Glenn J. Rix

A NON-RESONANCE METHOD FOR MEASURING DYNAMIC SOIL PROPERTIES

A non-resonance (NR) test method is introduced to determine dynamic soil properties at low strain amplitudes over a broad frequency range using a conventional resonant column/torsional shear (RC/TS) apparatus. The theoretical background of the NR method is presented and it is shown that the shear modulus and material damping ratio can be obtained from frequency response measurements between the applied torque and resulting rotational displacement of the specimen. By properly accounting for inertia effects, the NR method allows measurements at frequencies (e.g., 2–30 Hz) that fill the gap between conventional RC and TS tests. Experimental results are presented for aluminum alloy, polymethyl methacrylate (PMMA), and soil specimens over the frequency range of 0.01–30 Hz. Values for aluminum alloy and PMMA, used as calibration materials, obtained with the NR method agree well with reference values from the literature and help validate the approach. Tests on two soil specimens indicate that the NR method permits continuous measurements of shear modulus and material damping ratio of soils over a broad frequency range, which has the potential to yield improved understanding of viscoelastic soil behavior and provide dynamic soil properties over a range of frequencies appropriate for a variety of natural and man-made vibration sources.

Simultaneous measurement and inversion of surface wave dispersion and attenuation curves

Surface wave tests are non-invasive seismic techniques that have traditionally been used to determine the shear wave velocity (i.e. shear modulus) profile of soil deposits and pavement systems. Recently, Rix et al. [J. Geotech. Geoenviron. Engng 126 (2000) 472] developed a procedure to obtain near-surface values of material damping ratio from measurements of the spatial attenuation of Rayleigh waves. To date, however, the shear wave velocity and shear damping ratio profiles have been determined separately. This practice neglects the coupling between surface wave phase velocity and attenuation that arises from material dispersion in dissipative media. This paper presents a procedure to measure and invert surface wave dispersion and attenuation data simultaneously and, thus, account for the close coupling between the two quantities. The methodology also introduces consistency between phase velocity and attenuation measurements by using the same experimental configuration for both. The new approach has been applied at a site in Memphis, TN and the results obtained are compared with independent measurements.

Propagation of Data Uncertainty in Surface Wave Inversion

Although in recent years surface wave methods have undergone significant development that has greatly enhanced their capabilities, little effort has been spent to determine the uncertainty associated with surface wave measurements. The objective of this study is to determine how the uncertainty of the experimental data is mapped into the uncertainty of the shear wave velocity profile via the inversion algorithm. The methodology developed in this study for estimating the uncertainty of the shear wave velocity profile from surface wave measurements is based on the assumption that the experimental data are normally distributed. The validity of this hypothesis was experimentally verified using data gathered at two sites in Italy where surface wave tests were performed using linear arrays of multiple receivers. The experimental dispersion curve measured at the site was subsequently inverted to obtain the expected shear wave velocity profile together with an estimate of the associated standard deviation. The final results show that uncorrelated noise has a very little influence on multistation surface wave tests, confirming their robustness for applications in noisy environments.

Simultaneous Measurement of Surface Wave Dispersion and Attenuation Curves

In existing surface wave test procedures, experimental dispersion and attenuation curves are determined separately (i.e., uncoupled) using different source-receiver configurations and different interpretation methods. A new procedure based on displacement transfer functions is proposed in which dispersion and attenuation data are derived simultaneously (i.e., coupled) from a single set of measurements using the same source-receiver array. The new approach is motivated by the recognition that in dissipative media, Rayleigh phase velocity and attenuation are not independent as a result of material dispersion. Therefore, a coupled analysis of dispersion and attenuation is a more robust, fundamentally correct approach. The new approach is also more consistent with coupled inversion techniques to obtain the shear wave velocity and shear damping ratio profiles. The proposed approach is illustrated using data measured at a site in Atlanta, Georgia.

Overview of the 2010 Haiti Earthquake

The 12 January 2010 Mw 7.0 earthquake in the Republic of Haiti caused an estimated 300,000 deaths, displaced more than a million people, and damaged nearly half of all structures in the epicentral area. We provide an overview of the historical, seismological, geotechnical, structural, lifeline-related, and socioeconomic factors that contributed to the catastrophe. We also describe some of the many challenges that must be overcome to enable Haiti to recover from this event. Detailed analyses of these issues are presented in other papers in this volume.

Solution of the Rayleigh Eigenproblem in Viscoelastic Media

We present a technique for the solution of the complex-valued eigenproblem associated with the propagation of surface waves in general linear viscoelastic media. The new technique permits simultaneous determination of the Rayleigh dispersion and attenuation curves and the displacement and stress eigenfunctions for vertically heterogeneous, linear viscoelastic media with arbitrary values of material damping ratio. The technique is based on the Cauchy residue theorem of complex analysis that takes full advantage of the holomorphic properties of the Rayleigh secular function, which is viewed as an analytic mapping of the complex-valued Rayleigh phase velocity. Because the eigenvalue problem is solved directly in the complex domain with no simplifying assumptions, the algorithm implicitly accounts for the inherent coupling between phase velocity and attenuation of seismic waves as a result of material dispersion. The technique overcomes the limitations of previous algorithms that often break up the complex structure of the problem and/or require a priori information about the number of eigenvalues and their approximate value. The algorithm is validated via comparisons with closed-form solutions for a uniform half-space. Examples are also used to compare solutions obtained with the proposed technique and one based on the assumption of weak dissipation in strongly and weakly dissipative layered media.

Geotechnical site characterization in the greater Memphis area using cone penetration tests

Abstract The determination of seismic ground hazards in Memphis and Shelby County, Tennessee is facilitated by the use of electronic cone penetration tests that can provide up to four independent readings with depth from a single sounding. One series of soundings is being performed for site-specific mapping to determine the presence and extent of potentially-liquefiable sediments, in-situ soil resistance to liquefaction, and initial soil stiffness for ground motion amplification studies. Another series of soundings is being conducted in conjunction with field paleoliquefaction mapping in the New Madrid seismic zone to better define the intensity, magnitude, and geographic extent of ground failures caused by large past earthquake events, as well as information about the source sands. In this paper, an overview is given on the types of multi-channeled penetrometer data that are being collected, including vertical profiles of cone tip stress (qt), sleeve friction (fs), penetration porewater pressure (u1 or u2), downhole shear wave velocity (Vs), and/or electrical conductivity (ke). Representative soundings are presented from select sites to illustrate repeatability, data post-processing methods, and that derived downhole Vs profiles are generally in good agreement with non-invasive surface techniques at two Memphis test sites. Cyclic stress based procedures for liquefaction are discussed with relation to data from a paleoliquefaction site in Germantown, Tennessee, and estimates of the minimum magnitude of the historic event are discussed. While estimates of the earthquake magnitude are preliminary pending additional study on attenuation relationships and site response in the deep soils of the Mississippi Embayment, it is inferred from evaluation of in-situ test data using cyclic stress based techniques that the December 1811 New Madrid earthquake was likely larger than a Mw=7.5 event.

Geotechnical Aspects of Failures at Port-au-Prince Seaport during the 12 January 2010 Haiti Earthquake

Presented herein are the results of geotechnical investigations and subsequent laboratory and data analyses of the Port-au-Prince seaport following the Mw7.0 2010 Haiti earthquake. The earthquake caused catastrophic ground failures in calcareous-sand artificial fills at the seaport, including liquefaction, lateral spreads, differential settlements, and collapse of the pile-supported wharf and pier. The site characterization entailed geotechnical borings, hand-auger borings, standard penetration tests, and dynamic cone penetration tests. The laboratory tests included grain size and carbonate content tests. The observations and results presented herein add valuable field performance data for calcareous sands, which are relatively lacking in liquefaction case history databases, and the overall response of the artificial fills are consistent with predictions made using semi-empirical relations developed primarily from field data of silica sands.

Damage Patterns in Port-au-Prince during the 2010 Haiti Earthquake

The 2010 Haiti earthquake represents one of the most devastating earthquakes in history. Damage to structures was widespread across the city of Port-au-Prince, but its intensity varied considerably from neighborhood to neighborhood. This paper integrates damage statistics with geologic data, shear wave velocity measurements, and topographic information to investigate the influence of these conditions on the damage patterns in the city. The results indicate that the most heavily damaged areas in downtown Port-au-Prince are underlain by Holocene alluvium with shear wave velocities that average about 350 m/s over the top 30 m. The remainder of Port-au-Prince is underlain mostly by older geologic units with higher shear wave velocities. Damage was also concentrated on hillsides around Port-au-Prince. These pockets of damage appear to have been caused by a combination of factors, including topographic amplification, soil amplification, and failure of weakly cemented, steep hillsides.

An Initial Study of Surface Wave Inversion Using Artificial Neural Networks

An artificial neural network is proposed as an expeditious alternative to the trial-and-error and least-squares surface wave inversion techniques that are currently available. To use an artificial neural network for surface wave inversion, synthetic dispersion curves are calculated for representative shear wave velocity profiles using a theoretical wave propagation algorithm. An artificial neural network is then “taught” to map these dispersion curves back into their respective shear wave velocity profiles. Once the network has been successfully trained on these synthetic dispersion curves, experimental dispersion curves can be inverted by passing them through the neural network. Because the neural network requires only a single forward pass of the data, it performs inversions much more quickly than iterative procedures. To determine the feasibility of using an artificial neural network for surface wave inversion, a two-dimensional wave-propagation algorithm was used to create synthetic dispersion curves for 99 000 randomly generated, two-layer velocity profiles. A backpropagation neural network was then trained to associate the synthetic dispersion curves with their respective velocity profiles. The trained network was evaluated using synthetic dispersion curves for another 1000 randomly generated velocity profiles as surrogate experimental curves.