NDLI: In situ pore-pressure evolution during dynamic CPT measurements in soft sediments of the western Baltic Sea

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In situ pore-pressure evolution during dynamic CPT measurements in soft sediments of the western Baltic Sea

Content Provider	Springer Nature Link
Author	Seifert, Annedore Stegmann, Sylvia Mörz, Tobias Lange, Matthias Wever, Thomas Kopf, Achim
Copyright Year	2008
Abstract	We present in situ strength and pore-pressure measurements from 57 dynamic cone penetration tests in sediments of Mecklenburg (n = 51), Eckernförde (n = 2) and Gelting (n = 4) bays, western Baltic Sea, characterised by thick mud layers and partially free microbial gas resulting from the degradation of organic material. In Mecklenburg and Eckernförde bays, sediment sampling by nine gravity cores served sedimentological characterisation, analyses of geotechnical properties, and laboratory shear tests. At selected localities, high-resolution echo-sounder profiles were acquired. Our aim was to deploy a dynamic cone penetrometer (CPT) to infer sediment shear strength and cohesion of the sea bottom as a function of fluid saturation. The results show very variable changes in pore pressure and sediment strength during the CPT deployments. The majority of the CPT measurements (n = 54) show initially negative pore-pressure values during penetration, and a delayed response towards positive pressures thereafter. This so-called type B pore-pressure signal was recorded in all three bays, and is typically found in soft muds with high water contents and undrained shear strengths of 1.6–6.4 kPa. The type B signal is further affected by displacement of sediment and fluid upon penetration of the lance, skin effects during dynamic profiling, enhanced consolidation and strength of individual horizons, the presence of free gas, and a dilatory response of the sediment. In Mecklenburg Bay, the remaining small number of CPT measurements (n = 3) show a well-defined peak in both pore pressure and cone resistance during penetration, i.e. an initial marked increase which is followed by exponential pore-pressure decay during dissipation. This so-called type A pore-pressure signal is associated with normally consolidated mud, with indurated clay layers showing significantly higher undrained shear strength (up to 19 kPa). In Eckernförde and Gelting bays pore-pressure response type B is exclusively found, while in Mecklenburg Bay types A and B were detected. Despite the striking similarities in incremental density increase and shear strength behaviour with depth, gas occurrence and subtle variations in the coarse-grained fraction cause distinct pore-pressure curves. Gaseous muds interbedded with silty and sandy layers are most common in the three bays, and the potential effect of free gas (i.e. undersaturated pore space) on in situ strength has to be explored further.
Starting Page	213
Ending Page	227
Page Count	15
File Format	PDF
ISSN	02760460
Journal	Geo-Marine Letters
Volume Number	28
Issue Number	4
e-ISSN	14321157
Language	English
Publisher	Springer-Verlag
Publisher Date	2008-02-05
Publisher Place	Berlin, Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Geology
Content Type	Text
Resource Type	Article
Subject	Environmental Science Geotechnical Engineering and Engineering Geology Oceanography

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