Rotational jetting in clay

In dredging activities submerged jets are widely used for different purposes. Two main applications are considered in this study: 1) a jetting sword on a cable trencher and 2) jets located on a draghead. Both jetting systems are especially designed for jetting sands and achieving large excavation productions in sandy soils. However when cohesive soils have to be jetted, the production decreases drastically. In sandy soils the jet creates wide cavities, while the cavity width produced by a jet in cohesive soils like clay is very narrow. Different researchers found that the cavity depth decreases only a little by an increasing traversing velocity of the nozzle and the cavity width to remain more or less constant, i.e. the jet can process more clay than generally is supplied. This results in the fact that the highest production will be achieved at the highest traversing velocities. However, the traversing velocity of the nozzle is limited by the tool (trencher/draghead). With the use of a rotating jet, more soil is supplied to the jet. The affected area is increased and the combined traversing velocity (normal traversing part + rotating velocity part) is increased as well. In this research the possible increase in excavation production of clay of a rotating jet compared to a conventional non-rotating jet for two purposes is studied; a rotating jet on a trenching machine and a rotating jet on a draghead. This research exists out of two models and two experimental test setups. Bothmodels consist of two modules; a calculation module and a visualization module. The calculation module predicts the excavation production as a function of: 1) Jet pressure, 2) Undrained shear strength, 3) Traversing velocity, 4) Rotational velocity, 5) Nozzle angle, 6) Nozzle diameter. The model is based on the entrainment of water at the back-side of the jet and soil at the front-side of the jet. The more entrainment of soil, the greater the increase in jet mixture density and the higher the decrease in stagnation pressure, resulting in a smaller penetration depth. The visualization module shows the cavity shape based on the cavity parameters modeled in the calculation module. The jet is modeled as a cone based on the geometry calculated in the calculated module. The jet will travel along a trajectory depending on traversing velocity and rotational velocity. The trajectory were the jet has been present, is removed and visualized as excavated volume. Two experimental test setups are developed in order to validate the models. The main parameters varied were the traversing velocity, nozzle angle (45°/60°), rotational velocity and jet pressure. For the trenching setup the increase in excavation production found, lies in a range between 6 and 7 for a traversing velocity of 15 - 215 m/h. For the draghead setup the traversing velocity was varied between 35 and 1800 m/h. The increase in excavation production can be up to a factor 4. The calculation module for the trenching application is able to produce a good estimation of the excavation production for the 45° nozzle. The calculation module overestimates the excavation production of the 60° nozzle compared to the experimental results. For the draghead application the developed model is able to give results close to the corresponding experimental results for traversing velocities lower than 1000 m/h. For larger traversing velocities the model underestimates the excavation production compared to the experimental results. The results of the experiments including the developed models can be used as a base for the design of rotating jets on a jetting sword or draghead.Mechanical, Maritime and Materials EngineeringMarine and Transport TechnologyOffshore and Dredging Engineerin