Modern large dam projects often include strong-motion seismic arrays, which can be built in to the dam at the time of construction. Seismic data is benefical to the project, since it provides
In addition to this, the arrays provide general seismic data for defining earth-quake parameters, mechanisms and frequency characteristics. The most advanced arrays use both weak- and strong motion instruments to measure smaller vibrations as well as major events. Because arrays on dams are spread throughout the structure, data flow and processing is a central challenge.
These arrays provide significant contributions to our knowledge of dam behaviour, allowing engineers to refine designs towards the goal of minimal seismic response. Nevertheless, the number of dams incorporating instrument arrays is still relatively small.
Guralp Systems’ accelerometers provide a truly cost-effective way to build dense strong-motion broadband arrays. Combining these with our flexible digital hardware and telemetry systems, engineers and scientists now have access to a wider range of instrumentation options than has previously been possible.
The locations of instruments around a dam should be carefully chosen to maximise effectiveness. Consider a typical arch dam:
Three particularly important sets of measurements can be identified:
Dynamic response: Dams, like any structure, have vibrational modes which can be excited by ground motion. To measure these, instruments need to be placed near the points of maximum modal deflection, at positions A (for a minimal installation) and B. For more detail, especially on a non-symmetrical dam, instruments may be added at positions C.
Effective input motion: The interface between the dam and the ground is complex and often inadequately studied. To determine the effective input motion, instruments need to be placed along the interface (positions D), and possibly in foundation galleries (positions E).
The free field: Finally, the natural ground motion in the region needs to be monitored, both as a reference for the above two quantities, and to study the effects of the dam and reservoir on local seismicity. Instruments in the free field need to be close enough to the dam to be representative of the ground motions there, but far enough away that the dam itself has little effect.
As a real-world example, the Pacoima dam in California combines triaxial instruments at positions A and D with uniaxial (radial) sensors at F. Studies following an earthquake in 1994 showed this design to be effective, although the lack of a free field reference was considered a weakness.
A gravity dam is made up of a number of identical monoliths, which may be treated as moving independently. The variation of effective input motion across the base is a major factor.
Dynamic response: Positions A; B for more detailed results. Instruments within the dam can be placed in boreholes (for earth dams) or galleries.
Tridimensional response: Positions D, instruments placed at 1/4 points.
Effective input motion: Positions C, instruments placed at the base of monoliths and at each side of the canyon.
Behaviour of independent monoliths: Positions E, instruments placed on each monolith as required.
Embankment dams can also be considered as independent monoliths, so if bidimensional response is assumed, designers can apply the same basic scheme as for gravity dams.
Our latest surface strong motion instrument, the CMG 5T Compact, is a full three-component sensor housed in a case about the size of a standard CD.
The well-regarded original design CMG-5T is also available. This instrument can also be fitted with an internal CMG-DM24 digitizer module to form the CMG-5TD. The 5TD offers support for high sample rates and low latency data transmission, as well as direct data download over FireWire. At-a-glance status information is available on its optional LCD display as well as being transmitted with the data streams in GCF format. Full configuration and firmware updates can be carried out remotely over the standard serial interface.
Identical components to the 5T are also used in the CMG-5TB borehole accelerometer. Strong motion borehole instruments can provide important information on the sub-surface soil movements which cause a high proportion of the damage sustained by structures during an earthquake.
A CMG-5TB module can also be fitted as part of a combined weak- and strong-motion borehole instrument, providing unsurpassed resolution and dynamic range for both local events and teleseisms.
Guralp Systems hardware works as a modular system using standard protocols, giving engineers the flexibility to design the network to their needs. A single installation might use any of a wide range of technologies:
Stations using any of these options can be combined to form the full array. Where arrays are installed during dam construction, a local area network is often installed as part of the basic infrastructure. Alternatively, RS232, RS422, fibre-optic, Wi-Fi (802.11b) or radio links can be used. Wireless technology is particularly attractive where laying dedicated cabling would be impractical, or for temporary experiments.
