Bios Paper Rel 08 ENG

8/10/2019 Bios Paper Rel 08 ENG

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From Research to Design in European Practice, Bratislava, Slovak Republic, June 2 4, 2010

VERIFICATION OF FLEXIBLE FACING AND SOIL NAILING BIOS: BEST IMPROVEMENT OF SLOPES

Giorgio GIACCHETTI, Paola BERTOLO

consultant , Belluno - Italy2Officine Maccaferri SpA, Via Kennedy 10 - Zola Predosa - Bologna - Italy

Abstract: Flexible facing systems have been used for many years for the consolidationof steep vegetated slopes, but designers still do not have a simple design approachwhich is coherent with the soil nailing philosophy. To fill this gap, Maccaferri hasdeveloped the BIOS system. This design approach enables flexible facings to be quicklyverified in engineering practice.

1 Introduction

The use of steel mesh for facing soil nailed slopes has been increasingly frequentover recent years. The system, known in the technical literature as Flexible Facing(CIRIA, 2005), has, without doubt, advantages of an aesthetic nature and it may be usedto successfully consolidate slopes generally with vegetation. However, the best practiceis still not mature as is often shown by the lack of design approaches or the sometimesimproper use of the meshes. This paper analyses the general behaviour of the flexiblefacing and proposes the new BIOS (Best Improvement Of Slopes) design approachwhich has been widely used by Maccaferri for the design of excavation slopes and theconsolidation of natural slopes.

2 The Concept of Soil Nailing

The aim of soil nailing is to improve the soil stability when there are unfavourablestability conditions. The stability is achieved by inserting reinforcement bars into the soil,which are then grouted and fixed soundly to the ground for their entire length (nailing).The nail is not tensioned and is static or passive. The nailing mobilises friction forcesalong the entire length and contributes to the improvement of the stability conditionswhen there are displacements in the soil (Schlosser F. et al., 2002; Soulas R., 1991; BS8006; Byrne, R.J et al., 1998). The stabilising friction forces are generated passively with


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Figure 1: Results of the axial-symmetric FEM model of the anchor: (A) Maximum forcefields and direction of the displacements below the plate; (B) Total displacement fieldsand trend of the vertical pressures YY below the plate (blue line) and far from the plate

(purple line). Any result from such tests simply measure the type of compressiveresistance of the soil beneath an anchor plate of certain dimensions and is largely

regardless of the mesh performance

4 Behaviour of the Flexible Facing

Various studies and laboratory tests have been carried out with regard to thebehaviour of the meshes (Ruegger R., & Flumm D., 2000; Bonati & Galimberti 2004;Torres et al., 2000; Muhunthan B. et al., 2005) using various sized samples fastened totest frames with a range of constraint conditions. The results of the research highlightthat the movement of the meshes subject to puncturing, at the point of application of theload, ranges from several tens of centimetres to several metres, with a non-lineardevelopment of the response, the trend of which depends mainly on the combination ofthe mesh weaving, the size of the test sample and the type of constraint with which thesample is fixed. There are large displacements during the initial phase of application ofthe load in all the tests. Only subsequently does the mesh start to appreciably opposethe load. The mesh cannot therefore be modelled as a beam which is able to transmit


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pressures, uniformly distributed, on a surface by means of the nails. For this reason it

can only be considered as a passive element.Large displacements of the meshes are found in the case of tests carried out with 3 x 3m frames (Bonati and Galimberti, 2004): for example, with the same constraintconditions (fastened on all sides), the double-twist mesh reinforced with steel cables -Steelgrid - is very deformable compared with the HEA cable panel, but much more rigidthan the single-twist meshes with high-strength wire. With a load of 1,500 daN (1,5 tons,i.e. slightly less than a cubic metre of soil) the HEA panel deforms by 15 cm, theSteelgrid by 38 cm and the single-twist mesh by 55 cm. Full scale puncturing tests havebeen carried out more recently at Pont Boset, Aosta (Italy), reproducing a constraintsystem frequently used for the consolidation of rock faces and soil slopes, andsimulating the load on the mesh by means of a jack placed at 45 to the plane of themesh (Figure 2) (Bertolo et. al. 2007; Bertolo et. al., 2009).

Figure 2- Pont Boset test site developed by Turin Polytechnic in collaboration withOfficine Maccaferri (Bertolo et al. 2009)

The test is particularly relevant and interesting as it summarises in a realistic manner,the different types of puncturing that is applied on e meshes (puncturing forces at thecentre of the sample and puncturing with the anchor plate). On one hand, the resultsconfirm the non-linear behaviour of the meshes and on the other, they highlight that inreality the displacements are much greater than those envisaged on the basis of testscarried out on samples at a reduced scale or fastened in a perfect manner, such as

those within a laboratory (Figure 3).Regarding the real capacity of the mesh to push against the surface and stabilize

the slope, a very simple and direct experience can be done: the force felt while shakingor lifting the mesh of any installation is zero or quite negligible, and in the meantime thedeflection is wide, even if the mesh has been pre tensioned.


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5 Design Models

Taking the results of the in-situ tests into consideration, simplified numericaldesign models have been developed, which enable the effectiveness of the flexiblefacing (with different types of meshes) to be assessed. For example, a natural slope hasbeen considered with: regular morphology angle of inclination 54 height of 14.0 m, highly weathered rock bulk density of 24.0 kN/m3 cohesion of 20 kPa shear resistance angle of 20 Youngs modulus of 800 kPaThe calculation has been carried out with the non-associated flow rule obtaining a globalsafety factor of 1.05 (Phase2 7.0, 2009). By inserting solely anchors at a 3 x 3 m grid,but not the flexible facing, the global safety factor for the slope increases to 1.30. Theinstability of the soil between the nails only manifests itself in the long term, as areaction to the softening processes, gradual in-ravelling of the slope face. Only afterthis new instability condition has been established does the flexible facing begin tocontribute and collaborate with the anchors.

Figure 3: Graphs of the punch tests on 1x1 m samples in the laboratory (blue line), 3x3m samples in the laboratory (purple line) and 3 x 3 m samples in situ (green line), in the

case of double-twist mesh (on the left) and single-twist high performance mesh. Thedisplacement depends on the size of the sample and the constraints. The dramatic

displacement of the in-situ single-twist mesh may be seen.


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Figure 4: Total displacements on the slope and the relative total displacement graphsrecorded with double-twist mesh (purple line), high-performance (high tensile strength 1770 N/mm2) chain link mesh (green line) and without the flexible facing (blue line).Whilst the double-twist mesh is effective due to its stiffness, the single-twist chain link

mesh has practically no effect.

The short-term stability of the material between the nails is a necessary condition toenable the installation of flexible facingIn the simplified numerical models, the flexible facing is represented as a membranewith a known strength and displacement modulus, connected to the ground by means ofan interface with zero tensile strength (the displacement modulii of the meshes havebeen determined in the Pont Boset tests). Running the numerical model with the soil in

softening conditions and with a flexible facing, it may be seen that: the tensile strength of the flexible facing has no influence on the geotechnical system the membrane deflection of the flexible facing plays an important role in the stability

of the softened soilIn the example shown in Figure 4 the displacements of the slope have been calculated: with no facing (blue line) with a flexible facing consisting of a high-performance single-twist mesh (green line) with a flexible facing consisting of a double-twist mesh (purple line).


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The displacements obtained show that the single-twist mesh, even though it has a high

tensile strength, does not counteract the soil displacements, whilst the double-twistmesh actually works in coniction with the anchors in counteracting the totaldisplacements of the slope due to its high membrane stiffness. In addition, the flexiblefacing which is not very deformable minimises the stripping processes which affect thenails. The final result is that the double-twist mesh maintains the global safety factor ofthe slope approximately 10% higher than that for the single-twist mesh.

6 Simpli fied Approach: BIOS

The use of sophisticated numerical models requires effort and processing timeswhich may not be considered reasonable in normal design practice; the preferredapproach is therefore a limit equilibrium method where the design models used arenecessarily simplified. The conceptual approach commonly used assumes that thesystem of mesh and anchors, in some way, plays an active role in the consolidation ofthe slope(Cravero et al., 2004; Saderis A., 2004; Flumm D. & Ruegger, 2001; Castro,2008); however, this approach is not in accordance with either the concept of soil nailingor with the proven passive behaviour of the meshes (Figure 5 and ).Models have been proposed more recently which also consider the displacement of themesh in the consolidation of rock faces (Bertolo & Giacchetti, 2008, Valfr, 2007). In thecase of soil nailing with a flexible facing, once the nails have been designed with specificsoftware, it is necessary to separately check the service abilities and ultimate limit states

of the facing in accordance with EN 1997-1. In the Maccaferri BIOS approach, the loadstransmitted from the soil to the flexible facing have been calculated with the two wedgesmethod, with the assumption that the forces act directly on the steel mesh as adistributed load and that the two wedges are contained in the space that exists betweentwo adjacent nails. The angle of inclination of the two wedges is varied in thecalculations to maximise the applied force. The total force will be (Majoral et al., 2008):

21 TTTtot += (1)

with:( ) ( ) ( )[ ]

( )1111111111

1'tantan1

cos/'tan'tantan

+

++=

KUQWT (2)


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Figure 5: Example of improper use of flexible facing. A large landslide occurred after

application of the facing with chain link; the flexible facing at the base of the landslidedeformed extensively without exercising any preventive containment pressure, and

without collaborating with the nails.

Figure 6 : Behaviour of revetments: (a) Double twist mesh forming wide pockets becausethe debris load; the cable net is useful because increases the mesh stiffness. (b)

landslide crack below high performance mesh - chain link; the soil can move becausethe mesh deforms extensively without exercising any preventive containment pressure.

In few days the mesh will form a large pocket.

( ) ( ) ( )[ ]( )22

22222222

2'tantan1

cos/'tan'tantan

+

++=

s

ss KUQW

T (3)

where:

W1 (kN) Weight of wedge 1;W2 (kN) Weight of wedge 2;Q1 (kN) Overload acting on wedge 1;


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Q2 (kN) Overload acting on wedge 2;

1 () Angle at the base of wedge 1;2 () 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;K1 (kN) Cohesion force acting at the base of wedge 1;K2 (kN) Cohesion force acting at the base of wedge 2;s Slip factor at the base.

and the safety factor is calculated with

( )( ) ( )( )2211

2222111121 'tancos'tancos

senWsenW

UWUWKKFS

++++=

The analysis of the facing is carried out in 4 phases:

6.1 Phase 1 Short-term solution

The short-term behaviour of the slope is analysed firstly, to check that the safetyfactor of the slope between two nails is greater than 1.0 (Fs > 1.0). This procedurechecks the appropriateness of the geotechnical input assumptions.

6.2 Phase 2 Long-term check

The unravelling of the soil which occurs in the medium-long term is simulatedassuming the progressive loss of the strength parameters c and until driving andresisting forces are equal (FS=1). The procedure enables the volume of unstable soilwhich may move between the nails to be evaluated.

6.3 Phase 3 Ultimate limit state check

This phase evaluates whether the steel mesh between two adjacent nails fails.The problem cannot be resolved with limit equilibrium procedure, so a comparison ismade between the unstable volume of soil in the long-term situation and the maximumvolume of soil which the mesh can support.The maximum volume of soil may not easily be calculated since the mesh takes up the

characteristic configuration of a sack bulging to a greater or lesser degree, the shape ofwhich depends on the ground slope and deformability of the mesh. It is thereforenecessary to introduce geometrical approximations of the deformed shape of the meshand set up the analysis with conditions based on the initial assumptions, with the limitsof deformability and strength of the structure determined from the tests.

The following initial assumptions apply ( Figure 7- left):- The deformed shape is divided into 3 sections:


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the first element, rectilinear, with length X inclined with an angle with respect to

the slope, the angle of which is indicated by ; the second element, curved, with length (+) r that characterises the sack

shape of the soil; the third element, rectilinear, lies on the slope, with the same inclination and a

length X-L;- The second stretched element is tangential to both the first and third elements of

the mesh;

- The mesh, when completely stretched, deforms and reaches a maximum length atthe failure limit of not more than:

LLLtot += (4)

where percentage deformation under failure conditions obtained from large scalepuncturing and tensile tests;L length of the mesh between two nails in a direction parallel to the slope.

- The area of the section corresponding to the sack is equal to that of the circularsector with an angle at the centre equal to (+) and radius r;

- The area 1 is obtained by resolving the following system of equations ( Figure 7-

right):( ) ( )LXrXLL +++=+ (5)

=2

tgXr (6)

( )

tancos

cos1

+

=sen

TP amm (7)

VP =

(8)

Figure 7: left: division of the mesh into segments right: deformation model of the

mesh


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( )

( ) ( ) ( )CperBperAperperrXr

1AREA +

+

= 2

2

(9)

with:

2/)( CBAper ++= (10)

( ) ( )

= sensen

XA

180 (11)

( )

sen

sen

XB

=

180 (12)

XC= (13)where:

L (m) length of the mesh; (kN/m

3) unit weight of soil;

() angle of inclination of the slope;a () friction angle of the soil; () friction angle of the soil-slope interface;EA (kN) axial stiffness of the mesh;Tmax (kN/m) maximum tensile strength of the mesh;Fsmesh factor of safety of the mesh;

Tamm (kN/m) permissible tensile strength of the mesh; maximum percentage deformation of the mesh.

Area 2 is determined by:

)'180(2

)'()(

1

12

+

=sen

sensenA2AREA (14)

Area 3 is the difference between the volume of long-term unstable soil and area 2. Thetotal volume thus 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 at risk.

6.4 Phase 4 Design limit state check

The designer must check whether the deformations of the flexible facing inducedby the expected long-term load, are acceptable. Since it is a load-deformation problem,the limit equilibrium method does not provide direct solutions. Consequently, the load-deformation curves, obtained from the puncturing tests, are used to determine the


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volume of soil which in turn determines the maximum permissible deformation (Figure

8). If the volume determined in this way is greater than that expected in the long term,the flexible facing satisfies the design requirements.

Figure 8: In the serviceability limit state the maximum admitted bulging is takenperpendicular to the average plane between the head of the nails.

The maximum acceptable deformation is decided upon by the designer on thebasis of one or more criteria of a geotechnical nature (effects of the deformation of themesh on the stripping of the anchors, triggering of erosion processes, effect ofsettlement induced at the boundary of the soil nailing) and a functional nature (maximumsize permitted for the debris bulge), and also considerations regarding the aestheticsand the maintenance of the intervention ().

Figure 9: Different situation of maximum acceptable bulging: (a) On slopes close to roadthe bulging shall not interfere with the traffic; the bulging could reach for instance 0.3-


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0.4 m. (b) Sometimes a low deformation of the facing is desirable in order to minimize

problems on infrastructures; in that case a very stiff mesh is required.

7 CONCLUSIONS

This paper proposes the BIOS system developed by Maccaferri for theconstruction of flexible facing using steel meshes. The system highlights that thefundamental property for this type of application is the membrane stiffness of the flexiblefacing, whilst its tensile strength has limited influence, as the forces at work aregenerally very low. With the BIOS system it is possible to reduce both the time fordesign of the facing and the cost of the solutions. However, it is absolutely essential thatthe designer verifies the context in which the facing is to be applied, analysing the stressconditions, ground stratigraphy, permissible deformations, water in the soil, and erosionphenomena.


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