Geotechnical engineering scenarios engaged in internal erosion or mineral dissolution were found worldwide, from CO2 sequestration projects to internal erosion of earth dams. Materials subjected to loss of solid phase from their internal fabric vary from shales and sandstones to unbonded soils. The thesis aimed to extend the understanding of the effect of dissolution and erosion on the geomechanical behaviour of shales and granular materials. The research was composed of two main pillars; the first pillar aimed at quantifying the effect of grain loss on granular soils, and the second pillar focused on understanding the chemical effect of CO2 on Opalinus Clay shales. The thesis introduces the case studies of dissolution and internal erosion phenomena encountered in geotechnical engineering practices. Along with the problems introduced, corresponding publications on laboratory and field experiments were presented. For the final part of the literature survey, the existing constitutive models and frameworks were presented, with their limitations being addressed. At the end of the scope of the thesis and layout was presented. The first pillar of the thesis aimed to quantify the effect of grain dissolution on the mechanical behaviour of sands. The work was, for the most part, experimental. Salt-sand mixtures were used, where the salt was the dissolving species, and the sand was the chemically inert species. A large experimental programme was extended into three separate chapters. Oedometer test was first performed where the dissolution induced volumetric response and the post-dissolution compaction characteristics were shown. A novel framework for the dissolution-induced mechanical response of granular materials was proposed along with the experimental results. Additionally, a constitutive model based on the framework and the experimental findings was formulated. The second experiment aimed at observing the effect of dissolution on the shear strength of sands tested with direct shear box apparatus. The experiment was designed based on the core philosophy of the state parameter concept. The final experimental campaign was focused on studying the effect of dissolution based on the critical state soil mechanics theory of sands. The experiment consisted of two subsets: the first focused on the volumetric response under an isotropic stress state, and the second focused on the shear response. The second subject was Opalinus clay shale extracted from Northern Switzerland. From the borelog, the section with high carbonate content was tested as sporadic carbonates lenses were deemed the most vulnerable in a caprock layer to CO2. To study the spontaneous mechanical response during the CO2 exposure, the shale sample was confined inside the high-pressure oedometer cell, where any deformation throughout the test was recorded. Also, since sealing capacity is one of the most crucial parameters for the success of CO2 trapping, the permeability was measured at each stage of the test. The CO2 exposure was forced by injecting carbonated water for approximately 70 days. In parallel with the oedometer tests, an exposure test was conducted by exposing one sample to CO2 and another to deaerated water for about a month. After the exposure, the samples were inspected through SEM, ÎŒCT scan and XRD analysis to detect any structural and mineralogical alteration.