 3: short-term <= 24 hours).
(note 4: based on averaging 200 – 1000 frames per reading).
(note 5: Specification is for vertical mode within +/-20 degrees of vertical. Vertical accuracy degrades with angular deviation from vertical as shown in “Accuracy” Section.

Dynamic Acceleration (Vibration) Measurements:
Range: +/-2 G
3dB Bandwidth: 50 Hz (Subject to change).
Noise floor of MEMs: 220 microG/root-Hz.

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Contact Us

TerraDat
Unit 1, Link Trade Park
Cardiff CF11 8TQ
United Kingdom

Phone: +44 (0)8707 303050
Email: web@terradat.co.uk

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