Christopher D. P. Baxter

Sample Preparation of Silts for Liquefaction Testing

One of the most important aspects of cyclic testing in the laboratory is using samples that are representative of their in-situ conditions. Since undisturbed samples of cohesionless soils are typically too difficult or costly to obtain, reconstituted samples need to be prepared using a method that most closely replicates the in-situ stress, density, and fabric. Research has clearly shown the effect of sample preparation methods on the liquefaction resistance of sands, and it is believed that wet pluviation methods most closely approximate the in-situ fabric of fluvial soils. For pure silts, however, these methods are limited because only very loose samples can be made. This paper introduces a new modified moist tamping method that can be used to reconstitute denser specimens of silt. It was found that samples tamped at an initial saturation level of about 55 % gave comparable cyclic strengths to samples prepared from a slurry and to specimens trimmed from an in-situ block sample. The method can be considered a cost-effective alternative for the liquefaction testing of silts.

Modeling of SMF tsunami hazard along the upper US East Coast: detailed impact around Ocean City, MD

With support from the US National Tsunami Hazard Mitigation Program (NTHMP), the authors have been developing tsunami inundation maps for the upper US East Coast (USEC), using high-resolution numerical modeling. These maps are envelopes of maximum elevations, velocity, or momentum flux, caused by the probable maximum tsunamis identified in the Atlantic oceanic basin, including from far-field coseismic or volcanic sources, and near-field Submarine mass failures (SMFs); the latter are the object of this work. Despite clear field evidence of past large-scale SMFs within our area of interest, such as the Currituck slide complex, their magnitude, pre-failed geometry, volume, and mode of rupture are poorly known. A screening analysis based on the Monte Carlo simulations (MCS) identified areas for possible tsunamigenic SMF sources along the USEC, indicating an increased level of tsunami hazard north of Virginia, potentially surpassing the inundation generated by a typical 100-year hurricane storm surge in the region, as well as that from the most extreme far-field coseismic sources in the Atlantic; to the south, the MCS indicated that SMF tsunami hazard significantly decreased. Subsequent geotechnical and geological analyses delimited four high-risk areas along the upper USEC where the potential for large tsunamigenic SMFs, identified in the MCS, was realistic on the basis of field data (i.e., sediment nature and volume/availability). In the absence of accurate site-specific field data, following NTHMP’s recommendation, for the purpose of simulating tsunami hazard from SMF PMTs, we parameterized an extreme SMF source in each of the four areas as a so-called Currituck proxy, i.e., a SMF having the same volume, dimensions, and geometry as the historical SMF. In this paper, after briefly describing our state-of-the-art SMF tsunami modeling methodology, in a second part, we parameterize and model the historical Currituck event, including: (1) a new reconstruction of the SMF geometry and kinematics; (2) the simulation of the resulting tsunami source generation; and (3) the propagation of the tsunami source over the shelf to the coastline, in a series of nested grids. A sensitivity analysis to model and grid parameters is performed on this case, to ensure convergence and accuracy of tsunami simulation results. Then, we model in greater detail and discuss the impact of the historical Currituck tsunami event along the nearest coastline where its energy was focused, off of Virginia Beach and Norfolk, as well as near the mouth of the Chesapeake Bay; our results are in qualitative agreement with an earlier modeling study. In a third part, following the same methodology, we model tsunami generation and propagation for SMF Currituck proxy sources sited in the four identified areas of the USEC. Finally, as an illustration of our SMF tsunami hazard assessment work, we present detailed tsunami inundation maps, as well as some other products, for one of the most impacted and vulnerable areas, near and around Ocean City, MD. We find that coastal inundation from near-field SMF tsunamis may be comparable to that caused by the largest far-field sources. Because of their short propagation time and, hence, warning times, SMF tsunamis may pose one of the highest coastal hazards for many highly populated and vulnerable communities along the upper USEC, certainly comparable to that from extreme hurricanes.

DSS Test Results Using Wire-Reinforced Membranes and Stacked Rings

Recently developed American Society for Testing and Materials (ASTM) standards for direct simple shear (DSS) testing require that the sample be laterally confined in either a wire-reinforced membrane or stack of thin rings. Although wirereinforced membranes are more commonly used in practice, it is uncertain how the results of the two confinement methods compare. This paper presents the results of direct simple shear tests performed on high plasticity clay and low plasticity organic silt to compare the effects of using either wire-reinforced membranes or Tefloncoated rings for lateral confinement. Comparisons are made for both the consolidation and shear phases of the test. The consolidation data suggest that the rings may provide increased lateral stiffness relative to the wire membranes. When appropriate system corrections were applied to the measured soil test data, the results of both confinement systems produced comparable results in terms of stress-strain behavior and strength.

Investigation of mass wasting on the continental slope and rise

Abstract A methodology is presented for the investigation of submarine slope instability. This approach integrates the results of a variety of tools and methods, including multibeam, side scan and subbottom acoustic data, geotechnical sampling and testing, and advanced dating techniques. For slides/slumps that have the potential for causing tsunamis, the use of long, large-diameter piston corers is a cost-effective way of obtaining good quality sediment samples. Using a Multi-Sensor Core Logger and geotechnical analyses, a baseline of sediment properties is developed from nearby stable, unfailed regions, which is then compared to more active areas where faulting and instability is suspected. Clear evidence of past and recent slope failures can often be obtained from the stress history of the sediment derived from consolidation test results and from careful identification of naturally deposited sediments overlying relic failures. This methodology was applied for the investigation of slope failures along a study corridor in deepwater Gulf of Mexico. Large failures were identified from multibeam data, and coring locations were chosen based on subbottom and side scan data obtained with a deep-tow system. Buried failures were identified from detailed bulk density measurements, geotechnical test results, and paleomagnetic and 14 C dating techniques. Dating of these failures can provide information regarding the triggering mechanisms of these events, which can be used to assess the current risk of new instabilities.

Use of A¯ =0 as a Failure Criterion for Weakly Cemented Soils

There is considerable uncertainty in the determination of effective stress strength parameters of cemented soils from undrained triaxial tests. Large negative excess pore pressures are generated at relatively large strains (typically 4–5% for cemented silty sand) in isotropically consolidated undrained (CIU) tests, which results in gas coming out of solution during shear and significant variability in the measured peak deviator stress. In this study, different failure criteria for weakly cemented sands were evaluated based on the results of CIU and isotropically consolidated drained triaxial compression tests conducted on samples of artificially cemented sand. The use of A¯ =0 as a failure criterion eliminates the variability between the undrained tests and also ensures that the mobilized failure strength is not based on the highly variable negative excess pore pressures. In addition, the resulting strains to failure are comparable to the strains to failure for the drained tests. Mohr-Coulomb strength par...

Strength of Weakly Cemented Sands from Drained Multistage Triaxial Tests

Characterizing the strength of weakly cemented and sensitive soils in the laboratory is difficult because of the difficulty in obtaining high-quality replicate samples necessary for defining the failure envelope. Multistage triaxial tests have long been used to reduce the variability caused by testing multiple samples; however, traditional criteria used for transitioning from one loading stage to another often lead to destructuring or failure in sensitive or structured soils. The objective of this paper is to present a methodology for conducting multistage drained triaxial tests on weakly cemented sands and estimating the resulting shear strength parameters. Both multistage and single-stage drained triaxial tests were performed on artificially cemented samples of a silty sand at two levels of densities and cementation. The use of dev/dea=0 as a termination criterion to move on to the next stage of loading and ev=0 as the failure criterion for the final stage of the shear resulted in an average error of 6%...

Probabilistic Smf Tsunami Hazard Assessment For The Upper East Coast Of The United States

The level of tsunami hazard to the east coast of the United States is not well understood. This information is critical for the population, emergency services, and industry of the region. Assessing this hazard is particularly difficult because of the lack of tsunamis in the historical record and the uncertainty regarding the return periods of large-scale events that have been proposed, such as a large transoceanic tsunami possibly caused by a collapse of the Cumbre Vieja volcano in the Canary Islands, or a large coseismic tsunami initiated in the Puerto Rican subduction zone. The most significant tsunami hazard in this region, however, may be due to local submarine mass failures (SMF), which could cause concentrated damage in coastal communities located near the failures. This paper presents results of a probabilistic analysis that estimates the hazard, expressed in terms of runup (at a given probability of occurrence), of SMF tsunamis triggered by earthquakes, on the upper northeast coast of the United States. A Monte Carlo approach is employed, in which distributions of relevant parameters (seismicity, sediment properties, type and location of slide, volume of slide, water depth, etc.) are used to perform large numbers of stochastic stability analyses of underwater slopes, based on standard geotechnical methods. When slope failure occurs, initial tsunami characteristic height and runup are estimated, based on earlier numerical work, for specified return periods of seismic events. The overall hazard associated with SMF tsunamis along the coast is found to be quite low at most locations as compared, e.g., to the typical 100 year hurricane storm surge in the region (5 m). Two sites, however, located off of Long Island, New York and Atlantic City, New Jersey, show an elevated risk of higher tsunami runup (5.0-7.5 m). These two sites should be the focus of more detailed studies.

Development of a technology type factor for jacket structures for offshore wind turbines in Rhode Island

A marine spatial planning approach was used to locate possible sites for offshore wind development in Rhode Island. In connection with the Rhode Island ocean special area management plan, a technology development index was developed by Spaulding et al. (2010) to quantify the technical challenges of a particular site relative to its potential power production. A component of this index is the technology type (TT) factor, which quantifies the relative cost of a structure/foundation system as a function of environmental loading, water depth, and soil conditions. This paper presents the development of TT factors for jacket type support structures that is proposed for supporting the offshore wind turbines in Rhode Island Sound. TT factors were calculated by the total weight of the jacket and piles for a given water depth and soil conditions normalized by the weight of a reference structure. Jacket structure weights were determined by a frequency driven finite element analysis using the program ANSYS. The struc...

Effective soil density for small-strain shear waves in saturated granular materials

AbstractThis paper presents an experimental investigation of the concept of effective density for the propagation of small-strain shear waves through saturated granular materials. Bender element tests and resonant column tests were conducted on various granular materials in dry and saturated conditions. Values of small-strain shear modulus for the dry condition are compared with corresponding values for the saturated condition, which are calculated using saturated density and effective density. For bender element tests, the use of saturated density produced errors as high as 28% in the shear modulus, whereas the use of effective density resulted in errors generally less than 5%. For resonant column tests, errors in the shear modulus that were obtained using saturated density were smaller than those for bender element tests because of the lower range of excitation frequency and effect of mass polar moment of inertia of the loading system. A quick chart is provided to help users determine if effective densi...

Role Of Soil Behavior On The Initial Kinematics Of Tsunamigenic Slides

Recent investigations on tsunami generation from submarine mass failures show that one of the most important factors influencing the source characteristics of the wave is the initial acceleration of the failure itself. In a number of these studies, a translational slide is modeled as a rigid body sliding down an inclined plane and basal resistance is neglected. In this paper, a similar rigid body model is proposed that incorporates basal resistance, which is related to the shear strength of the soil. Initial slide kinematics were investigated under two triggering mechanisms including overpressures at depth and rapid sedimentation. The model results show that soil behavior significantly influences the acceleration time history as well as the magnitude of the peak acceleration. The slide kinematics depend largely on the initial stress state and on the undrained residual shear strength of the soil along a potential failure surface, which highlights the importance of performing detailed geotechnical site investigations when assessing these geohazards. More research is needed to determine the influence of using more realistic basal friction models on the initial wave heights generated by submarine mass failures.