To construct an underground rail dump station at a port on a land reclaimed island in Australia, diaphragm walls and a base block designed of overlapping jet grout columns was used, aiming to allow excavation without dewatering. The base block varied between a depth of 8.9 and 19.4 m (surface RL ~6 m AHD) with material mainly consisting of sands. Proximity to the harbour provided a groundwater head boundary, with typical levels of 2 m AHD. Jet grout columns were designed with diameters between 2.1 and 2.7 m and a trial at the site was performed prior to construction. Failure of water tightness tests prior to excavation resulted in a second layer of jet grout columns being installed immediately below the original base block. During excavation of the material within the dump station gaps in the base block led to boiling of the sand, often before the base block level was exposed, preventing further excavation or construction. Ultimately an external dewatering and reinjection system was required to allow further excavation and construction of a concrete base slab on top of the base block. Investigation post construction by PSM highlighted the importance of jet grout column trial interpretation, the influence of column diameter, tilt and position variability and the consequence of small gaps in the base block. Monte Carlo techniques, based on observed as-built column variability, were used to demonstrate the impact of variability on the continuity of the base slab. These indicated that near perfect construction was required if the project objective was to be achieved.