Calibrated 3D Computational Modeling of Soil-Structure Systems and Liquefaction Scenarios

Three-dimensional (3D) computational simulation is increasingly allowing for insights into the mechanics of seismic soil-structure system response. Calibration is being facilitated by field, full-scale, and centrifuge model laboratory data. Computational algorithms and scenario-specific graphical user-interfaces are gradually permitting the routine adoption of such geometrically realistic simulation environments. This paper presents an overview of salient recent 3D soil-foundation-structure earthquake response simulations. Developments related to graphical user-interfaces (OpenSeesPL, http://cyclic.ucsd.edu/openseespl) are summarized, demonstrating the current and evolving capabilities towards performance-based earthquake engineering (PBEE). From an OpenSeesPL-generated lateral push-over analysis of a large pile-group, it is shown that corner piles may shoulder a significantly higher level of load (axial, shear, and bending). Evolution of large tensile forces in these piles may warrant careful consideration. Modeling of liquefaction response mechanisms are also discussed, highlighting the role of cyclic mobility and influence of permeability in dictating the level of associated ground shear deformations, and related countermeasure performance

Passive Earth Pressure Force-Displacement Relationships

During a strong earthquake, passive earth pressure can provide resistance to excessive displacements along bridge abutments and pile caps. To account for this contribution, the force-displacement relationship is required, in addition to the peak resistance value. Experiments were performed at the University of California, San Diego to record the passive earth pressure force-displacement relationship behind a 1.7 meter tall vertical wall section. The experimental configuration of the soil container and wall system is described first. Backfill consisting of dense well-graded silty sand was placed in the soil container which measured 5.6 meters long, 2.9 meters wide and 2.15 meters deep. A finite element (FE) model is calibrated next, on the basis of this experimental response. FE analysis is then employed to compute the backfill resistance considering a range of representative backfill soils and depths. Results from these simulations help to illustrate the significant dependence on soil type and supported backfill depth on the passive force-displacement response. Calibrated hyperbolic model parameters are provided to represent the simulated passive resistance for use in practical applications

Liquefaction-Induced Lateral Spreading and Dilative Soil Behavior

During liquefaction, strength and stiffness degradation in sloping liquefied soil may lead to significant cycle-by-cycle shear strain accumulation. Accuracy in quantifying the magnitude of accumulated permanent shear strain is the key to satisfactory modeling of liquefaction-induced lateral spreading. Commonly used stress-space constitutive models may not be easily calibrated to reproduce the observed magnitudes of permanent shear deformation, since the shear flow phase is often accompanied by a minor change in shear stress magnitude. In a newly developed constitutive model, the observed large post-liquefaction shear-strain accumulation is accomplished by introducing a perfectly plastic zone into a multi-yield surface stress-space framework. After the perfectly plastic strain accumulation phase, the tendency for dilation may result in significant regain in shear stiffness and strength. This aspect of soil behavior is also modeled within the aforementioned constitutive model framework. This new model is integrated in an effective-stress fully coupled two-phase (solid and fluid) Finite Element computer code. In this paper, results of numerical simulations conducted using this computational program are discussed. A one-dimensional version of the program is now available on the Internet for on-line execution (http://casagrande.ucsd.edu)

Seismic Response on Dense and Loose Sand Columns

Densification (Compaction) of loose saturated soils has been the most popular method of reducing earthquake related liquefaction potential. Compaction of a foundation soil can be economical when limited in extent, leading to a case of an “island” of improved ground (surrounded by unimproved ground). The behavior of the densified sand surrounded by liquefied loose sand during and following earthquakes is of great importance in order to design the compacted area rationally and optimize both safety and economy. This problem is studied herein by means of dynamic centrifuge model tests. The results of two heavily instrumented-dynamic centrifuge tests on glycerin-water saturated models of loose and dense sand, prepared adjacent to each other are discussed. Observed model response provided an improved understanding of dynamic liquefaction behavior of a densified ground surrounded by a loose liquefiable ground. The test results suggest the following concerns about “Islands” of densified soil: 1) there is a potential strength loss in the densified zone as a result of pore pressure increase due to migration of pore water (or fluid) into the island from the adjacent (loose) liquefied ground; 2) there is a potential for lateral deformation (sliding) within the densified island as the surrounding loose soil liquefies

Survey on detecting and preventing web application broken access control attacks

Web applications are an essential component of the current wide range of digital services proposition including financial and governmental services as well as social networking and communications. Broken access control vulnerabilities pose a huge risk to that echo system because they allow the attacker to circumvent the allocated permissions and rights and perform actions that he is not authorized to perform. This paper gives a broad survey of the current research progress on approaches used to detect access control vulnerabilities exploitations and attacks in web application components. It categorizes these approaches based on their key techniques and compares the different detection methods in addition to evaluating their strengths and weaknesses. We also spotted and elaborated on some exciting research gaps found in the current literature, Finally, the paper summarizes the general detection approaches and suggests potential research directions for the future