Improvement of engineering properties of expansive highway subgrades with waste paper mill treatment water sludge

In this thesis, the geotechnical and engineering properties of three different highway subgrade soils with different swelling-shrinkage potentials were tried to be improved by using waste paper mill treatment water sludge (WPS). For this purpose, firstly, WPS was added to the subgrade soils at the rates of 3%, 6%, 9%, 12% and 15% by dry weight and comprehensive laboratory analyses including plasticity, swelling-shrinkage, compaction, unconfined compressive strength (UCS), California bearing ratio (CBR), direct shear box, falling cone, consolidation and resilient modulus (Mr) were performed on soil specimens. Scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analyses were carried out to examine the changes in the microstructure of the soils with the use of WPS. Different artificial neural network (ANN) models were developed to predict UCS and Mr values using the experimental results obtained within the scope of the study. Finally, the modeling of the subgrade soils was done with the Plaxis 2D program and finite element analysis was performed. When the test results are examined, it is seen that the WPS content significantly reduces the plasticity and swelling shrinkage potential of the soils. The compaction parameters are found to be affected by the soil type. It is deduced that the load-bearing capacity of the soils can increase with the curing period and WPS content. It is determined that the consolidation parameters are influenced by the soil type and WPS content. When the Mr test results are examined, it can be concluded that the stiffness of the subgrade soils increases with a certain amount of WPS. The results obtained by SEM, EDX and XRD analyses showed that the microstructures of subgrades with WPS revealed cementitious gels formation with flocculated particles as a result of the cation exchange and pozzolanic reactions that occurred between the soil and WPS grains, and new mineralogical compositions were determined to emerge. ANN models developed to estimate the UCS and Mr parameters showed better performance with higher coefficients of determination (R2) and lower error rates compared to multiple regression models. The results of the Plaxis 2D analyses showed that the stabilization of the subgrade soils with WPS reduced the deformations and strains caused by the traffic loads and provided a more rigid road platform. As a consequence, the data obtained showed that WPS can be used to improve the geotechnical and engineering properties of the subgrade soils used in this thesis.