MUHAMMAD FAIZ BIN RAHMAT

AF110041

ADVANCE GEOTECHNIC

SECTION 2

Centrifuge Modelling in Geotechnical Engineering

Geotechnical centrifuges are used for research in geotechnical science, an area of civil engineering concerned how geological materials (dirt and rock) interact with the foundations of built structures such as bridges, roads, and houses. In a research laboratory, engineers use centrifuges to study the affect of gravity on soil samples or small-scale models of structures. The experiments serve to measure properties such as the strength, stiffness and capacity of foundations for bridges and buildings, the stability of hillsides and seawalls, etc. Small models do not weigh the same as a full size structure, of course, but the forces created by the centrifuge can artificially recreate the affects of gravity to provide accurate results.

A geotechnical centrifuge is used to test models of geotechnical problems such as the strength, stiffness and capacity of foundations for bridges and buildings, settlement of embankments, stability of slopes, earth retaining structures, tunnel stability and seawalls. Other applications include explosive cratering, contaminant migration in ground water, frost heave and sea ice. The centrifuge may be useful for scale modeling of any large-scale nonlinear problem for which gravity is a primary driving force.

Geotechnical centrifuge tests are important for understanding - and predicting - the effects of earthquakes on buildings, bridges, roads and the ground upon which they stand. That knowledge will enable us to build earthquake-resistant structures and reinforce existing structures to withstand the force of earthquakes.

Yield Surfaces and Plastic Flow Rules in Geomechanics

In order to ensure safe geotechnical building methods, specific applications require certain foundations and structure reinforcements. Tests are quite expensive to carry out, so simulation can be really useful and even essential. Many numerical models have been developed to give a deep insight into soil behavior.

Quick Note on Geotechnical Engineering

There seems to be a general trend when it comes to construction. Offshore structures are constructed in deeper and deeper waters; buildings are constructed increasingly close to each other; offshore wind turbines are developed in deep waters far away from the coasts, which are likely to experience extreme loading conditions. Therefore, in recent decades, geotechnical engineers have developed numerical simulations to cope with this construction trend and ensure safe building methods.

Plasticity and Geomaterials

Materials for which strains or stresses are not released upon unloading are said to behave plastically. Several materials behave in such a manner, including metals, soils, rocks, and concrete, for example. These give rise to an elastic behavior up to a certain level of stress, the yield stress, at which plastic deformation starts to occur.

The elastic-plastic behavior is path-dependent and the stress depends on the history of deformation. Therefore, the plasticity models are usually written connecting the rates of stress, rather than stress, and the plastic strain. The most widespread and well-known plasticity model throughout the industry is based on the von Mises yield surface for which plastic flow is not altered by pressure. Therefore, the yield condition and the plastic flow are only based on the deviatoric part of the stress tensor.

However, this model is no longer valid for soil materials since frictional and dilatation effects need to be taken into account. Lets see how this can be worked out and briefly explain the different soil plasticity models available in the COMSOL Multiphysics simulation software.

Plasticity of Soils and Rocks

For materials such as soils and rocks, the frictional and dilatational effects cannot be neglected. This whole class of materials is well known to be sensitive to pressure, leading to different tensile and compressive behaviors. The von Mises model presented above is thus not suitable for these types of materials. Instead, yield functions have been worked out to take into account the behavior of frictional materials.