Evaluation of the effect of particle cementation on the stiffness of uniform sand was carried out by measuring compression wave velocities (VP) and shear wave velocities (VS) on both clean and artificially cemented specimens. Piezoelectric transducers (PT) were used to perform the majority of the measurements. Shear wave velocity (VS), shear moduli (G) and material damping ratio (D) of clean and artificially cemented specimens were also determined using resonant column (RC) testing. Linear (shearing strains ≤ 0.001%) and nonlinear (shearing strains > 0.001%) behavior of the specimens were evaluated in the resonant column tests. The sand selected for this investigation is commonly known as Hickory sand, from the Hickory formation, western Llano uplift, Texas. This material was selected for its grain geometry and gradation; it consists of uniformly graded sand with rounded particles. The sand specimens were artificially cemented with a solution of hydrated sodium silicate and water. Sodium silicate is an alkaline compound obtained from the reaction of sodium hydroxide and silica. All artificially cemented specimens and uncemented hickory sand specimens were formed by pluviation through air. The microstructure of the specimens was visually assessed with images obtained from both optical and scanning electron (SEM) microscopes. These images confirmed that the procedure used to form artificially cemented specimens provides cementation around the contacts while some grain-to-grain contact appears to be preserved. Seismic and drained strength measurements on Hickory sand specimens were obtained from different cement concentrations and compared with results from clean sand specimens. Among the findings of this investigation are: (1) the procedure to artificially cement sand specimens in the laboratory was successful, (2) the slopes (nP and nS) obtained from the relationships between compression and shear wave velocities with effective isotropic confining pressure in log-log scale decrease as the cement content increases, and (3) as increasing amounts of cement are added to the sand particles, the nonlinearity of the specimens increases up to certain amount of cement, after which the nonlinearity of the specimen decreases and tends towards rock-like behavior. / text

Oceanic lithosphere constitutes the upper boundary layer of the Earth’s convecting mantle. Its structure and evolution provide a vital window on the dynamics of the mantle and important clues to how the motions of Earth’s surface plates are coupled to convection in the mantle below. The three-dimensional shear-velocity structure of the upper mantle beneath the Atlantic Ocean is investigated to gain insight into processes that drive formation of oceanic lithosphere. Travel times are measured for approximately 10,000 fundamental-mode Rayleigh waves, in the period range 30-130 seconds, traversing the Atlantic basin. Paths with >30% of their length through continental upper mantle are excluded to maximize sensitivity to the oceanic upper mantle. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. Phase velocity shows a strong dependence on seafloor age, and removing age-dependent velocity from the 2-D maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. Depth-dependent variations in vertically-polarized shear velocity (Vsv) are determined with two sets of 3-D models: a layered model that requires constant VSV in each depth layer, and a splined model that allows VSV to vary continuously with depth. At shallow depths (~75 km) the seismic structure shows the expected dependence on seafloor age. At greater depths (~200 km) high-velocity lithosphere is found only beneath the oldest seafloor; velocity variations beneath younger seafloor may result from temperature or compositional variations within the asthenosphere. The age-dependent phase velocities are used to constrain temperature in the mantle and show that, in contrast to previous results for the Pacific, phase velocities for the Atlantic are not consistent with a half-space cooling model but are best explained by a plate-cooling model with thickness of 75 km and mantle temperature of 1400oC. Comparison with data such as basalt chemistry and seafloor elevation helps to separate thermal and compositional effects on shear velocity.