Scour prediction in non-uniform soils: undrained shear strength and erodibility

Scour development in non-uniform soils is still an area of great uncertainty and remains a challenge for designing structurally efficient and effective foundations in the marine environment. Scour risk in cohesive soils is made more uncertain by effects such as weathering and time-scale to scour. For large volume installation of foundations such as those related to offshore wind farm developments there is a limit to the amount of detailed geotechnical information that can be collected as part of the project. Therefore, reliance in data such as undrained shear strength, derived from cone penetration tests, supplemented with borehole data collected at a limited number of sites across the wind farm and laboratory analysis of soil samples becomes the principal source of geotechnical information. Hence, the question arises as to whether the undrained shear strength be used as a proxy for the erodibility of a soil as proposed in the approach of Annandale (1995). This paper will present evidence from both field and laboratory measurements of undrained shear strength and scour potential to test the hypothesis of undrained shear strength as a proxy for scour

Measures to reduce dynamic plunge pool pressures generated by a free jet

Spillways often include a flip bucket as terminal energy dissipator, combined with a plunge pool. To prevent scour at the river bed for large jet fall heights additional measures are required, such as terminal chute widening or increase of the plunge pool depth. The effect of these measures was investigated in hydraulic modeling. The dynamic plunge pool bottom pressures in the jet footprint area were systematically recorded. Both, the time-averaged and the fluctuating dynamic pressure heads are considered as references for the jet scour poten-tial, beside the related pressure coefficients. The investigated measures were proven to be effec-tive in terms of reduced pressures, especially in combination. The relevant parameters of the herein presented measures were systematically varied. This research relates to the Kárahnjúkar spillway model investigation, in which the principal working conditions as canyon topography, jet fall head and discharge spectrum were determined