Considerations When Building on Landfills Landfills
Prepared by:
Tom MacDougall
April 5, 1999 Brigham Young University Provo, UT
Abstract Building on landfills is becoming more common as more landfills are being closed, and land near urban areas is in higher demand. The considerations that should be taken into account when developing on a landfill are : s
large total and differential settlements and the effects these have on foundations and the structure;
s
deep foundation capacity, durability, and driveability; and
s
safety hazards.
Four conclusions are derived from the research and compilation of the special considerations that should be taken into account. 1. More studies should be performed so that designers have better guidelines. Data is scarce for building on landfills and it would help if an authority would compile much of the information into a solitary location. 2. If a shallow foundation is chosen, factors such as total and differential settlement need to be taken into account. Further, in most shallow foundations, site improvements should be made. 3. In deep foundations downdrag, lateral resistance, corrosion, and driveability should be accounted for. 4. Special safety issues cannot be overlooked, namely: gas inhalation, soil contamination, groundwater contamination, and possible explosions. The design engineer should take into account these special considerations. As the practice of developing landfills becomes more common, hopefully these guidelines can be followed and refined.
Table of Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Special Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Settlement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Deep Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
Introduction
Closed landfills are becoming increasingly more common. As the population grows, more trash is produced, more landfills are full, more landfills close, and a vacant space over the landfill is left. This vacant space is generally near the population center. Many times it is not known what to do with this vacant land overlaying a capped MSW (Municipal Solid Waste) landfill. Some businessmen are realizing the opportunities for developing these prime locations. However, normal building practices do not apply. Instead of building on top of soil, now there is a large layer of refuse which will bear the load and be the surrounding matrix for utilities. Therefore, when building on a closed landfill, there are special geotechnical considerations that must be taken into account.
Background
Before we discuss the special considerations of building on a landfill, we will review some of the history of waste dumps. It is important to understand how landfills were made, how the sites were chosen, and what was put in them in order to know why we should make special considerations. Throughout the history of the United States, the way refuse has been deposited has varied greatly. Only recently have landfills been engineered. Until the first half of this century, “ . . . nearly all wastes were discarded in open, unengineered, dumps”(Daniel, 1993). The place for waste was chosen based on convenience and existing characteristics (such as an old mine, low-lying areas, or just near-by open space).
For the most part, waste was just dumped on these locations. Sometimes, wastes were burned to conserve space. However, the waste was rarely covered until all dumping was through, and engineering was void in trash dumps (i.e. site selection, liners, leachate collection systems, etc.). After World War Two, sanitary landfills became common. In a sanitary landfill, a cover of soil (about 200mm thick) is placed over a “cell” of refuse (about 3m-5m thick) at the end of everyday. This helped reduce the number of rodents and increased health around the landfill. This practice went relatively unchanged until the late 1970’s, when the Environmental Protection Agency (EPA) started to require engineered, sanitary landfills (Daniel, 1993). Since the EPA began regulating waste disposal, significant changes have been made to landfill design and construction. Most modern landfills are engineered similar to the cross-section shown in figure 1. Single or double compacted clay liners are required. Figure 1. An Engineered Sanitary Landfill Liner
Geomembranes and geofabrics are required for leak prevention and added strength. Leak detection systems are included in modern landfills. Gas collection schemes have been
implemented. When a landfill is closed, cover systems are designed to keep out water and rodents, and to withstand the forces created by settlement. Regulations are enforced as to what can be placed in municipal landfills. Lastly, better records are being kept as to the extent and composition of municipal landfills. Along with the different ways landfills have been designed, landfill contents have changed. It used to be that people would dump just about anything into a landfill: old cars, demolished building refuse, chemicals by-products, etc. In the past 25 years, the EPA has restricted what can be placed in a municipal landfill. Junked cars, large tanks, many chemicals, and other waste are now restricted from municipal landfills. Unfortunately, in many old landfills, waste, such as corrosive chemicals, and large steel objects are present. These things make building on these landfills especially challenging. Another change with landfills is the site chosen. Landfill sites were previously just placed where convenient. People gave little or no regard for the geology, hydrogeology, or much else. The main considerations were access and convenience. Now, extensive studies are performed to determining suitable sites. Furthermore, many social aspects are taken into account. By using nearby, accessible open space, or by filling in nearby ravines and making them flat, land near urban areas has been left undeveloped. This contributes to closed landfills being prime locations for development.
Special Considerations
Like any building project, landfill development has successes and failures. Disasters occur when special site characteristics are overlooked. The geotechnical factors that must be considered when building on a closed landfill are: s
large total and differential settlements and the effects these have on foundations and the structure;
s
deep foundation capacity, durability, and driveability; and
s
safety hazards.
Settlement Large total settlements will occur in a closed landfill. Similar to structures on conventional sites, the magnitude of the settlement is a controlling design factor. A number of studies have been performed to understand settlement processes in landfills. Sowers (1968, 1973) identified mechanisms that contribute to the large settlements that occur in landfills. Landfill settlement is a time-dependant issue extending over several decades. . . The waste is compressed by its self-weight, overburden, and external loads, such as that induced by compaction, leading to a reduction of its void. In addition, because of the difference in particle size of the waste materials, smaller particles ravel into the void of the larger particles, especially during compaction. A large reduction in volume occurs due to waste decomposition, through biological and chemical processes. The decomposition process is complicated by many factors, such as the types of material, moisture content, temperature, and so forth (summarized by Ling et al., 1998).
These factors can cause settlements as large as 25% or more of the initial MSW fill thickness (Dunn, 1995). A brief overview of the different prediction methods is provided by Stulgis et al. (1995). Sowers (1968,1973) first proposed the application of a “soil mechanics” type approach to the analysis of settlements. Others such as Sheurs and Khera (1980) have presented data which show MSW to behave in a manner similar to peat, which is not unexpected in that peat and most MSW is composed of organic material to varying degrees. Edil et al. (1990) presented data and analysis procedures using a rheological model based upon the Gibson and Lo theory or the power creep law. . . Recently, two similar studies collected data from a number of MSW landfills and developed empirical models for the prediction of landfill settlements. Bjarngard and Edgers (1990) proposed a model based upon [their empirical data] . . . Fassett et al. propose a very similar model. . . (Dunn after Stulgis et al., 1995) A more detailed discussion of landfill settlement is complex and beyond the scope of this paper. Moreover, Fassett et al. (1994) concludes, due to the scatter of data and the numerous ways to calculate landfill settlement, that reported values in settlement studies may only really be useful for application in site specific analyses. Total settlement is not the only type of subsidence issue when building on landfills. Due to variations in the thickness and composition of the waste, large differential settlements may also occur. A study of various landfills between 8m to 10m thick showed differential settlements can be around 25% (over a 30m horizontal area) for a period of 15 years (Rinne et al., 1994). Settlement is a time-dependant issue and therefore the rate of settlement should be considered. Studies on settlement rates for landfills by Bjarngard and Edgars (1992), Fassett et al. (1994), and Stulgis et al. (1995), show that the initial compression due to
self-weight and surcharge loads usually occur within 100 days. Relating to longer settlement rates Dunn states: Chemical and biological decomposition processes generally take years to reach completion, with the specific rates varying widely depending upon characteristics of a landfill site and the MSW it contains. Settlements due to raveling are generally difficult to predict and seem to contribute a small percentage to the total landfill settlement as compared to other mechanisms. (1995)
Further settlement can occur in the soil layers beneath the landfill. This is most common in soft clays and should be considered when designing the foundation. Now that we have covered the rates and mechanisms of landfill settlements, we can discuss the special settlement problems of landfill construction and some ways to solve them. Shallow foundations and the structure they support are effected most by settlement. When considering light structures(one or two story wood frames), an engineer may choose a shallow foundation. There are three main types of shallow foundations: spread footings, tied footings, or mat foundations. Spread or tied foundations can easily be damaged due to the total or differential settlements. If a mat foundation is chosen, total settlements generally will not damage the structure as much as differential settlements. Common failures in foundations due to settlements
Figure 2. Shallow Foundation Failures from Settlement
are shown in figure 2. Common damages in structures caused by settlements are shown in figure 3. Engineers have designed ways to get around the damaging problems caused by landfill settlements. In almost every case when building a shallow
Figure 3. Damage in structures due to settlement
foundation on a landfill, at least partial excavation and replacement of compacted fill should occur. “In most cases, it is desirable to provide a layer of engineered fill sufficiently thick to provide essentially all of the bearing capacity” (Dunn, 1995). When this is not a reasonable choice, one must rely on the MSW for strength. The analysis of MSW bearing capacity is difficult due to the lack of information and consistency. The data available is from Jessenberger and Kockel (1991), Mitchell and Mitchell (1992), and Howard and Landva (1992). However, this data is quiet limited and usually in the case that the MSW must bear some of the load, it is better to select a different foundation type. Another technique for overcoming damages caused by settlement in shallow foundations is adding re-leveling jacks to the columns. In an investigation for the feasibility of building on a landfill in New York, the engineer suggested was one of the possible solutions to overcoming differential settlement. A re-leveling jack works by attaching the column to a long bolt as shown in figure 5. Every so often, after settlement has occurred, the column heights can be readjusted to level the structure. These can
compensate for small differential settlements and are usually used in conjunction with other mitigative measures.
One more
Figure 5. A Re-leveling Jack used to Level Columns.
technique used to reduce settlement (and its damaging effects) is dynamic compaction (DC). “The purpose of dynamic compaction is to densify the landfill, thereby decreasing post-construction settlement” (Gifford et al., 1992). Areas of greater bearing capacity should be densified more. Also, DC can reveal when it is necessary to excavate and replace. Springy weight behavior upon impact is an indication of concentrations of large wood or steel. These pieces should be removed, because they inhibit effective densification of adjacent areas. The DC may also reveal soft spongy areas. These areas [should] either be removed and replaced with more suitable material, or coarse gravel with cobbles [should] be pounded into the spongy areas (Gifford et al., 1992).
Further types of mitigative measures used to compensate for settlement damages to shallow foundations are: placing a surcharge, pressure grouting directly under the footings, and adding floor hinges to the structure to avert cracking. Placing a surcharge can be expensive due to the time it takes to get results. However, this can be effective for discovering areas of large differential settlements. Pressure grouting can help stabilize
localized areas. However, due to the content variability in MSW, this could be unsuitable for certain sites because of chemical incompatibility, large voids, etc. Adding floor hinges can help lessen the damage to structures. The only reasons for not choosing them are price and a foundation system that doesn’t require hinges (Mat, Piles, etc).
Deep Foundations If the site and structural improvements discussed above are uneconomical or will not perform adequately, a deep foundation system (usually driven piles) should be selected. There are different types of deep foundation systems. However, I will focus mainly on driven pile foundations. The problems with deep foundations in landfills are capacity (vertical and lateral), durability, and derivability. Pile bearing capacity has both vertical and horizontal components. We will first cover the vertical component. Conventional methods mostly apply when designing the vertical capacity of piles. Dunn recommends, “First, a suitable geotechnical investigation should be completed to characterize the bearing materials underlying the landfill and their strengths. Conventional vertical pile capacity analyses can then be utilized to calculate pile capacity” (1995). For procedures on vertical pile capacity analysis, many sources exist (see Coduto1994). However, MSW cannot be relied on to provide skin friction. In fact, the MSW causes a negative skin friction. This negative skin friction should be a major consideration and is often referred to as downdrag. Downdrag is an important factor in pile design. It adds load to the pile and if ignored can cause foundation failures. Downdrag is caused by the surrounding MSW settling at a faster rate than the pile. This movement around the pile “pulls” downward
on the pile and adds additional load. Vesic (1977) notes that this negative skin friction can fully develop with movements of only 15 mm (0.6 in). Dunn states that because of Vesic’s findings, “. . . downdrag loads will be developed along essentially the full length of a pile through MSW unless some type of mitigation is provided” (1995). Unfortunately, limited data is available for determining (or even estimating) the downdrag on piles due to MSW settlement. “The limited observations suggest that the downdrag force may be about 10% of the weight of overlying refuse. For example, if the 3
3
depth of refuse is 50 ft and the unit weight of 40 lf/ft (6.3kN/m ), then the downdrag 2
stress is 200 lb/ft (9.6kPa)” (Oweis and Khera, 1998). Other findings on downdrag force show that using a conservative value of shear strength can be a good estimate since the negative friction cannot exceed the shearing force. If the downdrag forces become too great for conventional piles, mitigation techniques must be used. Common friction reducing techniques include: casing a pile, predrilling and backfilling with bentonite, or coating a pile with bitumen. These different methods are shown in figure 6. Pile casings, often referred to as “double piles” consist of an inner pile surrounded by an outer pile. The outer pile can be made of a variety of materials.
Common materials are steel pipe, corrugated metal pipe, and plastic pipe. The inner pile supports the loads from the structure while the outer pile takes the load from the downdrag forces. The space between the piles can be filled with bentonite for extra stability and protection. This system should not be chosen if significant lateral load capacity is required. Pre-drilling is another technique used to reduce downdrag on piles in MSW. A steel mandrel is driven down through the MSW and then extracted. The resulting hole can be filled with a bentonite slurry. Sometimes the hole may just be left open, but usually collapse is an issue. Again, if lateral capacity is required, this system should not be chosen. A third common method for mitigating the effects of downdrag is covering the pile with a bitumen coating. Other friction reducing coatings are available, yet bitumen is the most common. In normal soils, this can greatly reduce downdrag effects (approximately 75%). However, bitumen covered concrete piles driven in refuse show that a reduction of 30% to 60% is reasonable (Rinne et al., 1994). The concerns with bitumen coated piles in refuse are, to be effective, the bitumen must not be scrapped off during driving, the bitumen must stay viscous over large temperature differences, and the chemicals in the MSW can’t degrade or dissolve the bitumen. Other methods used to meet vertical force requirements are the use of end bearing piles (usually with belled tips) and to simply just extend the length. A belled tip can provide greater end support and the pile can be designed to use only the end bearing for load support. However, driving a bell-tipped
The lateral capacity of piles is reduced significantly when driven in refuse. Once again, limited data is available on this subject. From the data we have, “it is logical to consider that the refuse is somewhat reinforced by various objects as evidenced by the near-vertical slopes in refuse over 25 ft. The active lateral earth pressure coefficient , K a, is expected to be less than that of loose sand and a value of 0.2 appears reasonable for unsoaked refuse” (Oweis and Khera, 1998). Dunn further comments, In most pile-supported structures, the resistance of the pilesoil system and pile cap or grade beams provide lateral load capacity. However, only that portion of the foundation system which can be ensured to remain embedded, and thus in contact with the soil or MSW, can be counted upon to provide resistance to lateral loads. Mitigation techniques used to reduce downdrag loads may thus greatly reduce lateral load capacity. Additionally as a result of sizable surface settlements, soil contact areas of grade beams and pile caps may be significantly reduced over time with an associated reduction in passive resistance to lateral loads. (1995) Therefore, to offset the reductions in capacity, pile caps should be made larger and grade beams should be added (Dunn, 1995). Not only do piles’ vertical and lateral load capacities in MSW present interesting problems, pile durability is an important consideration when constructing on a landfill. The pH in landfills can be high enough or low enough to corrode or disintegrate steel, concrete, and timber. Some acidity tests should be taken. Then the results can be used as a guide to choosing a pile type. Measures can be taken to account for pile corrosion. A common method of pile design is a dual-system pile: consisting of a steel pipe encasing concrete. The steel casing provides support during driving and protection from the corrosive chemicals. The load support given by the steel is ignored. Inside, acid resistant, high-density concrete
should be used. Another option is to increase the pile wall thickness. This is to account for the corrosion. An additional method is to use a composite reinforces pile. This pile type has a casing of carbon fiber or HDPE wrapped around concrete. The composite on the outside acts as a barrier to the chemicals. A method used to preserve timber piles is to soak them in creosote. This is a fairly expensive option because pre-augering is necessary to drive them. The final factor that should be considered when choosing a deep foundation is driveability. Driving piles through refuse is usually difficult. Techniques such as pre-augering or pre-drilling may facilitate the process. The use of heavy pile sections coupled with the protective tips and a large hammer with more than 32,000 ft-lb. of delivery energy also may be effective. The use of vibrating hammers is unlikely to be effective because of obstructions. Despite all this, foundation designs need to include pile deviations of 5 ft or more form planned locations to accommodate for the relocation that may be necessary to avoid unexpected obstructions. (Oweis and Khera, 1998) In a few instances, it may be necessary to excavate out large objects that impede pile driving.
Safety Not only are there special considerations with settlement and deep foundations, various safety issue arise when developing on a landfill. Gas inhalation, soil contamination, ground water contamination, and even possible explosions due to sparks are some of the safety considerations that should be taken into account. Landfills produce large amounts of methane gas. This can pose problems during the construction phase, but is more likely to cause difficulties to the inhabitants of the
structures. A gas collection system like the one shown in figure 7 will localize the gas vapor which can then be collected and safely dealt with.
Figure 7. A Common Gas Collection System.
Soil contamination is most threatening during the construction phase. Common environmental precautions should be taken during the drilling and pile driving stages. There is really no way to get around dealing with contaminated soil. Yet by following common practices in environmental engineering, health risks can be reduced. Groundwater contamination should be considered when driving piles through landfill liners. A hole in the liner or just a disturbance in the soil, can increase permeability and cause a significant increase in leachate found in the groundwater. The casing system discussed earlier can be used with a bentonite slurry to provide a seal around the pile. This seal can reduce the amount of leachate that seeps out near the piles. In the case of older landfills, where liners were not used, groundwater contamination most likely has already occurred and should be taken into account.
Explosions at construction sites on landfills are rare, yet very dangerous. If there is enough gas present, sparks from the pile driving process can cause explosions. Ways to reduce this hazard are by selecting a hydraulic hammer, realizing the possibility by know the amount of gas in the area, using good pile cushions, and replacing pile cushions often (before the catch fire). The engineer should consider these when providing the specifications on pile driving. During the construction phase in general, heavy equipment and any other ignition sources should carry special precautions.
Conclusions
The considerations that should be taken into account when developing on a landfill are : s
large total and differential settlements and the effects these have on foundations and the structure;
s
deep foundation capacity, durability, and driveability; and
s
safety hazards.
Building on landfills is becoming more common as more landfills are being closed, and land near urban areas is in higher demand. The conclusions based on the above information follow. 1. More studies should be performed so that designers have better guidelines. Data is scarce for building on landfills and it would help if an authority would compile much of the information into a solitary location. It was common to
find subjects throughout my research that said there is little conclusive data on this subject. 2. If a shallow foundation is chosen, factors such as total and differential settlement need to be taken into account. Further, in most shallow foundations, site improvements should be made. 3. In deep foundations downdrag, lateral resistance, corrosion, and driveability should be accounted for. 4. Special safety issues cannot be overlooked, namely: gas inhalation, soil contamination, groundwater contamination, and possible explosions. The design engineer should take into account these special considerations. As the practice of developing landfills becomes more common, hopefully these guidelines can be followed and refined.
