Dynamic Response and Soil Properties

Assessing the liquefaction potential of silts is a challenging problem for practicing engineers. A common approach to performing such assessments is to apply a fines content "correction" factor to field-based correlations that were developed primarily for sands that relate cyclic resistance and in-situ indices (i.e., standard penetration test (SPT) N-value, cone penetration test (CPT) q_c-value, or shear wave velocity (V_S)). This paper compares the cyclic resistance estimated using these methods for a site in Providence, Rhode Island which is characterized by deep deposits of non-plastic silt. Standard penetration tests with energy measurements were performed in conjunction with seismic cone penetration tests, and representative samples were collected for laboratory testing. A site-specific V_S-based cyclic resistance correlation was developed from cyclic triaxial tests with shear wave velocity measurements and was used as a baseline for comparing the results of current SPT- and CPT-based simplified approaches. The analysis suggests that the existing SPT- and CPT-based methods, provide reasonable predictions of cyclic resistance of the non-plastic silt when the recommended fines content corrections are applied.

The potential for soil liquefaction in a gravelly alluvium beneath a large embankment dam was assessed in this case history. Clearwater Dam is located in southeastern Missouri, near the New Madrid seismic zone. The 154-ft high dam is built on a 30-ft thick deposit of saturated, gravelly, alluvial soils; previous studies indicated that this foundation soil was susceptible to liquefaction. However, gravelly soils are difficult to characterize using conventional field test methods. In 2005, the potential for liquefaction during an Operating Basis Earthquake was re-evaluated using short-interval SPTs and large diameter samples. The results showed that although gravel was present throughout, characterization of the deposit based on data from historic site borings was representative. Because the gravel-sized particles appear to be sufficiently dispersed in the finer matrix and small enough relative to the sampler diameter, properly interpreted SPT data can be used to assess liquefaction potential in this particular deposit. For the design earthquake used in the 2005 study, significant liquefaction of the foundation soils is judged to be unlikely.

Fully coupled, porous solid-fluid formulation and related modeling and simulation issues are presented in this work. In particular, coupled dynamic field equations with u – p – U formulation are used to simulate pore fluid and soil skeleton responses. Present formulation allows, among other features, for water accelerations to be taken into account. This proves useful in modeling dynamic interaction of media of different stiffness (as in soil-foundation-structure interaction). Fluid compressibility is also explicitly taken into account, thus allowing excursions into modeling of limited cases of non-saturated porous media. One of the main challenges in modeling saturated soils is the appropriate modeling of elastic-plastic behavior of soil skeleton. This challenge is met with the use of an advanced material model and is discussed at some length. Illustrative examples describing dynamical behavior of porous media (saturated soils) are presented. Of particular interest is the verification and validation (V&V) of current models and simulations, and examples will be used for that purpose.

This paper deals with pre- and post-liquefaction behaviour of a silica sand tested under simple shear (SS) loading conditions. Post-liquefaction response was investigated taking into account two main features: evaluation of liquefaction-induced volumetric strains resulting from the "dissipation" of pore water pressures; assessment of re-liquefaction characteristics of sand which has previously experienced liquefaction under the first loading. To this purpose, loose and dense reconstituted specimens were subjected to a first cyclic loading in undrained conditions. After reconsolidation, cyclic shear stresses with the same amplitude were again applied to the specimens (second loading or re-loading). With respect to the first liquefaction, a relationship between the normalized cyclic shear stresses and the normalized phase transformation strength obtained from monotonic tests was established regardless of relative density of material. The liquefaction-induced deformations obtained after re-consolidation were found to be well in agreement with those predicted by other approaches proposed in literature. Finally, it was also found that the change in liquefaction resistance of liquefied sand was affected by the relative density of the tested sand.

The technique of using paleoliquefaction features for seismic analysis involves systematically searching for liquefaction features throughout large geographic areas, and then using those findings to quantitatively back-calculate the causative strength of shaking, i.e., earthquake magnitude (M) and peak ground acceleration (pga). Despite important contributions from previous studies to assessing regional seismic hazards, all paleoliquefaction studies performed to date are hampered by issues of uncertainty. These uncertainties relate to both the techniques used for back-analysis, but also to the collection and interpretation of field data. In this paper, we outline many of these uncertainties and briefly present preliminary efforts to quantify them.

Many regional liquefaction hazard maps are predominantly based on surficial geology to identify susceptible units. Geologic units are characterized by their age, depositional environment, and mapped liquefaction effects from previous earthquakes. In this study, we evaluate the distribution of liquefaction potential index (LPI) values with respect to surficial geologic unit in three regions: Alameda County, California, San Bernardino County, California, and the Evansville area, Indiana. These three areas have been characterized with a dense set of cone penetration test (CPT) data by the U.S. Geological Survey (USGS). We find the LPI distributions within a given geologic unit are highly variable and significant overlap exists between geologic units. As an additional tool, we can use geostatistical methods to interpolate between calculated LPI values to evaluate the variability of liquefaction potential both within and across geologic units. Direct interpolation of liquefaction potential enables us to map variations within a single geologic unit, which may be useful for geologic units that have variable soil types or other characteristics that affect liquefaction potential.

In a typical seismic dam safety evaluation of an existing generally competent dam on potentially liquefiable foundation, SPT, CPT, BPT or V_s tests are often conducted in the vicinity of toes of dams to infer if any liquefiable soils may exist in the foundation of the dam footprint. In order to use the simplified procedure for liquefaction potential evaluation, the data from these field tests are normalized to a reference vertical overburden stress. In the current practice, "free-field" condition is generally assumed in the determination of normalized penetration resistance and shear wave velocity, neglecting rotation of principal stresses in soils in the vicinity of big structures. This paper attempts to incorporate these effects using a simple finite element model for a case of a generic earth dam. It is proposed that effective mean stresses be used while normalizing penetration resistances and V_s instead of vertical effective stresses based on free-field assumption. A simple finite element model can be used to estimate effective mean stresses under or in the vicinity of a structure.

A 6.8M earthquake occurred in Niigata in October 2004 caused a huge damage that includes triggering of hundreds of landslides. Some of those landslides caused natural damming of the associated streams. Higashi Takezawa is one of the largest landslides that caused natural damming. Shear strengths of soil layers above and below the sliding surface of that landslide were remarkably different. Pseudo static analysis of the landslide showed that about 700 gal of peak ground acceleration (PGA) was enough to cause the slide moving at the time of earthquake. After the mobilization of residual shear strength, the stability was calculated and found to be reduced considerably, and about 350 gal of PGA was enough to keep the mass moving at that condition. However, if the earthquake has occurred in dry season or if the soil mass has not been softened by the excessive rainfall during the typhoon, the earthquake of that magnitude is not sufficient to cause failure. Dynamic safety factor of the landslide decreased exponentially with the increase in water pressure. This study is very useful to evaluate the stability of other potential landslides so that precaution can be taken against disaster during next earthquake events.

The objective of the author's on-going research is to explore the feasibility of determining reliable in situ curves of shear modulus as a function of strain using the dynamic test. The purpose of this paper is limited to investigating what material stiffness is measured from a dynamic test, focusing on the harmonic excitation test. A one-dimensional discrete model with nonlinear material properties is used for this purpose. When a sinusoidal load is applied, the cross-correlation of signals from different depths estimates a wave velocity close to the one calculated from the secant modulus in the stress-strain loops under steady-state conditions. The variables that contributed to changing the average slope of the stress-strain loop also influence the estimate of the wave velocity from cross-correlation.

Current design practice for structures subject to earthquake loading regards dynamic Soil-foundation-Structure Interaction (SFSI) to be beneficial to the behavior of structures. Including the flexibility of the foundation and soil reduces the overall stiffness of a SFS system and can therefore reduce peak loads caused by a given ground motion. This might be true in (some) many cases. However, there is the possibility of SFS system going into resonance with the exciting earthquake motions as a result of a shift of the natural frequencies of the SFS-system. This can lead to much larger inertial forces acting on a structure. In that case, the SFSI is not beneficial, but rather detrimental to the seismic response of the structures (and of the SFS system). In this paper we present methodology and an analysis of beneficial and detrimental effects SFSI can have on seismic response. It will be shown that an interaction between SFS system and the seismic motions plays crucial role in determining if SFSI will be beneficial or detrimental. In other words, SFSI can be both beneficial and detrimental for a particular SFS system, depending on the characteristics of the seismic motions exciting the system.

Design of foundations in earthquake prone areas needs special considerations. Shallow foundations may experience a reduction in bearing capacity and increase in settlement and tilt due to seismic loading. The reduction in bearing capacity depends on the nature and type of soil and ground acceleration parameters. In the case of piles, the soil-pile behavior under earthquake loading is generally non-linear. The nonlinearity must be accounted for by defining soil-pile stiffness in terms of strain dependent soil modulus. Several behavioral and design aspects of shallow foundations and pile groups subjected to earthquakes have been critically reviewed.

The ground vibration mitigation measures for the passage of high-speed trains on soft ground are reviewed in this paper. The vibration characteristics induced by high-speed trains in the bridge structures and the related vibration criteria which house vibration-sensitive instruments and tools are introduced. Various vibration mitigation measures are reviewed for their screening effect from the related numerical studies. Based on the numerical studies, the open trench may be potentially effective in reducing the vibration level; however, it has bad influence on the stability of the railway or building structures, and it is also difficult to maintain after installation. The hard-type in-filled trench with the depth greater than Rayleigh wavelength of soils is considered to be more practical for the use of vibration mitigation measures in areas under soft ground conditions. This study also attempts to develop a design procedure and engineering evaluation for the selection of these vibration mitigation measures.

Little research has been done to understand the behavior of the soil during vibratory compaction and quality assurance and quality control (QA/QC) testing with light weight deflectometer (LWD) devices. A better understanding of the dynamic behavior of the soil in response to the loadings imparted by these devices will aid in the development of continuous compaction control (CCC) as well as the refinement and acceptance of rapid spot test devices such as the LWD. To this end, earth pressure cells (EPCs) and modified linear voltage displacement transducers (LVDTs) were installed between lifts of lightly plastic clay to measure in-situ stress and strain. With these sensors, the ground response was subsequently recorded during vibratory compaction. In addition, data was recorded during post compaction QA/QC testing with a prototype LWD device. The importance of EPC spacing, laboratory calibration, and field installation along with LVDT modification and field installation are discussed in detail. Preliminary vertical stress and strain results from a single layer study are presented.

Silica gel, a typical desiccant widely used in the industry to absorb moisture, has been used as transparent soil in geotechnical research. It is a porous inert colloid form of silica made in different sizes in a beaded or angular shape. The research presented in this paper aims to investigate the dynamic properties of silica gel ranging from 0.5 to 5 mm using a resonant column device. The purpose of this study is to examine the similarity of dynamic behaviors between silica gel and sand so that the silica gel mixed with appropriate pore fluids can be used to simulate a soil-foundation structure system in shaking table tests. Silica gel was tested under different confining pressures (50 kPa, 100kPa, 200kPa, and 400kPa) at two different void ratios representing loose and dense states, respectively. The test results show that the dynamic properties of silica gel are different from those of sand. A possible reason for this is the highly yielding behavior of silica gel.

Material attenuation is an important dynamic engineering property of soil. Previous seismic surface wave methods for estimating this property have relied on an active source of energy that must be deployed at the site. This paper introduces a passive source method of estimating material attenuation of Rayleigh seismic surface waves. The method utilizes the estimated direction of arrival, phase and amplitude of a wave impinging on a two-dimensional array of sensors to estimate the material attenuation. To demonstrate the procedure, the material attenuation and wavefield models will be shown with simulated and experimental data. Finally, several considerations regarding limitations of this method will be discussed. The procedure is shown to yield good results when applied to the experimental data under consideration.

This research deals with the question of whether, and under what circumstances, the presence of underground structures should be taken into account for the earthquake resistant design of neighbouring surface structures. In order to investigate the effect of underground structures on surface seismic motion, a series of dynamic plane-strain numerical analyses were conducted, considering a circular tunnel embedded in a viscoelastic half-space, and a harmonic SV-wave excitation. The numerical methodology, based on the Finite Difference Method, aims at quantifying the effect of the soil medium characteristics, excitation frequency, tunnel diameter, depth of construction, and relative flexibility of the lining compared to that of the surrounding soil. Conclusions contain preliminary criteria identifying the cases when the presence of an underground structure should be considered in the design of a surface structure.