An ejector or water jet pump is particularly suitable for pore water pressure relief applications in fine soils since it can operate at relatively low pumped flowrates, creating a vacuum at depth inside the well. A field trial of an ejector well dewatering system was carried out in laminated glacial lake deposits at Conwy, North Wales, in connection with the construction of the A55 Conwy Crossing. During the trial groundwater extraction flow rates and pore water pressures were monitored. Some of the ejector wells terminated in the bedrock underlying the lake deposits: in many of these, the groundwater extraction flow rates were high and the drawdowns achieved were relatively low. Back-analysis using conventional methods modelled closely the observed response of near-by piezometers. On the other hand, wells terminating in the lake deposits yielded low groundwater extraction flow rates and vacuums at about 500 millibar developed. In this case, conventional analysis substantially overpredicted both the rate and magnitude of drawdown in the surrounding ground. The reasons for this are as yet uncertain: nonetheless, the results of the trial demonstrate clearly the influence of the hydraulic boundary conditions on the performance of a dewatering system.