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.
Motion on the dam can be detected relative to the five local reference stations. The utility adds data from the broader array of statewide receivers into both the real-time and post-processing-engine computations for the regional monitoring work. It's an impressive array of information and tools for analysis, and Schrock's team uses it to determine if motion is due to the receivers on the dam rather than movement of the local reference stations.
The links to the statewide network add a high level of performance to the monitoring system at no additional cost: “The regional network infrastructure was already there, so new monitoring sites could be added for just the cost of receivers, mounting, Integrity Management software modules, power, and communications,” explains Schrock.
DETECTING THE UNEXPECTED
On July 29, 2008, an earthquake showed up in Schrock's rapid-motion-engine data a few minutes after the event occurred. By Los Angeles standards, it was moderate: magnitude 5.4.
The quake's epicenter was more than 950 miles south of Tolt Dam. A quick check against local seismographs confirmed that the change in dam data coincided with the quake. The next day, a magnitude 3.0 quake occurred about 50 miles northwest of the dam and was again detected by the receivers.
These two events signify an important — if unexpected — bonus. The Pacific Northwest often experiences slow, deep earthquakes, and the system proved itself capable of understanding the surface effects of such events. GNSS provides actual measurements of displacement that aren't obtainable from seismic or other sensors, providing a more accurate picture of what takes place when the shock of an earthquake passes through the dam.
In January, a series of storms presented an opportunity to analyze positional data from the monitoring system under extraordinary conditions. Record levels of snowmelt and rainfall prompted the utility to discharge high volumes of water from the dam. Within minutes of a call to the survey team, there was good news: less than 3 cm (1 inch) of movement, with no permanent displacement, was registered.
MORE APPLICATIONS FOR SEATTLE
The Tolt Dam project includes calibrating the receivers and determining thresholds for alerts. Installation and commissioning took place over seven months, and the monitoring system was fully operational by June 2008. The project was funded by Seattle Public Utilities' dam safety budget and an academic partner. The utility is developing protocols for sharing GNSS information with data from other sensors already connected to the dam's emergency warning system.
The utility also plans to install GNSS monitors on a series of concrete dams in northeast Washington that will tie to the statewide network, and is considering other sites and structures as well.
“GNSS monitoring is an important advance for public works,” says Schrock. “It's a good fit for bridges, skyscrapers, landslides, critical slopes, seawalls, and even buoys.”
— Stenmark, a licensed surveyor (LS), is a freelance writer and consultant working in the AEC and technical industries. He has more than 20 years experience in applying advanced technology to surveying and related disciplines.Navigating satellite technology
How the components of Seattle Public Utilities' systems interact.
GPS (Global Positioning System) refers to the navigation and positioning satellites operated by the United States. The advent of analogous systems from Russia (known as GLONASS), China (COMPASS), Japan (QZSS) and the GALILEO system proposed by the European Union, has given rise to the relatively new acronym “GNSS” (Global Navigation Satellite System).
GNSS includes all existing systems and augmentations and leaves room for more to be added in the future. Receivers that can track GPS as well as GLONASS satellites and eventually GALILEO satellites are designated as GNSS units. Receivers that track only the U.S. satellites are called GPS receivers. While GPS and GNSS receivers are quite different internally, the operation and results from the user's point of view are nearly identical.
When a receiver is permanently mounted for fiducial purposes (i.e., for the network) it is called a “Continuously Operating Reference Station” or CORS. A station comprises a receiver, antenna, permanent (or semi-permanent mount) together with power and communications facilities. A receiver/antenna that is mobile (i.e., on a folding tripod or pole) or on a semi-permanent mount in a temporary or campaign style is referred to as a “rover.” Tolt Dam's monitoring receiver and antenna combinations are permanent installations used for monitoring rather than fiducial reference purposes. By virtue of their continuous outputs they are known as “Continuously Operating Monitoring Stations” or COMS.
The term VRS (Virtual Reference Stations), or RTN (Real-Time Networks), is widely used to describe real-time correction networks; Trimble is the original provider of a VRS network solution and has a trademark on the term. VRS networks are comprised of a number of GPS or GNSS receivers installed in a region and linked to a central control center to create a real-time network for precise positioning.
Without a VRS network, surveyors, engineers, and contractors must use additional GNSS receivers as base stations and single-baseline differential techniques to obtain accurate positions. The VRS eliminates the cost and inconvenience of individual base stations while providing robust, reliable results.