In order to relieve the busy traffic arteries of Antwerp (Belgium), the closing of the ring around Antwerp was put to study by BAM (Beheersmaatschappij Antwerpen Mobiel). The design of this construction requires detailed insight in the behaviour of the overconsolidated Boom clay layer during and after dewatering and removal of the covering soil layers. For this purpose a fully monitored excavation test was performed in order to assess the soil behaviour and specific techniques to be used in anticipated projects in the Antwerp area. This paper will specially present and evaluate the results regarding swell and pore pressures recorded in the Boom clay. From the monitoring results it can be derived that the Boom clay shows a quick response to a fluctuation in water level in the overlaying aquifer. The Boom clay also shows a severe pore pressure reduction as a result from loss on effective stress during excavation. These two effects should be considered separately. The underpressure is caused by swell hindered by a very low permeability of the Boom clay. Up to 50% of the effective stress reduction can be transferred into a pore pressure reduction. This condition results in a long term dissipation and swell process towards a gradient of approximately 25 kPa pressure increase per m depth. The pore pressure converges towards the original hydraustatic pressure in the Boom clay.