Stress-History Effects on Shear Modulus of Soils

Scouring of soil around large-diameter monopile will alter the stress history, and therefore the stiffness and strength of the soil at shallow depth, with important consequence to the lateral behavior of piles. The existing study is mainly focused on small-diameter piles under scouring, where the soil around a pile is analyzed with two simplified approaches: (I) simply removing the scour layers without changing the strength and stiffness of the remaining soils, or (II) solely considering the effects of stress history on the soil strength. This study aims to investigate and quantify the scour effect on the lateral behavior of monopile, based on an advanced hypoplastic model considering the influence of stress history on both soil stiffness and strength. It is revealed that ignorance about the stress history effect (due to scouring) underestimates the extent of the soil failure wedge around the monopile, while overestimates soil stiffness and strength. As a result, a large-diameter pile (diameter D = 5 m) in soft clay subjected to a souring depth of 0.5 D has experienced reductions in ultimate soil resistance and initial stiffness of the p-y curves by 40% and 26%, and thus an increase of pile head deflection by 49%. Due to the inadequacy to consider the stress history effects revealed above, the existing approach (I) has led to non-conservative estimation, while the approach (II) has resulted in an over-conservative prediction.

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Shear modulus and damping of soft Bangkok clays

Canadian Geotechnical Journal ◽

10.1139/t02-048 ◽

2002 ◽

Vol 39 (5) ◽

pp. 1201-1208 ◽

Cited By ~ 27

Author(s):

Supot Teachavorasinskun ◽

Pipat Thongchim ◽

Panitan Lukkunaprasit

Keyword(s):

Shear Modulus ◽

Seismic Analysis ◽

Damping Ratio ◽

Soft Clay ◽

Cyclic Stress ◽

Large Strains ◽

Cyclic Triaxial ◽

Stress History ◽

Load Frequency ◽

Equivalent Shear Modulus

The shear modulus and damping ratio of undisturbed Bangkok clay samples were measured using a cyclic triaxial apparatus. Although abundant literature on this topic exists, selection of the most suitable empirical correlation for a seismic analysis cannot be done unless site specific data are obtained. The apparatus used in this research can measure the stressstrain relationships from strain levels of about 0.01%. The equivalent shear modulus measured at these strains was about 80% of the value obtained from the shear wave velocity measurements. The degradation curves of the equivalent shear modulus fell into the ranges reported in the literature, for clay having similar plasticity. The damping ratios varied from about 45% at small strains (0.01%) to about 2530% at large strains (10%). The effects of load frequency and cyclic stress history on the shear modulus and damping ratio were also investigated. An increase in load frequency from 0.1 to 1.0 Hz had no influence on the shear modulus characteristic, but it did result in a slight decrease in the damping ratio. The effects of the small amplitude cyclic stress history on the subsequently measured shear modulus and damping ratio were almost negligible when the changes in void ratio were taken into account.Key words: soft clay, shear modulus, damping ratio, cyclic triaxial test, cyclic stress history.

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A semi-empirical relationship for the small-strain shear modulus of soft clays

E3S Web of Conferences ◽

10.1051/e3sconf/20199204005 ◽

2019 ◽

Vol 92 ◽

pp. 04005

Author(s):

Vashish Taukoor ◽

Cassandra J. Rutherford ◽

Scott M. Olson

Keyword(s):

Shear Modulus ◽

Effective Stress ◽

Void Ratio ◽

Empirical Relationship ◽

Small Strain ◽

High Plasticity ◽

Bender Element ◽

Stress History ◽

Small Strain Shear Modulus ◽

Semi Empirical

The small-strain shear modulus (Gmax) is a soil property that has many practical applications. The authors compiled a database of Gmax measurements for 40 normally consolidated to slightly overconsolidated low to high plasticity clays. Using these data, the authors propose a semi-empirical relationship between Gmax, effective stress (σ'v or σ'c), preconsolidation stress (σ'p) and in-situ void ratio (e0) for four ranges of plasticity index (Ip): Ip < 30%, 30% ≤ Ip < 50%, 50% ≤ Ip < 80% and 80% ≤ Ip < 120%. With results from bender element tests on a Gulf of Mexico clay subjected to multiple load-unload consolidation loops, the authors were able to validate the proposed relationships for 30% ≤ Ip < 50% and 50% ≤ Ip < 80%. The proposed relationship for 30% ≤ Ip < 50% and 50% ≤ Ip < 80% captures changes in laboratory Gmax resulting from variations in effective stress (σ'c), maximum past stress (σ'v,max), and void ratio. The proposed relationships are a simple and efficient tool that can provide independent insight on Gmax if the stress history of a clay is known, or on stress history if Gmax is known.

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