Srinivasa S. Nadukuru

Static Fatigue, Time Effects, and Delayed Increase in Penetration Resistance after Dynamic Compaction of Sands

Dynamically compacted sands often exhibit a drop in cone penetration resistance immediately after compaction, but a gradual increase in the resistance occurs in a matter of weeks and months. An explanation of the former is sought in analysis of the stress state immediately after a dynamic disturbance, and a justification for the latter is found in the micromechanics process of static fatigue (or stress corrosion cracking) of the micromorphologic features at the contacts between sand grains. The delayed fracturing of contact asperities leads to grain convergence, followed by an increase in contact stiffness and an increase in elastic modulus of sand at the macroscopic scale. Time- dependent increase in small-strain stiffness of sand under a sustained load is a phenomenon confirmed by earlier experiments. It is argued that the initial drop in the cone penetration resistance after dynamic compaction is caused by a drop in the horizontal stress after the disturbance. The subsequent gradual increase in the penetration resistance is not a result of increasing strength, but it is owed to the time-delayed increase in stiffness of sand, causing increase in horizontal stress under one-dimensional strain conditions. This process is a consequence of static fatigue at contacts between grains. The strength of sand after dynamic compaction increases as soon as the fabric of the compacted sand is formed and is little affected by the process of grain convergence in the time after compaction. Contact stiffness, with its dependence on static fatigue, holds information about the previous loading process, and it is a memory parameter of a kind; this information is lost after a dis- turbance, such as dynamic compaction, in which new contacts are formed. The scanning electron microscope (SEM) observations, discrete element simulations, and energy considerations are carried out to make the argument for the proposed hypothesis stronger. DOI: 10.1061/ (ASCE)GT.1943-5606.0000611.

Three-dimensional limit analysis of slopes with pore pressure

AbstractThree-dimensional (3D) slope stability analyses are not performed routinely because they involve increased effort compared with two-dimensional analyses, and because the latter yield more conservative results. However, in cases of distinct limitations of the width of the failure region (e.g., in the case of excavations), an assessment of safety using 3D stability analyses may be more appropriate. The kinematic approach of limit analysis for the assessment of stability of slopes is presented, based on a recently conceived rigid-rotation 3D mechanism modified to include below-toe failures, which are common for gentle slopes. The presence of pore-water pressure in the slope is included approximately using the scalar parameter ru. The results are presented in terms of the critical height (or dimensionless coefficient γH/c), and also in the form of charts and a closed-form approximation formula that allow for direct assessment of the safety factor for slopes with constrained width. Not surprisingly, th...

Arching in Distribution of Active Load on Retaining Walls

Traditional methods for calculations of active loads on retaining structures provide dependable forces, but these methods do not indicate reliably the location of the resultant load on the walls. The Coulomb method does not address the load distribution because it utilizes equilibrium of forces, whereas the Rankine stress distribution provides linear increase of the load with depth. Past experimental studies indicate intricate distributions dependent on the mode of displacement of thewall before reaching the limit state. The discrete element method was used to simulate soil-retaining structure interaction, and force chains characteristic of arching were identified. Arching appears to be the primary cause affecting the load distribution. A differential slice technique was used to mimic the load distributions seen in physical experi- ments. The outcome indicates that rotation modes of wall movement are associated with uneven mobilization of strength on the surface separating the moving backfill from the soil at rest. Calculations show that the location of the centroid of the active load distribution behind a translating wall is approximately 0.40 of the wall height above the base, but for a wall rotating about its top point, the location of the resultant is at approximately 0:55H. In the third case, rotation about the base, the location of the calculated centroid of the stress distribution on the wall is slightly below one-third of the wall height. DOI: 10.1061/(ASCE)GT.1943-5606.0000617.

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.

Static Fatigue at Grain Contacts: A Key Cause of Time Effects in Sand

Behavior of disturbed sand deposits is known to be time dependent, but the nature of this dependency is still the subject of research. A hypothesis is suggested, which considers delayed fracturing of the micromorphological features on grain surfaces in contact as the primary cause of time effects in sand. Investigation of micromorphology was carried out first, followed by grain-to-grain load tests and modified consolidometer tests. Experimental evidence is consistent with the suggested hypothesis.

3D Analysis of Steep Slopes Subjected to Seismic Excitation

Design of slopes and analysis of existing slopes are carried out routinely using approximations of plane strain and substitution of quasi-static load for the seismic excitation. A three-dimensional analysis of slopes is used here, based on the kinematic theorem of limit analysis. A 3D rotational mechanism with a failure surface passing through the slope toe was developed, applicable to steep slopes. A quasi-static approach is used and an example of charts for the assessment of the factor of safety for slopes with predefined width of the failure mechanism is shown. Critical acceleration is also calculated for 3D slopes, and a sliding block analysis is carried out to develop a solution for displacements of slopes subjected to seismic excitation.

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.

Performance and damage evolution of plain and fibre-reinforced segmental concrete pipelines subjected to transverse permanent ground displacement

Abstract This paper presents the results of three full-scale experiments performed on segmental concrete pipelines subjected to permanent ground displacement. The first pipeline was made of reinforced concrete pipes and the second pipeline was made of steel fibre-reinforced concrete pipes. The third pipeline was made of a combination of fibre-reinforced and reinforced concrete pipes. An array of sensing techniques was used to assess the damage evolution in pipelines and their overall performance. Three stages of damage were observed. In the first stage, damage was concentrated in the joints near the fault line. In the second stage, the damage occurred in all joints along the pipeline. While in the first two stages damage was mainly concentrated at the bell and spigot joints of the pipe segments, the third stage of damage was characterised by severe damage and rupture of the body of pipe segments located in the immediate vicinity of the fault line. The modes of failure for the plain and fibre-reinforced concrete pipelines were similar in the first and second stages of damage. However, in the pipeline constructed using both plain and fibre-reinforced concrete pipe segments, the damage was concentrated in the standard reinforced concrete pipe segments.

Static Fatigue or Maturing of Contacts in Silica Sand

Silica sands are known to exhibit a time-dependent response to applied loads, particularly after they were disturbed, for example, due to compaction. This behavior was documented by a time-dependent increase in shear wave velocity in sand subjected to sustained loads. The change of material properties with increasing time is often referred to as sand aging. While several hypotheses have been proposed to explain the aging process, none has been generally accepted by the research community. The hypothesis advocated in this paper is that static fatigue at contacts between the grains may be a key factor in time-dependent behavior of silica sand. An apparatus was constructed to load individual sand grains, and the time-dependent deflection under sustained load was monitored. The rate of deflection was found dependent chiefly on the surface texture of the grains (roughness), with rougher surfaces at contacts being more susceptible to larger deflection. The process of static fatigue occurring at the contacts is also referred to in this presentation as contact maturing. The results of grain scale testing in the custom-constructed apparatus are consistent with the hypothesis, which implies that contact maturing is a plausible contributor to aging of silica sand.

Contact Fatigue: A Key Mechanism in Time-Dependent Behavior of Sand After Dynamic Compaction

Geotechnical characterization of soils typically relates to the macroscopic description. This is because properties at the macroscopic scale directly relate to the behavior of soils and the performance of built infrastructure. In this presentation, attention will be drawn to the microscopic characterization of sand grains, which is believed to be very important in determining the causes of time- dependent behavior of silica sand. In particular, the morphology of the sand grain surfaces will be characterized using Scanning Electron Microscopy and Atomic Force Microscopy. When particles with rich surface morphology come into contact, the prime response is fracture. However, after initial damage (fracturing and crushing), fracturing continues for days. This has been found in tests where a single contact between two grains was subjected to a sustained load. The process of the time- dependent interaction between two grains subjected to a contact load will be referred to as contact fatigue. The physics underpinning this process is sought in the rate process concept, applied to fracture kinetics. Quantitative results from testing individual contacts and from tests on specimens of silica sand will be presented. An attempt is made at explaining a peculiar behavior of silica sand deposits after in-situ dynamic compaction.