Safety Analysis of Structural Foundations Built on Abandoned Solid Waste Sites

ABSTRACT This study adopted experimental design to investigate the suitability of abandoned solid waste site soil (ASWSS) as a foundation material for building construction. Measurements of geotechnical properties of stratified random soil samples of ASWSS and adjoining natural soil (NS) at depths 1.5, 2.0, 2.5, 3.0 and 3.5 m were obtained from six test points in Kaduna, Nigeria. The soil samples were subjected to sieve analysis, Atterberg limits (liquid limit, plastic limit, plasticity index and shrinkage limit), compaction, consolidation, triaxial, specific gravity tests as well as chemical characterization. Data treatment of ASWSS was carried out by applying 15% upper trim and 15% lower extended mean. These were done to forestall the effects of ‘reinforced earth scenario’ (unusual high strength spots caused by mix matrices of soil and fibrous materials) and unnoticed randomly distributed weak spots. Design data were evaluated in accordance with the provision of European code (Eurocode 7). The responses of ASWSS and NS to loadings were investigated by carrying out spread foundation designs on both of them using the same loading and geometric conditions. The two sets of designs were subjected to safety measurements by first order reliability method and Monte Carlo simulation respectively. The comparative reliability of ASWSS and NS with respect to structural loading was obtained in forms of reliability index and probability of failure. Significant differences in the geotechnical properties of ASWSS and NS were observed. The liquid and plastic limits of ASWSS fell in the ranges of 28 – 32% and 25 – 37% respectively. The angles of internal resistance ranged from 7 - 15º for ASWSS and 8 º - 17º for NS. Clay and silt accounted for up to 90% of ASWSS in some cases while as low as 9 kN/m2 cohesion was recorded. The composition of organic matters in ASWSS was found to be in the range of 2.1 – 5% while that of calcium/magnesium ranged between 106 mg/kg and 1000 mg/kg. Corrosive agents of sulphate and carbonate were found in the ranges of 235 – 903 mg/kg and 20 – 50 mg/kg respectively. The main mineral composition was quartz (silicon oxide), rutile (titanium oxide) and stolzite (lead tungsten oxide). Design values of cohesion, angle of internal resistance and unit weight of soil were obtained in the ranges of 9.5 – 12 kN/m2 , 7 - 20º and 12.9 – 14.3 kN/m3 respectively for ASWSS. The safety of foundation designs on ASWSS and NS was obtained in terms of reliability index and probability of failure. Despite the record of small probability of failure of 0.00013, corresponding to reliability index of 3.75, there were few cases of zero reliability indices corresponding to probability of failure of 0.5 on ASWSS. These values placed ASWSS in the category of ‘hazardous to high’ safety index in the standard performance classification formats. Sulphate resistant cement, large reinforced concrete basement or foundations covering large areas and a minimum foundation depth of 2.0 m are recommended for all structural foundations built on abandoned solid waste sites.