Another in the series, SCS Advice from the Field. It is possible to simplify landfill maintenance and create air space.
Many landfill designers continue to incorporate terraces on the outside slopes of landfills, but not always for sound reasons. Sometimes, terraces are necessary to maintain landfill slopes in stable condition, due to the low shear strength of the foundation soils, or when required according to the specific state or local solid waste rules.
At times designers propose terraces on slopes to collect and convey surface water runoff from a landfill’s higher slopes to a low point on the terrace where the downchute system is located. On paper, it’s easy to show nicely sloping terraces toward a low point, with transverse slopes toward the landfill slope, to control surface water. However, terraces cause significant operational issues for landfill operators. Some of these problems are very apparent, and some realized when a portion of the landfill slope is scheduled to receive a permanent final cover.
Consider these factors during permitting and design, as follows:
It is difficult to shape sloping terraces during waste placement operations; terraces can end up formed horizontally. When it is time to close the landfill’s side slope, significant amounts of soil are placed along the terrace to make it slope toward a low point where the downchute system is located. Normally, permit drawings do not include sufficient details to illustrate these technical issues, and the operator would not have the specific knowledge of such issues at the time of closing the slope.
During waste placement, difficulties arise for the equipment operator (dozer pushing waste and compactor compacting and shaping surfaces) to shape the breaklines and compress waste properly to form the terrace. Lack of compaction near the outside breakline of the terrace makes it susceptible to excessive settlement and can cause the terrace to change shape over time.
Operators shape the transverse slope of the terrace either horizontally, or sloping away from the landfill slope to manage surface water during the landfill’s operational phase. In either case, the slopes could end up formed differently, or in the opposite direction of the slopes in the permit drawings. Closure of the landfill slope requires special attention along with large quantities of soil to shape the terrace similar to what is in the permit drawings. Again, the landfill operator would not know of the additional work and the soil quantities necessary to fix the terrace transverse slope properly.
Settlement in waste causes previously shaped terraces, at a certain elevation ending up lower than the originally shaped terraces. Over time, the terrace originally constructed at a certain elevation and following the permit documents ends up lower in elevation due to waste settlement. Continuously occurring settlement can cause the misalignment of terraces formed at different intervals. At the time of closing, the terrace misalignments become a major problem for the engineer and contractor to meet elevations and shapes previously permitted.
Downchute pipes extend from the highest terrace to the lowest terrace and the surface water management system at the perimeter of the landfill. The downchute pipes are designed to cross the width of each terrace and pickup surface water from each terrace. However, the pipe design is complicated by the terrace transverse slopes toward the landfill slope causes construction complications and increasing the risk of failing to properly collect surface waters at the low point. This particular risk can become drastic when considering waste settlement changing the surface geometry at the inlets to the downchute system, causing costly repairs.
Over the terrace surface, the geocomposite drainage layer in the final cover follows the transverse slope toward the landfill slope and across the width of the terrace. Water in the geocomposite from the higher slope and water from the terrace reach the inside edge of the terrace, with nowhere to go except to follow the longitudinal slope of the terrace along the interior edge. Geocomposite is not designed to carry such a large quantity of water along the interior edge for the entire length of the terrace. Inevitably, problems arise, and potential failures can occur along the terrace. The solution is to install a toe drain along the interior edge of the terrace that collects and conveys water in the geocomposite layer to the low point in the terrace. This toe drain adds another complication to the design of the piping system at the low point of the terrace, where the down chute system is located. Additionally, the cost of the toe drain construction goes up significantly due to logistical complications during construction of the toe drain along the terrace in the middle of the slope-, including the placement of gravel around the toe drain pipe before the geomembrane and geocomposite are covered with the overlying soil.
The access road, to the top of the landfill, normally crosses several terraces located on the landfill slope. The slope surface geometry at the intersection of the access road with the terraces becomes complicated, affecting the alignment of the access road at each intersection point.
Leachate seeps can potentially appear at breaklines on landfill slopes. The inside edges of a terrace are considered breaklines in the landfill slope and are highly susceptible to leachate seeps appearing on the surface. It is common to observe leachate seeps at terraces on landfill slopes. Unfortunately, leachate ponding on the terrace surface from the seeps can easily mix with surface water runoff on the terrace. It’s then carried to the landfill perimeter surface water management ditches and detention/retention areas.
Significant leachate seeps at terraces may require a leachate toe drain system below the final cover geomembrane along the entire length of the terrace; adding cost and another level of complication at the low point of the terrace where the downchute system is located. The leachate in the toe drain system needs to drain to another system at the low point of the terrace, to discharge to the landfill leachate collection system or another liquid management system.
To uncomplicated operations, more landfills are designed without terraces on the slope. Before slope closure management of the surface water runoff is achieved by temporary tack-on berms on the slope and temporary downchute pipes that are constructed and maintained easily. After closure the surface water management is achieved by permanent tack-on berms at certain spacing on the slope; berms are constructed as specified in the permit documents. The swale on the upper side of the tack-on berm conveys surface water runoff from the higher slopes to the low point of the swale on the slope.
The downchute system at the low point of the tack-on berms is simple to construct. These downchutes connect to lateral pipes from lower-level swales collecting surface water from these swales before discharging to the perimeter surface water management system. The aforementioned design does not require significant maintenance.
Terraces decrease potential airspace within the permitted footprint of the landfill. Wasted airspace on landfill slopes is substantial and can be in the order of tens of millions of dollars depending on the size of the landfill. Owners/Operators request airspace loss calculations to emphasize the financial impact of terraces on their bottom line.
One recent evaluation for a 170-acre, 250-ft tall landfill with seven terraces lost approximately 7,500,000 cubic yards of air space. The tipping fee of $80 per ton results in an estimated value of $64.00 per cubic yard of compacted waste. Therefore, the estimated value of the airspace loses due to the terraces at this landfill site is estimated to cost $480,000,000, nearly half-a-billion dollars of the bottom line.
This author, Ali Khatami, Ph.D., P.E. with SCS Engineers, has over 30 years of landfill design and construction for municipalities and private firms. He has first-hand knowledge of the satisfaction and cost-effectiveness of no-terrace systems by landfill operators he has worked with over three decades; many of whom changed their permits to eliminate terraces to take advantage of the airspace and operational benefits.
Temporary Landfill Caps
Temporarily capping landfill slopes is becoming a common measure for landfill operators. There are many benefits to closing landfill slopes with geomembrane on a temporary basis. One of the benefits is delaying construction of the final cover. Following is a discussion of the steps that should be taken to determine whether temporarily capping the slope with geomembrane and postponing the final cover construction is a better financial/operational decision.
Constructing the final cover is costly, and it is considered an unavoidable expense that has no return on the money spent. Therefore, some operators perform a financial evaluation to determine whether the final cover construction costs can be delayed (provided, of course, that such delays are acceptable to the regulating agency). When evaluating whether to delay the final cover, the cost of maintaining the slopes during the postponement period should be considered. The operator must look at the financial aspects of either closing the slopes with a temporary geomembrane or of leaving the slopes open during the postponement period.
Temporary Landfill Capping Option
The benefits of temporarily capping the slopes during the postponement period may include:
The other side of the coin is the expense associated with the temporary cap. There may be repair costs associated with the geomembrane every few years in order to ensure that the temporary cap remains intact.
Leaving Slopes Open Option
The option of leaving the slopes open during the postponement period involves maintenance expenses such as:
The benefits of leaving the slopes open are twofold: first, the operator will save the costs of constructing the temporary cap; and second, the operator will gain additional airspace as waste settles during the postponement period.
Experience with the Temporary Capping Option
As discussed above, both options provide the benefit of gaining additional airspace during the postponement period. Constructing a temporary cap involves the costs of materials and installation, including the geomembrane and the ballasting system that keeps the geomembrane in place. Generally, the financial and non-tangible benefits of a temporary cap that remains in place five years or longer are more attractive than leaving the slopes open; therefore, most operators choose to install a temporary cap. The next step in the financial evaluation should be comparing the costs of the temporary cap to permanently closing the slopes without postponement.
Final Step in the Financial Evaluation
The next question is whether it makes financial sense to postpone the construction of the final cover.
Waste settlement during the postponement period and the resulting airspace are considered the determining financial factor in choosing the right option. If the present worth value of the airspace generated from waste settlement during the postponement period is greater than the cost to construct the temporary cap at the present time, then the temporary cap option would make financial sense; otherwise, the final cover should be constructed without postponement.
It should be noted that the length of the postponement period plays a very important role in this financial equation. Longer postponement periods have the potential for a greater gain in airspace. Another incentive that should be factored into the financial evaluation is the potential return on the money set aside for the final cover construction during the postponement period.
To assist with this financial evaluation, landfill operators are encouraged to discuss these options with their landfill engineers. Settlement models can be performed to calculate the amount of airspace that may be generated during the postponement period as well as the present worth value of the generated airspace. The returns on the final cover construction costs during the postponement will just be “icing on the cake.”
Read the related Advice From the Field blogs from the landfill and LFG experts at SCS Engineers: