Influence of end caps on the unconfined compressive strength of bio-cemented sand

Microbially induced carbonate precipitation (MICP) has been widely studied for enhancing soil strength, particularly through unconfined compressive strength (UCS) testing of treated sands. Many studies report correlations between UCS and calcium carbonate (CaCO3) content, significant variability exists, often attributed to reagent properties, soil characteristics, injection strategies, and environmental conditions. However, the role of experimental testing procedures—especially end preparation and capping methods—has been largely overlooked. When preparing cylindrical specimens for UCS testing, conventional grinding methods often fail to achieve the required surface flatness, owing to the relatively weak bio-cementation bond. This shortcoming necessitates the use of appropriate capping methods to minimise stress concentrations induced by surface irregularities. In this study, we examined the effects of using two different end capping methods on the UCS of bio-cemented sand specimens: (1) bonded gypsum plaster caps (ASTM C617/C617M-15), and (2) unbonded silicone (rubber) caps (ASTM C1231/C1231M-15). The two end caps produced significantly different mechanical responses. Unbonded silicone-capped specimens exhibited more ductile behaviour, lower peak strengths, and stress-strain curves with multiple peaks and troughs—attributed to near-surface damage as the cap deforms. In contrast, bonded gypsum-capped specimens exhibited higher peak strengths at lower axial strains and smoother stress–strain curves. These behaviours are attributed to the much higher stiffness of cured gypsum plaster relative to the bio-cemented sand. Consequently, failure in these bonded specimens typically occurred at the weakest point (i.e. the least dense section as evidenced by X-ray radiography), with some showing well-defined shear bands across the sample length. The study also demonstrated that the measured UCS of bio-cemented sand is highly sensitive to density variations along the sample length, in addition to CaCO3 content. To minimise the influence of surface irregularities on UCS results, it is therefore recommended that core specimens be capped with gypsum plaster in accordance with ASTM C617/C617M-15. The capping method should also be reported when presenting UCS data as it can significantly influence the mechanical response.