Theodoros Triantafyllidis

Prediction of Permanent Deformations in Pavements Using a High-Cycle Accumulation Model

The present paper discusses the application of a high-cycle accumulation (HCA) model originally developed for sand for the prediction of permanent deformations in an unbound granular material (UGM) used for base and subbase layers in pavements. Cyclic triaxial tests on precompacted samples of an UGM have been performed in order to validate and calibrate the model. The stress amplitude, the initial density, and the average stress were varied. The test results are compared to those of air-pluviated samples of sand (subgrade material). Some significant differences in the behavior of both materials under cyclic loading are outlined. It is demonstrated that the functions describing the intensity of accumulation can be maintained for an UGM with different material constants, but that the flow rule must be generalized in order to describe the anisotropy. Recalculations of the laboratory tests show a good prediction of the modified HCA model.

Comparison of Cyclic Triaxial Behavior of Unbound Granular Material under Constant and Variable Confining Pressure

Cyclic stresses due to passing wheels impose an accumulation of permanent strains in layers of unbound granular materials (UGMs) of flexible pavements. The hollow cylinder triaxial test would be the most appropriate test to simulate the in situ stress conditions but it is difficult to perform on UGMs due to their large maximum grain size. The simpler axisymmetric cyclic triaxial test does not consider the shear stress components. It can be performed with a constant confining pressure (CCP) or a variable confining pressure (VCP). CCP and VCP tests are commonly assumed to deliver similar residual and resilient strains as long as the average stress is the same. Thus, the simpler CCP test is mostly used in pavement engineering. However, this assumption is based on limited test data in the literature and may not be on the safe side. The present paper documents a comparative study of CCP and VCP tests on UGM. The study is mainly dedicated to the permanent deformations. The results show that only for some special stress paths do both types of tests deliver similar permanent axial or volumetric strains. For some other stress paths the CCP tests may underestimate the permanent axial strain in comparison to the corresponding VCP test.

A Large-Scale Soil-Structure Interface Testing Device

A large-scale testing device for the experimental investigation of soil-structure interactions was developed. It allows an observation of the evolution of deformations and the measurement of the stresses in the soil-structure contact zone under 1 g-conditions. The tests are designed as boundary value problems for benchmark tests of numerical simulations. They can be used to validate contact formulations and constitutive equations for the soil. The dimensions (soil height >1.5 m) provide a sufficiently high stress level for simulations with constitutive models for soil. The major part of the device is an instrumented wall section with modifiable surface roughness representing, e.g., a part of a pile shaft or a steel sheet. This wall can be displaced quasi-statically relative to an adjacent soil body. The distribution of shear and normal stress and the wall displacements are measured. Digital image correlation (DIC) is used to evaluate the soil deformations. The paper focuses on the basic concept, the design, and the instrumentation details of the testing device. Additionally, some selected test results are shown.

Interpretation of Vibratory Pile Penetration Based on Digital Image Correlation

A combined interpretation of force measurements together with the evaluation of dynamic motion around the pile based on digital image correlation (DIC) is performed to identify soil deformation during vibratory pile driving in model tests. The tests are executed under water-saturated 1g-conditions. We prove the occurrence of the so-called cavitational pile driving but without the experimental evidence of the forming of a cavity under the pile tip. Using the DIC results, first attempts are made to evaluate the volumetric cyclic deformation of soil around the pile tip during the vibro-penetration. The results show an alternation of contractancy and dilatancy in proximity of the pile tip with volumetric peak-to-peak strain amplitudes up to 2 %. They indicate drained or at least partially drained conditions. Based on the test results, existing phenomenological interpretations of soil deformation due to pile penetration are reviewed.

Shearing of Materials with Intermittent Joints

The strength of fractures is much lower as a rule than that of intact rock. As a result they play a controlling part in the mechanical behaviour in general and the failure in particular of rock mass. Though a large volume of experimental data is available on the shear resistance of joints, as well as on the propagation of single cracks, the same is not true for the mechanical behaviour of intermittent joints. The experimental data available in this case are limited and the strength of rock mass with intermittent joints is usually modelled using averaged values of cohesion or assuming the fractures to be continuous. In the present work, the results of simple shear tests on a series of gypsum specimens with pre-existing cracks are presented. Twelve different crack orientations and two normal stresses were tested. The hypothesis of averaged cohesion and the theory of fracture mechanics are used to reproduce the results. It is found that fracture mechanics provides a more suitable model for the experimental results, especially when crack interaction is taken into account.

Dynamic Problem for the Deformation of Saturated Soil in the Vicinity of a Vibrating Pile Toe

A numerical study conducted recently by the authors showed that the vibration of a pile in saturated granular soil leads to the formation of a zone with nearly zero effective stresses (liquefaction zone) around the pile toe. The dynamic problem was solved with the finite-element program Abaqus/Standard using a hypoplasticity model for soil with the assumption of zero soil permeability and without a mass force. A question which still remained open was the influence of the soil permeability and the gravity force on the solutions. In the present study, the problem is solved with nonzero permeability and gravity, and the solutions are compared with those obtained earlier. For this purpose, a user-defined element has been constructed in Abaqus to enable the dynamic analysis of a two-phase medium with nonzero permeability. The solutions show that high permeability and gravity do not prevent the formation of a liquefaction zone around the pile toe in spite of the fact that a build-up of the pore pressure is inhibited by the pore pressure dissipation.

Estimation of the Strength of Stratified Rock Mass

Stratified rock formations are often encountered in engineering structures with large dimensions, such as tunnels, caverns, etc. In the present work, the question of how and when such materials fail is addressed. It is argued that average values cannot provide appropriate measures for the strength of layered rock, as local stresses and strains differ greatly in the various constituents and one may fail, while others remain intact. The present work attempts, therefore, to formulate a solution covering both elasticity and failure in three dimensions, though simple enough to be easy to use in engineering applications. To this end, the local stress and strain state in each constituent were evaluated, assuming the constituents to be orthotropically elastic up to the point of failure. Subsequently, several failure criteria are considered and an example is presented to illustrate the model’s response. A discussion on the effect of geometry and on the values of local versus global quantities follows.

FE Simulation of Model Tests on Vibratory Pile Driving in Saturated Sand

The present study reports the extensive comparison of model tests with numerical simulations of vibro-driven pile installation in saturated sand. The purpose of the study is to validate existing simulation techniques and to investigate the ability of those to reproduce effects experimentally observed during pile installation. A limited number of cycles has been considered and the focus is placed on the cyclic evolution of soil deformations and stresses. Two axisymmetric FE models have been developed for the simulation of model tests. In the first, the pile-soil interaction is modeled in a simplified way by applying a sinusoidal displacement boundary condition at the soil-pile interface close to the pile toe. The second model simulates the performed model tests more realistically by including the pile-oscillator system. The \(\mathbf {u}\)-p formulation has been adopted in both models for the dynamic analysis of fluid-saturated solids with nonzero permeability. A hypoplastic constitutive model with intergranular strain has been selected to describe the mechanical behavior of the soil. The soil displacements and the evolution of pile resistance are compared. The good agreement between the results confirms that the pile installation process can be satisfactory reproduced numerically.

Experimental Investigation of Vibratory Pile Driving in Saturated Sand

An experimental study using half-model tests to investigate the vibro-penetration of piles in saturated sand is presented. In the tests, a model pile with half-circular cross section is penetrated along an observation window by means of a vibrator. The use of a high speed camera and a sophisticated image acquisition system enables the observation of the penetration process with sufficient temporal and spatial resolution. A consistent evaluation of pile head and toe motion is achieved using a combined interpretation of acceleration measurements and Digital Image Correlation (DIC) analysis. The application of DIC to evaluate cyclic soil deformations reveals the relation of typical displacement patterns in the soil and characteristic phases of pile penetration. Pore water pressure measurements at two fixed locations show the dependence of pore water pressure evolution on the penetration mode and soil density. The extensive measurements allow an improved interpretation of the typical penetration modes during vibratory pile driving.

Stress Paths on Displacement Piles During Monotonic and Cyclic Penetration

In this contribution, a study on the behavior of instrumented model piles in slow, cyclic penetration tests using a cylindrical full model test set-up is presented. The tests are performed under 1g-conditions in a uniform medium sand. A hydraulic driving system enables a displacement controlled penetration similar to the pile motion during vibro-driving at strongly reduced frequency. The pile instrumentation allows the measurement of shaft and tip force during the driving process. Systematic variation of soil density and displacement amplitude reveals the occurrence of typical stress paths of vibratory pile penetration. By comparison with results from monotonic and vibratory penetration tests, the influence of the penetration mode is deduced. Results from FE simulations applying a hypoplastic soil model help to illustrate the strong requirements and the considerable challenges to obtain realistic simulations of cyclic pile penetration processes. Some hints towards a further numerical modeling of the tests are given.