The use of geotechnical instruments is essential for ensuring the long-term stability of landfills slopes. These instruments provide real-time data that can be used to make informed decisions regarding potential intervention actions and manage/mitigate potential risks.
By Timothy Mitchell, P.E., Zach Metzler, E.I.T., and Brad Nagy
Several waste disposal facilities across the U.S. have recently experienced slope instability issues. Many of these slope failures can be attributed to the accumulation of liquids or increased pore pressures in the waste mass, resulting in changed stress conditions and/or an overall reduction of strength of the waste mass. Slope failures at disposal facilities can have massive financial implications for landfill owners, with potential remediation efforts costing in the $10 to $100 million range. More importantly, slope failures have resulted in environmental releases or fatalities. Events like these have major impacts on site operations, waste company financials, and the solid waste industry.
Often, early signs of instability are identified through visual observations made by onsite personnel. The drawback with this monitoring technique is that it tends to be subjective and relies on the personnel performing the observations to physically view the slope instability issue. Geotechnical instrumentation is used as a more quantitative method to monitor for slope instability and/or be used to track slope movement trends when signs of instability are observed. Routine visual observations of slopes remain crucial to monitoring slope stability but are supplemented with the addition of survey monitoring to monitor horizontal or vertical movement, installation of piezometers to monitor pore pressures within the waste slope, and/or the installation of thermistors in the waste. Traditional data gathering is limited by the frequency of site visits, which makes continuous monitoring labor intensive and costly. Recent advancements with data loggers and remote communication capabilities often allow for real-time data viewing.
We have been involved with several recent projects in which remote monitoring programs have been developed using various surface and subsurface instrumentation methods including piezometers, inclinometers, GPS monitors, thermistors, and remote data communication base stations. The remote monitoring capability of these instruments and the cloud-based data reduction software allows for real-time, continuous monitoring of the data for a number of project stakeholders. This results in an increased frequency of data gathering, while reducing labor costs and keeping onsite personnel away from potentially dangerous field conditions.
Geotechnical instrumentation can play a crucial role in the design, construction, and monitoring of landfills. These instruments help verify assumptions made during the design process, assist monitoring for changing conditions during construction activities, and ensure the stability and safety of landfill slopes. Avoiding an ultimate landslide helps sites prevent environmental contamination, continue safe site operations, and provide valuable data for ongoing site management. This article is intended to explore the diverse types of geotechnical instruments that are used in landfills..
Types of Geotechnical Instruments
The use of geotechnical instrumentation at landfill sites involves the implementation of various sensors and devices to monitor the physical properties and behavior of the waste mass. These instruments have been used in civil engineering projects, including landfills, to assess and manage risks associated with ground movements, liquid levels/pore pressures, and subsurface temperatures. The data collected from these instruments is in turn used for making informed decisions during the design, construction, and operational phases of a landfill. The remainder of this section will briefly describe the various geotechnical instruments that can be used at landfills, what parameters they monitor, and why the diverse types of instruments are used.
Piezometers/Vibrating Wire Piezometers
Piezometers are used to measure the pore pressures within the waste mass. Pore pressures can result from elevated liquid levels or landfill gas (LFG) pressures. Elevated pore pressures influence stability of slopes and should be monitored at sites. There are several types of piezometers, but those typically used at landfill facilities include open standpipes piezometers and vibrating wire piezometers.
Open standpipe piezometers are installed by boring into the waste mass, installing a screened well casing, and backfilling the annular space with aggregate. Boring in waste using traditional hollow-stemmed auger drilling can be difficult in municipal solid waste (MSW) landfills due to 鈥渇ibrous鈥 portions of the waste mass binding the augers casings. As such, 鈥渘on-traditional鈥 boring techniques are often used, including direct push or sonic coring to install open standpipe piezometers in waste. Open standpipe piezometers can then be outfitted with a pressure sensor and data logger to monitor subsurface pore pressures over time.
Vibrating wire piezometers operate by measuring the frequency of a vibrating wire, which changes in response to variations in the physical properties being monitored. Vibrating wire sensors are commonly used in piezometers. Their ability to provide precise measurements over extended periods of time makes them valuable for monitoring the pore pressures in landfill slopes and the early detection of potential issues associated with said pore pressures.
Stability Monitoring Points, GPS Sensors, and Settlement Plates
Stability monitoring points (SMPs), global positioning sensors (GPS)units, and settlement plates are used to monitor horizontal and vertical movement of landfill slopes over time. Incremental and cumulative movements over time (i.e., movement rates) are used to determine if slope movement trends are increasing, decreasing, or remaining steady. By doing so, a site can determine how imminent a threat of slope instability may be. Additionally, a site can use this data to determine how effective deployed mitigation techniques are.
SMPs are typically installed in a grid-like pattern or at critical areas of landfill slopes and are monitored routinely. SMPs can be monitored with traditional surveying, but results of the survey are delayed for surveyor processing, data reduction, engineering review, and presentation of survey results. Automated surveying and data interpretation/presentation is preferred and can be used to create real-time data monitoring.
GPS Units can be used similarly to the SMPs described and are better when routine cost effective surveying is not available at a site. The number of GPS Units deployed is often cost limited, as each sensor requires an upfront capital investment (e.g., $5,000 to $10,000 per point). Additionally, GPS Units are less accurate but can be used with 鈥渕oving averages鈥 of the data or machine learning algorithms to weed out artificial noise/spikes in the data. GPS Units also measure the tilt of the instrument at the installed location, which is useful to determine if a rotational movement is occurring.
Settlement plates are typically installed at various depths within the landfill and are connected to surface markers. By measuring the distance between the surface markers and the plates, engineers can determine the amount of settlement that has occurred. This information is crucial for maintaining the integrity of the landfill components or to monitor an area with excessive settlement. Settlement monitoring is essential for understanding the behavior of waste materials. Excessive settlement can lead to surface cracking, which can allow water infiltration and increase the risk of elevated pore pressures.
Inclinometers and Tiltmeters
Inclinometers are used to measure the angle of slope or tilt. They are crucial for monitoring the stability of landfill slopes and detecting any potential deep seated or surficial landslides or ground movements. Typical inclinometers can be installed within a boring to measure deep seated movements in a waste slope. Our experience has been that these inclinometers have traditionally been installed in non-MSW waste fills, but in limited cases have been installed in MSW landfills.
Additionally, tiltmeters can be installed near the ground surface. Tiltmeters are typically mounted to a post and installed to three feet below the ground surface to monitor tilt of the ground surface. These are used to monitor ground movement, typically near an excavation or area of known movement.
Thermistors
Thermal monitoring is important for sites with elevated temperature concerns. Remote monitoring of temperature probes can be accomplished. Thermistors can be installed at specific depths to monitor subsurface temperatures in a landfill and nested thermistors can be installed to provide a profile view of the temperatures. Like the other instruments, the temperature probes can also be connected to a remote monitoring system.
Remote Sensing
Recent advancements in remote sensing have expanded the capabilities of geotechnical instrumentation in landfills. These technologies allow for the monitoring of remote areas and provide high-frequency data on ground movements. Remote sensing data can be integrated with ground-based geotechnical instruments to monitor areas susceptible to slope stability issues. Additionally, remote sensing data can be used to monitor changes over time, allowing for the detection of long-term trends and patterns.
Importance of Geotechnical Instrumentation in Landfills
Landfills are complex engineered structures that require careful design and monitoring to ensure their stability. Geotechnical instruments provide real-time data on the behavior of the landfill, which allows engineers to detect and address any issues before they become critical. For example, inclinometers can detect slope movements that may indicate a potential landslide, thus enabling timely interventions to stabilize the area. Similarly, stability monitoring points can identify areas of increasing movement, which would indicate the stability of a slope is decreasing. Also, piezometers are used to monitor liquid levels/pore pressures during construction or over time to determine if they pose a threat to the stability of a slope/area.
The use of geotechnical instruments is essential for ensuring the long-term stability of landfills. These instruments provide real-time data that can be used to make informed decisions about potential remediation actions and/or mitigate potential risks. Most sites currently do not utilize geotechnical instrumentation in the design and construction of landfills but should consider the implementation of such tools.
Providing Real-Time Data
The use of geotechnical instrumentation is important within the design and operation of landfills. Additionally, a good engineering partner can develop action levels (i.e., triggers) and appropriate responses for the geotechnical instrumentation data.
It is important to integrate geotechnical instrumentation into the overall landfill management plan. Oftentimes, geotechnical instrumentation is installed in response to stability issues at sites instead of being installed proactively before issues arise.
The use of geotechnical instruments is essential for ensuring the long-term stability of landfills slopes. These instruments can provide real-time data that can be used to make informed decisions and manage/mitigate potential risks. | WA
Timothy Mitchell, P.E. is Principal at Civil & Environmental Consultants, Inc. (CEC). With over 18 years of experience supporting landfill facilities with engineering support. More recently Tim鈥檚 focus has been implementing geotechnical instrumentation for multiple waste management facilities.
Zach Metzler, E.I.T. is Project Manager at CEC, Inc. with over 8 years of experience in the solid waste industry. Recently, his concentration has been on the installation and maintenance of subsurface monitoring equipment, particularly in the support of excavating coal combustion residual (CCR) materials.
Brad Nagy is Project Consultant at CEC, Inc. with over 3 years of experience within the solid waste field. His experience includes slope monitoring instrumentation implementation, slope stability analysis, landfill construction, and environmental compliance.
For more information about starting a geotechnical instrumentation monitoring program at a landfill, e-mail [email protected].