why any construction can't be carried out for next 20 years on a landfill

Abstract: Increasing demand for developable space in urban areas has created increased interest in construction on top of old landfills . Landfill redevelopment projects can include hard uses such as commercial, industrial, or infrastructure facilities a nd soft uses such as athletic fields, golf courses, and amphitheatres. Engineering challenges asso ciated with landfill redevelopment include foundation design and landfill gas migration control. The large total and differential settlement often associated with landfills is an integral part of these challenges. Due to the large settlement potential, landfill redevelopment using shallow f oundations is generally restricted to low-rise structures of one or two stories with raft foundati ons. Construction of taller structures using pile foundations is generally restricted to landfills w ithout an engineered bottom liner system. Both deep and shallow foundations systems must be provided with protective measures against landfill gas migration. Despite the significant challeng es associated with post-closure development on top of landfills, both hard and soft uses of old landfills are becoming increasingly common. Keywords: Foundations, Gas, Landfills , Redevelopment, Settlement, 1. INTRODUCTION Until recently, it was general practice to avoid cl osed and abandoned landfill sites. However, as developable space becomes scarce in urban areas, development on top of and adjacent to old landfills has become increasingly common. Sometim es, such development is driven by economic opportunity (cheap or well-located land), other times by necessity (the only available space or suitable location). Development of old landfills includes both hard and soft uses. Hard uses include building, roadway, and infrastructure development. Figure 1 depicts a retail store built on top of an old landfill south of San Francisco, Ca lifornia. Soft uses include golf courses, other recreational facilities (athletic fields), and amph itheatres. Figure 2 shows a golf course built on top of a landfill in Fullerton, California. The engin eering challenges associated with development of old landfills include structural challenges su ch as foundation design and utility alignment and environmental challenges such as mitigation of e xplosion and health risks and air, soil, and groundwater impacts. 2. BACKGROUND According to the Concise Oxford Dictionary, a landfill is defined as follows: Landfill , n . 1. waste material etc. used to landscape or reclaim areas of ground. 2. the process of disposing of rubbish in this way. 3. an area filled in by this process. For the purposes of this paper, the third definition is the operable one used herein. Landfills, in various forms, have been used for many years. Th e first recorded regulations to control municipal                   Figure 4. Waste fill settling away from a building. Figure 5. Utilities hung from building slab. direct shear results on the interface between domes tic waste and concrete indicating an interface friction strength value of 30 kPa. However, one h as to be very cautious about the limitation of this type of test since it does not reproduce the real behaviour of waste in a landfill. Dunn (1995) recommended that field pullout tests be completed on a series of test piles to develop site specific shear strength values which can then be used to re fine the downdrag analyses. He also suggested that it is desirable to instrument the piles dr iven as part of the testing program to allow measurement over time of the actual downdrag loads which develop in the piles. There are several methods to mitigate the downdrag problem. Some of the methods that have been suggested in the literature include the use of friction reducing coatings on piles, use of double pile system, or pre-drilling with an oversize hole that is filled with bentonite slurry (Dunn, 1995). Coating piles with bitumen to reduce downdr ag is often used in conventional soils. In materials such as MSW, Rinne et al. (1994) reported that the reduction of downdrag for bitumen- coated, pre-cast prestressed concrete piles was on th e order of 30% to 40%. An important issue, which should not be overlooked when the bitumen option is used is the range of temperatures existing in the landfill. High temperatures (50 o to 70 o C) are often reported in landfills. However, landfill temperatures tend to decrease with time and, in a mature landfill, typically range between 20 o to 40 o C, depending on the nature of the waste and la ndfilling practice. If bitumen coating is to be considered, waste temperatures should be inv estigated thoroughly since the performance of the bitumen coating can be adversely affected by high temperatures. The decomposition of the waste and the wa y it has been deposited can also induce horizontal movements inside the landfill. To date, very little attention has been given to the effect of a lateral load (inside the landfill) on the ove rall performance of pile foundations in landfills. Maertens and Bloemmen (1995) presented a case hist ory related to the installation of precast prestressed concrete piles through a landfill. The pile length was around 17 m. The cross section of the piles ranged from 0.22 m x 0.22 m to 0.35 m x 0.35 m. During the installation, 25% of the piles broke and had to be replaced by additional piles. Two possible failure mechanisms were identified: 1) Large bending moment generated in the pile shaft due to deflection from obstructions in the waste; 2) accumulation of wastes such as plastic, metals, etc. at the tip inducing an uneven stress distribution below the pile tip. It is also possi ble that piles broke due to large tension stresses developed during easy driving conditions in low strength waste. This case history illustrates that a landfill should be regarded as a system with a large deformation potential that can produce both horizontal and vertical loads.   6.3 Gas Protection Measures to Buildings Gases such as methane (CH 4 ) and carbon dioxide (CO 2 ) are produced in most landfill sites. These gases can migrate into buildings or confin ed spaces and may accumulate to explosive concentrations. Methane gas is explosive at con centrations above 5 to 15 % by volume in air; these are the lower and upper explosive limits respectively. If methane concentrations are greater than 15% it is not explosive, however when it migrates it will, at some locations, become diluted into the concentration range where it can explode. La ndfill gases also carry low concentrations of non- methanogenic organic compounds (NMOCs). Some of these NMOCs are known to be carcinogenic in trace concentrations (e.g., benzene, vi nyl chloride). Hence, there is serious health concerns associated with chronic exposur e to even low levels of landfill gas. The movement of gases in porous media occurs by two major transport mechanisms: advective flow and diffusive flow. In diffusive flow, gas moves in response to a concentration gradient. In advective flow, the gas moves in res ponse to a gradient in total pressure. To equalise pressure, a mass of gas travels from a region of hi gher pressure to a lower one. In the context of landfills, the primary driving force for gas migration, especially through cover systems, is advective flow. Advective flow develops from pr essure differentials due to both internal gas generation and natural fluctuati ons in atmospheric pressure (barometric pumping). Indeed, falling barometric pressures tend to draw gas out of the landfill, increasing the gas concentration near the surface layers. A number of recent events have brought the h azards associated with landfill gas very much into public view. The best known of these were the Losco e, U.K, (Williams & Aitkenhead, 1991); Skellingsted, Denmark, (Kjeldsen & Fisher, 1995) and Masserano, Italy (Jarre et al., 1997) incidents, which resulted in extensive property damage and loss of lives. The Loscoe explosion in the United Kingdom for example, took place after atmospheric pressure dropped by 29 mbars in approximately 7 hours. The same phenomenon caused the Skellingsted and Masserano explosions. Elevation of the leachate/water table and temper ature gradients can also give rise to pressure differences and lead to gas migration. The potential for a landfill to produce gas should not necessarily be a restriction on whether the site can be developed. There is a wide range of available gas protection methods to suit different types of developments, depending on th e level of risk that can be tolerated. In Australia, there are no guidelines specifying measures to be taken to protect building structures in or around landfills. However, in California, regula tions require a building protection system that includes a membrane barrier beneath the structure and an alarm system within the structure for facilities built within 300 m of a landfill. Elsew here, and in the UK in particular, guidance documents have been produced following landfill gas related incidents. Table 4 provides a scope of the protection measures that can be taken to m itigate landfill gas problems in the UK. Referring to Table 4, it is important to stress that the monitoring of a gassing site should be carried out over a period of time and under varying weather conditions . In most of the cases presented by Wilson and Card (1999), ventilation of the underfloor subs pace is the primary method of providing gas protection, with secondary protection provided by a barrier to gas migration above the subspace. An alarm system may also be placed inside the st ructure to warn occupants of gas accumulation. Figure 6 conceptually illustrates an advanced la ndfill gas building protection system. Probably the most important aspect in this type of constructi on measures is the long-term maintenance strategy plan put in place to guarantee their performance over a long period of time (i.e, until the landfill stops producing gas). Indeed whatever measure is selected must be able to protect the building structure for the useful life of the facility.   Air Sensor Structure Owner To Gas Treatment (if needed) Waste Sensor Alarm System Figure 6. Potential landfill gas alternative protection system (Geosyntec Consultants patent pending). Table 4: Scope of protection measures (modified from Wilson and Card, 1999). Limiting CH 4 con. (%by vol) Limiting CO 2 con. (%by vol) Limiting borehole gas volume of CH 4 or CO 2 (l/h) Residential building Office/commercial/industrial development < 0.1 < 0.1 <0.07 No special pr ecautions No special precautions <1.0 <1.5 <0.7 Well constructed ground or suspended floor slab, geomembranes sealed around penetrations, passively underfloor sub- space and wall cavities Reinforced cast in situ ground slab. All joints and penetrations sealed. Possibly geomembrane. Granular layer below slab passively vented to atmosphere with interleaved geocomposite strips or pipes <5.0 <5.0 <3.5 Well constructed suspended or ground slab. Gas resistant geomembrane and passively ventilated underfloor sub- space Reinforced concrete cast in-situ ground slab. All joints and penetrations sealed. Waterproof/gas resistant geomembrane and passively ventilated underfloor sub-space <20 <20 <15 Well cons tructed suspended or ground slab. Gas resistant geomembrane and passively ventilated underfloor sub- space, oversite capping and in ground venting layer Reinforced concrete cast in-situ ground slab. All joints and penetrations sealed. Gas resistant geomembrane and passively ventilated underfloor sub-space. <20 <20 <70 Specific gas resistant geomembrane and ventilated underfloor void, oversite capping and in ground venting layer and in ground venting wells Reinforced concrete cast in-situ ground slab. All joints and penetrations sealed. Gas resistant geomembrane and passively ventilated underfloor sub-space. In ground venting wells <20 <20 >70 Not suitable unless gas regime is reduced first and quantitative assessment carried out to assess design of protection measures in conjunction with foundation design Reinforced concrete cast in-situ ground slab. All joints and penetrations sealed. Gas resistant membrane and actively ventilated underfloor sub-space, with monitoring. In ground venting wells Con.=concentration   7. REDEVELOPMENT FOR SOFT USE The engineering issues associated with redevel opment for soft use are to some extent less challenging than those associated with hard use. Soft uses (e.g., parks, golf courses) generally involve outdoor open space. Hence, the potential for accumulation of explosive levels of landfill gases is less for soft uses than for hard uses. However, if gas is not properly controlled, it can still present explosion and health risks and may adver sely affect the vegetation often associated with soft uses (e.g., turf grass). Therefore, gas migration control is still an important issue for soft use. Landfill settlement also remains a significant issue for soft use. Site utilities, paved areas, and foundations for ancillary facilities are all sensitive to large total and/or differential settlement. Furthermore, some soft uses may be even less to lerable of differential settlement than hard uses (e.g., athletic fields). Therefore, as in hard u se, the impact of post-closure settlement must be carefully considered when planning a soft use project. The most significant difference between soft u se and hard use is that soft use frequently involves vegetation and irrigation. Particularly in arid and semi-arid climates, post-closure uses such as golf courses and athletic fields can require the addition of significant amounts of water to the top of the landfill for irrigation purposes. If the landfill cap does not provide appropriate infiltration resistance, increased infiltration ma y occur, leading to increased gas generation, settlement, and groundwater impacts. While a cover system can generally be engineered to provide sufficient infiltration resistance, the construction of a low-permeability cover (e.g., a geomembrane) on top of an inactive site can exacer bate both landfill gas migration at the perimeter of the cap and the landfill gas impacts to groundwater beneath the landfill. There are many instances where low-permeability cover construction has increased lateral gas migration and/or gas impacts to groundwater. Therefore, post-closure development for soft use requires consideration not only of infiltration and gas migration contro l through the top of the landfill, but also gas migration control at the perimeter of the cap and beneath the landfill. As in post closure development for hard use, post-development maintenance and monitoring is an important consideration for soft uses. Annual inspections to detect and remediate damage to the landfill cover system, including the barrier layer, the gas control system, and the surface water control system, and to restore grades and repair utilities impacted by settlement must be provided for. Gas migration and groundwater monitoring are also key elements of the post- closure plan. Monitoring data and annual insp ection reports should be reviewed by qualified engineers to determine if the landfill cover is performing as designed and if preventative or corrective actions are required. 8. CASE STUDIES 8.1 Tecnoparc de Montreal, Canada (Rollin & Fournier, 2001) Movies studios, storage facilities, and administrativ e buildings were constr ucted on a landfill site that was active from 1870s until the 1960s. The site has fairly stabilised, very low concentrations of landfill gas were detected in 41 boreholes: CH 4 (0 to >50,000ppm); SO 2 and H 2 S <0.25-3.0 ppm); and CO <0.25-111 ppm). Five build ings covering a total area of 10,312 m 2 were built on piles and a geomembrane was installed on a collec tion and evacuation granular layer consisting of 150 mm diameter drainage pipes embedded in a 500 mm thick layer of 20-40 mm diameter material. A vacuum pump (100 cfm) was installe d on the roofs of each building to continuously vent biogas contaminated air (Figure 7). A pref abricated bituminous geomembrane was selected to act as gas barrier due to the fact that it was easy to install (a large number of protruding elements, 367 piles, 187 pipes, 838 structural steel rods, as well as many sump pits, needed to be safely sealed), and attach to concrete structures (the geomembrane was mechanically attached to 1,032 metres of peripheral concrete walls). For safety pur poses, 37 methane detectors were also installed   in different locations of the buildings. After one year of monitoring, no methane had been detected in the five buildings. Figure 7 Remedial system under a building. 8.2 Redwood City Office Park, California Miller and Vogt (1999) discussed the construction of an office park in Redwood City, California, USA, where the major design element was the inst allation of friction piles to support a 20-building complex. 40 m long pre-cast concrete piles were driven through an old landfill into the underlying soils over a one year construction period. 110 pil es were installed for each building foundation, a total of 2,200 piles were installed for the whole complex. 8.3 Gaffey Street Landfill, Wilmington, California Evans, et al. (2000) describe redevelopment of an inactive landfill in the Wilmington section of the City of Los Angeles, USA, as athletic fields. One of the primary redevelopment concerns was that the irrigation associated with post closure use woul d significantly increase infiltration to the waste, resulting in increased gas production, settlement, and groundwater impacts. The combined irrigation and rainfall necessary to sustain healthy tu rf grass in the semi-arid Los Angeles climate is approximately 140 cm per year compared to the mean annual rainfall of approximately 32 cm. Detailed water balance analyses were conducted us ing an unsaturated flow model to design an appropriate soil cover for the site. Results of th e water balance analyses indicated that a monolithic evapotranspirative cover could not provide adequate resistance to infiltration. However, a capillary break cover could provide sufficient infiltration resistance provided that the irrigation system was properly controlled (i.e., the turf was not over-watered ). In addition to inhibiting infiltration, the capillary break also provided a means for collecting and venting or treating (as necessary) landfill gas. To mitigate the potential for overwatering, landfill redevelopment included a “smart” irrigation system in which the irrigation controller was connected to a flow meter, a self tipping rain bucket, and an evapotranspi ration gauge. Daily irrigation values are automatically calculated based upon precipitation and evapotranspiration over the previous 24 hours. The flow meter also has the capability of sensing line br eaks in the irrigation system. Post-closure monitoring also includes neutron probe soil moisture sensors w ithin and beneath the cap to evaluate the effectiveness of the smart irrigation system. Non-vegetated areas of the landfill (e.g., roadwa ys, parking lots, basketball courts) were capped with an asphaltic concrete low-permeability barrier layer. The asphaltic concrete included a Vacuum pump or turbine Methane detector Methane detector Venting system Biogas Granular layer Geomembrane   resin-impregnated fabric interlayer to inhibit cr acking. The post-closure maintenance plan includes annual sealing of cracks in the asphaltic concrete and quarterly evaluation of the soil moisture probe data. 8.4 McColl Superfund Site, Fullerton, California (Collins, et al., 1998) The McColl Superfund Site in Fullerton, California, provides an example of a hazardous waste landfill redeveloped for productive use (Figure 8 and 9) . This 8.8-ha site c ontained 12 unlined pits containing highly acidic petroleum waste sludge (pH less than 1.0). While some parts of the site were closed as vegetated open space, some areas we re redeveloped as a golf course. The cap in both the open space and golf course areas included a composite geomembrane/geosynthetic clay liner infiltration barrier. Due to the low bearing capacity of the waste, the foundation layer beneath the cap in the golf course areas included two layers of geogrid reinforcement (Hendricker, et al., 1998). The foundation layer also included gas extraction pipes connected to a blower and an activated carbon treatment unit. The cap was tied into a soil -bentonite slurry wall that completely encircled the site. Figure. 8 McColl Site (Circa 1995) with Sump Figure 9. McColl Site in 1998As Part of Los Boundaries. Coyotes Country Club. 5. CONCLUSIONS Post-closure development of landfills includes bot h hard uses such as commercial, industrial, and infrastructure facilities and soft uses such as gol f courses and athletic fields. Post-closure development of old landfills includes a variety of engineering challenges. These challenges include accommodating the large total and differential settle ments typically associated with landfills and controlling the migration of landfill gas. Post-clo sure total settlement can approach 20 percent of the waste thickness, with differential settlement up to half that value. Shallow foundation systems for construction on top of landfills are typically limited to relatively light structures one or two stories tall, due to settlement considerations. D eep foundations bearing on firm strata beneath the waste may be used to support heavier structures. However, deep foundation systems are generally limited to landfills that do not have engineered bottom liner systems. Even though buildings on deep foundations may not settle significantly, the design engineer must still accommodate the relative settlement between the landfill and the st ructure. Both deep and shallow foundation systems require engineered systems to control landfill gas migration. These building protection   systems typically include a membrane barrier beneat h the slab, a venting system beneath the barrier to minimize the build-up of gases beneath the barrier, and an alarm system within the structure. 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