The Noordtunnel is an immersed tunnel open to roadway service since 1990 in The Netherlands. Over the past thirty years of its operation time, a significant differential settlement behaviour has been observed, and this ongoing settlement potentially imposes safety concerns to the tunnel, such as joint leakage. However, the underlying factor triggering this differential settlement behavior remains unknown. To ensure the tunnel's serviceability, this thesis aims to investigate the underlying causes of the occurring excessive settlement, predict future settlements, and assess its impact on tunnel structural safety within its designed lifetime.

The analysis starts by reconstructing the settlement time history of the Noordtunnel. The process involves determining the most reliable reference point and performing back analysis to estimate the settlement magnitude during the unmeasured period. Further, the settlement history is reconstructed by combining all the settlement data of all the periods. It is found that the settlement history at all immersion joints show a logarithmic trendline, with a maximum estimated settlement of about 94.36 mm occurring at immersion joint 2. Subsequently, the soil profile and geotechnical parameters were determined for the simulation. The provided Cone Penetration Test (CPT)s data and borehole ensure the soil profile depicted in the given situation map. Additionally, in the absence of laboratory data, the Hardening Soil Model (HS Model) and Soft Soil Creep Model (SSC) parameters are estimated based on CPT - NEN Table 2b correlations.

Afterward, the Two Dimensions (2D) Finite Element Method (FEM) simulations were carried out in PLAXIS (a commercial simulation software), while considering the variations of load acting on top of the subsoil. Two types of soil constitutive law were chosen to simulate the settlement in the Noordtunnel: HS Model and SSC Model. The optimum model, which can simulate the settlement behaviour in the field, was selected by aligning the simulation outcomes to the reconstructed historical settlement. The simulation results show that only settlement at immersion joint 2 has the same tendency as the SSC Model, while the other immersion joints tend to have a similar tendency to the HS Model. The simulation outcomes also indicate that excessive settlement at immersion joint 2 occurs due to the soft soil underneath the tunnel and the natural sedimentation on top of it. The soft soil is responsible for at least 20 mm of settlement, while the sedimentation contributes to a minimum of 8 mm of settlement during 30 years of tunnel operation. Subsequently, the sensitivity analysis is conducted to examine how much the simulation outcomes may deviate when accounting for soil variability in the field. Due to the narrow distribution of the reference values, adjusting the most sensitive parameter will only deviate the results by a maximum of 5.48% at immersion joint 2 and 4% at the other immersion joints. These results indicate that the model is robust enough and expected to generate reasonable future settlement predictions. An additional settlement of 15.13 mm of settlement is predicted to occur at immersion joint 2, while 4 to 5 mm of additional settlement is anticipated at the other immersion joints over the tunnel's remaining lifespan.

The differential settlement at the tunnel longitudinal direction has triggered element tilting and further induced compression and decompression to the GINA gasket at immersion joints. It has been observed that while uneven settlement contributes to joint decompression, the external forces acting on the GINA gasket remain considerably lower in magnitude compared to the overall friction force. Therefore, the impact of uneven settlement on the water tightness is generally minimal. Additionally, considering the limitations of the current monitoring procedure, an optimized monitoring plan based on the Distributed Optical Fiber Sensor (DOFS) system is proposed. Finally, future recommendations to improve the current thesis are also put forward.

The 1st Heinenoordtunnel is an immersed tube which has been open for service in the Netherlands since 1969. Over the past 50 years, a large and increasing settlement has been measured along the tunnel alignment. The greatest value of settlement (65 mm) occurs in tunnel element 1 from 1978 to 2018. The maximum differential settlement has reached 30 mm between element 1 and 2 which causes a potential threat to the tunnel serviceability. In addition, Distributed Optical Fiber Sensors (DOFS) was used to measure daily joint deformation of immersed tunnel. The result of DOFS has shown a periodic settlement under tide.
The aim of this thesis is to investigate the long-term and daily settlement behavior of Heinenoordtunnel. The study starts with the geological information investigation, followed by settlement analysis using 2D Finite Element Modeling. Both long-term settlement and daily cyclic settlement are simulated and compared with the monitoring data.
In long term settlement analysis, the results show that the consolidation settlement of tunnel element is trivial (<0.5mm) over the 50 years operation. The calculated result does not match with the monitoring data under a constant load assumption over the operational period. Therefore, three additional factors are considered which can be the possible causes of the excessive settlement. Those factors are the growth of traffic volume, fluvial deposition on riverbed and secondary compaction on the foundation. The main results are summarized as follows:
(1) The settlement due to extreme traffic growth is between 10 and 20 mm which increases gradually from 1979 to 2018.
(2) The fluvial deposition causes an average settlement of 1.68 mm over the past 40 years of operation.
(3) The secondary compaction result is in between 5.30 and 10.70 mm, while the exact value depends on the selection of creep parameter.
(4) The combined settlement is in between 18.27 and 30.62 mm that is significantly less than the monitoring data.
Based on the above findings, the combination of those three factors cannot provide a sound explanation on monitoring data. Hence, a sensitivity analysis will be conducted in the final stage of the research.
In the daily settlement response simulation, a coupled flow simulation is applied on both traverse and longitudinal direction under tidal fluctuation. The summary of results is indicated as below:
(1) A phase delay of excess pore water pressure occurs for soil under the low permeable layer. This causes periodic rebound and settlement on the tunnel element
(2) The amplitude of tunnel cyclic settlement is in between 0.143 and 0.215 mm
(3) The deformation of intermediate immersion joint is under 0.059 mm
Since the difference between calculation and monitoring results is over 0.2 mm in joint 6, it is important to conduct sensitivity analysis on the Plaxis model.
3
To verify the importance of soil variability, the sensitivity analysis is conducted by reducing the stiffness parameter with two standard deviation value in long term settlement analysis and one standard deviation value in daily settlement analysis. The long-term settlement become twice as the initial value after the parameter adjustment. The difference between monitoring data and Finite Element result reduces by 30 to 192 percent of the original value. On the other hand, the daily settlement of tunnel reduces by 4 to 17 percent under the one standard deviation increment. The daily response of tunnel is closer to the monitoring data after parameter adjustment.
In conclusion, the three factors considered in this research causes a significant change on the total settlement of immersed tunnel. The periodic response of tunnel settlement is verified from the coupled flow mode. Finally, some recommendations are made for future study.