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In underground mining with backfill, it is essential to evaluate the minimum required strength of side-exposed backfill. An analytical solution proposed by Mitchell and coworkers in 1982 is largely used. Since 2012, Li and coworkers have published several improved solutions, accounting for several limitations in the Mitchell 1982 model. However, the improved solutions have not fully been validated by experimental results. Experimental work is necessary. As such, box instability tests were performed using an uncemented paste backfill under undrained conditions with a high aspect ratio stope. The unconfined compressive strength of the backfill was measured. Comparisons between experimental results and analytical results were made. A part of the experimental results was used to obtain the missing parameters through calibration. The other part of experimental results was then used to test the predictive capability of the calibrated analytical solutions. The results show that the improved analytical solutions provide a better agreement with the experimental results than the original Mitchell solution, at least for short-term stability of side-exposed backfill submitted to quasi-unconsolidated and undrained conditions. More experimental works are necessary to test the validity of the improved solutions under wider conditions.