The Calculations on Haarajoki Test Embankment with the Finite Element Program Plaxis 6.31
The Finnish National Road Administration has organised an international competition to calculate the behaviour of the Haarajoki test embankment. This paper concentrates on calculations with the finite element program PLAXIS. The evaluation of the calculat
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A. Aalto, R. Rekonen and M. Lojander Helsinki University of Technology, Espoo, Fjrlland
ABSTRACT: The Finnish National Road Administration has organised an international competition to calculate the behaviour of the Haarajoki test embankment. This paper concentrates on calculations with the fmite element program PLAXIS. The evaluation of the calculation parameters is discussed. The main interest is put on preconsolidation pressure and the Poisson's ratio. The subsoil is slightly overconsoldiated and therefore the exact value of the .preconsolidation pressure is very important. The choice of Poisson's ratio has a great effect on the stress paths and displacements. The results of the calculations are compared with the measurements of the test embankment. INTRODUCTION The test embankment was instrumented and constructed during July and August 1997 (FinnRa, 1997). The measurements of the settlement plates, piezometers and inclinometers are also available in the internet (www.tieh.fi/pailas/). Several oedometer tests (Incremental Loading and Constant Rate of Strain) and triaxial tests (Consolidation test, undrained and drained shear) were carried out in the laboratory of Soil mechanics and Foundation Engineering at the Helsinki University of Technology. Calculations by several different methods were made before the construction of the embankment (NaaUinen et al, 1997). After the ftrst measurements some modifications of
A. Cividini (ed.), Application of Numerical Methods to Geotechnical Problems © Springer-Verlag Wien 1998
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A. Aalto, R. Rekonen and M. Lojander
the parameters have been done. The modification of the permeability and the elastic parameter (kappa) is discussed in the article of Naatlinen et al ( 1998). The profile of the preconsolidation pressure of Haarajoki is typical for the Finnish clay layers. The upper clay layer, dry crust, is heavily overconsolidated. Under the dry crust the pre-consolidation pressure is diminishing towards the minimum value. In deeper layers the pre-consolidation pressure profile is similar to the overburden pressure profile. The value of the Poisson's ratio is commonly determined from the results of drained triaxial tests. In the calculation program PLAXIS it is necessary to use the real elastic value of the Poisson's ratio. The effect of the change of the value of the Poisson's ratio for one layer (2 ... 5m) from 0.1 to 0.35 is shown in the article.
2. SOIL PROPERTIES The Haarajoki embankment is 2.9 meters high, 100 meters long and the width at the top is 8 meters. The geometry of the test embankment can be shown in the paper by Naatanen & al 1998. The fill material is gravel with a density of 21 kN/m3 • The embankment is lying on layers of overconsolidated clay and silt with an altogether depth of 20 meters. The ground water table is at the surface and the excess pore pressures are between -3 and 10 kPa have been measured in this area. The subsoil is divided in to 9 geotechnical layers for the calculations. The layers are shown in Table 1. On the top there is a 2 meters
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