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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.
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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.
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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
Distance from Corner of Excavation (m)
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
Days from Completion of Sheet Pile Wall Installation
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
Days from Completion of Sheet Pile Wall Installation
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
Days from Completion of Sheet Pile Wall Installation
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
Distance from Corner of Excavation (m)
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
Figure A-35 Comparison of Utility Survey Point U-2 Movement with
Approximated Ground Movement Values 143
Figure A-36 Comparison of Utility Survey Point U-3 Movement with
Approximated Ground Movement Values 144
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Figure A-37 Comparison of Utility Survey Point U-4 Movement withApproximated Ground Movement Values 144
Figure A-38 Comparison of Utility Survey Point U-5 Movement with
Approximated Ground Movement Values 145
Figure A-39 Comparison of Utility Survey Point U-6 Movement with
Approximated Ground Movement Values 145
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
of the Excavation 151
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Figure A-52 Maximum Tensile Stress Along West Gas Main at the Completion
of the Excavation 152
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