There may be a better method to anchor a landfill bottom liner than the traditional anchor trench.
By Reg Renaud
There are many types of flexible membrane liners and various uses for liner materials, but the one thing they all have in common is that they must be anchored in place. For example, most Title D landfills constructed per current regulations use an impermeable membrane (e.g. PVC or HDPE) as a bottom liner or in the cover or cap, to prevent infiltration of rainfall and, thereby, reduce leachate generation. The liner is usually anchored around the edges or at key points to prevent it from moving excessively during construction or as a result of settlement of underlying waste, or during a major earthquake.
Currently, most liners are anchored with anchor trenches, which are usually effective in most cases. However, there are situations where liners have been disqualified for a particular project because an anchor trench could not be used because there was not enough room, the soil type or the slope was too steep or point load anchors would overstress the liner. Also, one of the concerns with anchor trenches and point load anchors, is that they anchor the liner in place and allow for no movement of the liner itself, rather the anchor holds the liner at the stress points and as settlement occurs the liner begins to stretch. The system performance then relies on the designer’s ability to predict the total movement and the liner materials capability to stretch without tearing or “pulling out” the anchor. If stretch does occur, the liner properties can be weakened; therefore, a thicker/heavier membrane liner must be designated in the design.
Buried anchor trenches do not reveal the loading on any particular point along the trench until signs of failure/movement are apparent and then it may be too late. The holding capacity of an anchor trench can vary over time as the soil is saturated by rain or as it dries out.
“Earth anchors” have been used in some slope applications, but this requires perforating the liner with a threaded shaft to sandwich the liner between two steel plates. If it is necessary to readjust the anchor before failure occurs, the old perforation must be sealed, and a new perforation made in a new location. These perforations provide a failure point in the liner and a possible location for leakage. It can be labor intensive and costly to readjust an anchor trench or an earth anchor perforation.
A New Approach
There is a development of a new approach to anchoring landfill bottom liners particularly for canyon fill landfills. This approach can also be used for water containment such as reservoirs and large ponds. At key segments in this article, the readers will be asked to respond to questions pertaining to a particular component of this system. The hope is that readers will participate and provide feedback.
#1: Were the anchoring failures due to overstress, soil type or faulty liner, etc.?
Some applications may require an anchor to be forgiving as the underlying layer on which the liner is placed moves for whatever reason, or there is uneven loading on the liner. Usually, if the foundation layer under a liner moves or the loading changes and the liner moves beyond its yield, the liner must be re-anchored, or the liner fails. If site conditions indicate that readjustments are going to be necessary as the foundation settles or the liner loading changes, such as in a landfill, it would be better not to penetrate the liner or bury the liner in an anchor trench.
Current liner systems require a geotextile material be placed between the liner and the subgrade for drainage purposes and to provide a cushion for the liner material. Many designs call for a double liner due to the possibility of the first liner failing during loading, seismic activity or other forms of damage.
#2: Once tested, would there be a need for a second liner installation after implementing a Magnetic Anchor System (MAS)?
Although this article is focused on anchoring landfill liners, the integrity of the liner system is only as good as the underlying subgrade slope it is placed on. If the supporting slope fails, so does the liner system. As studies on past slope/liner failures suggest there may be a chicken and the egg relationship involved here. One may influence the other by applying excess or uneven loading on the other. However, if the magnetic anchor system allows slippage between these structures, this could mitigate the factors that cause failures.
#3: Was there any situation where the need for isolation between the liner system and the slope interface was investigated?
As stated previously, the standard anchor trench can hold the liner in place too firmly and not allow movement as the liner is loaded, especially when loaded unevenly. Due to this condition, the liners are designed to allow for stretching during loading. Therefore, the liner must be many millimeters thicker than if there was no stretching occurring. This adds costs especially to a large project.
#4: What is the basic stretch factor of most liner systems and what would the estimated cost savings be by using a thinner liner?
An alternative to having to re-anchor a stressed liner by resetting an anchor trench would be to use a magnetic liner anchor system. A duckbill earth anchor has been used in a point load anchor application in the past where the liner was penetrated and clamped in place. In an MAS, a duckbill earth anchor would still be used to anchor a modified steel plate to the subgrade.
Magnetic Anchor System
A Magnetic Anchoring System consists of the following parts:
• Duckbill anchor with cable and clamps
• Modified 0.25-inch steel plate with recessed cable anchor point
• Drive hammer and drive rod
• Duckbill cable puller and load indicator
• Magnet with spring plate
The duckbill anchor with a cable attached (see Figure 1) comes in various sizes and would be sized to match the required loads for the site conditions. The anchor can be placed using a large hammer or a jack hammer depending on soil conditions. Once the anchor is driven to the desired depth, the drive rod is extracted with only the cable end remaining out of the hole. Setting the anchor and attachment to the modified plate will be discussed later in the article. Other types or brands of earth anchors may also be used.
Duckbill Anchor: Modified Steel Plate
Once the duckbill anchor is installed, it is then set in place by pulling on the cable using a jack or a hydraulic piston jack with a pressure gauge to measure the amount of pull being applied to the duckbill anchor. This will verify that the anchor will resist the calculated load from the liner and geotextile. To prevent the jack system from sinking into the subgrade, a modified steel plate would be placed over the hole from the duckbill anchor.
A modified steel plate with a hole in the center with another smaller plate welded on the subgrade side would be attached to the cable. This smaller plate would have a smaller hole to allow the anchor cable to pass through. The jacking system is placed on top of this plate to prevent the jack from sinking into the subgrade. Once the duckbill anchor is set and the load has been achieved, a cable clamp is tightened on the cable. Excess cable is cut off. The clamp would sit in a recessed cup that would allow a flat surface for the geotextile and the liner to pass over.
The reason the steel base plate is called modified is due to the shape of the plate. Figure 2 shows the modified steel plate with the corners at the top of the plate cut off. The purpose of this is to reduce the magnetic hold of the magnet and the magnet with the spring plate. When the magnet is initially placed on the liner, the holding force is reduced since there is less steel under the magnet. This will allow the liner to move downward under initial loading. It will also allow the liner to maintain its thickness as loading begins. As the liner continues to be loaded, the liner moves downward with the magnet and will encounter more of the steel plate. The magnetic holding power will increase enough to put the liner in tension, but not cause it to stretch. The magnet would continue to be pulled down the plate until it reaches the bottom portion of the plate where it would be reset. The length of the steel plate can be elongated to extend the time between resetting the magnet to the top position. To know when to reset the magnet, the length of the plate would be measured above the plate and marked. When the mark moves downward to where the top of the plate is, it is time to reset the magnet.
Magnetic Spring Plate
Large rare-earth permanent magnets can be shaped to provide hundreds even thousands of pounds of frictional resistance. Neodymium Iron Boron or NdFeB magnets can pull 1,000 times its weight. However, with the proper design, this method should be very forgiving and will allow sliding as the landfill or underlying foundation settles without damaging the liner. The amount of magnetic force required is determined by the amount of area and weight of liner each anchor is required to hold in place and the angle of the slope. The magnetic resistance should not exceed the tensile strength of the liner materials used. The thickness of the liner material and geofabrics being used will create a “magnetic air space” between the steel bottom plate and the magnet, which will be important when calculating the required pulling power of the magnets to hold the liner.
The size of the magnets is dependent on the amount of resistance required—they can be as big as two square feet or larger. Figure 3 shows a magnet without the spring plate. Figure 4 shows the spring plate attached to the magnet to increase the force footprint on the liner. The spring plate can extend their influence over a greater area. Over time, if the magnets do not perform as desired such as holding too firm or not firm enough, the magnets can be quickly changed to a different strength or size.
A properly designed permanent magnet made of the correct material can retain its magnetization for decades if it is protected from the effects of the environment (e.g. coatings) and does not experience a reverse applied field of magnitude (demagnetization). When waste reaches the elevation of the magnets they can be removed and reused on the next cell.
#5: What suggestions do you have on other approaches of shapes and sizes of magnets or spring frames?
If the MAS is to be used, then the standard anchor trench is not practical since it will not allow extra liner material to move as liner slippage occurs. Therefore, the MAS should be used where the anchor trench would be located (Figure 5).
The MAS can also be used during placement of liner and geofabric materials in windy conditions to hold down the liner/fabric prior to loading it. Once loading begins, the magnets are removed and the bottom steel plates can be used as settlement monitoring plates.
There have been some concerns in the past as to the effects of earthquakes on liner systems (Northridge Earthquake Report, CAIWMB). Currently, if a liner does not fail during a seismic event, then the design was adequate. But if the stress was close to the failure point of the anchoring system, an anchor trench may not show any indication until total failure has occurred. The MAS could be used in research studies to measure the movement of the liner during an earthquake. The main advantage would be that the liner would not spring back and mask the amount of movement during a seismic event. If the magnet is set close to the maximum holding capacity of the liner’s yield, then any deviation of the magnet can be measured after the seismic event and show the amount of movement of the liner. Laboratory tests, such as a slope board on a shaker table, can be developed using the MAS technique to simulate various conditions. An electromagnet can be used instead of a permanent magnet so that the holding capacity can be adjusted to meet various conditions and membrane thicknesses. Another electromagnet can be used at the bottom of the liner to apply the simulated load on the liner.
An Alternative Solution
A magnetic anchor system was evaluated as one of the final cover liner system options for a superfund landfill in southern California. Soil was chosen instead of geomembranes as the final cover due to slope conditions, so this system was not implemented. Some laboratory studies have been performed on this method, but no field studies have been attempted. However, with liner/fabric materials being used in new applications everyday (i.e. landfill bioreactors, ponds, etc.) and their inherent unknowns, this method may prevent some future liner failures. The MAS may be as cost effective as anchor trenches even on very large liner systems and is not intended to replace anchor trenches, but there may be more projects available with this alternative means of anchoring liners.
Reg Renaud is President of STI Engineering, Inc. (Silverado, CA) and has been involved in the development and use of in situ testing instruments used in geotechnical and environmental investigations since 1978. He has been an instructor of in situ testing at several universities. He began developing the Landfill Gas 3-D Profiling Method in 1984. Based on in situ data from these profiles and with the understanding of the dynamic conditions inside landfills, he developed the patented Steam Injection Bioreactor Landfill method. From this development he patented the Steam Injected Biomass Reactor (SIBR). If you wish to respond to the questions in the article or make any comments, contact Reg at (714) 649-4422, e-mail firstname.lastname@example.org or visit www.airspacerecovery.com.
Robert M. Koerner, 1998, Designing with Geosynthetics, Prentice-Hall Inc.
Dr. Peter Campbell, 1998. Basic Permanent Magnetism, The Demagnetization Curve, Permanent Magnet Materials, The Most Common Classes. www.magnetweb.com.
Kettleman Landfill Slope and Liner Failure (1988): https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=2264&context=icchge
Northridge Earthquake Report, 1994, California Integrated Waste Management Board, Charlene Herbst.
Magnet Sales 2001, Calculating the pulling force of permanent magnets. www.magnetsales.com.