Engineering geological aspects of replacing a solid waste disposal site with a sanitary landfill

ENGINEERING GEOLOGY ELSEVIER

Engineering Geology 44 (1996) 203-212

Engineering geological aspects of replacing a solid waste disposal site with a sanitary landfill Kamil Kayabah University of Ankara, Department of Geological Engineering, Ankara 06100, Turkey Received 11 October 1995; revision 19 April 1996; accepted 19 April 1996

Abstract

The current solid waste disposal site in the Mamak district of Ankara is being engulfed by the growing city. All varieties of solid wastes, including medical wastes, are stored at the present site in an irregular manner. Topographical and geological conditions at Mamak waste site are favorable for constructing a sanitary landfill. Located at the edge of a topographical depression, the site is underlain by the natural hydraulic barriers such as clay and metagreywacke. The terrestrial clay has a permeability of 10 -7 to 10 -8 cm/s and low to moderate values of CEC. The proposed sanitary landfill to replace the present solid waste site has a capacity of storing solid waste over 50 years. The details of base liner, final cover, toe embankment, and drainage of leachate and gas are presented in the paper.

1. Introduction Research conducted in the past decades indicate that leachate can contaminate groundwater (e.g., Husain et al., 1989; Assmuth and Strandberg, 1993). Natural low-permeability layers beneath a disposal site have a limited capacity for purifying the leachate as far as its cation exchange capacity (CEC) is concerned (Baghci, 1990). The current practice with sanitary landfill sites is to try to hydraulically isolate the mass of waste in order to minimize the effects of disposal site on groundwater. Municipal, industrial, and medical waste produced in Ankara between 1960 and 1979 was disposed of at a site 9 km from the city center (Fig. 1). Soon after the closure of this site, a new site was selected 2 km from the old one and about 11 km from the city center. Solid wastes were dumped carelessly in both sites. New residential 0013-7952/'96/$15.00 © 1996 Elsevier Science B.V. All fights reserved PII S0013-7952 (96)00071-3

areas were planned to be constructed nearby by the local government, and currently the solid waste disposal site for Ankara is being engulfed by the growing urban development. This has forced the municipality to relocate the current disposal site. By 1997, the site will be discontinued and reclaimed. Uluatam et al. (1984) researched the feasibility of utilization of Ankara waste as an energy source through incineration or as raw material for producing fertilizer through composting. They came up with a conclusion that it is not feasible to produce either energy or fertilizer because the ash content of the waste was very high. Instead, they proposed a sanitary landfill to be constructed. Several other reports by the Ministry of Environment and the Metropolitan Municipality of Ankara focused on investigating the possible ways of closing the current waste site with the minimum effect on the environment.

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Preliminary investigations at the present disposal site showed that the geological and topographical conditions and other criteria for selecting a sanitary landfill site are all favorable, except that it is surrounded by a large adjacent housing development. This study aims at investigating geological and geoteehnical aspects of designing the present solid waste disposal site as a sanitary landfill. Also presented in the study are the details of the proposed sanitary landfill.

2. The current situation at the disposal site

All varieties of waste, such as domestic, medical, commercial and industrial, have been dumped at the site, and it has an uncontrolled environmental impact in the form of visual pollution, odor, leachate, fire and smoke, slope instability, and virus spreading vectors (by vermin, etc.). The site is spread over 31 hectares. It covers the northeast slope of a large topographic depression area (Fig. 2). The solid waste accumulation reached up to 50 m at its thickest point (Ankara Metropolitan Municipality, 1994a). A large scale topographic map of the site (Fig. 3) was constructed using an alidade. The lateral extension of solid waste on the slope face is about 800 m. The waste brought daily to the site is pushed downslope by bulldozers. The slope face often undergoes instability problems. During a slope failure of waste in March 1993, two workers engaged with

collecting the recyclable materials lost their lives by the sliding mass of waste. The slope angle of the waste is usually 30 35 °. Two slides are observed in the spread mass of waste. The one located at the southeastern corner of the disposal site causes slope instability in the underlying clay due to the surcharge created by waste while the larger slide takes place in the mass of waste itself (Fig. 3). Deep and wide tension cracks are associated with the larger slide. These cracks are formed inside the clay material used as daily or weekly cover material. The leachate emergence points are observed at the skirts of sliding areas. The highest discharge ratio among these three is about 1-2 liters/s. During dry seasons, this leachate is absorbed by the soft soil at the adjacent cultivated area. Following the rainy seasons, the leachate reaches Imrahor Creek, 2 km from the site, in much more diluted form. The creek water is used for irrigation and watering livestock. Emergence of leachate was reported from the older waste site, several kilometers northwest of the present solid waste site. Chemical analyses on leachate, stream water, and samples from domestic wells in that area indicate moderate to severe contamination in stream water and particularly domestic wells (Table 1). Heavy metals existing in creek water are carried into human body either through direct use of well water or food chain. A report issued by Ankara Metropolitan Municipality (1994b) indicates the existence of three types of PCB (polychlorobiphe-

K Kayabah/Engineering Geology 44 (1996) 203-212

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nyl) in the leachate emerging from the older disposal site merging into Imrahor Creek. PCBs which can enter the food chain through accumulation in plants and animals pose a great threat to human health. One of the pylons of an electric power line which passes through the disposal site lost its stability, most likely due to a local slope failure in the clay underneath the waste. Future instability problems with other pylons is inevitable unless the power line is re-routed.

3. Stratigraphy and hydrogeoiogy at the site The bedrock is composed of highly jointed, moderate to highly weathered Paleozoic metagreywacke. This description is valid for the upper portion of metagreywacke because the boreholes

drilled at the site were terminated as the drilling reached the bedrock. No borehole data are available for the fresh, unweathered deeper section of bedrock. Terrestrial Pliocene clay deposits overlie the metagreywacke (Fig. 4). Residual clays formed as the weathering product of bedrock are also observed. Boreholes drilled at the southwestern side of the disposal site in a relatively flat area (Fig. 3) indicate that the bedrock surface is of significant local relief. Pliocene terrestrial clay shows a great variation in thickness ranging from 7 m to greater than 35 m, where boreholes were drilled (Ankara Metropolitan Municipality, 1993). The thickness of this clay formation is estimated to reach up to 100 m. Metagreywacke crops out at lower altitudes at the bottom of the depression (Fig. 4). Terrestrial deposits are mainly clay consisting of small amounts of gravel, sand, and silt. A 5-m

K. Kayabah/Engineering Geology 44 (1996)203-212

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thick clayey gravel lens is also reported to exist in terrestrial clay (Ministry of Environment, 1992). The bottom of the topographic depression area is largely covered by alluvial material. Geological clay barriers or compacted natural clay barriers are a part of a sanitary landfill design. The required hydraulic conductivity for such impervious layers is frequently 10- 8 < k < 10- 6 cm/s (Chapius, 1990). The slug tests performed at 10 boreholes indicate that the hydraulic conductivity of terrestrial clay is about 10 -~ to 10 -s cm/s. The clayey gravel lens has a hydraulic conductivity range of 10 -4 to 10 -s cm/s (Ministry of Environment, 1992). Drilling records indicate the existence of groundwater at this level. In

addition, groundwater was reported to exist in the altered greywacke. In the area where boreholes were drilled, the depth to groundwater is only several meters (Fig. 4). However, based on engineering investigations, the groundwater is not economically exploitable at the site. Additional boreholes would be required to determine the lateral and vertical position of groundwater in the metagreywacke. X-ray diffraction analyses show that the type of clay in the terrestrial deposits are mainly mixed layers of chlorite-montmorillonite with some kaolinite. The higher the CEC of clay liners underneath sanitary landfill sites, the more dissolved solids are removed from leachate. At

K. Kayabah/Engineering Geology 44 (1996) 203-212

207

Table 1 Results of chemical analyses on water samples from leachate seeps, Imrahor Creek, and domestic wells (Ankara Metropolitan Municipality, 1994b) Constituent

Concentration at water samples from:

pH Conductivity (la.O/cm) Suspended matter (mg/1) COD (mg/1) CN (rag/1) SO4 (mg/l) Mn (mg/l) Fe (mg/l) Zn (mg/1) Cu (mg/l) Ni (mg/1) Cr (mg/l) Cd (mg/1) Pb (mg/1)

Leachate seeps

Imrahor Creek

Domestic wells

Typical value in municipal landfdl leachates i

63.9-7.40 750-2355 2.8-3.4 0.50-4.20 0.70 0.30 0.20-18.0 0.10 0.80

7.85-8.26 1666-1747 8-600 30-45 0.008 202.5-315 0.03-0.04 0.01-0.21 0.48-2.44 0.12-2.00 0.30 0.40 0.10 0.80

7.42-8.42 2200-10 225 1.13-52.44 0.24-6.47 <0.01-0.70 0.30-0.69 0.08-2.63 <0.10-0.1 <0.01-0.80

3.7-8.9 480-72 500 2-140 900 6.6-99 000 2-140 900 ND-6 ND-400 ND-4000 ND-731 ND-9.0 ND-7.5 ND-5.6 ND-0.4 ND-14.2

Drinking water limit b 7-8.5 800 25 0.05 0.05 0.3 5.0 1.0 0.02 0.05 0.01 0.05

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Fig. 4. Diagrammatic cross-section along the line between A and B in Fig. 2. M a m a k site, C E C v a l u e s o f u n d e r l y i n g c l a y r a n g e f r o m 26 t o 48 m E q / 1 0 0 g f o r t h e f o u r s a m p l e s tested u s i n g t h e s o d i u m a c e t a t e ( N a O A c ) m e t h o d . T h e a v e r a g e C E C v a l u e is 34 m E q / 1 0 0 g. T h e

C E C o f m o s t a b u n d a n t c l a y m i n e r a l s s u c h as illite a n d c h l o r i d e h a v e C E C v a l u e s b e t w e e n 10 a n d 40 m E q / 1 0 0 g ( T a b l e 2). T h e C E C v a l u e s o f t h e coll e c t e d soil s a m p l e s f r o m c l a y u n d e r l y i n g t h e w a s t e

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K. Kayabah/Engineering Geology 44 (1996) 203-212

are found to be satisfactory compared to that of most common clays on earth.

4. Sanitary landfill design The following are the conditions favorable for developing a sanitary landfill for Ankara at the present solid waste site: (a) the existence of a thick clay formation (> 25 m) with a hydraulic conductivity between 10 -7 and 10 -s cm/s; (b)the absence of economically usable groundwater; (c) the relatively high values of CEC values with the terrestrial clay; and (d) suitable topographic conditions. In addition to the factors listed above, soil material required for daily or weekly covering needs is abundant in the area. There is another criterion to be considered for site selection to design a sanitary landfill: the distance from such features as a lake, river, flood plain, marsh, national park, reservation area, airport, and major highways. This criterion is met for the proposed sanitary landfill. The engineering criteria for the proposed sanitary landfill includes: (a) base liner; (b) toe embankment; (c) final cover; (d) leachate collection system; (e) gas extraction wells; (f) surface water drainage. The proposed sanitary landfill was selected adjacent to the present solid waste site and will fill the northern half of a topographic depression. The NE and NW edges of the landfill would be surrounding topographic barriers while it is underlain by natural barriers (Figs. 5 and 6). The other two edges, at the southeast and southwest, are planned to be covered by artificial 1:3 slopes, which is common for most landfills. Various design configurations for base liner were Table 2 Major clay minerals and their cation exchange capacities (after Grim, 1968) Clay mineral

Cation exchange capacity (mEq/100 g)

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proposed by Baghci (1990) (Fig. 7). Among those, the configuration shown in Fig. 7(b) was selected for the proposed sanitary landfill. Although the site has a considerably thick clay layer and no economical groundwater exists, the use of synthetic membranes is still required owing to the growing interest in preventing environmental pollution. Another reason for selecting this configuration is to minimize possible damage to the membrane by the machines operating in the waste. The overlying clay liner and the drainage blanket would provide enough protection for the synthetic liner. The most commonly used synthetic membrane types for the design of sanitary landfills are highdensity polyethylene (HDPE), low-density polyethylene (LDPE), and polyvinyl chloride (PVC). Polyethylene-type membranes are durable against chemicals, sturdy, and easily seamable, while they have low performance at low temperature and

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against physical damage. PVC-type membranes are easier to work with, sturdy, and easily seamable. However, their performance is low at high temperatures and against chemicals. The polyethylene type of membranes are most often used for constructing sanitary landfills (Baghci, 1990). The construction of a toe embankment is proposed for the southeastern and southwestern edges of the landfill, to prevent possible toe erosion where the artificially built slope meets the ground surface. A 1.5-2-m high and 1:2 sloped embankment would serve this purpose (Fig. 8). However, depending on the properties of construction material, the height and dip of the embankment can be readjusted. A configuration for the final cover is presented in Fig. 8. The composition and function of each layer forming the final cover would be as follows: (a) a planted organic soil layer (top soil) to combat erosion; (b) a protective layer for shielding the synthetic membrane against physical damage; (c) a synthetic membrane for preventing rainwater infiltration; (d) a clay liner as a base material for

the synthetic material; (e) a smoothing layer (grading off the irregular waste surface). The collection of leachate would be accomplished by drainage pipes installed in the drainage blanket on top of the synthetic membrane (Figs. 8 and 9). The spacing between the pipes is dependent on the amount of leachate produced within the waste (this may require detailed investigation). The leachate collection pipes would merge into an outlet on the southeastern side of the landfill, where the leachate could be treated or transferred elsewhere. The proposed landfill is about 110 m high at its deepest point. It is vital that the usual landfill gas be removed. The composition of landfill gas is dominated by C H 4 ( > 80%). Methane can explode when its concentration in air is between 5 and 15%. Preventing possible explosions requires the use of extraction through suction. The frequency of gas extraction wells depends on how much gas a landfill of this size would produce and the hydraulic conductivity of municipal waste. The proposed gas extraction wells are usually the ana-

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logue of groundwater wells. Some published studies (e.g., Oweis et al., 1990; Powell et al., 1992) indicate that municipal waste has a hydraulic conductivity of 10 - 3 to 3 × 10 - 3 c m / s . However, it may vary depending on the composition and the degree of compaction of waste. At the closure stage, it is necessary to cover and completely isolate the body of waste from the environment. To prevent erosion of the final cover, Powell et al. (1992) proposed a surface water

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and sand. The remaining usable volume is about 92 × 106 m 3. At the current site, the volume of the solid waste accumulated so far is about 4.5 x 10 6 m 3. Assuming that the density of compacted waste is about 0.75 tons/m 3, the proposed landfill is capable of storing about 66 x 10 6 tons of solid waste. The amount of solid waste produced in Ankara is predicted to reach one million tons sometime between 2000 and 2005 (Fig. 11) (BELKO Inc., 1994). According to the trend shown in Fig. 11, the proposed landfill has a lifetime of over 50 years. The final configuration of the proposed sanitary landfill would have a cross-section as presented in Fig. 12.

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Fig. 10. Surfacedrainage details (from Powellet al., 1992). collection system shown in Fig. 10. A large portion of the partially perforated pipes would be buried in the slope. The slope on the upper side of the pipe is covered by a geotextile material to prevent possible scouring. The pipes are installed in a chevron pattern in the direction following the strike of the slope. The purpose for doing so is to prevent malfunction of the pipe system due to differential settlement inside the landfill. The volume of the proposed landfill is about 115 x 10 6 m 3. The ratio of waste/interlayer is 4:1 with regard to volume. Thus, 20% of the total landfill space would be filled by a mixture of clay

5. Conclusions and discussion

Investigations focused on the soil composition, hydraulic conductivity, topographic conditions, and other location criteria show that the current disposal site is suitable for design as a sanitary landfill. Terrestrial clay exceeding 25 m in thickness and the alluvium filling the bottom of the topo-

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Fig. 12. Cross-section showing the proposed landfill at its completion stage. g r a p h i c d e p r e s s i o n w o u l d be the source o f construction m a t e r i a l as well as the source o f interlayer material. It is believed that there is little e c o n o m i c a l l y exploitable groundwater. However, additional b o r e h o l e s are necessary for e x p l o r i n g the p o s i t i o n o f g r o u n d w a t e r in m e t a g r e y w a c k e . T h e existence o f a thick clay layer a b o v e the m e t a g r e y w a c k e , a n d the use o f a synthetic m e m b r a n e w o u l d eliminate possible g r o u n d w a t e r c o n t a m i n a t i o n even if there was e x p l o i t a b l e g r o u n d w a t e r . The c o n s t r u c t i o n o f a s a n i t a r y landfill a n d e m p l o y m e n t o f m o d e r n d e p o s i t i o n techniques s h o u l d m i n i m i z e the possible e n v i r o n m e n t a l pollution. S o m e large m e t r o p o l i t a n areas (e.g., Chicago, H o n g K o n g ) have sizable s a n i t a r y landfills in o p e r a t i o n inside the m e t r o p o l i t a n area. These landfills were built tens o f kilometers a w a y f r o m the city center in the past, b u t n o w they have been engulfed by the g r o w i n g u r b a n d e v e l o p m e n t . The r e l o c a t i o n o f the M a m a k ( A n k a r a ) solid waste site is also due to the r e a s o n t h a t it was engulfed by the g r o w i n g city. T h e a u t h o r emphasizes that the M a m a k d i s p o s a l site c o u l d serve for years to c o m e p r o v i d i n g p r o p e r m a i n t e n a n c e is taken.

Acknowledgment The a u t h o r is i n d e b t e d to Dr. i l h a m i 15nver ( U n i v e r s i t y o f A n k a r a , Soil Science D e p t . ) for C E C analyses; to Dr. Israfil K a y a b a h for X R D d e t e r m i n a t i o n s , a n d to the a u t h o r i t i e s o f A n k a r a

M e t r o p o l i t a n M u n i c i p a l i t y for p r o v i d i n g r e g a r d i n g M a m a k d i s p o s a l site.

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References Ankara Metropolitan Municipality, 1993. Preliminary geotechnical report on Mamak solid waste disposal site. Ankara Metropolitan Municipality, 1994a. Feasibility report on Mamak solid waste disposal area. Ankara Metropolitan Municipality, 1994b. Rehabilitation project for Tuzluqayxr old solid waste disposal area. Assmuth, T.W. and Strandberg, T., 1993. Ground water contamination at Finnish landfills. Water Air Soil Pollut., 69, 179 199. Baghci, A., 1990. Design, construction, and monitoring of sanitary landfill. Wiley & Sons, New York, 284 pp. BELKO Inc., 1994. Improvement report on the solid waste management project of Ankara. Chapius, R.P., 1990. Sand-bentonite liners: predicting permeability from laboratory tests. Can. Geotech. J., 27, 47-57. Freeze, R.A. and Cherry, J.A., 1979. Groundwater. PrenticeHall Inc., New Jersey, 604 pp. Grim, R.E., 1968. Clay Mineralogy, 2nd edn. McGraw Hill, New York. Husain, T., Hoda, A. and Khan, R., 1989. Impact of sanitary landfill on groundwater quality. Water Air Soil Pollut., 45, 191 206. Ministry of Environment, 1992. Geotechnical report on the rehabilitation of Mamak solid waste disposal site. Oweis, I.S., Smith, D.A., Elmwood, R.B. and Greene, D.S., 1990. Hydraulic characteristics of municipal refuse. ASCE J. Geotech. Eng., (116)4, 539-553. Powell, G.E., Watkins, A.T. and Manley, B.J.W., 1992. Restoration of a large urban landfill in Hong Kong. Geotechnique, (42) 1, 37-47. Uluatam, S.S., Yurteri, C. and Billur, N., 1984. Effectively collection, evaluation, and elimination of solid waste of Ankara. Nature J. Turkey, B, (8)1, 100-110.