[american society of civil engineers first international conference on transportation engineering -...

1

APPLICATION OF STRESS-RELIEF METHOD IN THE CONTROLLING

ENGINEERING OF PIPELINE-ACROSS LANDSLIDE

MA Qingwen1,2, Qian Xiangli3, Wang Chenghua1

1Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China; email:[email protected] 2Graduate University of Chinese Academy of Sciences, Beijing 100049, China; 3Southwest University of Science and Technology, Mianyang 621002, China; email: [email protected]

Abstract: Once the pipeline is laid in the body of landslides, part or even the total forces of landslide will act on the pipeline and the pipeline may twist or rupture with fatal demolishment. Taking the K317 landslide in Lan-Cheng-Yu pipeline as research object, the dynamical mechanisms between landslide and pipeline are studied before and after slope excavation by the methods of model test and numerical simulation. The results indicate that stress-relief method is fit for the controlling engineering of landslide when the pipe is laid in the body of landslides with the extending orientation perpendicular to the main slide direction. One or several of gouges, excavated in the upper of the slope, should be paralleling to the extending orientation of pipeline. The optimal depth of excavation ditch and distance for pipeline can be obtained by model test and numerical simulation. The investments of controlling engineering can be decreased largely by using of stress-relief method. Keyword: Pipeline; Landslide; Stress relief; Dynamical mechanisms

1 Introduction Stress-relief method is named relieving the slide forces by excavating

one or several ditches in the middle or rear part of landslides before slide surface is completely formed. By this way the mechanical equilibrium of slope is broken and new equilibrium is rebuilt (Cevik, 2003). The slide forces of landslide would reduce and readjust accordingly. This method is fit for the controlling of shallow soil landslide with little slide forces when the pipeline crosses the landslide in the middle or front part with its extending direction is perpendicular to the main slide direction. If the

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ICTE 2007

Copyright ASCE 2007 International Conference on Transportation Engineering 2007 (ICTE 2007)

International Conference on Transportation Engineering 2007

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landslide is large, the reverse forces of pipeline can be negligible and the stress-relief method is of no use.

The stress-relief method is seldom applied because of the limits of construction field. For example, if stress-relief method is applied to mitigate the damage of road landslide, it may take a long time for the slope to reach new mechanical equilibrium after excavating. In this process, there will be a lot of soil masses collapsing accumulating and blocking traffic. But, it is no problem to the pipeline because it mostly crosses the unmanned mountains areas and is laid under the surface for certain depth. The masses of collapse run over or accumulate on the overlaying soil of pipeline. It is not dangerous but safe to pipeline for the argument of laying depth (Leynes, 2005).

The investment of stress-relief method is little because it only needs to excavate a narrow ditch. Accordingly, the need of technology is high. The main difficulties are how to find the optimum location, width, depth of ditch and when to excavate. It is closely related to the mitigation effect and engineering investment. So dynamical mechanics is analysis, the model tests and dynamical simulations are executed as follows.

2 Mechanics analysisThe slide forces are upheld by two parts: pipeline and soil below the

pipeline. Due to the fact that the pipeline has certain strength and there still exists an unbroken part on bedding-plane or flanks in landslide at the critical failure stage, a horizontal soil arching is developed, which lead to restriction of deformation or braking of movement. In landslide soil arch acts as a locked segment that gives direct support to soils over crown and prevents them from slipping downward resulting in compacting and swelling of soil mass here (Cheng, 2004). It makes the pipeline become the area of stress concentration. Once slide forces exceed a limited value, the landslide will occur wholly (Fig.1).

Potential slide surface

Pipeline

New slide surface

Ditch of cutting

Collapse zoneRelaxtion zone

High stress zone

Fig.1 Sketch map of stress relief of slope

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3 Model Test 3.1 model preparation

According to the need of experiment and actual topography, the soil landslide of K317 in Lan-Cheng-Yu pipeline is chosen as the research object. A geological mechanic model is built, and its factors are established according to the relevant characters of soil, shape of slide plane, gradient of topography and deformation traits (Zhang, 2001).

In analogy to the loess traits, we mix quartz sand, barite powder, clay and water with the ratio 3.5:4:1.5:1 as the test model. The PVC pipe is chosen as the sentimental pipe and diameter D is appointed as 2 cm. The pipe is laid in the toe of landslide and vertical to the main slide direction. The factors of stress and strain are monitored with the strain gauges.3.2 Process and control

After building up the model, we excavate the slope over the pipe, the ditch should be parallel to pipeline. The width of ditch is 2D, and depth isform 2D to 4D. Five group models are made according to the distance between pipe and ditch. They are respectively 8D, 6D 4D 2D D. Thenwe raise the test bed step by step until the gradient of this model is up to 40°, in this process we should observe the deformations of the surface of the model, the forming and developing of tiny cranny. At the same time, we read the stress value from the pre-laying displacement counters in different deforming stages until the model starts to move in whole. 3.3 Result analysis of model test

(1) As shown of Fig.2, when the slope is not excavated, the stresses and strains of pipe increase linearly until the gradient of this model is up to 22°, and the pipe has tiny bending. Then the stresses and distortions of pipe increase steeply with the augment of gradient of model till 33°. If the gradient of this model increases more, the stresses of pipe will have a momentous invariableness. Then the pipe cracks abruptly.

The gradient of model(º)

Stre

ss(k

Pa)

Fig.2 The sketch of change of stress of pipeline (2) Once the slope is excavated, the stresses of pipe increase linearly

too. But the abstract value of forces acting on the pipeline is litter than

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the case before excavation till the gradient of this model is up to 22°, and the pipe has tiny bending. With the increasing of gradient of model the stress of pipe increases steeply, but its ratio of augment is litter than that of slope before excavating. The peak value of stress is litter too, and it will decrease steeply once new slide surface forms. Compared to the point that the stress is decreased evidently, the gradient of model after slope excavating is much litter than that of the breach point before excavating.

(3) The location of excavating has important impact on the forces acting on the pipeline. The litter the distance between pipeline and excavation location is, the smaller the forces acting on pipeline are. The bigger the width and depth of slope excavation is, the more wide the influence zone and unload of stresses are. In order to avoid scratching the safeguard of the pipeline by sharp broken rocks in the slide zone, it is better to keep a safe distance for more than 2D.

4 Numerical SimulationIn order to study the influence zones of slope excavating, changes of

stress and strain of slope and pipeline, a numerical simulation model is executed by using FLAC3D program Xiao, 2003 .4.1 Establishment of computing model

The numerical simulation model is simplified in view of the complexity of making of three dimension model. The height of model is 2.5m, slope ratio is 1:1.67, and the materials of loess landslide are even. Only self-weight is taken account. All rounds and the bottom surface of model are restrained in single direction, except that the slope face is free. Mohr-Columb and Elastic criterion are respectively used as the yielding criterion of the soil and pipeline. Displacements of one point and the maximum unbalance forces are monitored by strength-reducing factor method to judge that the outcome is whether or not convergent. The origin stresses filed should be firstly computed before slope excavating. Then, compute the stresses after excavation, which can be carried out through the command “Null” of FLAC3D. At last, analyze the stresses of slope and pipeline by changing excavation parameters, such as width, depth and location with the help of analysis of shear and stress of interface. 4.2 Material parameters choosing

According to the soil tests data and analogy of engineering practices, the parameters are chosen under the danger stage of landslide (table 1).

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Table 1 Material mechanical properties Gravity

(kN/m3)

Elastic

(kPa)

Poissons

ratio

Cohesion

(kPa)

Friction

(°)

Dilation

(°)

Soil 20.0 5.0×103 0.37 12.0 17.7 0

Pipeline 78.5 2.1×108 0.3 - - -

Interface - 5.0×103 0.37 8.0 10.0 0

4.3 Data analysisFig.3 to Fig.8 show the comparison of the shape of slide surface and

shear stresses of pipeline and landslide before and after excavation. Where, H is the depth of excavation and D is the diameter of pipeline.

(1) Fig.3 and Fig.4 show the pipeline bears all the slide forces and moves with the whole landslide before excavation. Once the slope is excavated, as shown in Fig.5 and Fig.6, new slide surface forms and the forces acting on pipeline decrease evidently.

(2) Comparing Fig.7 and Fig.8 with Fig.3 and Fig.4, there are not any differences of slide surface forces acting on the pipeline between before and after excavation when the depth of excavation is little. The depth of ditch plays an important role. When the bottom of ditch is equal or lower than the elevation of pipeline, the stresses change greatly. Or,

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the stresses change little. However, the width of ditch plays a little role in the relief of stress for the lock effect of pipeline. If the distance between pipeline and ditch is bigger, the depth should be increased accordingly to ensure the same influence of stress relief.

(3) By comparing the potential slide surface before and after excavation the potential instable zone of the slope is identified with the help of FLAC3D program, which make it easy to find the concentration area and excavate in slope.

5 Conclusions(1) Stress-relief method is fit for the controlling of pipeline-across

landslide, especially for little and shallow landslide when the main slide direction is perpendicular to the pipeline.

(2) The depth of ditch and distance between and pipeline are two important factors to the stress-relief method. The width of excavating ditch contributes seldom to relief of stress. The distance should not less than 2D. The elevation of the bottom of excavation ditch should be less than that of pipeline. The stress-relief method must be carried through the work of geological investigations in detail and the accurate data of field tests of rock and soil, especially through the key factors of rational slide surface and anti-shear parameters of soil.

References

Cevik E, Topal T (2003). “GIS-based landslide susceptibility mapping for a problematic segment of the natural gas pipeline”. Environmental Geology, 44(8): 949-962.

Cheng Q G, Zhang Z Y, Cui P. (2004) “Dynamical mechanism and stability criterion of landslide under lockup of soil arching” Chinese Journal of Rock Mechanics and Engineering, 23(17):2855-2864

Leynes R D, Pioquinto W P C, Caranto J A. (2005) “Landslide hazard assessment and mitigation measures in Philippine geothermal fields” Geothermic, 34(2): 205-217.

Xiao S G, Zhou D P. (2003) “Determination and numerical analysis method of relaxation region for cutting slope” Journal of Southwest Jiaotong University, 38(3):318-342.

Zhang D C. (2001) “The force summing analysis of buried pipeline under landslide condition” Petroleum Planning & Engineering , 12(6): 1-3.

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