Coupled specimen and fiber dimensions influence measurement on the properties of fiber-reinforced soil

•Specimen dimension-related precautions should be taken to reuse natural and synthetic waste fibers.•The type of fibers, specimen diameter, and fiber length can affect the engineering properties of the earthen samples.•The trends and values obtained in 71-mm reinforced earthen specimens are not comparable to the 100- and 151-mm specimens.•A 100 mm diameter and 200 mm length can be chosen as optimum for specimens to measure and design soil structures. The coupled influence of fiber length and specimen dimensions on the results of unconfined compressive strength (UCS) tests is measured in this study. Three types of fiber, namely waste tire textile fibers (WTTF), date palm (DP) and polypropylene fibers (PP) in three different lengths have been prepared and added to a sandy soil. Three different specimen diameters of 71, 100 and 151 mm were tested. The results prove that the type and length of fiber, and diameter of the specimen strongly affect the results. Specimens with a diameter of 100 and 150 mm show identical trend with a much more limited difference compared to the specimens with a diameter of 71 mm. The effect of fiber length is observed to be a function of specimen dimension and type of fiber. In all cases, fiber length is most influential in smaller specimens. This parameter is most and least influential in DP and WTTF, respectively.