Proceedings of the Second World Landslide Forum – 3-7 October 2011, Rome
Giorgio Giacchetti (1), Alberto Grimod (2),
David Cheer(3)
Soil Nailing with flexible structural facing:
design and experiences.
(1)
(2)
(3)
Alpigeo Consultants, Belluno, via Barozzi 45, Italy
(giorgio.giacchetti@alpigeo.it)
Alpigeo Consultants, Belluno - Italy
Officine Maccaferri Technichal Department, Bologna (Italy)
Abstract Officine Maccaferri has developed BIOS, a
simplified as well as realistic design approach for the
calculation of the flexible structural facing of soil nailing. The approach shows that the most important
property of this kind of application is membrane stiffness of the mesh. With the procedure of BIOS is possible to reduce the timing of design and get a cost effective intervention. Anyway the designer judgment is
required for a better evaluation of the critical factors
like the slope morphology, the admissible displacement and settlement, the presence of water and erosion processes.
Keywords soil nailing, flexible structural facing
Preliminary remarks
For years the wire mesh has been frequently utilized
as flexible structural facing on soil nailing. The system,
known in technical literature as structural flexible facing
(Phear A., 2005), surely offers aesthetic advantages
and can be successfully utilized over all for the
stabilization of slopes with vegetation. The present paper analyses the general behaviour of the flexible facing and proposes the new calculation approach BIOS
(best improvement of slopes) which has already widely
utilized by the Officine Maccaferri for the design of cuts
and natural slopes.
The concept of nailing
The aim of soil nailing is to improve the soil stability
when there are unfavourable stability conditions. The
stability is achieved by inserting reinforcement bars in
the soil, which are then grouted and fixed soundly to
the ground for their entire length (nailing). The frequency and the length of the nails must be calculated
in accordance with EN 1997 1.
The nailing mobilises friction forces along the entire length and contributes to the improvement of the
stability conditions when there are displacements in
the soil (Schlosser F. et al., 2002; Soulas R., 1991; BS
8006; Byrne, R.J et al., 1998). The stabilizing friction
forces are passively generated when the soil rupture
starts.
The protection of the exposed surface of the soil
reinforced by the nails is obtained with the facing, the
Figure 1 Example of wide punch displacement of flexible
structural facing loaded by the soil. The cables reduced the
mesh displacement.
aim of which is to hold the soil between the nails, prevent erosion phenomena and assume an aesthetic
function. Obviously the facing, within the limits of its
intrinsic deformability, can only collaborate with the
passive action of the nails (figure 1). In facts, it is not at
all comparable to a stiff structure (E.g.: shotcrete or
precast elements), which limits the soil displacement in
an optimal way. The preferential ambit of application of
the structural flexible facing is the natural slope, where
there are not considerable variations of the stress state
and a vegetal protection already lives.
Generally, on slope steeper than 60° the facing
has a temporary character. Anyway the design of the
flexible structural facing requires a certain attention in
order to minimize the problems related to the intrinsic
properties of the meshes and to the limits of their applications.
Figure 2 The forces due to the prestress - if any - act
tangentially to the surface. The resultants vectors developed
on the edges, are absolutely negligible.
G. Giacchetti, A. Grimod, D. Cheer – Flexible structural facing
Limits of the mesh application
Figure 3 Test facility of Pont Boset developed by Technical
University of Torino in cooperation with Officine Maccaferri
(Bertolo et Al. 2009)
Intrinsic properties of the meshes
The punch tests are fundamentals for the modelling of
the meshes applied on soil nailing. In these terms,
several Authors carried out tests with samples having
different sizes and restrained within test frames in a
different way (Ruegger R., & Flumm D., 2000; Bonati &
Galimberti, 2004; Muhunthan B. et Al., 2005). The
most interesting tests have been developed in Pont
Boset (Aosta – Italia), where a realistic restrain constituted by a raster 3.0 m x 3.0 m of nails, frequently
adopted for the consolidation of rocky and soil slopes,
and a punch device dipping 45° on the mesh plane
(figure 3) were implemented (Bertolo et. Al. 2007;
Bertolo et. Al., 2009). The following analyses show first
of all, both the scarce meaning of the laboratory tests
with small size samples, and the necessity to reproduce the restrain condition in which the meshes are
applied (Majoral et Al., 2008). Secondly, the results
show that the meshes develop an appreciable contrast
after they reach a displacement of several decimetres
with null load. For example, the displacement load is
worth 0.4 m in the case of the double twist mesh with
inscribed circle diameter 83 mm and wire 500 MPa,
and 0.8 m in the case of the single twist mesh, with inscribed circle diameter 65 mm and wire 1770 MPa
(figure 4). In that condition, it is manifest that whilst a
soil displacement trigger the nails work (passive intervention), the facing does not yet offer a stabilizing
contribution; actually it will start when the displacements are few decimeters.
In spite the evidences on site do not confirm it at all or
contradict it, it is quite frequently believed that the pre
stretching of the facing allows to develop active pressures which contribute to the slope stabilization.
The pre-stretching of the facing is theoretically
carried out pre-tensioning the nails; that is got whether
screwing down the nut on the nail plate, so that the
mesh is pushed into the concavities of the ground
surfaces (likewise the quilt pushes on the mattress), or
tangentially stretching the mesh on the edges of the
revetment. In the first case, the nails stretching does
not determine advantages, since to any pressure of the
plate on the mesh, necessarily corresponds an equals
and contrary force which tries to lift the nail (figure 2); it
follows that any stabilizing force is developed into the
geotechnical system. In the second case, the pre
stretching could be implemented on planar surfaces in
principle, but if the nails are already installed, or if the
ground surface is just uneven, on facts it became impossible to obtain because of the frictions on the asperities (Ferraiolo and Giacchetti, 2004). In both these
cases, the intrinsic deformability of the mesh invalidates the effect of the pre-stretching (figure 4).
At last it should be remembered that, even if it
was possible to pre-stretch the mesh, the developed
forces would be tangential to the mesh plane, and
some pressure could be developed against the protuberance of the ground surface only (figure 2). However
they are non relevant pressures, so that it is possible
to lift the mesh from the ground contact, simply using
the fingers. In a certain way, the mesh allows to
enlarge the contacts surface of the anchor plate
(Besseghini et. Al., 2009) but, always because the
above reasons, that increment remains absolutely
negligible.
Some implications
Some important implications for the design approach
of the structural flexible facing came out as corollary of
the above:
− From the geotechnical point of view, the mesh
absolutely has a passive behaviour. It cannot be
modelled as shotcrete which is fit to transmit almost
uniform pressures on the ground surface by means
of the nails.
− Taking into account of the mesh displacement, the
stripping becomes the most insidious rupture way.
It is better to attribute null or a negligible bearing
capacity to plate and mesh system. All the more
reason, the utilization of tie back anchors on the
meshes become unadvisable: they would scarcely
be functional, or ineffective.
− The difference of behaviour between meshes basically depends upon the kind of fabric, and not upon
the steel grade of the constitutive wire; the choice
of a mesh to be utilized as facing must take into account of the fabric properties and not simply of the
wire ones.
2
G. Giacchetti, A. Grimod, D. Cheer – Flexible structural facing
− The tensile stress stretching the meshes are almost
always 3 times lower than the nominal tensile resistance of the facing. Then the tensile resistance
has marginal importance in the mesh choice.
− The membrane stiffness plays a primary role into
the facing choice: the higher the stiffness is, the
more effectiveness the facing is.
− The overlapping of a cable net on the mesh facing
is always recommendable. The cable netting, which
is much more stiffer than the mesh, reduces the
membrane deformability and really helps to distribute the stress of the meshes by the nails. That is
why a mesh with cables woven in the fabric surely
performs the best.
− With the structural flexible facing, the nails could
have a certain difficulty to cooperate each others in
the consolidation. That is why the nail spacing
should be reduced to no more than 1.0 - 1.5 m
(Joshi, 2003). With larger spacing, each anchor
does its work as single, lonely reinforcement and
the flexible structural facing shades to soft facing
(Phear A., 2005). The spacing should not anyhow
exceeds 3.0 m. In order to control the excessive
deformability of the facing, intermediate auxiliary
anchors could be always introduced among the
deep ones (Phear A., 2005)
Simplified approach: BIOS
The design of the nails for the soil nailing can easily
developed with several calculation procedures.
Figure 4 Graphics of puncturing tests (a) on sample 1x1 m in
laboratory, (b) on samples sized 3x3 m in laboratory, and (c)
on samples 3 x 3 in field facility, in the case of the double
twist hexagonal mesh DT (upper graph) and of the single
twist one, higher tensile wire (lower graph). The displacement depends both on the sample size and the restrain kind
type. It is noticeable the dramatic deformability of the single
twist mesh on site that makes it almost useless as facing.
After the nailing has been calculated, the structural
flexible facing can be calculated. Nevertheless such
verification is not at all simple, since it requires the
utilization of complicated numerical models, with effort
and consumed time not reasonable in the design
praxis, overall if the design is aimed to interventions of
modest size. Because of that, at the present, the Limit
equilibrium methods are preferable which are necessarily very simplifying even if they consider in a certain
way the displacement. In the approach “Best Improvement of Slopes” (BIOS) the loads transmitted by the
ground to the facing have been calculated with the
“two wedges method”, while the displacements are extrapolated via the experimental results of the punch
tests. The solution would require a finite element
analysis, but a realistic solution can be find maximizing
the forces acting on the geotechnical system. Obviously the procedure quite rough, but it is more than
enough considering the low accuracy level of the input
data, the reliability of the results and the velocity of
calculation.
BIOS develops the analysis of the facing in 4
stages (in appendix there is the calculation procedure):
1) Verification of the input data: this first stage
analyses the slope behaviour in the short term in order
to verify that the safety factor of the slope between two
nails is greater than 1.0 (Fs > 1.0). The procedure allows to control the quality of the geotechnical input
data and, in case, to correct them, adjusting the geotechnical parameters or changing the nail spacing.
2) Verification of the instable volume: the software
simulates the soil softening which happens in the long
term. For that, the progressive reduction of the resistance parameters c’ e ϕ' is carried out up to the resisting forces are equals to the driving forces (FS = 1).
The procedure allows to determine the maximum instable volume of soil which can move down between
the nails (figure 3).
3) Ultimate limit state: the maximum volume of soil
that can move between the nails (see previous point 2)
is compared to the minimum one needed to break
down the mesh. If the volume between the nails is
smaller than the one that breaks down the mesh, then
the mesh fulfil the problem; on contrary, the facing
does not satisfy the problem.
4) Serviceability limit state: this stage analyzes the
mesh displacement by means of the curves load- displacement. The graphics allows to determine the volume of soil related to the maximum admissible displacement. If that volume is larger than the one waited
on the long term, the facing satisfy the requirements of
design. The maximum design displacement is assumed upon one or more geotechnical criterion (effects of the mesh displacement on the stripping, triggering erosion processes, effects of settlements induced in the neighbour), functional (maximum encumbrance of debris pockets) and aesthetic.
3
G. Giacchetti, A. Grimod, D. Cheer – Flexible structural facing
Figure 5 Displacement models with the segments of deformed mesh.
Next developments
The flexible structural facing represents a very interesting solution since it offers several technological and
environmental advantages. The tests on site showed
many applicative limits of the flexible facing and allowed the development of simplified calculation approach. Further investigations are needed for a better
comprehension of the interaction between nail and
mesh and for implementing more accurate behaviour
models of the flexible facing.
Bibliography
Bertolo P., Ferraiolo F., Giacchetti G., Oggeri C., Peila
D., e Rossi B., (2007). “Metodologia per prove in
vera grandezza su sistemi di protezione corticale dei
versanti” GEAM Geoingegneria Ambientale e mineraria, Anno XLIV, N. 2, Maggio-Agosto 2007.
Bertolo P., Oggeri C., Peila D. (2009). “Full scale testing of draped nets for rock fall protection” Canadian Geotechnical Journal, No. 46 pp. 306-317.
Bertolo P. , Giacchetti G. (2008). “An approach to the
design of nets and nails for surficial rock slope
revetment” in Interdisciplinary Workshop on Rockfall
Protection, June 23-25 2008, Morshach, Switzerland.
Besseghini F., Deana M., Di Prisco C., Guasti G.
(2008). “Modellazione meccanica di un sistema
corticale attivo per il consolidamento di versanti di
terreno” Rivista GEAM Geoingegneria ambientale e
Mineraria, Anno XLV, N. III dicembre 2008 (125) pp.
25-30 (in Italian)
Bonati A., e Galimberti V. (2004). “Valutazione sperimentale di sistemi di difesa attiva dalla caduta
massi” in atti “Bonifica dei versanti rocciosi per la
difesa del territorio” - Trento 2004, Peila D. Editor.
BS 8006 (1995). “Code of practice for Strengthened/reinforced soils and other fills”.
Byrne R.J, Cotton D., Porterfield J., Wolshlag C., e Ueblacker G.,(1998). “Manual for design & construction
monitoring of soil nail walls” U.S. Department of
transportation – Federal Highway Administration.
FHWA A-SA-96-06R – Washington D.C
Castro D. (2008). “Proyetos de investigaciòn en la Universidad de Cantabria” - II Curso sobre protecciòn
contra caida de rocas – Madrid, 26 – 27 de Febrero.
Organiza STMR Servicios técnicos de mecànica de
rocas.
Cravero M. Iabichino G., Oreste P.P., e Teodori S.P.
(2004). “Metodi di analisi e dimensionamento di
sostegni e rinforzi per pendii naturali o di scavo in
roccia” in atti “Bonifica dei versanti rocciosi per la
difesa del territorio” – Trento 2004, Peila D. Editor.
EN 1997 1, (2005). “Eurocode 7: Geotechnical design”
- Part 1: General rules
Ferraiolo F., e Giacchetti G., (2004). “Rivestimenti
corticali: alcune considerazioni sull’applicazione
delle reti di protezione in parete rocciosa” in atti
“Bonifica dei versanti rocciosi per la difesa del
territorio” – Trento 2004, Peila D. Editor.
Flumm D., Ruegger R. (2001). “Slope stabilization with
high performance steel wire meshes with nails and
anchors
–
International
Symposium
Earth
reinforcement, Fukuoka, Japan.
Giacchetti G., Majoral R. Bertolo P., (2009). “Verificacion del revestimiento structural flexible en un soil
nailing – BIOS: best improvement of slopes - VII
Simposio Nacional sobre Taludes y Laderas
Inestables - Barcelona, 27 – 30 de Octubre de 2009.
Giacchetti G., Bertolo P. (2010). “Approccio al calcolo
dei sistemi di reti con chiodi per il consolidamento
delle pareti rocciose” Rivista GEAM geoingegneria
Ambientale e Mineraria Anno XLVII, April 2010 – pp
23 – 35
Joshi B. (2003). “Behaviour of calculated nail head
strength in soil nailed structures”. Journal of
geotechnical and geoenvironmental engineering,
ASCE, pp. 818 – 819
LCPC (2001). “Parades contre les instabilités rocheuses” - Guide technique - Paris.
Majoral R., Giacchetti G., Bertolo P. (2008). “Las
mallas en la estabilizaciòn de taludes” II Curso
sobre protecciòn contra caida de rocas – Madrid, 26
– 27 de Febrero. Organiza STMR Servicios técnicos
de mecànica de rocas.
Muhunthan B., Shu S., Sasiharan N., Hattamleh O.A.,
Badger T.C., Lowell S.M., Duffy J.D. (2005).
“Analysis and design of wire mesh/cable net slope
protection - Final Research Report WA-RD 612.1”
Washington State Transportation Commission
Department of Transportation/U.S. Department of
Transportation Federal Highway Administration.
Phear A., Dew C., Ozsoy B., Wharmby N.J., Judge J.,
e Barley A.D. (2005). “Soil nailing – Best practice
guidance” CIRIA C637, London, 2005.
Ruegger R., e Flumm D. (2000). “High performance
steel wire mesh for surface protection in
combination with nails and anchors” – Contribution
to the 2nd colloquium “Construction in soil and rock”
– Academy of Esslingen (Germany). “
Saderis A., (2004). “Reti in aderenza su versanti rocciosi per il controllo della caduta massi: aspetti
tecnologici e progettuali” Tesi di Laurea in
4
G. Giacchetti, A. Grimod, D. Cheer – Flexible structural facing
Ingegneria per l’Ambiernte e il Territorio, non
pubblicata, Politecnico di Torino.
Schlosser F., (Chairman) (2002). “Additif 2002 aux
recommandations
clouterre
1991
pour
la
conception, le calcul, l’exécution et le controle des
soustènements realises par cluage des soil”,
Presses Ponts et chaussées.
Soulas R., (Chairman) (1991). “Recommandations
clouterre 1991 pour la conception, le calcul,
l’exécution et le controle des soustènements
realises par cluage des soil”, Presses Ponts et
chaussées.
Valfrè A. (2006). “Dimensionamento di reti metalliche
in aderenza per scarpate rocciose mediante
modellazioni numeriche” GEAM Geoingegneria
Ambientale e mineraria, XLIII: 47-53.
Figure 6 Geotechnical model with wedges
Appendix: stability and displacement of the
mesh
Officine Maccaferri has developed the software BIOS
for the automatic computation of the mesh capacity
which uses the “two wedge method” for the calculation
of the instable soil mass, in the hypothesis that the two
wedges lye within the space delimitated by two
adjacent nails; in order to maximize the driving force,
the software automatically searches the worst wedges
combination. It is assumed that the debris develops
distributed load on the facing, so that the total force
acting shall be (fig. 6):
Ttot = T1 + T2
[(W1 + Q1 ) ⋅ (tan θ1 − tan ϕ '1 ) + (U1 ⋅ tan ϕ '1 − K1 ) /cosθ1 ]
(1 + tan θ1 ⋅ tan ϕ '1 )
[2]
(W2 + Q2 ) ⋅ ( tan θ 2 − λs ⋅ tan φ '2 ) + λs ⋅ (U 2 ⋅ tan φ '2 − K 2 ) / cosθ 2 
(1+ λs ⋅ tanθ2 ⋅ tan φ '2 )
[3]
where:
W1 (kN)
Weight of wedge 1;
W2 (kN)
Weight of wedge 2;
Q1
(kN)
Overload acting on wedge 1;
Q2
(kN)
Overload acting on wedge 2;
θ1
(°)
Angle at the base of wedge 1;
θ2
(°)
Angle at the base of wedge 2;
U1
(kN)
Resultant of the pressure of the water
acting at the base of wedge 1;
U2
(kN)
Resultant of the pressure of the water
acting at the base of wedge 2;
T2 =
(kN)
K2
(kN)
λs
Cohesion force acting at the base of
wedge 1;
Cohesion force acting at the base of
wedge 2;
Slip factor at the base.
The safety factor is calculated with:
FS =
K 1 + K 2 + (W1 ⋅ cos(θ1 ) − U 1 ) ⋅ tan ϕ '1 +(W2 ⋅ cos(θ 2 ) − U 2 ) ⋅ tan ϕ ' 2
W1 ⋅ senθ1 + W2 ⋅ senθ 2
[4]
[1]
where:
T1 =
K1
In order to calculate ultimate limit state deformation of
the mesh, the following initial assumptions apply:
- The deformed shape is divided into 3 sections: the
first limb, rectilinear, with length X inclined with an
angle α with respect to the slope, the angle of
which is indicated by β; the second limb, curved,
with length (π+α) r that characterises the sack
shape of the soil; the third limb, rectilinear, lies on
the slope, with the same inclination and a length XL;
- The second stretched limb is tangential to both the
first and third limbs of the mesh;
- The mesh, completely stretched, deforms and
reaches a maximum length at the failure limit of not
more than:
[5]
Ltot = L + ε ⋅ L
Where:
ε
percentage deformation under failure conditions obtained from large scale puncturing tests and
tension;
L
distance of the mesh between two nails in a
direction parallel to the slope.
5
G. Giacchetti, A. Grimod, D. Cheer – Flexible structural facing
with:
per = (A+ B+ C) / 2
[11]
A=
X
⋅ sen( β − α )
sen (180 − β )
[12]
B=
X
⋅ senα
sen(180 − β )
[13]
C= X
Figure 7 Geotechnical model with instable soil divided in
elemental areas
- The area of the section corresponding to the sack is
[14]
where:
L
γ
β
ϕ’a
δ
(m)
3
(kN/m )
(°)
(°)
(°)
equal to that of the circular sector with an angle at
the centre equal to (π+α) and radius r (fig. 7);
The area 1 is obtained by resolving the following
system of equations:
EA
Tmax
(kN)
(kN/m)
Fsmesh
Tamm
(kN/m)
L + ε ⋅ L = X + ( Π + α ) ⋅ r + ( X − L)
[6]
ε
α 
r = X ⋅ tg 
2
[7]
P = Tamm ⋅
senβ − cos β ⋅ tan δ
P = γ ⋅V
AREA1 =
(1+ cosα )
length of the mesh;
unit weight of soil;
angle of inclination of the slope;
friction angle of the soil;
friction angle of the soil-slope
interface;
axial stiffness of the mesh;
maximum tensile strength of the
mesh;
factor of safety of the mesh;
permissible tensile strength of the
mesh;
maximum
percentage
deformation of the mesh.
Area 2 is determined by:
A2 ⋅ sen(θ 1 − β )⋅ sen(β − φ ')
2 ⋅ sen(180 − θ 1 + φ ')
[8]
AREA2 =
[9]
Area 3 is the difference between the volume of
long-term unstable soil and area 2. The total volume
thereby obtained must be compared with the unstable
volume under the long-term conditions; if the unstable
volume is greater than that necessary for failure of the
mesh, the flexible facing will be put at risk.
( Π + α ) ⋅ r 2 + X ⋅ r − per ⋅ per − A ⋅ per − B ⋅ per − C
(
)(
)(
)
2
[10]
[15]
6