soil nailing in germany

8/13/2019 Soil Nailing in Germany

1/56

University of Applied Sciences Munich / Germany Prof. Dr.-Ing. G. Gssler Department of Civil Engineering Presentation at Universidade de Braslia 03/2010 1

Soil Nailing in Germany: Development, Design, Execution

[Eigene Dateien\Brasilia Powerpoint\Soil Nailing in Germany-63-Brasilia.ppt]


2/56


2. Design: Verification of stability of a nailed wall2.1 General design considerations

2.2 Example: Verification of stability of a 10 m high wall

1.1 Principle static analyses

1.2 Model tests1.3 Field tests

Summary and conclusions

3. Execution: Several examples in soil and soft rock 3.1 Practical cases of nailed cuts near to the vertical3.2 Practical cases of nailed slopes3.3 Quality control and warnings

Scope of Presentation

Introduction

1. Development: German research programme


3/56


Introduction

Soil nailing: is a technique of reinforcing natural ground by the insertion of slender tension-carrying elements called nails.

Applications: excavation pits, permanent retaining structures and (natural) slopes.

Origin and idea of in situ reinforcing cuts or slopes:

1972: First recorded case of a nailed wall in France:temporary stabilising of a 70 cut in cohesive sand(commercial project, no subsequent research! )

1975 - 1981: Research and development projectSoil Nailing in Germany ( Univ. of Karlsruhein joint venture with the contractor BAUER)

1979: First international publication on soil nailing by the Univ. of Karlsruhe and BAUER ( In this

publication the termSoil nailing

was created )

1986 1990: National Research Project Clouterre in France

1976 - 1981: Research and development project

Lateral Earth Support System (Univ. of Cal.)Fig.: First test wall in history of soil

nailing pushed to failure (1976)


4/56


Construction method

(2) face stabilization using shotcrete(3) installation of nails and grouting

The procedure is repeated untilthe required depth is reached.

Three basic processes are involved insoil nailing:

(1) excavation in steps

h

0.5-0.7 h

(1)

.

..

.

.

(2)

(3)

(1)

Fig. 1: Installed temporary nail of a testwall in sand (digged out after test)

Fig. 2: Temporary nail after Germanapproval (System BAUER)


5/56


tension!

slip surface

displacementsmall

STEEP CUTS:TENSION NAILS

1

t e n s i o n !

d i s p l a c e m e

n t

s m a l l 2aTENSION NAILS

2FLAT SLOPES

bending!

s l i p s u r

f a c e

d i s p l a

c e m e n t

l a r g e !

w e a t h

e r e d

t o p l a y

e r

s o l i d

g r o u n

d

2b

DOWEL NAILS

Principle mode of operation of soil nails

Remark: Nails in case 1 and 2a act also as bending elements carrying shear forces at the slip line;but: These shear forces are of secondary order and are neglected in the following design example!


6/56


1. German research and development programmecomprised:

4. Derivation of partial safety factors ona statistic-probabilistic basisandDevelopment of a practical designmethod2. Model tests in sand

1. Static analyses on the basis of kinematicalfailure mechanisms of rigid earth blocks

3. Field tests in sand and clay


7/56


Single wedge(may be chosen for low nailingdensities or rough calculations)

Note: For given parameters (soil, load, nailing) the most unsafe failure mode is the one that, under vari-ation of the slip lines, requires the absolute maximum of the pull-out resistance per unit nail length T m,k !

1.1 Static analyses on the basis of kinematical failure mechanisms of rigid earth blocks

Several kinematic failure models for nailed walls were investigated.

An overview is given on the four different potential failure mechanisms that turned out to be critical.

Note: the following failure modes were - theoretically investigated by stability analyses but also- verified in model tests and full scale tests

Two-part wedge with circular slip lines(theoretically the least safe mechanism in case ofvery high surcharge in the rear or in case ofearthquake

Two-part wedge withstraight slip lines(shall be chosen in

non-cohesive soils)

Slip circle(shall be chosen in cohesivesoils as well as in non-cohe-sive soils)


8/56


1.2. Model tests

1.2.1 Simple wedge failure mechanism with a straight slip line

Fig.: Model test in Sand with low nailing density


9/56


Fig. 1: Observed 2-part-wedge mechanism with straightslip lines in a model box filled with sand

Fig. 2: Kinematics of earth block (1) and (2)

1.2.2 The 2-part-wedge failure mechanism with straight slip lines

This failure mode was observed in a model test, where the rigid facing of the nailed construction (h = 40cm) in dense sand was progressively moved to the air side.


10/56


a) Observed 2-part-block with circular slip linesin a model box filled with sand

b) Kinematics of earth block (1) and (2) with centre points and radii of the three slip circles

Fig.: Model test of a nailed wall in sand with a high load beyond the nailed zone

1.2.3 The 2-part-wedge failure mechanism with curved slip lines

This failure mode was observed in a model test, where a nailed wall (h = 40 cm) was progressivelysurcharged by a water pressure cushion beyond the nailed zone until failure occurred.


11/56


What is the explanation for this special failure mode?

Answer: The high earth pressure force E of wedge (2) due to the high area load, acting upon the nailed block in its upper part, leads to an overturning moment


12/56


1.3 Field tests

Fig.: Field tests in sand and clay


13/56


Fig. 1: Front view of the first test wall in history ofsoil nailng with measuring facilitiesHeight: 6.0 m; Nail lengths: 3.0 m to 4.0 mLoad: Strip load in the rear (hydraulic jackets)

Fig. 2: Seperation of the proper test wall from thesurrounding ground by thin slurry walls toachieve 2-D conditions

Test A: Strip load in the rear


14/56


Fig. 1: Cross-section of test wall A with inclino-meter boreholes to locate the slip lines

Fig. 2: Cross-section of test wall A in moment of failure(Wall was weakened by further excavation andpulling-out of bottom nail row)

Failure mode: Two-part wedge mechanism withstraight slip lines

Test A: Measuring results


15/56


Test A

Fig.: Field test A: Nailed wall after failure: total displacement ca. 20 cm(Movement of wall during failure parallel to itself is clearly to be seen)


16/56


Measurements: 1 Displacements using inclinometer and geodetic instruments

2 Earth pressures using hydaulic cells3 Nail forces using electr. strain gauges

Fig. 1: Cross-section of instrumented test wall B

Test B: Strip load near the crest

Fig. 2: Cross-section of wall B with horizontal defor-mation after completion (under dead weight)

u = horizontal displacementh = height of the wall

Non-cohesive soils u/h 2.5 - 3 (Cohesive soils u/h 3 - 5 )


17/56


18/56


Test B: Strip load near the crest

Fig. 6: View of test wall B after failure from theside (Movement of wall during failure inrotation clearly to be seen)

Fig. 5: Test B: Cross-section with ruptureline located in the inclinometerboreholes after failure


19/56


Fig.: Cross-section of test wall B with axial nail forcesafter completion and during loading until failure

Test B


20/56


21/56


Test G in clay

Fig.: View at test wall G in slightly overconsolidated clay with loading equipment


22/56


Test G

B1, B2, B3 .... Boreholes for inclinometer measurements (horiz. & vertical deformations)

Fig.: Test G in overconsolidated clay: Measured horizontal and vertical deformations of facingand nailed soil after completion (i. e. under dead weight)


23/56


2.1 Presenting the design example: cross-section and soil parameters

2.2 Preconsiderations on the type of design approach

2.3 Derivation of design values

2.4 Identification of the unsafest failure mechanism2.5 Determination of the required characteristic nail resistance

2.6 Further stability checks for the total structure

2. Design: Verification of stability of a nailed wall with shotcrete facing

Scope:


24/56

University of Applied Sciences Munich / Germany Prof. Dr.-Ing. G. Gssler Department of Civil Engineering Presentation at Universidade de Braslia 03/2010

2.1 Design example: Permanent nailed wall in sand with a transient surface load

Given geometric data:Height of the wall: h = 10 m;Horizontal nail length at the bottom: l = 6.0 mWall inclination (to the vertical): = 10;

Nail inclination: = 10Vertical nail spacing: s v = 1.10 m

Horizontal nail spacing: s h = 1.25 m

Acting characteristic parameters:Transient surface load: q k = 23 kN/mUnit weight of soil: k = 18 kN/m

Resisting characteristic parameters:Friction angle of soil: k = 35; cohesion: c k = 0Lowest value in a series of pulling-out tests of nails in-situ:

F P = 160 kN (Index P denotes P roof load)

Bonded length of all test nails: l b = 5.0 mHence: the ultimate pull-out resistance per unit nail lengthis yielded by: T Pm,k = F P lb = 160 / 5.0 = 32 kN/m

Question: Is this nailed wall safe after EC-7 and DIN 1054?


25/56


26/56


2.4 Identification of the unsafest failure mechanism for the given example

As shown above, the stability proof of a nailed wall with conditions as given in the example can be based on

- the two-part wedge mechanism with straight slip lines,

or as well as on - the slip circle

NOTE: As the two-part wedge failure mechanism is more comprehensible for a presentation,

it will be used in the following design procedure!


27/56


h' = 5,8 m

(1) = 35

qd

1st Variation (1) = 35

Actions or driving forces:

Wd

Wd = 694 kN/m

K a : active earth pressure coefficient

K a = 0,308

Pd = q d b = 30 4.2 = 126 kN/m

Pd b = 4.2 m

Ea,d = h ( h + 2q d) K a (d)d = 29

Ea,d

Ea,d = 5.8 (18 5.8 + 2 30) 0.308 = 147 kN/m

?

?

?

?


28/56


29/56


(2) = 40 Z d Q

d

Zd = T m,d (0.8 + 2.0 + 3.2 + 4.3 + 5.6)

Zd = T m,d li9

5

275 = T m,d 15.9

Tm,d = 275 15.9 = 17.3 kN/m/m

fr (2) = 40

Z d = 2 7 5 k N / m

Q

Wd =649 kN/m

Pd =126

d

Ea,d = 115 kN/m

l 9 = 5 .6 m

l 6 = 2 .0 m

l 8 = 4 .3 m

l 7 = 3.2 m

l 5 = 0 .8 m

Wd

Pd

d = 29

Ea,d

h = 5.0 m

2nd Variation (2) = 40

= 5


30/56


(3) = 45 Z d

Q

d

Z d = 30 0 k N / m

Q

Wd

Pd

Ea,d

l 9 = 5 .6 m

l 6 = 2 .6 m

l 8 = 4 .6 m

l 7 = 3.6 m

l 5 = 1.6 m

Wd

Pd

Ea,d

h = 4.0 m

3rd Variation (3) = 45

l 4 = 0 .6 m

16.118.630045

17.3 = MAX!15.927540

16.913.623035

[kN/m/m][m][kN/m][]

Tm,d = Zd / li liZd

design pull-out resistance: T m,d,max = 17.3 kN/m/m

characteristic pull-out resistance:with partial safetyfactor (DIN 1054,Table 3)

N = Tm,k

Tm,d

Tm,k,max = N Tm,dTm,k,max = 1.40 17.3T m, k,max = 24.2 kN/m/m


31/56


Remember: established horizontal spacing was: s h = 1.25 mThus, the required characteristic pull-out resistance per unit nail length of the physical

nail is yielded by:T m,k = 24.2 1.25 = 30.25 kN/m

Remember: ultimate pull-out resistance, verified in a series of load tests was:

Result of slip line variation:Tm,k,max = 24.2 kN/m/m (i.e. per unit length of a virtual nail per unit width of the wall)

T Pm, k = 32 kN/m (P indicates: P roof load)It can be seen: T m, k,max < T Pm,k which is OK!

The maximum ultimate pull-out resistance per unit nail length in the design is less than ultimate

pull-out resistance per unit nail length in the load test in-situ.Conclusion: 1. The design is safe!

2. The design is also economic, as

the utilisation factor for the nails: T m,k / T Pm, k = 30.25 / 32 = 0.95 or 95%

2.5 Determination of the required characteristic nail resistance


32/56


(a) Groundfailure

(a) Groundfailure

(c) Overall (or external) stability

(c) Embankmentfailure

M

(b) Failure against horizontal sliding (often involved in varia-ion of slip staight lines)

(b) Horizontal sliding

2.6 Further stability checks for the nailed wall

After DIN 1054, sec. 12.4.4 (2) the nailed (or reinforced) part ofthe soil has to be considered as a solid block or gravity wall.

If in doubt, the following stability checks have to be made:

NOTE: Verification of nails and facing cannot be presented here (please, see publications!)

DC N il


33/56


DC-Nailwww.dc-software.comMunich/ Germany

Fig.: Checking internal stability by two-wedge failure mechanisms with straight slip lines andexternal stability by slip circles using German computer program DC-Nail

Anlisis de Suelos EnclavadosProgram DC-Nail

Anlisis de suelos enclavados de acuerdo aDIN 1054:2005 (basado en EUROCODE 7),DIN 1054:1976

Diseo de revestimiento de hormignlanzado de acuerdo a DIN 1045, DIN 1045-1

Anlisis con factores de seguridad parcial ocon seguridad global

Versiones del programa en Alemn, Ingls,Francs, Italiano, Espaol

Anlisis de acuerdo con el mtodo general dedeslizamiento de bloques

Determinacin de la estabilidad interior yexterior

3 E i S l l i il d f k h d k


34/56


3. Execution: Several examples in soil and soft rock or weatherd rock

3.1 Practical cases of nailed cuts near to the vertical

a) Auger drilling for the nails b) Pull-out test of a nail

Fig. 1: One of the first permanent nailed walls near to the vertical in sand/ gravel,executed by the German contractor BAUER near Stuttgart, 1985

3 1 P ti l f il d t t th ti l (1 st ti ti )


35/56


3.1 Practical cases of nailed cuts near to the vertical (1 st continuation)

a) Shotcreting b) Drainage provided by drainmats (System ENKA)

Fig. 2: Permanent nailed wall near to the vertical in sand/ gravel as before(Nails coverd by shotcrete to prevent corrosion)


36/56


a) Permanent soil nail with corrugated plastic sheath

for complete corrosion protection

Fig. 2: Permanent nail (System BAUER, Germany) b) Permanent nail on site

Permanent nailed wall with revetment wall


37/56


a) b)Fig. 2: Cross-section of the nailed cut (a) after completion,

(b) in final state with a revetment wall to be greened

Fig. 1: Permanent nailed cut in mountaineous areain Southern Germany (Nails heads coveredby shotcrete, only weep hole pipes to be seen)(National Road widening scheme project)




38/56


Fig. 3: Wall shortly after completion: Greening yet to comeFig. 1: Mounting of the prefabricated concrete

elements by a crane


Fig. 2: Fixing of concrete elementsat the top


39/56


Fig.: Permanent cutting slope in Keuper marl (South-West Germany) for a new National Road;due to significant creeping property of marl: nail lengths 23 m; calculation by slip circles


40/56


Fig 1: Permanent nails without plastic sheath; corrosion protectiononly by in-situ grouting; diameter of GEWI tendon: 50 mm(Degradation due to corrosion incorporated in the design)

Fig. 2: Permanent facing with double reinforce-ment layers (earth-sided and air-sided)

3 2 Practical cases of nailed slopes


41/56


Landslide is a nightmare for every geotechnical engineer!

3.2 Practical cases of nailed slopes

F ig.: Landslide due to fossil slip surfaces reactivated in a cutting (Marlstone)


42/56


F ig.: Reinforcing a 30 cut in marlstone to prevent sliding on potential geologic slipsurfaces possibly reactivated by excavation in front of a tunnel portal of theGerman high speed railway line Mannheim - Stuttgart


43/56


INCLINOMETREBOREHOLE

SOIL-NAILS

2 0 m

1 5 m

2 4 m

GEOLOGIC-SLIP-

SURFACES

10

DRAIN TUBES

EXTENSO-

METRE (GREENED)(GREENED)

NATURAL GROUND LEVEL

FREE SURFACE

1 : 1 ,7 5

1 : 1, 1 5

NATURAL GROUND LEVEL

FREE SURFACE

1 : 1 ,7 5

1 : 1, 1 5

1 : 1, 1 5

F ig.: Cross-section of the 30 cut (i.e. 1:1.75) in marlstone to prevent sliding on potentialgeologic slip surfaces reactivated by excavation

(Slip surfaces were not localized but expected due to the experienced landslide shown before)


44/56


F ig.: View at geologic slip surface found in reality during excavation process: Nailing absolutely necessary for reasons of safety!


45/56


Fig.: Special drilling rig for 18 m long nails constructed by BAUER

Nail setting


46/56


Fig. 3

Fig. 2

Fig. 5

Fig. 1

Fig. 4


47/56


48/56


Fig. 2: Setting of instrumented nail for pulling-outtest to verify the assumptions of design

Fig. 1: Analysis for the worst case: Geologic slip surface assumed at thebottom of the slope (assumed failure: two-part wedge mechanism)

Question: Assumption of mean pull-out resistance per unit nail length T m (kN/m) correct?


49/56


Fig. 2: Test loading facilitiesFig. 1: Axial force distribution along the instrumented test nailfor different loads (soil: weathered Keuper Marl)


50/56


F ig.: Completed nailed slope with hidden nails under green surface(Picture taken few months before opening to traffic, 1991)

Quality control:


51/56


- Work: according to the plans?

- Excavation steps: following theadmissible depth and the time sequence?

- Free standing of boreholes?(if not: casing!)

- Nails: Length correct?Diameter of the steel

tendon correct?

- Soil conditions: according to geotechnical report?

Put the following questions on the site:

Missing quality control:Nails too short! Facing too thin! =>

Quality control:P h f ll i i h i


52/56


- Work: according to the plans?

- Excavation steps: following theadmissible depth and the time sequence?

- Free standing of boreholes?(if not: casing!)

- Nails: Length correct?Diameter of the steel tendon correct?

- Grouting water-cement-factor correct?

- Shotcrete: thickness?

- Mounting of mats: sufficient overlapping?

- Soil conditions: according to geotechnical report?

Put the following questions on the site:

- Pull-out tests of nails: performance correct?results according to the design?

Benefits of Soil Nailing:


53/56


g

- Economic advantage

- Construction flexibility

- Small construction equipment

- Special solutions: Remedial work

Fig.: Restoring and stabilizing historicmasonry walls using soil nails


54/56

Summary and Conclusions:


55/56


2. The ultimate limit state design of nailed walls and slopes is based on kinematic

failure mechanisms proved in model and field tests of the research programme.

1. The research and development programme in Germany (1976 1981) yielded

numerous results on the bearing behaviour and the stability of nailed walls.

3. The application of the principles of new European Standard EC 7-1 GeotechnicalDesign combined with the detailed rules in German Standard DIN 1054

provides a safe and practicable limit state design of soil nail constructions.

4. Execution of soil nailed constructions needs experience and good knowledge of the

ground conditions.

5. Soil nailing involves numerous benefits but also some limitations.

6. Today soil nailing is a well established construction method in many contries allover the world.

There is a wide field for its application in Brazil, too!

Last but very important conclusion:S il ili d i d i !


56/56


Not guys doing it like that:

Soil nailing needs experienced engineers !

Thank you very much for your attention!

Cartoon by courtesy of Sddeutsche Zeitung,

Germany

soil nailing in germany

Documents