This thesis is a preliminary geological and geotechnical study of the Zwin tidal area expansion plan. The existing Zwin is a silted-up tidal flat which ends at the International Dike in the southern boundary. The Willem Leoopoldpolder area located south of the International dike is to be converted into part of the Zwin intertidal environment by raising the dikes along the east, west and south border to 11m10 TAW and lowering the International Dike to to 4m10 TAW. Investigations are based on 9 boreholes and 65 cone penetration tests (CPT) carried out along the proposed dikes locations. The main objectives aim at understanding the history and morphology of the Zwin tidal flat, sedimentary processes and structures of the tidal environment, and subsurface exploration by using CPT tests. Sediments are characterized as a function of cone resistance (qc) and sleeve friction (fs). Subsurface profiles are made based on the characterization in correlation with CPT results. Soils classifications and behaviours are interpreted by using charts developed by Robertson et al., 1986 and Olsen, 1995. Overall results are global information on soil data for making a proper choice of location and design. Therefore, proper geotechnical parameters to be obtained can be selected for the final soil investigations.