## 2 Materials and Methods  
### Site Investigation for Establishing Landfill Waste Properties  
In the case of Radiowo landfill, the main challenge was to precisely determine the geotechnical parameters of waste built into the slope so the further stability analyses results could be reliable. For this purpose, the mechanical parameters of waste were investigated by using three main groups of tests. These were back-analyses, trial loading, and geotechnical in situ tests (cone penetrometer test, CPT; weight sounding test, WST; dynamic probe test, DPT). Based on the obtained results, the parameters could be established and adopted for further computation. For back-analysis methods, there were three cross-sections selected and surveyed on the landfill itself. After investigation of the landslide that occurred in the past, by knowing its factor of safety and having a precisely documented slip surface, it was possible to back-calculate the mechanical parameters of the slope-filling material. During the failure, the location of layers of the weakest parameters (investigated by using CPT and WST test) was confirmed. To further investigate the mechanical parameters of waste, there was also a trial embankment constructed, filled with waste, and overloaded with concrete slabs to induce the slope failure so another set of back-calculations for confirming the CPT and WST test results could be performed. Such broad verification of material properties allowed reliable computations of the factor of safety for other slopes on the landfill.  
The in situ tests for Radiowo landfill were performed in 1993-1999. The investigation was performed to determine waste mechanical parameters for the stability analysis and settlement prediction, as well as the estimation of bearing capacity for the landfill access road foundation. The other group of field tests was a geotechnical investigation including DPT, WST, and CPT tests [4]. The WST was generally performed in the vicinity of the main road constructed on the landfill body. Tests were repeated when a thickness of waste of 5 m was reached. The results were used for the quality control of the road foundation compaction. The average amount of N\({}_{20}\) (number of blows for 20 cm penetration) for fresh waste was 10 and 5 for old waste. The amount of N\({}_{20}\) increased twice as much when the disposing waste was separated with sand layers. The CPT investigation reached a depth of approximately 25 m. Based on those results, the effective internal friction angle for waste felt was within the range of \(\varphi^{\prime}\) = 25-45\({}^{\circ}\). However, in some particular locations, the values dropped down to \(\varphi^{\prime}\) = 20-24\({}^{\circ}\). These values were obtained after having considered waste as non-cohesive soils. The test results confirmed the presence of waste cohesion. The CPT test interpretation for waste, analogically to cohesive soils, gave a total shear strength of \(\tau_{\text{fu}}\) = 80 kPa for non-composed and \(\tau_{\text{fu}}\) = 90 kPa for municipal waste. The geotechnical parameters of waste material are presented in Table 1.  
Figure 1: Lateral reinforcements of landfill slope to improve the factor of safety (FOS).