2091
Role of the facing on the behaviour of soil-nailed slopes under surcharge loading
Rôle du parement sur le comportement des pentes de sol cloué sous surcharge
Sanvitale N., Simonini P., Bisson A., Cola S.
Department of Civil, Environmental and Architectural Engineering - University of Padova - Italy
ABSTRACT: Soil nailing is an economic and efficient method to reinforce soils, involving the insertion of threaded bars into natural
unstable slope for increasing the overall stability or into cut slopes during the top-down process of excavation. The retained soil, the
resisting reinforcements and the external facing are the main components of a soil-nailed structure. Their composite interactions
determine the performance of soil-nail construction in terms of deformations and stability. Even if the international codes deal about
the possibility of use rigid or flexible external facing, the role of facing stiffness is not sufficiently studied and evaluated. To this aim,
some tests with various facing types, differing in stiffness and continuity, were carried out so far in 1g small scale physical model.
The experimental results show the importance of both flexional and axial stiffness of facing in controlling the deformation of the wall
during excavation and the maximum surcharge applicable at the rear of wall.
RÉSUMÉ: Le clouage du sol est une méthode économique et efficace pour renforcer le sol en place: il consiste en l'insertion de barres
d'acier filetées ou d'autres barres dans les pentes naturelles instables ou dans des talus au cours du processus de l'excavation pour
augmenter la stabilité globale. Le sol soutenu, les barres résistantes et le parement extérieur sont les principales composantes d'une
structure du sol cloué. Leurs interactions mutuelles déterminent la performance du
soil nailing
en termes de déformations et de
stabilité. Même si les codes internationaux considèrent la possibilité d'utiliser des parements extérieurs rigides ou flexibles, le rôle de
la raideur du parement n'a pas été suffisamment étudié et évalué. Avec ce but ont été réalisées des épreuves dans un modèle physique
avec des parements différents en rigidité et continuité. Les résultats expérimentaux soulignent l'importance de la raideur en flexion et
en traction-compression du parement extérieur dans le contrôle de la déformation de la paroi pendant l'excavation, et la valeur
maximale de la charge applicable à l'arrière du mur.
KEYWORDS: soil nailing, facing, retraining wall, soil reinforcement, physical model.
1 INTRODUCTION
Soil nailing is an economical and efficient reinforcement
technique used as a remedial measure in unstable natural slopes
or as retraining structure for excavated slopes. In comparison
with other retaining structures, soil nailing permits to reduce the
excavated soil volume, saving construction materials and
realization time. Its first applications were proposed in France
for the Couterre project (Plumelle et al. 1990) and in Germany
(Stocker 1976). Even if many studies and researches have been
already performed and several national codes or guide lines
exist, the role of facing in controlling the deformation of
excavated front or the slope overall stability is not completely
understand yet.
On this matter, the new code EN 14490:2010 indicates the
possible use of three types of facing which are:
-
hard facing
which has to fulfil the function of stabilizing the
slope between the nails and shell therefore be dimensioned
to sustain the maximum expected destabilizing forces;
-
flexible facing
designed to provide the necessary restrains to
the areas of slope face between the bearing plates as well as
the erosion control;
-
soft facing
with the primary function of controlling slope
erosion in conjunction of vegetation.
Even if the EN 14490:2010 reports some examples for the
three types of facing, it does not give precise indications on how
to evaluate the forces applied by soil on facing or the stiffness
of facing.
In order to improve the understanding of the role of facing
on the resistant mechanisms of soil nailing, this paper presents
the results of an experimental program carried out on a 1g small
scale physical model of sandy slope reinforced with soil nailing
and brought to collapse by surcharge loading: in the tests six
facings, differing for continuity and stiffness, were utilized to
restrain the soil between nails.
2 PHYSICAL MODEL
The model face is 39.5 cm wide and 40 cm high, with a wall dip
angle of 80° (Figure 1). The soil is medium-fine sand from
Adige river with the following characteristics (Gottardi and
Simonini 2003): mean particle diameter
D
50
= 0.42 mm, no-
uniformity coefficient
C
u
=
D
60
/
D
10
=2.0, specific bulk weight
G
s
= 2.71, minimum and maximum dry specific weight
d,min
=
13.6 kN/m
3
and
d,max
= 16.5 kN/m
3
, peak friction angle
'
peak
=42-43° and critical friction angle
'
crit
=35°.
The sand is prepared in homogeneous layers into a
caisson with pluvial deposition method reaching a relative
density of about 85%. During deposition the caisson is
maintained inclined at 20° to deposit homogenously the sand
also at rear of facing and to simplify the nail installation. In the
meanwhile, the cover is fixedly bonded in vertical position,
using four wooden blocks behind which will be subsequently
removed to simulate the excavation.
The deposition is temporarily stopped to install the nails
in 4 horizontal lines and 3 vertical lines, with spacing
s
v
= 10.2
cm and
s
h
= 13.2 cm respectively.
The nails are 32.5 cm long aluminium tubes, with an
external diameter of 6 mm and covered with 1 mm thick layer
of glued Adige sand. They are perpendicularly connected to the
