Predicting the breaching production of a slope during a wet mining process

Abstract

An underwater slope can collapse by liquefaction, wedge failure or breaching. Breaching is the most unknown process and has a low predictability. Dredge mining is strongly influenced by the breaching process. If breaching can be accurately predicted, the efficiency of the dredging production can be maximised.It is the objective to know what field- and lab tests must be conducted to be able to predict the breaching behaviour of a slope. In this report, a model is developed to predict the breaching behaviour of a slope after a single cut at the toe of the slope is made. The model is used to research the influence of several soil parameters and geometrical parameters on the breaching behaviour. The most important criterion is the difference between stable and unstable breaching. An empirical relation is created which can be used to estimate the critical cut height. This is the height at which a smaller cut height creates a stable breach and a larger cut creates an unstable breach. The critical cut height is dependent on the particle size distribution represented by the D50 and the D-10, the particle density and the slope angle. By using these parameters, the breaching process can be predicted. The critical cut height can be used to predict what the best cut depth for a certain slope and dredger is. After the first cut, usually several more cuts are made in order to keep the production at a high level. This process is not extensively researched in the past, so a second simplified model is created which predicts the breaching production in 3D and can incorporate multiple cuts. The model incorporates a swinging motion by the cutter, which is essential for a dredging operation. The model can demonstrate different phenomena which might occur during a dredging operation. It will illustrate examples of consequences of a slow or fast swinging motion, large or small delay times between swings and differences in swing widths. The input parameters necessary for the model can be obtained from field- and lab tests. Several field- and lab tests are advised after analysis of the model. The tendency of the soil to either breach or liquefy can be estimated by a state parameter analysis using CPT data. It might however be more suitable to use an SPT, in this case the SPT data gives a direct indication of the relative density which can also be used to estimate the ability to breach. A sieving is necessary to obtain the particle size distribution in order to obtain the D50 and the D10. A pycnometer test is necessary in order to obtain the particle density. The angle of repose can be determined with a lab test or a correlation with SPT data. In order to get a better estimate of the permeability, it is advised to do an auger hole test to measure the permeability more accurately. The data from these tests are suitable to predict the breaching behaviour of any submerged slope around the world.