H O S T E D B Y The Japanese Geotechnical Society

Soils and Foundations

Soils and Foundations 2015;55(6):1466–1473

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Large-scale field trial to explore landslide and pipeline interaction

Feng Wenkaia,n, Huang Runqiua, Liu Jintaoa, Xu Xiangtaoa, Luo Minb

aState Key Laboratory of Geohazard Prevention and Geoenviroment Protection, Chengdu University of Technology, Chengdu 610059, ChinabGas Transportation Department of PetroChina Southwest Oil and Gas Field Corporation, Chengdu 610213, China

Received 31 January 2012; received in revised form 6 April 2015; accepted 29 July 2015Available online 17 December 2015

Abstract

Oil–gas pipelines for natural gas transmission from West to East China will inevitably undergo landslide especially in Southwestern China.Studies on the interaction between landslides and pipelines, particularly on the stress and deformation laws for pipelines under the action oflandslides, are of great importance. A large-scale landslide model with a prototype gas pipeline subjected to the same internal pressure as inoperation, was constructed outdoors to test and monitor the stress and strain in the pipeline together with landslide deformation. It is concludedthat the pipeline stresses change in close relation with the displacement of the landslide. The relationship can be described with an exponentialfunction. The induced stress distribution and deformation along the pipeline is in the form of a saddle. The most critical stresses on the pipelineare concentrated on both sides of the landslide border and in the central part of the landslide. These results are helpful to provide technical supportfor numerical simulations and for pipeline design, construction and remediation.& 2015 The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. All rights reserved.

International geotechnical classification numbers: B03; E14; H08

Keywords: Oil–gas pipeline; Large-scale landslide model test; Interaction of landslide and pipe line; Deformation of landslide; Deformation of pipeline

1. Introduction

As global economic development results in greater demand forenergy, many countries are facing increasing pressure to ensurethe safe transport of energy, especially by pipelines. Greatattention is therefore placed on the safety assessment (Hossamet al., 2010; Huang et al., 2011; Xiao et al., 2012), monitoring(Ma et al., 2011), and design of pipelines (Ma et al., 2007). In thisregard, landslides are gaining much attention because of theirpotentially devastating effects on the integrity of oil–gas

0.1016/j.sandf.2015.10.0115 The Japanese Geotechnical Society. Production and hosting by

g author.ss: fengwenkai@cdut.cn (F. Wenka).der responsibility of The Japanese Geotechnical Society.

pipelines. Deng et al. (1988) simplified pipelines inside andoutside a slope as beams and developed a method to assessinternal stresses and deformations. Using the pipe-soil interactionmodule in ABAQUS/Standard, Zhang and БыковЛИ (2001)found a relation between maximum Mises stress and slide lengthand displacement in loess landslides. Others (Calvetti et al., 2004;Guo, 2005; Abolmaali et al., 2011) carried out investigations intothe interaction between pipeline and soil body using the samepipe-soil interaction module in ABAQUS/Standard and theinterface constitutive relation.Zang (2007) established an index system to assess pipeline

safety in landslide areas, which was helpful to determine piperisks. Liu (2008) used finite element numerical simulations andmechanical theory analyzes on three ideal landslide types namely

Elsevier B.V. All rights reserved.

Fig. 1. Engineering geology plan of the landslide and pipeline model.

Fig. 2. Schematic diagram of the main section of landslide model.

W. Feng et al. / Soils and Foundations 55 (2015) 1466–1473 1467

axial, lateral and deep circular slides and developed preliminaryanalytical theories on pipeline deformation and strain. Feng andHuang (2009) used strength theory to assess pipeline safety basedon monitoring of slope surface displacements and strains in pipes.Jin and Li (2010) and Jung and Zhang (2011) analyzedperformance of buried pipelines under the impact of landslides,active faulting and seismic wave-triggered disasters.

Peter (1999) considered that the force on a pipe is a functionof the relative displacement between the pipeline and thesliding body. Challamel and de Buhan (2003) simplified theinteraction between the pipeline and the three-dimensionalslope in a landslide. Karimian (2006) described stress-strainstates of pipelines influenced by axial and lateral tensile forces.Manolis et al. (1995), Datta (1999) and Lee et al. (2009)investigated the mechanical behavior and resilience of pipe-lines under the influence of seismic action. Vazouras et al.(2011) used finite element software to analyze the behavior ofburied pipes going through active strike-slip faults.

In brief, though previous studies on pipelines have deliveredresults and understandings of potential failure mechanisms, mostof them were limited to numerical simulations. Detailed studiesare rarely found to combine an actual landslide with pipedeformation monitoring for assessing pipeline safety conditionsand few studies focused on monitoring of pipelines after largedeformation or total destruction. This is possibly because remedialmeasures are normally taken immediately after even a slightdeformation is observed according to standard safety requirements.Therefore, there is no data available for analyzing large deforma-tions and the failure process in case of an emergency. On the otherhand, some researchers (Majid and William, 1998; Calvetti et al.,2004; Kinash and Iseley, 2008; Li et al., 2011; Rojhani et al.,2012), using similitude theory, investigated the interaction ofpipelines and landslides by adopting physical simulation testing.However these physical simulations still require validation becauseof the reduced scale and the materials selected for landslide andpipeline. Large-scale landslide and pipeline model tests are thebest choice to obtain complete and continuous data about pipelineand landslide interaction.

This paper presents a large-scale field model with a pipelineburied through a potential landslide. The model test followspractical pipeline operations, actual deformation and failure ofthe landslide.

2. Large-scale test model set up

2.1. Basic test model

The large-scale model was built at Chengdu University ofTechnology. The foundation of the model consisted of soil thatbelongs to the Chengdu alluvial-proluvial plain. The soil usedfor the construction of the landslide body was medium-hardclay soil with some rubble and breccia.

The volume of the model landslide was about 500 m3 (thelength and width are about 10 m, and thickness about 5 m). Theplan shape of the landslide mass was arched, while the slidingsurface was prepared in advance and approached a straight plane.The tilt angle of the sliding surface and slope surface was in the

range between 151 and 201 (Figs. 1–3). The base of the slopemodel was the actual soil at the site, which was mainly brownishyellow clay with cohesion of 13.73 kPa, internal friction angle of4.9 degrees, elasticity modulus of 4.20 MPa, Poisson ratio of 0.3,and bulk density of 18.95 kN/m3. The soil of the landslide wasartificially made with the cohesion of 5.03 kPa, internal frictionangle of 3.5 degrees, elasticity modulus of 2.88 MPa, Poissonratio of 0.33, and bulk density of 20.98 kN/m3. The strength forboth soils was tested by using unconsolidated-undrained triaxialtests and triaxial compression tests. During the set up of thelandslide test model and installation of the pipe and monitoringinstruments (Wei and Yan, 2008; Kinash and Iseley, 2008; Maet al., 2011), the toe of the slope was supported by an earth wall(Fig. 2) to prevent sliding. The test started by excavating theretaining earth wall step by step.The pipe steel used for the test was of Grade L245NB. Its

yield strength is 245 MPa, tensile strength 415 MPa, minimumelongation 21%, surface roughness about 0.63 μm, and elasti-city modulus 210 GPa. The pipeline was 32 m long, and eachend reached at least 10 m outside the landslide boundary. Thediameter was 325 mm and the wall thickness was 8 mm. Thepipeline, with normal internal pressure of 2.5 MPa, was buriedat a depth of 1.5 m in a ditch perpendicular to the slidedirection.

Fig. 3. Full view of the landslide and pipeline model after the experiment.

Fig. 4. Installation plan of the strain gauges.

Fig. 5. Cumulative displacement at the landslide surface of the N-3 inclinometer. Fig. 6. Cumulative displacement at the landslide surface of the N-7 inclinometer.

W. Feng et al. / Soils and Foundations 55 (2015) 1466–14731468

2.2. Monitoring scheme and test process

The main purpose of the monitoring was to get a relationshipbetween landslide deformation and stress and strain observed in thepipe. Nine inclinometers were installed in the test model tomeasure internal slope deformations (Fig. 1), and 24 vibratingwire strain gauges welded on the pipe wall were used to monitorand measure pipe stresses and strains (Fig. 4). Continuousmeasurements were taken during excavation stage.

In order to observe and explore the interaction between theslope and the pipeline, the test was divided into 6 stages: 1)preliminary observation and measuring; 2) observation andmeasuring of the first excavation of the retaining wall (1stexcavation) to decrease the Safety Factor; 3) complete removalof the retaining wall (2nd excavation) to create a free face forthe potential landslide; 4) Infiltration of water in the back scarp

to promote sliding; 5) excavation of the collapsed material(Fig. 2) (3rd excavation), which hindered the development ofthe landslide; and 6) complete removal of the collapsed freeface material (4th excavation).

3. Results

3.1. Analysis of slope deformation and failure characteristics

Figs. 5 and 6 show the monitoring results of the inner slopedisplacements recorded by inclinometers N-3 and N-7. Thehorizontal displacement was consistent with the sliding direc-tion of the landslide, and perpendicular to the direction of thepipeline. The direction of the vertical displacement wasconsistent with the direction of gravity. The combined

Fig. 7. The cumulative horizontal displacement in depth measured with inclinometer N-3 and N-5.

Fig. 8. Strong deformation of the slope after the 4th excavation.Fig. 9. The temporal changes in stress measured by selected strain gauges onthe pipeline.

W. Feng et al. / Soils and Foundations 55 (2015) 1466–1473 1469

displacement was the resultant of the horizontal and verticaldisplacement vectors (Wei and Yan, 2008).

The monitoring results show a significant increase indisplacement after the 2nd excavation. The increase indisplacement on the slope surface was about 60–143 mm.Inclinometers N-3 and N-7 measured 143 mm and 133 mmrespectively (see Fig. 5 and Fig. 6). The surface displacementincreased along the slope surface from the back scarp to thefront, showing a “pull-type” slide-deformation. The inclin-ometer profiles N-3 and N-5 show the depth of deformation(Fig. 7), which is almost consistent with the designed depth ofthe slip zone (See Fig. 1 and Fig. 2).

During the infiltration of water at the back scarp, largehorizontal and vertical displacements were recorded. Max-imum displacements were measured by inclinometers N-3 andN-7, namely, 1280 mm and 1035 mm respectively (Fig. 5 and

Fig. 6). Deep inner slope displacement increased greatly andwas distributed as an inverted triangle. Deformations becameshallower forwards the front edge of the landslide (Fig. 7).The results also indicate that the pipeline was able to resist

the loading imposed by the deformations. The soil body abovethe pipe showed shallow deformation by sliding, while the soilunder the pipe showed no obvious sliding or extrusion.After complete removal of the collapsed material (the 3rd

and 4th excavations), the slope exhibited significant additionaldeformation, and the pipeline was subjected to larger bendingdeformation (Fig. 8).The buried pipeline showed both large deformations and

features associated with failure: 1) along the pipeline, the partof the soil retained by the pipeline formed a passive wedge, whilethe rest extruded downward (Fig. 8); 2) under favorable surface

Fig. 10. Stress distribution of the pipeline at different stages of the test.

W. Feng et al. / Soils and Foundations 55 (2015) 1466–14731470

condition, a part of the soil mass moved over the pipeline, in thesame direction as the landslide, and formed a shallowsliding mass.

3.2. Analysis of pipe stress characteristics

The stress-time curves of monitoring points S-3, S-10, S-17,and S-20 are representative of the deformation history of thepipe, as shown in Fig. 9, where the tension stress is positiveand compression stress is negative. The changes in stress of thepipe roughly correspond with the different stages of the test.

According to the measured data at each stage of the test thestresses on the pipeline changed as follows (Fig. 9):

1) Before the 1st excavation (Oct. 8, A.M.) of the retainingwall, the stresses on the pipeline were almost zero. Themaximum tension stress increment during the 1st excava-tion is 8.1 MPa (S-20) and the maximum compressionstress increment was �12.2 MPa (S-3).

2) The 2nd excavation (Oct. 10, A.M.) of the retaining wall ledto a clear stress increase. The maximum increment of thetension stress is 67.2 MPa (S-20), which was accompaniedby an almost symmetrical maximum compression stressincrement of �68.3 MPa (S-22).

3) The water infiltration stage of the test (Oct. 12–14) resultedin a significant increase of the stresses in the pipe. Themaximum tension stress increment was 209.6 MPa (S-5) andthe maximum compression stress increment was �327.8MPa (S-15). The curves show again almost a symmetricalincrease at this stage towards a nearly constant value.

4) The 3rd (Oct. 19, PM) and 4th (Oct. 20, PM) excavations ofthe collapsed material had also a strong influence on thestresses of the pipe. The largest stress increments occurredduring these stages due to the severe deformation of thepipeline leading to a failure of the strain gauges.

The stress distribution along the pipeline for the differenttest stages is shown in Fig. 10.

The figure indicates that: 1) the stress distribution along thepipeline was saddle-shaped with more or less left-right sym-metry corresponding to the overall pipeline deformation; 2)within the landslide, the external side and lower part of pipelinewere strained. Outside the landslide boundaries, the internalsides and upper parts were strained, and the stress disappearedon both ends of the pipeline; 3) the influence of water infiltrationon the pipeline's front side stress in the horizontal direction,caused by deformation, is larger than the stress on the pipeline'sunderside in the vertical direction; 4) The 3rd and 4th excava-tions of the collapsed material had a large influence on thestresses in the pipeline. As the lower part of the pipeline becamesuspended after the excavation, the influence of excavation onpipeline's underside stress in the vertical direction is larger thanthat of pipeline's front side in the horizontal direction; and 5) atthe end of the test, the stress on the middle section of thepipeline is larger than the yield stress of the pipeline.In accordance with the “Code for design of gas transmission

pipeline engineering (GB50251-2003)” and the “Code for designof oil transmission pipeline engineering (GB50253-2003)” andrelated provisions, a pipe should resist a maximum axial stress of80% of the minimum yield strength. The tensile strength of thispipeline appeared to be more than 500 MPa. Although the pipehas yielded, and the bending degree is around 3%, the pipelinedid not break or leak. The fact that in the design only strengthcriteria are considered is a subject for discussion. Future researchmaybe focus on the formulation of a reasonable combination ofdeformation and strength criteria for the design of these pipelines.

3.3. Relationships between landslide deformation and pipelinedeformation and stress

3.3.1. Relation between landslide surface deformation and themaximum axial stress on the pipelineA relation between landslide surface displacement and

tension stress in the pipeline can be obtained from displace-ments from different inclinometers along the longitudinal

Table

1Landslid

esurfacedisplacementandstress

developm

entof

thepipe

lineat

consecutivetest

stages.

After

2ndtim

eexcavatio

nBeforewater

infiltration

After

water

infiltration

Before3rdtim

eexcavatio

nAfter

3rdtim

eexcavatio

nAfter

4thtim

eexcavatio

n

Lon

gitudinalsection

A-A

’

Surface

horizontal

displacementat

the

locatio

nof

N-7(m

m)

92.1

385.51

1145

.01

1208.03

1384.33

1816

.66

Stressof

S-5

strain

gauge(MPa)

51.27

98.96

227.55

246.10

336.73

–

Lon

gitudinalsection

B-B’

Surface

horizontal

displacementat

the

locatio

nof

N-3(m

m)

151.33

227.05

1420

.26

1483.42

1789.63

–

Surface

horizontal

displacementat

the

locatio

nof

N-4(m

m)

100.02

167.90

1003

.71

943.00

1100.02

1119

.87

Surface

horizontal

displacementat

the

locatio

nof

N-5(m

m)

51.04

95.68

419.05

423.22

427.11

432.45

Stressof

S-6

strain

gauge(MPa)

67.5

72.2

356.0

426.9

654.0

–

Lon

gitudinalsection

C-C’

Surface

horizontal

displacementat

thelocatio

nof

N-9(m

m)

58.78

145.39

659.01

675.51

714.77

827.06

Stressof

S-7

strain

gauge(MPa)

66.9

75.4

239.7

246.7

286.7

–

Fig. 11. Relationship between the surface horizontal displacement at thelocation of inclinometer N-7 and the stress measured with the strain gauge S-5located in profile A-A’.

Fig. 12. Relationship of the surface horizontal displacement at the location ofinclinometer N-3, N-4, N-5 and stress measured with strain gauge S-6 locatedin profile B-B’.

W. Feng et al. / Soils and Foundations 55 (2015) 1466–1473 1471

section of the landslide combined with pipe stress changes inthe same section. We will ignore such factors as depth ofpipeline, pipe diameter, sliding zone depth and landslide width.The analysis was carried out only on inclinometers and straingauges which were not broken and did not record valuesbeyond the measuring range (Feng and Huang, 2009).Table 1 shows values of the relationship between surface

displacement and the tensile stress on the pipeline, located inthree profiles: A’-A’, B-B’ and C-C’ (Figs. 11–13).The figures revealed that: 1) tension stress on the pipeline

increases exponentially with slope surface displacement in thesliding direction. The coefficients and exponents of theexponential equations of the trend lines have nearly the samevalue for inclinometers at the same distance from the pipeline.It can be concluded that within a certain distance from the

pipeline, slope surface deformation (especially within thestronger deformation zone) is to some extent related to stresseson the pipeline. The surface displacement is the mostconspicuous and easy to capture information. A robustrelationship between pipe stresses and surface displacementswould be a basis for a fast judgment in pipeline riskassessment for experienced experts. The relationships

Fig. 13. Relationship of the surface horizontal displacement at the location ofinclinometer N-9 and stress measured with strain gauge S-7 located inprofile C-C’.

Fig. 14. Strong down warping of the pipeline after the experiment.

Fig. 15. The final deformation of the pipeline.

Fig. 16. Schematic diagram of the generation of forces and cracks at theboundary of the landslide induced by the deformation of the pipeline.

W. Feng et al. / Soils and Foundations 55 (2015) 1466–14731472

presented here are not unique since they are affected by manyfactors such as the nature of the soil, pipeline depth, diametervariation, etc. Further systematic research will be carried out tofind a rapid evaluation method for establishing a relationshipbetween surface deformation and stress on pipelines.

3.3.2. Interaction between landslide and pipeline deformationDeformations will be imparted on the pipeline as the

landslide displaces around and across its length. The pipelinewill then bend downwards taking a symmetrical saddle-shapedprofile (Figs. 14 and 15). Due to the pattern of deformation, thepipeline will also be subjected to substantial torsional stresses,in addition to tensile and bending stresses, making excesstorque a viable mode of failure. It is popssible that the torquefailure mode of the pipeline will occur rather than tensilefailure or crushing damage after bending deformation.

Pipeline resistance has an obvious influence on slopestability and the deformation and failure mode: 1) soil massdrifts and slides over a small section along the pipe; 2)downwards extrusion of the soil at the rear of the pipelinecombined with the pipeline bending down are driving forcesfor torque failure of the pipe.

The pipeline prevents the free movement of the slope andcauses both outsides of landslide border to deform more alongthe pipe. Moreover, the pipeline bends in the opposite directionnear both sides of the landslide boundaries. This bringschanges to the stress pattern of the sliding slope at the frontand rear sides of the pipeline (Fig. 16).

1) The pipeline section bending in the slide direction produceslateral extrusion forces near the landslide boundaries on thefront side of the pipeline. This also generates diagonalcracks near the landslide boundaries in front of the pipeline(Fig. 16).

2) The pipeline section bending backwards near the boundaryalso provides an arching effect, which causes the directionsof stresses and displacements to turn towards the inside oflandslide (Fig. 16). Simultaneously, the pipeline sectionoutside the landslide is also moving forward through stablesoil, causing the rear of the pipe to become exposed(forming cracks parallel to the pipe). With the increase ofslip deformation, lateral shear cracks become more serious(Fig. 16). The impact on the pipeline sides widens thesliding area to some degree and creates a rapid expansion ofthe soil deformation along the pipe line. It gradually helps

W. Feng et al. / Soils and Foundations 55 (2015) 1466–1473 1473

the pipeline to expand further. The deformation andexpansion, however, is also related to the self-deformationof the pipeline (Figs. 3, 8 and 14).

4. Conclusion

With a pipeline crossing the landslide, the deformation andfailure mode of the landslide shifts from an original coherentslide to a shallow drift slide above the pipe and a deepextrusion deformation below the pipe. The deformation of thelandslide was mirrored by deformations of the pipeline fordifferent test stages. The stress distribution along the pipelineshows a saddle shape.

An exponential relationship was observed between surfacedisplacement and pipeline stress. Surface locations on thelandslide, at equal distance from the pipeline have nearly thesame response with similar coefficients and exponents indescribing the observed relationship.

The characteristics of pipe bending deformation are con-nected with the pipeline stress pattern and the failure mode:bending and breaking or twisting and breaking. However, theexistence of the pipeline increased the stability of the slope,spread deformations towards the edges, reducing the maximumdisplacements in the direction of the landslide.

Finally, future research should consider design criteria interms of a reasonable combination of deformation andstrength.

Acknowledgment

This research is supported by National Natural ScienceFoundation of China (Grant number: 41172278) and by theNational Basic Research Program of China (Grant number:2008CB425801). Great support also comes from the South-west oil–gas Corporation Petro China (Grant number:XNS07JS2008-37). The authors would like to express grati-tude to all others for the help we have received during theresearch. I also gratefully acknowledge the help from ZUOYaya, YANG Fan, GUO Zhen and others. Last, my thanksshould also go to Qiao Qiuchun from the Chengdu Universityof Technology, China, professor Theo van Asch from theUtrecht University, Netherlands, Professor Yongxin Xu fromUniversity of the West Cape, South Africa, and AssistantProfessor Yanrong Li from University of Hong Kong, China,for their help with the English translation and revision.

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