Srinivasa S. Nadukuru

Abstract: 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. © 2012 American Society of Civil Engineers. CE Database subject headings: Sand (soil type); Soil compaction; Fatigue; Corrosion; Cracking; Cone penetration; Discrete elements. Author keywords: Sand behavior; Dynamic compaction; Static fatigue; Stress corrosion cracking; Micro-fracturing; Grain convergence; Multiscale analysis; Static cone penetration; Sand aging; Discrete element simulations.

Abstract: 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. © 2012 American Society of Civil Engineers. CE Database subject headings: Arches; Retaining structures; Load distribution; Numerical analysis; Discrete elements. Author keywords: Arching; Retaining walls; Active load; Numerical analysis; Discrete element method.

Abstract: Three-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...

Abstract: This paper describes results of an experimental study that used sensing methods for monitoring damage along segmental concrete pipelines resulting from permanent ground displacement across a simulated earthquake fault. The literature contains examples of such damage occurring during actual earthquakes, significantly impacting the functionality of the pipelines. Detecting the location of the damage and the extent of the damage in pipelines can significantly accelerate post-earthquake repair efforts. In this paper, electrical sensing methods, magnetic sensing, and acoustic emission are used to monitor structural damage in a segmental concrete pipeline during a large-scale test. In this test, the segmental concrete pipeline was subjected to a concentrated transverse permanent ground displacements (PGDs). The majority of the damage to the pipe segments was localized at the joints, especially the bell sections while the damage to the spigots was minimal. The damage extended away from the joints in the pipe segments in the immediate vicinity of the fault line. Telescoping (i.e., crushing of the bell-and-spigot) was a primary mode of failure that was observed. The results of this study indicate that electrical sensing methods (including the use of conductive grout), magnetic sensing, and acoustic emission, employed alone or in combination, can detect and quantify the damage in segmental concrete pipelines.

Abstract: Rapid assessment of damage to buried pipelines from earthquake-induced ground deformation is a crucial component to recovery efforts. This paper reports on the first year of a four-year study aimed at developing rapid, reliable, and cost-effective sensing systems for health monitoring and damage detection for buried concrete pipelines subjected to ground deformation. A custom-designed sensing strategy was implemented in a ground rupture experiment with a scaled-down concrete pipeline. The behavior of the pipeline, including the failure modes and damage inflicted to the pipe segments, was monitored during the test. Two modes of failure were identified in the test: (1) compression associated with telescoping-type deformation and (2) bending at the pipeline joints closest to the fault plane. Consequently, future research toward advancing sensing technology for concrete pipelines will likely focus on the behavior of the joints.

Abstract: Fluid ingress is a primary factor that influences freeze-thaw damage in concrete. This paper discusses the influence of fluid ingress on freeze-thaw damage development. Specifically, this paper examines the influence of entrained air content on the rate of water absorption, the degree of saturation, and the relationship between the saturation level and freeze-thaw damage. The results indicate that whereas air content delays the time it takes for concrete to reach a critical degree of saturation it will not prevent the freeze-thaw damage from occurring. The results of the experiments show that when the degree of saturation exceeds 86–88%, freeze-thaw damage is inevitable with or without entrained air even with very few freeze-thaw cycles.

Abstract: Most analytical models for the design of piled embankments or load transfer platforms with geosynthetic reinforcement (GR) include two calculation steps. Step 1 calculates the arching behaviour in the fill and step 2 the load-deflection behaviour of the GR. A calculation method for step 2 based on the results of model tests has been published by Van Eekelen et al. (2012a,b). The present paper analyses and presents a new model for step 1, which is the arching step. Additional tests, which are also presented in this paper, were conducted for this purpose. The new model is a limit-state equilibrium model with concentric arches. It is an extension of the models of Hewlett and Randolph (1988) and Zaeske (2001). The new model results in a better representation of the arching measured in the experiments than the other models mentioned, especially for relatively thin fills. Introducing GR in a piled embankment results in a more efficient transfer of load to the piles in the form of an arching mechanism. The load is then exerted mainly on the piles and the GR strips between the piles, on which the load is approximately distributed as an inverse triangle. The new model presented in this paper describes this behaviour and is therefore meant to describe the situation with GR. The new model provides a physical explanation for observations of the arching mechanism, especially the load distribution on the GR. Other observations with which this model concurs are the dependency on fill height and friction angle. The amount of arching increases with increasing subsoil consolidation and GR deflection. The paper describes how the new model relates to the development of arching as a result of subsoil consolidation.

Abstract: This paper outlines the development of a large-area sensing skin for damage detection in concrete structures. The developed sensing skin consists of a thin layer of electrically conductive copper paint that is applied to the surface of the concrete. Cracking of the concrete substrate results in the rupture of the sensing skin, decreasing its electrical conductivity locally. The decrease in conductivity is detected with electrical impedance tomography (EIT) imaging. In previous works, electrically based sensing skins have provided only qualitative information on the damage on the substrate surface. In this paper, we study whether quantitative imaging of the damage is possible. We utilize application-specific models and computational methods in the image reconstruction, including a total variation (TV) prior model for the damage and an approximate correction of the modeling errors caused by the inhomogeneity of the painted sensing skin. The developed damage detection method is tested experimentally by applying the sensing skin to polymeric substrates and a reinforced concrete beam under four-point bending. In all test cases, the EIT-based sensing skin provides quantitative information on cracks and/or other damages on the substrate surface: featuring a very low conductivity in the damage locations, and a reliable indication of the lengths and shapes of the cracks. The results strongly support the applicability of the painted EIT-based sensing skin for damage detection in reinforced concrete elements and other substrates.

Abstract: Natural disasters, in particular earthquakes, can cause damage to pipelines with disastrous humanitarian, social, economic, and ecologic consequences. Thus, real-time, automatic, or on-demand asses...

Abstract: Existing numerical modeling of three-dimensional (3D) slopes is performed mainly by using the shear strength reduction (SSR) technique based on the linear Mohr–Coulomb (MC) criterion, whereas the n...