A CONSIDERATION ON THE PORE PRESSURE IN CLAYS UNDER UNDRAINED SHEAR

Many of the world reserves of fossil fuels are located at various water depths in fine-grained sediment under the seabed. The fine-grained sediment contains relatively large biogas bubbles, which has been posing challenges to the stability of offshore foundations supporting oil and gas platforms. Although fine-grained gassy soil was found to exhibit different undrained shear strengths (cu) by altering the initial pore pressure, ui (relevant to water depth), systematic studies concerning the effect of ui on undrained shear behaviours of the soil are still lacking. This study reports a series of undrained triaxial tests aiming to compare and investigate the responses of reconstituted fine-grained gassy soil with the same consolidation pressure ([Formula: see text]), but at a wide range of varying ui (0–1000 kPa). The shearing-induced excess pore pressure (Δu) in the gassy specimens highly depends on ui. It can be either smaller than that of the saturated specimen with the same [Formula: see text] (due to partial dissipation of Δu into relatively large bubbles at low ui) or larger than that of the saturated specimen (related to collapse of relatively small bubbles at high ui). Consequently, the presence of bubbles had beneficially increased cu at relatively low ui (ui/[Formula: see text] < 0.6), and vice versa. The critical stress ratio of the reconstituted fine-grained gassy soil, however, did not appear to be altered by ui.

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Evaluating the sealing potential of young and thin mass-transport deposits: Lake Villarrica, Chile

Geological Society London Special Publications ◽

10.1144/sp500-2019-155 ◽

2019 ◽

Vol 500 (1) ◽

pp. 129-146 ◽

Cited By ~ 2

Author(s):

Jasper Moernaut ◽

Gauvain Wiemer ◽

Achim Kopf ◽

Michael Strasser

Keyword(s):

Shear Strength ◽

Mass Transport ◽

Pore Pressure ◽

Sediment Cores ◽

Free Fall ◽

Undrained Shear Strength ◽

Excess Pore Pressure ◽

Minor Role ◽

Mass Transport Deposits ◽

Undrained Shear

AbstractSubaqueous mass-transport deposits (MTDs) can be important elements in hydrocarbon systems, forming potential reservoirs or seals. Most research has targeted outcrops or moderately to deeply buried MTDs and, therefore, the petrophysical properties of near-seafloor MTDs, and their influence in the trapping and release of shallow fluids, is poorly studied. Here, we investigate shallow MTDs in Lake Villarrica (Chile) by combining sub-bottom profiles, free-fall penetrometer data, pore pressure dissipation tests and geotechnical properties of sediment cores. Low undrained shear strength under a surficial MTD indicates underconsolidation caused by sudden loading and rapid sealing. Larger, buried MTDs show acoustic signatures of free gas at their base, indicating effective sealing. This is supported by degassing core gaps just below MTDs and by excess pore pressure ratios c. 30–70% within MTDs. Acoustic windows below rafted blocks suggest local fluid escape. MTDs exhibit elevated undrained shear strength and reduced porosity compared to surrounding sediments, but are comparable to upslope source sequences. This suggests that MTD sealing capacity in Villarrica relates to the apparently overconsolidated nature of the slope sequence, leaving a minor role for shear densification. This study shows that shallow MTDs can form a relatively rapid seal for fluid migration, locally degraded by rafted blocks.

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Behavior of a Cemented Plastic Clay as an Embankment Foundation

Canadian Geotechnical Journal ◽

10.1139/t74-004 ◽

1974 ◽

Vol 11 (1) ◽

pp. 46-58

Author(s):

Guy Lefebvre ◽

Liguori M. Lefebvre ◽

Peter Rosenberg

Keyword(s):

Shear Strength ◽

Pore Pressure ◽

Applied Pressure ◽

Undrained Shear Strength ◽

Glacial Lake ◽

Triaxial Tests ◽

High Plasticity ◽

Degree Of Consolidation ◽

Undrained Shear ◽

Varved Clay

A 32 ft (10 m) high embankment has been built at Matagami, Quebec, on a varved clay deposit of the glacial lake Barlow–Ojibway. This paper reports on some aspects considered in the design, mainly the cementation and the high plasticity of the clay. The variation of settlements and pore pressure is presented and indicates that, 8 months after construction, the pore pressure dissipation is very small while the measured settlements reach 18 in. (45 cm).Triaxial tests, with various consolidation times, were carried out on specimens cut from undisturbed block samples obtained in the same area, in order to study the effect of consolidation on the shear strength of these cemented clays. Results indicate that the undrained shear strength decreases during consolidation until the degree of consolidation reaches 50%. This phenomenon together with the non-dissipation of pore pressure in the field is attributed to the collapse of the cemented structure when the applied pressure exceeds Pc.Similar behavior reported for two other cases of embankments built on deposits of the same origin, leads to the conclusion that the stage construction method is not suitable for embankment on cemented clays, at least those from the glacial lake Barlow–Ojibway, because no gain in shear strength is recorded over a normal consolidation period.

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Undrained shear behaviour of sands subjected to large shear displacement and estimation of excess pore-pressure generation from drained ring shear tests

Canadian Geotechnical Journal ◽

10.1139/t05-017 ◽

2005 ◽

Vol 42 (3) ◽

pp. 787-803 ◽

Cited By ~ 19

Author(s):

Yasuhiko Okada ◽

Kyoji Sassa ◽

Hiroshi Fukuoka

Keyword(s):

Pore Pressure ◽

Triaxial Compression ◽

Excess Pore Pressure ◽

Compression Tests ◽

Shear Tests ◽

Ring Shear ◽

Ring Shear Tests ◽

Triaxial Compression Tests ◽

Undrained Shear ◽

Excess Pore

Undrained shear behaviour of fine silica and weathered granitic sand subjected to large shear displacement is examined. Parallel experiments using ring shear and the triaxial compression tests on soil specimens through a wide range of initial void ratios were conducted to investigate undrained shear strength as the key factor in the flow-like motion of landslides. The steady-state undrained shear strengths achieved in ring shear tests were, in general, smaller than those in the triaxial compression tests, probably because of the excess pore-pressure generation by grain crushing within the shear zone that occurred in ring shear. Very low steady-state shear strengths were achieved, however, in triaxial compression tests on the dense silica sand in which well-defined shear surfaces developed in the cylindrical specimens. In these triaxial compression tests, shear deformation must have been concentrated on these surfaces to generate excess pore pressure similar to that found in ring shear tests. An attempt was made to estimate excess pore pressure generated in undrained ring shear tests using the results of drained ring shear tests. The equivalent normal stress calculated as the ratio of volumetric strain in the drained test to the coefficient of volume change was introduced as a parameter for the estimation of excess pore-pressure generation for the large shear displacement that is usually found in landslides. Equivalent normal stress from drained tests was almost the same as the generated excess pore pressure in undrained tests with up to 1 m of shear displacement, at which the steady state was reached.Key words: undrained shear strength, excess pore pressure, equivalent normal stress, ring shear test, triaxial compression test, liquefaction.

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Effective stress paths and pore-pressure responses during undrained shear along the bedding planes of varved Fort William Clay

Canadian Geotechnical Journal ◽

10.1139/t91-097 ◽

1991 ◽

Vol 28 (6) ◽

pp. 804-811 ◽

Cited By ~ 4

Author(s):

K. D. Eigenbrod ◽

J. B. Burak

Keyword(s):

Pore Pressure ◽

Deformation Behaviour ◽

Vertical Axis ◽

Elastic Behaviour ◽

Triaxial Tests ◽

Stress Range ◽

Bedding Planes ◽

Stress Levels ◽

Anisotropic Elastic ◽

Undrained Shear

Previous studies of the varved sensitive clays from the Thunder Bay area indicated that the geotechnical properties were influenced by their layered structure. When sheared along the bedding planes, double strength envelopes were apparent, with strength parameters depending on the stress levels applied. Thus, a series of consolidated isotropical undrained triaxial tests with pore-pressure measurements was carried out on specimens with the varves inclined to the vertical axis. A number of unusual results were observed: (i) low B-values for presumably saturated samples; (ii) failure of the specimens along the clay seams at stress levels above precompression load, exhibiting highly compressive behaviour and considerable cohesion; (iii) failure of the specimens in the silt seams at stress levels below the precompression load, exhibiting dilatancy and a lower cohesion than in the normally consolidated range; (iv) slightly anisotropic elastic deformation behaviour almost up to failure for testing in the precompression range; however, strongly anisotropic, largely nonelastic behaviour in the normally consolidated stress range, indicating larger stiffnesses vertically than horizontally. The stress paths in the normally consolidated stress range suggested also that consolidation of the clay seams occurred during undrained shear owing to internal dissipation of pore pressures into dilating silt seams. Key words: varved sensitive cemented clay, pore-pressure response, undrained shear, internal pore-pressure dissipation, anisotropic elastic behaviour, critical-state conditions.

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