Small-strain Stiffness

A method is proposed to estimate the degradation of the first natural frequency of vibration of earthen dams with increase in strain levels induced due to seismic events. A synthesised wave, referred to as the 'sum of sines', generated by the superposition of sinusoidal waves with frequencies ranging from 0.01 Hz to 25 Hz, is scaled to different peak accelerations and used to simulate seismic excitations at the base of the dam. The natural frequency is then determined by studying the response of the structure in the frequency domain. For this study, four dams with varying complexities of geometry, constituent material properties, and known natural frequencies were at first selected from previously published literature to establish the validity of the proposed method for determining natural frequencies of the dams at small strain levels. Plane strain models of these structures were constructed and analysed, using a commercially available finite element method-based software that is capable of performing time-history analyses. Results from the analyses indicate a good agreement between the natural frequencies predicted using the proposed method and the frequency values reported in the literature for the corresponding structures at small strain levels. The method was later used to determine the strain-dependent natural frequency of a hydraulic-fill dam in North Texas. Twenty-one different earthquake conditions, with different peak ground accelerations, frequency contents, and mean periods, were used to thoroughly validate the applicability of the developed methodology. Numerical analyses indicate that the strain-dependent variation of the first natural frequency follows a similar trend as that obtained using the 'sum of sines' excitation when the dam is subjected to widely different earthquake conditions.

This paper illustrates the use of the seismic dilatometer (SDMT) to assess the decay of in-situ stiffness withstrain level in different soil types. The approach adopted in this study relies on the ability of the SDMT to provide routinely at each test depth both a small strain stiffness (G0 from VS) and a working strain stiffness (constrained modulus MDMT derived from the usualDMTinterpretation). At various test sites, working strain DMT moduli are compared with reference stiffness decay curves back-figured from (i) the behavior observed under a full-scale test embankment (at Treporti) or footings (in Texas), (ii) from laboratory tests (at L’Aquila, Fucino plain and Po plain) and (iii) various combinations of insitu and laboratory testing techniques (Western Australia). Typical ranges of the shear strains DMT associated with working strain DMT moduli are inferred to assist construction of stiffness - strain decay curves for different soil types.

The shear modulus of cemented soils at very small strain (G 0) was studied. For artificially cemented clay, G 0 was found to be independent of the mean effective stress until the yield stress. After yield, a significant effect of structure degradation on G 0 was observed. The experimental data were interpreted by an equation, which relates G 0 of cemented soils to mean stress, apparent overconsolidation ratio and the state of structure (sensitivity). The equation was also found to represent G 0 of cemented sands.

The quality and acceptability of the results of the spectral analysis of surface waves (SASW) field test are dependent upon the coherence function, in conjunction with the transfer function. The coherence value is a measure of the quality of the test, and the transfer function denotes the phase lag between signals that are received by the geophones. This article presents a methodology for obtaining high-quality field test data, using a source capable of producing constant impact energy for several repeated impact strikes in an SASW test. Both laboratory and field investigations were performed to assess the effectiveness of the constant impact energy source at improving the coherence value. Tests were conducted on the surface of the soil compacted in a metal box and on the crest of an earthen dam using (a) handheld hammers, resulting in variable impact energy and (b) a drop hammer, dropped from a predetermined fixed height, resulting in constant impact energy. The variation in the shear wave velocity (Vs) profile obtained using the two testing methodologies and its impact on the seismic response analysis of an earthen embankment structure were studied. The SASW tests performed in the laboratory using constant impact energy were more efficient and repeatable than those performed using varying impact energy, and the results showed increased coherence values over a wide range of frequencies. A similar improvement in coherence data was observed in the field studies, and the Vs profiles were found to be significantly different for tests conducted using both methodologies. It was observed that the peak and spectral accelerations at the crest of embankments are significantly different when embankments with different Vs profiles are subjected to seismic excitation. This study emphasizes the importance of performing the SASW test, using a constant impact energy source to obtain a reliable estimate of Vs profiles of subsurface layers.

This paper illustrates the use of the seismic dilatometer (SDMT) to assess the decay of in-situ stiffness with strain level in different soil types. The approach adopted in this study relies on the ability of the SDMT to provide routinely at each test depth both a small strain stiffness (G0 from VS) and a working strain stiffness (constrained modulus MDMT derived from the usual DMT interpretation). At various test sites, working strain DMT moduli are compared with reference stiffness decay curves back-figured from (i) the behavior observed under a full-scale test embankment (at Treporti) or footings (in Texas), (ii) from laboratory tests (at L’Aquila, Fucino plain and Po plain) and (iii) various combinations of insitu and laboratory testing techniques (Western Australia). Typical ranges of the shear strains DMT associated with working strain DMT moduli are inferred to assist construction of stiffness strain decay curves for different soil types.

Bender/extender element testing of unsaturated compacted speswhite kaolin samples was used
to investigate the influences of mean net stress p , suction s , specific volume v and degree of saturation Sr
on shear and compression wave velocities, Vs and Vp , and hence on small strain elastic values of undrained
shear modulus and undrained constrained modulus. Testing was performed in a suction-controlled triaxial
cell and involved combinations of isotropic loading and unloading stages and wetting and drying stages, in an
attempt to isolate the influence of each unsaturated state variable. The results showed that increases of Vs are
caused by increases of p and decreases of v . Combined increases of s and decreases of Sr during drying
also lead to increases of Vs, and conversely combined decreases of s and increases of Sr during wetting lead
to decreases of Vs. Separating the effects of changes of s and Sr on Vs was not possible, because the evidence
on the influence of a change of Sr at constant s was contradictory. Increases of Vp are caused by increases
of p and Sr and decreases of v , whereas the influence of a change of s at constant Sr is unclear.

This paper illustrates the use of the seismic dilatometer (SDMT) to assess the decay of in-situ stiffness with strain level in different soil types. The approach adopted in this study relies on the ability of the SDMT to provide routinely at each test depth both a small strain stiffness (G0 from VS) and a working strain stiffness (constrained modulus MDMT derived from the usual DMT interpretation). At various test sites, working strainDMTmoduli are compared with reference stiffness decay curves back-figured from (i) the behavior observed under a full-scale test embankment (at Treporti) or footings (in Texas), (ii) from laboratory tests (at L’Aquila, Fucino plain and Po plain) and (iii) various combinations of insitu and laboratory testing techniques (Western Australia). Typical ranges of the shear strains �DMT associated with working strain DMT moduli are inferred to assist construction of stiffness - strain decay curves for different soil types.