Deep braced cuts utilities.pdf

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Technical Report Documentation Page

1. Report No. A450, A464 2. Government Accession No. 3. Recipient's Catalog No.

5. Report Date December, 20034. Title and Subtitle Analysis of Effects of Deep Braced Excavations onAdjacent Buried Utilities

7. Author/s Richard J. Finno, Kristin M. Molnar, Edwin C. Rossow 8. Performing Organization Report No.

10. Work Unit No. (TRAIS)9. Performing Organization Name and AddressDepartment of Civil and Environmental Engineering

Northwestern University

2145 Sheridan Road

Evanston, IL 60208

11. Contract or Grant No.

DTRS98-G-0016

13. Type of Report and Period CoveredFinal Report, April 1, 2002

September 30, 2003

12. Sponsoring Organization Name and Address

U.S. Department of Transportation

Research and Special Programs

Administration

400 7thStreet, SW

Washington, DC 20590-0001

14. Sponsoring Agency Code

15. Supplementary Notes

16. Abstract Ground movements resulting from deep braced excavations impose the risk of damage toadjacent buried pipelines. Accurate assessment of the effects these movements have on pipelines allows

potential damage to be avoided or mitigated. A predictive process for determining the stresses occurring

in a pipeline adjacent to deep braced excavations is presented. The method can be used to establishrational criteria for determining allowable maximum values for excavation-induced ground movements.

The ground movement distribution around the excavated area is predicted using a complimentary error

function, an assessment of the maximum ground deformation, and knowledge of the geometry of the

excavation. The pipeline is assumed to move with the ground enabling the behavior of the pipeline to be

represented by the ground surface movements at its location. Conservative analyses for determining thebending stresses and joint rotations along a pipeline caused by its deformation are established. Allowable

values for both the tensile bending stress and joint rotation resulting from the excavation-induced

movements are presented for comparison with the computed maximum values. The predictive

methodology is applied to three gas mains surrounding a deep braced excavation in downtown Chicago

and four cast iron mains from various excavations in Chicago. For these cases, the calculated bending

stresses in the pipelines were significantly smaller than allowable values, but the joint rotations were

observed to be the more critical case.

17. Key Words Excavation, Pipelines,

Deformation, Seismicity, Infrastructure

18. Distribution Statement No Restrictions

9. Security Classification (of this report) Unclassified 20. Security Classification (of this page)Unclassified

21. No. Of Pages 168 22. Price


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ANALYSIS OF EFFECTS OF DEEP BRACED

EXCAVATIONS ON ADJACENT BURIED UTILITIES

By

Kristin M. Molnar

Richard J. FinnoEdwin C. Rossow

By

School of Civil and Environmental Engineering

Northwestern University

Evanston, IL

December, 2003


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ABSTRACT

Ground movements resulting from deep braced excavations impose the risk of damage to

adjacent buried pipelines. Accurate assessment of the effects these movements have on pipelines

allows potential damage to be avoided or mitigated. A predictive process for determining the

stresses occurring in a pipeline adjacent to deep braced excavations is presented. The method

can be used to establish rational criteria for determining allowable maximum values for

excavation-induced ground movements. The ground movement distribution around the

excavated area is predicted using a complimentary error function, an assessment of the maximum

ground deformation, and knowledge of the geometry of the excavation. The pipeline is assumed

to move with the ground enabling the behavior of the pipeline to be represented by the ground

surface movements at its location. Conservative analyses for determining the bending stresses

and joint rotations along a pipeline caused by its deformation are established. Allowable values

for both the tensile bending stress and joint rotation resulting from the excavation-induced

movements are presented for comparison with the computed maximum values. The predictive

methodology is applied to three gas mains surrounding a deep braced excavation in downtown

Chicago and four cast iron mains from various excavations in Chicago. For these cases, the

calculated bending stresses in the pipelines were significantly smaller than allowable values, but

the joint rotations were observed to be the more critical case. Excessive leakage was observed at

a rotation of 6 x 10-3

radians for a cast iron pipeline with lead caulked joints.

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ACKNOWLEDGEMENTS

A number of people and organizations were instrumental in providing the data from the

Lurie Research Center project that form the basis of this work. Inclinometer data were obtained

by Construction Testing & Instruments, Inc., and Professionals Associated obtained the vertical

and lateral survey data. Turner Construction Company was the general contractor and Case

Foundation Company was the excavation support subcontractor. The help and interest of Dr.

Jerry Parola and Ms. Dhooli Raj of Case and Mr. Ron McAllister of Turner made this work

possible. Mr. John Brzezinski and Ms. Jo LeMieux-Murphy of the Facility Management group

at Northwestern provided access to the project and were very helpful throughout its duration.

Northwestern University students who generously gave their time to assist in the field

monitoring effort included Frank Voss, Tanner Blackburn, and Terry Holman. Jill Roboski of

Northwestern University developed the method to estimate surface settlement profiles along

lines parallel to an excavation. The authors also thank Professor Thomas ORourke from Cornell

University who generously provided us with a number of reports he authored related to pipelines.

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TABLE OF CONTENTS

Abstract i

Acknowledgements ii

Table of Contents iii

List of Tables iv

List of Figures v

List of Symbols vi

INTRODUCTION 1

BACKGROUND 2

Pipe Material 2Joints 4

Initial Stresses 5

ANALYSIS OF PIPES 6

Approach 6

Bending Stresses 8Joint Rotations 11

Allowable Stresses and Joint Rotations 12

PREDICTION OF EXCAVATION-INDUCED MOVEMENTS 15

Summary of Procedure 15

CASE STUDIES 16

Lurie Center 16

Chicago Excavation Case Studies 19

CONCLUSIONS 21

REFERENCES 23

TABLES 26

FIGURES 32

APPENDIX A: LURIE CENTER DATA 40

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LIST OF TABLES

Table 1 Engineering Properties for Piping Materials 26

Table 2 Failure Rotations for Selected Cast Iron and Ductile

Iron Joints 27

Table 3 Allowable Bending Stresses from Excavation-Induced

Movements 28

Table 4 Allowable Joint Rotations for Cast Iron and Ductile

Iron Joints 29

Table 5 Strength Reductions at Location of Line Pipe Welded

Joints 30

Table 6 Description of Cast Iron Pipelines Parallel to ChicagoExcavations 31

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LIST OF FIGURES

Figure 1 Schematic of Joint Rotation for Joint j of Rigid Pipeline 32

Figure 2 General Layout of Lurie Center Site Instrumentation and

Adjacent Gas Mains 33

Figure 3 Comparison of Ground and Pipeline Movement during

Excavation at Lurie Center 34

Figure 4 Ground Displacements at Location of Gas Mains along North,

South and West Walls after Completion of Excavation 35

Figure 5 Maximum Tensile Stress in Pipelines Adjacent to North, South

and West Wall During Excavation 36

Figure 6 Maximum Relative Rotation Encountered Along North, South,and West Pipelines During Excavation for 3.6m Pipe Sections 37

Figure 7 Observed and Predicted Maximum Bending Stresses and Joint

Rotations for Final Stages of Construction 38

Figure 8 Rotations in Cast Iron Pipelines Adjacent to Excavations in

Chicago 39

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LIST OF SYMBOLS

A0, A2, B2 coefficients for no slippage between pipe and soil equations

A0*, A2*, B2* coefficients for slippage between pipe and soil equations

B dimensionless constant related to lateral stress ratio

C dimensionless constant related to lateral stress ratio

dc thickness of caulking in joint

dc/ds rotation to cause metal binding failure in lead caulked cast iron joints

di inner diameter of pipe cross section

dl depth of lead

do outer diameter of pipe cross section

ds depth of bell

dw depth of packing material

distance between rubber gasket and end of pipe

E modulus of elasticity

EA circumferential extensional stiffness per unit length

EI circumferential bending stiffness per unit length

Fu ultimate stress

Fy yield stress

h relative distance from data point to node of grid for radial basis interpolation

H depth of pipe

HDB hydrostatic design basis value

I moment of inertia of cross section

j stress exponent

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K lateral stress ratio

L length of diagonal of extent of data range for radial basis interpolation

original length

Lji distance along pipeline between points i and j (i,j = 1, 2, 3, n)

m modulus number

M moment

Newmark coefficient

Ms constrained modulus of soil

Mt tangent modulus of soil

Mx moment around x-axis

Mz moment around z-axis

n number of survey points along pipeline

N number of data points in radial basis interpolation

thrust

Boussinesq coefficient

p internal pressure

Pr interaction load at interface of pipe and soil

q uniform pressure

r mean radius of pipe

R horizontal distance from top of pipe to application of load

R2 smoothing factor for radial basis interpolation

t wall thickness

Tr interaction shear at interface of pipe and soil

UF extensional flexibility ratio

VF bending flexibility ratio

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w curvature of deflection curve w

W concentrated wheel load

xi distance to extreme fiber at point i under Mz(i = 1, 2, 3, n)

x(Yi) curvature of lateral movement profile at point i (i = 1, 2, 3, n)

Xi total lateral movement at survey point i (i = 1, 2, 3, n)

Yi distance from origin to survey point i along pipeline (i = 1, 2, 3, n)

z distance from centroidal axis of deflection curve

zi distance to extreme fiber at point i under Mx(i = 1, 2, 3, n)

z(Yi) curvature of vertical movement profile at point i (i = 1, 2, 3, n)

Zi total vertical movement at survey point i (i = 1, 2, 3, n)

coefficient of thermal expansion

i relative rotation between two adjacent pipe sections at joint i

L change in length

T change in temperature

strain

ji differential lateral movement between points i and j (i,j = 1, 2, 3, n)

Poissons ratio

angle from horizontal of pipe cross section

ALLOW allowable joint rotation from excavation-induced ground movements

i angle from horizontal for cross section of pipe at point i (i = 1, 2, 3, n)

imax angle of principal plane of pipe cross section at point i (i = 1, 2, 3, n)

L rotation to cause excessive leakage in lead caulked joint for analysis

max rotation to cause metal binding failure in rubber gasket iron joints

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M rotation to cause metal binding failure in rubber gasket joints for analysis

radius of curvature of centroidal axis of deflection curve

ji differential vertical movement between points i and j (i,j = 1, 2, 3, n)

effective stress of soil

ALLOW allowable stress from excavation-induced ground movements

B design bending stress of pipe material

H hoop stress (circumferential stress)

i bending stress at point i (i = 1, 2, 3, n)

INITIAL initial stress in pipeline prior to stress analysis

imax maximum longitudinal bending stress in pipe cross section at point I

r reference stress (atmospheric pressure = 100kPa = 1atm)

v vertical soil stress

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INTRODUCTION

Buried pipelines within urban environments are exposed to large ground movements as

the result of adjacent excavations. These excavations may include tunneling, trenching, or deep

open cuts. The ground movements resulting from these conditions impose stresses on the

pipeline that could lead to its failure. Deep braced excavations are known to cause significant

ground movements at great distances away.

Most commonly buried utilities are exposed to ground movements from trench

construction for the installation or repair of an adjacent pipeline. Much work has been done in

this area to determine the patterns of movement caused by trench construction and the stresses

encountered in the pipeline. Field experiments (Carder, et al. 1982, Carder and Taylor 1983 and

ORourke and Kumbhojkar 1984), beam on elastic foundation analysis (Crofts, et al. 1977 and

Tarzi, et al. 1979), and finite element simulation (Nath 1983 and Ahmed, et al. 1985) have been

performed to determine a pattern of behavior for ground movements from trench construction

and their effects on adjacent pipelines.

Information regarding the effects of pipelines adjacent to deep braced excavations is very

limited. Methods for predicting the magnitude and location of maximum ground movements

around an excavation have been determined from field observations (Clough and ORourke 1990

and Hsieh and Ou 1998). Maynard and ORourke (1977) report field observations for four cast

iron gas mains in Chicago exposed to ground movements from neighboring deep excavations.

They reported excessive leakage from one of the mains making it necessary for it to be taken out

of operation.

If excavation-induced ground movements are sufficiently large, imposed stresses on a

pipeline can cause failure. One therefore needs to determine the effects of an excavation on a

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pipeline prior to construction so that rational criteria for preventing damage can be included in

the design of the excavation support system. A method for predicting the longitudinal bending

stresses and joint rotations incurred by a pipeline from ground movements resulting from an

adjacent excavation is presented. The complimentary error function (Roboski and Finno, 2004)

is presented for producing a distribution of movements parallel to an excavation wall based on

the excavation geometry. The ground distributions are imposed on the surrounding pipelines.

Equations are derived for bending stress and joint rotation analyses to determine the distribution

of stresses along the pipeline.

Distributions of the bending stresses and joint rotations are determined by the analysis.

The maximum values can be determined and compared to allowable values determined from

previous experimental and empirical observations. This approach provides a rational method of

establishing excavation-induced ground movement limits when surrounding utilities are the

critical structure impacted by the excavation.

BACKGROUND

Pipe Materials

The most common metallic materials found in pipelines in urban areas are cast iron,

ductile iron and steel. The more modern installations use plastics because of their flexibility.

Polyethylene is a popular material for buried utility installations for gas and water transportation.

General engineering properties for these materials are presented in Table 1.

Cast iron is the oldest metal found in pipelines, and is relatively common since many

pipelines installed over 100 years ago are still in operation. Cast iron pipe was formed by pit

casting, the predominant process until the 1930s when the centrifugal cast process was

developed. Of the two manufacturing processes, centrifugal cast pipe has a greater strength due

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to a better distribution of graphite flakes within the iron matrix. The tensile stress-strain behavior

of both types of cast iron exhibits irrecoverable deformations at low strains. There is no apparent

yield point, and brittle failure occurs at relatively low strain values.

Cast iron pipe was the dominant pipe material until the 1950s when ductile iron was

introduced. The metallurgy of the materials is very similar, but ductile iron has increased

strength and ductility from carbon that exists as small spheroids. The presence of the carbon in

this form introduces fewer discontinuities into the matrix. Ductile iron also does not exhibit a

yield point. Angus (1976) observed a true elastic range of stress values in which irrecoverable

deformation did not occur. When the material begins to plastically deform, the graphite nodules

do not deform with the matrix and the useful cross sectional area is reduced which in turn

reduces the apparent pipe stiffness.

At the end of the 19th

century, steel, or line, pipe was implemented into the transport of

gas and oil. Pipe sections are manufactured as welded or seamless. Welded pipe has two halves

of pipe longitudinally welded together. Seamless pipe contains no welds and is produced by

either hot piercing or cupping and drawing.

Steel differs from cast iron and ductile iron in that it exhibits a yield point at the end of a

region of linear elastic behavior, with a well-defined modulus elasticity. The stress-strain

behavior is dependent on the carbon content within the alloy. The yield point becomes less

sharp, the yield plateau becomes less prominent, and the ultimate stress increases as the carbon

content increases.

In the more recent installations of pipelines, plastics have been used due to the great

lengths of pipe available and its flexibility and durability. Plastics are solid materials with one or

more polymeric substances that can be formed by flow (Plastics Pipe Institute, 1993). Common

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plastics used in piping are polyvinyl chloride (PVC) and polyethylene (PE). Polyethylene pipe is

commonly used for the transportation of water and gas. The behavior of polyethylene under

stress is complex due to its viscoelastic properties and its dependency on the load duration,

environment, and temperature. The shape of the stress-strain curve is highly dependent on the

load duration due to its material rate-dependent behavior. The Plastics Pipe Institute (1993)

define a flexural, short-term, and long-term modulus for polyethylene dependent on the nature of

the load and the rate at which the load is applied.

Joints

The methods of joining pipe sections also have steadily improved over time allowing

greater rotations without attendant loss of service. Joining mechanisms may be used to allow

rotations or the pipe sections may be joined to one another directly.

Table 2 show typical failure rotations for cast iron and ductile iron joints. Early

installations of cast iron pipe were joined with rigid connections and were constructed with

metal-to-metal contact within a bell and spigot connection to prevent leaks, while allowing very

little rotation. Semi-rigid joints were the more predominant mode of joining cast iron pipe and

were constructed by adding a packing and caulking material within a bell and spigot connection.

The packing material was soft, usually jute or yarn, to permit a certain amount of flexibility

within the joint. The caulking held the packing in the joint and, for most installations, was lead.

These joints were able to withstand small rotations before failure was observed. Joints allowing

exhibiting flexible behavior within a cast iron pipeline are constructed by adding a rubber gasket

to eliminate leaks; these joints allow greater rotations without loss of service. Rubber gaskets are

used in push-on and mechanical joints.

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Ductile iron pipe is predominantly joined with rubber gasket joints similar to that of cast

iron pipes. There are rubber gasket push-on joints and bolted-gland mechanical joints. For more

adverse conditions there is a ball and socket joint, which is free to rotate up to 0.27 radians (15

degrees).

Rubber gasket joints are available for steel pipe as well, but the more common method of

joining steel pipe sections is by welding. Lap, single-butt, and double-butt welds are the more

common types of joining welds and produce a joint with strength very similar to that of the

strength of steel. Of the three types of welded joints, the single-welded lap joint introduces the

greatest reduction in the strength of the steel of approximately 25 percent.

Similar to that of welding steel pipe, polyethylene is joined by fusion, wherein two ends

of adjacent pipes are melted to a fluid-like state and then forced together to join a continuous

section after cooling. Melting of the ends of the pipe can be from direct heat from a hot surface

or a coiled wire. The fusion joint is equal in strength to that of the rest of the polyethylene pipe.

Initial Stresses

The installation of a buried pipeline introduces stresses within the pipe upon which are

added additional stresses caused by movements associated with excavations. These stresses can

be a result of any combination of an operating internal pressure, the soil cover load, cyclic or

static surface loads, the installation procedure, previous ground movements, or environmental

effects.

Taki and ORourke (1983) analyzed the effects internal pressure, thermal fluctuations,

repeated loading, and installation procedures on cast iron mains. They determined typical

amounts of tensile or bending strain induced by these conditions for low pressure pipelines.

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From these calculations, they suggest assuming that a buried pipeline has an initial bending

strain value between 0.02 to 0.04 percent.

Internal pressures can cause a circumferential tensile stress due to the imbalance of

interior and exterior pressures (Timeshenko 1951). The stresses induced by the soil cover (e.g.

Carder et al. 1982; Carder and Taylor 1983) and static or cyclic loads (e.g. Pocock et al. 1980)

can cause ovalling of the pipe cross section with attendant stresses that vary around the pipe.

The installation of pipelines in the ground can create a stress or joint rotation from uneven

bedding or a curved laying pattern (e.g. Pocock et al. 1980). Prior to an adjacent construction,

the previous construction history may have resulted in movements of the pipeline causing

bending stresses and joint rotations (e.g. Maynard and ORourke 1977). The environment in

which the pipeline is buried can cause different stresses to be imposed. A pipeline installed in a

location with fluctuations in temperature can cause strains in the pipeline (Attewell 1986).

Moisture changes in the soil surrounding the pipeline can cause corrosion to occur which could

weaken the strength of the pipe walls (e.g. Sears 1986).

ANALYSIS OF PIPES

Approach

Pipelines parallel to deep excavations undergo deformations due to the displacement of

the surrounding soil. For most utilities that parallel a large excavation in an urban environment,

the pipeline can be assumed to move with the soil. Carder, et al. (1982) and Carder and Taylor

(1983) conducted field experiments with 100 mm diameter cast iron pipelines buried 0.75 m

deep parallel to trench excavations in different types of soil. They determined that the

movements of the pipelines calculated from the data obtained from strain gauges along the

pipeline were similar to that of the ground displacements observed. Nath (1983) conducted a

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three-dimensional finite element analysis on cast iron pipelines ranging from 75 to 450 mm in

diameter buried 1 m deep parallel to open trench excavations of differing dimensions. He

concluded that pipelines with diameters of 150 mm or less move with the ground providing little

or no restraint to movements, whereas larger pipes provided some restraint against the movement

of the surrounding soil.

For pipelines whose movements are consistent with the displacements of the surrounding

soil, one can make assumptions concerning joint flexibility to analyze the effects of ground

movements on the pipe. By assuming a pipe is either flexible, wherein it is assumed a pipe

connection does not affect the mechanical behavior of the pipe, or rigid, where all the movement

is assumed to occur at a joint, one can bound the response of the pipe to the imposed

deformations. The question becomes defining the critical condition, either excessive bending

stresses for the flexible condition or large rotation at a joint for the rigid condition, possibly

leading to excessive leakage or fracture at a joint. Special care should be taken when applying

this approach to larger diameter metal pipelines because they tend to restrain movements more

than that of small diameter pipelines, and the restraint provided by the pipe will result in higher

stresses.

Flexible pipe is assumed herein to move along with the ground causing bending within

the pipe sections and no rotation at the joints. With the introduction of more ductile materials,

bending stresses have become less of a concern; however, this is still of great concern for the old

cast iron mains that can fail as a brittle fracture at a low strain, or in cases of pipes subjected to

high pressures. The displacements of the pipe sections are assumed to be small allowing for

axial displacements to be neglected. The deflection within a pipe is assumed to follow the

Bernoulli-Navier theory of bending wherein the plane normal cross sections remain plane and

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perpendicular to the deflected centroidal axis and the transverse normal stresses are negligible

(Baant and Cedolin, 1991).

Rigid pipe is assumed herein to deform along with the ground displacement profiles as

rigid links connected by points that are free to rotate. The effects of the ground movements on

the pipe are concentrated in the joints as relative rotations between adjacent pipe sections. The

pipe sections are assumed to have a large flexural rigidity thus preventing any curvature to

develop. The joints are assumed to have no rotational rigidity allowing free rotation. The

rotation at the joints is assumed to be longitudinal due to bending of the pipeline, therefore

torsional behavior is neglected.

Bending Stresses

Flexible pipes exposed to ground movements develop bending stresses within the pipe

sections. The bending stresses obtained from this analysis should be compared to established

allowable values to determine the structural adequacy of the pipeline. If there are high internal

pressures, a Mohr circle analysis should be made to find the maximum of the combined stresses.

A displacement profile of the pipeline should be established within a convenient local

coordinate system, as shown in Figure 1a. The origin is situated adjacent to the corner of the

excavation, the positive x-axis represents the lateral movement towards the excavation, the

positive y-axis is the longitudinal axis of the pipeline, and the positive z-axis is directed upwards.

Following the assumption that the pipeline moves along with the ground, the lateral and vertical

ground surface movements at the location of the pipeline determine the lateral and vertical

displacement profiles of the pipeline.

Bending is of greatest concern at the location of the displacement profiles where the

curvature is the largest. The curvature is calculated at a point j from the lateral and vertical

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displacement profiles causing a differential movement of point j with respect to two adjacent

points along the pipeline by the following equations:

( )( )

( )( )

( )ki

ji

ji

kj

kj

ik

ij

ij

jk

jk

jL

LL2

YY

Y-YXX

Y-YXX2

)Y("x

=

=

(1)

( )( )

( )( )

( )ki

ji

ji

kj

kj

ik

ij

ij

jk

jk

jL

LL2

YY

Y-Y

ZZ

Y-Y

ZZ2

)Y("z

=

=

(2)

where ji= Xj-Xiis the differential lateral movement between points i and j (i,j = 1,2,3,..n), Lji=

Yj-Yiis a characteristic length defined as the distance along pipe between points i and j, and ji=

Zj-Ziis the differential settlement between points i and j. The displacements are defined in the

local coordinate system as Xiand Zirepresenting the total lateral and vertical movements at point

i, respectively, and Yiis defined as the distance from the origin along the pipeline to point i. In

defining a characteristic length, Lji, for the curvature calculations, the distance should be large

enough to reasonably reflect the curvature in the pipeline. A characteristic length of

approximately 6.1 m or greater has shown to give an adequate representation of the curvature

values for a pipeline undergoing bending.

Treating the lateral and vertical profiles separately allows a simple three-dimensional

analysis for determining the principal planes of the pipe cross sections. This provides a more

accurate calculation of the maximum stresses along the pipeline than calculations from the

resultant movements due to the continual change of the angle of the principal planes along the

pipeline.

For a pipe in bending there is a distribution of tensile and compressive stresses within the

cross section. The maximum tensile stress existing in the pipe is critical due to the greater

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strength of many pipe materials in compression. The distribution of normal stresses, i, within

the pipe cross section exposed to both lateral movements in the x-direction and vertical

movements in the z-direction, assuming the pipeline behaves as an elastic beam, is found by:

I

zM

I

xMixiz

i = (3)

in which Mxand Mzare the moments around the x- and z-axis, respectively, with the x-axis

horizontal and the z-axis is vertical, and I is the moment of inertia about the neutral axes of the

cross section of the pipe. The terms xiand zi represent the distance to the most extreme fiber

with respect to the moment. Substituting the expression for bending moment with small

deflections, M=EIw", the equation for stress at a point i along the pipeline becomes:

)(Yx"Ez)(Yz"Exiiiii

= (4)

where x"(Yi) and z"(Yi) are the curvatures of the lateral and vertical displacement profiles at

point i, respectively. To express equation (4) in terms of the pipe radius, r, and the angle from

the positive x-axis, one can write:

[ ]iiiii

)cos(Yx")sin(Yz"Er += (5)

To calculate the maximum tensile stress within the cross section, this expression has to be

maximized. By taking the derivative of the stress with respect , the expression for the angle of

the principal plane can be found as:

=

)(Yx"

)Y(z"tan

i

i1

imax (6)

The value of the maximum tensile stress is determined by substituting the results of (6) into (5).

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Joint Rotations

It is assumed in this limiting case that rigid pipelines conform to the ground movements

through rotations at the joints due to the infinite stiffness of the pipe sections. To determine if

there is failure at a joint, the relative rotation between the two adjacent pipe sections needs to be

calculated. Since the pipeline is assumed to follow the ground movements, the joints are located

along the displacement profile at distances equal to the length of the pipe sections. The

differential movements, as defined earlier, need to be determined to determine the change in

slope along the pipeline at the joint.

The rotation at a joint can be calculated using vector mechanics. The pipe sections can

be represented as vectors that intersect at the joint. The angle between them can be calculated if

the differential movements are known. Figure 1b shows the vector representation of two

adjacent pipe sections along a pipeline that have undergone both lateral and vertical movements

relative to joint j. The differential lateral and vertical displacements, jiand ji, are as previously

defined for the bending stress analysis. The characteristic length, Lji, is defined by the pipe

section length of the pipeline. The majority of cast iron mains consist of pies 3.6 m in length.

Ductile iron pipe sections range in length of 5.5 to 6.1 m. If the pipe section length is unknown,

a conservative assumption of 6.1 m should be used. This will yield the largest rotations because

of the greater differential displacements. Using this established convention, the rotation at joint

j, j, can be calculated using the following expression:

( ) ( )2kj

2

kj

2

kj

2

ji

2

ji

2

ji

kjjikjjikjji1

jcos

++++

++=

LL

LL (7)

For buried pipelines, the locations of the joints may be unknown. In this case, in order to

determine the maximum potential joint rotation along a pipeline, a joint should be assumed at

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one end of the displacement profile. Assuming small displacements, the next joint should be

located on the displacement profile a distance of a pipe section length along the pipeline. The

line connecting these two points represents the pipe section between these two points. This

should be continued to the other end of the profile. Once the rotations are calculated, the same

procedure should be repeated after offsetting the location of the first joint. Multiple analyses

should be completed with the offset increasing until it is as long as the length of a pipe section.

From these analyses, the maximum potential joint rotation can be determined.

Allowable Stresses and Joint Rotations

The failure of the pipeline from excavation-induced ground movements can occur from

excessive bending stresses or large rotations at the joints. Stresses and rotations from preexisting

conditions must be considered when allowable values for imposed stresses, ALLOW, and

rotations, ALLOW, are established, for bending stresses and rotations, respectively. These

allowable values are presented in Tables 3 and 4.

For cast iron pipe, the failure in a pipe due to bending occurs as an abrupt brittle fracture.

Attewell et al. (1986) recommend a maximum design stress for cast iron under direct tensile load

equal to one-quarter of the ultimate tensile strength of the material. For cast iron exposed to

bending, a rupture factor of 1.6 needs to be applied which results in a maximum design stress

equal to 40 percent of the ultimate tensile strength of the material or a factor of safety of 2.5.

For ductile iron in bending, which is a much more flexible material than cast iron, the

Ductile Iron Pipe Research Association (2001) recommends using a factor of safety of 2.0 for

calculations involving bending. Attewell, et al. (1986) recommend using a value of 85 percent of

the minimum yield strength of the material since the failure of the material would not occur at

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the yield point, but when plastic yielding exists through the section. This definition corresponds

to a safety factor of 1.2.

Steel line pipe is usually the material used in the transportation of oil and gas under

extremely high internal pressures. Under these conditions, a combined stress analysis should be

used to determine an allowable design stress. For safety considerations, pipelines installed in

urban environments generally are maintained at low internal pressures and a combined stress

analysis is unnecessary. Steel pipe for these installations can be assumed to follow the ground

movements behaving as a continuously supported beam. For laterally supported beams

subjected to bending moments, the allowable stress can be calculated as 60 percent of the yield

stress which is equivalent to a factor of safety of 1.67.

Polyethylene pipe design strengths are established through an internal pressure analysis.

The Plastics Pipe Institute (2000) recommends using a hydrostatic design basis value to

determine a limiting strength for the pipe material. They recommend using a factor of safety of 2

resulting in allowable hydrostatic design values for PE80 and PE100 grades as 4.3 MPa and 5.5

MPa.

Failure at a pipe joint can occur in the form of an excessive amount of leakage or a

fracture of the pipe joint itself. These are critical behaviors for only cast iron and ductile iron

pipelines since steel and plastic pipe joining procedures ensure joints that are of equivalent

strength to that of the material. Experimental data have been obtained to determine rotations at

which failure of a joint could occur.

The majority of cast iron pipe installations are joined with lead-caulked joints. These

semi-rigid joints allow some rotation because of the soft packing material, but failure can occur

when the packing is forced from the joint resulting in excessive leakage. Fracture of the joint

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can occur when the rotation is large enough to cause metal-to-metal contact between the bell and

the spigot inducing large bending stresses. Maynard and ORourke (1977) observed excessive

leakage from a cast iron main exposed to ground movements with a joint rotation of 0.006

radians (0.34 degrees). With the application of a safety factor of 1.25 to be conservative, this can

define an allowable ground movement-induced rotation of approximately 0.0048 radians (0.275

degrees).

For rubber gasket joints for both cast iron and ductile iron the failure at the joint occurs as

metal-to-metal contact. This rotation is dependent on the size of the pipe leading to different

dimensions within the joint. Leakage is no longer a primary concern due to the flexibility of the

rubber material to seal holes that could possibly form from movement at the joint. From

observed conditions of cast iron and ductile iron mains with flexible joints following installation,

Attewell, et al. (1986) suggest assuming an initial rotation within the joint up to 0.026 radians

(1.5 degrees). This value is reflected in the allowable ground movement-induced rotations

shown in Table 4.

The majority of steel pipe is joined by welding with a minimal loss in strength along the

pipeline from this method. Table 5 shows typical percentages for the reduction in strength for a

steel pipe for different welds presented by Watkins and Anderson (2000). The greatest strength

reduction of 25 percent of the strength of the steel is for single welded lap joints. This strength

reduction should be applied to the design bending stress for the pipe material at the location of

the joint for the bending stress analysis.

The thermoplastic behavior of polyethylene allows for the material to be heated and

reformed to another shape without losing strength. The fusing of two pipes by heat or

electrofusion produces a joint with equal or greater strength of that of the material.

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PREDICTION OF EXCAVATION-INDUCED GROUND MOVEMENTS

Semi-empirical methods have been developed for determining the values of maximum

movement resulting from a deep braced excavation. For application of the stress analysis

presented earlier, the distribution of the movements along the excavation are necessary for

determining the displacement profile of the pipeline.

A method for predicting the distributions of the lateral and vertical movements by the

application of the complementary error function is presented by Roboski and Finno (2004).

They determined that the ground movement distribution might be adequately represented with

the following formula:

=B

Ax

2

11)x(

maxerfc (8)

where (x) is the settlement or lateral movement at distance x from the corner of the wall, maxis

the maximum movement, A is the distance to the inflection point of the function to the corner of

the wall, and B is an empirical shape factor. Positive values of settlement should be used and

lateral movement should be considered positive towards the excavation. The value of A is

determined from a relationship with the ratio of the depth of the excavation to the length of the

wall, He/L. The value of B can be calculated from the value of A from the following expression:

8.2

A2

L

B

= (9)

A thorough explanation of the derivation and application of this procedure is presented by

Roboski and Finno (2004).

Summary of Procedure

To determine the magnitudes of stresses and rotations in a pipeline caused by excavation-

induced movements:

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1. Determine the maximum lateral ground movements from semi-empirical or detailed

methods. Estimate the maximum vertical movement from the value of the lateral

movement, and develop a proposed ground surface settlement profile at the pipeline

location based on procedures summarized in the previous section.

2. The stresses and rotations in the pipeline can be determined from the two limiting

conditions. The pipeline is assumed flexible and a bending analysis is conducted to

determine the maximum tensile stress occurring in the pipe. The pipeline is then

assumed to behave rigidly, and a joint rotation analysis made to determine the largest

possible rotation along the pipeline.

3. The maximum tensile stress and joint rotation values should then be compared to the

allowable values given in Tables 4 and 5, respectively. If the values predicted fall below

the minimum allowable values, the pipeline should be safe under the imposed ground

deformations.

CASE STUDIES

Lurie Center

Construction of the Lurie Medical Research Center included a deep braced excavation in

downtown Chicago. The dimensions of the excavation were approximately 82 m by 69 m by

12.8 m deep. The support system for the excavation was a sheet pile wall with three levels of

tiebacks. A more complete description of the project is reported in Finno and Roboski (2004).

The area surrounding the Lurie Center site is heavily populated with underground utilities

transmitting water, waste, gas, electric lines, and telecommunication cables. The analyses

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presented herein focuses on gas mains along the north, west, and south walls of the excavation.

The locations of the instrumentation and gas mains with respect to the excavation are shown in

Figure 2. The instrumentation around the site consisted of eight inclinometers, 150 surface

points, and 18 embedded soil anchors, and 30 points on utilities. Detailed ground movement

measurements were collected throughout the excavation process and ground movement

distributions were computed from the data by a radial basis interpolation.

The gas mains along the west and north walls of the excavation are old ductile iron mains

with mechanical joints. The main along the west wall is a 500 mm diameter pipeline located 5.5

m from the excavation wall. The main along the north wall is a 150 mm diameter pipeline

located approximately 15.5 m from the north wall of the excavation. Along the south wall, there

is a 300 mm diameter main approximately 8.1 m from the southern edge.

The pipelines were assumed to move with the ground and provide no restraint against the

soil. Therefore, it was assumed that the ground displacement profiles at the locations of the gas

mains defined their movement. To support this assumption, comparisons were made between

settlement readings taken from the survey points located directly on the gas mains and the

approximated settlement profile at the pipeline location. Only the values for vertical movements

were compared because lateral readings from the gas mains were unable to be obtained. Figure 3

shows a comparison between the pattern of the pipeline determined from a survey point located

directly on the pipeline and that of the observed ground surface movement above for the pipeline

located along the west wall of the Lurie Center excavation. Excluding an initial offset of the

observed ground movement, the displacement patterns are the same within the accuracy of the

optical survey measurements. Because the pipe along the west wall had the largest diameter, the

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ground displacement profiles at the locations of the three gas mains can be then assumed to be

the movements of the pipelines.

Figure 4 shows the ground movements observed at the locations of the gas mains at the

end of the excavation, where the maximum vertical and lateral movements were the greatest.

This should correspond to the maximum stresses in the pipeline. Only settlement values only for

the gas main to the north of the excavation are reported because the pipeline was located further

from the excavation than the furthest row of lateral survey points.

Assuming that the pipelines behave flexibly, a bending stress analysis was completed for

the ground displacement profiles in Figure 4. Figure 5 shows the variation in time of the

maximum tensile stress for the three gas mains throughout the excavation process. There is a

gradual increase in the magnitude of the maximum tensile stress for all three gas mains. The

magnitude of the maximum tensile stress for all mains at the completion of the excavation is

representative of the maximum stress incurred for the duration of the construction. The

maximum tensile stresses in the north, south, and west gas mains were 2.5, 10 and 25 MPa,

respectively. For cast iron and ductile iron pipelines, these values are well below the allowable

stress values of the pipe for design (Table 3). From the bending stress analysis, it can be

concluded that no failure due to bending would occur in these gas mains due to the excavation.

Assuming the gas mains adjacent to the Lurie Center excavation behave as rigid

pipelines, an analysis on the relative rotation at the joints must be completed to determine if the

rotations are within the allowable limits. The joints for the north and west gas mains are known

to be mechanically bolted joints. The type of joint for the cast iron pipeline along the south wall

is unknown.

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Figure 6 shows the maximum joint rotations for the north, south, and west gas mains for

the time length of the construction for pipe section lengths of 3.6 m. As shown in Table 4, lead

caulked joints began to show large amounts of leakage at rotations equaling 0.006 radians (0.34

degrees). From Figure 6, it is apparent that the rotations in the joints from the ground

movements for the north and west gas mains remained below the critical rotations for rubber

gasket joints. The joint rotations for the south gas main, which was cast iron, reached critical

magnitudes for lead caulked joints.

A comparison of the maximum stress and joint rotations obtained from the ground

movements computed as a radial basis interpolation and of pipe displacement on the error

function model showed very similar results. Figure 7 shows the comparison of the predicted,

calculated, and allowable maximum bending stresses and joint rotations values for the gas main

along the west wall of the excavation, a 500 mm diameter ductile iron gas main with mechanical

joints. The maximum calculated stress from the complementary error function-based ground

movements for the final stages of the construction were within 8 MPa of those computed from

the radial basis approximation using the observed ground movements. The joint rotation analysis

yielded similar results. The joint rotations from the complementary error function-based ground

movements differed from those computed as a radial basis function by approximately 2.5 x 10-3

radians (0.14 degrees). Similar conclusions concerning the effects of the excavation on the

pipelines at this site can be drawn when computing stresses and rotations with both methods.

Chicago Excavation Case Studies

Maynard and ORourke (1977) present ground surface movement data for different

braced excavations in Chicago where cast iron pipelines were impacted by the effects of

excavations. Table 6 presents the information for the four cast iron mains and their approximate

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maximum movements. The ground movements were observed within a distance of

approximately 3 m behind the pipeline to eliminate the effects of the edge of the trench on the

data. This produced ground movement data representative of the movement of the pipelines.

The displacement profiles for the four pipelines show the maximum movements

occurring near the center of the excavation with large curvatures at the edge. From a bending

stress analysis, assuming a modulus of elasticity of 100 MPa and negligible change in rotation of

the principal planes, the largest stresses occurred at the edge of the excavation where the largest

curvatures were located. The characteristic length for determining the curvatures for each

pipeline was taken as the distance between the data points along that pipeline. The distances for

the four mains ranged from 7.6 to 15.2 m, which is a reasonable characteristic length for the

calculation of curvature being that they are greater than 5.5 m, which was determined to be the

lower bound. For two of the pipelines the maximum calculated bending stresses proved to be

greater than the minimum value of allowable bending stress from excavation-induced ground

movements for cast iron presented in Table 3. However, there was no evidence of fracture of the

pipes due to excessive bending showing the conservativeness of the bending stress analysis.

For an analysis of the rotations at the joints along the pipelines, it was assumed that the

joints were located at the data points. Since only the total vector movement data was available,

the relative rotations could be calculated from the changes in slope of the displacement profiles.

Figure 8 shows the rotations calculated at the data points. The open symbols represent

mechanical joints and the filled symbols denote lead caulked joints.

For the two mains joined by lead caulked joints, the 300 mm diameter pipeline showed

the largest relative rotation at a joint of 6 x 10-3

radians (0.34 degrees) at which leakage at the

joints was observed and was taken out of service. The mains equipped with mechanical joints

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experienced a maximum rotation of 8 x 10-3

radians (0.46 degrees). This rotation is greater than

that observed to cause failure at a lead caulked joint, however, for mechanical joints it is within

the allowable limits of 0.044 radians (2.5 degrees). Both pipelines joined with mechanical joints

experienced larger relative rotations yet from observations they did not show excessive leakage,

illustrating the benefits of the more flexible joints.

CONCLUSIONS

Based on the analyses presented herein and the data obtained at several deep excavations

in Chicago, the following conclusions can be made.

1. From comparison of ground displacements interpolated from collected

data and field observed movements of buried utilities, it was shown that the

pipeline tracked the movement of the surrounding soil within the accuracy of the

optical survey data. For computation of bending stresses and joint rotations

induced in pipelines from ground movements related to a deep braced excavation,

the pipeline displacement profile may be assumed to be that of the displacement

of the surrounding soil when the displacement in the pipe is small in relation to

the length of the pipeline, i.e. sin . When utilizing this assumption, special

consideration of construction activity, differential soil behavior, and local effects

must be taken into account.

2. A methodology was presented for computing the longitudinal bending

stresses and joint rotations induced in a pipeline from an adjacent deep braced

excavation. The validity of the method for calculating the bending stresses and

joint rotations is illustrated by comparisons of calculated ground movement

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values and direct observations made in the field for the Lurie Center excavation

and the various Chicago excavations presented by Maynard and ORourke (1977).

The method proved to be conservative for both bending stress and joint rotation

analysis.

3. The more critical condition for a cast iron or ductile iron main considered

herein is excessive rotation at a joint. The bending stress analysis on the ground

movements presented by Maynard and ORourke (1977) showed large

longitudinal tensile stresses with no observed cracking or rupture. The small

calculated joint rotation of 6 x 10

-3

radians (0.34 degrees) proved to cause

excessive leakage in a lead caulked joint.

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REFERENCES

Ahmed, I. (1990). Pipeline Response to Excavation-Induced Ground Movements. PhD thesis,

Department of Civil and Environmental Engineering, Cornell University, Ithaca, NY.

American Petroleum Institute. (1991). Specification for Line Pipe, 39th

Ed. American Petroleum

Institute, Washington, DC.

Attewell, P.B., Yeates J., and Selby, A. R. (1986). Soil Movements Induced by Tunneling and

Their Effects on Pipelines and Structures. Blackie and Son, Ltd., London.

Bonds, R. W. (2003).Ductile Iron Pipe Joint and Their Uses. Ductile Iron Pipe Research

Association, Birmingham, AL.

Carder, D. R., Taylor, M. E., and Pocock, R. G. (1982). Response of a Pipeline to Ground

Movements Caused by Trenching in Compressible Alluvium.Department of the

Environment Department of Transport, TRRL Report LR 1047, Transport and Road

Research Laboratory, Crowthorne.

Carder, D. R. and Taylor, M. E. (1983). Response of a Pipeline to Nearby Trenching in Boulder

Clay.Department of the Environment Department of Transport, TRRL Report LR 1099,

Transport and Road Research Laboratory, Crowthorne.

Clough, G. W. and ORourke, T. D. (1990). Construction Induced Movements of In-Situ

Walls.Design and Performance of Earth Retaining Structures, Proceedings of a

Specialty Conference at Cornell University, ASCE, New York, 439-470.

Croft, J. E., Menzies, B. K., and Tarzi, A. I. (1977). Lateral Displacement of Shallow Buried

Pipelines due to Adjacent Deep Trench Excavations. Geotechnique, 27(2), 161-179.

Finno, R. J., and Roboski, J. F. (2004). Three-Dimensional Responses to a Tied-back

Excavation Through Clay.

23


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Hsieh, P. G. and Ou, C. Y. (1998). Shape of Ground Surface Settlement Profiles Caused by

Excavation. Canadian Geotechnical Journal, 35, 1004-1017.

Maynard, T. R. and ORourke, T. D. (1977). Soil Movement Effect on Adjacent Public

Facilities. Preprint No. 3111, ASCE Annual Meeting, San Francisco, CA.

Nath, P. (1983). Trench Excavation Effects on Adjacent Buried Pipes: Finite Element Study.

Journal of Geotechnical Engineering, ASCE, New York, NY, 109(11), 1399-1415.

Plastics Pipe Institute. (1993). Engineering Properties of Polyethylene. PPI Handbook of

Polyethylene Piping, Plastics Pipe Institute, Washington, DC.

Plastics Pipe Institute. (2003). Specifications, Test Methods and Codes for Polyethylene Piping

Systems. PPI Handbook of Polyethylene Piping, Plastics Pipe Institute, Washingtion,

DC.

Plastics Pipe Institute. (2000).Model Specification for Polyethylene Plastic Pipe, Tubing and

Fittings for Water Mains and Distribution. Plastics Pipe Institute, Washington, DC.

Roboski, J. F., and Finno, R. J. (2004). Distributions of Ground Movements Parallel to a Deep

Excavation.

Salmon, C. G., and Johnson, J. E. (1996). Steel Structures: Design and Behavior, Emphasizing

Load and Resistance Factor Design, 4th

Ed. HarperCollins College Publishers, New

York, NY.

Sears, E.C. (1968). Comparison of the Soil Corrosion Resistance of Ductile Iron Pipe and Gray

Cast Iron Pipe.Materials Protection, 7(10), 33-36.

Tarzi, A. I., Menzies, B. K., and Crofts, J. E. (1979). Bending of Jointed Pipelines in Laterally

Deforming Soils. Geotechnique, 29(2), 203-206.

24


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Timoshenko, S. and Goodier, J. N. (1951). Theory of Elasticity. McGraw-Hill Book Company,

Inc., New York, NY.

Untrauer, R. E., Lee, T. T., Sanders, Jr., W. W., and Jawad, M. H. (1970). Design Requirements

for Cast Iron Soil Pipe.Bulletin 199, Engineering Research Institute, Iowa State

University, Ames, IA.

Watkins, R. K. and Anderson, L. R. (2000). Structural Mechanics of Buried Pipes, CRC Press,

New York, NY.

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Tables

Pipe Material

207 20Centrifugally Cast Iron

300 420Ductile Iron

207Steel Grade A

241Steel Grade B

0.28

0.29

Poisson's

Ratio

Coeff. of

Thermal Exp.(per C)

Vertically Pit Cast Iron 0.26

114 14

166-180

200

200

Modulus of

Elasticity(GPa)

83 14

15-18 31Polyethylene PE80 0.42552-758

414Steel Grade 414 200

21-24 31Polyethylene PE100 0.42758-1103

145 20 11 x 10-6

0.26 11 x 10-6

11 x 10-6

0.29

0.29

12 x 10-6

12 x 10-6

12 x 10-6

2 x 10-6

2 x 10-6

Reference

Ahmed (1990)

DIPRA (2001)

API (1991)

API (1991)

Ahmed (1990)

PPI (2003)

API (1991)

PPI (2003)

331

413

517

Yield

Stress, Fy(MPa)

Ultimate

Stress, Fu(MPa)

Table 1 Engineering Properties for Piping Materials

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Material

Cast Iron

Ductile Iron

Limiting Rotation

Leakage, rad. (deg.) Failure, rad. (deg.)

0.09-0.1 (5-6)

0.07-0.09 (4-5)

0.07 (4)

0.05-0.09 (3-5)

0.03-0.14 (2-8)

0.26 (15)

Reference

See Note 1

Attewell, et al. (1986)

Attewell, et al. (1986)

Bonds (2003)

Bonds (2003)

Bonds (2003)

0.0094-0.017 (0.54-1.0)

Joint

Lead-Caulked

Rubber-Gasket

Mechanical

Rubber-Gasket

Mechanical

Ball and Socket

Note 1: Adapted from Untreaur, et al. (1970), O'Rourke and Trautmann (1980), Harris and O'Rourke (1983), andAttewell, et al. (1986)

Table 2 Failure Rotations for Selected Cast Iron and Ductile Iron Joints

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Pipe Material

Pit Cast Iron

Spun Cast Iron

Ductile Iron

Grade A Steel

Grade B Steel

YieldStrength

(Fy), MPa

UltimateStrength

(Fu), MPa---

---

300

207

241

145

207

420

331

414

Grade 414 Steel

PE80

414

---

517

8.6

PE100 --- 11

Initial Stress(INITIAL),

MPa13.8 - 38.6

20.7 - 52.4

33.1 - 71.7

41.4 - 82.8

41.4 - 82.8

41.4 - 82.8

0.13 - 0.26

0.28 - 0.56

Factorof

Safety2.5

2.5

1.67

1.67

1.67

2.0

2.0

Design BendingStress (B), MPa

0.4Fu

0.4Fu

0.8Fu

0.6Fy

0.6Fy

0.6Fy

0.5HDB

0.5HDB

58

82.8

124.2

144.6

248.4

4.3

5.5

Allowable Stress(ALLOW), MPa

19.3 - 44.1

30.3 - 62.1

41.4 - 82.8

62.1 - 103.4

165.4 - 206.8

4.04 - 4.17

4.94 - 5.22

12

Adapted from Taki and O'Rourke (1984) assumed initial longitudinal bending strain of 0.02 to 0.04%1

Allowable bending stress from excavation-induced ground movement = ALLOW= B- INITIAL2

Polyethylene designed for internal pressure. Allowable values expressed as Hydrostatic Design Basis (HDB).3

3

1.2 336 264.3 - 302.9

Table 3 Allowable Bending Stresses from Excavation-Induced Movements

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Mode ofFailure

Leakage Lead-Caulked

Rubber-Gasket Push-on

Mechanical

Rubber-Gasket Push-on

Mechanical

Joint Type

MetalBinding

(metal-to-metal

contact)

Failure Rotations

Radians Degrees

0.0094 - 0.016 0.54 - 0.92

4 - 5

4

3 - 5

2 - 8

Ball and Socket 12.5 - 15

Lead Caulked 5 - 60.09 - 1.0

0.07 - 0.09

0.07

0.05 - 0.09

0.035 - 0.14

0.22 - 0.26

Allowable Rotations, ALLOWRadians Degrees

2.5 - 3.5

2.5

1.5 - 3.5

0.5 - 6.5

11 - 13.5

3.5 - 4.50.06 - 0.08

0.044 - 0.06

0.044

0.026 - 0.06

0.009 - 0.11

0.19 - 0.24

1

2 3

1

Observed from laboratory tests to cause excessive leakage.2

Observed from field data to cause excessive leakage (initial rotation already occurred).3

Material

Cast Iron

Ductile Iron

ALLOWrepresents allowable excavation-induced rotation with assumed 0.026 rad. (1.5 deg.) initial rotation (Attewell, etal., 1986) for flexible joints, ALLOW= METAL BINDING- INITIAL. ALLOW=LEAKAGE/F.S. for lead caulked joints where F.S. = 1.25.

0.0048 0.275

Table 4 Allowable Joint Rotations for Cast Iron and Ductile Iron Joints

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Butt Welded Joint

Single Welded Lap Joint

Double-Welded Lap Joint

1

2

For a full-penetration weld through thickness of pipe.1

For a gap smaller than 3.2 mm.2

Welded JointPercentStrength

Reduction

0

25

20

Table 5 Strength Reductions at Location of Line Pipe Welded Joints

(Watkins and Anderson, 2000)

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Water Main

Lead-Caulked

Lead-Caulked

Mechanical

MechanicalGas Main

1938

pre-1900

1960

1935

300

1200

900

150

2.1

2.3

1.7

1.4

207

310.5

207-276

1.75

WL-1

WL-2

WM-1

GM-1

Diameter

(mm)

Depth

(m)

InternalPressure

(kPa)

Main Type Year Joints Symbol

Table 6 Description of Cast Iron Pipelines Parallel to Chicago Excavations

(Maynard and ORourke, 1977)

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Figures

Figure 1 Coordinates for Bending and Joint Rotation Analyses

z

x

y

i j

k

ji

kj

Lji

L kjji kjj

z

x

y

i j

k

ji

kj

Lji

L kjji kj

b. Rotation

a. Bending

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Figure 2 General Layout of Lurie Center Site Instrumentation andAdjacent Gas Mains

N

Gas Main

Inclinometer

Surface Point

Soil AnchorUtility Point

LEGEND

LURIE MEDICAL RESEARCHCENTER EXCAVATION

E. Superior St.

N.

FairbanksCt.

E. Huron St.

Existing Pedestrian Tunnel

0 2 4 8Scale in meters

PrenticeWomen's

Hospital

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0

10

20

30

40

50

60

0 50 100 150 200 250 300

Days from Completion of Sheet Pile Wall Installation

Settlement(mm)

Ground Surface Settlement from Contours Survey Data from Pipe

Figure 3 Comparison of Ground and Pipeline Movement during Excavation at Lurie Center

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0

10

20

30

40

50

60

70

80

-20 0 20 40 60 80 100

Distance from Corner of Excavation (m)

Settlemen

t(mm)

North South West

0

1020

30

40

50

60

70

80

90

-20 0 20 40 60 80 100


Later

alMovement(mm)

South West

Figure 4 Ground Displacements at Location of Gas Mains along North, South, and

West Walls after Completion of Excavation

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0

5

10

15

20

25

30

0 50 100 150 200 250 300


Max.TensileStress(Mpa)

North Pipeline South Pipeline West Pipeline

Figure 5 Maximum Tensile Stress in Pipelines Adjacent to North, South, and West WallDuring Excavation

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0

1

2

3

4

5

6

7

0 50 100 150 200 250 300


Rotation(x

10-3rad.)

North Pipeline South Pipeline West Pipeline

Allowable Rotation Failure Rotation

Figure 6 Maximum Relative Rotation Encountered Along North, South, and West Pipelines

During Excavation for 3.6 m Pipe Sections

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0

50

100

150

200

250

300

100 150 200 250 300

Days of Construction of Sheet Pile Wall Installation

TensileStress(MPa)

Radial Basis Error Function Maximum Allowable Stress

a) Maximum Tensile Bending Stresses

0

5

10

15

20

25

30

100 150 200 250 300


Rotation(x10-3

rad.)

Radial Basis Error Function Maximum Allowable Rotation

b) Maximum Joint Rotations

Figure 7 Observed and Predicted Maximum Bending Stresses and Joint Rotations for Final

Stages of Construction for Gas Main Along West Wall

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0

1

2

3

4

5

6

7

8

9

-5 0 5 10 15 20 25 30 35 40 45 50


Rotation(10-3 rad.)

WL-1 WL-2 WM-1 GM-1

Note: Leakage Observed in WL-1.

Figure 8 Rotations in Cast Iron Pipelines Adjacent to Excavations in Chicago

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APPENDIX A: LURIE CENTER DATA

TABLE OF CONTENTS

Table of Contents 40

List of Tables 42

List of Figures 44

A.1 Introduction 48

A.2 Pipeline and Material Properties 49

A.2.1 Cast Iron 49A.2.2 Ductile Iron 55

A.2.3 Steel 57A.2.4 Polyethylene 60

A.3 Case Study: Lurie Medical Research Center 63

A.3.1 Ground Movements due to Excavation 64A.3.2 Ground Movements at Locations of Gas Mains 66

A.3.3 Ground Movement and Pipeline Movement Comparison 69

A.4 Stress Conditions on Buried Pipelines 73A.4.1 Initial Stresses 73

A.4.1.1 Hoop Stress 74

A.4.1.2 Ring Stresses from Soil Cover 75A.4.1.3 Traffic Loads 81

A.4.1.4 Stresses from the Installation Procedure and Adjacent

Construction History 83A.4.1.5 Environmental Effects 84

A.4.2 Analysis of Effects of Ground Movements from Adjacent

Excavations on Pipeline 87

A.4.2.1 Soil Pipeline Interaction 90A.4.2.2 Sign Convention and Definition of Terms 91

A.4.2.3 Calculation of Bending Stress for Flexible Pipeline 92

A.4.2.4 Calculation of Relative Rotation at Joint for RigidPipeline 98

A.5 Summary 104

A.6 Conclusions 108

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References 111

Tables 115

Figures 126

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LIST OF TABLES

Table A-1 Tensile Strength and Stress-Strain Properties for Cast Iron 115

Table A-2 Typical Dimensions for Lead Caulked Joints 115

Table A-3 Typical Dimensions for Flexible Joints for Cast Iron Pipe 116

Table A-4 Experimental Results for Rotation at Leakage for Lead Caulked

Cast Iron Pipe Joints 116

Table A-5 Tensile Strength and Stress-Strain Properties for Ductile Iron 117

Table A-6 Typical Dimensions for Flexible Joints for Ductile Iron Pipe 117

Table A-7 Strength Reductions at Location of Welded Line Pipe Joints 118

Table A-8 Typical Mechanical Properties for Polyethylene Gas Distribution

Pipe 118

Table A-9 Locations of Underground Utilities with Respect to Lurie Center

Excavation 119

Table A-10 Definitions of Stages of Construction 119

Table A-11 Magnitudes of Maximum Ground Movements Surrounding Lurie

Center Excavation 120

Table A-12 Magnitudes of Maximum Ground Movements at Locations of

Gas Mains Adjacent to Lurie Center Excavation 120

Table A-13 Locations of Utility Survey Points with Respect to Nearest Corner

of Excavation 121

Table A-14 Sample Hoop Stress Calculations for Gas Mains Adjacent to

Lurie Center Excavation 121

Table A-15 Overpressure Stresses for Gas Mains Adjacent to Lurie CenterExcavation 122

Table A-16 Allowable Bending Stresses from Excavation-InducedMovements 123

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Table A-17 Allowable Excavation-Induced Joint Rotations for Semi-Rigid

and Flexible Cast Iron and Ductile Iron Joints 123

Table A-18 Typical Engineering Properties for Piping Materials 124

Table A-19 Dimensions and Maximum Tensile Stress Values in Gas Mains

Adjacent to Lurie Center at End of Excavation 124

Table A-20 Dimensions and Maximum Joint Rotation in Gas Mains Adjacent

to Lurie Center at End of Excavation 125

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LIST OF FIGURES

Figure A-1 Typical Stress-Strain Curves for Cast Iron 126

Figure A-2 Typical Lead Caulked Cast Iron Joint 126

Figure A-3 Typical Flexible Iron Joints 127

Figure A-4 Stress-Strain for Cast Iron and Ductile Iron 127

Figure A-5 Rotational Stiffness of Ductile Iron Rubber Gasket Joint 128

Figure A-6 Typical Joint Welds for Line Pipe 128

Figure A-7 Stress-Strain Curve for Polyethylene Under Controlled

Conditions 129

Figure A-8 Typical Joining Methods for Polyethylene Pipe 129

Figure A-9 General Layout of Lurie Center Site Instrumentation and

Adjacent Underground Utilities 130

Figure A-10 Vertical Ground Movements Along North Wall on Day 146 131

Figure A-11 Lateral Ground Movements Along North Wall on Day 146 131

Figure A-12 Vertical Ground Movements Along North Wall on Day 192 132

Figure A-13 Lateral Ground Movements Along North Wall on Day 192 132

Figure A-14 Vertical Ground Movements Along North Wall at End of 133

Excavation

Figure A-15 Lateral Ground Movements Along North Wall at End of 133

Excavation

Figure A-16 Vertical Ground Movements Along South Wall on Day 157 134

Figure A-17 Lateral Ground Movements Along South Wall on Day 157 134

Figure A-18 Vertical Ground Movements Along South Wall on Day 203 135

Figure A-19 Lateral Ground Movements Along South Wall on Day 203 135

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Figure A-20 Vertical Ground Movements Along South Wall at End of 136

Excavation

Figure A-21 Lateral Ground Movements Along South Wall at End of 136

Excavation

Figure A-22 Vertical Ground Movements Along West Wall on Day 146 137

Figure A-23 Lateral Ground Movements Along West Wall on Day 146 137

Figure A-24 Vertical Ground Movements Along West Wall on Day 185 138

Figure A-25 Lateral Ground Movements Along West Wall on Day 185 138

Figure A-26 Vertical Ground Movements Along West Wall at End of 139Excavation

Figure A-27 Lateral Ground Movements Along West Wall at End of 139Excavation

Figure A-28 Maximum Magnitude of Vertical Movement During Excavation

at Location of Gas Main Adjacent to North Wall 140

Figure A-29 Maximum Magnitudes of Movements During Excavation at

Location of Gas Main Adjacent to South Wall 140

Figure A-30 Maximum Magnitudes of Movements During Excavation atLocation of Gas Main Adjacent to West Wall 141

Figure A-31 Re-zeroed Settlement Values Along North Gas Main atCompletion of Excavation 141

Figure A-32 Re-zeroed Settlement Values Along South Gas Main atCompletion of Excavation 142

Figure A-33 Re-zeroed Settlement Values Along West Gas Main atCompletion of Excavation 142

Figure A-34 Comparison of Utility Survey Point U-1 Movement with

Approximated Ground Movement Values 143





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Figure A-37 Comparison of Utility Survey Point U-4 Movement withApproximated Ground Movement Values 144





Figure A-40 Free-Body Diagram of Forces Resulting from Internal Pressure 146

Figure A-41 Sign Convention for Thrust, Moment, Displacement, and Stress

Equations 146

Figure A-42 Local Coordinate System Convention for Pipeline Analysis 147

Figure A-43 Definitions of Dimensions and Differential Ground MovementDesignations 147

Figure A-44 Pipeline Profiles with Established Local Coordinate System for

Analysis of Bending Stress for Flexible Pipeline 148

Figure A-45 Pipe Cross Section and Sign Convention 148

Figure A-46 Comparison of Characteristic Lengths of 6.1, 9.2, and 15.3 m for

Stress Analysis Along Gas Main Adjacent to North Wall DuringExcavation 149

Figure A-47 Comparison of Characteristic Lengths of 6.1, 9.2, and 15.3 m forStress Analysis Along Gas Main Adjacent to South Wall During

Excavation 149

Figure A-48 Comparison of Characteristic Lengths of 6.1, 9.2, and 15.3 m for

Stress Analysis Along Gas Main Adjacent to West Wall During

Excavation 150

Figure A-49 Maximum Tensile Stress in Pipelines Adjacent to North, South,

and West Walls During Excavation 150

Figure A-50 Maximum Tensile Stress Along North Gas Main at Completion

of the Excavation 151

Figure A-51 Maximum Tensile Stress Along South Gas Main at Completion


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Figure A-52 Maximum Tensile Stress Along West Gas Main at the Completion


Figure A-53 Pipeline Profiles with Established Local Coordinate System for

Analysis of Joint Rotations for Rigid Pipeline 152

Figure A-54 Schematic of Joint Rotation at Joint j of Rigid Pipeline 153

Figure A-55 Maximum Relative Rotation Encountered Along North, South,and West Pipelines During Excavation for 3.6 m Pipe Sections 153

Figure A-56 Comparison of Maximum Relative Rotation in North Gas Mainfor 3.6 and 6.1 m Pipe Sections 154

Figure A-57 Comparison of Maximum Relative Rotation in South Gas Mainfor 3.6 and 6.1 m Pipe Sections 154

Figure A-58 Comparison of Maximum Relative Rotation in West Gas Mainfor 3.6 and 6.1 m Pipe Sections 155

Figure A-59 Joint Rotations and Pipeline Settlement Along Pipeline Adjacent

to North Wall at Completion of Excavation 156

Figure A-60 Joint Rotations and Pipeline Settlement Along Pipeline Adjacent

to South Wall at Completion of Excavation 157

Figure A-61 Joint Rotations and Pipeline Settlement Along Pipeline Adjacentto West Wall at Completion of Excavation 158

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A.1 Introduction

Within urban environments, buried pipelines may be exposed to large ground movements

either by tunneling, mining, or open cut constructions, such as trenching or deep braced

excavations. These large ground movements can induce deformations in pipelines resulting in

stresses within the pipeline. These stresses, if excessive, could result in damage or complete

failure of the pipeline.

Failure in a pipeline due to excavation-induced ground movements could be caused by

large bending stresses in the pipe or relative rotations between two adjacent pipe sections at a

joint. Large bending strains in a pipe and rotations in a joint are the result of significant

differential movements along the pipeline. This mainly occurs near the edge of an excavation

due to the transition of the pipeline from being restrained to it being free to move with the

ground.

A conservative analysis of the effects of the ground movements from deep braced

excavations is presented. The analysis considers two different methods of deformation

separately, either curvature in the pipe or rotation at the joints. The displacements along the

pipeline are used to calculate the maximum stresses and rotation imposed on the pipeline. These

values can be compared to established allowable values to determine whether the pipeline could

be damaged or remain safe and in operation.

This analysis is presented and applied to two case studies where data was obtained from

excavations in downtown Chicago. Maximum calculated values are computed and compared to

the allowable values established from previous experimental and empirical studies. Conclusions

are drawn for the stresses that were imposed on the pipeline due solely to the ground movements

resulting from the deep braced excavations.

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A.2 Pipeline Material and Properties

Four different materials used in pipeline engineering will be discussed; cast iron, ductile

iron, steel, and polyethylene. An overview of the manufacturing methods, engineering

properties, stress-strain behavior, standard dimensions, and joining methods for all four pipeline

materials will be presented and compared.

A.2.1 Cast Iron

Many cast iron gas pipelines in use today have been in operation for over 100 years. The

initial growth of the use of gray cast iron pipe in the pipeline industry in the United States

occurred around 1816. Foundries for production of cast iron specifically in the form of pipe

began in the eastern states and spread quickly westward. The metallurgic composition of cast

iron is an alloy of iron and carbon with a percentage of silicon and manganese. The carbon

exists in the form of graphite flakes and gives the material much of its strength.

Two main manufacturing methods were used in the production of cast iron pipe: pit

casting and centrifugally, or spin, casting. The majority of cast iron pipes installed during the

duration for which cast iron was the main piping material were pit cast. This was due to the late

introduction of centrifugal casting in the 1920s. In the pit casting process, the molten mixture of

metals was poured into either a horizontal or vertical mold and allowed to set in place as it

cooled. Vertical pit casting was the preferred method of manufacturing due to the longer

sections of pipe that could be cast. Horizontal casting was limited by the flexural rigidity of the

mold core where bending of the core could cause inconsistent wall thickness along the length of

the pipe. Vertical pit casting increased the length of pipe sections available from 1.2 to 1.5 m to

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lengths of up to 3.6 m, in turn decreasing the amount of joints necessary. Both horizontal and

vertical pit cast pipe was available in a range of diameters from 75 to 1500 mm.

In the 1920s, the process of centrifugally casting gray cast iron pipe was introduced and

became the primary manufacturing method of cast iron pipe by the early 1930s. This procedure

involved the pouring of the molten material into a horizontal spinning mold. The rate of rotation

of the mold was controlled to obtain the desired thickness of the pipe. The centrifugal forces

generated by the spinning produced a material with a much more consistent cross section and

even distribution of impurities within the pipe section. The graphite flakes characterizing the

strength of the pipe were more evenly distributed to produce a much stronger and more

consistent material. Cast iron pipe was available in pipe sections 3.6 to 6.1 m in length and in

sizes ranging from 75 to 1200 mm diameter.

The stress-strain behavior of cast iron exhibits a brittle behavior with no yield point and

an abrupt fracture at failure. Under an applied stress there exists no completely elastic behavior

for any stress value. Cast iron undergoes an amount of plastic strain under the application of an

increment of load. The total strain at all stress values is composed of both elastic and plastic

components. The elastic strain behavior is characterized by a curve of recoverable strain, which

does not agree with the traditional straight-line path of linear elastic materials. Therefore, the

definition of a modulus of elasticity of material is more challenging to calculate and is usually

defined by an initial tangent modulus.

A typical stress-strain curve for both pit cast and centrifugally cast iron presented by

Attewell, et al. (1986) is shown in Figure A-1. The lack of elastic behavior by cast iron is clearly

visible from the non-linearity of the elastic strain curves. The brittle behavior of both pit cast

and centrifugally cast iron is apparent with rupture failure occurring at a relatively low value of

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axial strain. Centrifugally cast iron shows approximately a 50 percent increase in strength over

that of pit cast. This can be attributed to its quick solidification, uniformity of its cross section,

and the isolation of impurities accomplished through the improved method of production.

Cast iron has been shown to behave differently in tension and compression. This can

have a large effect on the flexural behavior of the pipe. Schlick and Moore (1936) conducted 12

tests on specially cast plates of four different grades of strength with 13, 23, and 32 mm

thickness in direct tension and compression. From the 12 tests, it was shown that the

compressive strength of cast iron to be on average 3.6 times greater the tensile strength with a

standard deviation of 0.31. The difference in the behavior of cast iron in tension and

compression is most notable for strains above 0.1 percent, when the slope of the tensile curve

decreases at a faster rate than that of the compression curve.

Due to the complex behavior of cast iron, extensive testing has been done to determine

appropriate engineering properties to represent its response to loading conditions. Ahmed (1990)

compiled the test results and established recommended ranges for values for the ultimate stress,

the initial tangent modulus, and the failure strain for pit cast and centrifugal cast iron. Table A-1

shows his suggested values for both pit cast and centrifugally cast iron. The variation of the

values is the result of the improvements in the production process from pit to centrifugal casting.

Cast iron pipe failure due to excessive bending occurs as an abrupt brittle fracture at low

strains. Attewell, et al. (1986) recommend a maximum design stress for cast iron under direct

tensile load equal to one-quarter of the ultimate tensile strength of the material. For cast iron in

bending, a rupture factor of 1.6 needs to be a

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