Large diameter marine Deep Mixing

North of the city of Sundsvall, Sweden, GeoMind has designed a 600-meter-long rubble-mound structure founded on large diameter wet deep mixing columns. The work was initiated by an evaluation of competitive technical solutions with the aim of minimizing the technical and work environment risks, costs as well as time for construction. The soil comprised of silty, partly quick clay and sulphide clay. In addition to conventional soil investigations, the design included disturbed and undisturbed soil sampling to be able to optimize binder type and quantity with the aid from laboratory mixing and strength testing. The design resulted in a column pattern with mainly non-overlapping columns with the area coverage of roughly 90 % with a required average unconfined strength of 140 to 230 kPa in the columns. The governing failure modes were sliding and overall stability. The wet deep mixing columns were executed by Bauer Spezialtiefbau GmbH. Execution was carried out from a pontoon using large diameter, single shaft equipment. During execution, the stepwise wet grab and core sampling revealed a challenge to achieve sufficient homogeneity and strength in the silty sulphide clay pockets that were erratically encountered, mainly in the upper soil volume. Modification of the mixing tool, mixing process parameters as well as adding a double stroke sequence in the upper part to the mixing process resulted in significant improvement of the homogeneity as well as the undrained shear strength. Detailed testing of strength versus time was performed in the laboratory where seismic, unconfined and triaxial tests were performed. After sufficient curing, the rubble was constructed by subsequent filling from one side to the other.