*=srEChemicals

ART 208 1989 02/2

:HNICAL INFOMATION

F.RMATT.N TEKN$F*ED.T POIYETHYLENE,*,o*ro}3[i[H,::X??llH',?[r*o*

DIMENSIONING PRINCIPLES AND LAYINGINSTRUCTIONS FOR BURIEDPLASTIC PIPES

GENERAL

Plaslic pipes have specific properties and therebyotherdemandson dimensioning calculations and

laying instructions than lor example concrete and steel pipes. ln general the plastic pipes are

flexibte, that is they are deformed under load. This is positive in the rleaning that lhe pipe has the

abilily together with the back-{illing material to cause a horisontal earthpressure lowards lhe sidewall of the pipe, which increase the ability of the pipe to carry the load.

VERTICAL LOAD ON A BURIED PIPE

The important dimensions for a buried pipe are explained in Figure 1. The total vertical load, O, on

the buried pipe is the sum o{ 3 different loads.

O =Qs+Qr+O",MPa

O = Total vertical loadOs = Soil load, see Figure 2

Qr = Tralfic load, see Figure 3O,v = Water load, see Figure 4

The first step is to calculate lhe total vertical load, O, on the pipe. This can be done using the

diagrams in Figure 2 - 4.

Example 1

Consider a llexible pipe ol PEHD that is to be layed at a depth of 6.0 meter. The ground water level

is 4.0 meter below ground level. Determine the totalvertical load on lhe pipe.

Figure 2 gives:Figure 3 gives:Figure 4 gives:

That is:

Qs = 0.021 + 0.075 = 0.096 MPaOr= 0Q* = 0.020 MPa

Q = 0.096 + 0.020 = 0.1 16 MPa

Neste Chemicals lnt SA/NVl, Avenue de Bile/Bazellaan, I

B.I I40 BRUSTLSBelgiumTel. +32 22 44 42 llTelex 62270 NCC B

Neste Oy ChemicalsPr 320sF-06101 PoRVooFinlandTel. +358 !5 lB712Telex l72l neste sf

Neste Polyeten AB

5-444 86 STENUNGSUNDSwedenTei. +45 303 86 000Telex 2402 nestepe s

DEFORMATION OF A BURIED PIPE

The vertical deformation, 8", o{ a buried pipe can be calculated by the lollowing lormula:

,6v

-=Do 0.083

-aEs s+uue

Modulus ol elasticity o{ lhe pipe, MPa' see Figure 6

Wallthickness' mm

&,DoEs

= Vertical PiPe delormation' mm

= Original PiPe diameter, mm

= Total vertical load, MPa

= S.*nr todulus ol supporting soit' MPa' see Figure 5

Stiffnessfactor = T .

E" \ D /:t@

,S =

Ee

Figure 5 gives:Figure 6 gives:

LG

taved under *,u ,"rnr-.oniition, i, in e*"ttipiti' frtu-1*i"i" ol compaction is assumed to be

aiproximatety 80 % t*ni"n-.orr.sponds to roSit iiiri''gi' Deiermine the verticalpipe deformation'

6v.

Es = 1.25 MPaE = tgO MPa for Neste NCPE 2467'BL at 50 years

Now the stiflness laclor, S, can be calculated:

s: 2 .1so (:!\':o.oo563 1.2s \110 /

Finally the verlical pipe delormation, 6v, can be calculated' o = 0'1 16 MPa' the Same agin Example

1.

^oOv:-Eg

D 0.083 0.116..110.0.083

s+0.122 1.25.(0.0056+0122)= 6.6 mm

The pipe delormation ino/ool originally pipe diameter is then:

6.6= 60/o

'110

ThedeformationoflhepipeafterbackfillingisrecommendednottoexceedB%,

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NESTE

'w

STRAIN IN THE PIPE WALL

The strain in the pipe wall can be calculated by the lollowing lormulas:

8=6,(#'(+)is to be used when lhe stitlness lactor, s, is less or equal to 0.012:

e = Strain in the PiPe wall

e E Wallthickness, mm

D = OriginalPiPe diameter' mm

6, = VerticalPiPe deformalion, mm

r= 18 e) (?) (+)'(025's+ooo3)

istobeusedwhenthestiflnessfactor,s,isgreaterthan0.0l2:

Es=Secantmodulusofsupportingsoil,MPa,seeFigure5E = Modulus ol elasticity ol the pipe, MPa' see Figure 6

S = Stiflness factor, see Page 2

Allowed initial strain in the pipe wall is 1 .5 o/o lor HDPE and 2.0 % lor M DPE'

ALLOWED PRESSURE ON THE PIPE CONCERNING THE RISK OF BUCK'

LING

The pipe wall must have suflicienl stif{ness to prevent lhe pipe_lrom collapsing due to buckling' The

allowed pressure on a flexible pipe can be calculated by the lollowing lormula:

o": 4 '/-1'E\'" /:-\*vq- F \ 3 / \o/

= Allowed Pressure on the PiPe, MPa

= Factor ol salety

= Tangent moduius of supportinO soila MPa, see Figure 7

= Modilus of elasticity ol the pipe, MPa, see Figure 6

= Walllhickness, mm

= Originalpipe diameter' mm

,r'

OaFEr

EeD

Example 3consider a llexible pipe of NCPE 2467-8L that is to be layed at a depth ol 6'0 meter' The ground

waler level is 4.0 meter below ground level. The degree of compaction is.assumed to be

"jf io*iratu ty B0 %(correspondinf to loose {illing). Determine the lowest required value of the ratio

e/D and calculate the pipe deformltion 6u/D and the Strain in the pipe wall'

6v=

D4"

The required wartthict(ness is determined by the risk o{ buckring when the ground water level is at

it's highest Point'

Fisure 1s:"::: t;= i;i ilFi ",

Neste NCPE 2467'BL at 50 vears

Figure 6 gives:t-sl f= 2.0b"!'o.t't 6 MPa, the same as in Example 1 '

Formula 1 gives:

0116= + F5'1'g0-)'"' (J +

(tJ; (#)"=oo28 =28o/o

As a security due to handring or the pipe itis recommended that e/d should no be p'etow 0'03' This

means thar pipe crass ii ic-orresponding ,;5-;;';;; rigu" al iu to oL used in this example'

The pipe delormation is calculated as in Example 2

. = L. 190 (0.031)3 = o'oo3o

3 1.25

0.116 0.083 = 0.062 = 6.20/o

1.rs o.oo3o+o'122

Now linary the strain in the pipe wa, c11 be carcurated. since the stiflness ractor, s' is ress rhan

0.012 the lollowing t"t"'t'i" itio Ot used' see page 3'

t = 6'0.031 '0'062 = 0'0115 = 1'15%

This is below the allowed initial strain in the pipe wall'

NESTE

Figure 1

Ground level

D:UV:ft=

O:H=

Original pipe diameter, mm

Vertical piPe deformation, mm

Distance between the ground water level and

the pipe center, rn

Total vertical load, MPa

Height of fill above the PiPe, m

Ground water level

Figure 2

0.15

0.10

05, MPa

0.05

g

B

H,M

Ground water levelbelo-w the PiPe.

llr-v-r--1/t Ground rabove th

-/

ater levelpipe

I

I

I

Soil pressure vs height of backfilling

NESTE

0.04

Q1, MPa

0.03

Figure 3

Traffic load from 14 ton distributedon 2 wheels with a distance of 1.g m

14 ton

H,m

0.01

Traffic pressure vs height of backfilling.

With a height of backfilling above 3 m the contributionfrom traffic load to the totalvertical load on the pipe isnormally negligeabel.

1- 4tFjQ,I

i\iI

:

I

I

I\,

Figure 4

1 bulo-j g',-^"1 l"uol'Ground water pressure vs head of ground waterfor ground water level above the pipe.

NESTEChemicals

Figure 5E5, MPa

//

5:i,"J-ffi

-t'

--+-€---

012345Secant modulus of supporting soil vs height of backfillingfor non-cohesive filling material i.e' sand and grave!

r - -. Ground water levelat ground level

-

Ground water levelbelow pipe

67H,M

75 % Filling from ground levelwith shovel

80 % Corresponds loose filling and almost no compaction (dumping from truck)

85 oh Corresponds soft comPaction

90 % corresponds hard compaction (under areas with heavy trafficdone with a machine)

9

Flgure 6

MODULUS OF ELASTICITY, E, FOR DIFFERENTNESTE PIPE GRADES

NCPE 2467 210 MPa (50 year value)

NCPE 2467-BL 190 MPa (50 year value)

NCPE 2418 185 MPa (50 year value)

10

NESTEChemicals

Figure 7

E,, MPa

7

Jr-.'

I ^"."-

_L^"{:]_'o\o' Iqo'

Iq\Q I ^ro(/-t--- . ?(o:-lno*ff

6

H,m

Tangent modulus of supporting soil vs height of backfillingfor non-cohesive filling material i.e. sand and gravel

-

Ground water level at ground level

r orQ;ound water level below pipe

75 oh Filling from ground level with shovel

80 o/o Corresponds loose filling and almost no compaction(dumping from truck)

85 % Corr:asponds soft compaction

90 o/o Corresponds hard compaction (under areas with heavy traffic done with a machine)

11

RECOMMENDED DIMENSIONS FOR

Ref. Swedish Slandard SS 3403

Flgure 8

BURIED HDPE PIPES

ISO PIPE SERIESSwedsh designationOUTER DIAMETER

40

50

75

90

110/ -

160

20.0

250

2<tr

404

500

630

ln lhis Swedish Standard "Pipes and f itlings otbuildings" the minimum wall thickness is 3.0operation.

PE for buried sewers and discharge systems insidemm, due to handling of the pipe during the laying

s-16L-2

WALL

3.0

3.0

3.0

3.0

3.4

4.9

6.2

7.7

9.7

12.3

15.3

19.3

s-12.5T:4 ,

WALL

3.0

3.0

3.0

3.5

4.2 /6.2

7.7

9.6

12,1

15.3

19.1

24.1

s-10E-16

W.ALL

6.6

9.5

11.9

14.8

18.7

oe -,

29.6

37.2

12

NESTE

'-a

HDPE PIPES IN SANDYSOILRecommended pipe class under different

laying depths and traffic load

" Backfilling with rnacadam, 4-16 mmBackfillinng with excavations

lf no sand or macadam is available make the trench broader at least 2D on eachside of the pipe.

Height ofbackfilling

(m)

Open areasCarparks

Local StreetsSporadic heavy

traffic

Main roadslntensiv heavy

tratfic

1.0

2.0

3.0

4.0

5.0

6.0

s12. s12.5

s-16

s12.5

s10

s12.5

s10 s10 s10

13

HDPE PIPES IN CTAYSOILRecommended pipe ctass under different

laying depths and traffic load

'!

' Backfiiling with macadam, 4-16 mmBackfilling with excavations*r Backfillingwithsandandgravel, 0_16mm "

lf no sand or macadam is avairabre make the trench broaderat least 2 D on each side of the pipe.

Height ofbackfilling

(m)

Open areasCarparks

Local streetssporadic heavy

traffic

Main roadslntensive heavy

traffic

1.0

2.0

3.0

4.0

5.0

6.0

..1tr,

l

s10

sl0"ll.*s1o I I sro

\-'- \ -,- \ -'-

t4

NESTE

HDPE PIPES IN LOOSE CLAYSOILRecommended pipe class under different

laying depths and traffic load

Height ofbackfilling

(m)

Open areascarparks

Localstreetssporadic heavy

traffic

Main roadslntensiv heavy

traffic

1.0

2.0

3.0

4.0

5.0

6.0

s10

sl0 s10-- s10

' Backfilling with macadam,4-16 mm*. Backfilling with excavations"' Backfilling with sand an gravel, 0-16 mm

lf no sand or macadam is available make the trench broaderat least 2-D on each side of the pipe.

15

1.

2.

3.

4.

5.

6.

LAYING OF PE SEWAGE PIPES

Preparation of narrow trench'

Sand or gravel bedding

Firm bottom supporting

lnilial backlilling around sides

Protective backfilling. No hard compacling above the pipe

Final backfilling with compacting

Proper laying is lhe last step in making a reliable sewage system.

lf no sand or gravel is available as backfilling material make the trench broader at least 2 Don each side of the pipe

i6