No matter the type, monitoring systems provide more than measurement
By Boris Skopljak and Riley Smith
Deformation monitoring is a key component to site safety and is required for many construction projects around the globe. Oftentimes, manual monitoring by a site surveyor will suffice, but there are other instances when automated, real-time monitoring would provide the critical safety component while also affording several productivity gains. Ultimately, selecting the right monitoring system is important for project success.
Why monitor deformation?
Deformation monitoring, the task of measuring an object’s movement over time, plays a crucial role in assisting the construction process and understanding a structural asset’s health. Owners, operators, and contractors rely on monitoring systems to measure and report on the movement of certain structures or earthen formations to ensure public safety, protect the asset and make informed decisions.
Manual and automated monitoring systems
Manual (also called periodic or campaign-based monitoring) monitoring schemes are not new in the industry. Typically, these are performed when a low frequency of measurement (e.g., once a week or month) and installation of instruments on site is not required (e.g., tripod, total station, and data collector). Manual monitoring is generally used for monitoring an object during the construction process by observing a defined set of discrete points within or surrounding a construction site or during specific operations such as moving a large load over a bridge where the project duration can be less than a day. These types of projects typically require a survey crew to be on site at all times. Often, manual monitoring can be used to detect and track trends on slow, subtle movements such as natural land drift and seasonal structure movement.
With advancements in connectivity and power supply options, automated monitoring systems have become the norm for infrastructure construction and asset maintenance. When safety of life and fast reaction times are required after movement has been detected, an automated monitoring and alarming system is required. Automated systems allow for continuous measurement and analysis, often combining a number of geodetic (e.g., Global Navigation Satellite Systems (GNSS) and total stations) and geotechnical (e.g., crack, tiltmeter, and inclinometer) sensors. Instruments are permanently or semi-permanently installed on site (e.g., concrete pillar in housing for total station mount connected to permanent infrastructure) to reduce the need for repeat site visits by enabling autonomous data collection and integration. To ensure owners and contractors timely reactions to any unexpected movements, data can be collected at frequencies ranging from hundreds of measurements per second to hourly or daily updates.
Factors that drive decisions for implementing a deformation monitoring system
Graphs, charts, reports, and alarms showing physical movement and trends is the primary output of deformation monitoring, but there are several other notable returns on investment (ROI) when employing either manual or automated systems. The most compelling reasons to implement a deformation monitoring system include:
- Compliance with legal and regulatory requirements: This is the most common driver for implementing a fully automated 24/7 deformation monitoring system. This is typically directed at the state or federal level by authorities such as DOTs (Departments of Transportation) issuing licenses for construction or operations such as mining. These regulatory bodies typically specify implementation of automated systems and require a certain movement exceeding the threshold to be reported with a clear escalation and mitigation path to protect the safety of everyone involved.
- Protecting safety of life: In highly dynamic environments such as infrastructure, mining, or areas where there is a known landslide or avalanche potential, sudden or unexpected movement can have catastrophic impact on site workers or the general public. Monitoring systems collect data that provides critical information in order to help prevent or anticipate a catastrophe. Very often there is guidance from regulatory bodies, but the implementation of accident prevention is the responsibility of general contractors or owners/operators.
- Increased productivity by eliminating multiple site visits and manual data interpretation: Installing an automated monitoring system removes the need for regular site visits to take measurements once the system is set up. There is no more traveling to and from the site or multiple equipment setups. This allows the site surveying team to take on additional projects with the same manpower while still keeping tabs on critical movement at the monitored site, thereby enhancing team productivity. Automatic reporting and real-time alarms are also gained from automated systems, which keep stakeholders informed of movement levels and conditions.
- Increased project control and visibility of asset conditions: Similar to the safety benefits afforded by a monitoring system, the data collected can alert the project team of trends and potential structural failures as a result of any movement. Having the ability to prevent a failure before it happens helps to ensure a project stays on track and on budget. In the case of dams or mines, assets can be observed over a longer period, from months to years. Having historical surveying data can aid in very accurate modelling of asset behavior and serve to identify trends that support preventive versus scheduled maintenance, resulting in optimizing the cost to maintain the asset.
Along with the benefits, there are several additional points to consider. Manual monitoring systems require an on-site operator, and manual upload and storage of the data. There is also a reduced repeatability of measurements, mainly due to environmental factors, and less ability for the field survey crew to make strategic decisions while on site.
In the case of automated monitoring, it is important to account for additional up-front costs associated with the installation and preparation of the system. There are several requirements that must be met, including 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 accessibility. 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 instrumentation 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 solution 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 permissions 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 connectivity 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 extremely 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 monitoring 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 sensor 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 measurements. 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 frequency 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 depending 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 impossible 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 engineering and infrastructure, such as rail or tunnel construction.
Foundation pile installation: In the case of short term construction activities 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:
- Site size up to 200×200 meters. Small, local construction site with 10-20 points to be monitored.
- Site is easily accessible.
- Power and connectivity is available but can be unreliable due to changing construction site conditions.
- The surrounding area is stable but construction activities pose movement concerns. Movement levels of concerns are in the centimeter-level range.
- Every 15 – 30 minutes.
- Nearby construction equipment such as piling rigs and excavators.
- The required accuracy to detect movement is one centimeter or greater.
- The site layout is constantly changing making it difficult to install permanent instruments and targets.
- Data is required immediately while in the field to warn construction workers of movement risks.
Concrete curvature dam: This type of project measures the structural health and seasonal movement in a concrete curvature dam. An automated monitoring system was necessary due to the safety and structural health concerns of the dam structure and the length of the project (greater than five years). Daily measurements for multiple points over long distances were required and even though communications posed a challenge, an automated system was preferred to ensure repeatability of measurements over the lifetime of the structure and real-time alarming and notifications of movement.
- Site size is 500m x 500m with 200+ points to be monitored
- Permitting and time windows to access the site is limited.
- Power and connectivity can be accessed from local dam infrastructure operations.
- The expected movement is millimeter-level, but alarms should be raised when centimeter level movement is detected.
- Measurements are required once per hour.
- Movement is occurring from seasonal change causing stress in the dam structure.
- The expected movement is millimeter-level per day or week.
- The site provides a large footprint to install the proper targets and instruments for reliable measurements.
- Data is required within a day of measurement for proper analysis.
No matter the project size, most construction sites will benefit from either a manual or automated monitoring system. Determine the ROI potential and discuss these options with a monitoring service subcontractor to help implement the right system for the project. If anything, the construction crew will be happy to have another layer of safety added to protect both them and the project.