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There are few empirical measurements of velocity, shear velocity, sand transport, morphological change on the windward slopes of dunes.This thesis compares field measurements on a barchan dune in Oman with calculations using a mathematical model (FLOWSTAR) and measurements in a wind tunnel. All three techniques demonstrate similar patterns of velocity, confirming the acceleration of flow up the windward slope, deceleration between the crest and brink and significant flow deceleration upwind of and at the toe of the dune. The FLOWSTAR model is unable accurately to predict airflow at the brink and its predictions near the surface are highly susceptible to small-scale terrain irregularities. The measurements of shear velocity in the field and those predicted by the FLOWSTAR model reflect observations of previous studies including the widely reported upwind reduction in shear velocity. Such a reduction in shear velocity upwind of the dune should result in a reduction in sand transport and sand deposition. This is not observed in the field. Wind tunnel modelling using a near-surface pulse-wire probe suggests that the field and FLOWSTAR methods of shear velocity derivation are inadequate. The wind tunnel results exhibit no reduction in shear velocity upwind of or at the toe of the dune. This maintenance of upwind shear stress may be caused by concave (unstable) streamline curvature, which is not taken into account by the field and FLOWSTAR techniques. From this hypothesis, a new model of dune dynamics is developed relying on the establishment of an equilibrium between windward slope morphology, streamline curvature and streamwise acceleration.

The system for predicting tyre performance on sand, measuring sand strength with a cone penetrometer and using non-dimensional empirical curves developed by the Waterways Experiment Station (WES) of the U. S. Army was investigated. A series of tyre tests on dry Cresswell sand were carried out and the results were in complete disagreement with the WES system, in both its original and revised forms. It was therefore decided to try to discover the basic soil mechanics of such a system and modify it accordingly. Critical State Soil Mechanics describes two types of soil behaviour, dilating and weakening or compacting and strengthening. It was found that the first of these processes occurred in most situations likely to be found naturally, compaction occurring only in the loosest states obtainable under laboratory conditions. Under dilating conditions sand strength is described by the density, γ (Gamma). and the angle of internal friction, Ø. The angle of friction, for a single sand, was found to vary over a very wide range, depending on the state of compaction and the confining pressure. Density does not vary greatly. The cone penetrometer gradient, G, was found to be related to Ø at a low confining pressure, and the relationship was well described by the theory of Durgunoglu and Mitchell (1975). This lead to the idea that tractive performance would be dependent on Ø, which would be lower the higher the tyre contact pressure. A series of tyre tests on a single tyre on two sands showed clearly that performance depended on both tyre pressure and tyre load. The WES numeric only contains pressure. It was therefore decided to include both parameters by expressing performance by several curves depending on the tyre deflection. Deflection. being expressed as a ratio of tyre diameter rather than tyre section height. The new system was shown to describe all of the WES data better and more logically than their system. The reason why the system cannot describe performance in Yuma and Mortar sands with the same single curve as for Leighton Buzzard and Cresswell remains a mystery.

Since the horrific attacks on September 11th 2001, the United States government and research community have been focused on how to better protect US assets across the Globe. This push for safety led the research community to develop “F2656-07 Standard Test Method for Vehicle Crash Testing of Perimeter Barriers” in 2007 which standardized the method of validating a perimeter security barrier’s ability to withstand an impact from an attacking vehicle. Many of these security barriers rely on weak cohesionless soils to stop attacking vehicles. Designers currently rely heavily on hand calculations and engineering judgment when sizing these installations. This simplified analysis is generally used because of the complex nature of these soils under impact. These soils could be simulated in advanced finite element simulations; however, traditional modeling techniques will not allow for the simulation of these complex behaviors. Due to the complex nature of these simulations, new modeling techniques need to be evaluated and their use needed to be perfected. From this, a new method for creating a predictive simulation of a low speed impactor into a weak cohesionless soil was generated. This paper presents the development of a method by which a predictive simulation was created using only standard soil tests parameters. This paper also presents measured data from physical impact tests utilized to validate the method by which the simulation was generated. Next, the paper gives a detailed comparison of the results of the physical testing and the simulated impacts. The paper finally gives a summary of where the method is successful and where it needs improvement. The resulting methodology developed in this paper defines a reasonable process for creating a predictive simulation of a rigid impactor penetrating weak cohesionless sands. This finding is validated by a reasonable correlation between the measured and simulated impact penetrations. This paper also highlights the high variability of measured penetrations when testing with these soil materials.