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The Stephenson Probe

A penetrometer-installed strong-motion accelerometer system.

This is a tool in the suite of instruments and services that GNS Science can provide to the world in mitigating the effects of earthquakes on buildings.

For more information contact: Joe Manning.

A novel subsurface strong-motion accelerometer system intended to sense the shaking of the ground under soft-soil sites during strong earthquakes was developed by Bill Stephenson from the GNS Science. The instrument combines the attributes of a similar but larger and more costly instrument along with the low-cost installation method called "penetrometry". The net result was an economical solution to the task of investigating site effects on earthquake shaking, at relatively shallow, soft-soil sites. Penetrometry, is a method that essentially pushes or drives an instrument into the ground at a fraction of the cost of traditional drilling operations.

Traditional methods of acquiring subsurface accelerograms

  • Traditional accelerometers are bulky, expensive
  • Seismic accelerations are only recorded on the surface
  • Borehole instrumentation and installation is notorious for its expense
  • Drilling and casing a hole is not a simple endeavour

Immediate Benefits of using a Stephenson probe

  • An order of magnitude in cost savings over conventional methods
  • Ease of installation and low cost of installation
  • Cost reduction is such that the probes are considered to be disposable
  • 3-Dimensional view of how ground conditions vary significantly with depth

The Stephenson Probe is a non-recoverable triaxial strong motion accelerometer module, intended for sites and depths where cone penetrometry is viable. By using penetrometry, installation costs are cut by an order of magnitude compared with drilling and casing.

The cost of incorporating seismo-resistant designs into new structures only contributes 2-5% of the total cost of construction, while retrofitting old standing structures is significantly greater. Analysing the site conditions through new instrumentation enables engineers to design structures more cost-effectively.

For large buildings this suggests that foundations may have to deal with variable shaking at depth. The implications for how these structures may have to be designed for such complicated conditions could have profound implications for the industry as a whole.

SOME INSTALLATIONS

1. Porirua, New Zealand

Operator – GNS Science
Purpose - field test of prototype probe
Site description - 12m estuarine silts over weathered rock
Accelerometers – Surface, 12m

2. Wainuiomata, New Zealand

Operator - GNS Science
Purpose - research - investigation of basin waves
Site description - 22m lacustrine organic silts and clays, over 8m silts, over 40m fluvial deposits, over rock
Accelerometers - Surface , 12m, 22m

3. Texcoco, Mexico

Operator - Universidad Nacional Autónoma de México
Purpose - research - characterise site response
Site description - at least 40m lacustrine clays interbedded with tephra layers
Accelerometers – Surface, 2m, 10m, 20m, 30m, 40m