Bone quality and the conditions that affect it make up a large field of study. One specific area of interest is the loss in bone strength during exposure to microgravity. The femoral neck (FN) region in particular is an important region of study since a FN failure has such a detrimental effect on mobility. The objective of this study was to characterize the effects of microgravity and recovery on the FN in the adult male hindlimb unloaded (HU) rat model. This was done through peripheral quantitative computed tomography (pQCT), mechanical testing in two different loading conditions, and estimated strength indices. Adult male Sprague-Dawley rats (6-mo) were grouped into baseline (BL), ambulatory cage control (CC) and hindlimb unloaded (HU); HU and CC animals were further divided into sub-groups (n=15 each): HU euthanized after 28 days of suspension, and HU euthanized after 28, 56, and 84 days of recovery with CC groups being euthanized at each of these time points. The excised right and left femoral necks were both scanned ex vivo using pQCT. Quasi-static mechanical testing was performed with the right femurs positioned vertically and the left femurs positioned laterally at a -10 degree angle. A series of strength indices was used to attempt to predict the mechanical testing results, including a compression index, a bending index and an alternative combination of the two. HU exposure led to 6.3 percent lower bone mineral content (BMC), compared to BL and 7.8 percent lower total volumetric bone mineral density (vBMD) at the FN. The vertical or axial loading showed a 17.1 percent drop in mechanical strength due to HU exposure. The lateral loading test revealed a 5.4 percent drop in strength, showing that HU had a greater effect on the axial loading configuration. Also, after just 28 days of recovery, the axial loading test revealed a complete recovery of strength. None of the strength indexes completely predicted the mechanical behavior of the FN. In the right femur, the combined index had the highest correlation with an R value of 0.94. The bending strength index had the highest correlation in the left lateral testing with an R value of 0.98. However, in all the cases, the strength indexes failed to predict the mechanical behavior at all the time points. In general, the strength indexes provide valuable input, but fail to replace mechanical testing.

Placement and compaction of the backfill behind an earth retaining wall may induce a vertical shear force at the soil-to-wall interface. This vertical shear force, or downdrag, is beneficial for the stability of the structure. A significant reduction in construction costs may result if the downdrag is accounted for during design. This potential reduction in costs is particularly interesting in the case of U.S. Army Corps of Engineers lock walls. A simplified procedure is available in the literature for estimating the downdrag force developed at the wall-backfill interface during backfilling of a retaining wall. However, finite element analyses of typical U.S. Army Corps of Engineers lock walls have shown that the magnitude of the downdrag force may decrease during operation of the lock with a rise in the water table in the backfill. They have also shown that pre- and post-construction stress paths followed by interface elements often involve simultaneous changes in shear and normal stresses and unloading-reloading. The hyperbolic formulation for interfaces (Clough and Duncan 1971) is accurate for modeling the interface response in the primary loading stage under constant normal stress. However, it has not been extended to model simultaneous changes in shear and normal stresses or unloading-reloading of the interface. The purpose of this research was to develop an interface model capable of giving accurate predictions of the interface response under field loading conditions, and to implement this model in a finite element program. In order to develop the necessary experimental data, a series of tests were performed on interfaces between concrete and two different types of sand. The tests included initial loading, staged shear, unloading-reloading, and shearing along complex stress paths. An extended hyperbolic model for interfaces was developed based on the results of the tests. The model is based on Clough and Duncan (1971) hyperbolic formulation, which has been extended to model the interface response to a variety of stress paths. Comparisons between model calculations and tests results showed that the model provides accurate estimates of the response of interfaces along complex stress paths. The extended hyperbolic model was implemented in the finite element program SOILSTRUCT-ALPHA, used by the U.S. Army Corps of Engineers for analyses of lock walls. A pilot-scale test was performed in the Instrumented Retaining Wall (IRW) at Virginia Tech that simulated construction and operation of a lock wall. SOILSTRUCT-ALPHA analyses of the IRW provided accurate estimates of the downdrag magnitude throughout inundation of the backfill. It is concluded that the extended hyperbolic model as implemented in SOILSTRUCT-ALPHA is adequate for routine analyses of lock walls. / Ph. D.