of the system. There are several requirements that must be met, includ- ing the availability of a power supply, communications system and a public Internet Protocol (IP) for the data display, and a safety system to protect the equipment from theft or vandalism. Determining what deformation monitoring system is the best fit? Several factors should be considered when determining which type of monitoring system is the best solution for a construction project. These include level or risk, the need for real-time alerts and site accessibil- ity. Following is a list of questions to ask when determining whether manual or automated is the best fit for a project. 1. Site conditions and monitoring object characteristics a. Size of the area/object to be monitored? Size of the area to cover will have a key impact on the choice of instru- mentation and financial feasibility. For example, a landslide with 1-2 km2 potential movement area would require deployment of a larger number of sensors and consideration of modified instruments such as long range total stations, radar or aerial solutions that would ensure measurements can be made across a wide area and over long distances. b. Site accessibility and location? In the case of limited site accessibility, an autonomous monitoring solu- tion is advantageous because after the initial setup and configurations, other than periodic calibrations and maintenance, the system operates continuously. This removes the need for frequent site visits where per- missions need to be obtained. With remote sites it is also important to ensure equipment safety and stability using enclosures and theft prevention systems. In urban canyons and underground environments, where use of GNSS or aerial technology may not be possible, optical based solutions may be the only available solution. c. On-site power and connectivity infrastructure? Automated monitoring systems require on-site power and connectiv- ity for reliable data collection while manual monitoring data collection does not. When LAN (Local Area Network) data is not available, the most common solution for transfering data is cellular via GSM (global system for mobile communication). When choosing a cellular network, the area coverage and network strength are critical factors to consider. In ex- tremely remote areas, satellite cellular solutions are more affordable. For power, direct access to the AC power may not always be possible and this is why solar panels have become very common. A local provider can help with keeping shipping and installation costs low and assist in locating the panels in the optimal solar position. It is always important to plan for unexpected communication or power outages; the monitor- ing system needs to include data or power backups like the Settop M1 where information gets stored locally until the connectivity to the server is resumed. Geodetic and geotechnical sensors including wireless tilt or tilt with laser are often implemented to compliment each other and provide additional measurement redundancy. 2. Measurement requirements and response times a. Expected level of movement in operating conditions? To meet regulatory and project requirements, it is important to have a solid hypothesis on the expected level of movement and if there is any seasonality in object behaviour to consider. For example, dams are
very susceptible to temperature variations or level of water pressure applied. It is important to understand what has the potential for causing movement and set realistic expectations at the start of the project. The expected movement level can determine the most cost-effective sen- sor to meet the requirements. Setting alarm thresholds that will trigger warnings and SMS or email messages to responsible or affected project members requires a careful design. b. Frequency of measurement? This is typically dictated by the window of time provided to respond after movement is detected, ranging from several updates every second, to lower frequencies of 15 minutes to daily, monthly or yearly measure- ments. These can also be dictated by project or regulatory requirements. Automated and manual systems provide fast measurement times but where low frequency is required (measurements once a month), a manual system can be more cost effective. In the case of higher fre- quency of measurement, GNSS solutions are more advantageous as they provide absolute position with updates of up to 20Hz while the total station instrument can provide more accuracy information, but can take 15 minutes or more between two consecutive position updates depend- ing on the number of targets and measurements between rounds. c. Measurement accuracy required to detect movement? For surveyors, this is usually the key variable, as often the goal is chasing millimeters. It is common for project specifications to state near impos- sible accuracies asking for 0.0001 ft. or fractions of millimeters, which require special setups, network configurations and data redundancy. In these cases, post processing techniques and network adjustments are required to achieve the highest accuracy requirement. Monitoring a landslide that is considered to be moving “fast” (e.g., 1-2 cm/day) the level of precision required will be lower than for highly precise engineer- ing and infrastructure, such as rail or tunnel construction. Project examples Foundation pile installation: In the case of short term construction ac- tivities like a foundation pile installation, manual monitoring systems can serve the best needs for the project. Manual systems can be quickly deployed, requiring no existing infrastructure and can monitor high frequency of data over a short period of time. This project example would employ a robotic total station and field software, like Trimble Access Monitoring for semi-automated data collection: 1. Site size up to 200x200 meters. Small, local construction site with 10-20 points to be monitored. 2. Site is easily accessible. 3. Power and connectivity is available but can be unreliable due to changing construction site conditions. 4. The surrounding area is stable but construction activities pose movement concerns. Movement levels of concerns are in the centimeter-level range. 5. Every 15 - 30 minutes. 6. Nearby construction equipment such as piling rigs and excavators. 7. The required accuracy to detect movement is one centimeter or greater. 8. The site layout is constantly changing making it difficult to install perma- nent instruments and targets. 9. Data is required immediately while in the field to warn construction work- ers of movement risks.
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January 2021
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