Stability of Stored Municipal Waste for Different Sealing Systems

Sealing the base and slopes of the landfill creates an impermeable sealing barrier, protecting the ground against the penetration of leachates and landfill gases into the lower layers of the ground and groundwater, as well as draining the resulting leachate to the treatment system. High-density polyethylene (HDPE) geomembranes are one of the synthetic materials used as artificial sealing barriers in municipal waste landfills. One of the disadvantages of HDPE geomembrane is its smooth surface, which results in a low value of interface shear strength obtained for multilayered liner systems. This fact was especially noticed after the slope-stability failure of a Class I hazardous waste landfill at Kettleman Hills in California [12, 16]. The failure was caused by insufficient shear strength between layers of mixed storage seals. The interaction of smooth geomembrane and compacted clay layer was characterized by a very low residual value of the interface friction angle equalled 8°. Nowadays, geomembranes with textured surfaces are produced to prevent slippage along phases with mixed seal systems. Factors that affect landfill stability can be divided into internal (geological) and external (geo-environmental). More detailed they can be assessed as [7]: engineering properties of waste, structural features of the waste body (leachate and landfill gas) and dynamic engineering geological processes (earthquake, rainfall, leachate, excavation, overloaded).

Important elements affecting the stability of the landfill are the geometric dimensions (height, width and inclination of the slope of the waste massif), as well as their physical and mechanical parameters. Municipal waste collected in landfills is a material that is very diverse in terms of morphology and density. Fresh municipal waste has a density in the range of 0.4−1.0 Mg/m³, while landfill waste has a density of 0.8−1.2 Mg/m³ [25]. The most common ranges of strength parameters are about 20−35° for the angle of internal friction and 15−40 kPa for the cohesion resistance [26]. The slightly different values of strength parameters are given by Dixon et al. [3]: 15−42° for the angle of internal friction and 0–28 kPa for cohesion intercept. The shear strength of municipal waste varies over time, which is mainly related to its compression and decomposition of organic substances. The period of about 1.5–3 years after the ending of deposition corresponds to a change in the intensity of the processes of bio-decomposition [6]. Attention should also be paid to the gradual decrease in the strength parameters of waste due to the progressing decomposition of municipal waste.

Many publications have been assigned to the issue of the stability of landfills, e.g. [2, 8, 9, 10, 13]. Both the limit equilibrium methods based on a cylindrical (circular) slip surface and the finite element method (FEM) can be used to analyze the stability of landfills. In the most used limit equilibrium methods, the factor of safety (F) determined from the ratio of the stabilizing resistances and the destabilizing effect of actions is to be greater than the permissible value of the stability factor, which in the case of landfills should be taken in the range of 1.2 to 1.3, depending on the importance of the facility and threats to the adjacent areas. The slopes of municipal landfills with the factor F < 1.3 are considered unstable in terms of stability.

The construction of municipal waste landfills in Poland is currently regulated by the Act of 14 December 2012 on waste and the Regulation of the Minister of the Environment of 30 April 2013 on landfills [15, 19]. The regulation [15] stated that the operation of the landfill should ensure “geotechnical stability of the stored waste”, however, the method of ensuring this has not been defined. The slopes of the landfill in the post-operational phase should also be subject to the assessment of stability “determined by geotechnical methods”.

The aim of the work is to verify the stability of municipal waste stored in a landfill of a specific structure, assuming variables related to the waste massif, such as height, width of the crest and slope inclination of the massif. Two different materials were considered as a mineral sealing layer: compacted highly plastic clay and compacted fly ash that meets the conditions of the material for the construction of the sealing layer [20, 23]. The analysis may be helpful in determining a procedure for waste placement storage.

The calculation results are presented, depending on the geometry of the slope and the calculation method in Table 3 for compacted clay as a material in sealing, and Table 4 – for fly ash as a part of sealing. The results are shown as the degree of utilization for the limit state (Λ) and as the factor of safety (F). Parameters indicating the lack of stability of the waste massif are marked in red. Parameters differentiated for clay and fly ash are marked in bold.

In the vast majority of calculation cases, slightly lower values of the factors of safety F and greater degrees of utilization were obtained using the Fellenius/Petterson method, due to the assumption of zero shear and normal forces between the design blocks. The dimensions and shape of the mass of waste which are assessed as stable according to other methods, in the case of the Fellenius method, they are evaluated as unstable (underestimation of the factor of safety and overstatement of the degree of utilization). Earlier literature reports indicated much lower F values in the case of the Fellenius method compared to other methods than those obtained for the slope of municipal waste lined with a sealing layer.

Analyzing the calculations made in accordance with DA3 of Eurocode 7 [4], it was found that with the assumed structure and geometric dimensions of the landfill, the waste mass can be considered stable at the storage height H = 5 – 50 m, the width of the crown of the waste lump B equal to 10 and 50 m and the slope inclination waste massif α from 20° to 25°, independently on kind of sealings. After increasing the inclination of the waste slope to 30°, the slope is stable at B = 10 m, and when B = 50 m − only at the height of waste storage H = 5 – 10 m. In the case of a further increase of the slope inclination α to 45°, the stability condition is met only with the width of the slope equal to 10 m and the height H of the stored waste equal to 5 m.

Stability calculations considering the factor of safety may be more or less rigorous compared to the calculations according to Eurocode 7 (DA3), depending on the permissible value of the stability factor adopted. Assuming that the factor of safety should be F_p = 1.2−1.3 [8, 10], structures of waste slopes with the slope of the waste massif α ≥ 45° and crown width B = 50 m can be considered unstable, while in the case of B = 10 m, the slopes up to a height of 10 m are stable. The stability condition is also not checked by slopes with an inclination of α = 30°, a crest width of B = 50 m and a height of 50 m. These conditions are practically identical to the calculations according to Eurocode 7. Considering that F_p should be at least equal to 1.3, the stability condition is met only by the construction of slopes with an inclination of α ≤ 25°, in the case of both the width of the crown and the height of the massif H ≤ 50 m. Slopes with a greater inclination α = 30° meet the condition stability if the height of the stored waste is 10 m or less.

In the case of the analyzed slope inclination α equal to 20° and 25°, the increase in the height of the stored waste does not reduce the stability of the waste mass. The height of the slope starts to affect the stability of the waste at α = 30° and height H = 30−50 m, to reach full impact at α = 45°.

It should be noted that the location of the critical slip lines varies depending on the geometrical dimensions of the waste body, and is generally independent of the adopted calculation method. However, the location of the slip line is affected by the type of layer sealing the slope and the base of the landfill. In the case of classic clay sealing, the slope inclination α = 20°−25° and the height of the massif H = 5−30 m, the slip plane runs in the sealing of the excavation slope, regardless of the width of the waste massif. After increasing H to 50 m, when B = 10 m, circular slip lines go along the slope seal (Fig. 3a), but when B = 50 m lines go across waste massif (Fig. 3b). At slope inclination α = 30° the slip line crosses sealing at excavation slope when H = 30 m and B = 10 m (Fig. 3c), and when H = 30 – 50 m and B = 50 m – runs across waste. In the case of inclination α = 45° the circular slip line appears in the waste massif from the value of H = 5 m, and the structure is not stable (Fig. 3d).

Figure 3.

In the case of sealing with fly ash as the mineral layer of the sealing, similar courses of circular slip lines (Fig. 4) and mostly identical values of safety factors and utilization degrees were obtained, within the scope of calculations performed (Tables 3 and 4). Variation of the slip line was observed in the case of the slope inclination α = 30° and massif shape determined by B = 10 m, H = 30 m. For fly ash sealing the slip line does not cross the sealing but goes along the sealing (Fig. 4a), but in both cases the structure is stable.

Figure 4.