In situ tensile fracture toughness of surficial cohesive marine sediments |
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| Authors: | Bruce D Johnson Mark A Barry Bernard P Boudreau Peter A Jumars Kelly M Dorgan |
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| Institution: | (1) Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada;(2) Darling Marine Center, University of Maine, Walpole, ME 04573-3307, USA;(3) Scripps Institution of Oceanography, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0202, USA;; |
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| Abstract: | This study reports the first in situ measurements of tensile fracture toughness, K
IC, of soft, surficial, cohesive marine sediments. A newly developed probe continuously measures the stress required to cause
tensile failure in sediments to depths of up to 1 m. Probe measurements are in agreement with standard laboratory methods
of K
IC measurements in both potter’s clay and natural sediments. The data comprise in situ depth profiles from three field sites
in Nova Scotia, Canada. Measured K
IC at two muddy sites (median grain size of 23–50 μm) range from near zero at the sediment surface to >1,800 Pa m1/2 at 0.2 m depth. These profiles also appear to identify the bioturbated/mixed depth. K
IC for a sandy site (>90% sand) is an order of magnitude lower than for the muddy sediments, and reflects the lack of cohesion/adhesion.
A comparison of K
IC, median grain size, and porosity in muddy sediments indicates that consolidation increases fracture strength, whereas inclusion
of sand causes weakening; thus, sand-bearing layers can be easily identified in K
IC profiles. K
IC and vane-measured shear strength correlate strongly, which suggests that the vane measurements should perhaps be interpreted
as shear fracture toughness, rather than shear strength. Comparison of in situ probe-measured values with K
IC of soils and gelatin shows that sediments have a K
IC range intermediate between denser compacted soils and softer, elastic gelatin. |
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