anchored (tie back) retaining walls and soil nailing in brazil

www.geotecnia.unb.br/gpfees

Anchored (Tie Back) Retaining Walls and

Soil Nailing in Brazil

Summer Term 2015 Hochschule Munchen

Fakultat Bauingenieurwesen


LAYOUT

Details and Analysis of Anchored Walls

Details and Analysis of Soil Nailing

Examples of Executive Projects

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ANCHORED “CURTAIN” WALLS

(Tie Back Walls)

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Introduction Details:

• Earth retaining structures with active anchors

• A.J. Costa Nunes pioneer work in 1957

• 20 – 30 cm thick concrete wall face tied back

• Ascending or descending construction methods

• Niche excavation

• ACTIVE anchor

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Excavation Procedure

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Molding Joints 7/60


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Stability Analysis

Verification of failure modes:

• Toe bearing capacity

(NSPT < 10)

• Bottom failure

• Wedge or generalized failure: limit equilibrium analyses

• Excessive deformations

• Anchor stability and punching

• Structural failure

• Construction failures (e.g. during excavation)

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Stability Analysis Methodologies

Wedge Method:

• Kranz (1953) is the pioneer

• One or two wedges

• Ranke and Ostermeyer (1968) German Method

• Nunes and Velloso (1963) Brazilian Method

• Hoek and Bray (1981)

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Clayton et al (2001)

Kranz (1953) Method:

•FS in relation to each anchor

•FS= max allowable / actual anchor load

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Hoek and Bray (1981) Method:

•Simple geometries

•Homogeneous soils

•FS by vertical and horizontal equilibrium

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Pre-design Charts:

• Safety Factor = 1.5

• Surcharge q = 20 kPa

• Unit Weight = 18 kN/m3

• Preliminary analyses

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TECNOSOLO (1964)

Original Report 3310

Nunes and Velloso (1963) Method:

•FS for an existing Culmann wedge

But modified to have

•FS in relation to cohesion vector

esStableForcInstable

Cohesion

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22 Example

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Stability Analysis Methodologies Complex Cases:

• Numerical or analytical tool

• Limit equilibrium approach

• Non homogeneous soils

• Complex load and geometries

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Bishop (1955)

Geoslope Slopew

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Anchor Spacing:

•Counterbalance Instability x Stability Forces

•Anchor force to yield general FS > 1.5

•Length > “critical” plane

Micropiles

•Whenever there is low capacity soils at wall base

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Surcharge 26/60


Stresses and Deformation Analysis

Tools:

• User friendly numerical FEM programs

• Distinctive models

• Laboratory parameters

• Pre and post processors

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Example:

•Águas Claras Site – Fed. District, Brazil

•Porous clay over soft soil

•Close to train rail

•15 m height and 4 anchor layers

•Staged analyses

•Laboratory parameters

•Mohr Coulomb model

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SOIL NAILING

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Introduction Basics of Design:

• Reinforcement of soil with thin elements: nails

• Pre-bored sub horizontal hole, with grout

• Originated from shotcrete flexible support in tunnels

• Active zone is formed around excavation

• Started in Brazil in 1970 and France 1972 (sol cloué)

• PASSIVE anchors = “nails”

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Experience and Construction Method

Experience:

• High and successful experience in Brazil

• Use for man made, residual and saprolitic slopes in Hong Kong

• Not suitable for very loose sands or soft clays

Construction:

• Similar as tieback walls: top – down excavation stages (1-2 m)

• Vertical or inclined slopes – depends on geology

• Installation of nails, mesh, drains and shotcrete

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Installation of Nails:

• After driving or drilling

• Short nails (3 m) by hand hammers

• Corrosion protection aspects

• Driving is not adequate with boulders

• Common drilling with 50-100mm ´s

• 20-32 mm steel bars

• > 100 kPa lateral friction

• Pneumatic drill rigs are used

• Light drill rigs are desired

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Construction Details Nail Head:

• With or without steel plate and wrenches

• Small torque of 5 kN is incorporated as residual load

• Inclinations of 10-20 degrees

• Embeddement in a cast-in-place concrete niche

• Grounting with or without (gravity head) pressures

Geocompany (2009)

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Souza et al. (2005)

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Slope Facing:

• Shotcrete is applied through dry or wet mix

• Thickness of 50-150 mm

• One or two steel meshes

• Steel reinforced shotcrete (SFRS) is also used:

fibers 30-50 mm lingth, 0.5 mm dia.

dosage 35-60 kg/m3

good for slope irregularities

• Vegetation combined with nails

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Details:

• Wall

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Details:

• Nail

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Details:

• Injection

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Details:

• Frontal Spacing

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Comparisons

With Tieback Walls:

• Generally do not use prestressed active anchors

• Uses passive low prestressed nails (5-10 kN)

• Load transference by friction along entire length

• Very low loads on shotcrete facing compared to tieback walls

• Inclined or vertical facings

• Length of nails 60-120% of height (shorter than walls)

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With Reinforced Walls:

• Top-down versus upwards construction sequence

• Distinct displacement patterns (0.1 - 0.3 % of height)

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Advantages Economy: • Cost effective technique, as low as 50% of a tieback wall Rate of Construction: • Fast rate specially with SFRS shotcrete

Deformation: • 0.1 – 0.3% of height at top of wall for well designed structures

Flexibility: • Deformation can be controlled with combined use of anchors

Reliability: • Already proved in residual and saprolitic soils in Brazil • Increases stability in unsupported slopes with weak surfaces

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Limitations Displacements: • May render unacceptable deformations close to structures Construction: • Needs temporary stability of excavated face

Geology: • Risky solution for weak materials or very height walls

Durability : • Corrosion protection of nail is fundamental

Testing and post-execution intervention: • Generally not possible with nails. • Post execution corrective injection is still not widely used

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Analysis of Nailed Structures

Theoretical Methods:

• Several approaches and simplifications

• Active and passive zones

• Global Limit Eq. (slice) analysis with nail effects

• Circular, bilinear, linear surfaces

• Tension only or with bending effects in nails

• Constant or variable soil-nail interface friction

• Winkler type analysis for nail or force vectors

• Single or multiple surfaces – FS optimization

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Effect of Injection Phases


Modified after

Souza et al. (2005)

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Computer Programs

Benchmark Tests:

• Comparative comparisons are made

• Talren is the most widely used

• Prosper is a research tool

• Clouage and Nixesc are french softwares

• Rstabl adopts Bishop and Janbu´s method

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Results:

• Influence of bending is rather small

• Janbu´s method tends to yield lower SF´s

• Few differences between methodologies

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Nailing Software (Czech Republic):

• Good experience and successful results in Brasília porous clay

• Nice research and design tool

• User friendly

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0 4.00[m]

Length of structure = 3.20mGeometry of structure Structure load

21.08

25.00[kPa]

0

Cut1

0.50

1.00

1.00

0.70

Max. M = 4.32kNm/mBending moment

3.43

-3.43

4.32

-5.00 5.00[kNm/m]

0

Max. Q = 11.14kN/mShear force

-6.87

-6.87 6.87

6.87-6.81

-10.71 11.14

-25.00 25.00[kN/m]

0

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Examples

Icaraí Beach, Niteroi-RJ:

• 25mm bars in 90 mm holes – 150 mm shotcrete, inclined 75° – 1.5 m spacings (H:V) and two steel meshes

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Railway, São Paulo-SP:

• 25mm bars in 75 mm holes – 50 mm shotcrete, inclined 75° – 2.5m x 2.0m (H:V)

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Ortigão et al. (1993)

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Cindacta Project – Friburgo-RJ

Executive Design Project 57/60



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Soil Nailing

Tie Back Wall

Active Anchor

Passive Anchor



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Tie Back Wall

Soil Nailing


REFERENCES

• Ortigão & Sayão (2004). Handbook of Slope Stabilisation, Springer, New York, 478 p.

• Hunt, R. E. (1986). Geotechnical Engineering Techniques and Practices, McGraw Hill, New York, 729 p.

• Personal pictures.

• Internet pages.

• Executive Design projects from ACRosa Engenharia de Consultoria Ltda., Rio de Janeiro, Brazil.

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anchored (tie back) retaining walls and soil nailing in brazil

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