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Thin-walled thermoplastic pipes
Thin-Wailed Thermoplastic Pipes J J J~irvenkyl~i
Uponor Innovation, Nastola, Finland.
Abstract
Stressed skin and rib.reinforced shell structures optimize plastics pipe performance. Computation of engineered pipes is easy as they can be substituted for by orthotropic pipes having the same rigidity characteristics. The flexible pipe theory lends itself very well to the concept of thin walled plastics pipes predicting a l O0-year design life. Practical experiences are studied as well as other dimensioning aspects, e.g. buckling, impact resistance and minimum wall thickness. Combinations of various construction principles and materiaLs have different performance characteristics making market segmentation optimal
Construction prindples of engineered pipes The development of engineered pipes began in Germany in the sixties when alternative methods for producing big bore PVC pipes were studied and a spiral wound pipe was developed at the late sixties. The so called oil crisis in the seventies gave start for a vivid activity in this felcl as the costs for plastics raw materials were feared to increase more rapidly than the substitute materials, i.e. clay, concrete and asbestos. The principle of saving raw material is mainly based on the I-beam principle but attempts have been made, mainly in the U.S. to increase the Young's modulus by very high rifler contents. The latter way is questionable because these 'polymer concrete' pipes cannot economically be made so stiffthat they would work like rigid pipes. This leads to the fact that strain is bound to exist in the pipe wall and this causes some doubt for the long term durability. ]t must be noted that the strain based dimensioning philosophy works only with pressure pipes made of high quality plastics compounds with moderate filler contents. The figure 1 shows the basic equations concerning the reaction forces that the pipe develops during deformation. This is true even for installed pipes as well, with the additional forces that comes from the soil reaction. It can be seen that the force is a linear function of deformation ( ) and system stiffness (ST). This means that every time an incrementing deformation takes place it needs a stronger force. In other words the strength of the system can be increased either by increasing the stiffness or by allowing greater deformation. This fact tells us why the most economic way for the society to get better pipelines would be to increase the allowed deformation to a more reasonable level. As the deformation is a direct function of geometries and strain, the equation tells the basic correlation between rigid structures and flexible pipes. The strength is the same regardless of whether it has been achieved by high stiffness (concrete) and low allowable strain (~), or by high deformation and low stiffness. Thus it can be concluded that the best way of increasing the stiffness is to increase the moment of inertia (1) as much as possible by increasing the profile height (e). The limit here comes from local buckling and from maximum allowed strain (~) at maximum allowed deformation.
Good tools for evaluating the performance of engineered pipes are: - piping efficiency defined as flow era divided by pipe weight, which tells the customer 'the price of the bore' - reinforcing efficiency defined as moment of inertia divided by the second power of average crossectional area, which tells how effectively the stiffness is gained. When comparing the results for engineered pipes and conventional ones, one immediately sees that the stressed skin and rib reinforced shell structures show the modern path to performance optimization in plastic pipe engineering. Computation of pipes made of corrugated or ribreinforced shells becomes easy when they can be substituted for by orthotropic pipes having the same rigidity characteristics. So is the case directly on macro level, i.e for designing pipelines.
m D~etlec~10n Sir( (:
m
mm
Fig 1 Basic equations for deflected pipe
Flexible p i p e t h e o r y Many researchers have recognised that the load on newly buried structures is not as great as it will be after substantial time has elapsed. That is, the load or pressure increases with time until the equilibrium load is reached. This gradual increase in load is accompanied by seasonal
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 D E C E M B E R 1989
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physically that the backfilling material will always settle. What amazes in the Table 1 is the variety of material One of the earlier (and still valid!) methods of good design constants that are in use for the same product and same was to adjust the rate of pipe creep to be about the same material. Especially the values proposed by WRc seem to as for surrounding soil and hence safeguard the correct be too much on the conservative side despite of the fact function(23L The other extreme will be seen in road that they are as such correct and measured values. What building techniques where the deformation is completely is to be remembered is that the deformation and buckling controlled by allowable settlement in road level. A plastic formulas have been empirically derived from much pipe installed by that quality of compacting would not higher values and these short term measured and long term extrapolated values are misleading in this context. A deform! The current understanding of the role of stiffness is that reliable short term value, even if conservative, may be the rate of creep in the stiffness controls the rate at which established by the use of ISO short term dynamic ring the pipe-soil system reaches its equilibrium. Hence the stiffness testing and by deriving the respective E-modulus correct method of classifying flexible pipes is based on from that. Similarly the long term value will be established short term values. The actual stiffness of the pipe is in from the 1000 hours creep testing and extrapolation to 2 most cases only needed because of the compacting years stiffness. This value corresponds closely to the other needed for building up the passive modulus. If the pipe is 50 years moduli given in Table 1. too soft there will be the risk for negative initial deformation It must be understood, when knowing how the soil-pipe which should be avoided as it calls for rectangular interaction works, that measured and then extrapolated ovalization. In other words the flexible plastic pipe works 50a E-modulus values really lack physical meaning in this only as a chemically inert liner in the newly built hole! This context. The deformation process is a sequential creep fact is well proven by Kay et al.(241by a series of utilising and relaxation phenomena with the result that after a few rubber hoses. The arching capability of the soil gives the years the pipes are practically not loaded. possibility for extremely high fillsto be supported with little deformation if the lining is sufficiently flexible both in Deflection calculations for engineered pipes bending and axial compression. It has been shown as well Designing the installation techniques to meet the requirethat especially corrugated and ribbed pipes with rather low ments for maximum allowed deformations is as straightstiffness withstand greater loading than could be predicted forward for engineered pipes as for conventional pipes. from the deformation formulas(25,26LThis may be due to An engineered pipe of certain stiffness class behaves with good compacting over the pipe crown together with load- good enough accuracy as a conventional solid wall shedding softer sidefull and the fact that compacted filling pipe(29.30L material between the ribs adds to the buckling strength. This preferred practice is to follow tabulated max. A striking example of the unnecessity of getting covering depths. The only difference is that for ribconfused by the creep is served by Bishop0~ in a test reinforced pipes the nominal size, NS, gives a closer series that was conducted in embankment conditions for approximation than the outside diameter when calculating both PVC pipes and steel pipes of same stiffness. The the loading and needed trench width. This is a nice feature several years long test showed that the creep of PVC does as the inside diameter is pretty near the next standard not have any effect on the deformation which is controlled diameter downwards, which makes it possible to follow by the short term stiffness. This verifies the correctness of both the preferred outside diameter range and the the ISO draft for lightweight pipes (2a) where the classi- nominal size range making standardization easy. For all fication is based on short term measurement preferably other types the representative figure is the real outside by a dynamic method (27>. Some discussion has been diameter. raised concerning the testing time and the degree of Designing the pipe to meet the required minimum deflection. The values used for short and long term stiffness per pipe class is trickier as straightforward properties are shown in Table 1. calculation does not apply. The higher profile height calls
Table L Material data used for structural design Material
192
Unit
Long
Short term
term
modulus
modulus
PVC PVC PVC PVC PVC
N/mm N/mm N/mm N/mm N/mm
3000 3000 3600 3000 2800
1000 2000 1750 1500 500
HDPE HDPE HDPE HDPE
N/mm N/mm N/mm N/mm
1000 900 1000 800
100 200 150 150
Short term bending stress
Long term bending stress
Legend
90
10 50
(12) (44) (1 O) (19) (15)
35
5 13.5
(12) (44) (10) (1 9)
CONSTRUCTION & BUILDING MATERIALS VoI. 3 No. 4 DECEMBER 1989
perturbations due to rain, snow etc. The increase in load takes place because the distrurbed soil moves towards equilibrium with the surrounding insitu soil and with the buried structure (i). Extensive long term studies on the deformation behaviour of conventional solid wall PVCu pipes <2,3,4~ show indeed that after initial backfilling the deflection of the pipes will stabilize and often follows a Aformed curve as a function of time. This is typical to all flexible pipes installed underground. The effective strength of the pipe-soil system is remarkably high. For example Moser et A1/5~have shown that a rigid pipe will fail under such soil conditions and loading, while SDR 35 PVCu pipe only deflects 5% and maintains its performance. This load-shedding property of plastic pipes is enhanced by creep as the apparant modulus reduces at a greater rate than that of the soil. The absence of sudden failures under load is typical to plastic pipelines. AS the load is increased the wall stresses are redistributed due to occurring yield properties of plastics. Load can thus be increased over that at initial yield point before complete collapse occurs. The plastic material will hence theoretically take a load 50% in excess of the failure load of a rigid pipe ~6~.As can be seen for plastic pipes creep is not a liability but an asset for imposed deformations <5~.
Flexible Pipe Calculation Theories The aim of structural design of pipelines is to create a stable interaction between the pipes and the surrounding soil, in which the pipe shape remains approximately circular. What limit for that allowed ovality is safe? When the Scandinavian limits for the long term deformation of the pipe were set~% the basis was the tightness of the joints. The limit was based on the knowledge that if the initial deformation is below 8% then the long term deformation will be below 15% if the prescribed installation techniques were followed. This knowledge was based on extensive field tests with SDR 34 and 41 pipes with cohesive and friction soils used even as dug backfilling materials. Ten years long l a b o r a t o ~ 8~tests backed up the decision as they showed that high quality PVC is able to sustain very high strains without rupture. Because the joints were tested according to the standard with a 15% deformation such a performance limit would not create problems. AS the reduction in internal cross sectional area from circular to elliptical shape will be only 3.15% at 15% deformation this was regarded as a minor problem. Because buckling as the reason for collapse was unknown for PVC pipes with less than 25% of deformation the limits could be safely set. Hence the 15% max. deformation (at any point) is rather safe and well proven limit. In a recent analysis c9)made by FEM techniques and using very conservative values for initial modulus the conclusion was that in all circumstances, an initial relative compression of a vertical pipe diameter of less than 6% corresponded to a safe 50-year design. In Germany the deformation limit is set to 6% by the rather complicated ATV code of practice ~°~ although plastic industry backed up with sound proof struggles to increase the limitations on a more realistic ieveF TM. In Switzerland c~2~and Austria ~3~ the same limitation is valid. Although the Austrian code of practice is quite refined it has a very important benefit which should be considered when the ISO rules are settled; it is namely available as a
PC-program as well! The practice in the USA is almost as conservative with 7.5% of the long term iimitV4~, in the newly published WRc design guide 15~ the long term deformation is limited to 6% as well. it seems that these national regulations will follow with considerable lag to the ISO code of practice, compiled by leading geotechnical experts, which r e c o m m e n d s 12.5T as the upper limit for long term deformation. Today for example in Finland the market share of plastic sewer pipes is over 78% and no noteworthy failures have occurred. The reason for the good success of underground plastic sewer pipes in Scandinavia might be that the code of practice for structural design and installation is kept simplelle.~71. The viability of the pipes is shown tabulated with soil type and filling height as parameters. Thus the overengineering with problems is avoided. Now that underground PVC pipes have been in use for more than 50 years and monitored installations date back more than 20 years the time is ripe to forget the 50-year design base and speak about more realistic design life. The needed extrapolation from 50a (or 20a) to 100a is indeed not a quantum leap and it is very well motivated. In a very important report Janssonl~8~ gives full proof to justify this by analyzing practical installations and by material technical investigations. The Norwegian conference on this theme was as reassuring ~9~. Engineered sewer pipes take advantage of the most m o d e m installation techniques and they lend themselves well to the flexible pipe theory. As the quality control for accepted designs is more rigorous than before we can conclude that they will for sure show a life time of 100 years and over. W h a t s t i f f n e s s is needed? All the existing more or less detailed design principles and manuals rely on p h e n o m e n a well described for by example Spangler, those theories are still valid with sufficient accuracy~2°t. It must be emphasized that these theories are empirical and thus the coefficients have been calculated afterwards to match the known material values. A g o o d example of this is the teething problems created by the Young's modulus to be used in the equations. Now that the theory has been compiled using supposed E-values it will for sure give false results if new, actually measured values are to be put in it. Originally the equation worked with calculated E*[(21). Having in mind the inevitable problems in calculation of the product E*I for engineered pipes it is clear that the real measured stiffness must be used in the calculations as proposed by Prevost ¢22~. As the measured results are lower than the theoretical values originally used and created by different test methods, the chosen test method should give test results that are meaningful and subsequently are in harmony with proven practice. The stiffness of the pipe itself is in the region of one percent of the system stiffness. By inserting varying Emodulus values in the deformation equations one can immediately see how small is the difference in outcome. This fact can be seen even in practical long term deformation measurements. Why is stiffness then needed at all? The question is justified when analysing what the different equations give for a pipe stiffness zero. Typically even in that case the deformation is in the region of a few percent. This means
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
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for lower modulus to be used and the geometry causes that Poisson's ratio is less than normally. It has been proposed to be zeroC3~L Buckling
of engineered
e=2xMo* (~ * X ~ J R m where e = strain
(~ Zmax Rm
/
f
f
5
'iME h
100
1000
Fig 2 Sensible limits for deformation at any point
STRAIN
)~=15% DEFORMATION
/
4
I
/
1
0,850.5~
/ ~
I
I-I I I
5
50 101
6
8
25 2016,7 12.5 51 /.13/*,/~ 26
10 10 9 21
8
16PN 6,25S 13,5de/e RELATIVE WALLTHICKNESS
Fig 3 Strain as function of SDR and deformation
= co-efficient; 1.5 for elliptically deformed pipes in soil, 2.14 for parallel plate method, 3.012 normal m a x i m u m and 4.28 for rectangular
deformation = deflection in percentage = distance from neutral axis to point of interest = pipe radius to neutral axis
Figure 3 gives an idea of the magnitude of strain at the outer pipe wall for different deformations. It can be seen that for typical engineered pipe with profile height taken from ISO wall thickness series $9 the maximum strain is
194
% 15
pipes
It has been stated that in normal practice for conventional SDR<5| solid wall pipes the buckling of the pipeline is hardly ever an existing problem. The reason for this is that the deformation at which the pipe might become unstable exceeds 25%. The main part of known failures concern pressure sewage pipelines. However, some aspects of the practice call for attention in this sense: - The dimensioning against pipe buckling shall have a reasonable safety margin. For example i n Scandinavia where a long term deformation of 15% is allowed the margin to range where buckling could occur is in the magnitude of two. In the Swedish functional standard for stormwater pipes and culverts (3~,which covers all types of engineered pipes as well, the requirement for deformation without buckling is 30% if normal plastic pipe installation technique is to be used. - Buckling analysis can be made on stress basis or strain basis. Remembering that uneven loading or stiff soil can cause much tighter radius than is the case for elliptical deformation leads automatically to much higher strains than predicted. - Flaws in the material or irregularities (typically on inside) cause high stress concentrations which together with for example local point point loading can cause faults similar to pure buckling. A bdttle fracture at the crown of the pipe has been one of the most common failure modes133,341. - All too flexible pipes, ring stiffness St below 2.5 kN/m 2, tend to fail by buckling load especially in stiff soil. - Traffic loads can cause fatigue and hence premature buckling. Considerable discussion is going on whether the buckling is critical or not ~2°35~with new dimensioning methods as o u t c o m e (9,36,37). This is even m o r e so in the case of engineered pipes where by nature the strains and s o m e t i m e s the stresses are bound to be on a higher level. The construction principle add as well new buckling m o d e s that m u s t be regarded before the pipe is constructed, and tested before the product is sold. The leading idea is that the specifying engineer should not be obliged to do it as, for a well dimensioned product, this should be unnecessary as far as normal sewage conditions prevail. The strain in the pipe wall is a function of profile height and deformation and can be expressed as follows:
Mo
PIPE DEFORMATION
on a totally different level than the previously reported critical strain for PVC, 0.85%. The fact that these high strains can be allowed, already from beginning, is due to relaxation. This theory was developed during the development time for ribbed pipes and is well proven by long term testsOSL The work done by Moser gives similar indication<391. These high strains cause immediate buckling in t h e different pipe sections if they are not adequately dimensioned as is the case for m a n y earlier engineered pipes.
C O N S T R U C T I O N & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
Especially double-walled (DWP) pipes are prone to outer corrugation buckling. Because the outmost end of the ribs can yield without buckling, ribbed constructions are safer in this sense. As the waterway is only a little constrained in solid wall ribbed pipes this calls for even better safety against buckling than for conventional thickwalled pipes. This effect is enhanced by the fact that the ribs stay cool despite of warm foul water. This is easily proved by Box Loading Test, in which ribbed pipes typically reach half the deformation that is normal for equivalent thickwalled counterparts. The calculation of buckling risk of engineered pipes is done by normal manner using the basic formula: ac,=r/xk*
~/2" E * ( t / b ) 2
where ~ = plasticity correction factor k = buckling co-effident depending upon boundary conditions of the buckling element t = thickness of the buckling element b = width of the buckling element The use of this basic equation implies two difficulties. The exact Young's modulus is seldom known and the buckling co-efficients given in various sources tend to give not so accurate results. The Jansson strain theory~1 lends itself, however, quite well to these calculations. For the purpose of designing engineered pipes a new optimization program was developed which works with an iterative method based on the basic finding that if the from the
~
~
E
N
E
beginning allowed max strain or stress is exceeded the program recalculates a lower modulus and then the deformation and strain again until acceptable equilibrium is reached. The design method was verified with real pipes in bucking tests according to the ISO document and results were encouraging; buckling did not occur at all even with very high deformations up to 80%. The buckling criteria that the force needed to deflect the pipe fails to grow or the sample looses its elliptical shape occurred simultaneously when testing conventional DWP pipes. These showed a clear buckling at the inside and the corrugation already at a deformation between 15 and 25%. Regarding standardization of engineered pipes this buckling test is very important as the mathematical calculation methods cannot be standardized, if a even more stringent buckling test would be needed for very low stiffness pipes the idea of Prevost ~ seems worthwhile. A test could be conducted in the same manner, with stiff side plates, as shown in Figure 5 which clarifies the difference in deformation when the pipe is properly supported123~. Tests made with this principle show that for both conventional solid wall pipes and for ribbed solid wall pipes the force needed to press the pipe to maximum allowed long term deformation (15%) is roughly three times as high as in normal buckling test where the pipe is free to deform. No buckling was noticed in the modified test either. It can be mathematically shown that ring buckling
R FENER
NEU[lU~.AX~S / WHENBENT j BUCKLINGOF WATERWAYBETWEENFLANGES
J
BUCKLINGOFTHE FLANGES
Fig 4
RING
!
" ~
BUCKLING
-'lllllllllllllllllllll,
DUCKLINGOF THE FLANGES
-rm |
|
-
BUEKLINGOFTHE FLANEES
Typical ways of buckling (the sandwich modes aft. 45)
CONSTRUCTION & BUILDING MATERIALSVol. 3 No. 4 DECEMBER 1989
195
~J
lq ZV=t DIST=6.[6 ¥rmOK
/
///////////////..
q / Drn}3 6.~22~ T I T / Fig 5
.o.o rq ( Om) 3
Deformation of pipe with and without sidesupport
always occurs at a given compressive deformation which is virtually independent of pipe wall geometry. Hence engineered pipes do not differ in their calculation methods from conventional pipes. This is true on the condition that pipe walls have been correctly dimensioned so that local buckling phenomena do not occur at lower deformation than ring buckling. So is the case for qualified engineered sewer pipes.
=_
Fig 6
Pressure resistance
Generally the engineered pipes for non pressure use show a lower internal pressure resistance than their thickwailed counterparts. However, if the stiffening principle makes sense, some interesting features are seen: - If the pipe is produced by winding processes some orientation takes place and the bursting pressures are somewhat higher than predicted. The same is of course valid as well if the single corrugated pipe is produced with a blow ratio greater than one. - Ifthe pressure test is carried out with an internal pressure equivalent to that of a plain walled pipe with same internal diameter and the same mass it can be seen that the stiffeners are contributing to the bursting time A good principle for non pressure pipes is that the wall thickness is enough to guarantee at least a nominal pressure of PN 1.
Impact resistance When testing an engineered plastic sewer pipe one should be aware that the ISO TIR test is not directly applicable to all constructions. This is due to the small test tup in use which can fall either on the corrugations or between the ribs. Hence the group to be analysed by the method lacks uniformity and the mathematics don't apply. In fact this is not needed either because this test is developed for monitoring the excellence of the processing - not the pipe, and for this purpose more effective methods exist. However, the impact resistance is an important property in many applications and that is why ISO proposes a modified ASTM staircase method for the purpose. This test is to be regarded more applicable with the 500 m m tup as well as it definitely simulates the real performance better than other alternatives. Generally impact strength of a product is a function off wall thickness. Hence it would be logical that engineered
196
I--
FEM analysis of a breakaway ultra.rib
pipes show inferior values to those of conventional pipes. However some parameters disturb the situation: - possible orientation increases heavily the impact strength - in solid wall constructions the profile height function as wall thickness, the real wall thickness plays only a minor role - pipe stiffness decreases the impact strength when testing as the pipe cannot flex as easily. Anyhow, a thin-walled stiff product should not be able to reach the same performance as the solid wall pipes do. This seems to be true for double-walled pipes made of rigid plastics. The construction behaves as an eggshell if the wall thicknesses are low. Tests made with solid wall pipes show two to three fold performance in this respect with very little loss compared to thickwalled pipes. Same kind of results have been reported for small diameter ribbed pipes, too (311. A very useful feature of solid wall ribbed pipes is the clever utilization of the notch sensitivity of PVC. If some ribs break off when handling, as the often do, a standard rib breaks in such m o d e that the crack will go through the pipe wall destroying it. The ultra-rib (fig 6) has a console in the root which safeguards that it will only knock off with maintained performance, stiffness and tightness, as result. Minimum wall thickness
Where conventional solid wall pipes were classified this was done merely by stiffness grounds. The pipe that fulfilled these requirements was sufficient on other grounds as well, i.e. the abrasion resistance, the impact strength, handling properties, buckling properties etc.
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
'¢~'" '~'" I
I
I.o
I
I
v
Effect of settlernent on rigid and flexible p ~ e in filL In the photograph at the lel~ pertaining to rigid p~e. S represents the amount of settlement of fill by consolidation, in the photograph at the righ~ pertaining to flexible p~e, D [ ~ in Eq (8.3)], represents the deflection of the p~oe as it yields under loacL were good enough for the purpose and hence there are many aspects of design that are 'forgotten' in the matching standards. When the required ring stiffness is derived by other means than wall thickness and hence the wall thickness can be chosen, there are many properties that still will be touched, for example: abrasion resistance - resistance against wear caused by mechanical rodding - resistance against rubber ring denting - longitudinal stiffness (handling, uneven settlement) - shear strength (uneven settlement, joints to rigid structures) -
Fig 8
- axial tensile strength (thermal elongation, uneven settlement) internal pressure resistance (steep grades, water jet cleaning) resistance against warm fou] water - impact strength The task to develop a criteria and a testing method for all of these aspects separately was regarded as too ambitious in the ISO task group for light weight pipes even if the goal to establish a rnatedal independent fully functional performance standard is a lucrative one. It was decided that setting a m i n i m u m wall thickness which is -
-
Flexible p~es redistribute loads with relaxation as result
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
197
.... i
l¸
;,:, o
h,o
¢, o
200 o
4o~ ,:,
~(,,3 ,, ,, 3
INNER DIaMEIER i:.,mJ
U L : r ~ - RIB (SDR 34} •
~H
I STC,~M WATER DWP
CONyEH[IOIIAI ~CIR J4
[
Weights of class T (SDR 34) pipes.
~oo o
400 c
~ ~ ,~
iNnER DIAt~EIER , m , T + ".ONvENllONAL SDR 54 ~ CON,Lt~',,;)n~l ![,p 41 ,~ EONYENTICNAL SDR 51 ~ ASSOLLITE i~lN I~ALL
, k r H : , [;i H f S : ~ 341
Fig 10 Sewer pipe wall thickness.
3SA
Engineered pipes can be substituted for, by orthotropic pipes of identical rigidity. Combinations of various principles and materiaLs impart different performance characteristics. calculations are made. Even when this layer has wared off the pipe shall fulfil other performance requirements. Tests made with mechanical rodding devices(40) show Abrasion resistance and wear that tears up to 0.5 are possible. On the basis of these facts Numerous different test methods seem to give quite the Swedish independent Council for Quality Control of consistent results. The problem lies more on the difficulty Plastic Sewer Pipes (KP-Radet) has given the minimum to define how much sand will pass through a real pipeline acceptable wall thicknesses for plastic pipes(41L T h e s e during its design life of 100 years. S o m e studies show that wall thicknesses can be calculated from equation no measurable wear has been noticed in 10 to 20 year old Emin=0.005* Di+0.5mm. PVC pipelines. The estimated wear during 50 years usage will be about 0.25mm. Thus an optimized pipe should Dents c a u s e d by rubber ring pressure have at least 0.5 m m thick abrasion layer when new and Depending on the construction of the pipe these forces this layer should not be taken into account when stability can be focused directly on the waterway wall. In this case based on c o m m o n sense and experience will make unnecessary testing obsolete.
198
CONSTRUCTION & B U I L D I N G MATERIALS VoL 3 No. 4 D E C E M B E R 1989
the time for subjective and objective valuations easily come into picture. If the pipes are to be ranked, the following must be evaluated: customer needs in different market segments easiness of production and quality control how do different construction/material combinations perform in various dimension ranges. These variables have been discussed earlier ~421in the sight of PVC sewer pipes, in the size range which is fitting Longitudinal stiffness and tensile strength Axial tensile and compression forces as well as shear intensive all wound pipes loose attraction because of jointing difficulties. The solid wall concentric fib-reinforced occur in sewer pipes in the following circumstances: PLC sewer pipe offer many advantages over the conwhen the pipes are lifted when pipes are tramped on during installation and they ventional pipes in the size range 110 to 1000 mm. The concept seems to be interesting even having telecomlay upon uneven bedding deliberately made Iongtitudinal bending in order to munication pipes in mind. When regarding medium range 200-600 mm ID storm make changes in alignment to avoid obstructions differential settlement of a manhole or rigid structure to water pipes or road culverts, where silt tightness is good enough and high robustness is required the HDPE-pipes which the pipe is rigidly connected seem(431to be a good option if medium stiffness 3-6 kN/m 2 is uneven settlement of the pipe bedding - ground movement associated with tidal or ground water enough (as it usually is). If impact strength is not of primary importance the double wall PVC pipes offer a cost conditions effective alternative. erosion of bedding due to pipeline leakage For big bore stormwater (ID>600) pipes the closed seasonal variation in soil conditions due to changes in profile PVC pipe seems to be a good choice as the wall moisture content (limited to expansive or organic soil) seasonal height variation of pipeline due to the lifting thicknesses are large enough to tolerate handling. In these effects of frost (limited to certain expansive or organic soil sizes the low stiffness, profiled or ribbed, pipes are competitive as well. However, the minimum stiffness types in cold climates) - improper installation procedures, e.g. non uniform should preferably exceed 20 psi. foundation, unstable bedding, inadequate embedment, References consolidation 1. Bishop, R R, "13me dependent performance of buried I~C pipes', These forces can lead to the collapse of pipeline, International Conference on Underground Rastid Pipe, New buckling of the wall structure, exceeding of the allowed Orleans 1981 tensile/compressive - stresses and strains as well as 2. Jfirvenk'ylfi, J J, 'Muoviset maaviemEriputket-tutkittua tekniikkaa'. Rakennustaito 14,1980 shearing problems. 3. Vervomingsmetingen aan operationale PVC-straatrioleringen. The optimum axial stiffness of a pipe can be described Stichting KOMO, Rijswijk 1982 with border values: 4. Joekes D et al, 'Deflection of PVC sewer pipes and a new method - The pipe shall have a certain minimum stiffness in order for measuring and specifying stiffness of plastic pipes'. 6. Int. Conf. to Plastic Pipes, 1985 5. Moser AP et al, 'Design and performance of pvc pipes subjedcted - the pipes can be lifted without getting too much to external soil pressure;, Utah State University 1977 bent 6. Stephenson D, 'Flexible pipe theory applied to Thin-Wall PVC the pipes can be installed to the wished grade easily Piping', Municipal Engineer, March/April 1980 as they stay straight 7. 'NUVG-Report', Stockholm 1980 - The pipe should not be too stiff because: 8. Koski, L M, 'Stress Relaxation and Deflection Recovery in Plastics Pipes over a Period of 10 years.' 1st Int. Conf. from Materials - the load-shedding property of plastic pipes call for Science to Construction Materials Engineering. RILEM. Paris. ring and axial flexibility 1987 the installation becomes easier if the pipes can be 9. Hjelmquist E. et al, 'Long-Term Deformation and Failure of Buried longitudinally bent at site Plastic Pipes'. Journal of Engineering Mechanics, Vol 113, No 7, - transport of long pipes becomes easier ASCE 1987 10. Richtlinie fur die statische berechnung von Entw~sserungskan~len - the pipe must be able to follow uneven settlements of und -leitungen. Abwassertechnische Vereinigung e.V.Arbeitsblatt the bedding (no flexible pipe is stiff enough to bridge a 127 dezember 1984 over uneven spots) 11. Nowack R, 'Statische Berechnung von erdverlegten Entw~sserungsthe proposed minimum wall seems hazardous especially for countries where root penetration is a severe problem and for soft pipe materials type HDPE. Because of the creep in plastics materials there should exist a long term tightness test and this is incorporated in the ISO draft in the form of Box Loading Test. This test has proven its value during the development time for engineered pipes many times in other aspects as well.
-
-
-
-
-
-
-
-
-
-
-
-
-
Without going deeply into mathematical analysis it can be said that a pipe with a wall thickness of roughly E = 0 . 0 0 5 * d i + l m m fulfils all functional requirements well. The main contributing fact is that most plastic materials relax soon and hence the stresses will not get excessively large.
12. 13. 14. 15.
S u m m a r y
The amount of different designs is infinite. So far some 25 different constructions have been tried. All these variants are modifications to six basic construction principles(42). When considering which option of the pipes is the 'best',
16. 17.
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
kan~len und -leitungen aus F'VChart und PE hart', 3R International, 24 Jahrang, Heft 8/1985 Heirerki W, 'Die statische Berechnung erdvedegter Kanalisationsrohre', 3R International. 21 Jahgang, heft 1/2, 1982 Statische berechnung erdverlegter rohrleitungen im Siedlungsund Industriewasserbau. Draft for OENORM B 5012. (Dec. 86) Handbook of PVC pipe. Uni-Bell PVC Pipe Association, 1986, p.183 Water Authorities Association Sewers and Water Mains Committee: 'Guide to the water industry from the structural design of underground non-pressure uPVC pipelines', WRc External Report ER 201E, October 1986 SPF teknisk rapport 01, Sveriges Piastf6rbund 1988 Molin J, 'Dimensioneringsnomogram f6r markavloppsr6r av styv PVC och PE', VBB Malm6 1986
199
18. Jansson L-E, 'Hur gammalt kan ett plastrSr bli!', KP-Radet 1987.27 litI. ref (english translation available soon) 19. Meland T, 'Loeser vi probleme med plastroer?', Cinference paper 9-11 januari 1984, Norges tekniske hoegskole 20. Prevost RC et al, 'Design of non-pressure very flexible plastic pipe', 'Pipes & Pipelines international. November-december 1985, pl 217. January-February 1986 p 19-20 21. Howard, A K, 'The usbr equation predicting flexible pipe deflection', Int. Conf. on Underground Plastic Pipe, New Orleans 1981 22. Prevost, R C, 'Design of non-pressure, very flexible buried pipe;, Pipes & Pipelines International. May-June 1983. p7-10 23. lmhotf W, et al, 'Abwasser-KanaFle aus Hartpoly~thylen. Teil 1: Erdaut]ast und WAnddickenberechnung', Kunststoff'e 57. Jahrgang 1967. Heft 1 24. Kay,J Net al, 'Compressive conduits under deep fill loads-a-model study', The University of Adelaide. Report no. r58, October 1983 25. Kay, J N, 'Minimum acceptable soil cover heights for ribbed, spirally-wound pvc drainage and sewer pipe installed in a trench and subject to severe vehicle loading'. The University of Adelaide, February 1984 26. Stigberg R, 'MarkfSrlagda plastr6r typ DSA, belastningsf6rsSk', Statens Viigverk, V~stra byggnadsdistriktet, 1982 27. ISO draft proposal 9969 28. ISO draft proposal: light weight underground PVC sewer pipes 29. Tidd, A et al, 'Evaluation of the structural performance of Ultra-PJb uPVC pipe when used on conjunction with conventional and alternative bedding and sidefill materials', WRc External Report ER 277E, September 1987 30. Tammirinne M, et al, 'Deformation measurements on ribbed uPVC sewer pipes embedded in soil', Technical Research Centre of Finland, Geotechnical Laboratory, 1986 31. Ragab A-R et al, 'Evaluation of the Mechanical Behaviour of Plain and Spirally Stiffened Polyvinyl Chloride Pipes', Journal of Testing and Evaluation, JTEVA, Vol 13, No 2, 1985
32. 'Materialneutral speciflkation av r6r och r6rdelar av plast avsedda art anv~ndas som anl~ggningsr6r', SPR Verksnorm 500 utgava 1,1988 33. Marshall, (3P et al, 'The analysis of practical failures of plastic pipelines'. The 3rd international plastic pipes symposium, Southampton 1974 34. Marshall, (3 P, 'Guidelines on the use of Polyethylene Pipe for use in Sewer Applications', Int. Conf. on the Planning, Construction, Maintenance & Operation of Sewerage Systems, University of Reading, September 1984 35. Howard, A, 'Diametral elongation of buried flexible pipe'. Int. Conf. on Underground Plastic Pipe. p191-201, New Orleans 1981 36. O'Reilly, M Pet al, 'Loading tests on buried plastics pipes to validate a new design method', Transport and Road Research Laboratory, UK 1982 37. Prevost, RC et al, 'Instability of buried flexible pipe - Part 1', Pipes & Pipelines International, May-June 1988 38. Jansson L-E, 'The relative strain as a design criterion for buried uPVC gravity sewer pipes', Proc. Int. Conf. Advances in Underground Pipeline Engineering, ASCE/madison 1985 39. Moser A P, 'Strain as a Design Basis for PVC Pipe?', Int. Conf. on Underground Plastic Pipe, New Orleans, 1981 40. Molin J, 'Report on underground plastic service sewage pipes', VBB Malm5 1976 41. Jansson L-E, 'Vad I~r man kr~va av ett l~ttviktsr6r av plast?', Norsk Ws/Energi och lnnerklima, vol 30, no 9, 1987, 9 litt.ref. 42. J~irvenkyl~ J J, 'Rib-Reinforced PVC pipes for gravity sewers', Pipes and Pipelines International, January-February 1987 43. J~irvenky[~ J J, 'Plastics pipes-prelude to improved road technology', Road and traffic magazine, Helsinki 1985 44. Lauer Hi, 'Stress analysis of buried rigid PVC and rigid PE pipe and sewers', 3R International, Heft 2/1978 45. Caprino G, et al, 'Sandwich for structural applications', Macplas International, 1987, p. 137-141
B O N D I N G A N D R E P A I R OF C O M P O S I T E S Proceedings of a one day seminar organised jointly by Butterworth Scientific Ltd and Rapra Technology Ltd 14 July 1989, NEC, Birmingham The ever-increasing use of polymer based composite materials and their joining with adhesive materials or otherwise have led to a greater interest in the lifetimes and the applications of these materials in safety-critical areas. There are many problems associated with the bonding and the repair of polymer resin-based composites and the interest surrounding this topic was a major reason for this seminar. 1989
88 pages approx 297 x 210mm Softcover 400 illustrations £35.00
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200
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
Uponor Innovation, Nastola, Finland.
Abstract
Stressed skin and rib.reinforced shell structures optimize plastics pipe performance. Computation of engineered pipes is easy as they can be substituted for by orthotropic pipes having the same rigidity characteristics. The flexible pipe theory lends itself very well to the concept of thin walled plastics pipes predicting a l O0-year design life. Practical experiences are studied as well as other dimensioning aspects, e.g. buckling, impact resistance and minimum wall thickness. Combinations of various construction principles and materiaLs have different performance characteristics making market segmentation optimal
Construction prindples of engineered pipes The development of engineered pipes began in Germany in the sixties when alternative methods for producing big bore PVC pipes were studied and a spiral wound pipe was developed at the late sixties. The so called oil crisis in the seventies gave start for a vivid activity in this felcl as the costs for plastics raw materials were feared to increase more rapidly than the substitute materials, i.e. clay, concrete and asbestos. The principle of saving raw material is mainly based on the I-beam principle but attempts have been made, mainly in the U.S. to increase the Young's modulus by very high rifler contents. The latter way is questionable because these 'polymer concrete' pipes cannot economically be made so stiffthat they would work like rigid pipes. This leads to the fact that strain is bound to exist in the pipe wall and this causes some doubt for the long term durability. ]t must be noted that the strain based dimensioning philosophy works only with pressure pipes made of high quality plastics compounds with moderate filler contents. The figure 1 shows the basic equations concerning the reaction forces that the pipe develops during deformation. This is true even for installed pipes as well, with the additional forces that comes from the soil reaction. It can be seen that the force is a linear function of deformation ( ) and system stiffness (ST). This means that every time an incrementing deformation takes place it needs a stronger force. In other words the strength of the system can be increased either by increasing the stiffness or by allowing greater deformation. This fact tells us why the most economic way for the society to get better pipelines would be to increase the allowed deformation to a more reasonable level. As the deformation is a direct function of geometries and strain, the equation tells the basic correlation between rigid structures and flexible pipes. The strength is the same regardless of whether it has been achieved by high stiffness (concrete) and low allowable strain (~), or by high deformation and low stiffness. Thus it can be concluded that the best way of increasing the stiffness is to increase the moment of inertia (1) as much as possible by increasing the profile height (e). The limit here comes from local buckling and from maximum allowed strain (~) at maximum allowed deformation.
Good tools for evaluating the performance of engineered pipes are: - piping efficiency defined as flow era divided by pipe weight, which tells the customer 'the price of the bore' - reinforcing efficiency defined as moment of inertia divided by the second power of average crossectional area, which tells how effectively the stiffness is gained. When comparing the results for engineered pipes and conventional ones, one immediately sees that the stressed skin and rib reinforced shell structures show the modern path to performance optimization in plastic pipe engineering. Computation of pipes made of corrugated or ribreinforced shells becomes easy when they can be substituted for by orthotropic pipes having the same rigidity characteristics. So is the case directly on macro level, i.e for designing pipelines.
m D~etlec~10n Sir( (:
m
mm
Fig 1 Basic equations for deflected pipe
Flexible p i p e t h e o r y Many researchers have recognised that the load on newly buried structures is not as great as it will be after substantial time has elapsed. That is, the load or pressure increases with time until the equilibrium load is reached. This gradual increase in load is accompanied by seasonal
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 D E C E M B E R 1989
191
physically that the backfilling material will always settle. What amazes in the Table 1 is the variety of material One of the earlier (and still valid!) methods of good design constants that are in use for the same product and same was to adjust the rate of pipe creep to be about the same material. Especially the values proposed by WRc seem to as for surrounding soil and hence safeguard the correct be too much on the conservative side despite of the fact function(23L The other extreme will be seen in road that they are as such correct and measured values. What building techniques where the deformation is completely is to be remembered is that the deformation and buckling controlled by allowable settlement in road level. A plastic formulas have been empirically derived from much pipe installed by that quality of compacting would not higher values and these short term measured and long term extrapolated values are misleading in this context. A deform! The current understanding of the role of stiffness is that reliable short term value, even if conservative, may be the rate of creep in the stiffness controls the rate at which established by the use of ISO short term dynamic ring the pipe-soil system reaches its equilibrium. Hence the stiffness testing and by deriving the respective E-modulus correct method of classifying flexible pipes is based on from that. Similarly the long term value will be established short term values. The actual stiffness of the pipe is in from the 1000 hours creep testing and extrapolation to 2 most cases only needed because of the compacting years stiffness. This value corresponds closely to the other needed for building up the passive modulus. If the pipe is 50 years moduli given in Table 1. too soft there will be the risk for negative initial deformation It must be understood, when knowing how the soil-pipe which should be avoided as it calls for rectangular interaction works, that measured and then extrapolated ovalization. In other words the flexible plastic pipe works 50a E-modulus values really lack physical meaning in this only as a chemically inert liner in the newly built hole! This context. The deformation process is a sequential creep fact is well proven by Kay et al.(241by a series of utilising and relaxation phenomena with the result that after a few rubber hoses. The arching capability of the soil gives the years the pipes are practically not loaded. possibility for extremely high fillsto be supported with little deformation if the lining is sufficiently flexible both in Deflection calculations for engineered pipes bending and axial compression. It has been shown as well Designing the installation techniques to meet the requirethat especially corrugated and ribbed pipes with rather low ments for maximum allowed deformations is as straightstiffness withstand greater loading than could be predicted forward for engineered pipes as for conventional pipes. from the deformation formulas(25,26LThis may be due to An engineered pipe of certain stiffness class behaves with good compacting over the pipe crown together with load- good enough accuracy as a conventional solid wall shedding softer sidefull and the fact that compacted filling pipe(29.30L material between the ribs adds to the buckling strength. This preferred practice is to follow tabulated max. A striking example of the unnecessity of getting covering depths. The only difference is that for ribconfused by the creep is served by Bishop0~ in a test reinforced pipes the nominal size, NS, gives a closer series that was conducted in embankment conditions for approximation than the outside diameter when calculating both PVC pipes and steel pipes of same stiffness. The the loading and needed trench width. This is a nice feature several years long test showed that the creep of PVC does as the inside diameter is pretty near the next standard not have any effect on the deformation which is controlled diameter downwards, which makes it possible to follow by the short term stiffness. This verifies the correctness of both the preferred outside diameter range and the the ISO draft for lightweight pipes (2a) where the classi- nominal size range making standardization easy. For all fication is based on short term measurement preferably other types the representative figure is the real outside by a dynamic method (27>. Some discussion has been diameter. raised concerning the testing time and the degree of Designing the pipe to meet the required minimum deflection. The values used for short and long term stiffness per pipe class is trickier as straightforward properties are shown in Table 1. calculation does not apply. The higher profile height calls
Table L Material data used for structural design Material
192
Unit
Long
Short term
term
modulus
modulus
PVC PVC PVC PVC PVC
N/mm N/mm N/mm N/mm N/mm
3000 3000 3600 3000 2800
1000 2000 1750 1500 500
HDPE HDPE HDPE HDPE
N/mm N/mm N/mm N/mm
1000 900 1000 800
100 200 150 150
Short term bending stress
Long term bending stress
Legend
90
10 50
(12) (44) (1 O) (19) (15)
35
5 13.5
(12) (44) (10) (1 9)
CONSTRUCTION & BUILDING MATERIALS VoI. 3 No. 4 DECEMBER 1989
perturbations due to rain, snow etc. The increase in load takes place because the distrurbed soil moves towards equilibrium with the surrounding insitu soil and with the buried structure (i). Extensive long term studies on the deformation behaviour of conventional solid wall PVCu pipes <2,3,4~ show indeed that after initial backfilling the deflection of the pipes will stabilize and often follows a Aformed curve as a function of time. This is typical to all flexible pipes installed underground. The effective strength of the pipe-soil system is remarkably high. For example Moser et A1/5~have shown that a rigid pipe will fail under such soil conditions and loading, while SDR 35 PVCu pipe only deflects 5% and maintains its performance. This load-shedding property of plastic pipes is enhanced by creep as the apparant modulus reduces at a greater rate than that of the soil. The absence of sudden failures under load is typical to plastic pipelines. AS the load is increased the wall stresses are redistributed due to occurring yield properties of plastics. Load can thus be increased over that at initial yield point before complete collapse occurs. The plastic material will hence theoretically take a load 50% in excess of the failure load of a rigid pipe ~6~.As can be seen for plastic pipes creep is not a liability but an asset for imposed deformations <5~.
Flexible Pipe Calculation Theories The aim of structural design of pipelines is to create a stable interaction between the pipes and the surrounding soil, in which the pipe shape remains approximately circular. What limit for that allowed ovality is safe? When the Scandinavian limits for the long term deformation of the pipe were set~% the basis was the tightness of the joints. The limit was based on the knowledge that if the initial deformation is below 8% then the long term deformation will be below 15% if the prescribed installation techniques were followed. This knowledge was based on extensive field tests with SDR 34 and 41 pipes with cohesive and friction soils used even as dug backfilling materials. Ten years long l a b o r a t o ~ 8~tests backed up the decision as they showed that high quality PVC is able to sustain very high strains without rupture. Because the joints were tested according to the standard with a 15% deformation such a performance limit would not create problems. AS the reduction in internal cross sectional area from circular to elliptical shape will be only 3.15% at 15% deformation this was regarded as a minor problem. Because buckling as the reason for collapse was unknown for PVC pipes with less than 25% of deformation the limits could be safely set. Hence the 15% max. deformation (at any point) is rather safe and well proven limit. In a recent analysis c9)made by FEM techniques and using very conservative values for initial modulus the conclusion was that in all circumstances, an initial relative compression of a vertical pipe diameter of less than 6% corresponded to a safe 50-year design. In Germany the deformation limit is set to 6% by the rather complicated ATV code of practice ~°~ although plastic industry backed up with sound proof struggles to increase the limitations on a more realistic ieveF TM. In Switzerland c~2~and Austria ~3~ the same limitation is valid. Although the Austrian code of practice is quite refined it has a very important benefit which should be considered when the ISO rules are settled; it is namely available as a
PC-program as well! The practice in the USA is almost as conservative with 7.5% of the long term iimitV4~, in the newly published WRc design guide 15~ the long term deformation is limited to 6% as well. it seems that these national regulations will follow with considerable lag to the ISO code of practice, compiled by leading geotechnical experts, which r e c o m m e n d s 12.5T as the upper limit for long term deformation. Today for example in Finland the market share of plastic sewer pipes is over 78% and no noteworthy failures have occurred. The reason for the good success of underground plastic sewer pipes in Scandinavia might be that the code of practice for structural design and installation is kept simplelle.~71. The viability of the pipes is shown tabulated with soil type and filling height as parameters. Thus the overengineering with problems is avoided. Now that underground PVC pipes have been in use for more than 50 years and monitored installations date back more than 20 years the time is ripe to forget the 50-year design base and speak about more realistic design life. The needed extrapolation from 50a (or 20a) to 100a is indeed not a quantum leap and it is very well motivated. In a very important report Janssonl~8~ gives full proof to justify this by analyzing practical installations and by material technical investigations. The Norwegian conference on this theme was as reassuring ~9~. Engineered sewer pipes take advantage of the most m o d e m installation techniques and they lend themselves well to the flexible pipe theory. As the quality control for accepted designs is more rigorous than before we can conclude that they will for sure show a life time of 100 years and over. W h a t s t i f f n e s s is needed? All the existing more or less detailed design principles and manuals rely on p h e n o m e n a well described for by example Spangler, those theories are still valid with sufficient accuracy~2°t. It must be emphasized that these theories are empirical and thus the coefficients have been calculated afterwards to match the known material values. A g o o d example of this is the teething problems created by the Young's modulus to be used in the equations. Now that the theory has been compiled using supposed E-values it will for sure give false results if new, actually measured values are to be put in it. Originally the equation worked with calculated E*[(21). Having in mind the inevitable problems in calculation of the product E*I for engineered pipes it is clear that the real measured stiffness must be used in the calculations as proposed by Prevost ¢22~. As the measured results are lower than the theoretical values originally used and created by different test methods, the chosen test method should give test results that are meaningful and subsequently are in harmony with proven practice. The stiffness of the pipe itself is in the region of one percent of the system stiffness. By inserting varying Emodulus values in the deformation equations one can immediately see how small is the difference in outcome. This fact can be seen even in practical long term deformation measurements. Why is stiffness then needed at all? The question is justified when analysing what the different equations give for a pipe stiffness zero. Typically even in that case the deformation is in the region of a few percent. This means
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
193
for lower modulus to be used and the geometry causes that Poisson's ratio is less than normally. It has been proposed to be zeroC3~L Buckling
of engineered
e=2xMo* (~ * X ~ J R m where e = strain
(~ Zmax Rm
/
f
f
5
'iME h
100
1000
Fig 2 Sensible limits for deformation at any point
STRAIN
)~=15% DEFORMATION
/
4
I
/
1
0,850.5~
/ ~
I
I-I I I
5
50 101
6
8
25 2016,7 12.5 51 /.13/*,/~ 26
10 10 9 21
8
16PN 6,25S 13,5de/e RELATIVE WALLTHICKNESS
Fig 3 Strain as function of SDR and deformation
= co-efficient; 1.5 for elliptically deformed pipes in soil, 2.14 for parallel plate method, 3.012 normal m a x i m u m and 4.28 for rectangular
deformation = deflection in percentage = distance from neutral axis to point of interest = pipe radius to neutral axis
Figure 3 gives an idea of the magnitude of strain at the outer pipe wall for different deformations. It can be seen that for typical engineered pipe with profile height taken from ISO wall thickness series $9 the maximum strain is
194
% 15
pipes
It has been stated that in normal practice for conventional SDR<5| solid wall pipes the buckling of the pipeline is hardly ever an existing problem. The reason for this is that the deformation at which the pipe might become unstable exceeds 25%. The main part of known failures concern pressure sewage pipelines. However, some aspects of the practice call for attention in this sense: - The dimensioning against pipe buckling shall have a reasonable safety margin. For example i n Scandinavia where a long term deformation of 15% is allowed the margin to range where buckling could occur is in the magnitude of two. In the Swedish functional standard for stormwater pipes and culverts (3~,which covers all types of engineered pipes as well, the requirement for deformation without buckling is 30% if normal plastic pipe installation technique is to be used. - Buckling analysis can be made on stress basis or strain basis. Remembering that uneven loading or stiff soil can cause much tighter radius than is the case for elliptical deformation leads automatically to much higher strains than predicted. - Flaws in the material or irregularities (typically on inside) cause high stress concentrations which together with for example local point point loading can cause faults similar to pure buckling. A bdttle fracture at the crown of the pipe has been one of the most common failure modes133,341. - All too flexible pipes, ring stiffness St below 2.5 kN/m 2, tend to fail by buckling load especially in stiff soil. - Traffic loads can cause fatigue and hence premature buckling. Considerable discussion is going on whether the buckling is critical or not ~2°35~with new dimensioning methods as o u t c o m e (9,36,37). This is even m o r e so in the case of engineered pipes where by nature the strains and s o m e t i m e s the stresses are bound to be on a higher level. The construction principle add as well new buckling m o d e s that m u s t be regarded before the pipe is constructed, and tested before the product is sold. The leading idea is that the specifying engineer should not be obliged to do it as, for a well dimensioned product, this should be unnecessary as far as normal sewage conditions prevail. The strain in the pipe wall is a function of profile height and deformation and can be expressed as follows:
Mo
PIPE DEFORMATION
on a totally different level than the previously reported critical strain for PVC, 0.85%. The fact that these high strains can be allowed, already from beginning, is due to relaxation. This theory was developed during the development time for ribbed pipes and is well proven by long term testsOSL The work done by Moser gives similar indication<391. These high strains cause immediate buckling in t h e different pipe sections if they are not adequately dimensioned as is the case for m a n y earlier engineered pipes.
C O N S T R U C T I O N & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
Especially double-walled (DWP) pipes are prone to outer corrugation buckling. Because the outmost end of the ribs can yield without buckling, ribbed constructions are safer in this sense. As the waterway is only a little constrained in solid wall ribbed pipes this calls for even better safety against buckling than for conventional thickwalled pipes. This effect is enhanced by the fact that the ribs stay cool despite of warm foul water. This is easily proved by Box Loading Test, in which ribbed pipes typically reach half the deformation that is normal for equivalent thickwalled counterparts. The calculation of buckling risk of engineered pipes is done by normal manner using the basic formula: ac,=r/xk*
~/2" E * ( t / b ) 2
where ~ = plasticity correction factor k = buckling co-effident depending upon boundary conditions of the buckling element t = thickness of the buckling element b = width of the buckling element The use of this basic equation implies two difficulties. The exact Young's modulus is seldom known and the buckling co-efficients given in various sources tend to give not so accurate results. The Jansson strain theory~1 lends itself, however, quite well to these calculations. For the purpose of designing engineered pipes a new optimization program was developed which works with an iterative method based on the basic finding that if the from the
~
~
E
N
E
beginning allowed max strain or stress is exceeded the program recalculates a lower modulus and then the deformation and strain again until acceptable equilibrium is reached. The design method was verified with real pipes in bucking tests according to the ISO document and results were encouraging; buckling did not occur at all even with very high deformations up to 80%. The buckling criteria that the force needed to deflect the pipe fails to grow or the sample looses its elliptical shape occurred simultaneously when testing conventional DWP pipes. These showed a clear buckling at the inside and the corrugation already at a deformation between 15 and 25%. Regarding standardization of engineered pipes this buckling test is very important as the mathematical calculation methods cannot be standardized, if a even more stringent buckling test would be needed for very low stiffness pipes the idea of Prevost ~ seems worthwhile. A test could be conducted in the same manner, with stiff side plates, as shown in Figure 5 which clarifies the difference in deformation when the pipe is properly supported123~. Tests made with this principle show that for both conventional solid wall pipes and for ribbed solid wall pipes the force needed to press the pipe to maximum allowed long term deformation (15%) is roughly three times as high as in normal buckling test where the pipe is free to deform. No buckling was noticed in the modified test either. It can be mathematically shown that ring buckling
R FENER
NEU[lU~.AX~S / WHENBENT j BUCKLINGOF WATERWAYBETWEENFLANGES
J
BUCKLINGOFTHE FLANGES
Fig 4
RING
!
" ~
BUCKLING
-'lllllllllllllllllllll,
DUCKLINGOF THE FLANGES
-rm |
|
-
BUEKLINGOFTHE FLANEES
Typical ways of buckling (the sandwich modes aft. 45)
CONSTRUCTION & BUILDING MATERIALSVol. 3 No. 4 DECEMBER 1989
195
~J
lq ZV=t DIST=6.[6 ¥rmOK
/
///////////////..
q / Drn}3 6.~22~ T I T / Fig 5
.o.o rq ( Om) 3
Deformation of pipe with and without sidesupport
always occurs at a given compressive deformation which is virtually independent of pipe wall geometry. Hence engineered pipes do not differ in their calculation methods from conventional pipes. This is true on the condition that pipe walls have been correctly dimensioned so that local buckling phenomena do not occur at lower deformation than ring buckling. So is the case for qualified engineered sewer pipes.
=_
Fig 6
Pressure resistance
Generally the engineered pipes for non pressure use show a lower internal pressure resistance than their thickwailed counterparts. However, if the stiffening principle makes sense, some interesting features are seen: - If the pipe is produced by winding processes some orientation takes place and the bursting pressures are somewhat higher than predicted. The same is of course valid as well if the single corrugated pipe is produced with a blow ratio greater than one. - Ifthe pressure test is carried out with an internal pressure equivalent to that of a plain walled pipe with same internal diameter and the same mass it can be seen that the stiffeners are contributing to the bursting time A good principle for non pressure pipes is that the wall thickness is enough to guarantee at least a nominal pressure of PN 1.
Impact resistance When testing an engineered plastic sewer pipe one should be aware that the ISO TIR test is not directly applicable to all constructions. This is due to the small test tup in use which can fall either on the corrugations or between the ribs. Hence the group to be analysed by the method lacks uniformity and the mathematics don't apply. In fact this is not needed either because this test is developed for monitoring the excellence of the processing - not the pipe, and for this purpose more effective methods exist. However, the impact resistance is an important property in many applications and that is why ISO proposes a modified ASTM staircase method for the purpose. This test is to be regarded more applicable with the 500 m m tup as well as it definitely simulates the real performance better than other alternatives. Generally impact strength of a product is a function off wall thickness. Hence it would be logical that engineered
196
I--
FEM analysis of a breakaway ultra.rib
pipes show inferior values to those of conventional pipes. However some parameters disturb the situation: - possible orientation increases heavily the impact strength - in solid wall constructions the profile height function as wall thickness, the real wall thickness plays only a minor role - pipe stiffness decreases the impact strength when testing as the pipe cannot flex as easily. Anyhow, a thin-walled stiff product should not be able to reach the same performance as the solid wall pipes do. This seems to be true for double-walled pipes made of rigid plastics. The construction behaves as an eggshell if the wall thicknesses are low. Tests made with solid wall pipes show two to three fold performance in this respect with very little loss compared to thickwalled pipes. Same kind of results have been reported for small diameter ribbed pipes, too (311. A very useful feature of solid wall ribbed pipes is the clever utilization of the notch sensitivity of PVC. If some ribs break off when handling, as the often do, a standard rib breaks in such m o d e that the crack will go through the pipe wall destroying it. The ultra-rib (fig 6) has a console in the root which safeguards that it will only knock off with maintained performance, stiffness and tightness, as result. Minimum wall thickness
Where conventional solid wall pipes were classified this was done merely by stiffness grounds. The pipe that fulfilled these requirements was sufficient on other grounds as well, i.e. the abrasion resistance, the impact strength, handling properties, buckling properties etc.
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
'¢~'" '~'" I
I
I.o
I
I
v
Effect of settlernent on rigid and flexible p ~ e in filL In the photograph at the lel~ pertaining to rigid p~e. S represents the amount of settlement of fill by consolidation, in the photograph at the righ~ pertaining to flexible p~e, D [ ~ in Eq (8.3)], represents the deflection of the p~oe as it yields under loacL were good enough for the purpose and hence there are many aspects of design that are 'forgotten' in the matching standards. When the required ring stiffness is derived by other means than wall thickness and hence the wall thickness can be chosen, there are many properties that still will be touched, for example: abrasion resistance - resistance against wear caused by mechanical rodding - resistance against rubber ring denting - longitudinal stiffness (handling, uneven settlement) - shear strength (uneven settlement, joints to rigid structures) -
Fig 8
- axial tensile strength (thermal elongation, uneven settlement) internal pressure resistance (steep grades, water jet cleaning) resistance against warm fou] water - impact strength The task to develop a criteria and a testing method for all of these aspects separately was regarded as too ambitious in the ISO task group for light weight pipes even if the goal to establish a rnatedal independent fully functional performance standard is a lucrative one. It was decided that setting a m i n i m u m wall thickness which is -
-
Flexible p~es redistribute loads with relaxation as result
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
197
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400 c
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iNnER DIAt~EIER , m , T + ".ONvENllONAL SDR 54 ~ CON,Lt~',,;)n~l ![,p 41 ,~ EONYENTICNAL SDR 51 ~ ASSOLLITE i~lN I~ALL
, k r H : , [;i H f S : ~ 341
Fig 10 Sewer pipe wall thickness.
3SA
Engineered pipes can be substituted for, by orthotropic pipes of identical rigidity. Combinations of various principles and materiaLs impart different performance characteristics. calculations are made. Even when this layer has wared off the pipe shall fulfil other performance requirements. Tests made with mechanical rodding devices(40) show Abrasion resistance and wear that tears up to 0.5 are possible. On the basis of these facts Numerous different test methods seem to give quite the Swedish independent Council for Quality Control of consistent results. The problem lies more on the difficulty Plastic Sewer Pipes (KP-Radet) has given the minimum to define how much sand will pass through a real pipeline acceptable wall thicknesses for plastic pipes(41L T h e s e during its design life of 100 years. S o m e studies show that wall thicknesses can be calculated from equation no measurable wear has been noticed in 10 to 20 year old Emin=0.005* Di+0.5mm. PVC pipelines. The estimated wear during 50 years usage will be about 0.25mm. Thus an optimized pipe should Dents c a u s e d by rubber ring pressure have at least 0.5 m m thick abrasion layer when new and Depending on the construction of the pipe these forces this layer should not be taken into account when stability can be focused directly on the waterway wall. In this case based on c o m m o n sense and experience will make unnecessary testing obsolete.
198
CONSTRUCTION & B U I L D I N G MATERIALS VoL 3 No. 4 D E C E M B E R 1989
the time for subjective and objective valuations easily come into picture. If the pipes are to be ranked, the following must be evaluated: customer needs in different market segments easiness of production and quality control how do different construction/material combinations perform in various dimension ranges. These variables have been discussed earlier ~421in the sight of PVC sewer pipes, in the size range which is fitting Longitudinal stiffness and tensile strength Axial tensile and compression forces as well as shear intensive all wound pipes loose attraction because of jointing difficulties. The solid wall concentric fib-reinforced occur in sewer pipes in the following circumstances: PLC sewer pipe offer many advantages over the conwhen the pipes are lifted when pipes are tramped on during installation and they ventional pipes in the size range 110 to 1000 mm. The concept seems to be interesting even having telecomlay upon uneven bedding deliberately made Iongtitudinal bending in order to munication pipes in mind. When regarding medium range 200-600 mm ID storm make changes in alignment to avoid obstructions differential settlement of a manhole or rigid structure to water pipes or road culverts, where silt tightness is good enough and high robustness is required the HDPE-pipes which the pipe is rigidly connected seem(431to be a good option if medium stiffness 3-6 kN/m 2 is uneven settlement of the pipe bedding - ground movement associated with tidal or ground water enough (as it usually is). If impact strength is not of primary importance the double wall PVC pipes offer a cost conditions effective alternative. erosion of bedding due to pipeline leakage For big bore stormwater (ID>600) pipes the closed seasonal variation in soil conditions due to changes in profile PVC pipe seems to be a good choice as the wall moisture content (limited to expansive or organic soil) seasonal height variation of pipeline due to the lifting thicknesses are large enough to tolerate handling. In these effects of frost (limited to certain expansive or organic soil sizes the low stiffness, profiled or ribbed, pipes are competitive as well. However, the minimum stiffness types in cold climates) - improper installation procedures, e.g. non uniform should preferably exceed 20 psi. foundation, unstable bedding, inadequate embedment, References consolidation 1. Bishop, R R, "13me dependent performance of buried I~C pipes', These forces can lead to the collapse of pipeline, International Conference on Underground Rastid Pipe, New buckling of the wall structure, exceeding of the allowed Orleans 1981 tensile/compressive - stresses and strains as well as 2. Jfirvenk'ylfi, J J, 'Muoviset maaviemEriputket-tutkittua tekniikkaa'. Rakennustaito 14,1980 shearing problems. 3. Vervomingsmetingen aan operationale PVC-straatrioleringen. The optimum axial stiffness of a pipe can be described Stichting KOMO, Rijswijk 1982 with border values: 4. Joekes D et al, 'Deflection of PVC sewer pipes and a new method - The pipe shall have a certain minimum stiffness in order for measuring and specifying stiffness of plastic pipes'. 6. Int. Conf. to Plastic Pipes, 1985 5. Moser AP et al, 'Design and performance of pvc pipes subjedcted - the pipes can be lifted without getting too much to external soil pressure;, Utah State University 1977 bent 6. Stephenson D, 'Flexible pipe theory applied to Thin-Wall PVC the pipes can be installed to the wished grade easily Piping', Municipal Engineer, March/April 1980 as they stay straight 7. 'NUVG-Report', Stockholm 1980 - The pipe should not be too stiff because: 8. Koski, L M, 'Stress Relaxation and Deflection Recovery in Plastics Pipes over a Period of 10 years.' 1st Int. Conf. from Materials - the load-shedding property of plastic pipes call for Science to Construction Materials Engineering. RILEM. Paris. ring and axial flexibility 1987 the installation becomes easier if the pipes can be 9. Hjelmquist E. et al, 'Long-Term Deformation and Failure of Buried longitudinally bent at site Plastic Pipes'. Journal of Engineering Mechanics, Vol 113, No 7, - transport of long pipes becomes easier ASCE 1987 10. Richtlinie fur die statische berechnung von Entw~sserungskan~len - the pipe must be able to follow uneven settlements of und -leitungen. Abwassertechnische Vereinigung e.V.Arbeitsblatt the bedding (no flexible pipe is stiff enough to bridge a 127 dezember 1984 over uneven spots) 11. Nowack R, 'Statische Berechnung von erdverlegten Entw~sserungsthe proposed minimum wall seems hazardous especially for countries where root penetration is a severe problem and for soft pipe materials type HDPE. Because of the creep in plastics materials there should exist a long term tightness test and this is incorporated in the ISO draft in the form of Box Loading Test. This test has proven its value during the development time for engineered pipes many times in other aspects as well.
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Without going deeply into mathematical analysis it can be said that a pipe with a wall thickness of roughly E = 0 . 0 0 5 * d i + l m m fulfils all functional requirements well. The main contributing fact is that most plastic materials relax soon and hence the stresses will not get excessively large.
12. 13. 14. 15.
S u m m a r y
The amount of different designs is infinite. So far some 25 different constructions have been tried. All these variants are modifications to six basic construction principles(42). When considering which option of the pipes is the 'best',
16. 17.
CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989
kan~len und -leitungen aus F'VChart und PE hart', 3R International, 24 Jahrang, Heft 8/1985 Heirerki W, 'Die statische Berechnung erdvedegter Kanalisationsrohre', 3R International. 21 Jahgang, heft 1/2, 1982 Statische berechnung erdverlegter rohrleitungen im Siedlungsund Industriewasserbau. Draft for OENORM B 5012. (Dec. 86) Handbook of PVC pipe. Uni-Bell PVC Pipe Association, 1986, p.183 Water Authorities Association Sewers and Water Mains Committee: 'Guide to the water industry from the structural design of underground non-pressure uPVC pipelines', WRc External Report ER 201E, October 1986 SPF teknisk rapport 01, Sveriges Piastf6rbund 1988 Molin J, 'Dimensioneringsnomogram f6r markavloppsr6r av styv PVC och PE', VBB Malm6 1986
199
18. Jansson L-E, 'Hur gammalt kan ett plastrSr bli!', KP-Radet 1987.27 litI. ref (english translation available soon) 19. Meland T, 'Loeser vi probleme med plastroer?', Cinference paper 9-11 januari 1984, Norges tekniske hoegskole 20. Prevost RC et al, 'Design of non-pressure very flexible plastic pipe', 'Pipes & Pipelines international. November-december 1985, pl 217. January-February 1986 p 19-20 21. Howard, A K, 'The usbr equation predicting flexible pipe deflection', Int. Conf. on Underground Plastic Pipe, New Orleans 1981 22. Prevost, R C, 'Design of non-pressure, very flexible buried pipe;, Pipes & Pipelines International. May-June 1983. p7-10 23. lmhotf W, et al, 'Abwasser-KanaFle aus Hartpoly~thylen. Teil 1: Erdaut]ast und WAnddickenberechnung', Kunststoff'e 57. Jahrgang 1967. Heft 1 24. Kay,J Net al, 'Compressive conduits under deep fill loads-a-model study', The University of Adelaide. Report no. r58, October 1983 25. Kay, J N, 'Minimum acceptable soil cover heights for ribbed, spirally-wound pvc drainage and sewer pipe installed in a trench and subject to severe vehicle loading'. The University of Adelaide, February 1984 26. Stigberg R, 'MarkfSrlagda plastr6r typ DSA, belastningsf6rsSk', Statens Viigverk, V~stra byggnadsdistriktet, 1982 27. ISO draft proposal 9969 28. ISO draft proposal: light weight underground PVC sewer pipes 29. Tidd, A et al, 'Evaluation of the structural performance of Ultra-PJb uPVC pipe when used on conjunction with conventional and alternative bedding and sidefill materials', WRc External Report ER 277E, September 1987 30. Tammirinne M, et al, 'Deformation measurements on ribbed uPVC sewer pipes embedded in soil', Technical Research Centre of Finland, Geotechnical Laboratory, 1986 31. Ragab A-R et al, 'Evaluation of the Mechanical Behaviour of Plain and Spirally Stiffened Polyvinyl Chloride Pipes', Journal of Testing and Evaluation, JTEVA, Vol 13, No 2, 1985
32. 'Materialneutral speciflkation av r6r och r6rdelar av plast avsedda art anv~ndas som anl~ggningsr6r', SPR Verksnorm 500 utgava 1,1988 33. Marshall, (3P et al, 'The analysis of practical failures of plastic pipelines'. The 3rd international plastic pipes symposium, Southampton 1974 34. Marshall, (3 P, 'Guidelines on the use of Polyethylene Pipe for use in Sewer Applications', Int. Conf. on the Planning, Construction, Maintenance & Operation of Sewerage Systems, University of Reading, September 1984 35. Howard, A, 'Diametral elongation of buried flexible pipe'. Int. Conf. on Underground Plastic Pipe. p191-201, New Orleans 1981 36. O'Reilly, M Pet al, 'Loading tests on buried plastics pipes to validate a new design method', Transport and Road Research Laboratory, UK 1982 37. Prevost, RC et al, 'Instability of buried flexible pipe - Part 1', Pipes & Pipelines International, May-June 1988 38. Jansson L-E, 'The relative strain as a design criterion for buried uPVC gravity sewer pipes', Proc. Int. Conf. Advances in Underground Pipeline Engineering, ASCE/madison 1985 39. Moser A P, 'Strain as a Design Basis for PVC Pipe?', Int. Conf. on Underground Plastic Pipe, New Orleans, 1981 40. Molin J, 'Report on underground plastic service sewage pipes', VBB Malm5 1976 41. Jansson L-E, 'Vad I~r man kr~va av ett l~ttviktsr6r av plast?', Norsk Ws/Energi och lnnerklima, vol 30, no 9, 1987, 9 litt.ref. 42. J~irvenkyl~ J J, 'Rib-Reinforced PVC pipes for gravity sewers', Pipes and Pipelines International, January-February 1987 43. J~irvenky[~ J J, 'Plastics pipes-prelude to improved road technology', Road and traffic magazine, Helsinki 1985 44. Lauer Hi, 'Stress analysis of buried rigid PVC and rigid PE pipe and sewers', 3R International, Heft 2/1978 45. Caprino G, et al, 'Sandwich for structural applications', Macplas International, 1987, p. 137-141
B O N D I N G A N D R E P A I R OF C O M P O S I T E S Proceedings of a one day seminar organised jointly by Butterworth Scientific Ltd and Rapra Technology Ltd 14 July 1989, NEC, Birmingham The ever-increasing use of polymer based composite materials and their joining with adhesive materials or otherwise have led to a greater interest in the lifetimes and the applications of these materials in safety-critical areas. There are many problems associated with the bonding and the repair of polymer resin-based composites and the interest surrounding this topic was a major reason for this seminar. 1989
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CONSTRUCTION & BUILDING MATERIALS Vol. 3 No. 4 DECEMBER 1989