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Full scale field tests for strength assessment of peat
Essais in situ en vraie grandeur pour évaluer la résistance d’une tourbe
Zwanenburg C., Van M.A.
Deltares
ABSTRACT: The costs for improving the safety of the Dutch Dike system are significant. The safety of about 40 km of dikes along
the lake 'Markermeer' have to be increased according to the calculation methods based on current knowledge. However, the
Markermeer dikes were already in place when the Markermeer was still part of the Zuiderzee. Water levels in the past have been
higher then current design water levels. In this area, the subsoil consists mainly of peat and soft soils. The uncertainty associated with
measuring the strength of soft soil (in laboratory testing) and constitutive behavior in computer models is a research issue in the safety
assessment. In order to close the knowledge gap between the real dike and the small laboratory tests, full-scale field tests in the peat
area have been executed. The large-scale field test, the so-called 'container tests' and intensive monitoring are executed in order to
reduce the uncertainty of the strength parameters in the assessment of the stability of dikes on peat subsoil. This paper discusses
results of the lab and field tests and shows how DSS tests and ball penetrometer tests correspond well to the actual strength found in
the field tests.
RÉSUMÉ : L’amélioration de la sécurité du système de digues hollandaises a un coût significatif. La sécurité d’une quarantaine de
km de digues le long du lac de Markermeer a été augmentée selon les méthodes de calcul basées sur nos connaissances actuelles.
Cependant, les digues de Markermeer étaient déjà en place quand le lac faisait encore partie du Zuiderzee. Dans le passé, le niveau
d’eau était plus haut que celui adopté dans les calculs faits à présent. En ce lieu, le sous-sol est constitué principalement de tourbe et
de sols compressibles. L’incertitude associée à la mesure de la résistance de sols compressibles (en laboratoire) et aux lois de
comportement dans les modèles numériques est le sujet de recherche dans le cadre de l’évaluation de la sécurité des digues. De façon
à combler l’écart entre une digue réelle et les petits essais en laboratoire, des essais à échelle réelle ont été conduits dans une zone
tourbeuse. L’essai in situ en vraie grandeur, appelé « essai au conteneur » a été exécuté et un système de surveillance intensive a été
mis en place de façon à réduire l’incertitude sur les paramétres de résistance dans l’évaluation de la stabilité des digues construites sur
les sols tourbeux. Le papier discute les essais en laboratoire et sur le terrain et montre que les résultats des essais de cisaillement
simple direct et les essais au pénétromètre à balle correspondent bien à la résistance observée lors des essais in situ.
KEYWORDS: dike technology, peat, field tests
1 INTRODUCTION
Despite the fact that for centuries dikes have been built along
rivers and lakes on soft ground, the stability assessment of these
structures is still inaccurate. Due to uncertainties in sub soil
behaviour and the daily practice of engineers making safe
estimates of each not well known parameter, low factors of
safety are calculated for dikes that have functioned properly for
many years or even centuries. One of these examples is the dike
between Amsterdam and Hoorn, approximately 30 km north of
Amsterdam, along lake Markermeer. The dike originates from
medieval times and its last reinforcement was made in 1926.
Since the closure of the Zuiderzee, now known as IJsselmeer
and Markermeer, occurrence of extreme high water levels is
strongly reduced. As a consequence, present day design water
levels are lower, but consist of a longer duration than occurred
in the past. Despite the history of this dike, calculations show a
low stability factor and indicate dike reinforcement. The design
of the required reinforcement incorporates large stability berms
with lengths up to 30 m. Such reinforcement has a strong
impact on the villages and small towns along this dike section.
The calculated large stability berms followed from
uncertainty in the peat characteristics. Laboratory tests on peat
samples are difficult to interpret and result in a large scatter. To
optimize the design, a series of field tests is conducted. The aim
of these field tests is to make a comparison between the strength
found in the field to the laboratory test results.
2 CHARACTERIZATION OF THE PEAT SUBSOIL
Figure 1 shows the subsoil profile. The depths in this profile are
related to the reference datum NAP which is approximately
main sea level. The ground level is approximately NAP -1.4 m.
On top a thin clayey layer is found with a thickness of 0.2 to 0.5
m, followed by a peat deposit of approximately 4.5 m, a deposit
of several clay layers with a thickness of 4.5 m, a thin basal peat
layer and finally a thick pleistocene sand deposit. The water
table is nearly at ground level, NAP -1.5 m to -1.6m.
Figure 1. Typical CPT, ball penetrometer test and soil profile
