The influence of coarse content on the erosion of clay lumps during dredge pipe transport

Abstract

In dredging and land reclamation, there are growing economic and environmental pressures to make beneficial use of fine-grained/cohesive soils. Due to these pressures, there is renewed interest in researching ways to optimise the beneficial use of fine-grained soils in dredging and reclamation.
Stiff cohesive clayey soils soil break up into lumps rather than fluidising when being dredged from the sea floor. These lumps are hydraulically transported in the dredge pipe, and erode to a spherical shape due to the forces of scour and abrasion. The presence of clay lumps has a large impact on various parts of the dredging process, such as clogging of dredging equipment, head loss in dredge pipes, wear of equipment, bulking factor, and geotechnical properties (strength, consolidation) of a reclamation. Accurately estimating the presence and prevalence of clay lumps in early phases of a dredging project can significantly improve project outcomes.
The presence of clay lumps is for a large part dependent on the degree to which clay lumps erode during transport through dredge pipes. Currently, methods for predicting this consist of practical experience or design charts, and mostly result in a binary prediction rather than a quantified prediction. Alternatively, physical tests can lead to more quantified predictions, but are expensive and time-consuming. In order to improve predictions of clay lump erosion in the dredge pipe, an analytical approach may be taken. This could consist of a clay lump erosion model (CLEM) equation, representing remaining solid lump mass over time.
Inputs of a CLEM would consist of model parameters that can be correlated to soil parameters and other factors such as dredge pipe transport velocity. These correlations can be found by using an index test apparatus (clay lump erosion test) that simulates dredge pipe transport of clay lumps. Such a test can be used quantify the relationship between a soil parameter and clay lump erosion.

There is no consensus as to which equation is the most suitable as a CLEM, and no quantified correlations exist between soil parameters and clay lump erosion. In this thesis, a suitable CLEM equation is identified and the influence of coarse content (%>63 microns) on clay lump erosion is studied. 35 clay lump erosion tests were performed on various soils. Using this data, a suitable CLEM is selected consisting of an exponential decay curve, where a single model parameter c represents erosion resistance. For 17 of the clay lump tests, soil lump samples of varying coarse content were prepared by mixing clay and sand in various proportions. Fitting the selected CLEM to the results of these tests resulted in a clear relationship between coarse content and c. Lower coarse content corresponds to a higher c, indicating a higher erosion resistance.
These results consist of a starting point in a broader framework for prediction of erosion of clay lumps in the dredge pipe based on soil parameters and dredge pipe transport factors. In order to use a CLEM to make predictions of clay lump erosion in actual dredging projects, the relationship between clay lump erosion test results and clay lump erosion in actual dredge pipes must be studied.
Besides selecting a suitable CLEM equation and studying the relationship between coarse content and clay lump erosion, this research also resulted in a broad compilation of knowledge on clay lump erosion. Also, valuable experience was gained in conducting an experimental study on the relationship between a soil parameter and clay lump erosion in the dredge pipe.