Analysis of Variations in Deflection and Stresses in Buried PVC Pipe Protected With an Eps Geofoam Layer

Transcription

1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 9(3): Scholarlink Research Institute Journals, 2018 (ISSN: ) jeteas.scholarlinkresearch.com Journal of Emerging Trends Engineering and Applied Sciences (JETEAS) 9(3): (ISSN: ) Analysis of Variations in Deflection and Stresses in Buried PVC Pipe Protected With an Eps Geofoam Layer Adesogan, S.O and Alawode, K.J Department of Civil Engineering, University of Ibadan, Nigeria Corresponding Author: Adesogan, S.O Abstract This study examines the behaviour of buried Polyvinyl Chloride (PVC) pipe under static traffic load, through detailed deflection and stress analysis, when a layer of EPS geofoam is placed in the trench with the pipe, using finite s method of analysis. The finite program, ABAQUS, was selected for the three dimensional static analysis of the buried pipe. In the model, pipes were buried at 600mm, 700mm, 800mm, 900mm and 1000mm in saturated sandy soils, moist sandy soils, saturated cohesive soils and moist cohesive soils.eps geofoam 19, 29 and 46 were considered in the study to investigate the influence of reducing compressibility of the geofoams on deflection and stress at the crown of the pipes. A road section, with 100mm asphalt concrete, 200mm base and 100mm sub-base was included on top of the soil, caring a vehicular surface load of 1100kPa in all cases of the analysis. The results indicate that there was a 0.14%reduction in stresses and 1.32% reduction in vertical deflection at the pipe crown with the use of the EPS geofoam when compared with the condition when no EPS geofoam was installed. The pipe in the sandy soil had higher stresses and vertical deflection than those in corresponding cohesive soil at all buried depths considered in this study because the sandy soil has a higher friction angle than cohesive soil. Also, the pipes in the saturated soils had lower stresses and vertical deflection, compared to those in moist soils. Hence, the study concludes that finite method is effective for the analysis of buried pipes and EPS Geofoam can be used with buried pipes for reduction of stresses and vertical deflection Keywords: buried pvc pipe, eps geofoam, finite method, variation in deflections, stress analysis. INTRODUCTION Failure of buried pipes is a major concern for the government, private firms and individuals, and the society because the transportation of fluids such as water, gas, crude oil and sewage through buried pipes has become an important part of modern infrastructure. To protect this important infrastructure, it is necessary to reduce stresses and vertical deflection as a result of live loads (such as traffic loads) and dead loads (weight of backfill soil) on it.a significant number of numerical, laboratory, and full scale studies have been published on the behaviour of buried rigid and flexible pipes (Spangler, 1941; Burns and Richard, 1964;Hashash and Selig, 1990;Moore, 1994; McGrath et al., 1999; Gassman, Schroeder and Ray, 2005 and, Elshimi and Moore, 2013) The load applied to a pipe can easily be thought of, as simply the weight of the column of soil, and any surface loads directly over the pipe. Marston and Anderson (1913) in his paper on The Theory of Loads on Pipes in Ditches and Tests of Cement and Clay Drain Tile and Sewer Pipe demonstrated that the load on a pipe in a trench was in some cases lower than the weight of the soil prism above the pipe. Marston and Anderson demonstrated that through arching (reduction in load imposed on a pipe because of its flexibility) action, The side pressures of the filling materials against the sides of the ditch develop frictional resistance, which helps carry part of the weight and, The pipe must be strong enough to carry safely the entire weight of the ditch filling materials above the top of the pipe, less the friction of the filling against the sides of the ditch (Marston and Anderson, 1913).McGrath et al. (1999) working with Burns and Richard (1964) solution demonstrated that plastic pipes had a greater tendency, than corrugated steel pipes (CSP), to have positive arching. Geosynthetics, factory-manufactured polymer materials in sheets (e.g., geotextiles, geofoams and geogrid reinforcement) or cells (e.g., geocells), can also be used above a pipe or underground utility line to reduce surface loading (such as footings, highway traffics, and rails), lower settlement, and prevent damage by excavation equipment (Corey, 2015).EPS geofoam is a type of geosynthetic, and has proven to be quite durable when exposed to common natural s. Polystyrene is non-biodegradable, and is inert in both soil and water (Horvath 1994).The deformation in the EPS geofoam provides a mobilization of shear strength in the backfill (positive soil arching) and reduces the expected vertical earth pressure. It is therefore used for it compressible inclusion functions. The use of geofoam to reduce 105

2 earth pressure have also been used on concrete culverts below high fills, Vaslestad et al. (1993), Yang et al. (2005), Zhang et al. (2006) and McAffeeet al. (2008). In their studies, Yoshizaka and Sakanoue (2003) report a 33 to 60 percent reduction in the lateral soil-pipe forces when geofoam was used as lightweight trench backfill cover for pipe undergoing horizontal displacement. Choo et al. (2007) have explored the use of geofoam as a lightweight cover system for buried steel pipelines subjected to vertical fault offset. They used centrifuge testing of scaled models to evaluate the benefits of geofoam in reducing pipeline stresses undergoing vertical offset. The most useful general analytical tool foranalyzing problems involving a compressible inclusion (eg. EPS Geofoam) is computer software that solves a continuum using the finite- method (Horvath 1997). The inception of finite and finite difference modeling coupled with advances in computational geomechanics allowed researchers to investigate and predict the behavior of buried pipes by means of numerical models. Sun et al. (2005)in their work on Reduction of Stresses on Buried Rigid Highway Structures Using the Imperfect Ditch Method and Expanded Polystyrene Geofoam, conducted the research with numerical analysis and concluded that EPS has a great effect in reducing the vertical soil pressure above and below a culvet. Stephen Singh (2016) using numerical methods in a coputer program, also affirm in his work on Pressure Reduction on Wide Culverts with EPS Geofoam Backfill, that there is a pressure reduction with the use of EPS geofoam. Hyuk Lee (2010) in his dissertation on Analysis of buried pipelines, concluded that the soil s material physical property influences the stresses on buried PVC pipes.soils can be grouped into sandy soils and cohesive soils. Sandy soil is often called frictional soil or drained soil and cohesive soil is normally classified as undrained soil. Therefore, examination by FEM (ABAQUS) related to the buried pipelines performance in different soil types, and different EPS geofoam protection layer is appropriate. This research is appropriate for the conference in Colloquium Three: Sustainable Development in Health, Environment Science, Climate Change, and Project Planning (SDHECP).The knowledge of the impact of backfill will help reduce cases of infrastructural failures METHODOLOGY The model consist of a 160mm diameter PVC pipe, embedded in a Soil backfill (saturated sandy soils, moist sandy soils, saturated cohesive soils and moist cohesive soils) at depths of 600mm, 700mm, 800mm, 900mm and 1000mm. There is a road pavement layer consisting of asphalt concrete, base and sub-base at 100mm, 200mm and 100mm thicknesses respectively. Three geofoam types were used in the study; EPS 19, 29 and 46, all of which have a thickness of 100mm and placed immediately after the road layers. The pipe was modelled as a shell and assumed to beisotropic, elastic and perfectly elasto-plastic while joints between PVC pipes were disregarded. The soil was assumed to beelasto-plastic characterised by Mohr Coulomb theory. Asphalt concrete Base Sub Base EPS Geofoam PVC Pipe 160mm Backfill Soil Trench wall 300mm 400mm 300mm 1000mm 500mm Figure 2.1 Dimension of the model in ABAQUS 100mm 200mm 100mm 1100mm Figure 2.2 Boundary conditions of the model in ABAQUS Constraint Type and Boundary Conditions A general contact was used between the parts of the model with an isotropic friction coefficient of 0.5. The bottom surface of the 3D-FE model is completely fixed in order to restrain horizontal and vertical movements, while the side surfaces are pinned to restrict movement in the horizontal 106

3 direction. The pipeline is pinned at both ends surfaces. Type of Elements In this study, the linear 3D stress was considered for all the parts except for the pipe that has a shell. Table 2.1 Types of Elements Material Element Type First Order (Linear Asphalt 8 node brick Interpolation) Base 8 node brick Sub base 8 node brick EPS Geofoam 8 node brick Backfill 8 node brick Pipe 4 node doubly curved general purpose shell S4 Figure 2.3 Mesh of the model in ABAQUS Materials Property EPS Geofoam The EPS 19, 29 and 46 are used in this study to study the effect of their inclusion on the stresses and deflection of the PVC pipe. Their properties is in accordance with ASTM D6817. Table 2.3 Property of EPS Geofoam Density(Kg/ ) Young s Modulus (KPa) EPS EPS Poisson s Ratio Trench Wall 8 node brick EPS Size of Elements The density of the meshed s has an effect on the accuracy of analysis. The sizes of the s in this study are presented in table 2.2 Table 2.2 Size of s Material Global size of meshed s Asphalt Base Sub base EPS Geofoam Backfill Pipe Trench Wall Number of meshed s Road The road properties used in this study are given in table 2.4 Table 2.4 Properties of road layers Depth (mm) Density(Kg/ ) Young s Modulus (MPa) Poisson s Ratio Asphalt Concrete Base Sub-Base Soil A soil model using the Mohr-Coulomb theory is used in order to execute a study that will give a benefit for understanding interaction between soil and buried structure straightforwardly. 107

4 Table 2.5 Properties of soil(liu et al., 2010) Type of Soil Mech Properties Term Value Sandy Soil Elastic Property Density (Kg/ ) Young s Modulus (Mpa) Poisson s Ratio Cohesive strength (C - kpa) 17 Plastic Property Friction angle ( - deg) 37 Dilation angle (ѱ - deg) 2 Cohesive Soil Elastic Property Density (Kg/ ) Plastic Property PVC Pipe The PVC pipe used in this study has a diameter of 160mm, with a Mass density of 1467Kg/ a Young s modulus of 2800pa and a Poisson ratio of Young s Modulus (MPa) Poisson s Ratio Cohesive strength (C - kpa) 252 Friction angle ( - deg) 25 Dilation angle (ѱ - deg) Loads In general, several loads and load combinations affect buried structures. In this study, only static loads were considered. These are traffic pressure and gravity loads.the self-weight of both pipeline and soil was considered by adding gravity, 9.81 N, to the created ABAQUS model. In order to examine the critical deformation of pipeline, the internal pressure of pipeline will not be taken into account in this study because the most deformation of buried pipeline occurs in the absence of pipe flow. The eight wheel HB wheel load is used as the traffic load. Based on BS :2006 and BS 9295:2010.The load on each wheel is kn and spread over an contact area to the road of m2, to specified contact pressure of 1100kPa. Thus, a uniform surface load of 1100 kpa was used as the traffic load. RESULTS AND DISCUSSION Static analysis was executed using the model under static loads as outlined in section 3.4. The tables of the graphs in this section are in Appendix. Vertical Deflection of Buried Pipe The pipeline was deformed in a heart shape and followed the vertical deflection prediction by Spangler regarding flexible pipes in Typical deformation of the pipeline under static load is shown in Figure 4. Figure 3.1a 3.1b Figure 3. 1 Typical pipeline deformation under static loads Figure 3.1a is the front view of the pipeline while Figure 3b is an isometric view of the pipeline. The results by ABAQUS indicates that the maximum vertical deflection occurs at the crown of the pipe which is marked red and orange, and the minimum occurs at the bottom of the pipe which is marked blue. The calculated pipeline vertical deflection meant the own vertical deflection of the pipe as it was the distance moved by the nodes at the top of the pipe. Influence of Buried Depth on Vertical Deflection The distance from the surface of the road to the crown of the pipe is referred to as the buried depth. Figures 3.2, 3.3, 3.4 and 3.5 shows the vertical deflection of the crown of the PVC pipe across 108

5 depths of 600mm to 1000mm in moist sandy, saturated sandy, moist cohesive and saturated cohesive soils respectively. The vertical deflection of the pipe crown reduced with increasing buried depth across all soil types and across all EPS geofoam types. There was a 1.84% average vertical deflection reduction between 600mm and 1000mm buried depth. This is due to reduction in the impact of live loads on buried structures as depth increases in the soil. This indicates that it is better to bury pipe deeper in the ground. However, it is necessary to consider stress statement, economy and so on when considering buried depth of pipeline in actual construction field because it is impossible to conclude that the deepest buried pipeline is the safest and to construct all of pipeline deeply for the only purpose of safety. Figure 3.4 Buried Depth against Vertical deflection of Pipe Crown in Moist Cohesive Soil Figure 3.2 Buried Depth against Vertical deflection of Pipe Crown in Moist Sandy Soil Figure 3.5 Buried Depth against Vertical deflection of Pipe Crown in Saturated Cohesive soil Influence of Soil Type on Vertical Deflection Figures show the vertical deflection of buried pipe crown for EPS 19, EPS 29, EPS 46 and No EPS conditions in saturated sandy, moist sandy, saturated cohesive and moist cohesive soil. The pipe in the sandy soil had higher vertical deflections than that in cohesive soil at both the 600mm and 700mm buried depth. This changes at the 800mm, 900mm and 1000mm buried depth where the vertical deflection of the pipes in the cohesive soil exceeded that of the pipes in the sandy soil. Also, the pipes in the saturated soils had lower vertical deflections compared to those in moist soils. Figure 3.3 Buried Depth against Vertical deflection of Pipe in Saturated Sandy Soil This meant that whereas the water in saturated sandy soil is allowed to move easily due to the effect of big grain of sandy soil and non-cohesion when adding static loads, it is impossible for the water in saturated cohesive soil to move easily because minute particles 109

6 and cohesion of cohesive soil interrupt the movement of saturated water. Influence of EPS Geofoam Types on Vertical Deflection There was a general reduction in the vertical deflection of the pipe crown with each type of EPS geofoam when compared with the No Geofoam condition. Figures shows the vertical deflection of the pipe crown with each type of EPS Geofoam and the No Geofoam condition. The EPS 19 case had the least vertical deflection of pipe crown at all the buried depths and soil types considered in this study. This is because it has the least density and least Young s modulus, making it more compressible when compared to the other types of EPS Geofoam. Averagely, there was an increase in vertical deflection of the PVC pipe crown with increasing density of the EPS Geofoam. There was a 1.32% reduction in vertical deflection of the pipe crown with the use of the EPS 19 Geofoam on the average Figure 3.8 Vertical deflection of 800mm buried Pipe Crown in Saturated and Moist, Sandy Soil and Cohesive Soil Figure 3.9 Vertical deflection of 900mm buried Pipe Crown in Saturated and Moist, Sandy Soil and cohesive soil Figure 3.6 Vertical deflection of 600mm buried Pipe Crown in Saturated and Moist, Sandy Soil and Cohesive Soil Figure 3.7 Vertical deflection of 700mm buried Pipe Crown in Saturated and Moist, Sandy Soil and Cohesive Soil Figure 3.10 Vertical deflection of 1000mm Buried Pipe Crown conditions in Saturated and Moist, Sandy Soil and Cohesive Soil 110

7 Stress in Buried Pipe The term stress used in this section generally means the Mises equivalent stress. Figure 3.11 Typical Pipe Stresses in the PVC Buried Pipe Figure 3.13 Buried Depth against Stresses at Pipe Crown in Saturated Sandy soil Figure 3.11 is an isometric view of the deformed pipe with the crown having the least stress marked blue while the springlines have the maximum stress marked yellow-green. Influence of Buried Depth on Stresses The stresses at the pipe crown reduced with increasing buried depth across all soil types and across all EPS geofoam types. There was a 0.5% average stress reduction between 600mm and 1000mm buried depth. This is due to reduction in the impact of live loads on buried structures as depth increases in the soil. The reduction was low because the range of buried depth considered in this study was also low. Figure 3.14 Buried Depth against Stresses at Pipe Crown in Saturated Cohesive Soil This indicates that higher stress caused higher vertical deflection in buried PVC pipes. It therefore gives credence to the rule that it is better to bury pipe deeper in the ground. However, it is necessary to consider other factors when considering buried depth of pipeline in actual construction field. Figure 3.15 Buried Depth against Stresses at Pipe Crown in Moist Cohesive Soil Figure 3.12 Buried Depth against Stresses at Pipe Crown in Moist Sandy Soil Figures show the stresses of buried pipe crown for EPS 19, EPS 29, EPS 46 and No EPS conditions in saturated sandy, moist sandy, saturated cohesive and moist cohesive soil. The pipe in the sandy soil had higher stresses than those in corresponding cohesive soil at all buried depths considered in this study. This is because the sandy soil had a higher friction angle when compared with the cohesive soil. Also, the pipes in the saturated soils had lower stresses compared to those in moist soils. 111

8 Influence of EPS Geofoam types on Stresses There was a general reduction in the stresses of the pipe crown with each type of EPS geofoam when compared with the No Geofoam condition. Figures shows the stresses at the pipe crown with each type of EPS Geofoam and the No Geofoam condition. The EPS 19 case had the least stress of pipe crown at all the buried depths and soil types considered in this study. This is because it has the least density and least Young s modulus, making it more compressible when compared to the other types of EPS Geofoam. Averagely, there was an increase in stresses at the PVC pipe crown with increasing density of the EPS Geofoam. There was a 0.14% reduction in stresses of the pipe crown with the use of the EPS 19 Geofoam on the average. Figure 3.19 Stresses of 900mm buried Pipe Crown Figure 3.20 Stresses of 1000mm buried Pipe Crown Figure3.16 Stresses of 600mm buried Pipe Crown Figure 3.17 Stresses of 700mm buried Pipe Crown Figure 3.18 Stresses of 800mm buried Pipe Crown 112 CONCLUSIONS AND RECOMMENDATIONS The following conclusions can be drawn from this study 1. With increasing buried depth of a PVCpipe, there is a reduction in the vertical deflection and stresses at the crown of the buried PVC pipe. 2. The introduction of an EPS geofoam layer in the backfill causes a reduction in the vertical deflection and stresses at the crown of the buried PVC pipe. Therefore EPS geofoam be used with buried pipes for reduction of stresses and vertical deflection. 3. The most compressible EPS geofoam gives the highest reduction in vertical deflection and stresses at the crown of the buried PVC pipe. 4. The type of soil (sandy or cohesive) and its moisture content affect the vertical deflection and crown stresses in Buried PVC pipes. The following tasks are recommended for possible future studies: 1. An experimental analysis can be conducted using the same dimensions in this research to validate the results obtained in this research. 2. More types of EPS geofoam, layers of EPS geofoams and different sizes of PVC pipes can be used to investigate the influence of geofoams and pipe diameter on underground pipe deflection and stresses.

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