The persistent increase of container vessel dimensions requires ports to adapt their wet infrastructure in order to compete with their competitors. However, in some cases waterway tunnels are expected to restrict the maximum depth. The Beneluxtunnel in Rotterdam is an example of such a tunnel. To allow more depth, the possibility of lowering this tunnel is studied. The Beneluxtunnel consists of two immersed tunnels constructed in 1967 and 2002. They consist of prefabricated elements that are immersed and connected to land parts on either side of the waterway. To allow lowering, various design options have been proposed in this study: • Regarding the land part, the options are proposed (1) to do nothing, (2) to make local adjustments utilizing the space provided by the dewatering cellar or (3) to apply general lowering which would affect the entire structure. • Regarding the immersed part, the options are proposed (1) to adapt the river bottom, (2) to lower the elements while they remain immersed by utilizing the limited freedom of rotation in its joints or (3) to re-float the elements to be re-immersed on a lowered bed. • Regarding traffic requirements, an option is to lower the maximum speed which would allow for steeper slopes. These options and their combinations have been analysed to determine their feasibility and value. To do so, first all aspects regarding the functional design are determined. The relations between traffic requirements and navigation channel dimensions have been determined, resulting in an overview of possible vertical profiles for all relevant design combinations. Apart from a first estimate of the effectivity of the design options, it also showed that the 1st Beneluxtunnel is governing because of the shorter length of its immersed part. For the technical design, the design of the original tunnel is analysed and for some important aspects, the structural capacity has been determined. This information is used to determine the effects of lowering on the tunnel and to estimate the associated lowering capacity in terms of vertical displacement profiles. This analysis shows that for all realistic design options, the structural capacity is sufficient. Apart from the lowering, also the effects and limitations of joint rotations have been determined. Both for the segment joints and the immersion joints, the rubber water stops are critical in determining the maximum rotation. The applied rotations must however be less as additional settlement could increase them. The vertical profiles associated with these rotations show significant depth increase, ranging up to 4.5 m. ? To determine the technical feasibility, the design must also be constructible. Hence, for four of the design options, the construction methods have been determined and proposed: • To lower the immersed part while it remains immersed, the soil must be removed below the elements by special dredging equipment. Also the supporting tiles must be removed. The large longitudinal pressure is expected to allow this without supports. However, for the lowering process, applying pre-tensioning and temporary supports is required. Special components connected to the pre-tensioning should allow to control the lowering process. Finally, the soil can be returned and the tunnel can be finished. • To re-float the elements, the closure joint must be re-opened. Next, the weight of the elements must be reduced by removing the ballast. Instead of re-floating it is chosen to lift the elements which allows better control of the process. The elements can now be transported to a location where they can be stored and adapted. When the bed is lowered and the land parts are finished, the elements can be re-immersed. • To locally adjust the land part, access is required which requires for the connecting element to be removed and for a watertight screen to be placed. Next, the old transition point can be demolished and the new transition point can be constructed within the existing abutment, utilizing the space provided by the dewatering cellar. • General lowering can be achieved by wet reconstruction. First, additional anchors have to be inserted in the walls. Next the land part can be flooded and the entire abutment and underwater concrete floor must be demolished. A new floor should be constructed at a few meters below, but first additional anchors must be inserted into the soil to provide vertical stability. Finally, water could be pumped out and everything could be reconstructed. Finally, the construction sequences are used to estimate the costs of the design options and their combinations. The results were combined with the maximum navigation depth increase to determine the most cost effective solution for different lowering ranges. Also the long and short term traffic costs (traffic jams) have been taken into account. The results are: Depth increase Land part Immersed part Costs (millions) 0.0 - 0.9 m Local adjustments on both sides Remain immersed € 295 0.9 - 1.9 m General lowering on one side Remain immersed € 415 1.9 - 3.2 m General lowering on both sides Remain immersed € 500 3.2 - 3.6 m General lowering on both sides Re-float € 665 > 3.6 m New immersed tunnel € 730 This study has proven that lowering the Beneluxtunnel is technically feasible and a cost effective method to increase the navigation draught.?