Dr Ian BatemanDirector

Interflow Pty Ltd

Limitations of AS/NZ2566.1 For The

Trenchless Technology Industry

Outline

1.Use of AS/NZ2566.12.Examples where AS/NZ2566.1 is • not conservative enough• too conservative

3.Conclusions and recommendations

Design in AUS/NZ Trenchless Industry

1. There is no specific design standard2. We borrow aspects from other standards

Fully Deteriorated

- Borrow from AS/NZ2566.1

- Assumes existing pipe has no strength

- Assumes liner acts like a buried flexible pipe

In Tact

- Borrow from ASTM F1216

- Assume existing pipe acts to enhance the liner strength

Overview of AS/NZ2566.1

Scope

“This Standard sets out a practice for the structural design of buried flexible pipelines which rely primarily upon side support to resist vertical loads without excessive deformation. The interactive pipe/embedment structure is considered only in the transverse direction. Structural performance is predicted in the long-term for pipes in trenches and embankments but not for jacked or bored lines.”

Overview of AS/NZ2566.1

AS/NZ2566.1 is designed to cover- installation of a flexible pipe into a trench- takes into consideration

- pipe characteristics (stiffness and material properties)

- embedment characteristics- design loads

- prescribes a method of performing a design

Design Method

Calculate Applied Loads - Soil load- External hydrostatic load- Internal pressure- Dead loads- Live loads (eg traffic)

Check the following- Deflection- Strain- Buckling

Buckling Condition

In most applications the governing equation is

(Applied Loads) x FOS = (St)1/3 x (E’)2/3

St = Pipe ring stiffnessE’=Modulus of Soil Reactivity

In Other Words …

The applied loads need to be resisted by

1. The ring stiffness of the pipe (liner)2. The surrounding soil

But, the effect of the soil is much more dominant

AS/NZ2566.1 In Our Trenchless Industry

• Used successfully for more than a decade• Hundreds of thousands of pipes re-lined• Industry provides cost effective solutions• Installers have effective and practical systems• Suppliers are able to produce products for

nearly all situations

So, what’s the problem?

Potential Problems

As the industry develops …

• We are faced with ever more challenging situations

• Suppliers develop more and more sophisticated products

Fall outside of the intent of AS/NZ2566.1

AS/NZ2566.1 – Not Conservative Enough

High Modulus Thin Walled Liners

- AS/NZ2566.1 allows us to determine the RING STIFFNESS of a liner that is needed

- RING STIFFNESS is a function of- The modulus of the material- The thickness of the material

High Modulus Thin Walled Liners

Example

- To achieve a desired RING STIFFNESS of 1,000 N/m/m

Modulus Of Liner Material

1,000 MPa

5,000 MPa

9,000 MPa

Thickness Required

3.4mm 2.0mm 1.7mm

Can achieve desire stiffness using thin, high modulus materials

High Modulus Thin Walled Liners

• This is reasonable if the liner of perfectly circular cross section

• Implicit in AS/NZ2566.1 is that pipes are supplied to site free of defects (then buried)

• In a trenchless application the pipe (liner) is formed inside a deteriorated host pipe

• The final shape of the liner is influenced by the shape of the deteriorated host pipe

High Modulus Thin Walled Liners

Liners can contain IMPERFECTIONS

x

Effect of Imperfections On Liner Stiffness

• Phenomenon is well studied (Moore,I et al)• Effect on liner stiffness is a function of

– Liner Thickness– Size of the Imperfection

With Typical Liner Materials

Reduction in Liner Stiffness vs Imperfections Size(E mod=1,000MPa)

0%10%20%30%40%50%60%70%80%90%

100%

0 5 10 15 20 25 30

Imperfection size (mm)

% O

f T

heo

reti

cal

Lin

er

Sti

ffn

ess 150mm

300mm

450mm

600mm

With A High Modulus Liner

Reduction in Liner Stiffness vs Imperfection Size(E mod=9,000MPa)

0%10%20%30%40%50%60%70%80%90%

100%

0 5 10 15 20 25 30

Imperfection size (mm)

% O

f T

heo

reti

cal

Lin

er

Sti

ffn

ess 150mm

300mm

450mm

600mm

Summarising…

1. With a high modulus material, a 150mm liner has almost zero stiffness with a 15mm imperfection

2. The effect is far less severe with traditional materials

3. The effect reduces as the diameter increases

Looking at this another way…

Theoretical

Thickness

With High Modulus Material

Liner Thickness Needed To Compensate For Imperfection (E mod=9000MPa)

0

2

4

6

8

10

12

0 5 10 15 20 25 30

Imperfection (mm)

Lin

er T

hic

knes

s (m

m)

150mm

300mm

450mm

600mm

Theoretical

Thickness

Theoretical

Thickness

With Traditional Materials

Liner Thickness Needed To Compensate For Imperfection(E mod=1000 MPa)

02468

101214161820

0 5 10 15 20 25 30

Imperfection (mm)

Th

ickn

ess

(mm

)

150mm

300mm

450mm

600mm

Theoretical

Thickness

How Deal With This Issue

Options

1. Set a minimum liner thickness of (say) 4mm2. Use an equation to de-rate the actual stiffness

and compensate for imperfections of a given size

3. Calculate theoretical thickness and add a constant (say) 2mm

AS/NZ2566.1 – Too Conservative?

Large Diameter Lining

- By number, >95% of all pipes rehabilitated are at diameters of 1,000mm or below

- By dollar, ~60 -70%

Returning to Our Design Equation…

(Applied Loads) x FOS = (St)1/3 x (E’)2/3

St = Pipe ring stiffnessE’=Modulus of Soil Reactivity

The industry’s default approach has been- Use values of E’ of between 2 and 5 MPa- Design a liner with sufficient Stiffness

Example

Stiffness VS Diameter

010002000300040005000600070008000

0 500 1000 1500 2000 2500 3000 3500

Pipe Diameter (mm)

Sti

ffn

ess

(N/m

/m)

Stiffness required to re-line a pipe 5m below surface assuming constant E’=4

** Long Term Stiffness

A Perspective On Pipe Stiffness

- Most large diameter plastic sewer and stormwater pipes will have LT Stiffness of less than 3,300N/m/m

- Flexible pipes with a long term stiffness of >8,000N/m/m do not exist

- Furthermore the reason for the stiffness is due to installation damage not deflection over time

- Flexible plastic pipes are commonly made up to 2,400mm diameter (LT stiffness~1,500 N/m/m)

Using a Typical Liner Material…

3000mm

2700mm

2400mm

2100mm

1800mm

1500mm

1200mm

900mm

600mm

300mm

Liner Thickness VS Diameter

020406080

100120140160

0 500 1000 1500 2000 2500 3000 3500

Pipe Diameter (mm)

Lin

er T

hic

knes

s (m

m)

At large diameters a solution would be not be possible and/or would be very expensive

~10% Diameter Loss

With E’=14 instead of E’=4

Liner Thickness VS Diameter

020406080

100120140160

0 500 1000 1500 2000 2500 3000 3500

Pipe Diameter (mm)

Lin

er T

hic

knes

s (m

m)

E’=14

E’=4

Changing the value of E’ has a major affect on what is possible

…. Even more dramatic effect on required stiffness

Stiffness VS Diameter

010002000300040005000600070008000

0 500 1000 1500 2000 2500 3000 3500

Pipe Diameter (mm)

Sti

ffn

ess

(N/m

/m)

E’=14

E’=4

How Is E’ Determined

• Selecting a realistic value of E’ has a huge bearing on the solution (and economics)

• AS/NZ2566.1 provides the following table• But there are other methods

AS/NZ2566.1 and E’

• Suggests a range of values between 1 and 20• Suggests the values are conservative• Suggests with cover heights greater than 10m

higher values should be used• Shows that the value increases as greater

compaction occurs

BUT, Trenchless Industry tends to use values of between 2 and 5

WHY?

Selection of E’

• Estimating E’ is difficult and time consuming• We often do not know what occurred during

initial pipe construction• We don’t know what has happened to the soil

during its lifetime• Cannot ensure 100% uniform support of the

liner by the host pipe and/or soil

The cost involved in estimating the actual E’ outweighs the cost of installing a stiffer liner – in smaller diameter pipes.

Estimating E’

• … but in large diameter pipelines this is probably not true.

• Estimating an appropriate value for E’ will have a significant bearing on the overall economics

• Not understanding the condition of the soil in large diameter pipelines can lead to serious consequences

Above a certain diameter it is worth determining a realistic value of E’

Silo Reduction Factors

• AS/NZ2566.1 allows the use of silo reduction factors when the depth of cover exceeds 10 times the diameter

• For small diameter pipes this seems reasonable

• At large diameters this becomes very conservative

Silo effects actually occur at much lower cover heights

ALSO, E’ has been shown to be related to depth

AS/NZ256.1 For Large Diameters

• Using a constant AND/OR low values of E’• Not applying silo reduction factors to soil loads

until 10 x D

VERY CONSERVATIVE EXPENSIVENOT POSSIBLE

How To Deal With This Issue

Suggestions• Continue to apply the current approach up to

a diameter that provides cost effective outcomes

• Above this diameter, establish more information about the condition of the soil (E’)

• Allow silo reduction factors below 10 times D

Alternatively…

If we don’t then we will have to …

• Ensure that large diameter pipelines are rehabilitated before they reach the fully deteriorated condition

• Use a different design method at large diameters (not AS/NZ2566.1)

Conclusions

1. The design approach borrowed from AS/NZ2566.1 has served the industry very well

2. As products and the industry have evolved some limitations of this approach have arisen

3. As these situations present themselves specifications should be enhanced with specific guidelines