Characterization of Spectral Analysis of Surface Waves Shear Wave Velocity Measurement Uncertainty

The spectral analysis of surface waves (SASW) method is a nondestructive test for characterization of the variation with depth of the shear modulus of soils. While the testing procedure is well developed, only one preliminary study has investigated measurement uncertainty associated with SASW, and the methods utilized to quantify measurement uncertainty were prohibitive to routine assessment. Knowledge of this uncertainty, and ability to include its assessment in routine testing, would allow for inclusion of SASW results in reliability-based design and in assessment of the spatial variability of shear modulus. In this study, a large sample of test data was collected from two test sites. Characteristic statistics, statistical distribution, and measurement uncertainty were determined for each phase of SASW. Using the empirical statistical properties and measurement uncertainty results as validation criteria, an analytically based uncertainty assessment system was developed. Phase angle, inverse phase angle, and phase velocity data typically display a coefficient of variation (COV) of 2%, and the COV for combined phase velocity data is typically 1.5%. The COV for shear wave velocity is typically between 5 and 10%, and thus the inversion appears to magnify measurement uncertainty. Phase angle, inverse phase angle, phase velocity, and combined phase velocity data are normally distributed. Shear wave velocity samples at a given depth are generally normally distributed. Using a small sample of experimental data and the analytically based process developed in this study, the measurement uncertainty of SASW test results can be assessed as part of routine testing.     

Determination of the Material Damping Ratio with the Bender Element Test

The bender element test has shown to be a reliable tool to determine the shear wave velocity. This paper aims to extend the bender element technique to also measure the material damping ratio in a triaxial cell. Three different interpretation techniques are developed for this purpose. The first technique uses a modal test of a cylindrical soil specimen in a triaxial cell, the second approach uses multiple arrivals of a bender element signal, and third, a self-correcting method using different travel paths is presented. These methods are applied on test results on undisturbed samples of a silt and a heavily overconsolidated clay, and show promising results for the assessment of the material damping ratio.     

Seismic Rotational Stability of Waterfront Retaining Wall Using Pseudodynamic Method

Design of waterfront retaining walls under seismic conditions is an important topic of research among the geotechnical engineering fraternity, and recently there have been studies in which the stability of rigid waterfront retaining walls has been assessed. However, an important aspect of seismic rotational stability of such walls is still missing from the literature archives. The present study shows the importance of rotational displacements for the design of the rigid waterfront retaining wall. Consideration has been made for the calculation of the hydrodynamic pressure as well as the seismic forces, both due to the seismic pressure and seismic wall inertia. These seismic forces have been calculated using the pseudodynamic approach. The free water condition has been considered in the analysis, and thus the hydrodynamic pressure has been considered to exist on the downstream face of the retaining wall as well, and a well-known expression approximating the effect of the inertia of the water due to the earthquake has been used for the estimation of this hydrodynamic pressure force. Simple expressions for the calculation of rotational displacement both during and after the earthquake have been proposed, and typical results have been obtained. It is observed that with an increase in the ratio of the water level to the total height of the wall from 0.50 to 1.00 the rotational displacement of the wall increases by about 110%. Similar trend of an increase in the value of the rotational displacement was observed for an increase in the values of the horizontal and vertical seismic acceleration coefficients. Also, the parametric study carried out in the analysis suggested that the rotational displacement is sensitive to other parameters such as the upstream water height, pore pressure ratio, soil, and wall friction angles. Due to nonavailability of the results in which rotational stability of the waterfront retaining wall under the seismic conditions has been studied, the results from the present analysis seem to bring out a unique approach.     

Use of Exact Solutions of Wave Propagation Problems to Guide Implementation of Nonlinear Seismic Ground Response Analysis Procedures

One-dimensional nonlinear ground response analyses provide a more accurate characterization of the true nonlinear soil behavior than equivalent-linear procedures, but the application of nonlinear codes in practice has been limited, which results in part from poorly documented and unclear parameter selection and code usage protocols. In this article, exact (linear frequency-domain) solutions for body wave propagation through an elastic medium are used to establish guidelines for two issues that have long been a source of confusion for users of nonlinear codes. The first issue concerns the specification of input motion as “outcropping” (i.e., equivalent free-surface motions) versus “within” (i.e., motions occurring at depth within a site profile). When the input motion is recorded at the ground surface (e.g., at a rock site), the full outcropping (rock) motion should be used along with an elastic base having a stiffness appropriate for the underlying rock. The second issue concerns the specification of viscous damping (used in most nonlinear codes) or small-strain hysteretic damping (used by one code considered herein), either of which is needed for a stable solution at small strains. For a viscous damping formulation, critical issues include the target value of the viscous damping ratio and the frequencies for which the viscous damping produced by the model matches the target. For codes that allow the use of “full” Rayleigh damping (which has two target frequencies), the target damping ratio should be the small-strain material damping, and the target frequencies should be established through a process by which linear time domain and frequency domain solutions are matched. As a first approximation, the first-mode site frequency and five times that frequency can be used. For codes with different damping models, alternative recommendations are developed.     

Correlation between Cyclic Resistance Ratios of Intact and Reconstituted Offshore Saturated Sands and Silts with the Same Shear Wave Velocity

The cyclic liquefaction resistance of intact medium dense specimens of sands and silts obtained from offshore platform sites was compared to that of specimens reconstituted to the same values of shear wave velocity. The shear wave velocity was measured using a new system that is comprised of torsional piezoelectric ceramic ring transducers mounted in a triaxial cell, a multiwave measuring device, and special watertight connectors. The relationship between cyclic resistance ratio and the number of cycles to liquefaction Nf of intact and reconstituted specimens was compared at the same values of consolidation pressure and shear wave velocity. There was good agreement between cyclic resistance ratios of intact and reconstituted specimens with similar values of shear wave velocity if liquefaction is defined as ? 6% peak-to-peak axial strain. The results of this study support the hypothesis that the cyclic liquefaction resistance of reconstituted specimens may be restored to in situ conditions when their shear wave velocity is restored to in situ values.     

Fast Stacking and Phase Corrections of Shear Wave Signals in a Noisy Environment

Hardware, software, and analysis of transient response of sources and receivers are presented for a piezoelectric bender element system designed to measure shear wave velocities in noisy environments. Signal-to-noise ratio is improved by signal stacking, wherein data vectors from many pulses are summed. A new fast-stacking algorithm enables signal quality to be improved much more rapidly than conventional stacking. Conventional stacking is accomplished by repeatedly sending an excitation pulse to a source, waiting for the signal and secondary reflections to pass the receiver and then introducing a subsequent excitation pulse. Using conventional stacking, it is important to wait for the signal and secondary reflections to die out before exciting subsequent pulses. In the new fast-stacking algorithm, a varied interval between consecutive pulses is used so that high quality signals can be obtained even if consecutive pulses are excited in rapid succession. Transient behavior of soil–bender interaction was characterized using closed-form analytical solutions of single-degree-of-freedom oscillators, numerical solutions using a beam-on-springs method, and measurements from an array of bender elements in a sand model. The time delay caused by soil–bender interaction was calculated to be half of the natural period of the bender element, and this theoretical time delay was supported by experimental data. This system makes it feasible to rapidly collect accurate shear wave velocity information so that transient changes in shear wave velocity can be monitored even if background noise is large.     

Shear-Wave Velocity Correlations for Puyallup River Alluvium

The suitability of selected cone penetration test-based shear-wave velocity correlations is assessed for use with Puyallup River alluvium in Puget Sound, Washington. The correlation models are found to be biased for Puyallup River alluvium; however, the functional forms of selected correlations were found to be readily adapted to local site conditions. The calibration for Puyallup River alluvium is presented and is found to perform satisfactorily for both down-hole SCPTu- and boring-based down-hole shear-wave velocity derived measurements. The distributions of prediction bias are presented for use in reliability studies. Recommendations are made for calibration of geologic-specific correlations using the functional forms of selected statistical shear-wave velocity regression models.     

On a Possible Role of Rayleigh Surface Waves in Dynamic Slope Failures

This contribution addresses the effect of a surface wave on the dynamic behavior of a slope. In particular, the interaction of a Rayleigh surface wave, possibly generated by an earthquake or nearby blasting, with a simple wedge-shaped slope is considered. A two-dimensional elastodynamic analysis suggests that the amplitudes and phase shifts of the surface waves reflected and transmitted at the crest strongly depends on the inclination of the slope face, and the superimposition of the reflected and incident waves may induce large stress amplification and thus produce open cracks in the top surface of the slope. The computational semianalytical results are used to investigate the generation mechanism of slope failure caused in the city of Sendai dynamically by the 1978 Miyagi-ken-oki, Japan, earthquake. Finally, the significance of the effect of Rayleigh wave propagation on dynamic slope stability is discussed in comparison with the influence of body waves.     

Seismic Compression of Two Compacted Earth Fills Shaken by the 1994 Northridge Earthquake

Seismic compression is defined as the accrual of contractive volumetric strain in unsaturated soil during strong shaking by earthquakes. We document and analyze two case histories (denoted school site and site A) of ground deformation from seismic compression in canyon fills strongly shaken by the Northridge earthquake. Site A had ground settlements up to about 18 cm, which damaged a structure, while the school site had settlements up to about 6 cm. For each site, we perform decoupled analyses of shear and volumetric strain. Shear strain is calculated using one-dimensional and two-dimensional ground response analyses, while volumetric strain is evaluated from shear strain using material-specific models derived from simple shear laboratory testing that incorporates important effects of fines content and as-compacted density and saturation. Analyses are repeated using a logic tree approach in which weights are assigned to multiple possible realizations of uncertain model parameters. At the school site, predicted settlements appear to be unbiased. At site A, the analyses successfully predict the shape of the settlement profile along a section, but the weighted average predictions are biased slightly too low. We speculate that the apparent site A bias can be explained by limited resolution of the site stratigraphy, bias in laboratory-derived volumetric strain models, and/or uncertainty in the estimated earthquake-induced settlements.     

Case Study: Seismic Upgrade of a Masonry Bell Tower Using Glass Fiber-Reinforced Polymer Ties

In many cases masonry buildings present structural problems related to development of local mechanisms under seismic actions. The solution to this weakness has to be chosen taking into account several aspects if the construction is ancient and is gifted of monumental significance. In this paper the case of the Bell Tower of Santa Maria del Carmine (Napoli, Italy) is discussed; the construction has been deeply examined by the writers performing experimental inquires in situ and theoretical analysis with three-dimensional models. While the results obtained in the hypothesis of compact behavior of the structure have pointed out a low risk condition under seismic actions, in contrast the study of local out-of-plane mechanisms, dealt with in detail in this paper, have evidenced an unsafe situation. To avoid such mechanisms, connective systems with tie rods made of glass fiber-reinforced polymer laminates have been designed to be inserted where local verifications are not satisfied. Design, application, and monitoring procedures of this innovative intervention are discussed in detail herein.     

Dynamic Modal Analysis and Stability of Cantilever Shear Buildings: Importance of Moment Equilibrium

The dynamic modal analysis (i.e., the natural frequencies, modes of vibration, generalized masses, and modal participation factors) and static stability (i.e., critical loads and buckling modes) of two-dimensional (2D) cantilever shear buildings with semirigid flexural restraint and lateral bracing at the base support as well as lumped masses at both ends and subjected to a linearly distributed axial load along its span are presented using an approach that fulfills both the lateral and moment equilibrium conditions along the member. The proposed model includes the simultaneous effects and couplings of shear deformations, translational and rotational inertias of all masses considered, a linearly applied axial load along the span, the shear force component induced by the applied axial force as the member deforms and the cross section rotates, and the rotational and lateral restraints at the base support. The proposed model shows that the stability and dynamic behavior of 2D cantilever shear buildings are highly sensitive to the coupling effects just mentioned, particularly in members with limited rotational restraint and lateral bracing at the base support. Analytical results indicate that except for members with a perfectly clamped base (i.e., zero rotation of the cross sections), the stability and dynamic behavior of shear buildings are governed by the flexural moment equation, rather than the second-order differential equation of transverse equilibrium or shear-wave equation. This equation is formulated in the technical literature by simply applying transverse equilibrium “ignoring” the flexural moment equilibrium equation. This causes erroneous results in the stability and dynamic analyses of shear buildings with base support that is not perfectly clamped. The proposed equations reproduce, as special cases: (1) the nonclassical vibration modes of shear buildings including the inversion of modes of vibration when higher modes cross lower modes in shear buildings with soft conditions at the base, and the phenomena of double frequencies at certain values of beam slenderness (L/r); and (2) the phenomena of tension buckling in shear buildings. These phenomena have been discussed recently by the writer (2005) in columns made of elastomeric materials.     

Scattering of Harmonic Waves by a Circular Cavity in a Porous Medium: Complex Functions Theory Approach

An analytical solution for the evaluation of scattering of waves by a circular cavity in infinite isotropic elastic porous media is presented. Two groups of complex functions for solid skeleton and pore fluid in a two-dimensional complex plane are introduced in order to solve the Biot equations. Stress, displacement, and pore pressure fields induced by incident and scattered waves in the medium and especially in the vicinity of the cavity are evaluated in this complex plane. The validation of the proposed solution is shown by various numerical examples. A parametric study including the effects of fluid compressibility changes, shear modulus, and permeability variations, several wave numbers, and wave types (fast, slow, and shear waves) is performed.     

Global Monitoring of Large Concrete Structures Using Acoustic Emission and Ultrasonic Techniques: Case Study

Global monitoring of civil structures is a demanding challenge for engineers. Acoustic emission (AE) is one of the techniques that have the potential to inspect large volumes with transducers placed in strategic locations of the structure. In this paper, the AE technique is used to characterize the structural condition of a concrete bridge. The evaluation of AE activity leads to information about any specific part of the structure that requires attention. Consequently, more detailed examinations can be conducted once the target area is selected. In this case, wave propagation velocity was used as a means to evaluate, in more detail, the condition of the region indicated by the AE analysis.     

Productivity of Ocean-Wave-Energy Converters: Turbine Design

A study was undertaken to identify which of a range of advanced Wells turbine configurations would maximize wave power productivity. The productivity is estimated of a monoplane with fixed guide vanes, a monoplane with variable-pitch blades, and a high- and low-solidity biplane with counterrotating rotors. Two control mechanisms are investigated for the variable pitch configuration. Raleigh distributions based on a mean annual pneumatic power rating of 500 kW are utilized to generate the short and long-term variations of input power to be matched with experimental turbine performance data obtained from a steady-state test rig. It was found that productivity was relatively insensitive to turbine configuration but that a low-solidity counterrotating turbine had the best performance characteristic providing high peak efficiency and gradual onset of stall.     

Scattering of SH Waves by Truncated Semicircular Canyon

The judicious region-matching technique is adopted to derive a series solution for shear horizontal waves scattering from a truncated semicircular canyon on the ground surface. A circular-arc auxiliary boundary is introduced to divide the analyzed region into two subregions. The antiplane motion of each subregion is represented in terms of an infinite series of cylindrical wavefunctions with unknown coefficients. By employing the Graf’s addition formula, boundary conditions on the curved canyon surface and continuity conditions on the auxiliary boundary, the unknown coefficients can be determined. Two modifications of the present theoretical derivation are done for the semicircular canyon and free surface cases. The plotted results reveal how the surface displacement amplitudes are influenced by varying the truncation depth.     

Static Cyclic Response of Masonry Walls Retrofitted with Fiber-Reinforced Polymers

The behavior of seven one-half scale masonry specimens before and after retrofitting using fiber-reinforced polymer (FRP) is investigated. Four walls were built using one-half scale hollow clay masonry units and weak mortar to simulate walls built in central Europe in the mid-20th century. Three walls were first tested as unreinforced masonry walls; then, the seismically damaged specimens were retrofitted using FRPs. The fourth wall was directly upgraded after construction using FRP. Each specimen was retrofitted on the entire surface of a single side. All the specimens were tested under constant gravity load and incrementally increasing in-plane loading cycles. The tested specimens had two effective moment/shear ratio, namely, 0.5 and 0.7. The key parameter was the amount of FRP axial rigidity, which is defined as the amount of FRP reinforcement ratio times its E modulus. The single-side retrofitting/upgrading significantly improved the lateral strength, stiffness, and energy dissipation of the test specimens. The increase in the lateral strength was proportional to the amount of FRP axial rigidity. However, using high amount of FRP axial rigidity led to very brittle failure. Finally, simple existing analytical models estimated the ultimate lateral strengths of the test specimens reasonably well.     

State of Research on Seismic Retrofit of RC Beam-Column Joints with Externally Bonded FRP

A considerable amount of research has been directed recently toward understanding and promoting the use of externally applied fiber-reinforced polymer (FRP) for the seismic retrofit of reinforced concrete (RC) structures. In this paper, a comprehensive review and synthesis of published experimental studies on the seismic rehabilitation of RC frame beam-column joints with FRP is presented, and the issues that need to be addressed for further research are discussed. In addition, the paper presents a simple design model for predicting the contribution of the FRP to the shear strength of retrofitted joints. The key element in the model is the derivation of an expression for the effective FRP strain, based on the calibration of test data reported in the literature. A total of 54 tests carried out worldwide were considered in the review, and a database of the published studies, encompassing all relevant design parameters, was assembled. The reported test results confirm the structural effectiveness of the FRP strengthening technique for the seismic retrofit of RC joints. However, there are some gaps which need to be addressed. For instance, there is a lack of a rationale explanation of the resistance mechanisms involved in the beam-column joints retrofitted with FRP. Such a rational explanation is a prerequisite for the development of more comprehensive and rigorous design procedure.     

Full Two-Dimensional Model for Rolling Resistance. II: Viscoelastic Cylinders on Rigid Ground

Following the previous paper, “Full two-dimensional model for rolling resistance: Hard cylinder on viscoelastic foundation of finite thickness,” addressing modeling of rigid cylinder rolling against viscoelastic foundation of finite thickness, this paper focuses on the development of a rigorous full two-dimensional semianalytical model for viscoelastic rollers with layered structure rolling against a rigid ground. In this model, the polar coordinate system is used, the solution is expanded into a set of Fourier series corresponding to the angular coordinate, the frequency domain master curves of G′ and G″ characterizing the general viscoelastic properties for a viscoelastic material are used to relate Fourier coefficients, and a special condensed structure model based on the Fourier series is developed to handle viscoelasticity and the rolling contact boundary condition. Examples are given to show the model capabilities to efficiently handle rolling resistance and contact stresses, and capture major characteristics of standing-wave phenomenon, such as sharp rise of rolling resistance, emergence of standing waves and material dynamic softening as the rolling speed approaches a critical value. The methodology may be of interest to industrial roller designers.     

ONDA: Computer Code for Nonlinear Seismic Response Analyses of Soil Deposits

This paper describes a newly developed computer code for performing one-dimensional nonlinear dynamic analysis (ONDA) of soil deposits. The code has been developed by revisiting the 1982 work by Ohsaki with the purpose of simulating the ground response to an earthquake of moderate intensity (i.e., values of peak ground acceleration on stiff soil on the order of 0.15 to 0.25g, which are typical of many sites in Italy). In the Ohsaki model a horizontally stratified soil deposit is idealized as a discrete mechanical system composed of a finite number of lumped masses connected with a series of springs and dashpots. Nonlinearity is modeled by assuming (1) a “backbone” curve that describes the initial monotonic loading of the stress-strain curve, and (2) a “rule” that simulates the unloading-reloading paths and stiffness degradation undergone by soil as seismic excitation progresses. Typically, the backbone curve is obtained from conventional cyclic undrained loading laboratory tests. The rule generally used is the so-called Masing criterion, which assumes that the unload-reload branches of the stress-strain curve have the same shape as the initial loading curve but are affected by a scale factor (n) equal to 2. In this work, the Masing criterion has been modified by assuming a scale factor (n) not necessarily equal to 2. It turns out that a factor n greater than 2 allows the simulation of cyclic hardening, while cyclic softening can be modeled by assuming decreasing values of n even smaller than 2. Pyke proposed in 1979 to use a scale factor (n) lower than 2 to simulate cyclic degradation. According to Pyke, the n parameter is a function of the mobilization factor. The generalization of the Masing criterion allows ONDA to properly simulate the phenomena of soil hardening and soil degradation, giving it the capability to compute the permanent strains developed during a seismic event. The procedure required to evaluate the model parameters is also described in the paper. Note that the laboratory tests examined gave values of n between 2 and 6 for a strain level not greater than 0.3%. In ONDA the numerical solution of the nonlinear equations of motion is obtained using the unconditionally stable Wilson θ algorithm (with θ ≥ 1.37). The new method has been used to predict the seismic response at two sites in Italy. For these case studies, the maximum input acceleration was not greater than 0.3g and the computed shear strains were less than 0.2%. The ONDA results have been compared with those computed with SHAKE, EERA (equivalent-linear analysis), and NERA (nonlinear analysis).     

Laboratory Investigation on Assessing Liquefaction Resistance of Sandy Soils by Shear Wave Velocity

Shear wave velocity (Vs) offers engineers a promising alternative tool to evaluate liquefaction resistance of sandy soils, and the lack of sufficient in-situ databases makes controlled laboratory study very important. In this study, semitheoretical considerations were first given based on review of previous liquefaction studies, which predicted a possible relationship between laboratory cyclic resistance ratio (CRRtx) and Vs normalized with respect to the minimum void ratio, confining stress and exponent n of Hardin equation. Undrained cyclic triaxial tests were then performed on three reconstituted sands with Vs measured by bender elements, which verified this soil-type-dependent relationship. Further investigation on similar laboratory studies resulted in a database of 291 sets of data from 34 types of sandy soils, based on which the correlation between liquefaction resistance and Vs was established statistically and further converted to equivalent field conditions with well-defined parameters, revealing that CRR will vary proportionally with (Vs1)4. Detailed comparisons with Vs-based site-specific investigations show that the present lower-bound CRR–Vs1 curve is a reliable prediction especially for sites with higher CSR or Vs1. The framework of liquefaction assessment based on the present laboratory study is proposed for engineering practice.