Typical survey monument
The old and replacement Calaveras Dams are located approximately 1,500 feet from the Calaveras Fault. This means that we need to make sure that there are devices in place to monitor the movements of both the current and future dams and the area around them.
The future Calaveras Dam will have a total of seven types of instruments inside it and around it. Today I will focus only on two types of external instruments that will be placed at Calaveras Dam to help us monitor and protect it -- survey monument and accelerographs.
I will explain the purpose of each and the role each device plays at Calaveras Dam.
Survey monuments are placed both around the exterior of a dam and later will be placed on the surface of the dam. They are monitored periodically to determine if they have moved from previous measurements, which would indicate any significant movement of the ground surface has occurred. Survey monuments also track the change in elevation of the ground surface, allowing us to determine the settlement of the new dam embankment. Some of the survey monuments placed at Calaveras Dam have survey prisms permitting the monitoring to be done remotely using a digital theodolite (aka total station). We will have a total of 65 survey monuments at Calaveras Dam.
Accelerographs are used to detect strong acceleration of the ground due to seismic forces. Accelerographs record the acceleration of the ground with respect to time. An Accelerograph is different than a seismograph. The difference is that an accelerograph doesn’t start working until something moves it. A seismograph is recording continuously. An is a recorder that uses an accelerometer. Accelerometers are much less sensitive than seismometers, but have a much greater range. There will be 4 accelerographs installed around the new Calaveras Dam.
These features are just some of the ways in which we are ensuring that the new Calaveras Dam serves our current and future generations of customers.
See you around the Valley!