Home to the largest portion of residents in Washington state, the Seattle area is located in an earthquake zone, making it critical to detect sudden physical changes in the area's water infrastructure — including Tolt Dam, which pro vides storage and management for nearly a third of the city's freshwater supply. Owned and operated by Seattle Public Utilities, the dam has a capacity of 56,000 acre-feet of water.
The earth embankment dam was built in the 1960s, with a multitude of monitoring systems installed over the years that include observation wells, piezometers, and seepage monitors. Connected to the dam's emergency failure warning system, the systems provide precise information on the dam's physical integrity but not in real time. They're not intended to act solely as an instantaneous warning system.
The utility had used traditional optical monitoring for years, taking observations annually. While the data's 1/4 -inch precision is excellent, the long intervals between measurements stymied engineers when trying to correlate dam motion with rapidly changing factors such as temperature, water levels, and wind conditions.
In early 2008, a team led by Project Surveyor Gavin Schrock, PLS, installed an automated Global Navigation Satellite System (GNSS; see sidebar on page 30) monitoring system that provides continuous observations and position updates every second. The dense data allows employees to correlate position measurements with events recorded by other sensors, both local to the dam and in the region around it, so they can differentiate between motion of the dam itself and tectonic motion of the region overall.
The system consists of five GPS receivers with high-precision antennas, three mounted along the dam's crest and two on the slope near its base. The receivers are connected to the Internet via Ethernet connections to a microwave transceiver at the top of the dam. To provide positional references for the monitoring, employees set up a network of five nearby reference stations. The entire network — GPS receivers and GNSS reference stations — is managed by computers located in downtown Seattle.
The utility's GNSS software, Trimble Integrity Manager from Trimble Navigation Ltd., employs three monitoring techniques:
- Server-based real-time kinematic (RTK), which differs from single-baseline RTK because it computes positions that are based on simultaneous observations from multiple points.
- To scan for unexpected motion, the software's “rapid motion engine” detects movement over hundreds of kilometers between reference stations. The software “learns” the normal behavior of a monitoring point and creates a model — referred to as a filter — of its typical or expected movement. When motion exceeds expectations, the system alerts an operator's desktop or mobile device.
- For deeper analysis, the software's “post-processing engine” processes data collected over long periods to compute position changes with precision of 3 mm (1/8 inch). By selecting and refining the larger datasets, the utility is developing a detailed picture of the dam's behavior.
The receivers are mounted on posts sunk into the dam's earthen cover but not as far as the dam's core. The positioning of these posts is on purpose. Any motion of the receivers could be caused either by slippage of the cover or by motion of the entire dam structure, and that's a critical distinction. To tell the difference, Schrock's team installed tilt sensors on each receiver mount to gather and correlate tilt-sensor data with receiver observations. If motion by a receiver is accompanied by tilting, they know the earthwork — not the entire dam — is moving.
The utility tested the system by placing a translation table (a solid flat surface with a precise grid on it) on the dam and mounting a receiver to it. Resulting data indicate that the server-based RTK could detect motion of a few millimeters within seconds of when the receiver was moved, an outcome that exceeded expectations.
QUAKES VS. DAM MOVEMENTS
While Tolt Dam itself exhibits very little motion, it's in a tectonically active region. When motion is detected, dam operators need to know if it's coming from the dam. The solution is to treat the regional motion as background noise and separate it from activity on the dam.
To accomplish this, the utility took advantage of an existing asset: a $2.2 million statewide cooperative Trimble VRS network of more than 90 GPS and GNSS receivers. The network was launched in 2002 to serve a wide range of positioning uses including land survey, precision agriculture, construction, science, and environmental mapping. One of the largest GNSS networks in the nation, it's also regarded as one of the most accurate and reliable. The utility uses it to “control the control” for the dam's monitoring system.