International Journal of Geomechanics

This paper presents the use of pseudodynamic method to compute the rotational displacements of gravity retaining walls under passive condition when subjected to seismic loads. The concept of Newmark sliding block method for computing the rotational displacements under seismic condition and the limit equilibrium analysis have been combined in this paper to evaluate the performance of a gravity retaining walls under seismic conditions. One of the main features of the paper is the adoption of a new procedure to evaluate seismic passive earth pressure considering composite curved rupture surface (which is the combination of arc of a logarithmic spiral and straight line) and the dynamic nature of earthquake loading, which is useful to predict rotational displacements accurately. It also determines the threshold seismic acceleration coefficients for rotation using Newmark’s sliding block method. It is shown that the assumption of planar failure mechanism for rough soil-wall interfaces significantly overestimates the threshold seismic accelerations for rotation and underestimates the rotational displacements.

Nanoindentation is a relatively new technique which has been used to measure nanomechanical properties of surface layers of bulk materials and of thin films. In this study, micromechanical properties such as hardness and Young’s modulus of asphalt binders and asphalt concrete are determined by nanoindentation experiments. Indentation tests are conducted on a base binder and two polymer-modified performance grade (PG) binders such as PG-70-22 and PG76-28. In addition, two Superpave asphalt mixes such as SP-B and SP-III are designed using these PG binders, and the corresponding mixes are compacted to prepare asphalt concrete. Aggregate, matrix (Materials Passing No. 4 sieve) and mastic (Materials Passing No. 200 sieve) phases of each asphalt concrete sample are indented using both Berkovich and Spherical indenters. In nanoindentation, an indenter penetrates into asphalt material and the load (milli-Newton) and the depth (nanometers) of indentation are recorded continuously. Indentation load versus displacement data are analyzed using Oliver and Pharr method to measure hardness and Young’s modulus. The unloading data of base binder is a straight line and therefore could not be analyzed using Oliver and Pharr’s method. However, the indentation data of the PG grade binders are successfully analyzed. Young’s modulus value is less than 3 GPa for mastic, 3 to 12 GPa for matrix, and greater than 12 GPa for aggregate studied herein. Based on the hardness data, mastic is 2 to 15 times softer than matrix materials, and matrix is 10 times softer than aggregate materials. The fact that the properties of the mastic can be measured while in the mixture, this study has great potential for realistic characterization of asphalt mixture components. In this study, spherical indenter is found to be suitable for asphalt binders based on the fact that the spherical indenter produces higher indentation depths than the Berkovich indenter. The study contributes significantly to the use of nanoindentation for transportation material characterization.

Pipelines and risers form an essential part of the infrastructure associated with offshore oil and gas facilities. During installation and operation, these structures are subjected to repetitive motions which can cause the surrounding seabed soil to be remolded and soften. This disturbance leads to significant changes in the operative shear strength, which must be assessed in design. This paper presents an analytical framework that aims to quantify the degradation in undrained shear strength as a result of gross disturbance—in this case through repeated vertical movement of a cylindrical object embedded in undrained soil. The parameters of the framework were calibrated using data obtained in a geotechnical centrifuge test. In this test a T-bar penetrometer, which is a cylindrical tool used to characterize the strength of soft soil, was cycled vertically in soil with strength characteristics typical of a deep water seabed. Using simple assumptions regarding the spatial distribution of “damage” resulting from movement of the cylinder, and by linking this damage to the changing undrained shear strength via a simple degradation model, the framework is shown to simulate well the behavior observed in a cyclic T-bar test. This framework can potentially be extended to the similar near-surface behavior associated with seabed pipelines and risers.

Arching is a common phenomenon that is encountered in backfilling behind retaining walls, trenches, or underground voids in the mine. Marston’s equation has been widely used and modified for computing stresses within backfills, duly accounting for the reduction in stresses due to arching. A critical appraisal of Marston’s equation and its improved forms reveal that the average vertical stress within the soil backfill at any depth is governed by the product of earth pressure coefficient $K$ and wall-backfill frictional coefficient $\mu$ , which does not vary significantly with variation in friction angle $\varphi$ of the granular soil backfill. The average normal stress depends on the value assumed for $\delta /\varphi$ and whether the lateral earth pressure coefficient has been assumed as $K_a$ , $K_0$ , or $K_{\text{Krynine}}$ . Therefore, it is not necessary to direct any effort toward determining the friction angle of the backfill precisely. Rather, attention should be paid to the value of $\delta /\varphi$ and the appropriate expression for $K$ . Thus, it can be stated that soil arching is almost insensitive to the friction angle $\varphi$ of the backfill.