Nonlinear identification of the soil response at Dahan downhole array site during the 1999 Chi–Chi earthquake

Records of the 1999 Chi–Chi earthquake, provided by the Dahan downhole array were analyzed, and nonlinear hysteretic stress–strain relations in the soil layers were estimated by the method previously used for studying the response of soils during the 1995 Kobe and the 2000 Tottori earthquakes. The obtained models of the soil behavior were applied for evaluating changes of the shear moduli in the soil layers and for the nonlinear identification of the soil behavior at Dahan site during the Chi–Chi earthquake. We found that reduction of the shear moduli in the soil layers did not exceed 5%, and the soil response was virtually linear. The content of the nonlinear components in the soil response was about 5% of the intensity of the response, and it was mostly due to the odd-order nonlinearities. A similarity in the stress–strain relations describing the behavior of soils during the 1995 Kobe earthquake, the 2000 Tottori earthquake, and the 1999 Chi–Chi earthquake was found, which indicates the possibility to describe the behavior of similar types of soils at different sites by similar stress–strain relations and predict soil behavior in future earthquakes.     

Effects of the constitutive relationship on seismic response of soils. Part II. The site amplification study

This paper is the first part of the general paper dealing with effects of constitutive modeling of cyclic stress–strain behavior of soils on site amplification. The paper concentrates on modeling of pseudo-static cyclic soil behavior in small to medium strain range. In order to fit the small strain data accurately, the chosen analytical stress–strain relationship should satisfy the specific small strain condition formulated for soils using the small strain data from the pseudo-static cyclic tests. Analysis of conventional relationships, in particular the Ramberg–Osgood (R–O) relationship, indicated that a failure to satisfy this condition lead to low accuracy of prediction of both tangent stiffness and damping ratio at small and medium strains. The logarithmic function originally proposed to describe static monotonic stress–strain behavior is applied to fit experimental cyclic backbone curves. Constructed to satisfy the formulated small strain condition for soils, this function has proven to be free from the limitations of the R–O and other relationships. When applied in combination with the Masing rules to predict damping ratios, it gives a good prediction in the small to medium strain range, where the Masing hypothesis is supported by experimental evidence.     

Cyclic shearing and liquefaction of soil under irregular loading: an incremental model for the dynamic earthquake-induced deformation

The paper presents a mathematical model for the deformation of soil under irregular cyclic loading in the simple-shear conditions. The model includes the possible change in the effective pressure in saturated soil due to the cyclic shearing, the reciprocal influence of the effective pressure on the response of the soil to the shear loading, and the pore pressure dissipation due to the seepage of the pore fluid. The hysteresis curves for the strain–stress relationship are constructed in such a way that they produce both the required backbone curve and the required damping ratio as functions of the strain amplitude. At the same time, the approach enables the constitutive functions involved in the model to be specified in various ways depending on the soil under study. The constitutive functions can be calibrated independently of each other from the conventional cyclic shear tests. The constitutive model is incorporated in the boundary value problem for the dynamic site response analysis of level ground. A numerical solution is presented for the dynamic deformation and liquefaction of soil at the Port Island site during the 1995 Hyogoken-Nambu earthquake.     

Stiffness degradation of natural fine grained soils during cyclic loading

Cyclic behavior of natural fine grained soils under a broad range of strains were investigated considering the effects of plasticity index and changes in confining pressures based on cyclic triaxial tests. A total of 98 stress controlled cyclic triaxial tests were conducted on normally consolidated and slightly overconsolidated samples. The investigation was divided into two parts. The first part consists of stress controlled cyclic triaxial tests under different stress amplitudes that were conducted to estimate the modulus reduction and the thresholds between nonlinear elastic, elasto-plastic and viscoplastic behavior. The second part involves the investigation of the undrained stress–strain behavior of fine grained soils under irregular cyclic loadings. The results showed that the elastic threshold is approximately equal to 90% of G_max. Another transition point was defined as the flow threshold where the value of tangent of shear modulus ratio changes for the second time. Simple empirical relationships to estimate the dynamic shear modulus and damping ratio was formulated and compared with the similar empirical relationships proposed in the literature. The results provide useful guidelines for preliminary estimation of dynamic shear modulus and damping ratio values for fine grained soils based on laboratory tests.     

Cyclic shear response of channel-fill Fraser River Delta silt

The cyclic shear response of a channel-fill, low-plastic silt was investigated using constant-volume direct simple shear testing. Silt specimens, initially consolidated to stress levels at or above the preconsolidation stress, displayed cyclic-mobility-type strain development during cyclic loading without static shear stress bias. Liquefaction in the form of strain softening accompanied by loss of shear strength did not manifest regardless of the applied cyclic stress ratio, or the level of induced excess pore water pressure, suggesting that the silt is unlikely to experience flow failure under cyclic loading. The cyclic shear resistance of the silt increased with increasing overconsolidation ratio (OCR) for OCR>1.3. The silt specimens that experienced high equivalent excess cyclic pore water pressure ratios (r_u>80%) resulted in considerable volumetric strains (2.5%–5%) during post-cyclic reconsolidation implying potentially significant changes to the particle fabric under cyclic loading.     

The effect of plastic fines on the pore pressure generation characteristics of saturated sands

Pore water pressure generation during earthquake shaking initiates liquefaction and affects the shear strength, shear stiffness, deformation, and settlement characteristics of soil deposits. The effect of plastic fines (kaolinite) on pore pressure generation in saturated sands was studied through strain-controlled cyclic triaxial tests. In addition to pore pressure generation, this experimental study also focused on evaluating the threshold shear strain for pore pressure generation and the volumetric compressibility of specimens during pore pressure dissipation. The results reveal that specimens having up to 20% plastic fines content generated larger values of pore water pressure than clean sand specimens. At 30% fines content, the excess pore water pressure decreased below that of clean sand. The threshold shear strain, which indicates the strain level above which pore pressures begin to generate, was assessed for different kaolinite–sand mixtures. The threshold shear strain was similar for 0–20% fines (γ_t0.006–0.008%), but increased to about 0.025% for 30% fines. The volumetric compressibility, measured after pore pressure generation, was similar for all specimens. The transition of behavior at fines contents between 20% and 30% can be attributed to a change in the soil structure from one dominated by sand grains to one dominated by fines.     

Shear moduli of volcanic soils

The shear modulus of soils is one of the important parameters in small strain level geotechnical problems (i.e. the study of earthquake effects and soil–structure interaction). In this paper, the shear moduli of crushable volcanic soils at small strain level were investigated in bender element and cyclic undrained triaxial tests. Comparison of results shows that the shear moduli from bender element tests agree well with those determined in cyclic triaxial tests. The influence of particle breakage, effective confining pressure, consolidation time, void ratio and fines content on the shear modulus are also discussed. In the present study, empirical equations for evaluating the shear modulus of granular materials with particle breakage are proposed based on the test results.     

Simulation of torsional shear test results with neuro-fuzzy control system

In the first part of this study, a series of stress-controlled hollow cylinder cyclic torsional triaxial shear tests were conducted on loose to medium dense saturated samples of clean Toyoura sand to investigate its liquefaction behavior. A uniform cyclic sinusoidal loading at a 0.1 Hz frequency was applied to air-pluviated samples where confining pressure and relative density was varied. Cyclic shear stress–strain changes, the number of cycles to reach liquefaction and pore pressure variations were recorded. Results indicate that the liquefaction resistances of uniform sands are significantly affected by the method of sample preparation and initial conditions.     

A modified parallel IWAN model for cyclic hardening behavior of sand

A modified parallel IWAN model, which includes a cyclic hardening function, is proposed and verified. The proposed model consists of elasto-perfect plastic and isotropic hardening elements. The model is able to predict cyclic hardening behavior through the adjustment of the internal slip stresses of its elements beyond the cyclic threshold, and satisfies Bauschinger's effect and the Masing rule with its own behavior characteristics. The cyclic hardening function is developed based on the irrecoverable plastic strain (accumulated shear strain) of dry sand during shearing, which is assumed to be a summation of shear strain beyond the cyclic threshold. Symmetric-limit cyclic loading and irregular loading tests were performed to determine model parameters and to verify the behavior of the proposed model. Finally, a one-dimensional site response analysis program (KODSAP) is developed by using the proposed model. The effects of cyclic hardening behavior on site response are evaluated using KODSAP.     

Simultaneous identification of time and space variant dynamic soil properties during the 1995 Hyogoken-Nanbu earthquake

Time and space variant soil properties at a liquefied site were simultaneously identified in the time domain by using borehole array strong motion records. During soil liquefaction at a site, soils usually show a wide variety of non-linear behavior along the depth as well as non-stationary behavior. Strong ground motion records were obtained at Port Island borehole array observatory, Kobe, during the 1995 Hyogoken-Nanbu earthquake. In this study, the instrumented soil was modeled by the equivalent linear MDOF system, and an extended Kalman filter with local iteration was employed for the identification of the soils. The identification process was successfully conducted, and the stress–strain relationships of the soils at the liquefied site were obtained from different depths all at once.     

Verification of predicted non-linear site response during the 1995 Hyogo-Ken Nanbu earthquake

The linear and non-linear responses of surface soil layers have been predicted through the simultaneous simulation test against the observed ground motions at the six sites in Kobe City during the 1995 Hyogo-ken Nanbu earthquake. The total stress analysis method and the effective stress analysis method have been applied for the rough and detailed verification of the predicted non-linear dynamic behavior at the PIS and RKI sites including the liquefaction phenomenon. The shear strain distribution along depth, the ratio of excess pore water pressure to initial effective stress, the liquefaction strength parameters to initial effective stress, and the stress–strain curve during the earthquake at the PIS site have been investigated when the predicted ground motion could simulate successfully the observed acceleration time histories and response spectra in the non-linear range.     

Liquefaction potential and strain dependent dynamic properties of Kasai River sand

The availability of efficient numerical techniques and high speed computation facilities for carrying out the nonlinear dynamic analysis of soil-structure interaction problems and the analysis of ground response due to earthquake loading increase the demand for proper estimation of dynamic properties of soil at small strain as well as at large strain levels. Accurate evaluation of strain dependent dynamic properties of soil such as shear modulus and damping characteristics along with the liquefaction potential are the most important criteria for the assessments of geotechnical problems involving dynamic loading. In this paper the results of resonant column tests and undrained cyclic triaxial tests are presented for Kasai River sand. A new correlation for dynamic shear damping (D_s) and maximum dynamic shear modulus (G_max) are proposed for the sand at small strain. The proposed relationships and the observed experimental data match quite well. The proposed relationships are also compared with the published relationships for other sands. The liquefaction potential of the sand is estimated at different relative densities and the damping characteristics at large strain level is also reported. An attempt has been made to correlate the G_max with the cyclic strength of the soil and also with the deviator stress (at 1% strain) from static triaxial tests.     

Computational modeling of cyclic mobility and post-liquefaction site response

Under seismic excitation, liquefied clean medium to dense cohesionless soils may regain a high level of shear resistance at large shear strain excursions. This pattern of response, known as a form of cyclic mobility, has been documented by a large body of laboratory sample tests and centrifuge experiments. A plasticity-based constitutive model is developed with emphasis on simulating the cyclic mobility response mechanism and associated pattern of shear strain accumulation. This constitutive model is incorporated into a two-phase (solid–fluid), fully coupled finite element code. Calibration of the constitutive model is described, based on a unique set of laboratory triaxial tests (monotonic and cyclic) and dynamic centrifuge experiments. In this experimental series, Nevada sand at a relative density of about 40% is employed. The calibration effort focused on reproducing the salient characteristics of dynamic site response as dictated by the cyclic mobility mechanism. Finally, using the calibrated model, a numerical simulation is conducted to highlight the effect of excitation frequency content on post-liquefaction ground deformations.     

A practical method for utilization of commercial cyclic testing apparatuses for computation of site response in central Adapazari

We suggest a practical method for estimating strain–modulus–damping relationships for utilization in equivalent-linear site response analyses, so that the necessity for more sophisticated sampling and testing procedures can be justified. The method employs the commercial cyclic testing apparatuses, which have limitations in low-strain ranges, and the in-situ seismic tests. The shear modulus at about 1% cyclic shear strain amplitude and the shear-wave velocity measured in-situ is used for building a hyperbolic relationship between shear stress and shear strain. An extension of Masing׳s rule and the constraint on hysteretic damping at 1% cyclic shear strain amplitude leads to a strain–damping relationship. By putting a particular emphasis on the soils of Adapazarı, a city famous for the concentrated damage on alluvium basin during the 1999 Kocaeli (Mw7.4) earthquake, we demonstrated the usefulness of the method, and concluded that the shear-modulus reduction and damping characteristics of Adapazarı soils can yield to site amplification factors greater than those predicted by strain–modulus–damping relationships presented in literature, and can more efficiently explain the concentration of damage on the alluvium basin. Through the comparisons of spectral amplification factors computed by equivalent-linear site response analyses, we justified the necessity to run a more sophisticated testing program on determination of cyclic stress–strain behavior of Adapazarı soils, and consequently to consider transient nonlinear site-response analyses in order to reduce the possible bias in calculation of spectral amplification factors.     

On the influence of the polarization and the shape of the strain loop on strain accumulation in sand under high-cyclic loading

This paper presents results of cyclic triaxial (CT) tests on sand with a simultaneous variation of the axial and the lateral stresses. Furthermore, a cyclic multidimensional simple shear (CMDSS) device and corresponding test results are discussed. For in-phase cycles with a constant strain amplitude it is demonstrated that the accumulation rate is independent of the polarization of the cycles in the strain space. Polarization changes lead to a temporary increase of the accumulation rate: they increase the effectiveness of compaction. For out-of-phase (e.g. elliptical) cycles the shape of the strain loop significantly influences the residual strain. A circular strain loop generates twice larger accumulation rates than a one-dimensional strain loop with identical span. The accumulation rate is not influenced by the circulation of the strain loop. It is shown that the direction of accumulation (so-called “cyclic flow rule”) is only moderately affected by the polarization and the shape of the cycles.     

Cyclic and post-cyclic monotonic behavior of Adapazari soils

The August 17, 1999 Kocaeli earthquake affected the city of Adapazari, which is located in the northwest of Turkey, with severe liquefaction and bearing capacity failures causing tilting of buildings, excessive settlements and lateral displacements. To understand the stress–strain behavior and pore pressure behavior of undisturbed soils during the earthquake, the cyclic and post-cyclic shear strength tests have been conducted on soil samples obtained from Adapazari in a cyclic triaxial test system within the scope of this research. Cyclic tests have been conducted under stress controlled and undrained conditions. Post-cyclic monotonic tests have been conducted following cyclic tests. The strength curves obtained in the experiments showed that the dynamic resistance of silty sand was found to be 45% lower than those of high plasticity soils (MH). The strength of clayey soils with the plasticity index of PI=15–16% was lower compared to the strength of high plasticity soils. Also, it was observed that silty sand soils had the lowest strength. The dynamic strength of the soils increased with the increase in plasticity.     

A united deformation-strength framework for Lightweight Sand–EPS Beads Soil (LSES) under cyclic loading

As a new artificial geo-material, lightweight soil is an ideal countermeasure to solve many geotechnical problems in soft ground by reducing upper load, and Lightweight Sand–EPS Beads Soil (briefly as LSES) is a typical representative. Generally used as backfill, which may bear earthquake loading or traffic loading, the behaviors of LSES under dynamic loading are worth to pay much attention on, though there are few relative studies about this at present. Based on laboratory cyclic triaxial test data presented in this study, a united framework is tried to set up, in which deformation and strength of LSES can be integrated by failure cycle number N_f that corresponds to the complete degradation of LSES structure. This framework describes both the development process of cyclic deformation and dynamic failure criteria in the state as the structure cannot afford further tension. Meanwhile, cyclic stress–strain relationships and Modulus Reduction Curves for LSES with different mixture ratios are also discussed. All distinct behaviors of LSES are considered to arise from its bond-dominated structure, which controls the two types of modulus reduction characteristics and brittle failure of LSES under cyclic loading.     

Volcano collapse promoted by progressive strength reduction: new data from Mount St. Helens

Rock shear strength plays a fundamental role in volcano flank collapse, yet pertinent data from modern collapse surfaces are rare. Using samples collected from the inferred failure surface of the massive 1980 collapse of Mount St. Helens (MSH), we determined rock shear strength via laboratory tests designed to mimic conditions in the pre-collapse edifice. We observed that the 1980 failure shear surfaces formed primarily in pervasively shattered older dome rocks; failure was not localized in sloping volcanic strata or in weak, hydrothermally altered rocks. Our test results show that rock shear strength under large confining stresses is reduced ∼20% as a result of large quasi-static shear strain, as preceded the 1980 collapse of MSH. Using quasi-3D slope-stability modeling, we demonstrate that this mechanical weakening could have provoked edifice collapse, even in the absence of transiently elevated pore-fluid pressures or earthquake ground shaking. Progressive strength reduction could promote collapses at other volcanic edifices.     

Pore-fluid pressures and frictional heating on a fault surface

This study considers the effects of heat transfer and fluid flow on the thernal, hydrologic, and mechanical response of a fault surface during seismic failure. Numerical modeling techniques are used to account for the coupling of the thermal, fluid-pressure, and stress fields. Results indicate that during an earthquake the failure surface is heated to a tempeature required for the thermal expansion of pore fluids to balance the rate of fluid loss due to flow and the fluid-volume changes due to pore dilatation. Once this condition is established, the pore fluids pressurize and the shear strength decreases rapidly to a value sufficient to maintain the thermal pressurization of pore fluids at near-lithostatic values. If the initial fluid pressure is hydrostatic, the final temperature attained on the failure surface will increase with depth, because a greater pressure increase can occur before a near-lithostatic pressure is reached. The rate at which thermal pressurization proceeds depends primarily on the hydraulic characteristics of the surrounding porous medium, the coefficient of friction on the fault surface, and the slip velocity. If either the permeability exceeds 10^–15 m² or the porous medium compressibility exceeds 10^–8 Pa^–1, then frictional melting may occur on the fault surface before thermal pressurization becomes significant. If the coefficient of friction is less than 10^–1 and if the slip velocity is less than 10^–2 msec^–1, then it is doubtful that either thermal pressurization or frictional melting on the fault surface could cause a reduction in the dynamic shear strength of a fault during an earthquake event.