By Ned Racine
If you want to take someone’s temperature, you use a thermometer and read the line of mercury (old school) or read the digits displayed. If you want to know how far you have driven, you use an odometer, assuming that you remembered to reset it to 0 when you began the trip.
If you are excavating a subway station and want to ensure that the skeleton of your station and the buildings adjoining it remain stable, you would use inclinometers and lasers and prisms and other geotechnical instruments—hundreds of them.
Photo Credit: Ned Racine
“I have never counted, but there are easily over 500 in the La Brea Station area,” said Peter McDonald, geotechnical instrumentation specialist for Metro’s Purple Line Extension project.
“I oversee the design-builder’s installation, monitoring and maintenance of the instrumentation system,” McDonald added. “They are primarily monitoring the excavation support systems, ground movement, and groundwater levels.”
“This monitoring is crucial because it is our first line of defense to protect the public by ensuring that buildings and utilities remain stable,” McDonald explained. “Simply put, this web of instrumentation acts as an early warning system against ground movement.”
The Purple Line Extension’s construction team uses several types of instruments to do the job, such as:
- Ground water level instruments—40 ground water sensors near La Brea Station excavation alone—which are recorded four times each day
- Settlement points, which are checked by a surveyor using lasers and prisms to ensure structures are not sinking or leaning
- Inclinometers, which measure any bending of the steel supports in the excavation walls
- Load-sensing instruments, which check that support structures are not experiencing extra stress
McDonald’s favorite instruments are the inclinometers. Inclinometers are installed vertically against a station wall, extending below the lowest level to be dug. Technicians measure the position of the inclinometers to extremely fine measurements when they are installed and monitor them for miniscule hints of bending.
One type of inclinometer resembles a metallic silver, 100-foot long snake. Once installed, the inclinometer rests in place for the duration of the project, recording any movement and sending readings via the internet to computers and smart phones. This allows near real-time evaluation 24 hours a day.
When McDonald began in construction—he served as a tunnel geologist on the original Red Line from Vermont Ave. to North Hollywood in 1994—much of his time was spent in the tunnels checking the instrumentation readings.
Now the Purple Line Extension uses state-of-the-art instruments, according to McDonald, some for the first time on a project of this kind. Some of those advances were driven by Southern Californian geology.
“Southern California has always been at the forefront of geotechnical engineering, and the number of instruments required by the instrumentation engineers for this project reflects that,” McDonald explained.
“Instrumentation engineers analyze the ground conditions and structures near excavation, and then specify the types and layout of instruments.” McDonald said. “So you cannot compare the number of our instruments with the instruments in another project because differences in geology and nearby structures make each project unique.”
The current devices were selected for the construction phase—many of them are installed on buildings and streets and will be removed—although the stations and tunnels will have their own instrumentation when the Purple Line Extension goes into operation in 2024.
“I like interpretation of the data,” McDonald said, when discussing what he likes about his job. “Each instrument has its own role, but they need each other to tell the complete story. Being able to ‘listen’ to the system and make accurate, defendable engineering decisions based on that story is very satisfying.”