Neoweb® Slope Protection Engineering Design

Neoweb® Slope Protection System • Reinforced cover protection • Erosion control for stable slopes • Extend local stability life-span • Vegetated landscape solution... Section IISlope Sta

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Engineering Seminar, PRS

July 2010

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Neoweb® Slope

Protection System

• Reinforced cover protection

• Erosion control for stable slopes

• Extend local stability life-span

• Vegetated landscape solution

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Section I INPUT

• Checklist

• Project Plans

• Objectives - Client’s preferences

• Goal & Cost

• Slope Definition

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I - INPUT

Checklist

• Summary of design properties

• Includes most/all design properties

• Enables relatively quick design for preliminary evaluation

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I - INPUT

Objectives - Client’s Preferences

• Understanding main objectives of client

Preferences, e.g.:

– Min cover thickness for vegetation

– Min cover thickness for geomembrane protection– Type of surface cover protection

• Vegetated

• Granular

• Concrete

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I - INPUT

Goal & Cost

• Cost of alternative solutions (if any) per sqm or linear meter of solution

• Determine go/no-go

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(1) Slope Type:

I - INPUT

Slope Definition

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Sub-base / Native Soil:

• Examining slope global stability

• Evaluating stakes resistance

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Loads / Actions on Slope:

• Loads on slope, e.g snow

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Section II

Slope Stability Examination

• Examining global slope stability for

rotational-circular failures

• Not considering Neoweb cover

• Target: ensuring the Neoweb is on a stable slope

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II - Slope Stability Examination

• Circular-Rotational Failure:

– Large mass of soil rotates and fails

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Why?

• Poor soil properties:

– Low friction angle

– Low cohesion under saturated

condition

• Challenging geometry:

– Height & inclination

• Crest loads

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Is the slope stable based on:

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• Unstable Slope - Circular Failure

• Factor of Safety < 1.30

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• Stable Slope - Circular Failure

• Factor of Safety ≥ 1.30

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• Effect of different soil properties on

identical slope geometries

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Stability Examination

Slope is Stable

Slope Protection System

Slope is stable Not

Earth Retention System

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24º 24º 24º (32º)

1V:2H (26.6º) 2V:3H (33.7º) 1V:1H (45.0º) 3

Example of Stable Slopes (FS ≥1.30):

5.0 (3.0) 24º (32º)

1V:1H (45.0º)

2.5 3.0 5.0

24º 29º 32º

1V:2H (26.6º) 2V:3H (33.7º) 1V:1H (45.0º) 5

3.5 6.0 9.0

24º 25º 28º

1V:2H (26.6º) 2V:3H (33.7º) 1V:1H (45.0º) 7

4.5 8.0 13.0

24º 26º 28º

1V:2H (26.6º) 2V:3H (33.7º) 1V:1H (45.0º) 10

* Properties of cut/native slope soil at saturated state

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Eroded slope Example

- solution is possible with Neoweb slope protection system

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Global slope failure – needs retention of the slope utilizing Neoweb Earth Retention System

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Section III

Neoweb Slope Protection Design

• Defining Neoweb Infill Type

• Defining Neoweb product: cell size and height

• Calculating Downslope Driving Force

• Calculating Downslope Resisting Forces

• Customizing Anchorage System for sufficient Factor of Safety

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III – Neoweb Design

Types of Infill Protection:

Vegetated Topsoil Topsoil-Granular

Granular Concrete

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Neoweb Infill Properties:

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Neoweb Selection:

2 key guidelines for choosing Neoweb type:

• Slope inclination

• Neoweb Infill Properties

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Neoweb Selection:

• Depends on 3 types of soil surface cover infill

• Considers Internal Friction Angle:

– Local Vegetated Topsoil (friction angle = 20°)*

– Vegetated Topsoil-Granular (friction angle = 28°)

– Gravelly soil (friction angle = 36°)

* If lacking information, refer to Case 1 (default).

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Case 1: Local

Vegetated Topsoil (friction angle=20°)

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Case 2: Vegetated

Topsoil-Granular (friction angle=28°)

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Case 2:

Gravelly soil (friction angle=36°)

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Basic principle for each 3 cases:

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Topsoil-Granular (φ=28°)

Gravelly soil (φ=36°)

Locate Slope Inclination

Choose NEOWEB Choose NEOWEB Choose NEOWEB

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The design is based on 1 meter strip width of Neoweb slope:

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Calculating Downslope Driving Force

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( S )

S slp

W = γ +

(2) Weight of Neoweb infill

and its extra cover= [kN/m]

Where:

• Lslp = Slope Length [m]

• γs = Unit Weight of Neoweb Infill [kN/m 3 ]

• D = Neoweb Cell Height [m]

• Zs = Thickness of Neoweb Infill extra cover [m]

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q = ⋅

(3) Total external loads

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( )cos α

T g

Calculating Force Components

(4) Perpendicular component force

(5) Parallel driving force component

on Slope = ( )sin α [kN/m]

T g

Where:

• Wg = Weight of Neoweb infill and its extra cover [kN/m]

• qT = Total Vertical Load on slope (e.g snow) [kN/m 2 ]

• α = Slope Angle [°]

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FS T

T = ⋅

• Ensuring Neoweb Stability against Driving Sliding Forces downslope

• Min Factor of Safety against Neoweb Sliding Fsl ≥ 1.30

sl a

ad T FS

T = ⋅

(6) Design Driving Force of Sliding =

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( )k L k C N

R I = a tan 1ϕ + slp 2

Where:

• k1 = Effective friction angle reduction coefficient between Neoweb infill to

1

Geosynthetic under layer [kN/m]; in lack of information use:

- k1 = 0.80 for non-woven Geotextile

- k1 = 0.60 (and down) for Geomembrane liner

- k1 = 1.0 No Geosynthetic

• φ = Effective Friction Angle of Neoweb Infill [°]

• C = Effective Cohesion of Neoweb Infill [kPa]; usually use 0, or very small values for soil with low friction angle

•K2 can be equal to k1 while almost neglecting Effective Cohesion value

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( ) ( )

L

R SHL = SHL + S γ tan 1φ + 2

Where:

• LSHL = Shoulder Length

** RSHL is limited to 10% of Ta

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Embedded Toe Resistance = R T

Passive Resistance: RT(PAS) = 0.5γSKP(2DT - D)D

Where:

DT = Embedding Depth

** RT is limited to 20% of Ta

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• In most cases interface resistance force is smaller

than design driving force, therefore:

RI +RSHL+ RT < Tad

• Hence, Anchorage System must be added to achieve

required Factor of Safety (1.30)

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Anchorage System

ANCHORAGE SYSTEM

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system

Anchorage System- Penetrable Slopes

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Anchorage System - Penetrable Slopes

Typical properties of a pin stake anchor:

Embedded length (eff.) = 500mm

Plastic Neo-Clip™

Diameter = 10mm

Downslope resistance = 0.5-0.6kN

Plastic Neo-Clip™ attached

** Minimum Stakes in either way

(for retaining Neoweb shape) is app 0.6-0.7 stakes/sqm.

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R = Total downslope stakes resistance for 1m strip

** Minimum Stakes Density in either way (for retaining Neoweb shape) is app.0.6 stakes/sqm.

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EXAMPLE LAYOUT:

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Checking Neoweb Stability

Downslope Interface friction resistance, R I

+

Crest Shoulder Interface friction resistance, R

Crest Shoulder Interface friction resistance, R SHL

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Checking Neoweb Stability - EXAMPLE

OK!

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Anchorage System- Impenetrable Slopes

1 Tendons (cables) are expanded in the

middle height of Neoweb cell prior

section expansion downslope.

2 Tendons are tied to clips/clamps

holding uphill cell wall

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3 Tendons are secured to a crest Deadman Anchor

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EXAMPLE LAYOUT:

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Tendons Type:

• Polyester or Galvanized Metal- depends on required tensile strength (metal tendon is stronger); default tendon diameter is 6mm.

Clips/Clamps:

• Plastic Neo-Clips™ are used with polyester Tendons;

• Galvanized Metal Clamps are used with Galvanized Metal Tendons;

** Minimum range of Clips/Clamps in either way

(for retaining Neoweb shape) is app 0.7-1.0 units/sqm.

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– The tendons are expanded in specific holes in the

middle of cell height– These sections are not default Neoweb section

– Hence required to be defined for a customized project

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• Stages for Design Tendons:

• Define tendon type (polyester/metal), diameter, ultimate tensile

strength and reduction factors

• Define tendons horizontal spacing (every cell/every 2 cells, etc…)

• Calculate un-factored applied force on 1 tendon

(depends on the spacing between tendons)

• If Allowed Tensile Strength of tendon is greater than applied force

OK!

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• Define crest anchor type:

• PVC pipe / concrete beam

• Most of resistance is achieved by height of deadman and buried thickness below ground level

• Define deadman’s dimensions:

• Width, height, unit weight, buried thickness

• If passive + friction resistance are equal or greater than applied

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Impenetrable Slopes - EXAMPLE:

OK!

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