Laboratory model tests in geotechnical engineering are routinely performed in small concrete or steel test tanks to examine the bearing capacity of footings, lateral earth pressures on retaining walls, behavior of reinforced foundation soils, and more. Results from these model tests are influenced by the shear forces generated on the sidewalls. The influence of such extraneous forces may completely change the interpretation of the test results or the failure mechanism. This paper presents a methodology for assessing the influence of such sidewall forces through numerical simulations. Through systematic numerical simulations, the influence of factors such as the relative size of the test tank, soil properties, and sidewall properties as investigated. The influence of sidewall friction and relative size of the test tank on the results as also investigated. Sidewall friction was found to a lesser effect for a wider test tank or for soil with a greater friction angle. Some recommendations on the relative tank sizes are also made on the basis of the results of this investigation. © 2012 American Society of Civil Engineers.