Material Behavior of Dredged Contaminated Sediments from Simple Laboratory and Oedometer Tests

Abstract

Abstract Contaminated sediments, near urbanized areas, usually have a high content of organic matter and therefore a high water content and low shear strength. During dredging, additional water is mixed with the sediment, increasing the water content even further. The resulting dredged material that has to be disposed will be very soft with a very low shear strength. As a result, capping of a disposal site with clean sediments will be a challenge and put restrictions on the thickness of the cap and the used placement technique. Simple laboratory tests have been conducted to evaluate the transition of newly dredged material from being a fluid to behaving like a continuum. The performed tests show that it is possible to put a sand layer on top of dredged material after a short initial consolidation phase. This indicates that dredged material at this stage is by definition a continuum. Results from special oedometer tests revealed that the time needed for primary consolidation for contaminated clay from the Bjørvika area in Oslo (Norway), was approximately 30 h. The undrained shear strength of dredged material after consolidation under the influence of a thin sand layer is difficult to measure using traditional methods. A simple laboratory test with a transparent cylinder and a tilting table was conducted to force a failure mechanism. From these results, the undrained shear strength can be back calculated. For Bjørvika clay, test results, using a 2–3 cm sand cap, indicated a shear strength of approximately 0.10–0.15 kPa after three days of consolidation. Thin sand layers will increase the shear strength of the dredged material and act as draining layers of excess pore pressure during disposal site operation. This will improve the stability of the final cap after site closure.