WHEN FINAL CAPS FAIL

Benjamin Siebecker

LANDFILL FINAL COVER failures can significantly affect your pocketbook and the environment. The cover's stability involves various geotechnical soil conditions and physical and environmental factors that can affect the site. If those elements are not interacting properly, the risk of failure can increase.

Final cover failure can show up in a variety of ways, including global slides on a geomembrane cap or geocomposite drain; subsoil piping and topsoil collapse; and swale berm washouts and gullies. Failures often occur because uncontrolled water is moving through or over the cover soils. It may be difficult to determine the reason and mechanism for a failure, but inadequate design, poor construction techniques, changed landfill conditions or unique weather conditions may be the culprit.

Investigating the Failure

To understand the reason for the failure and design a cost-effective repair, the failure mechanism can be determined by thoroughly inspecting the failure, reviewing the original design and assessing, in detail, the construction documents. Most slope failures are caused by subsurface soil washouts and by soil slippage on the geomembrane or geocomposite drain.

In one case study of a failure, the landfill's clay cap and the layer interface strengths were not the source of the problem. Instead, the landfill had settled, causing the drainage layer to become uneven. Confluent flow of water in the drainage layer caused excess head to develop and lifted the topsoil, which was held together by the root mat. The protective layer soil eroded and popped out through the topsoil and flowed into the diversion swale.

The topsoil then collapsed into the void with enough void remaining to channel water and continue washing out sand. The diversion swale filled with that sand, resulting in stormwater overtopping the berm and causing a down-gradient washout.

The drainage swales were constructed with a 1 percent slope, which was not steep enough to counter the settlement effects. Although the over-topping was limited to certain areas, the unlined swale's cap drain pipe was not protected from stormwater, and the filled drain pipe dumped water into the adjacent drainage layer. The resulting pressure caused the berm to rupture.

A topographic map helped to identify the settled areas. New drainage swales — constructed with liners and cap drains to remove excessive drainage layer water — were installed up-gradient to failing swales. New downchutes also were constructed at the failure sites.

A Rare Event

In another example of final cap cover failure, a landfill was hit with a significant amount of water — a 100-year storm's worth. A poorly maintained culvert caused water to flow over a road and into the landfill perimeter swale. This excess water eroded and exposed the toe of the landfill cap. The toe of slope was reconstructed with a durable toe drain to assure water was released from the final cover of the drainage layer.

Farther up on the same slope, a 2-acre patch of cover soil slid 15 feet down the geomembrane, at a point immediately below a drainage diversion swale. The benched diversion swale was constructed using a geotextile instead of a geomembrane for the swale liner. Water quickly flowed into the swale cap-drain pipe, and the 4-inch pipe capacity was exceeded. The pipe, under pressure, pressurized the drainage layer of the adjacent swale berm and inundated the lower slope drainage layer. The repairs included reconstructing the diversion swale; adding a new and larger cap drain in the swale; adding a competent swale liner; and using a geocomposite drain in the areas where the cover was replaced.

Other areas of drainage sand washouts were attributed to a rodent-infested composted top soil that allowed high infiltration rates and poor drainage in incorrectly installed cap-drain outlet pipes. The cap drain outlet pipes were fixed, and engineers recommended that the topsoil be compacted to crush the rodent holes.

Accidental Overload

Another landfill's final cap was affected by an accidental overloading of the geocomposite drainage layer. A truck-rutted access road caused concentrated flow of surface water to channel along the road's edge and eventually enter the drainage layer. The slope changed, which reduced the drain's capacity. The slip occurred under the protective soil layer, at the slope change, where the hydraulic pressure was the greatest. The movement stopped once the pressure was released. Repairs consisted of redressing the slopes and filling the rutted area.

The fourth example of landfill cover failure occurred when an additional cap was added above the established final cover area. An unprotected top exposed edge of the geocomposite drainage layer caused ponded storm water to enter the geocomposite drainage layer. The hydraulic pressure in the geocomposite lifted the existing final cover just enough to allow water to flow above the geocomposite and start eroding small channels.

The soil continued to erode from the bottom up, making larger channels, whereby the vegetated topsoil layer fell into the channels. Large deltas of silt were evident at the slope toe. The slope required substantial reconstruction.

The last example of final landfill cover failure, once again, involved heavy rain. Although the cap drains functioned well, the outlet pipes were restricted. Water backed up the outlet pipe to the first and second horizontal collection laterals, where water was released and blew out the cover soils. Repairs included replacing the outlet pipe with a larger pipe and repairing the slopes.

Often, final cover construction starts too late in the construction season. In this case, the geomembrane was placed in November and was covered with drainage sand. The construction work was halted until spring. Winter rain and snows saturated the sand and, with no cohesion, the sand flowed offsite, disturbing roads, streams and wetlands. The main lesson here is not to leave drainage sand unprotected.

Corrective Measures

Landfill final covers fail two basic ways: soil slides down the geomembrane or subsurface washes out, which deposit drainage sands into the site waterways and leave the cover soil full of collapsed tunnels. The corrective measures start at design and should focus on preventing head buildup by selecting cover soils carefully and removing the excess water from the subsurface layer.

Engineers should always add a safety factor for each aspect of the design, always documenting the work and asking themselves, “What can go wrong?” at every stage of the project. Do not let contractors change the design during construction just because they say it is too difficult to build. The extra construction costs always will be small compared to repair costs.

Benjamin Siebecker is senior engineer at the Andover, Mass.-based office of EMCON OWT., a division of Shaw Environmental Inc.

UNDERSTANDING FINAL LANDFILL COVER BASICS

A final landfill cover system includes four layers, which are sometimes made into three layers. The first layer at the bottom is the low-permeability cap, made of a clayey soil, a geomembrane or both. The cap limits moisture into the underlying waste material.

The second layer is a high-permeability subsurface drainage area made of either a geosynthetic net drain or clean sand. The drainage layer and its cap drainage pipes transmit infiltrated precipitation off the cap.

The third layer is a protective soil cover over the drainage layer.

The fourth layer is topsoil with grass vegetation.

Sometimes, the third and fourth layers are combined. A network of storm water diversion swales often is built into the landfill's side.

Each layer combination and slope condition require special considerations. Often, the location, regulations or client preferences limit the designer's choices. As a result, the as-built documents are essential to the failure assessment.

A stable final cover is governed by soil shear strength, which includes the material's internal shear strength and the external — or interface — shear strength with adjacent materials. Shear strength is a combination of friction strength and cohesion strength. Friction strength, defined by a material's friction angle, is a function of the soil particle angularity.

The factor of safety (FS) against failure, with an FS of less than 1 failing, identifies stability. The FS for a final cover design is usually 1.5, but under some circumstances, it can be as low as 1.3. For a 3 horizontal:1 vertical slope, the FS for a final cover with a fully drained sand drainage layer and an interface friction angle with a geomembrane of 28 degrees, you can anticipate an FS of 1.6. The FS drops to 1.13 with 1 foot sand of saturation. This illustrates the importance of controlling water in the drainage layer and knowing the shear strength properties of the materials.

Pre-construction testing with the actual materials must be conducted to confirm that the product will meet the design shear strength's requirements.

The water in the drainage layer reduces stability and should be kept to the minimum level needed to maintain an adequate FS. Installing a system of cap drain pipes in the drainage layer will help preserve this delicate balance.

In summary, each final cover layer combination and slope condition require special considerations. Often, the location regulations or client preferences limit the designer's choices. So the designer should not be lulled into a one-size-fits-all mentality.
— Benjamin Siebecker

WasteExpo Watch

Don't miss Siebecker's presentation, “Why Your Landfill Final Cover is Sliding Down the Hill” at WasteExpo's Waste Tech Landfill Technology Track in Las Vegas.

Date: Monday, May 2

Time: 9 - 10:15 a.m.

For more information, visit www.wasteexpo.com.