Thomas F. Zimmie

Centrifuge modeling of surface blast effects on underground structures

The effects of surface blasts on underground structures were studied through centrifuge model tests. Centrifuge scaling relationships make it possible to model the effects of large explosions, using a relatively small quantity of explosives under a high g-level. Centrifuge tests, conducted at 70 g, using 2.6 mg of TNT equivalent of explosives, resulted in explosions equivalent to those using 8.7 kN (0.9 tons) of TNT equivalent under normal (1 g) gravity. Strains induced at different locations of the model structure due to the explosion were measured using strain gages. Results indicated that the strains depend on the thickness and nature of the intervening medium. The presence of a polyurethane geofoam compressible inclusion barrier appeared to mitigate the impact of the explosion. Centrifuge model testing is useful in determining the effectiveness of different design alternatives, in studying the mitigating effects of different barrier systems, and in verifying and calibrating results of numerical models related to explosions and underground structures.

Mitigation of blast effects on underground structure using compressible porous foam barriers

Explosions on the ground surface induce strains on underground structures, such as tunnels and pipelines. Depending on the size of the explosion, distances involved, and the nature of the intervening material, these strains can be relatively high, thus causing severe damage to the structure. Barriers may be used to protect a structure, by reducing the strains created as a result of the explosion. The role of compressible protective barriers (made of polyurethane foam) and rigid barriers (made of concrete) in reducing the impact of a surface explosion, were studied. Results of physical model experiments, conducted on geotechnical centrifuge, are presented and analyzed to evaluate the effectiveness of different types of barriers. Numerical model analyses were also conducted to further understand the behavior of different types of protective covers compare them to a soil cover. Results of such studies can help design barrier systems capable of mitigating effects of explosions, thus protecting essential components of underground infrastructure.

Analyses, Simulations and Physical Modeling Validation of Levee and Embankment Erosion

We present a computer simulation of hydraulic erosion on levees, dams, and earth embankments, with emphasis on rill and gully initiation and propagation. We focus on erosion features that occur after an earthen structure is overtopped. We have developed a 3D fluid and hydraulic erosion simulation engine using Smoothed Particle Hydrodynamics (SPH). We present the results of digital simulations for different soil types. Furthermore, small-scale physical models of levees composed of different soils were constructed and tested experimentally. The digital simulations are compared to physical experimental results to validate the computer models.

Quantitative analysis of simulated erosion for different soils

Levee overtopping can lead to failure and cause catastrophic damage, as was the case during Hurricane Katrina. We present a computer simulation of erosion to study the development of the rills and gullies that form along an earthen embankment during overtopping. We have coupled 3D Smoothed Particle Hydrodynamics with an erodibility model to produce our simulation. Through comparison between simulations and between simulation and analogous laboratory experiments, we provide quantitative and qualitative results, evaluating the accuracy of our simulation.

Simulating Levee Erosion with Physical Modeling Validation

This paper studies rill and gully initiation and propagation on levees, dams, and general earth embankments. It specifically studies where these erosion features occur, and how long a particular embankment can sustain overtopping before breaching and catastrophic failure. This contrasts to previous levee erosion analysis, which has primarily concerned the final effects of erosion, such as soil loss, depth of scour and breach width. This paper describes the construction of scaled-down physical models of levees composed of different homogeneous sands, as well as sand- clay mixtures, and their laboratory testing. A 3-D laser range scanner captured the surface features of the physical model, before and after erosion. The resulting data is utilized in developing digital simulations of the rill erosion process. Those simulations combine 3-D Navier-Stokes fluid simulations and a segmented height field data structure to produce an accurate portrayal of the erosive processes, which will be validated by physical modeling.

CENTRIFUGE RESEARCH OF COUNTERMEASURES TO PROTECT PILE FOUNDATIONS AGAINST LIQUEFACTION-INDUCED LATERAL SPREADING

Repeated experience of foundation and superstructural damage during earthquakes associated with ground failure and liquefaction, have shown the importance of lateral spreading and especially of the presence of a shallow nonliquefiable soil layer as a cause of damage. Typically such strong nonliquefiable layer or crust, riding on the liquefiable soil where the lateral spread actually occurs, pushes laterally against the piles and pile cap, inducing large lateral forces, deformations and bending in the foundation. The effects depend on the free field deformation, characteristics of the nonliquefiable layer and foundation, and characteristics of the superstructure. For an existing pile foundation, the first step is to verify if the pile foundation has enough stiffness and strength to resist the soil lateral forces, and especially the passive thrust applied by the nonliquefiable layer, with acceptable foundation displacement and rotation and no damage to the foundation elements, in which case no countermeasures are needed. If this evaluation concludes that the foundation does not possess the required stiffness and strength to prevent distress, three mitigation strategies are often used: (i) Remediation countermea-sure of the liquefiable soil at the site to prevent the free field liquefaction and lateral spreading from occurring in the first place; (ii) retrofitting of the foundation by reinforcing it, for example by adding piles or strengthening the existing piles; and (iii) isolation of the foundation, and especially of the pile cap, from the nonliquefiable layer in the free field by a retrofitting countermeasure where a “soft fuse” such as a soft soil or other material or device are installed at shallow depths near the foundation, hence limiting the lateral force applied to the piles and pile cap to a low value which minimises the deformation and bending of the foundation. The paper discusses these countermeasure strategies through several series of centrifuge model tests of single piles and pile groups conducted with the participation of the authors at the RPI geotechnical centrifuge.

INSTRUMENTATION AND CALIBRATION OF GEOTEXTILES USED IN CENTRIFUGE MODELING OF SLOPES

Research on the application of geotextile strips used as reinforcement in marginally stable soft soil slopes was performed with a geotechnical centrifuge. A variety of instrumentation was used in the model studies. When centrifuge models are used, the elements being modeled must be scaled down by the level of centrifuge acceleration—that is, the g level used in the modeling process. This results in the use of very small geotextile reinforcing strips, which are difficult to instrument. In this study, small strips of nonwoven geotextile were driven into the model slope while the centrifuge was in flight. These strips were instrumented with miniature foil-type strain gauges. The following items are discussed in detail: various stages of installation of the strain gauges on nonwoven geotextiles, selection of suitable bonding techniques, use of thin coatings for gauge protection, and selection of suitable bridge excitation voltages for the gauges. The devices used in the calibration process, and the calibration results, are presented.

Physical and Numerical Modeling to Study Effects of an Underwater Explosion on a Buried Tunnel

AbstractThe effects of an underwater explosion on a tunnel buried below submerged ground were studied through a combination of physical model tests, utilizing a geotechnical centrifuge, and numeric...

Ocean Rise Instability in Coastal Clay Slopes and Possible Countermeasures: A Centrifuge Modeling Study

Increase in saturation in natural clayey slopes along coastal zones as a result of tsunamis or storm surges may cause flow slides or failures. One of the common treatments is to increase the overall stability by soil replacement and/or re-compaction, which is often difficult to implement, expensive, and, most importantly, damages the natural vegetation. In this paper, remedial effectiveness of a relatively economical and environmentally friendly method involving insertion of geotextile strips with drainage capability into natural clayey slopes is evaluated through a series of centrifuge tests. The test results demonstrate the effectiveness of the employed technique to increase the stability of slopes and their drainage capability as well as to reduce the deformations under surcharge loadings.

Effects of Surface Explosions on top of Earth Embankment Dams

An explosion on the crest of an earth embankment dam can create a crater causing a dam breach, eventually leading to extensive flooding downstream of the dam. Results are presented from a series of centrifuge model tests and numerical modeling to characterize crater formation and consequent breaching of an earth dam. The results of the centrifuge model tests were utilized to calibrate the numerical model, which can be used to conduct parametric analyses to study the effects of varying the soil characteristics or geometry in the future.