SDMSDocID 2027669SDMSDocID 2027669

2027669

Failure Analysis Associates

Stress Analysis of theSunrise Mountain LandfillPipeline

ExponentFailure Analysis Associates"

Stress Analysis of theSunrise Mountain LandfillPipeline

Prepared for

Republic Services of Southern Nevada770 East Sahara AvenueLas Vegas, Nevada 89104

Prepared by

Exponent320 Goddard, Suite 200Irvine, CA 92618

December 19, 2003

Doc. no. OC10290.001 GOTO 1203 RAP1

Contents

Page

List of Figures iii

List of Tables iv

Background 1

Finite Element Model 2

Applied Loads 3

Seismic Loads 3

Wind Loads 3

Operational Loads 3

Applied Load Summary 4Pipe Reaction Loads 4Pipe Stresses 4

Allowable Loads 6

Allowable Pipe Stresses 6

Allowable Soil Reactions 6

Conclusion 7

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List of Figures

Page

Figure 1. Overview of concrete cradle for anchor blocks 2 to 19 8

Figure 2. Two concrete cradle blocks forming the anchor blocks at position 2 to19 9

Figure 3. Steel reinforcement and steel anchors for concrete cradles 10

Figure 4. Overview of complete Finite Element Model (x-axis chosen pointingnorth and the z-axis pointing west) 11

Figure 5. Typical detail of Finite Element Model at cradle blocks withboundary conditions 12

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List of Tables

Page

Table 1. Summary of reaction loads (absolute values) for the three relevantload cases: the wind load case and two thermal expansion cases forpipe temperatures of 130°F and 150°F 13

Table 2. Summary of individual allowable soil reaction loads at each concretecradle block 13

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Background

The Sunrise Mountain Landfill pipeline consists of a 36-inch diameter pipe with aDimension Ratio (DR) of 32.5, and is made of a High Density Polyethylene (HOPE)plastic. At approximately every 300 feet, a pair of concrete cradle blocks anchors thepipeline1. Each cradle block is 5 feet wide, 3 feet long, and 3.5 feet high. The distancebetween the midpoints of each cradle of the pair is approximately 7 feet. Each cradleblock is embedded 3 feet into the surrounding gravel (i.e., ballast). An overview of ananchor block is given in Figure 1 and a typical pair of concrete cradle blocks is shown inFigure 2. A 24-inch wide steel strap secures the pipe to the concrete cradle block via 4steel anchors that transfer the loads to the steel reinforced concrete (see Figure 3). Toreduce stress concentrations at the edges of the strap and cradle, the pipe is wrapped witha flexible rubber pad that is approximately 40-inch wide and '/i-inch thick.

As part of the design, the pipeline was analyzed to meet earthquake requirements setforth in the 1997 edition of the Uniform Building Code (UBC). In addition, wind loadsand daily thermal cycling were considered in the design. For the purpose of accurateanalysis and minimization of the size and weight of the concrete cradle blocks, a detailedFinite Element Model (FEM) of the pipeline was developed. An overview of the modelis shown in Figure 4 and a typical model detail of the cradle block is shown in Figure 5.

The layout of the pipeline is such that the pipe serpentines, with arcs between eachanchor block (see Figure 4). The radius of each of those arcs ranges from 350 feet to 685feet. This allows the pipe to better accommodate thermal expansion. The serpentinelayout is expected to exhibit smaller lateral deflections at each midpoint between anchors(arc midpoint for serpentine layout) than would a straight HDPE-pipeline2 in buckling.The computed lateral pipe deflections3 at the arc's midpoint are expected to be smallerthan 55 inches and 75 inches for pipe temperatures up to 130°F and 150°F, respectively.Secondly, the pipe is axially more compliant than a straight HDPE-pipeline, thusreducing thermally induced stresses on the pipe and producing significantly smaller axialnormal stresses upon cooling below the installation temperature. For additionalflexibility considerations, the ballast system is designed to relieve excessive loads bydisplacement of the gravel.

1 An anchor block consists of two concrete cradles. Each anchor block is numbered from 2 to 19. Thepipeline has a total of 18 anchor blocks with 36 concrete cradle blocks. There is no concrete anchor blockat the above ground pipe inlet at position 1.2 It is assumed that the spacing of the anchor points is sufficiently wide to allow for buckling, otherwise theinduced thermal stresses will be very large and exceed the long-term fatigue limit of the pipe material.3 The pipe bows out at the arc's midpoint. This lateral deflection decreases to zero towards each anchorblock.

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Finite Element Model

A FEM of the above ground section of the pipeline was built using the commerciallyavailable pipe stress analysis software CAESAR II. The model follows the contour of thelandfill and reproduces the serpentine layout of the pipe. Approximately 850 elementswere used to model the roughly 5,000-foot long pipeline. To model the load transferfrom the pipe to the ballast, each node representing a concrete cradle block waselastically suspended in a horizontal plane and given a rotational stiffness with respect tothe vertical axis. Model and model assumptions for the seismic, wind, and thermalloadings are the same, with the following exceptions:

1. The pipe friction between the pipe and ballast system wasconservatively disregarded for the wind and seismic load cases.

2. Elastic modulus for the wind and seismic load cases was taken as130,000 psi.

3. Elastic moduli for the thermal expansion model were taken as 75,000psi and 85,000 psi for temperature differentials of 50°F and 70°F,which is commensurate with pipe temperature changes varying from80°F to 130°F or 80°F to 150°F, respectively.

The analysis provides operational stresses of the pipeline and ballast reaction forces.Computed forces serve to size the cradle blocks, pipe straps, and bolts. Several designanalysis iterations were performed to develop the current pipeline layout and anchoringsystem. Numerically computed pipe stresses are in good agreement with analyticallypredicted bending stresses due to the pipeline's thermal expansion. It is noted that thecomputed values must be superimposed on the installation stresses. However, thesestresses decay rapidly as the pipe's material relaxes over time. This relaxation ischaracterized by the literature-reported4, significantly lower long-term elastic modulus of30,0005 psi versus the larger short-term modulus of 130,000 psi at 70°F. Thus, the initialinstallation stresses and reaction forces will decrease over time. Yearly averagetemperature fluctuation between winter and summer are less severe than the analyzeddaily heating cycles since they occur over a long period of time, for which the elasticmodulus is smaller.

4 Plastic Pipe Institute, "Above Ground Applications for Polyethylene Pipe".5 This value was used to estimate the long-term pipe stresses due to installation alone.

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Applied Loads

As indicated above, load cases for the pipeline include seismic, wind, and daily thermalchanges. Wind loads and thermal forces provide the governing load case for the pipeline,since the seismic load case generates smaller stresses and reaction forces. The reactionloads for the governing load cases are summarized in Table 1, where the reaction loads inthe x-direction are aligned north-south and the reaction loads in the z-direction arealigned east-west. The resultant is the vector product of the two listed reaction forces andthe moment is with respect to the vertical axis.

Seismic Loads

Seismic loads were determined per Section 1632 of the UBC. Due to the nature anddesign of the storm drain pipeline, the seismic importance factor was taken as 1.0. Thisyields a lateral seismic design coefficient of 0.17g. It is noted that the computed seismicloads are always less than computed wind and thermal loads. Thus, seismic design forcesnever govern the overall design.

Wind Loads

The metrological data gathered at McCarran International Airport from 1961 until 1990indicate that the strongest winds are less than 60 miles per hour (mph) and typically fromthe south to southeast. Using these data one can compute an average drag force of lessthan 5 lb/ft2 for a cylinder in free perpendicular flow. Considering the pipeline'salignment with respect to the predominant wind direction further reduces the drag force.

Aerodynamic lift on the pipeline reduces the effective weight of the pipe, and thusreduces frictional restraint. Therefore, a conservative, worst-case analysis of the structurewas performed assuming no friction between the pipe and ballast. For design purposes, alateral distributed force of approximately 10 lb/ft was used. This distributed load isequivalent to a lateral wind velocity of 58 mph. Computed cradle block reactions aresummarized in Table 1.

Operational Loads

The daily heating and cooling of the pipeline induces significant thermal expansion andcontraction of the pipe. A detailed study of the encountered reaction forces andoperational stresses was conducted.

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Applied Load Summary

Pipe Reaction Loads

Thermal expansion forces at pipe temperatures of 150°F generate the largest reactions.Specifically, the pair of cradle blocks at anchor block locations 18 and 19 carry thehighest loads.

Generally, the wind load case only generates small moments and large axial forces withrespect to the cradle block, whereas the thermal expansion load cases are governed by themoment. Thus, the highest loaded cradle blocks are at position 18 and 19, which is dueto the pipe's layout and dictated by the site's local topography. All other cradle blocksare loaded at lower values and will have larger safety factors since the cradle blocks areall the same (see Table 1). The reaction loads presented in Table 1 were computed with astiff connection at the pipe's transition into the ground downstream of anchor block 19.A more compliant transition will yield larger reaction loads at cradle block 19 South, butwill reduce the reaction loads at cradle block 19 North. Since the model with the stifftransition yields the more conservative results, the following discussion is based on thisanalysis.

Pipe Stresses

Pipe stresses are composed of two types: the alternating stress due to the operation, andthe mean stress due to installation. Installation stresses depend mostly on the radius ofcurvature and are highest for sections with the smallest radius of curvature. Thepipeline's smallest radius of curvature is 350 feet, which induces a maximum mean stressof approximately 130 psi.

Wind loads generate alternating stresses of approximately 80 psi maximum. Themaximum total pipe stress is thus expected to be less than 210 psi during such an event.

For the worst-case thermal expansion load case (150°F), the pipe's maximum alternatingstress is computed to be 430 psi at anchor block 19. This stress drops to approximately325 psi for a pipe temperature of 130°F. However, at cradle blocks, the alternating stressneeds to be superimposed on the mean stress value. Between anchor block 18 and 19,this mean stress is approximately 66 psi. Thus, the maximum total stress is 391 psi and496 psi for a pipe temperature of 130°F and 150°F, respectively. For the rest of the pipe,thermally induced stresses are smaller. The second highest stress is encountered atanchor block 18 with an alternating stress of 382 psi and a mean stress of 75 psi, givingrise to a total maximum stress of 457 psi at 150°F. At the remaining anchor blocks, thetotal stress is less than 420 psi and typically in the 300 psi range for a temperature of150°F. Smaller stresses are predicted at a pipe temperature of 130°F.

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Another characteristic location is the arc's midpoint between each anchor block. There,total stresses are less than 320 psi, and typically in the 200 to 300 psi range fortemperatures of 150°F.

Between the pair of cradle blocks, alternating stresses are typically below 50 psi andsince the pipe is initially straight at this location, no installation stresses (mean stress)need to be superimposed. Thus, total stresses are predicted to be generally the lowest atthis location.

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Allowable Loads

Allowable loads were investigated considering two main cases: 1) allowable stresses forthe HDPE pipe, and 2) allowable bearing capacity of the pipeline's load retention system.For the latter, the analysis focused on the interaction loads between the ballast andindividual concrete cradle blocks, since the concrete cradles and straps have beendesigned with a large margin of safety.

Allowable Pipe Stresses

The pipe manufacturer quotes an ultimate tensile stress6 of 3,200 psi for the HDPE. Thepredicted pipe stresses are at all times several orders of magnitude smaller than theultimate tensile stress. However, fatigue due to daily cycling needs to be considered inthe analysis. The manufacturer provides a long-term allowable stress7 of 800 psi forpressurized above ground operation of the HDPE pipe at 140°F. Additional research didnot reveal specific references for the same HDPE pipe material and operating conditions.The study indicates that the 50,000 cycle fatigue strength8 of HDPE's is 375 psi attemperatures of 176°F. Based on these data, the long-term fatigue strength (alternatingstress) of the pipeline would be 450 psi for an approximate 30-year service life with dailycycling. It is noted that the pipe manufacturer states that the HDPE pipe material isresilient to UV-radiation.

Allowable Soil Reactions

The allowable soil reaction forces are given in Table 2. Soil reaction forces are based onconservative estimates of the active and passive soil pressures, but include friction due tothe weight of the cradle block.

6Tensile Strength per ASTM D-638, "DriscoPlex PE 3408 HDPE Data Sheet", Bulletin: PP 109,DriscoPlex, 2002.7 Engineering Manual, Chapter 2 Stress Rated Materials, page 40, Chevron Phillips Chemical CompanyLP, 2002.8 The investigators conducted these tests under highly accelerated conditions. If one considers one cycleper day, then a total of 50,000 cycles would theoretically yield a life larger than 100 years.

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Conclusion

Pipe stresses, as limited by the design features described herein, should provide long-termservice life. Fatigue of the pipe is expected to be a maximum at the end of the pipe,downstream of anchor block 17.

The concrete cradle blocks and the ballast system are expected to sustain typicallyencountered loads (i.e., seismic, wind, and thermal expansion and contraction loads).The most demanding loading condition is given by thermal cycling of the pipe up totemperatures of 150°F. To prevent any excessive damage accumulation in the pipe, theconcrete cradle blocks are designed to allow small relative rotations if such conditionsshould ever be reached. Cradle reaction forces due to the flow of water are very small incomparison to the other load cases.

In summary, the design was intended to provide a pipeline with a long-term fatigue life inconnection with an anchoring system that does not penetrate the landfill's cover and isunduly heavy. This is achieved by the pipeline layout and location of concrete cradleblocks presented in the design.

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ANCHORSTRAP

ANCHOR BOLTS

EXISTINGGROUND

PIPE ANCHOR SYSTEM

DETAILSCALE: NOT TO SCALE

Figure 1. Overview of concrete cradle for anchor blocks 2 to 19

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PIPE ANCHORSYSTEM

NOTE:1. CONTROL POINT AS SHOWN ON

TABLES ON DRAWINGS 3 AND 4.

DETAILSCALE: NOT TO SCALE

Figure 2. Two concrete cradle blocks forming the anchor blocks at position 2 to 19

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Figure 3. Steel reinforcement and steel anchors for concrete cradles

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Anchor Block 19

Figure 4. Overview of complete Finite Element Model (x-axis chosen pointing northand the z-axis pointing west)

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Figure 5. Typical detail of Finite Element Model at cradle blocks with boundaryconditions

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Table 1. Summary of reaction loads (absolute values) for the three relevant load cases: thewind load case and two thermal expansion cases for pipe temperatures of 130°F and150°F

Cradle Id

Cradle19191818171716161515141413131212111110109988776655443322

N/SSNSNSNSNSNSNSNSNSNSNSNSNSNSNSNSNSNSN

Wind Load Case

X-Reactionin (Ibf)2531994177410131624024430444024714473947314690502449944022418246604717474647074643440047464702463246935080506644554655430743214580443838243779

Z-Reactionin (Ibf)10883910601229363925617328461731137421672406384297273223869561067142312762204277746176317131523263713268288437250824792716128243

Resultantin (Ibf)2758624309428148214770436644835022493452045271503950034862481547574836495548775140520347464705561356535090508055255476432343515208520338263787

Momentin (Ibft)605145914145835923034023526731188357314107196184260318196173338437332997040044115415921636642224268408

Thermal Expansion Load Case 150F

X-Reactionin (Ibf)354644333795700023331914498392853038462965391267340996911765433190465341672829473298146399155738903906221156636703042116444825174750

Z-Reactionin (Ibf)111589786430492440107153432741384522539552854233402943084442522552494780453951275992451149794881577853493659375546215899368937565855572439173804

Resultantin (Ibf)3717100137466855846397405659942414553662660604251483559464495551568054865458666076626453658036734585053495340541846266103520448335970574246566085

Momentin (Ibft)38359407460656652713382312443974576338635344194414236943708348136324870486334653296459252874848408240515087500938283386369137564091457842833415

Thermal Expansion Load Case 130F

X-Reactionin (Ibf)243728612577505621751452379913899287721961731627294218211753225189173090207279718753353574260266326805401091255319775944814973466

Z-Reactionin (Ibf)80164394896374526755425326430543273409939973007301132913175391539993541333838834542319338553784423938202804289832374431279828514378429929692901

Resultantin (Ibf)256570465533629234485616500930573274500845613012342244143180408851373546333849624992329142875056427838293867394732824563378834694418429933254520

Momentin (Ibft)31241425290472237402508229831113285249625793000298826842681256126903505349225372464328138613577293429023643358427582491269427063069339130782503

Table 2. Summary of individual allowable soil reaction loads at each concrete cradleblock

IAxiai horcein Ibf

Allowables | 10000

Lateral Forcein Ibf9000

uomDinedForce in Ibf

13500

Moment inlb*ft

13400

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