Discussion of ‘Soil creep effects on ground lateral deformation and pore water pressure under embankments’

Settlement analysis of field cases is normally studied based on parameters interpreted from laboratory samples influenced by varying degrees of sample disturbance. Such disturbance is more pronounced in natural soft clays and could significantly affect the engineering properties of the soil, e.g. the over consolidation ratio (OCR) and compressibility index (C_c). Hence, it is vital to understand the role of sample quality in relation to soil characterisation for long-term settlement analyses. In this work, this is numerically illustrated by use of a simple creep model along with realistic parameter selection. This work takes its starting point on critical discussion of the work presented by Fatahi et al. (2013) and uses the opportunity to further clarify some important aspects of settlement/creep analyses in light of sample quality and parameter interpretation valid for the corresponding constitutive model.     

Formulation of non-coaxial plastic dissipation and stress–dilatancy relations for geomaterials

Acta Geotechnica - Stress–dilatancy theories play a central role in the modeling of the plastic dissipation of geomaterials. There exist several mathematical frameworks for describing the...     

High-resolution P-Cable 3D seismic imaging of gas chimney structures in gas hydrated sediments of an Arctic sediment drift

The newly developed P-Cable 3D seismic system allows for high-resolution seismic imaging to characterize upper geosphere geological features focusing on geofluid expressions (gas chimneys), shallow gas and gas hydrate reservoirs. Seismic imaging of a geofluid system of an Arctic sediment drift at the Vestnesa Ridge, offshore western Svalbard, provides significantly improved details of internal chimney structures from the seafloor to ∼500 m bsf (below seafloor). The chimneys connect to pockmarks at the seafloor and indicate focused fluid flow through gas hydrated sediments. The pockmarks are not buried and align at the ridge-crest pointing to recent, topography-controlled fluid discharge. Chimneys are fuelled by sources beneath the base of gas hydrate stability zone (GHSZ) that is evident at ∼160–170 m bsf as indicated by a bottom-simulating reflector (BSR). Conduit centres that are not vertically straight but shift laterally by up to 200 m as well as discontinuous internal chimney reflections indicate heterogeneous hydraulic fracturing of the sediments. Episodically active, pressure-driven focused fluid flow could explain the hydro-fracturing processes that control the plumbing system and lead to extensive pockmark formation at crest of the Vestnesa Ridge. High-amplitude anomalies in the upper 50 m of the chimney structures suggest formations of near-surface gas hydrates and/or authigenic carbonate precipitation. Acoustic anomalies, expressed as high amplitudes and amplitude blanking, are irregularly distributed throughout the deeper parts of the chimneys and provide evidence for the variability of hydrate and/or carbonate formation in space and time.     

Marine C reservoir ages for 19th century whales and molluscs from the North Atlantic

In order to compare radiocarbon dates on marine and terrestrial samples the former have to be corrected for a reservoir age. We present reservoir ages from dating 21 whales collected 1860–1901 and recalculating dates of 23 molluscs collected 1857–1926. Most of the whales were caught along the coast of Norway, but one is from France and one from Iceland. We assume the former mainly lived in the North and equatorial Atlantic and in the Norwegian Sea. Whales feed only on pelagic organisms and will provide the reservoir age for the open ocean surface water. However, they travel long distances and will integrate the reservoir ages of the different water masses along their way. Molluscs (dated from Norway, Spitsbergen and Arctic Canada) are stationary and monitor the sea water passing their dwelling site, but some also take up carbon from particulate food or sediment pore water. Coastal water also often contains some continental carbon. We present two different views on how to analyze and interpret the data. Mangerud recommends to use reservoir ages based on a combination of the whale and mollusc dates, i.e. 380±30 and 360±30 yr relative to Intcal04 and British oak, respectively, and a ΔR value of 20±30 for the surface water in the N-Atlantic and Norwegian Sea. Bondevik and Gulliksen maintain that the reservoir age—and ΔR—along the Norwegian coast is latitude dependant, with ΔR-values increasing from −3±22 in the South to 105±24 at Spitsbergen. Whales, reflecting North Atlantic open ocean surface water have lower ΔR (7±11) than most molluscs.     

Experimental study of strain localization in sensitive clays

For evaluation of slope stability in materials displaying strain-softening behavior, knowledge concerning the failed state material response is of importance. Here, soft sensitive clay is studied. Such clays behave contractant at failure, which for undrained conditions yields a strain-softening behavior governed by the generation of excess pore water pressure. Strain softening is further linked with material instability and the phenomenon of strain localization. In the case of shear band formation, internal pore pressure gradients are then expected to be present for globally undrained conditions in the sensitive clay due to its low permeability. In the present study, this hypothesis and its implications on the global response and shear band properties are investigated. Utilizing an experimental setup with a modified triaxial cell allowing for shear band formation, the effect of varying the displacement rate is studied. Onset of strain localization is interpreted to occur just before or at the peak shear strength. A strong rate dependency of the softening response is observed. Increasing displacement rates give raised brittleness in terms of the slope of the global softening curve due to accumulating pore pressure. Also, reduced shear band thickness and a shear band inclination approaching 45° are obtained for increasing rates. In the context of slope failure in such materials, the rate dependency in the post-peak state opens up for a large variation in behavior, all depending on time as an important factor.