Hans Mattsson

EFFECTS OF END RESTRAINT AND STRAIN RATE IN TRIAXIAL TESTS

Abstract Inhomogeneities caused by end restraint and insufficient drainage during conventional compression triaxial tests are analysed by a numerical method. A finite element model is presented to simulate the testing procedure. The soil-platen interaction is represented by contact elements which allow frictional sliding between contacting nodes. The soil mass is represented by the modified Cam clay model. Coupled hydro-mechanical analyses are carried out in order to simulate both drained and undrained tests. The distributions of stresses and strains in the specimen for different end conditions are compared with the ideal case where no end restraint exists, in order to find representative measuring positions in the sample. Different rates of axial strain are tested in order to study the inhomogeneities caused by insufficient drainage during drained tests. Simulated results show that both end restraint and insufficient drainage can cause the barrel-shape deformation of the specimen. Stress-strain and strength properties based on global measurements are not a good representation of the true material behaviour of one single soil element at constitutive level.

A METHOD TO CORRECT YIELD SURFACE DRIFT IN SOIL PLASTICITY UNDER MIXED CONTROL AND EXPLICIT INTEGRATION

SUMMARY When applying an explicit integration algorithm in e.g. soil plasticity, the predicted stress point at the end of an elastoplastic increment of loading might not be situated on the updated current yield surface. This so-called yield surface drift could generally be held under control by using small integration steps. Another possibility, when circumstances might demand larger steps, is to adopt a drift correction method. In this paper, a drift correction method for mixed control in soil plasticity, under drained as well as undrained conditions, is proposed. By simulating triaxial tests in a Constitutive Driver, the capability and eƒciency of this correction method, under di⁄erent choices of implementation, have been analysed. It was concluded that the proposed drift correction method, for quite marginal additional computational cost, was able to correct successfully for yield surface drift giving results in close agreement to those obtained with a very large number of integration steps. ( 1997 by John Wiley & Sons, Ltd.

On a constitutive driver as a useful tool in soil plasticity

Abstract A mathematical basis for the development of Constitutive Drivers in soil plasticity has recently been proposed by the authors. A Constitutive Driver is here understood as a computer program, containing a number of selected constitutive models, in which different laboratory and field tests can be simulated and model parameters optimised. As a pilot study of the mathematical concept, a Constitutive Driver for soils, in the form of a PC-program, has been developed. The paper discusses this particular program, i.e. its structure, the mathematical basis, included soil models and some application examples, to give an idea of how a general and user-friendly Constitutive Driver can be designed. Such a program can be used for practical, research and educational purposes. In fact, it is believed that so many important applications for Constitutive Drivers exist that it would be beneficial if such programs were easily accessible as complementary programs in commercial software.

Numerical Analyses of Earthquake Induced Liquefaction and Deformation Behaviour of an Upstream Tailings Dam

Much of the seismic activity of northern Sweden consists of micro-earthquakes occurring near postglacial faults. However, larger magnitude earthquakes do occur in Sweden, and earthquake statistics indicate that a magnitude 5 event is likely to occur once every century. This paper presents dynamic analyses of the effects of larger earthquakes on an upstream tailings dam at the Aitik copper mine in northern Sweden. The analyses were performed to evaluate the potential for liquefaction and to assess stability of the dam under two specific earthquakes: a commonly occurring magnitude 3.6 event and a more extreme earthquake of magnitude 5.8. The dynamic analyses were carried out with the finite element program PLAXIS using a recently implemented constitutive model called UBCSAND. The results indicate that the magnitude 5.8 earthquake would likely induce liquefaction in a limited zone located below the ground surface near the embankment dikes. It is interpreted that stability of the dam may not be affected due to the limited extent of the liquefied zone. Both types of earthquakes are predicted to induce tolerable magnitudes of displacements. The results of the postseismic slope stability analysis, performed for a state after a seismic event, suggest that the dam is stable during both the earthquakes.

Investigation of Increased Hydraulic Conductivity of Silty Till Subjected to Freeze–Thaw Cycles

The hydraulic conductivity of silty till increases when the till is subjected to freeze–thaw cycles. A dramatic increase normally occurs after the first freeze–thaw cycle, and the magnitude general ...