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Soil-Structure-Pipe Interaction with ParticularReference to Ground Movement Induced

Failures

J.L. OLLIFF and S.J. ROLFE

Montomery Watson

D.C WIJEYESEKERA and J.T. REGINOLD

University of East London

ABSTRACT

Soil-pipe interaction studies generally recognise the significance of deformations in thepipe due to soil loading, but not differential ground and structure movements, which caninduce excessive stress concentrations in the pipeline. Plastics pipes can suffer failure dueto such movements, though their flexibility makes them less vulnerable than rigid pipes.This study examines the problems of interaction between pipelines and the surroundingsoil medium, subjected to differential ground movement.A procedure for predicting pipeline settlements, based on strip foundation theory is

presented, and a detailed analysis is made of failures encountered by a group of submarineplastics pipelines.

INTRODUCTION

Structural pipeline design is a subject that has been in and out of fashion over theyears. The first description of the behaviour of buried flexible pipelines under load wasgiven by Clarke(1) in 1897. Most of the research work in the next 40 years was on thebehaviour and design of rigid pipelines in the cross-sectional direction, most famouslyby Marston, Spangler(2) and Schlick. The first analysis of flexible pipe behaviour takingaccount of both soil and pipe stiffness was published by Lazard(3) in 1935.

The behaviour of a buried pipeline will depend very much on how its stiffness compareswith the stiffness of the native soil in which it is to be buried. Although traditionally“rigid” materials are thought of as being concrete, clay and asbestos cement, and “flexible”materials are thought of as being the plastics, the differentiation is not as simple as this.The response of the pipes under load will be largely dependent on the behaviour of thenative soil. If the pipes are of medium stiffness (say 20,000N/m2), and buried in a stiff

1128 Plastics Pipes XI


soil (such as a dense gravel of Es , soil modulus

≈ 150MN/m2, K, soil stiffness = 111 MN/m2)

then the pipes will exhibit predominantly “flexible” behaviour, i.e. they will tend todeflect on loading. If, however, the same 20,000 N/m2 stiffness pipes are buried in a softsoil (such as a very soft clay of E

s ≈ 15 MN/m2, K ≈ 11 MN/m2), then the pipes will

exhibit predominantly “rigid” behaviour, i.e. they will tend to settle into their foundationon loading. Pipes exhibiting “rigid” behaviour are those which attract a backfill pressurewhich is higher than the overburden stress (γ * H) value. In analysis, the overburdenpressure on a rigid pipeline, γH, is multiplied by a load concentration factor, C

1 (closely

related to the CC used in the U.K. and American tradition), which is greater than unity.

The reason for the load on a rigid pipeline being greater than gH is because the soil to thesides of the pipes in the trench tends to compress more than the pipes themselves, andhence by friction, additional load is placed on the column of soil above the pipeline, seeFigure 1.

Equilibrium of the pipeline demands that the vertical load (pressure multiplied bydiameter) is matched by the foundation reaction, and the increase in vertical load on thepipes above the overburden value therefore increases the pressure on the foundation soilbeyond the previously existing overburden value. In addition, this load is transferred tothe underlying soil only over a small arc at the base of the pipeline, meaning that thegreater load distributed over a smaller area inevitably leads to the settlement of thepipeline into the underlying soil. The resulting settlement of the pipes into the foundationsoil, in turn reduces the value of C

1 until a new equilibrium is reached. This process of

interaction was taken account of by Marston and Spangler in their “settlement-deflection

Fig. 1 Soil at the sides of the pipeline compresses more than soil over the pipeline, anddue to friction, increases the load on the pipeline.

1129Soil-Structure-Pipe Interaction with Particular Reference to Ground Movement


ratio” (rsd

), which included a term (sf) denoting the settlement of the pipeline into its

foundation.

TRADITIONAL APPROACH

Generally speaking, if structural pipeline design is done at all, it is limited to thecross-sectional design. Settlement is rarely, if ever, considered, and when it is, the chancesare that the chosen method of analysis will be a “weight comparison” calculation. In this“traditional” approach the weights of the pipeline full of water, the pipe embedment andthe backfill are compared with the weight of the soil mass they replace in the trench,with the difference in weights leading to a settlement prediction. This approach is wrongbecause it overlooks one crucially important point - the pressure redistribution. As ananalogy, why must ladies not wear stiletto heels in halls with wooden floors? Becausethe heels mark the floors. Why do the heels mark the floors? Are the ladies too heavy towalk on the floor? The reason the heels mark the floors, of course, is because the ladies'weight is transferred to the floor over a very small area. The same principle applies topipelines. As well as attracting a pressure greater than the previous overburden value,which was present before the excavations began, “stiff ” pipelines often transmit theirload to the underlying soil over a width which is less than the pipeline diameter. Hencethe settlement which occurs is likely to be much greater in magnitude than the valuepredicted from a weight balance approach, which assumes uniform pressure distributionon the trench bottom.

There are a number of ways of mitigating the effects of pipeline settlement. BothBarnard4 and Leonhardt5 assumed a “zone of influence” around a buried pipeline, inwhich the pipe-soil interaction occurred, extending a maximum of 2 pipe diametersfrom the pipeline in every direction. If the native soil in this zone was very soft, therefore,it could be removed to a depth of 2 pipe diameters beneath the pipeline, and replacedwith an incompressible material in order to minimise settlement. This might be areasonable solution with a small diameter pipeline, but with large diameter pipelines thecost could be prohibitive.

Rigid, cement mortar pipe joints, as widely used in the nineteenth century, and oftenwell into the twentieth century, were eventually realised frequently to crack. The solutionwas the adoption of flexible mechanical joints, sealed by rubber rings, and joints of thistype were widely adopted for use with PVC and GRP pipes. The success of such jointsin solving many problems led most engineers to believe they could solve all problems.This resulted, for example, in recommendations that two such flexible joints should beprovided, close together, where pipelines approach and enter structures.6 The mistakenbelief was that these two flexible joints, and the short length “rocker” pipe betweenthem, would satisfactorily accommodate differential settlement between the pipelineand the structure. What was overlooked, of course, was that the shorter the “rocker”pipes were made, the greater would be the angles through which their joints would have



to rotate to accommodate a given amount of differential settlement. Short length maytherefore have reduced the risk of longitudinal bending failures in “rocker” pipes, but itdid this at the expense of increasing the risk of joint damage. The vulnerability of shortpipes to damage by settlement is well known, and is frequently observed in closed-circuit television surveys of clay pipe sewers.7 The mechanism by which damage occurshas been described by Rolfe.8

The traditional approach therefore exacerbates the problems of differential settlement.At the same time, the trend towards higher ring stiffnesses which has been followed inthe design of some plastics pipes, for example GRP and profile-wall polyethylene, hasbrought these pipes into the ‘semi-rigid’ category. Such pipes, when installed in softsoils, then attract backfill pressure concentrations which cause them to settle into theirfoundation soils.

The abilities of different types of pipe to accommodate the shear forces, bendingmoments, curvatures and joint rotations caused by settlement and differential settlement,vary greatly. This variation is evident between the different types of plastics pipe. Thelow elongation at break of GRP pipes makes them unsuitable for accommodating thelongitudinal curvature which tends to develop as pipelines try to follow groundsettlements. Polyethylene pipes, on the other hand, having very large elongations, areideally suited for use where large settlements, and differential settlements, will occur.

The widespread lack of awareness of these problems, amongst consulting engineers,contractors, and even some pipe manufacturers, often results in the adoption, or at leastproposal, of inappropriate pipe materials. The consequent failure, delays, and extra costs,are harmful to all parties.

STRIP FOUNDATION APPROACH (OLLIFF 1994)9

Awareness of the patently incorrect assumption of uniform pressure distribution implicitin the “traditional approach”, led the first two authors' company to develop a more rational,and safer approach.

This approach consists of taking the vertical soil load on the pipeline, as calculatedfor the normal cross-sectional design, and estimating the resulting settlements, byanalysing the pipeline as a strip foundation responding to that load. The procedure waspresented to the CEN committee attempting to draft a "common European method" forstructural pipeline design in 1994.9

The essential steps in the analysis are as follows:1. Calculate pipe-soil stiffness ratio (e.g by Greatorex10, 11) using Eqn. 1

( )ESEn ′+′= 8.0105/ (1)

2. Estimate vertical soil pressure concentrations factor using Eqn. 2

( ) ( ) HDnnC −−−+= 10875.01585.01 48.01

(2)



This formula closely reproduces the rigorous Marston-Spangler result, by combiningGreatorex's solution (from Eqn. 1) for the settlement-deflection ratio, with theconcentration factor proposed by Olliff.12

3. Calculate elastic settlement using Eqn. 3a (Spangler based methods) or Eqn. 3b (ATV/ONORM based methods).

( )( ) EDHDTCe ′γ−−=Ζ 211 (3a)

( ) 41 6.01 EDHCe ζγ−=Ζ (3b)

4. Calculate long-term consolidation settlement in cohesive soils using Eqn. 4

( ) ( ) ( )oLc eCTL ++−=Ζ 1/1log10009.0 1 (4)

This approach was first proposed by Olliff(9), and early verification of this approachwas obtained by comparing predicted elastic settlements with observed initial settlementsof a concrete pipe sewer in Hong Kong. The settlement predicted by Eqn. 3a was180 mm, whilst observed settlements averaged 134 mm, but reached a maximum of275 mm.

BEAM EFFECTS

The differential settlement between two points, distance l apart, is expressed in termsof angular distortion ∝ . This should not exceed the allowable angular distortion ∝

all.

alll

ss

l

ss α≤−−−=α2

32

1

21max (5)

Foundation settlements depend on the compressibility of the foundation, and themagnitude of the pressure concentration. The influence of these is likely to vary alongthe length of a pipeline, resulting in a variable settlement profile. These differentialsettlements cause bending moments in the pipe barrels, shear forces across the pipejoints, and angular rotation of the joints.

Frequently, adverse bending moments and shears induced by differential settlementarise at positions where the pipelines approach structures. Most pipe joints canaccommodate angular movements of 0.5 to 4.0°, so that the amount of differentialsettlement a rocker pipe can accommodate is its length multiplied by the tangent of arelatively small angle which may amount only to a few millimetres. Furthermore, thesettlements adjacent to structures can be relatively much larger, and accentuated by loosebackfill material beneath the pipeline, or raised ground levels in the formation aroundthe structure.

Vertical displacement of rocker pipe = L Sinθ (6)where L is the length of the rocker, and θ the joint rotation.



This relationship suggests that longer pipes are desirable to accommodate largedifferential settlements, but increasing the length of the pipe will also increase the shearforces at the joints, and the bedding moment in the pipe. The first “free” pipe can beregarded as a simply supported beam carrying a uniformly distributed load, in whichcase the maximum bending moment will be:

M = 0.125 Pv.da.L2 (7)

In the case of flexible pipes, the limiting bending moment is more likely to be controlledby consideration of stability against buckling, than bending stress. According toLundquist,13 the critical longitudinal bending moment for a pipe is given by the followingformula:

Mc = K.E.rm.t2/(1-v2) (8)The designer must first decide how much settlement is acceptable. This should include

consideration of the following possible constraints:• The bending strength of the pipes. (Note: the product standards for clay, concrete and

fibre - cement pipes all include minimum requirements for the moment of resistanceof small diameter pipes),

• The shear strength of the pipes. (Note: this may be particularly significant for thesocket of mechanical joints and for butt fusion joints in polyethylene pipelines) and

• Leak - tightness of mechanical joints. (Note: some pipe product standards includerequirements for shear force resistance whilst remaining watertight).

CALCULATION OF SETTLEMENT

Following Selvadurai14 (1984) and Fletcher and Herrmann15 (1971) we have thefollowing for near surface and deeply buried pipelines

Authors Selvadurai (1984)

Fletcher and Herrmann (1971)

J.L.Olliff (1994)

Shallow Foundation

( )21

65.0

S

s

v

EK

−=

( ) SEvCK ≅

( ) ( )2165.0 vvC +≅

Deep Foundation

( )( )( )SS

SS

vv

vEK

431

16

−−+

≈

( ) SEvCK ≅

( ) ( )22360.0 vvC +≅

Foundation Settlement

( )( ) 3’20.11 00 EDHDTCZ γ−−=

Where T = Thickness of bedding material under pipes

The foundation stiffness parameter “K” depends on the elastic constants, Elasticmodulus and Poisson’s ratio (E

S and ν

S) of the soil medium surrounding pipeline. It must

be emphasised that these approximate expressions are valid for situations in which thesoil/pipe material modular ratio satisfies the constraint (E

s/E) < 0.01. Also, in these

expressions the material parameters encountered are the elastic modulus and Poisson's



ratio of the soil surrounding the buried pipeline. The assumed linear elastic behaviourwill be valid only for soils, which are strictly valid for small strains within the vicinity ofthe buried pipeline. It may be noted that although the ground displacements are large theground strains around the pipeline region are expected to be small. The linear elasticmoduli for the soil medium can be estimated from results of triaxial tests conducted onsamples of the backfill material or the undistributed soil, Winterkorn and Fang16 (1974).The cell pressure in the triaxial test is taken as γH

0 where γ is the unit weight of the

backfill or the natural soil. Typical results for Es and ns are given by Bowles17 (1977) and

Selvadurai18 (1979).

CASE STUDY

This case study presents the failure, at the point of commissioning, of a network ofoffshore GRP pipelines, serving as intake pipelines installed in a powerhouse project.Information was gathered by one of the authors, who was employed by the contractorson the project, but the exact location details of the case study are not disclosed. Sandysilts and occasionally sands and silty sands cover the seabed which gently deepensreaching a maximum depth of 10 metres over the area that was actually investigated.Three main lithological units occur in the submarine strata over a depth of 40 m from thesea bed. Borehole investigations were carried out along the line of intake pipelines. Thenearest borehole to the shoreline was located at a distance of approximately 500 m,which is similar to the soil properties of the near shore pump house foundation area. Thefield geological data together with the interpreted logs from seven boreholes indicatethat the thickness and nature of the superficial and channel deposits vary over very shortdistances, characteristic of the near shore depositional processes in a channel. An unevennature of the bedrock (marl) was observed from seismic surveys and showed the presenceof channel deposits overlying the marl. The geotechnical investigations revealed thatthe superficial deposits are mainly composed of loose to very loose, soft to very softdark grey clayey sandy silts with an abundance of seaweeds and shells in some places.These gave unfavourable geotechnical conditions with highly compressible soils havingalmost no cohesion and very low angles of friction. The channel deposits were a morefavourable formation than the overlying superficial deposits due to the presence ofcemented horizons with an absence of fines and organic material. The dark grey marlbedrock was therefore a relatively homogeneous formation with reasonable geotechnicalcharacteristics. The presence also of a “Khaki” marl with higher plasticity andcompressibility than the dark grey hard marl occurred in the transition zone accentuatingthe threat of differential settlement.

The geotechnical properties for the superficial deposits overlying the marly bedrockis summarised as follows for the purpose of this paper:



• Bulk density: 1.6 – 1.9 Mg/m3 • Consolidation characteristics of Cc = 0.46• Clay fraction of 30-80 % • m

v = 0.6 – 1.5 m2/MN,

• Liquid limit of 30-50% • Es = 0.6 -1.7MN/m2.

• Plastic Limit of 0-27% • Unconfined compressive strength qu = 0.29-

0.35 MPa.Along the transition zone in the vicinity of the pumphouse, the trench for the pipeline

was excavated to the marly bedrock. The minimum depth of the trench was 200 mmbelow the elevation of the pipeline invert. Ground investigations suggested that therequired excavation to reach the bedrock level would not exceed 300 mm below thebase of the pipeline, and in most cases, some excavation of the marl was required inorder to satisfy the minimum bedding thickness requirement of 200 mm.

The Filament wound GRP pipes of stiffness 2500 N/m2 were installed in 12m lengths.These were of 2.7 m internal diameter with a wall thickness of 30 mm, and utilised GRPsleeve joints. The pipe laying was started from the point of the intake structures andprogressed towards the inlet area of the pumphouse. The design did not accommodateflexible joints or rocker pipe connections at the interface of the pipe with the structure inthe transition zone.

The water test was carried out on the cooling water intake system by flooding thePumphouse by opening one intake pipeline, while the other two pipelines were isolatedusing their stop gates. When the initial flooding was completed up to -5.20 m level, theother two pipelines were made active to successfully flood the pump house to 0.00 mlevel. The three 2.7 m diameter GRP pipes failed at the forebay inlet area as a consequenceof the flooding of the cooling water pumphouse. The failure took place at a distance of36.28 m from the inlet wall of the pumphouse, and was consistently at the crown of thejoints fracturing both the pipe and the coupling.

The mean thickness of the backfill layer (including the armour layers) was 4.46 m.Assuming that the average unit weight of the backfill was 19 kN/m3, and that of the

in-situ soil was 16.5 kN/m3. The net additional pressure at the invert of the pipeline was11.15 kPa.

The settlement of the underlying deposits was computed using the equation:

sEH

1σ∆=δ (9)

The ground settlement thus estimated was 30 mm for 2 m (H) of soft soil increasing to90 mm for 6 m (H) of soft soil beneath the pipelines.

NUMERICAL MODELLING

In numerical modelling, the quantitative description of physical phenomena isestablished with a system of ordinary or partial differential equations valid in a certainregion (or domain) with the imposition of suitable boundary and initial conditions. The



modelling acts to educate the intuition of the design engineer by providing a series ofcause-and-effect examples. Finite Difference Analysis (FD) is one of the simplest formsof such discretization processes. In a field such as geomechanics, the field data (such asin-situ stresses, material properties, and geological features) will never be knowncompletely. However, if extensive field data are available, then these can be incorporatedinto a comprehensive model that can yield design information directly. More commonly,however, the data-limited model does not produce such information directly, but providesinsight into mechanisms that may occur; the designer can then do simple calculations,based on these mechanisms, which estimate the parameters of interest or the stabilityconditions.

For the numerical modelling, the central pipeline out of the three was considered forFlac3D modelling, as illustrated in Figure 2. The pipeline is modelled as a structuralelement in which flexure takes place only in the longitudinal direction. The techniquesproposed by Selvadurai18 (1979), can be used to examine the flexural interaction betweenboth near surface and deeply embedded pipelines located in a transition zone. The trenchsection for the central pipeline was assumed rectangular. The pipeline was considered asa continuous beam fixed at both ends. The sign convention of compression as negativeand tension as positive was adopted.

The model is symmetrical about the global X-axis. The pipes, couplings, soil propertiesand boundary conditions are defined for a transition zone of 9 pipe lengths. The modelwas subjected to different settlement conditions for analysis in order to study thedevelopment of critical stress regimes.

A soil stiffness, K, of 572.6 kN/m2 (based on the Selvadurai,14 1984) was adopted inthe numerical analysis. The corresponding settlement of the structure (weight of 1360Tonnes) and the pipeline are 149.1 and 19.5 mm respectively. The differential settlementof 129.6 mm occurring over a transition length of 9 pipe lengths was modelled. Figure 2shows the results of the modelling to indicate a stress concentration to occur at the pipecrown at a third of the modelled transition length. The failure that occurred in the fieldwas also at the third coupling and affected both the coupling and the crown of the pipe(see Figures 3 and 4).

The model output analyse study illustrates that, the stress concentration at the crownof the pipe is due to the induced settlement. Similar patterns of stress concentrations areobserved at the crown level of the joint couplings as illustrated in the Figures 3 and 4.

According to Leonhardt's adaptation of Jaky's theory, the additional soil stress causedby the presence of the pipes, will be dissipated over a distance of about 2 pipe diameters.Consequently, if the compressible foundation soil at the bottom of a trench is removedfor this additional depth, and replaced by gravel, or well-compacted sand, the settlementswill be minimal. Such a solution is likely only to be practicable with small diameterpipes, and an alternative, which may be used with larger diameter pipes, is to constructa reinforced concrete slab over the full width of the trench, to redistribute the pressureconcentration.

Fig. 2 "Maximum stress distribution on the soil pipe model"



Fig. 3 “Fractured GRP coupling surface and the Numerical modelling output”.



Fig. 4 “Fractured GRP pipe surface and the Numerical modelling output”.





A further possibility is to support the whole, or part, of the pipeline on piles. If thissolution is adopted, however, account must be taken of the increase in the vertical soilpressure on the pipeline (due to the increased projection ratio).

None of the measures described above should be expected to eliminate differentialsettlements completely, and designers should therefore ensure that flexible joints areprovided at adequate intervals. This is particularly important at the approach to structures,where the first pipe flexibly jointed at each end should ideally be capable ofaccommodating the differential settlement without exceeding the allowable angularrotation of the joints.

CONCLUSIONS

• Established pipeline design procedures frequently ignore or underestimate thesettlements of soil masses, pipelines and structures.

• Settlement damage is frequently observed in old pipelines, and is the most commoncause of structural failure in new pipelines.

• Analysis of pipelines as strip foundations can provide a useful estimate of likelysettlements.

• Differential settlements between pipelines and structures are particularly dangerous.• Pipeline design should include analysis of settlements, and the provision of measures

to limit them and/or enable the pipelines to accommodate their effects.• The ability to accommodate settlements should be taken account of during the pipe

material selection process.• Major pipelines should be designed with the assistance of comprehensive ground

investigations, and more rigorous methods of analysing settlements.• The effective modulus of a pipeline foundation will vary from place to place, reflecting

inconsistencies in the placing and compaction of bedding material, variations inbedding thickness, and in sub-grade properties.

• Differential settlement is a serious hazard, needing consideration at the pipeline designstage.

• Pipeline, embedment, trench backfill and native soils can be modelled as an elasticsystem.

REFERENCES

1. D.D. Clarke, The Distortion of Rivetted Pipe by Backfilling, Proc. ASCE, 18972. M.G. Spangler, Underground Conduits: An Appraisal of Modern Research, Trans

ASCE, 113, 1948.3. R. Lazard, Ouvrages Circulaires Placée en Terre, Travaux 33, 1935.



4. R.E. Barnard, Design and Deflection Control of Buried Steel Pipe SupportingEarth Loads and Live Loads, Proc ASTM, 1957.

5. G. Leonhardt, Die Belastung von Erdverlegten Rohren unterschiedlicher Steifigkeitunter Berücksichtigung des Verformungsverhaltens des umgebenden Bodens(Loading of buried pipe of varying stiffness with emphasis on the deformation ofthe surrounding soil), 3R International, 1977.

6. Civil Engineering Specification for the Water Industry, 5th Edition.7. M.P. O’Reilly, R.B. Rosbrook, G.C. Cox and A. McCloskey, Analysis of Defects

in 180 km of Pipe Sewers in Southern Water Authority, Research Report 172,TRRL, 1989.

8. S.J. Rolfe, The Effects of Pipeline Settlement, World Water and EnvironmentalEngineering, 22(6), 1999.

9. J.L. Olliff, Pipeline Foundation Design, Document TC164/165/JWG1/TG1, CEN,1994.

10. C.B. Greatorex, Ductile Iron Pipelines-Embedment Design, Publication PJF268Section 5, Stanton & Staveley, 1979.

11. C.B. Greatorex, Personal Communication to J. Olliff, 1990.12. J.L. Olliff, European Structural Design Standardisation for Sewers and Water

Mains: A Report on Progress, Pipeline Management ‘93, 1993.13. E.E. Lundquist, Strength Tests of Thin-Walled Duralumin Cylinders in Pure

Bending, Technical Note 479, Nat. Advisory Committee on Aeronautics, 1933.14. A.P.S. Selvadurai, The Flexure of an Infinite Strip of Finite Width Embedded in

an Isotropic Elastic Medium of Finite Extent, International Journal of Numericaland Analytical Methods in Geomechanics, 8, 1984.

15. Fletcher and Herrman, Elastic Foundation Representation of Continuum, Journalof the Engg Mechanics Division, Proc. ASCE, 97, 1971.

16. Winterkorn and Fang, Hand book of Foundation Engineering, VAN NostrandReinhold, New York, U.S.A., 1974.

17. Bowles, Foundation Analysis and Design, McGraw Hill, New York, U.S.A., 1977.18. A.P.S. Selvadurai, Elastic Analysis of Soil - Pipe Interaction, Development in

Geotechnical Engineering Series, Elsevier Scientific Publication Co., 17, 1979.

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