Environmental Engineering Reference
In-Depth Information
Passive anchors within retaining walls to stabilize volcanic rock
slopes in road widening
M.a. Franesqui
Department of Civil Engineering, University of Las Palmas de Gran Canaria, Las Palmas, Spain
aBsTRacT: an economical and environmentally-friendly solution to stabilize jointed vertical rock
slopes in works of improvement and cross section widening of a local road section in Gran canaria island
(spain) is presented in this paper. Due to the mountainous relief of this territory, this road cross over
an extremely narrow section between two deep cliffs with vertical rock slopes on jointed phonolitic ign-
imbrites. The structural solution involves the construction of traditional gravity retaining walls with pas-
sive fully-grouted steel bar anchorages within its foundation. The rock mass nailing under the foundation
of the retaining walls and even the adjacent rock slopes is also designed. This system combines traditional
constructions of high simplicity with modern techniques of rock reinforcement.
1
inTRoDUcTion
and construction, specifically stated to design
an especially simple construction procedure in
order to be economical and systematically built
with safety without needing skilled labour, owing
to the complicated access to working site and
high unevenness of the slopes. Furthermore, the
maximum environmental and landscape protec-
tion was specified, given that 64% of the canary
territory is environmentally protected. in conse-
quence, solutions involving expensive and com-
plex cantilever structures to support the roadbed
widening outside the existing ground had to be
abandoned.
With this aim, this paper describes a com-
bined stabilization system, designed to widen the
road infrastructure and that includes numerous
rock slope reinforcement measures, achieving the
former requirements of minimum environmental
impact and maximum building simplification.
infrastructure maintenance and alignment and
roadbed improvement of local roads represent an
especially significant component of the total actions
carried out by local and regional road agencies
operating these road network levels.
The natural drainage in most of the volcanic
territories with particularly abrupt topography is fre-
quently shaped by a radial scheme with deep ravines
divided by extremely narrow basin watersheds. This
is a common relief pattern in Gran canaria island,
where communications between coasts and central
summits are fairly frequently performed along local
roads with alignments roaming on these constricted
watersheds.
These were the Gc-503 local road conditions,
with a sinuous non-homogeneous horizontal
alignment and a particularly narrow cross section
as a result of its historic origin as a rural bridleway
and an extremely mountainous relief. This road
is currently classified in the Gran canaria road
network as a minor way, but servicing to several
residential areas with increasing population. its
winding layout is daily run by the aforementioned
residents (annual average Daily Traffic [aaDT]
forecast for design life over 500 vehicles/day).
Thus, difficulties for cross section widening
reside in the rugged ground conditions of the
roadbed foundation, on an extremely narrow
section between two deep cliffs with 15-20 m high
vertical rock slopes on jointed rock masses.
Design specifications, dictated by the road insu-
lar administration agency for roadbed widen-
ing solutions including structural system design
2
GeoMechanical BackGRoUnD
Volcanic materials in the working area were formed
during the cycle i (Upper Miocene) and comprise
welded phonolitic ignimbrites of scoriaceous
nature, although highly fractured. ignimbrites
are originated from welded pyroclastic flow
deposits whose coarse fragments are flattened
and stretched. Their high welding level, one of
the main characteristics of this type of material,
is related to its deposition at temperatures higher
than 400°c-550°c, and the foliation and joints
are due to its cooling (González de Vallejo
et al
.
2007).
