Threshold Shear Strain for Cyclic Degradation of Three Clays

Cyclic threshold shear strain, γt, is small cyclic shear strain amplitude above which soil properties significantly change with the number of cycles, N, and below which such changes are for all practical purposes negligible. To date, three cyclic threshold shear strains have been experimentally verified: for cyclic settlement (cyclic compression), for residual cyclic pore water pressure, and for cyclic stiffening. Subject of the paper is testing of fourth cyclic threshold shear strain for cyclic degradation, γtd. When fully saturated soil is subjected in undrained conditions to moderate or large cyclic strain-controlled loading, its secant shear modulus, Gs, decreases with N. This is quantified for given cyclic shear strain amplitude, γc, by degradation index, δ = GsN /Gs1, where GsN = Gs at cycle N. Index δ and N are related via degradation parameter t = −(logδ / logN) which measures the rate of cyclic degradation. At γc \u3c γtd there is no cyclic degradation and t = 0. If γc \u3e γtd cyclic degradation takes place and t \u3e 0. With a special simple shear device for small-strain testing the variation of t with γc was examined and γtd evaluated for three clayey soils. Results show that γtd increases with plasticity index, PI. For PI=12 γtd=0.015%, for PI=26 γtd=0.04, and for PI=47 clay γtd=0.05%. Testing procedure and comparison to other types of γt are presented

Cyclic Compression of Compacted Clayey Sand at Small Cyclic Strains

Ten drained cyclic strain-controlled direct simple shear tests were conducted on compacted low-plasticity clayey sand to measure its cyclic compression properties. The soil had 37 % fines, liquid limit of 28% and plasticity index 14. The relative compaction of specimens prior to consolidation and cyclic shearing was between 80 and 90 %. Cyclic compression is expressed as the accumulation of vertical strain with the number of cycles, N. Vertical strain recorded at the end of every cycle, ενc, increased with the cyclic shear strain amplitude, γc, and N. Such behavior is typical and has been obtained by others on other types of soils. Amplitude γc was relatively small, ranging between 0.008% and 0.24%. Such small cyclic strains are common in moderate and large earthquakes. The effects of the dry unit weight, γd, and corresponding void ratio, e, vertical consolidation stress, σνc, and certain aspects of the degree of saturation, S, on ενc are evaluated. The test results revealed that for the applied conditions ενc increases with σνc and e (decreases with γd) and is smaller if S is increased above approximately 90%. For this soil the cyclic threshold shear strain of about 0.02% was obtained. Simple mechanisms that most likely govern the cyclic compression of compacted soils are discussed

Development of Database of Cyclic Soil Properties from 94 Tests on 47 Soils

Cyclic properties of 47 soils were tested in several investigations between 1994 and 2004 in the standard Norwegian Geotechnical Institute (NGI) direct simple shear (DSS) device and an NGI-type dual-specimen DSS (DSDSS) device for small strain testing. In each investigation many cycles of different amplitude, c, and frequency, f, were applied at different levels of vertical stress, v, and overconsolidation ratio, OCR. In DSDSS device many consecutive series of different small c=0.0003-0.01% were applied on the same specimens without changing their structure, because at such small c cyclic shearing is nondestructive. Consequently, the vast amounts of small-strain data were generated. This necessitated the development of new approach to data processing and analysis. New procedure for reading, checking, organizing, combining, comparing and analyzing the vast arrays of cyclic test data has been developed and structured into a database that has the cyclic loop as its elementary unit. Each cyclic loop in the database is characterized by the soils’ plasticity index, moisture content, void ratio, degree of saturation, v, OCR, c, f, secant shear modulus, damping ratio, and the shape of cyclic straining. Using the database very large number of cyclic loops can be compared to instantly obtain graphical presentation of different behavioral trends. The structure of the database and its application is summarized

General Report Session 2: Model Testing in Cyclic Loading

Geotechnical earthquake engineering problems and similar soil dynamics problems usually involve random three directional cyclic loads, complicated stratification of soil deposits and geometry of supported structures, heterogeneity, anisotropy, and nonlinearity of soils, large degradation of soil stiffness and strength accompanied by equally large displacements and deformations, complex interaction between different soil deposits and between the foundation soil and the supported structure, etc. On the other hand, the empirical and analytical methods used in conjunction with standard field testing or standard laboratory cyclic testing on small specimens, which are currently available in engineering practice, are relatively simple and limited to ideal conditions. In most cases, such methods can provide only a rough estimate of the response of a foundation-structure system to seismic or similar cyclic loads. Consequently, large scale field and laboratory testing, testing of soil-structure models using shaking table, and centrifuge testing play an important role in the advancement of soil dynamics design methods. Such testing, however, in comparison with standard experimental and analytical soil dynamics investigations, is quite expensive. It requires special facilities with an adequate technical support and it involves team work. The affiliations of the authors in this session, as well as the number of the authors of some papers, show that such complex and expensive studies are done mainly by large corporations or large institutes in cooperation with universities

Damping at small strains in cyclic simple shear test

Cyclic tests were conducted to study damping properties of two reconstituted sands and three laboratory-made clays at small cyclic shear strain amplitudes of gamma(c) approximate to 0.001-0.04%, employing a recently developed constant-volume equivalent-undrained direct simple shear device for small-strain testing. The tests were strain-controlled with an approximately sinusoidal shape of cyclic straining. The effects of cyclic strain amplitude (gamma(c)), frequency of cyclic loading (f), plasticity index (PI), silt content, vertical effective consolidation stress (sigma(vc)') and overconsolidation ratio (OCR) on the equivalent viscous damping ratio, lambda, were investigated. The results show that, for a given gamma(c), lambda decreases with sigma(vc)' and OCR, but both of these effects become smaller if PI increases. The effect of f on lambda was not observed for f approximate to 0.01-0.1 Hz. The results also show that below gamma(c) approximate to 0.005%, lambda for clays is larger than lambda for sands, which is exactly the opposite of the trend above gamma(c) approximate to 0.005% established previously. Such a reversal of the trend of lambda with respect to the type of soil is explained by the relative contributions of soil nonlinearity and soil viscosity to the area of the hysteretic cyclic stress-strain loop at small versus large cyclic strains