B.N. Madhusudhan

Wave Propagation Attenuation and Threshold Strains of Fully Saturated Soils with Intraparticle Voids

The prediction of ground response against wave propagation is essential for construction materials and the safe design of civil engineering infrastructures, such as, embankments, retaining walls, or foundations subjected to machine vibrations. For ground response analysis studies, the shear modulus and material damping, which are expressed as a function of shear strain, are the important properties of soils. The volumetric threshold strain is also a key property in order to evaluate possible permanent deformations or substantial increase in pore water pressure in saturated soils during dynamic loading. The paper presents dynamic test data derived from resonant column experiments on volcanic granular soils which are characterized by low unit weight and weak grains of intraparticle voids. These materials can be used as potential lightweight backfill in retaining walls or other applications with a demand in reduction of vertical or horizontal stresses to the ground and structural facilities. Additional experiments on quartz sands were conducted for comparison. The volcanic soils had much lower small-strain shear modulus than that of quartz sands and higher linearity in the range of medium strains, by means of normalized stiffness and material damping curves. The elastic and volumetric thresholds were shifted to larger strains for the volcanic soils in comparison to the quartz sands. Different prevailed micromechanisms possibly contributed to these observed trends.

Damping of Sands for Varying Saturation

AbstractA series of resonant column tests have been performed in the torsional mode of vibration to assess the effect of saturation, starting from the near dry state to the fully saturated state, on the damping ratio of sands corresponding to the threshold strain level. Tests were carried out on three different gradations of sand for various combinations of relative density and effective confining pressure. For fine sands, a certain optimum degree of saturation exists at which the damping ratio minimizes; it is known that a decrease in Sr from a fully saturated state leads to a continuous increase in the matric suction. With an increase in the relative density, the optimum degree of saturation for fine sand increases marginally from 1.38 to 1.49%, but does not show any dependency on the effective confining pressure. In contrast, the minimum values of the damping ratio for medium and coarse sands are found to always correspond to the near dry state. The damping ratio decreases continuously with increases i...

On determining the elastic modulus of a cylindrical sample subjected to flexural excitation in a resonant column apparatus

For resonant column tests conducted in the flexure mode of excitation, a new methodology has been proposed to find the elastic modulus and associated axial strain of a cylindrical sample. The proposed method is an improvement over the existing one, and it does not require the assumption of either the mode shape or zero bending moment condition at the top of the sample. A stepwise procedure is given to perform the necessary calculations. From a number of resonant column experiments on aluminum bars and dry sand samples, it has been observed that the present method as compared with the one available in literature provides approximately (i) 5.9%-7.3% higher values of the elastic modulus and (ii) 6.5%-7.3% higher values of the associated axial strains.

Performance of Fiber Reinforcement in Completely Decomposed Granite

Adding discrete fibers to soils can improve their strength; however, fiber reinforcement remains scarce in practice. Previous studies on the performance of soils reinforced with discrete fibers consist mainly of laboratory studies with either clay or, most often, uniform sand as the host soil, so there is a lack of data on other types of soils such as weathered soils, which tend to be well graded. Unlike uniform soils, which are generally dilative, well-graded soils usually show a contractive behavior. This study examines the effect of adding fibers to a completely decomposed granite (CDG) typical of many residual soils, which has the characteristics to be sensitive to material and sample preparation and also to be compressive during shearing. It is found that adding discrete fibers to the CDG homogenizes it because the reinforced soil is not sensitive to the method of material or sample preparation. It is also found that, despite its compressive nature, fibers mobilize extra strength compared with the unreinforced soil, and this effect does not reduce at large confining stresses. Adding discrete fibers to soils can improve their strength; however, fiber reinforcement remains scarce in practice. Previous studies on the performance of soils reinforced with discrete fibers consist mainly of laboratory studies with either clay or, most often, uniform sand as the host soil, so there is a lack of data on other types of soils such as weathered soils, which tend to be well graded. Unlike uniform soils, which are generally dilative, well-graded soils usually show a contractive behavior. This study examines the effect of adding fibers to a completely decomposed granite (CDG) typical of many residual soils, which has the characteristics to be sensitive to material and sample preparation and also to be compressive during shearing. It is found that adding discrete fibers to the CDG homogenizes it because the reinforced soil is not sensitive to the method of material or sample preparation. It is also found that, despite its compressive nature, fibers mobilize extra strength compared with the unreinforced soil, and this effect does not reduce at large confining stresses.

Geotechnical profiling of deep-ocean sediments at the AFEN submarine slide complex

The Atlantic Frontier Environmental Network (AFEN) submarine slide complex is located 95 km NW of the Shetland Islands, at a water depth of c. 1000 m. It is thought to have occurred some 2800 years ago. The slide complex is c. 4 km wide and 13 km in length, and lies on a shallow slope, between 0.7 and 2.5°. Previous sampling and laboratory testing of the very soft sediment has been limited, and did not penetrate the basal failure surface, nor the underlying sediments. This paper presents the first intensive geotechnical profiling of deep-sea sediments from the AFEN slide complex, carried out on core obtained during the July 2014 expedition of the R.V. Pelagia. Importantly, this core sampled through, and below, the failure surface of the AFEN slide, and was subjected to unusually closely spaced geotechnical testing using a range of index and classification testing techniques. These are shown to have great value, when taken together with standard sedimentological logging, in establishing ground profiles. From these data new mechanisms of triggering the long runout slope failure at AFEN are postulated, and explain how submarine slides here and elsewhere can have occurred on very shallow slopes, of the order of 1 – 2°.

Presence and Consequences of Coexisting Methane Gas With Hydrate Under Two Phase Water‐Hydrate Stability Conditions

Methane hydrate saturation estimates from remote geophysical data and borehole logs are needed to assess the role of hydrates in climate change, continental slope stability, and energy resource potential. Here we present laboratory hydrate formation/dissociation experiments in which we determined the methane hydrate content independently from pore pressure and temperature and from electrical resistivity. Using these laboratory experiments, we demonstrate that hydrate formation does not take up all the methane gas or water even if the system is under two phase water-hydrate stability conditions and gas is well distributed in the sample. The experiment started with methane gas and water saturations of 16.5% and 83.5%, respectively; during the experiment, hydrate saturation proceeded up to 26% along with 12% gas and 62% water remaining in the system. The coexistence of hydrate and gas is one possible explanation for discrepancies between estimates of hydrate saturation from electrical and acoustic methods. We suggest that an important mechanism for this coexistence is the formation of a hydrate film enveloping methane gas bubbles, trapping the remaining gas inside.