Aaron S. Bradshaw

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.

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.

Behavior of full-scale concrete segmented pipelines under permanent ground displacements

Concrete pipelines are one of the most popular underground lifelines used for the transportation of water resources. Unfortunately, this critical infrastructure system remains vulnerable to ground displacements during seismic and landslide events. Ground displacements may induce significant bending, shear, and axial forces to concrete pipelines and eventually lead to joint failures. In order to understand and model the typical failure mechanisms of concrete segmented pipelines, large-scale experimentation is necessary to explore structural and soil-structure behavior during ground faulting. This paper reports on the experimentation of a reinforced concrete segmented concrete pipeline using the unique capabilities of the NEES Lifeline Experimental and Testing Facilities at Cornell University. Five segments of a full-scale commercial concrete pressure pipe (244 cm long and 37.5 cm diameter) are constructed as a segmented pipeline under a compacted granular soil in the facility test basin (13.4 m long and 3.6 m wide). Ground displacements are simulated through translation of half of the test basin. A dense array of sensors including LVDTs, strain gages, and load cells are installed along the length of the pipeline to measure the pipeline response while the ground is incrementally displaced. Accurate measures of pipeline displacements and strains are captured up to the compressive and flexural failure of the pipeline joints.

Experimental study on the behavior of segmented buried concrete pipelines subject to ground movements

Seismic damage to buried pipelines is mainly caused by permanent ground displacements, typically concentrated in the vicinity of the fault line in the soil. In particular, a pipeline crossing the fault plane is subjected to significant bending, shear, and axial forces. While researchers have explored the behavior of segmented metallic pipelines under permanent ground displacement, comparatively less experimental work has been conducted on the performance of segmented concrete pipelines. In this study, a large-scale test is conducted on a segmented concrete pipeline using the unique capabilities of the NEES Lifeline Experimental and Testing Facilities at Cornell University. A total of 13 partial-scale concrete pressure pipes (19 cm diameter and 86 cm long) are assembled into a continuous pipeline and buried in a loose granular soil. Permanent ground displacement that places the segmented concrete pipeline in compression is simulated through the translation of half of the soil test basin. A dense array of sensors including linear variable differential transducers, strain gauges, and load cells are installed along the length of the pipeline to measure its response to ground displacement. Response data collected from the pipe suggests that significant damage localization occurs at the ends of the segment crossing the fault plane, resulting in rapid catastrophic failure of the pipeline.

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.

Influence of Dilation Angle on Drained Shallow Circular Anchor Uplift Capacity

AbstractAn experimental study of uplift capacity of 22 circular helical anchors installed in sand with peak friction angles between 40 and 50° was performed. Laboratory triaxial tests indicated that the dilation angle varied between 10 and 25° for these peak friction angles. To account for soil behavior exhibiting nonassociated flow (NAF), in which the dilation angle is much less than the friction angle, a limit equilibrium plane strain analytical solution for plate anchor uplift was updated and extended to axisymmetric conditions. Anchor test results were compared with upper bound (UB) plasticity solutions (based on associated flow) and the newly developed NAF limit equilibrium model. The UB solution overpredicted uplift capacity by more than a factor of 2, whereas the limit equilibrium model had a ratio of calculated to measured capacity of 1.15 and a coefficient of variation of 0.14. Although additional study is warranted, the consistency among the numerical, analytical, and experimental results gives ...

Load Transfer Curves from a Large-Diameter Pipe Pile in Silty Soil

This paper presents load transfer curves interpreted from a static load test performed on a large-diameter pipe pile in silty soils. In large-diameter driven piles and drilled shafts, appreciable movement is needed to mobilize toe resistance and thus settlement may control the design. Advanced load transfer methods require the prediction of the load transfer behavior along the shaft (i.e. t-z curve) and beneath the toe (i.e. q-z curve) of the pile. A number of generic load transfer curves for sand and clay are reported in the literature but limited information is available for large diameter piles and intermediate soils such as silt. This study develops t-z and q-z curves for silty soils from the analysis of a static loading test performed on an 1.8-meter diameter pipe pile in Rhode Island. The t-z curves from the test pile showed a softer load-movement response in comparison to those from slender piles in the literature. The results suggest that use of existing empirical t-z curves developed from slender piles in sands could lead to inaccurate load transfer analyses in large-diameter piles in silty soils.

New Density Normalization Approach for Evaluation of the Cyclic Resistance of Silts

This paper proposes a new normalization approach to replace relative density when evaluating the cyclic resistance of silty soils. Normalization of dry density is advantageous because it captures the combined effects of void ratio and fabric on strength for similar soil types. Relative density is commonly used to normalize the dry density or void ratio of sands with fines contents less than 15%, however there is currently no such standard for silts. The approach presented in this paper involves normalizing the dry density of a specimen by the maximum dry density obtained from a Modified Proctor test at a specific degree of saturation (or molding water content). Cyclic triaxial tests were performed on samples of four similar non-plastic silts from Rhode Island. Samples were prepared using a modified moist tamping approach at molding water contents corresponding to a degree of saturation of 55%. Previous work by the authors showed that samples prepared at this degree of saturation yielded the same cyclic resistance as samples prepared from a slurry and carved from an intact block. Samples of four different silts prepared to the same relative density had different cyclic strengths. Samples prepared to the same normalized density yielded the same cyclic strength suggesting that normalized density could be a useful approach for silty soils.

Underground Sensing Strategies for the Health Assessment of Buried Pipelines

Buried lifeline infrastructure including pipelines, tunnels, power and communication lines, among others, are vital to ensuring the operation of the national economy. Permanent ground displacement (PGD) from earthquakes and landslides is the most serious hazard to buried pipelines, prompting often slow and expensive methods of damage localization before repairs can be made. Due to the importance of these buried lifelines, it is critical that low-cost and rapid methodologies for damage detection and localization be developed. Monitoring systems embedded in and around the pipeline are an obvious approach but typically suffer from the cost and obtrusiveness of long cable requirements. The primary goal of this chapter is to illustrate novel sensing methods that can serve as the basis for monitoring buried pipelines exposed to PGD. In particular, the chapter focuses on the monitoring of segmented concrete pipelines, which typically experience damage at their joints due to PGD. Wireless telemetry is evaluated to validate wireless sensors for buried applications, thus reducing greatly the cost of dense sensor systems in regions of high PGD risk. An overview of current buried pipeline sensing technology is made and three experimental full-scale PGD tests are conducted to evaluate pipeline motion and damage detection methodologies in segmented concrete pipelines. Real-time monitoring of joint rotations and translations by potentiometers as well as direct damage measures of joint regions by acoustic emission and conductive surface sensors were made. Strain gages were used to successfully portray global load transfer throughout the pipeline, validated by load cell measurements at the pipe ends. The combined sensor information is successfully used to create a hypothesis for the damage evolution process of buried segmented concrete pipelines under PGD and to validate the use of wireless sensors for buried pipeline monitoring.

Scaling Considerations for 1-g Model Horizontal Plate Anchor Tests in Sand

This paper addresses scaling issues related to small-scale 1-g model tests on plate anchors in sand under drained loading conditions. Previous centrifuge studies from the literature have suggested that the results of conventional 1-g model testing are inaccurate because of scale effects. Other studies have suggested, however, that scaling errors can be reduced in 1-g model tests if the results are presented in dimensionless form and the constitutive response of the model soil is representative of the prototype behavior. There are no experimental studies in the literature that have tested the validity of this approach for plate anchors. A simple 1-g scaling framework was developed for vertically loaded, horizontal plate anchors. Small-scale 1-g model tests were performed on square plate anchors in dry sand, and combined with existing centrifuge and 1-g model test data from the literature to test the scaling approach for both capacity and deformation. The 1-g model tests provided a reasonable representation of the full-scale prototype behavior when the scaling approach was applied.