Bay Mud Research Articles

Probabilistic Seismic Hazard Analysis for Maximum Seismic Shear Stresses in Soils Using Improved Ground-Motion Parameters

Maximum seismic shear stresses (τmax) have been recognized as one of the important parameters in design practice. This study develops ground-motion parameters for τmax and implements these in probabilistic seismic hazard analysis to provide the τmax distribution of deep soil layers for design purposes. The application of improved ground-motion parameters for τmax is demonstrated at the Oakland International Airport, where a thick Young Bay Mud deposit exists under the artificial fill. Model biases in the predictive equations of seismic shear-stress reduction coefficients (rd) are evaluated by comparison with the site response analyses performed with a wide range of input ground motions. Based on these results, we introduce improved ground-motion parameters for τmax (Itau) as a linear combination of spectral accelerations, implemented in probabilistic seismic hazard analysis to calculate seismic hazard curves. Conditional mean spectra are calculated, given Itau at 10% in 50 years to illustrate the variations in frequency contents with depth compared with the uniform hazard spectra. Finally, τmax is calculated with depth by using hazard values of Itau and compared with the peak-ground-acceleration-based and uniform-hazard-spectra-based calculations. Analysis results show that τmax will be underestimated for deep soil layers by peak-ground-acceleration-based calculation if the median value of rd is used in design practice.

Extended UH Model: Three-Dimensional Unified Hardening Model for Anisotropic Clays

AbstractExtended UH model is a three-dimensional elastoplastic constitutive model that builds on a modification of the UH model with a unified hardening parameter, to account for the effect of anisotropy and the influence of K0 consolidation on the stress-strain-strength behavior of clays. The combined effects of anisotropy and stress magnitude are considered through the minimum value of angles (α) made by the spatially mobilized planes (SMPs) and soil depositional plane. The original transformed stress tensor is revised by incorporating the anisotropic peak stress ratio Mα, which is defined as a function of α, with the stress tensor σij. The comparison with test results on San Francisco Bay Mud demonstrates the capability of the proposed anisotropic criterion. The UH model is extended to account for the combined effects of inherent anisotropy and K0 conditions on the stress-strain-strength response of anisotropic clays by incorporating the revised transformed stress tensor σij˜, the potential strength ra...

Triaxial shear testing of polished slickensided surfaces

A series of consolidated-drained triaxial tests were performed on precut and polished clay specimens to measure their drained residual strength. Two soils were tested during this research program: Rancho Solano Clay and San Francisco Bay Mud. Specimens were tested using a specially modified triaxial compression device which was developed to minimize the effects of end-platen restraint on the measured strengths. Special attention was paid to the influence of changing specimen area and membrane effects during the test. Triaxial test results were compared with baseline measurements of drained residual shear strength that were made for each of the clay soils using Bromhead ring shear tests and polished-specimen direct shear tests. Residual strength values measured in the triaxial device were higher than the drained residual strengths measured in the Bromhead ring shear apparatus and the direct shear device, indicating that this test approach is more challenging than the use of direct shear tests conducted on polished slickensided surfaces. Comparison of single stage and multistage triaxial test data indicates that multistage triaxial testing may work well for specimens that fail along a well-defined plane, provided that careful attention is given to the effects of end platen restraint, membrane restraint, and changes in specimen area during shear.

Cold-climate slope deposits and landscape modifications of the Mid-Atlantic Coastal Plain, Eastern USA

Abstract The effects of Pleistocene cold-climate geomorphology are distributed across the weathered and eroded Mid-Atlantic Coastal Plain uplands from the Wisconsinan terminal moraine south to Tidewater Virginia. Cold-climate deposits and landscape modifications are superimposed on antecedent landscapes of old, weathered Neogene upland gravels and Pleistocene marine terraces that had been built during warm periods and sea-level highstands. In New Jersey, sequences of surficial deposits define a long history of repeating climate change events. To the south across the Delmarva Peninsula and southern Maryland, most antecedent topography has been obscured by Late Pleistocene surficial deposits. These are spatially variable and are collectively described as a cold-climate alloformation. The cold-climate alloformation includes time-transgressive details of climate deterioration from at least marine isotope stage (MIS) 4 through the end of MIS 2. Some deposits and landforms within the alloformation may be as young as the Younger Dryas. Southwards along the trend of the Potomac River, these deposits and their climatic affinities become diffused. In Virginia, a continuum of erosion and surficial deposits appears to be the product of ‘normal’ temperate, climate-forced processes. The cold-climate alloformation and more temperate deposits in Virginia are being partly covered by Holocene alluvium and bay mud.

A laboratory study of rheological properties of mudflows in Hangzhou Bay, China

Abstract A laboratory study of the rheology of mudflows in Hangzhou Bay, China, is reported in this paper. Both the steady and oscillatory (dynamic) rheological properties are studied using RMS-605 rheometer. A Dual-Bingham model is proposed for analyzing flow curves and compared with Worrall-Tuliani model. It is found that Dual-Bingham plastic rheological model is easier to implement than Worrall-Tuliani model and can provide satisfactory representations of the steady mudflows in Hangzhou Bay and other published data. The dependence of the yield stress and viscosity on sediment concentration is discussed based on the data from Hangzhou Bay mud and other published data. For the dynamic rheological properties of Hangzhou Bay mud, empirical expressions for elastic modulus and dynamic viscosity are provided in the form of exponential functions of sediment volume concentration, and comparisons with other published data also discussed.

Constitutive modeling of San Francisco Bay mud

Conventional laboratory tests on undisturbed samples of a highly plastic silt are described. Modified Cam Clay Model (MCC) is formulated and verified against the laboratory tests. Next, in-situ self-boring pressuremeter (SBPM) tests in the same soil are described. In order to simulate the SBPM tests, a finite element model is constructed incorporating the MCC model and the Biot's theory of consolidation. The numerical model is used to predict field SBPM tests. The predictions (or simulations) are compared with the field data from SBPM tests. The paper attempts to answer an important question in constitutive modeling: Can a constitutive model based on laboratory tests generate the response actually observed in the field? The paper concludes that it is possible to formulate a satisfactory model based on conventional laboratory tests. But, it is only when a permeability value determined by a field test is used, can an acceptable match be found between the data generated by the numerical model and the actual field data.

Firm foundations to build on

This brief article describes the reconstruction of a section of Interstate 880 (known as the Cypress Freeway) in Oakland, California. Tens of thousands of meters of expanded polystyrene (EPS) blocks were used to provide a firm foundation for seven on/off ramps. The geofoam is less expensive than conventional materials and because it was being used in a contaminated soil-area, its lighter weight made it a much more effective base material. The Cypress Freeway was destroyed in the 1989 Loma Prieta earthquake. It is built on 12.2 m of bay mud, which is a poor base for building roads because it settles significantly under heavy loads. By using EPS, only 0.60 m of soil had to be removed. After that, a 76.2-cm base of lightweight concrete was poured and the EPS blocks placed on top.

Nonlinear Site Amplification Factors for Constraining the NGA Models

Amplification factors computed from the equivalent-linear method using the program RASCALS are used to develop constraints on the nonlinear soil response for possible use by the NGA ground-motion model developers. The site response computations covered site conditions with averageVS30values ranging from 160 to 900 m/s, soil depths from 15 to 914 m, and peak accelerations of the input rock motion (VS30=1100 m/s) between 0.01 g and 1.5 g. Four sets of nonlinear properties of the soils are used: EPRI, Peninsular Range, Imperial Valley, and Bay Mud. The first two soil models are used forVS30≥270 m/s and the later two are used forVS30≤190 m/s. Simple parametric models of the nonlinear amplification factors that are functions of the PGA on rock andVS30are developed for the EPRI and Peninsula models.

Activated carbon amendment as a treatment for residual ddt in sediment from a superfund site in San Francisco Bay, Richmond, California, USA

Pesticide formulators formerly operating at Lauritzen Channel, a portion of San Francisco Bay near Richmond (CA, USA), caused contamination of sediment with dichlorodiphenyltrichloroethane (DDT). The present study evaluated the distribution of residual DDT in channel sediment six years following extensive remedial dredging. High DDT concentrations (up to 252 mg/ kg) were found in Young Bay Mud sampled across the channel. Particle analyses showed most of the contamination is contained in the clay/silt sediment fraction, and desorption tests showed that availability is greater for DDT metabolites than parent DDT. The present study examined the feasibility of using activated carbon amendment to sequester DDT from sediment, including an evaluation of reactivated carbon as a less costly alternative to virgin activated carbons. Treatment success of activated carbon amendment to sediment collected from Lauritzen Channel was measured by reductions in aqueous equilibrium concentrations and uptake in semipermeable membrane devices (SPMDs). Four different activated carbons were tested and, after one month of treatment with 3.2 weight % carbon, DDT aqueous equilibrium concentrations were reduced up to 83% and SPMD uptake was reduced up to 91%. Reactivated carbon was comparable with virgin carbons in all tests. Reduction in SPMD uptake of DDT by treatment with 3.2% reactivated carbon increased to 99% after 26 months of treatment. The effectiveness of activated carbon was dependent on the type, size, dose, and contact time. The results show the potential usefulness of activated carbon amendment as a follow-up remedial technology for management of residuals after dredging contaminated sediment.

Shear-Wave Velocity of Surficial Geologic Sediments in Northern California: Statistical Distributions and Depth Dependence

Shear-wave velocities of shallow surficial geologic units were measured at 210 sites in a 140-km2 area in the greater Oakland, California, area near the margin of San Francisco Bay. Differences between average values of shear-wave velocity for each geologic unit computed by alternative approaches were in general smaller than the observed variability. Averages estimated by arithmetic mean, geometric mean, and slowness differed by 1 to 8%, while coefficients of variation ranged from 14 to 25%. With the exception of the younger Bay mud that underlies San Francisco Bay, velocities of the geologic units are approximately constant with depth. This suggests that shear-wave velocities measured at different depths in these surficial geologic units do not need to be normalized to account for overburden stress in order to compute average values. The depth dependence of the velocity of the younger Bay mud most likely is caused by consolidation. Velocities of each geologic unit are consistent with a normal statistical distribution. Average values increase with geologic age, as has been previously reported. Velocities below the water table are about 7% less than those above it.

The effect of suspended sediment concentration on the settling velocity of cohesive sediment in quiescent water

A laboratory study was carried to qualitatively investigate the effect of suspended sediment concentration C on the settling velocity w s of cohesive sediment in quiescent water. A bay mud sample was mixed with water in a cylindrical container, and three optical back scatterance sensors were then used to measure suspended sediment concentrations of the mud–water mixture at three levels every 15 s for 5 h while sediments were settling in the quiescent water. Based on the measured sediment concentrations, the settling velocities at different concentrations were derived from the depth-integrated mass balance equation. This study has found that the settling velocity w s is independent of C in the free settling regime of C<0.3 g/l, and then increases nonlinearly with C in the enhanced settling regime of 0.3< C<4.3 g/l, and finally decreases sharply with C in the hindered settling regime of C>4.3 g/l. The maximum settling velocity occurs at C≈4.3 g/l and is about nine times faster than the settling velocity in the free settling regime. A single empirical formula is also proposed to calculate the settling velocities at different sediment concentrations.

Characterization of Failure in Cross-Anisotropic Soils

Drained true triaxial tests on dense Santa Monica Beach sand deposited with a cross-anisotropic fabric have been performed to study the failure condition in the principal stress space. The failure surface was assumed to be symmetric around the vertical axis (on the octahedral plane of the principal stress space), but varying as a function of the Lode angle. Data from previously performed consolidated-undrained true triaxial tests on San Francisco Bay Mud and data from triaxial compression, triaxial extension, and plane strain tests on Toyoura sand showed similar behavior in terms of effective stresses. A three-dimensional failure criterion is proposed for characterization of failure in cross-anisotropic soils, under commonly occurring conditions when loading and depositional directions coincide and no significant rotation of principal stresses occur. This cross-anisotropic criterion is developed using a coordinate rotation of the principal stress space and utilization of an existing isotropic failure formulation. Derivation of the three required parameters is explained and illustrated. The proposed criterion is compared with various experimental results; and it is demonstrated that the failure criterion for cross-anisotropic soils captures the experimental behavior with good accuracy.

Measurement of In Situ Hydraulic Conductivity and Coefficient of Consolidation with Prefabricated Vertical Drains

Prefabricated vertical drains (PVDs) are often used to improve soils by speeding up the rate of consolidation, and they are increasingly used for remediation of contaminated soils. When PVDs are installed in the ground, there is always a zone of disturbed soil surrounding them. The extent and the impact of this "smear" zone are difficult to quantify, and this uncertainty complicates the use of laboratory values in design. Consequently, obtaining in situ values for the hydraulic conductivity and horizontal coefficient of consolidation that include the effects of smear would be of great use to engineers who use PVDs in design. A method that utilizes PVDs to obtain in situ measurements of these parameters is described. The primary advantages of this method are its cost-effectiveness and the inclusion of smear in the results. A trial test was conducted on San Francisco Bay, California, mud. Some problems encountered during the initial phase of testing are described, and possible solutions are discussed. This promising method yields values that are logical and that compare well with published values for San Francisco Bay mud.

Anisotropic Elastoplastic Bounding Surface Model for Cohesive Soils

This paper presents an improved soil model based on anisotropic critical state theory (ACST) and bounding surface plasticity. The ACST of Dafalias was extended into 3-D stress space. In addition to the isotropic hardening rule, rotational and distortional hardening rules were incorporated into the bounding surface formulation with an associated flow rule. The projection center that is used to map the actual stress point to the imaginary stress point was specified along the K-sub-0 line instead of the hydrostatic line or at the origin of the stress space. A simplified form of plastic modulus was used and the proposed model requires a total of 12 material parameters, the same number as that of the single-ellipse time-independent version of the Kaliakin--Dafalias model. The model was validated against the undrained isotropic and anisotropic triaxial test results under compression and extension shearing modes for Kaolin Clay, San Francisco Bay Mud, and Boston Blue Clay. The effects of stress anisotropy and overconsolidation were well captured by the model. The time effect was not included in formulations presented here.

Influence of stiffness degradation on strength demands of structures built on soft soil sites

The effect of stiffness degradation on the lateral strength demands of inelastic single-degree-of-freedom systems subjected to soft soil records is investigated. The modified-Clough model is used to represent structures that exhibit significant stiffness degradation when subjected to reverse cyclic loading and the elastic-perfectly-plastic model is used to represent non-degrading structures. The study is based on two sets of ground motions. The first set includes 100 accelerograms recorded in various recent earthquakes on soft soil sites of Mexico City. The second set includes 16 ground motions recorded on bay mud sites in the San Francisco Bay Area during the 1989 Loma Prieta earthquake. A special emphasis is placed on the effect of stiffness degradation on strength reduction factors that permit the estimation of inelastic strength demands from elastic strength demands. Mean ratios of lateral strength demands of stiffness degrading systems to lateral strength demands of non-degrading systems for both sets of ground motions are presented. It is concluded that structures with stiffness degradation, and with periods of vibration shorter than the predominant period of vibration of the ground motion, can experience lateral strength demands larger than those of non-degrading structures in the same period range. Whereas for structures with periods of vibration equal or longer than the predominant period of the ground motion, the lateral strength demands are typically smaller for stiffness-degrading structures than those of non-degrading structures.

Soil Deformation and Excess Pore Pressure Field around a Closed-Ended Pile

Numerous field and laboratory research projects have examined loading conditions, total soil stresses, and excess pore pressures during and following pile installation. Recently, a large number of seismic retrofitting projects for bridges in California have revealed the need for well-documented field tests evaluating the effect of pile installation on the static and dynamic properties of soft clays. This paper documents the geotechnical field investigation and field monitoring program, which included measurements of excess pore pressure and horizontal deformations at three radial distances from a full-scale closed-ended pile driven into a deep deposit of Young Bay Mud. Significant excess pore pressures developed as a result of pile driving and they are a function of distance to the pile wall. Lateral deformations decrease with increasing distance from the pile and can be approximately described by the cylindrical cavity expansion method. Inclinometer measurements show lateral deformation towards the pile wall as the excess pore pressure dissipates. These measurements are essential in the calibration and validation of analytical techniques to predict changes in properties of the foundation soil.

Development of a Geomaterial from Dredged Bay Mud

The lack of construction materials in the coastal areas and the high cost of importing good-quality soil creates the need for developing new technologies to manufacture engineered fills using low-quality materials obtained from a seabed. This study presents results of a series of tests conducted to obtain field data related to basic engineering properties of a geomaterial comprised of the mud dredged from Tokyo Bay, mixed with lightweight additives, such a foam or expanded polystyrol beads, and portland cement as a binder. Due to discrepancies that may exist in laboratory-prepared samples, a field study was launched to replicate the actual construction conditions. Relationships are developed to correlate various engineering properties of the geomaterial in terms of strength, modulus of elasticity, and deformation characteristics.

Effects of Stress Rotation and Changes of B-Values on Cross-Anisotropic Behavior of Natural, K0-Consolidated Soft Clay

ABSTRACT A series of consolidated-undrained torsion shear tests was performed on large, hollow cylindrical specimens of undisturbed San Francisco Bay Mud to investigate the influence of stress rotation and changes in b-values on the stress-strain, pore pressure, and strength characteristics of a cross-anisotropic, natural clay. The specimens were first K0-consolidated and then sheared along various undrained stress paths to study the clay behavior in the full range of stress rotation from 0° to 90° relative to vertical. For this cross-anisotropic clay, the test results indicate that the major principal strain increment directions in physical space do not coincide with the directions of major principal stress at failure. The deviation between these two directions varied continuously and the maximum deviation was measured at approximately 20°. The normalized, undrained shear strength decreases systematically with inclination of the major principal stress at failure, ψ, from triaxial compression with ψ = 0° and b = 0 to triaxial extension with ψ = 90° and b = l. The effective friction angle remains constant at approx. 35° in the range of ψ from 0° to 30° after which increases gradually to almost 50° at ψ = 90°. These results are unlike those obtained for a laboratory prepared deposit of kaolin, which tended to behave more like an isotropic material despite the initial A0-consolidation. Thus, effects of aging are very important in the behavior of clays, and it may not be possible to simulate such effects with sufficient accuracy and reliability with laboratory prepared clay deposits.

Tsunami deposits in Holocene bay mud in southern Kanto region, Pacific coast of central Japan

Abstract Tsunami have probably deposited sand sheets that are intercalated in Holocene bay-floor mud exposed on the Boso and Miura Peninsulas, facing the convergent boundary of the Pacific, Philippine Sea, and Eurasian Plates. We have identified seven sand sheets at four drowned valleys, and correlated these by 137 radiocarbon dates of shells and wood. The sheets consist of poorly sorted muddy sand and well-sorted sand layers in fining upward sequences, containing abundant transported shells, rip-up clasts and wood fragments. The sheets erosionally overlie bioturbated bay-floor mud that contains molluscan shells in life position. Most of the sheets are less than 20 cm thick and rarely more than 50 cm thick. Some molluscan shells are older in these layers than in underlying mud. Both landward and seaward paleocurrents are shown in a few cases by imbrication of shells and by low-angle wedge shaped lamination. At least five of the sand sheets contain molluscan fossils derived from rocky coasts or shore platforms, although they intercalated in mud deposited within bays, at depths of 10–15 m. Two other sand sheets are dominated by open seashore ostracode assemblages, although they were deposited in the brackish inner bay and muddy central bay. Five of the layers may correlate with emergences recorded by nearby Holocene marine terraces. These correlations suggest that great earthquakes triggered the inferred tsunami. The tsunami occurred at intervals of 300–2000 years beginning about 10,000 years ago.

Precast Segmental Seismic Retrofit for the San Mateo Hayward Bridge

The retrofit of the San Mateo-Hayward Bay Bridge is part of the California Department of Transportation (Caltrans) effort to retrofit the toll bridges in the state of California. As part of this program, the use of precast segmental construction techniques in the San Mateo-Hayward Bay Bridge retrofit resulted in a constructable and durable retrofit for the rectangular foundations. The $102 million construction package included the seismic retrofit of 20 of these rectangular piers. This article provides an overview of the project design and construction phases. Precasting of the frames was performed on barges that were towed to the site and the frames were assembled and placed around the footings. Arms on the frames reached out to 8-12 ft (2.4-3.6 m) diameter piles driven through the bay mud to firmer soils so the system limited the excessive transverse deflections that would occur in a severe earthquake.