Pipe Zone Bedding and
Pipe Zone Bedding and Backfill: A Flexible Pipe Perspective
Backfill: A Flexible Pipe Perspective
Reynold WatkinsReynold Watkins11, Brent Keil, Brent Keil22, Rich Mielke, Rich Mielke33, Shah Rahman, Shah Rahman44
ABSTRACT ABSTRACT
Bedding and backfill materials pl
Bedding and backfill materials play a critical role in the loay a critical role in the long-term structural integrity of buriedng-term structural integrity of buried municipal pipelines.
municipal pipelines. Soil strength is resistance to soil slip, and is a Soil strength is resistance to soil slip, and is a function of soil friction angle andfunction of soil friction angle and any cohesion in the soil. It determines the stability of the soil. Soil stiffness is the modulus of soil any cohesion in the soil. It determines the stability of the soil. Soil stiffness is the modulus of soil elasticity, E', and is a function of soil type and the applied level of compaction. E' is also affected by elasticity, E', and is a function of soil type and the applied level of compaction. E' is also affected by depth of burial which increases
depth of burial which increases confining pressure. E' determines the ring deflection confining pressure. E' determines the ring deflection of flexibleof flexible rings.
rings. The Modified Iowa equation is used to predict rThe Modified Iowa equation is used to predict ring deflection of buried flexible pipes.ing deflection of buried flexible pipes. Common types of backfill material can range from native soils (which usually have fines, silt and Common types of backfill material can range from native soils (which usually have fines, silt and clay) to imported crushed rock, to
clay) to imported crushed rock, to soil-cement slurry (or flowable fill/CLSM). Whenever backfillsoil-cement slurry (or flowable fill/CLSM). Whenever backfill material other than native soils is specified on a project, the reason for doing so must be justified. material other than native soils is specified on a project, the reason for doing so must be justified. This paper reviews and recommends best practices for the selection of common bedding and This paper reviews and recommends best practices for the selection of common bedding and backfill materials for flexible pipes and conducts an economic analysis of the various options backfill materials for flexible pipes and conducts an economic analysis of the various options
utilized. Theoretical aspects of pipe-soil interaction as it relates to ring deflection of pipe for various utilized. Theoretical aspects of pipe-soil interaction as it relates to ring deflection of pipe for various soil types are discussed.
soil types are discussed.
INTRODUCTION INTRODUCTION
Flexible pipes, when buried,
Flexible pipes, when buried, are designed to deflect vertically. It is are designed to deflect vertically. It is through the process of verticalthrough the process of vertical deflection, which results in an equal horizontal expansion of the pipe, that a flexible pipe such as deflection, which results in an equal horizontal expansion of the pipe, that a flexible pipe such as gasket-joint or welded-joint spiral welded steel pipe (WSP) engages the passive resistance of gasket-joint or welded-joint spiral welded steel pipe (WSP) engages the passive resistance of surrounding soils. The pipe-soil system eventually reaches a point of equilibrium where the soil surrounding soils. The pipe-soil system eventually reaches a point of equilibrium where the soil above the pipe forms an arch and
above the pipe forms an arch and further deflection of the pipe ceases. In further deflection of the pipe ceases. In this pipe-soil interactionthis pipe-soil interaction dynamic, the role played by the soil far outweighs that of the pipe’s stiffness. In structural
dynamic, the role played by the soil far outweighs that of the pipe’s stiffness. In structural
mechanics, deflection of flexible pipes is referred to as ring deflection. In buried pipe systems, soil mechanics, deflection of flexible pipes is referred to as ring deflection. In buried pipe systems, soil quality and placement, and boundaries, are relatively imprecise. So prediction of ring deflection quality and placement, and boundaries, are relatively imprecise. So prediction of ring deflection typically does not justify very complex analyses. For practical design, predicted ring deflection is typically does not justify very complex analyses. For practical design, predicted ring deflection is roughly equal to, and no greater than, the vertical compression of the sidefill soil. Moreover, ring roughly equal to, and no greater than, the vertical compression of the sidefill soil. Moreover, ring deflection is limited by specification.
deflection is limited by specification.
It is important to note that the concern for soil pressure on a pipe is limited to empty pipe or gravity It is important to note that the concern for soil pressure on a pipe is limited to empty pipe or gravity flow pipelines where the conduit never flows full. WSP, with physical/mechanical characteristics flow pipelines where the conduit never flows full. WSP, with physical/mechanical characteristics such as a high tensile strength of 60 ksi, yield strength of 42 ksi, and allowable elongation of 22%, is such as a high tensile strength of 60 ksi, yield strength of 42 ksi, and allowable elongation of 22%, is used most often in municipal pressure systems. Examples include distribution and transmission of used most often in municipal pressure systems. Examples include distribution and transmission of raw and potable water, and sanitary sewer force mains. After installation, when a line is placed into raw and potable water, and sanitary sewer force mains. After installation, when a line is placed into service, pressure inside the pipe is typically much greater than soil pressure on the pipe. Internal service, pressure inside the pipe is typically much greater than soil pressure on the pipe. Internal
11 Professor Emeritus, Department of Civil and Environmental Engineering, Utah State University,Professor Emeritus, Department of Civil and Environmental Engineering, Utah State University,
Logan, UT 84322; Tel:
Logan, UT 84322; Tel: (435) 797-2864; Fax: (435) 797-1185; Email:(435) 797-2864; Fax: (435) 797-1185; Email: reynold@cc.usu.edureynold@cc.usu.edu
22
Corporate Chief Engineer, Northwest Pipe Company, 430 North
Corporate Chief Engineer, Northwest Pipe Company, 430 North 600 West, Pleasant Grove, UT600 West, Pleasant Grove, UT 84062; Tel: (801) 785-6922; Fax: (801) 785-6922; Email:
84062; Tel: (801) 785-6922; Fax: (801) 785-6922; Email: bkeil@nwpipe.combkeil@nwpipe.com
33 Director of Engineering, Northwest Pipe Company, 10512 New Arden Way, Director of Engineering, Northwest Pipe Company, 10512 New Arden Way, Raleigh, NC 27613;Raleigh, NC 27613;
Tel: (919) 847-6077; Fax: (919) 847-5977; Email:
Tel: (919) 847-6077; Fax: (919) 847-5977; Email: rmielke@nwpipe.comrmielke@nwpipe.com
44
Western Regional Engineer, Northwest Pipe Company, 1101 California Avenue, Suite 100, Corona, Western Regional Engineer, Northwest Pipe Company, 1101 California Avenue, Suite 100, Corona, CA 92881; Tel: (951)
pressure, therefore, supports the soil load. When designing a pressurized steel pipe system, an pressure, therefore, supports the soil load. When designing a pressurized steel pipe system, an
Engineer should first determine the pipe’s wall thickness based on the expected system performance Engineer should first determine the pipe’s wall thickness based on the expected system performance (working pressure, surge, etc.), then follow this up with a simple calculation to ensure that the pipe (working pressure, surge, etc.), then follow this up with a simple calculation to ensure that the pipe will remain within the allowable ring deflection limits in the specified soil backfill system. A third will remain within the allowable ring deflection limits in the specified soil backfill system. A third
step is to select the
step is to select the appropriate corrosion protection to ensure that the intenappropriate corrosion protection to ensure that the intended design-life of theded design-life of the pipeline is met.
pipeline is met.
Despite the facts already stated, the top
Despite the facts already stated, the topic of pipe-soil interaction receives a disproportionateic of pipe-soil interaction receives a disproportionate
attention from the design engineering community for flexible pressure pipelines. On many projects, attention from the design engineering community for flexible pressure pipelines. On many projects, large sums of money are unnecessarily
large sums of money are unnecessarily spent in an effort to improve the buried spent in an effort to improve the buried deflection calculateddeflection calculated from elaborate analysis that does not
from elaborate analysis that does not take into account that a pressure litake into account that a pressure line will re-round itself whenne will re-round itself when placed into service. There isn’t much discussion in publications that addresses this issue, nor are placed into service. There isn’t much discussion in publications that addresses this issue, nor are there good tools to provide a solution. The same quality of construction can be achieved by lower there good tools to provide a solution. The same quality of construction can be achieved by lower cost means if a clear understanding of some of the issues and associated costs exist. For example, cost means if a clear understanding of some of the issues and associated costs exist. For example, while flowable fill or Controlled Low Strength Materials (CLSM) provide a good bedding for both while flowable fill or Controlled Low Strength Materials (CLSM) provide a good bedding for both flexible and rigid pipelines, they can cost almost 20 times more than granular backfill material that flexible and rigid pipelines, they can cost almost 20 times more than granular backfill material that can be manipulated to provide a similar level of structural support to buried pipe. There has never can be manipulated to provide a similar level of structural support to buried pipe. There has never been a better time to understand the pertinent issues than now, when we are faced with a financial been a better time to understand the pertinent issues than now, when we are faced with a financial crisis of epic proportions, and funds are dwindling for critical public infrastructure projects. This crisis of epic proportions, and funds are dwindling for critical public infrastructure projects. This paper is intended to educate the reader on the basics of pipe-soil interaction, pipe and soil stability, paper is intended to educate the reader on the basics of pipe-soil interaction, pipe and soil stability, and the selection of appropriate backfill
and the selection of appropriate backfill systems for pressurized pipelines so systems for pressurized pipelines so that the knowledge canthat the knowledge can be related to the overall cost of a project.
be related to the overall cost of a project.
RING DEFLECTION RING DEFLECTION
The first analysis of soil pressures on buried pipe was proposed by Marston et. al (1913), Dean of The first analysis of soil pressures on buried pipe was proposed by Marston et. al (1913), Dean of
Engineering at Iowa State College, for cu
Engineering at Iowa State College, for culverts to de-water muddy rural roads. The Marston lverts to de-water muddy rural roads. The Marston loadload (Marston 1930) on buried pipe was the total
(Marston 1930) on buried pipe was the total weight of soil in the trench weight of soil in the trench above the pipe, reduced by above the pipe, reduced by frictional resistance of the trench walls, Figure 1a. The pipe had to support the Marston load. The frictional resistance of the trench walls, Figure 1a. The pipe had to support the Marston load. The pipes were rigid (concrete or clay). Marston's student, Merlin G. Spangler showed that flexible pipes were rigid (concrete or clay). Marston's student, Merlin G. Spangler showed that flexible corrugated steel pipe does not need to support Marston load because it deflects as the soil corrugated steel pipe does not need to support Marston load because it deflects as the soil
embedment is compressed during backfilling (Spangler 1941). For flexible pipe design, the load on embedment is compressed during backfilling (Spangler 1941). For flexible pipe design, the load on the pipe is PD, where P = P
the pipe is PD, where P = Pdd (dead load pressure at top of pipe) + P(dead load pressure at top of pipe) + Pll (live load pressure at top of (live load pressure at top of
pipe) and D is pipe diameter. The prism load, PD, is conservative because installers do not compact pipe) and D is pipe diameter. The prism load, PD, is conservative because installers do not compact soil against the pipe. The pipe is embedded in a "packing" of less dense soil that serves the same way soil against the pipe. The pipe is embedded in a "packing" of less dense soil that serves the same way as does packing around an item in a shipping container. The pipe is relieved of part of the prism as does packing around an item in a shipping container. The pipe is relieved of part of the prism load which the soil then picks up in arching action over the pipe, Figure 1b.
load which the soil then picks up in arching action over the pipe, Figure 1b.
Figure 1a, b: Marston Load (ASCE 2009), and Soil
Figure 1a, b: Marston Load (ASCE 2009), and Soil Arching (Watkins 2001)Arching (Watkins 2001)
Flexible pipes, when buried, are expected to undergo vertical deflection due to the dead weight of Flexible pipes, when buried, are expected to undergo vertical deflection due to the dead weight of soil and the effect of live loads, on top of the pipe. When deflection of a flexible conduit takes place, soil and the effect of live loads, on top of the pipe. When deflection of a flexible conduit takes place,
it engages the passive resistance of surrounding soils, and in doing so, the pipe and soil work it engages the passive resistance of surrounding soils, and in doing so, the pipe and soil work together to provide the necessary conditions
together to provide the necessary conditions for the pipe to maintain its for the pipe to maintain its long-term structurallong-term structural integrity. The deflection of flexible conduits is also referred to as ring deflection.
integrity. The deflection of flexible conduits is also referred to as ring deflection. Ring deflection is the percent change
Ring deflection is the percent change in diameter of the pipe or in diameter of the pipe or a measurement of the out-of-a measurement of the out-of-roundness of the pipe. In cement-mortar lined and coated pipe, excessive ring deflection can result roundness of the pipe. In cement-mortar lined and coated pipe, excessive ring deflection can result in cracking of the mortar from the pipe wall. System hydraulics is usually unaffected until a
in cracking of the mortar from the pipe wall. System hydraulics is usually unaffected until a deflection of 10 to 15 percent has occurred. Other negative imp
deflection of 10 to 15 percent has occurred. Other negative imp acts of gross ring deflection may acts of gross ring deflection may include leaks in gasket-sealed joints.
include leaks in gasket-sealed joints.
A classical example of the importance of soil in a buried pipe system is ring deflection. Per AWWA A classical example of the importance of soil in a buried pipe system is ring deflection. Per AWWA
Manual M11 (AWWA 2004), the maximum recommended ring deflection
Manual M11 (AWWA 2004), the maximum recommended ring deflection in steel pipe is in steel pipe is 5% if the5% if the pipe is bare or has flexible lining and coating, 3% if the lining is rigid and the coating is flexible, and pipe is bare or has flexible lining and coating, 3% if the lining is rigid and the coating is flexible, and 2% if both the lining and coating are rigid. Ring deflection is controlled primarily by the soil during 2% if both the lining and coating are rigid. Ring deflection is controlled primarily by the soil during the placement and compaction of embedment and backfill.
the placement and compaction of embedment and backfill.
Soil quality and placement, and boundaries are so imprecise that prediction of ring deflection does Soil quality and placement, and boundaries are so imprecise that prediction of ring deflection does not typically justify complex analyses. For practical design, predicted ring deflection is roughly equal not typically justify complex analyses. For practical design, predicted ring deflection is roughly equal to, and no greater than, the vertical compression of the sidefill soil, i.e. d =
to, and no greater than, the vertical compression of the sidefill soil, i.e. d = ЄЄ; where; whereЄЄis the verticalis the vertical
strain in the soil at the spring-line due to vertical stress. Stress-strain data from soils laboratories strain in the soil at the spring-line due to vertical stress. Stress-strain data from soils laboratories from uni-axial stress in confined compression tests are adequate and worst-case. At spring-line, the from uni-axial stress in confined compression tests are adequate and worst-case. At spring-line, the strain may be slightly less because stresses are bi-axial to an undetermined degree. But for design, strain may be slightly less because stresses are bi-axial to an undetermined degree. But for design, d =
d = ЄЄ..
The value of classical, complex analysis is the Modified Iowa equation, first derived by M. G. The value of classical, complex analysis is the Modified Iowa equation, first derived by M. G.
Spangler (1941), father of buried flexible pipe analysis,
Spangler (1941), father of buried flexible pipe analysis, and later modified by Reynold and later modified by Reynold WatkinsWatkins (Watkins et. al 1958). Shown below is the Modified Iowa equation.
(Watkins et. al 1958). Shown below is the Modified Iowa equation.
(1) (1)
Where: Where:
d
d = = ring ring deflectiondeflection D
DLL = deflection lag factor (1.0 when using prism load)= deflection lag factor (1.0 when using prism load)
K
K = = bedding bedding constant constant (use (use 0.10 0.10 for for steel steel pipe pipe applications)applications) P
P = = sum sum of of vertical vertical pressure pressure on on pipe pipe (dead (dead and and live live loads), loads), psipsi EI/r
EI/r33 = pipe ring stiffness, psi= pipe ring stiffness, psi E'
E' = = empirical empirical soil soil stiffness, stiffness, psipsi r
r = = pipe pipe radius, radius, inchesinches
(imprecision does not justify distinguishing mean radius from nominal D/2) (imprecision does not justify distinguishing mean radius from nominal D/2) I
I = = moment moment of of inertia inertia of of pipe pipe wall, wall, tt33/12/12
t
t = = thickness thickness of of pipe pipe walls, walls, in.in.
It can be seen that the ring deflection is a function of the vertical pressure on the pipe, the pipe ring It can be seen that the ring deflection is a function of the vertical pressure on the pipe, the pipe ring stiffness, and soil stiffness. For steel pipe with rigid lining and coating, the ring stiffness is
stiffness, and soil stiffness. For steel pipe with rigid lining and coating, the ring stiffness is ∑∑EI/rEI/r33,,
the sum of ring stiffness of the steel, and the mortar lining and coating. Flexible linings and coatings the sum of ring stiffness of the steel, and the mortar lining and coating. Flexible linings and coatings do not contribute to ring stiffness. Soil stiffness is 0.06 E'.
do not contribute to ring stiffness. Soil stiffness is 0.06 E'.
'' E E 06 06 .. 0 0 r r EI EI KP KP D D d d 3 3 L L + + = =
The Modified Iowa equation shows the relationship between soil stiffness and ring stiffness in the The Modified Iowa equation shows the relationship between soil stiffness and ring stiffness in the control of ring deflection. Values of E' are also listed in Manual M11 as “modulus of soil reaction” control of ring deflection. Values of E' are also listed in Manual M11 as “modulus of soil reaction” for some of the important soil classifications, Table 1.
for some of the important soil classifications, Table 1. Noteworthy is the prevailing effect of soil
Noteworthy is the prevailing effect of soil stiffness over ring stiffness. From the Modified Iowastiffness over ring stiffness. From the Modified Iowa equation, ring deflection for a plain steel pipe is d% = 10P/(EI/r
equation, ring deflection for a plain steel pipe is d% = 10P/(EI/r33+ 0.06E'). The first term in the+ 0.06E'). The first term in the denominator is ring stiffness (pipe
denominator is ring stiffness (pipe resistance). For a typical pipe with D/t resistance). For a typical pipe with D/t = 240, pipe resistance is= 240, pipe resistance is 1.45 psi. The second term in the denominator is soil resistance. If, for the poorest soil, E' = 500 psi, 1.45 psi. The second term in the denominator is soil resistance. If, for the poorest soil, E' = 500 psi, soil resistance is 30 psi. Pipe resistance is therefore only 5% of the total resistance. Soil resistance is soil resistance is 30 psi. Pipe resistance is therefore only 5% of the total resistance. Soil resistance is 95% of the total.
95% of the total.
Table 1: E' Values by Soil Type and Compaction from AWWA M11 Table 1: E' Values by Soil Type and Compaction from AWWA M11
Soil Soil Stiffness Stiffness Category Category Soil Type
Soil Type AASHTO SoilAASHTO Soil Groups Groups Depth of Depth of Cover Cover Compaction Level Compaction Level 85% 85% 90% 90% 95% 95% 100%100% SC1 SC1
Clean, coarse grained soils: SW, SP, GW, Clean, coarse grained soils: SW, SP, GW, GP,GP, or any soil beginning with one of
or any soil beginning with one of these symbolsthese symbols with 12% or less passing a No. 200 sieve with 12% or less passing a No. 200 sieve
A1, A3 A1, A3 2-5 2-5 700 700 1000 1000 1600 1600 25002500 55--1100 11000000 1155000 0 2222000 0 33330000 1100--1155 11005500 1166000 0 2244000 0 33660000 115 5 ++ 11110000 1177000 0 2255000 0 33880000 SC2 SC2
Coarse-grained soils with fines: GM, GC, SM, Coarse-grained soils with fines: GM, GC, SM,
SC, or any soil beginning with one SC, or any soil beginning with one of theseof these
symbols more than
symbols more than 12% fines. 12% fines. Sandy or Sandy or gravelly fine-grained soils: CL, ML (or CL-ML, gravelly fine-grained soils: CL, ML (or CL-ML, CL/ML, ML/CL) with more than 25% retained on CL/ML, ML/CL) with more than 25% retained on
a No. 200 sieve a No. 200 sieve
A-2-4, A-2-5, A-2-6, A-2-4, A-2-5, A-2-6, or A-4 or A-6 soils or A-4 or A-6 soils with more than 25% with more than 25% retailed on a No. 200 retailed on a No. 200 sieve sieve 2-5 2-5 600 600 1000 1000 1200 1200 19001900 55--1100 990000 1144000 0 1188000 0 22770000 1100--1155 11000000 1155000 0 2211000 0 33220000 15 15 + + 1100 1100 1600 1600 2400 2400 37003700 SC3 SC3
Fine-grained soils: CL, ML (or CL-ML, CL/ML, Fine-grained soils: CL, ML (or CL-ML, CL/ML, ML/CL) with 25% or less retained on
ML/CL) with 25% or less retained on a No. 200a No. 200 sieve
sieve
A-2-7, or A-4 or A-6 A-2-7, or A-4 or A-6 soils with 25% or soils with 25% or less retained on a less retained on a No. 200 sieve No. 200 sieve 2-5 2-5 500 500 700 700 1000 1000 15001500 55--1100 660000 1100000 0 1144000 0 22000000 1100--1155 770000 1122000 0 1166000 0 22330000 115 5 ++ 880000 1133000 0 1188000 0 22660000
Note: E' values in M11 are based on Hartley, J. D., and J. M. Duncan, “E' and its Variation with Depth,” Journal of Note: E' values in M11 are based on Hartley, J. D., and J. M. Duncan, “E' and its Variation with Depth,” Journal of Transportation, Division of ASCE, Sept. 1987.
Transportation, Division of ASCE, Sept. 1987.
From Table 1, it can be seen that many, if not most native soil types, are captured in the table and From Table 1, it can be seen that many, if not most native soil types, are captured in the table and therefore can be analyzed using the Modified Iowa equation.
therefore can be analyzed using the Modified Iowa equation.
It can be shown by analysis that in most cases native soils are capable of limiting pipe deflections It can be shown by analysis that in most cases native soils are capable of limiting pipe deflections below the recommended
below the recommended AWWA limits. AWWA limits. As an example, As an example, assuming most steel pipe assuming most steel pipe installations forinstallations for water transmission are buried with 3
water transmission are buried with 3 to 15 foot of cover, below are deflection calculto 15 foot of cover, below are deflection calculations for a 96”ations for a 96” diameter pipe using the lowest
diameter pipe using the lowest E' values available from the AWWA M-11 table.E' values available from the AWWA M-11 table.
Case 1 Assumptions: Case 1 Assumptions:
96” Diameter 96” Diameter
3% recommended max deflection 3% recommended max deflection 0.40” wall thickness (D/T of 240) 0.40” wall thickness (D/T of 240) 0.50” Mortar Lining; Flexible Coating 0.50” Mortar Lining; Flexible Coating
Case 2 Assumptions: Case 2 Assumptions:
96” Diameter 96” Diameter
2% recommended max deflection 2% recommended max deflection 0.40” wall thickness (D/T of 0.40” wall thickness (D/T of 240)240)
0.50” Mortar Lining; 1.0” Mortar Coating 0.50” Mortar Lining; 1.0” Mortar Coating
E'
E' Values Values From From Table Table 1: 1: E' E' Values Values From From Table Table 1: 1:
3 to 5 foot of cover minimum E' = 500 psi 3 to 5 foot of cover minimum E' = 500 psi Maximum deflection calculated is 1.3% Maximum deflection calculated is 1.3% 5 to 10 foot of cover minimum E' = 600 psi 5 to 10 foot of cover minimum E' = 600 psi Maximum deflection calculated is 2.2% Maximum deflection calculated is 2.2% 10 to 15 foot of cover minimum E'
10 to 15 foot of cover minimum E' = 700 psi= 700 psi Maximum deflection calculated is 2.8%
Maximum deflection calculated is 2.8%
3 to 5 foot of cover minimum
3 to 5 foot of cover minimum E' = 500 psiE' = 500 psi Maximum deflection calculated is 1.2% Maximum deflection calculated is 1.2% 5 to 10 foot of cover minimum
5 to 10 foot of cover minimum E' = 600 psiE' = 600 psi Maximum deflection calculated is 2.0% Maximum deflection calculated is 2.0% 10 to 15 foot of cover minimum E'
10 to 15 foot of cover minimum E' = 700 psi= 700 psi Maximum deflection calculated is 2.6
In all cases, Maximum deflection is below 3%. In all cases, Maximum deflection is below 3%. The recommended deflection limit is satisfied. The recommended deflection limit is satisfied.
Deflection exceeds 2% at 12 feet of cover. Deflection exceeds 2% at 12 feet of cover.
Further consideration is required for depths over Further consideration is required for depths over 12 ft.12 ft.
Noteworthy are the percentages of deflection that are controlled
Noteworthy are the percentages of deflection that are controlled by the soil (in the by the soil (in the denominator of denominator of Modified Iowa equation) —
Modified Iowa equation) — from 87% to 89% to 90% as soil cover, H, from 87% to 89% to 90% as soil cover, H, increases from 5 to 10 to 15increases from 5 to 10 to 15 ft. Allowable ring deflection for mortar lined
ft. Allowable ring deflection for mortar lined and coated steel pipe is and coated steel pipe is d(%) = 2%. For H = d(%) = 2%. For H = 15 ft of 15 ft of soil cover, if E' is i
soil cover, if E' is increased to 1200 psi by compaction (density) oncreased to 1200 psi by compaction (density) of embedment to 90%, then thef embedment to 90%, then the ring deflection becomes, d(%) = 1.7%, which is less than the 2% maximum allowable.
ring deflection becomes, d(%) = 1.7%, which is less than the 2% maximum allowable. If increasing the compaction of the
If increasing the compaction of the soil is impractical for the project, soil is impractical for the project, or if the complexity of theor if the complexity of the project warrants it, a more sophisticated analysis can be completed of the pipe-soil interaction. project warrants it, a more sophisticated analysis can be completed of the pipe-soil interaction. From that it can be verified if
From that it can be verified if native soils will work for the native soils will work for the project, or if other soil improvementsproject, or if other soil improvements will be needed to satisfy the deflection
will be needed to satisfy the deflection recommendations. recommendations. A brief summary of this analysis isA brief summary of this analysis is provided in this paper.
provided in this paper. A complete analysis can be found in the A complete analysis can be found in the ASCE Manuals and Reports onASCE Manuals and Reports on Engineering Practice No. 119 (ASCE 2009).
Engineering Practice No. 119 (ASCE 2009).
SOIL STABILITY SOIL STABILITY
Soil stability is soil strength based on friction angle,
Soil stability is soil strength based on friction angle, φφ, which can be determined in a soils laboratory., which can be determined in a soils laboratory. Soil strength is the resistance to soil slip. Soil strength is the ratio of maximum to minimum soil Soil strength is the resistance to soil slip. Soil strength is the ratio of maximum to minimum soil stresses at soil slip,
stresses at soil slip,σσmaxmax//σσminmin=(1+sin=(1+sinφφ )/(1-sin )/(1-sinφφ ) where ) whereφφ= friction angle. Cohesion in the soil= friction angle. Cohesion in the soil
increases soil strength. Cohesion is caused by a clay fraction, by carbonates, by organics, and even by increases soil strength. Cohesion is caused by a clay fraction, by carbonates, by organics, and even by moisture in fine grained soil. Note, for example, the strength of flowable fill with Portland cement moisture in fine grained soil. Note, for example, the strength of flowable fill with Portland cement added for cohesion.
added for cohesion.
Slope Stability: The concept of soil strength is demonstrated by the maximum slope,
Slope Stability: The concept of soil strength is demonstrated by the maximum slope, φφ, of a pile of , of a pile of soil. Any more soil dumped on the pile, would slip down the slope. The maximum slope is the angle, soil. Any more soil dumped on the pile, would slip down the slope. The maximum slope is the angle,
φ
φ. Figure 2 shows the soil friction angle,. Figure 2 shows the soil friction angle,φφ, for different qualities of soil., for different qualities of soil.
Figure 2: Soil Friction Angle,
Figure 2: Soil Friction Angle, φφ, for Various Soil Types, for Various Soil Types
φ
φ = 45= 45oo Select, granular soil. Slope,Select, granular soil. Slope,φφ, may be even greater if soil is well compacted., may be even greater if soil is well compacted.
φ
φ = 30= 30oo Good soil. The soil may be uncompacted, or possibly moist.Good soil. The soil may be uncompacted, or possibly moist.
φ
φ = 15= 15oo Poor soil. Poor soil may contain Poor soil. Poor soil may contain a high percentage of fines, and a high percentage of fines, and may be wet.may be wet.
φ
φ = 0= 0oo Mud. The soil is liquid, and has no slope angle,Mud. The soil is liquid, and has no slope angle, φφ
Soil Strength: For granular, cohesionless, soil, strength is
Soil Strength: For granular, cohesionless, soil, strength is σσmaxmax//σσminmin=(1+sin=(1+sinφφ )/(1-sin )/(1-sin φφ ). For active ). For active
soil resistance, Figure 3 shows how soil strength is increased by increasing the soil friction angle, soil resistance, Figure 3 shows how soil strength is increased by increasing the soil friction angle, φφ..
For example, soil strength is doubled by increasing soil friction angle from 30° to 45°. In poor soil For example, soil strength is doubled by increasing soil friction angle from 30° to 45°. In poor soil (low strength), a flexible pipe can be deflected by surface live loads, and loose soil embedment (low strength), a flexible pipe can be deflected by surface live loads, and loose soil embedment -especially loose soil under the haunches. It is possible for good soil with excessive fines, to become especially loose soil under the haunches. It is possible for good soil with excessive fines, to become poor soil when it gets wet. Water can get into the soil from a water table, storm-water, or leaks in poor soil when it gets wet. Water can get into the soil from a water table, storm-water, or leaks in pipes. If strengths for stability are critical, tests should be performed on saturated soil.
pipes. If strengths for stability are critical, tests should be performed on saturated soil.
Figure 3: Soil Strength Figure 3: Soil Strength PIPE STABILITY
PIPE STABILITY
Flexible pipes, by design, are expected to deflect when buried. Figures 4a, and 4b show a soil cube at Flexible pipes, by design, are expected to deflect when buried. Figures 4a, and 4b show a soil cube at the side of a flexible pipe. It has both active stress and passive resistance.
the side of a flexible pipe. It has both active stress and passive resistance.
Active Soil Stress Active Soil Stress
σ
σy y max ,max ,σσxxminmin
Pipe is deflected inwards Pipe is deflected inwards
Passive Soil Stress (resistance) Passive Soil Stress (resistance)
σ
σxxmax ,max ,σσy y minmin
Pipe is deflected outwards Pipe is deflected outwards
Figures 4a, b: Active and Passive Soil Stresses Figures 4a, b: Active and Passive Soil Stresses
Active Soil Stress: At active soil stress, the horizontal soil stress,
Active Soil Stress: At active soil stress, the horizontal soil stress, σσxx, is minimum. It presses against, is minimum. It presses against
the pipe. At soil slip, the pipe deflects inward. Active soil stress could be caused by a surface wheel the pipe. At soil slip, the pipe deflects inward. Active soil stress could be caused by a surface wheel load approaching the pipe.
load approaching the pipe.
Passive soil stress (passive resistance): At passive soil resistance, the horizontal stress, Passive soil stress (passive resistance): At passive soil resistance, the horizontal stress, σσxx, is, is
maximum. When soil slips, the pipe deflects outward. This instability could be caused by a live load maximum. When soil slips, the pipe deflects outward. This instability could be caused by a live load crossing over the pipe with less than minimum (adequate) soil cover.
crossing over the pipe with less than minimum (adequate) soil cover.
Pipe stability is controlled primarily by stability (strength) of the soil embedment. Pipe stability is controlled primarily by stability (strength) of the soil embedment.
Soil Tests: The literature is replete with soil tests. For buried flexible pipe-soil interaction, the Soil Tests: The literature is replete with soil tests. For buried flexible pipe-soil interaction, the following are the most important properties to be controlled. Appropriate laboratory tests are following are the most important properties to be controlled. Appropriate laboratory tests are available.
available. 1.
1. Soil strength - tri-axial tests; on both dry and wet soil.Soil strength - tri-axial tests; on both dry and wet soil. 2.
2. Soil density - to assure soil quality and required compaction at optimum moisture content.Soil density - to assure soil quality and required compaction at optimum moisture content. 3.
3. Soil compression - stress-strain diagram (confined compression, or tri-axial, or modified)Soil compression - stress-strain diagram (confined compression, or tri-axial, or modified) 4.
4. Particle size gradation - to control fines, to reduce soil particle migration, to allow drainage.Particle size gradation - to control fines, to reduce soil particle migration, to allow drainage. For cases where poor soils may be used for embedment or backfill, other tests, such as liquid limit For cases where poor soils may be used for embedment or backfill, other tests, such as liquid limit and plasticity index, may be of value. Chemicals in the soil could pose a problem. Conditions for and plasticity index, may be of value. Chemicals in the soil could pose a problem. Conditions for potential liquefaction require limits on the soil friction angle and on soil density. For most
potential liquefaction require limits on the soil friction angle and on soil density. For most installations, there is no need to encumber the required tests with these conditions.
installations, there is no need to encumber the required tests with these conditions.
IMPORTANCE OF SOIL IMPORTANCE OF SOIL
Basic properties of both pipe and soil are strength and stiffness: Basic properties of both pipe and soil are strength and stiffness:
Pipe
Pipe Soil Soil (granular)(granular)
strength
strength = = yield yield stress stress of of steel steel strength strength == σσmaxmax//σσminmin= (1 + sin= (1 + sin φφ )/(l - sin )/(l - sin φφ ) at soil slip ) at soil slip
stiffness = EI/r
stiffness = EI/r33 (ring (ring stiffness), stiffness), psi psi stiffness = stiffness = E'= E'= modulus modulus of of elasticity elasticity in in compressioncompression The importance of soil is demonstrated in the following example of allowable vertical external The importance of soil is demonstrated in the following example of allowable vertical external
pressures, P, including internal vacuum on a buried steel pipe, with 1.0-inch thick mortar coating pressures, P, including internal vacuum on a buried steel pipe, with 1.0-inch thick mortar coating and 0.5-inch thick mortar lining.
and 0.5-inch thick mortar lining.
Pipe Soil
Pipe Soil
D
D 96-inch 96-inch Diameter Diameter SC3 SC3 Soil Soil ClassificationClassification r
r 48-inch 48-inch (radius)(radius) γγ 115 pcf, soil density at 85% standard density 115 pcf, soil density at 85% standard density t
t 0.40-inch wall 0.40-inch wall thicknessthickness γγss 135 pcf, saturated soil density 135 pcf, saturated soil density
∑
∑EI/rEI/r33 7 7 psi psi (ring (ring stiffness) stiffness) E' E' Soil Soil StiffnessStiffness
E 30(10
E 30(1066 ) ) psi psi modulus modulus of of elast. elast. H H Height Height of of soil soil covercover
σ
σf f 42(1042(10 33
)
) psi, psi, yield yield stress stress SSEE Vertical Soil ModulusVertical Soil Modulus From the AWWA M11 manual, the
From the AWWA M11 manual, the allowable verticallowable vertical pressures on the al pressures on the buried pipe are shown in buried pipe are shown in Table 2.Table 2. Soil stiffness varies from lightly compacted, E' = 700 psi, to compacted, E' = 2300 psi. Allowable Soil stiffness varies from lightly compacted, E' = 700 psi, to compacted, E' = 2300 psi. Allowable external pressure, P, is based on
external pressure, P, is based on “failure,” with a safety factor included.“failure,” with a safety factor included.
Table 2: Allowable External Pressures, P Table 2: Allowable External Pressures, P
Soil
Soil Stiffness, Stiffness, E' E' P P @ @ H H = = 4-ft 4-ft P P @ @ H H = = 15-ft15-ft
700
700 psi psi 29 29 psi psi (dry) (dry) 24 24 psi psi (sat.) (sat.) 37 37 psi psi (dry) (dry) 30 30 psi psi (sat.)(sat.) 2300
2300 psi psi 52 52 psi psi (dry) (dry) 43 43 psi psi (sat.) (sat.) 67 67 psi psi (dry) (dry) 55 55 psi psi (sat.)(sat.) 0
Pressure, P, includes vertical soil pressur
Pressure, P, includes vertical soil pressure on the pipe, and any vacuum in e on the pipe, and any vacuum in the pipe. In the case of dry the pipe. In the case of dry soil, and height of cover, H = 15-ft, applied pressure is P = 12 psi. With vacuum, 14.7 psi, the
soil, and height of cover, H = 15-ft, applied pressure is P = 12 psi. With vacuum, 14.7 psi, the appliedapplied pressure is 26.7 psi which is well below the allowable P = 37 psi.
pressure is 26.7 psi which is well below the allowable P = 37 psi. In Table 2, internal vacuum, 14.7 psi, can be added to the soil
In Table 2, internal vacuum, 14.7 psi, can be added to the soil pressure without exceedinpressure without exceeding theg the allowable pressure, P. The only exception is t
allowable pressure, P. The only exception is the liquefied soil embedment (mud). he liquefied soil embedment (mud). Of possibleOf possible concern is height of cover, H = 4-ft, in saturated soil, for which allowable P = 24 psi. Saturated soil concern is height of cover, H = 4-ft, in saturated soil, for which allowable P = 24 psi. Saturated soil pressure on the pipe is 3.2 psi. With vacuum, 14.7 psi, applied pressure is 17.9 psi. The pipe is pressure on the pipe is 3.2 psi. With vacuum, 14.7 psi, applied pressure is 17.9 psi. The pipe is therefore adequate.
therefore adequate. The conditions in
The conditions in Table 2 meet allowable ring deflection, Table 2 meet allowable ring deflection, d = 2% for CML/CMC pipe. The worstd = 2% for CML/CMC pipe. The worst case for ring deflection is, H = 15-ft of dry soil, and E' = 700 psi vertical soil modulus for which case for ring deflection is, H = 15-ft of dry soil, and E' = 700 psi vertical soil modulus for which vertical compressio
vertical compression (strain) of sidefill soil is n (strain) of sidefill soil is 2.0%. Ring deflection should be controlled. The 2.0%. Ring deflection should be controlled. The AWWAAWWA values for allowable pressure, P,
values for allowable pressure, P, are conservative because of concern for out-of-roundness,are conservative because of concern for out-of-roundness, imprecision in soil properties, and non-uniform soil placement and compaction.
imprecision in soil properties, and non-uniform soil placement and compaction.
Ring deflection is about equal to the vertical compression of the sidefill soil. Vertical compression is Ring deflection is about equal to the vertical compression of the sidefill soil. Vertical compression is the average vertical strain,
the average vertical strain, εε, of the soil in the sidefill. Strain is, of the soil in the sidefill. Strain is εε==σσ//EE
SS where where σσis the vertical stressis the vertical stress
at springline, and E
at springline, and ESSis vertical soil modulus from a confined compression test in a laboratory.is vertical soil modulus from a confined compression test in a laboratory. For example, consider the case
For example, consider the case of H = 15 ft of dry soil, of H = 15 ft of dry soil, lightly compacted, unit weight = lightly compacted, unit weight = 115 pcf.115 pcf. The average vertical soil strain in the
The average vertical soil strain in the sidefill occurs at springline where vertical stress sidefill occurs at springline where vertical stress isis σσ= 115= 115 pcf(15 + 2.5)ft = 14 psi. From the soils lab, the vertical soil modulus is E
pcf(15 + 2.5)ft = 14 psi. From the soils lab, the vertical soil modulus is ESS≥≥700 psi. Therefore,700 psi. Therefore,
average strain in the sidefill is
average strain in the sidefill is εε==σσ//EE
SS= 14/700 = 2%. The allowable ring deflection, d = 2%, is not= 14/700 = 2%. The allowable ring deflection, d = 2%, is not
exceeded, but ring deflection should be controlled during installation. exceeded, but ring deflection should be controlled during installation. If native material can not meet the
If native material can not meet the requirement of the project, importing material or requirement of the project, importing material or flowable fill canflowable fill can be considered.
be considered.
FLOWABLE FILL FLOWABLE FILL
Flowable fill is soil-cement that flows under the pipe and becomes uniform bedding, which in turn Flowable fill is soil-cement that flows under the pipe and becomes uniform bedding, which in turn reduces the differential settlement of adjoining pi
reduces the differential settlement of adjoining pipes. Portland cement or fly ash pes. Portland cement or fly ash improvesimproves
flowability and adds strength. It is assumed that all voids are filled without the need for compaction flowability and adds strength. It is assumed that all voids are filled without the need for compaction due to the “flowing” characteristics of the material.
due to the “flowing” characteristics of the material. The term Controlled Low Strength MaterialThe term Controlled Low Strength Material (CLSM) is also used to describe flowable fill.
(CLSM) is also used to describe flowable fill. Advantages of flowable fill include:
Advantages of flowable fill include: 1.
1. The need to level and compact bedding for alignment is avoided. The need to level and compact bedding for alignment is avoided. 2.
2. Native soil can be used for bedding and embedment instead of imported select soil.Native soil can be used for bedding and embedment instead of imported select soil. 3.
3. Multifunctional trenching and installing is facilitated.Multifunctional trenching and installing is facilitated. 4.
4. Trenches can be narrow. Tunnels need to be only slightly larger in diameter than the pipe. Trenches can be narrow. Tunnels need to be only slightly larger in diameter than the pipe. 5.
5. Flowable fill helps to protect the pipe in the event of future excavations.Flowable fill helps to protect the pipe in the event of future excavations. 6.
6. Flowable fill provides additional stiffness.Flowable fill provides additional stiffness. Disadvantages of flowable fill include:
Disadvantages of flowable fill include: 1.
1. Cost of adding Portland cement and making the slurry is high. Delivery in mixers is costly.Cost of adding Portland cement and making the slurry is high. Delivery in mixers is costly. Costs can be higher by
2.
2. Delays occur in setting forms Delays occur in setting forms for each "pouring" and in curing time for each "pouring" and in curing time before backfilling.before backfilling. 3.
3. Due to cure time delays, productivity on a project is reduced, sometimes by as much asDue to cure time delays, productivity on a project is reduced, sometimes by as much as much as 40%, according to data
much as 40%, according to data received from Contractors.received from Contractors. 4.
4. During placement of the fill, care is required to prevent flotation of the pipe. Constraints forDuring placement of the fill, care is required to prevent flotation of the pipe. Constraints for prevention of flotation can cause variability o
prevention of flotation can cause variability of the geometry of the flowable fill embedment.f the geometry of the flowable fill embedment. 5.
5. Care must also be Care must also be taken during fill placement to prevent collapse by taken during fill placement to prevent collapse by external hydrostaticexternal hydrostatic pressure.
pressure. 6.
6. Flowable fill around the pipe may crack when internal pressure re-rounds the pipe. If theFlowable fill around the pipe may crack when internal pressure re-rounds the pipe. If the flowable fill is high-strength and cracks under soil movement, stress concentrations on the flowable fill is high-strength and cracks under soil movement, stress concentrations on the pipe are much greater for high-strength than for low-strength embedment.
pipe are much greater for high-strength than for low-strength embedment.
CLSM must be fluid enough to flow under the pipe, and strong enough to hold the pipe in shape. A CLSM must be fluid enough to flow under the pipe, and strong enough to hold the pipe in shape. A slump of 10 inches on a flow table of 12 inch diameter is often specified. Strength should be low slump of 10 inches on a flow table of 12 inch diameter is often specified. Strength should be low enough that future excavations do not
enough that future excavations do not damage the pipe. Unconfined strength of the CLSM damage the pipe. Unconfined strength of the CLSM within awithin a range of 40 psi to 100 psi is recommended. Only a small amount of Portland cement is added if the range of 40 psi to 100 psi is recommended. Only a small amount of Portland cement is added if the native soil has enough fines to make the slurry flowable. Most native soils have fines, silt and clay. native soil has enough fines to make the slurry flowable. Most native soils have fines, silt and clay. CLSM does not require concrete quality aggregate. As much as 60% silt in native soil with one sack CLSM does not require concrete quality aggregate. As much as 60% silt in native soil with one sack of Portland cement per cubic yard has been used successfully.
of Portland cement per cubic yard has been used successfully. Figure 5a shows bedding schematic for
Figure 5a shows bedding schematic for pipe using CLSM. Figure 5b is pipe using CLSM. Figure 5b is a graph of the ring deflectiona graph of the ring deflection term as a function of flowable fill angle,
term as a function of flowable fill angle, αα. The ring deflection term is dimensionless and, therefore,. The ring deflection term is dimensionless and, therefore, applies to any size of pipe. Assumptions of no side support are worst-case. From the graph, ring applies to any size of pipe. Assumptions of no side support are worst-case. From the graph, ring deflection is reduced as angle,
deflection is reduced as angle, αα, is increased., is increased.
Figure 5a, b: Flowable Fill
Figure 5a, b: Flowable Fill Bedding, Ring Deflection-BeddingBedding, Ring Deflection-Bedding Angle Relationship (ASCE 2009)
Angle Relationship (ASCE 2009) α
α angle angle of of flowable flowable fill fill r r radius radius of of the the ring ring d
d ring ring deflection, deflection, ( ( ΔΔy/D) y/D) PP ververtictical al prepressurssuree E
E modulus modulus of of elasticelasticity ity of of pipe pipe II moment moment of of inertiinertia a of of wallwall
Δ
Δy y vertical vertical displacement displacement of of the the crown, crown, A, A, due due to to PP Example:
Example:
What is ring deflection of a steel pipe for which D/t
What is ring deflection of a steel pipe for which D/t = 240, P=600 psi?= 240, P=600 psi? From Figure 5b: From Figure 5b: At Atαα=30°,=30°,d = 2.4%d = 2.4% At Atαα=60°,=60°,d = 0.8%d = 0.8%
Deflection at
Deflection atαα=60° is only one third of =60° is only one third of ring deflection atring deflection at αα=30°. The height of flowable fill at=30°. The height of flowable fill at
α
α=30° is 0.07D. When=30° is 0.07D. When αα=60°, the height of flowable fill =60°, the height of flowable fill is 0.25D.is 0.25D.
PIPELINE CONSTRUCTION COSTS PIPELINE CONSTRUCTION COSTS
From discussions with Contractors during the writing of this paper, the Authors discovered that it is From discussions with Contractors during the writing of this paper, the Authors discovered that it is difficult to obtain real meaningful cost information from the construction community as they do not difficult to obtain real meaningful cost information from the construction community as they do not quantify costs in a manner that is conducive to providing breakdowns on the different subroutines quantify costs in a manner that is conducive to providing breakdowns on the different subroutines of a pipeline installation. These subroutines can be categorized as excavating trenches, installing of a pipeline installation. These subroutines can be categorized as excavating trenches, installing pipe, installing pipe zone backfill, compaction of the backfill, installing trench backfill, rough grading pipe, installing pipe zone backfill, compaction of the backfill, installing trench backfill, rough grading and final grading. While each of these subroutines exists on all projects, the reality is that the cost for and final grading. While each of these subroutines exists on all projects, the reality is that the cost for each is interrelated, whereby each item can change
each is interrelated, whereby each item can change by a litany of variables that exist by a litany of variables that exist on every on every
project. Establishing median costs or unit pricing for large diameter pipeline construction projects is project. Establishing median costs or unit pricing for large diameter pipeline construction projects is difficult because of the wide range of variables and intangibles that impact costs, some of which are difficult because of the wide range of variables and intangibles that impact costs, some of which are discussed below. Ultimately, it is overall productivity that can be achieved by the entire construction discussed below. Ultimately, it is overall productivity that can be achieved by the entire construction crew that drives costs for installation on
crew that drives costs for installation on every project.every project.
A few significant considerations/variables that can impact productivity and ultimately installation A few significant considerations/variables that can impact productivity and ultimately installation
costs are listed below: costs are listed below: 1.
1. type of pipe materials permitted for use type of pipe materials permitted for use on a projecton a project 2.
2. pressure class of the pipe material pressure class of the pipe material specified --- higher pressure classes raise the cost specified --- higher pressure classes raise the cost of the pipeof the pipe 3.
3. depth of bury of the pipeline and how that impacts productivity depth of bury of the pipeline and how that impacts productivity 4.
4. impact of existing utilities, fence lines, etc. to productivity impact of existing utilities, fence lines, etc. to productivity 5.
5. in-situ soil conditions and how they effect productivity in-situ soil conditions and how they effect productivity 6.
6. cost of backfill materials/aggregates, which are impacted greatly by distance from quarry cost of backfill materials/aggregates, which are impacted greatly by distance from quarry (hauling costs)
(hauling costs) 7.
7. backfill compaction requirements and how they impact productivity backfill compaction requirements and how they impact productivity 8.
8. cost of flowable fill backfill materials if specified on a job, whether it has to be imported orcost of flowable fill backfill materials if specified on a job, whether it has to be imported or whether it can be manufactured on site, equipment necessary to manufacture it on site whether it can be manufactured on site, equipment necessary to manufacture it on site 9.
9. condition of the raw materials commodity markets and how that impacts cost of fuel, concrete,condition of the raw materials commodity markets and how that impacts cost of fuel, concrete, pipe, bedding etc.
pipe, bedding etc. 10.
10. easement restrictions, if any, and how that impacts productivity easement restrictions, if any, and how that impacts productivity 11.
11. in urban areas, if the route of the pipeline is it in the pavement, how the removals andin urban areas, if the route of the pipeline is it in the pavement, how the removals and restorations impact productivity
restorations impact productivity 12.
12. if roads can not be closed and traffic maintenance is required, how that impacts productivity if roads can not be closed and traffic maintenance is required, how that impacts productivity 13.
13. work time restrictions and how that impacts productivity work time restrictions and how that impacts productivity 14.
14. whether excess trench materials whether excess trench materials can be left at the can be left at the site, site, or location of the nearest or location of the nearest dumpdump 15.
15. in rural areas, what restrictions are there for access in rural areas, what restrictions are there for access and how many points and how many points of access existof access exist 16.
16. how do restoration requirements impact productivity how do restoration requirements impact productivity 17.
17. time frame for the pipeline to be time frame for the pipeline to be constructed and contract time limitationsconstructed and contract time limitations In their efforts to quantify construction
In their efforts to quantify construction cost differences between the use of native material cost differences between the use of native material versusversus granular import backfill versus CLSM, the Authors gathered the data shown in Tables 3 through 6. granular import backfill versus CLSM, the Authors gathered the data shown in Tables 3 through 6. The installed cost of native material versus importing can be seen in Tables 3 and 5 by comparing The installed cost of native material versus importing can be seen in Tables 3 and 5 by comparing
the “Cost of compaction labor and equipment per linear cubic yard” to the “Total cost per cubic the “Cost of compaction labor and equipment per linear cubic yard” to the “Total cost per cubic yard for embedment installed”.
yard for embedment installed”. The installed cost when import is included is between 2 The installed cost when import is included is between 2 and 3.5and 3.5 times the cost of simply using native material.
For 36-inch through 48-inch diameter pipe, the difference between the cost
For 36-inch through 48-inch diameter pipe, the difference between the cost of CLSM versus nativeof CLSM versus native material is approximately $86/cu.yard, a factor of 17 over the
material is approximately $86/cu.yard, a factor of 17 over the cost of using native material. cost of using native material. For 60-For 60-inch through 66-60-inch pipe, the cost
inch through 66-inch pipe, the cost difference of CLSM versus native material is roughly difference of CLSM versus native material is roughly $150/cu.yard, a factor again of 17 over the cost of granular material.
$150/cu.yard, a factor again of 17 over the cost of granular material. Construction productivity goesConstruction productivity goes down by a factor of almost 3 when CLSM is used instead of native or granular import backfill. down by a factor of almost 3 when CLSM is used instead of native or granular import backfill.
Table 3:
Table 3: Cost of Cost of Granular Fill Granular Fill (36-in thru (36-in thru 48-in) 48-in) Table 4: Table 4: Cost of Cost of CLSM CLSM (36-in thru (36-in thru 48-in)48-in)
Trench
Trench Width Width (ft.) (ft.) 6.5 6.5 Trench Trench Width Width (ft.) (ft.) 6.56.5 Embedment
Embedment Height Height (ft.) (ft.) 5 5 Embedment Embedment Height Height (ft.) (ft.) 55 Embedment Envelope (yd
Embedment Envelope (yd33 ) ) 1 1 Trench Trench Length Length 11
Pipe
Pipe Diameter Diameter (ft) (ft) 3.5 3.5 Embedment Embedment Envelope Envelope (yd(yd33 ) ) 11
Pipe
Pipe Volume Volume 0 0 Pipe Pipe Diameter Diameter (ft) (ft) 3.53.5 Pipe
Pipe Volume Volume 00
Conversion
Conversion & & Waste Waste Factor Factor 1.11.1 Cubic Yards Per Linear Foot
Cubic Yards Per Linear Foot 0.930.93 Conversion Conversion & & Waste Waste Factor Factor 1.11.1 Cubic Yards Per Linear Foot
Cubic Yards Per Linear Foot 0.930.93 Cost of compaction labor and
Cost of compaction labor and equipment/day
equipment/day $2,800.00$2,800.00 Productivity
Productivity (LF/day) (LF/day) 500 500 Cost Cost of of compaction compaction labor labor and and equipment/day equipment/day $9,500.00$9,500.00 Cost
Cost of of compaction compaction labor labor and and equipment/LF equipment/LF $5.60 $5.60 Productivity Productivity (LF/day) (LF/day) 500500 Cost of compaction labor and
Cost of compaction labor and equipment/Lyd
equipment/Lyd33 $5.22 $5.22 Cost Cost of of compaction compaction labor labor and and equipment/LF equipment/LF 200200
Cost of embedment materials/yd
Cost of embedment materials/yd33 $8.50 $8.50 Cost Cost of of compaction compaction labor labor and and equipment/Lydequipment/Lyd33 $28.50$28.50
Hauling/yd
Hauling/yd33 $6.00 $6.00 Cost Cost of of embedment embedment materials/ydmaterials/yd33 $26.57$26.57
Total Cost / yd
Total Cost / yd33for Embedment Installedfor Embedment Installed $19.22$19.22 Hauling/ydHauling/yd33 $65.00$65.00
Total Cost / yd
Total Cost / yd33for Embedment Installedfor Embedment Installed $91.57$91.57
Table 5: Cost of Granular Fill
Table 5: Cost of Granular Fill (60-in thru 66-in)(60-in thru 66-in) Table 6: Cost of CLSM (60-in thru Table 6: Cost of CLSM (60-in thru 66-in)66-in)
Trench Width (ft.)
Trench Width (ft.) 8.58.5 Trench Width (ft.) Trench Width (ft.) 8.58.5
Embedment Height (ft.)
Embedment Height (ft.) 77 Embedment Height (ft.)Embedment Height (ft.) 77
Embedment Envelope (yd
Embedment Envelope (yd33 ) ) 22 Trench Length Trench Length 11
Pipe Diameter (ft)
Pipe Diameter (ft) 5.55.5 Embedment Envelope (ydEmbedment Envelope (yd33 ) ) 22
Pipe Volume
Pipe Volume 11 Pipe Diameter (ft)Pipe Diameter (ft) 5.55.5
Pipe Volume
Pipe Volume 11
Conversion & Waste Factor
Conversion & Waste Factor 1.11.1
Cubic Yards Per Linear Foot
Cubic Yards Per Linear Foot 1.461.46 Conversion & Waste FactorConversion & Waste Factor 1.11.1
Cubic Yards Per Linear Foot
Cubic Yards Per Linear Foot 1.461.46
Cost of compaction labor and Cost of compaction labor and equipment/day
equipment/day $2,800.00$2,800.00 Productivity (LF/day)
Productivity (LF/day) 350350 Cost of compaction labor and equipment/day Cost of compaction labor and equipment/day $16,000.0$16,000.0
00
Cost of compaction labor and equipment/LF
Cost of compaction labor and equipment/LF $8.00$8.00 Productivity (LF/day)Productivity (LF/day) 400400
Cost of compaction labor and Cost of compaction labor and equipment/Lyd
equipment/Lyd33 $11.65$11.65 Cost of compaction labor and equipment/LFCost of compaction labor and equipment/LF 150150
Cost of embedment materials/yd
Cost of embedment materials/yd33 $8.50$8.50 Cost of compaction labor and equipment/LydCost of compaction labor and equipment/Lyd33 $66.67$66.67
Hauling/yd
Hauling/yd33 $6.00$6.00 Cost of embedment materials/ydCost of embedment materials/yd33 $97.11$97.11
Total Cost / yd
Total Cost / yd33for Embedment Installedfor Embedment Installed $26.15$26.15 Hauling/ydHauling/yd33 $65.00$65.00
Total Cost / yd
Total Cost / yd33for Embedment Installedfor Embedment Installed $162.11$162.11
CONCLUSION & RECOMMENDATIONS CONCLUSION & RECOMMENDATIONS
1.
1. For all buried pipe projects, consideration must first be given to the use of native soils for theFor all buried pipe projects, consideration must first be given to the use of native soils for the backfill. In most typical situations, proper installation and backfill compaction at burial depths of backfill. In most typical situations, proper installation and backfill compaction at burial depths of 3 to 15 ft, can make use o
3 to 15 ft, can make use of native soils, even when they f native soils, even when they meet the description of the worst soimeet the description of the worst soilsls permitted by Table 1. Only when these conditions can not be met should an engineer consider permitted by Table 1. Only when these conditions can not be met should an engineer consider
further analysis and importing backfill material. The cost savings of using native materials over further analysis and importing backfill material. The cost savings of using native materials over importing or using CLSM can be significant.
importing or using CLSM can be significant. 2.
2. The cost of importing granular backfill materials can be as high as 2 to 3.5 times the cost of The cost of importing granular backfill materials can be as high as 2 to 3.5 times the cost of using native materials.
using native materials. 3.
3. For pipe diameters in the 36-inch through For pipe diameters in the 36-inch through 48-inch range, the cost of using CLSM can 48-inch range, the cost of using CLSM can be 17be 17 times higher than the cost of
times higher than the cost of using native materials. For 60-inch through 66-inch diameter pipe,using native materials. For 60-inch through 66-inch diameter pipe, the cost difference of CLSM versus native materials is
the cost difference of CLSM versus native materials is again higher by a factor of 17.again higher by a factor of 17. 4.
4. Construction productivity can be reduced by a factor of almost 3 when CLSM is utilized insteadConstruction productivity can be reduced by a factor of almost 3 when CLSM is utilized instead of native or granular import backfill materials.
of native or granular import backfill materials. 5.
5. While engineers often search for and associate an E' value with CLSM/flowable fill, and while While engineers often search for and associate an E' value with CLSM/flowable fill, and while the Authors have seen values ranging from
the Authors have seen values ranging from 3000 psi to recommendations as high as 3000 psi to recommendations as high as 25,000 psi25,000 psi (Howard 1996), in reality this is meaningless. E', as used in the Modified Iowa equation, is a (Howard 1996), in reality this is meaningless. E', as used in the Modified Iowa equation, is a horizontal modulus of elasticity; it is not linear and is not constant. The Modified Iowa equation horizontal modulus of elasticity; it is not linear and is not constant. The Modified Iowa equation does not typically apply to soils with cohesion. A CLSM backfill system, on the other hand, is does not typically apply to soils with cohesion. A CLSM backfill system, on the other hand, is essentially “rigid” and therefore, has cohesion. E', therefore, does not apply to CLSM backfill essentially “rigid” and therefore, has cohesion. E', therefore, does not apply to CLSM backfill systems.
systems. 6.
6. The topic of pipe-soil interaction receives The topic of pipe-soil interaction receives a disproportionate attention from the designa disproportionate attention from the design
engineering community for flexible pressure pipelines. The concern for soil pressure on a pipe is engineering community for flexible pressure pipelines. The concern for soil pressure on a pipe is limited to empty pipe or gravity flow pipelines where the conduit never flows full. In pressure limited to empty pipe or gravity flow pipelines where the conduit never flows full. In pressure piping systems, the internal pressure is typically much greater than soil pressure on the pipe; piping systems, the internal pressure is typically much greater than soil pressure on the pipe; internal pressure essentially supports the soil load.
internal pressure essentially supports the soil load. 7.
7. The prism load is used for calculating the deflection in a buried, non-pressurized flexible pipe. The prism load is used for calculating the deflection in a buried, non-pressurized flexible pipe. The prism load is the
The prism load is the maximum maximum long-term load that a buried flexible long-term load that a buried flexible pipe can experience afterpipe can experience after installation. Per AWWA M11, deflection of steel pipe is calculated using the prism load. In installation. Per AWWA M11, deflection of steel pipe is calculated using the prism load. In pressure pipes, long-term deflections are prevented by internal
pressure pipes, long-term deflections are prevented by internal pressure. Design deflection lag pressure. Design deflection lag factor, D
factor, DLL=1.0.=1.0.
8.
8. Ring deflection is about equal to the vertical compression of the embedment. This is worst-case.Ring deflection is about equal to the vertical compression of the embedment. This is worst-case. When using a rigid lining, and/or rigid coating for the corrosion protection of steel pipe, the When using a rigid lining, and/or rigid coating for the corrosion protection of steel pipe, the
additional stiffness decreases ring deflection. additional stiffness decreases ring deflection. 9.
9. Resistance provided by the pipe material stiffness is typically only 5% of the total resistance toResistance provided by the pipe material stiffness is typically only 5% of the total resistance to deflection. The remaining 95% of the resistance to
deflection. The remaining 95% of the resistance to deflection is provided by the sdeflection is provided by the soil.oil. 10.
10. For granular, cohesionless soil, the soil strength is increased by increasing the soil friction angle,For granular, cohesionless soil, the soil strength is increased by increasing the soil friction angle,
φ
φ. Soil strength is doubled by increasing . Soil strength is doubled by increasing φφfrom 30from 30ooto 45to 45oo.. 11.
11. The percentage of high PI soils in a backfill system (certain clays), must be limited by The percentage of high PI soils in a backfill system (certain clays), must be limited by specification in order to prevent loss of
specification in order to prevent loss of strength when the soil gets wet.strength when the soil gets wet.
ACKNOWLEDGEMENTS ACKNOWLEDGEMENTS
The Authors would like to thank Mr. Scott Parrish, Vice President, Garney Construction, Kansas The Authors would like to thank Mr. Scott Parrish, Vice President, Garney Construction, Kansas
City, MO, for his assistance with the construction costs discussed in this paper. City, MO, for his assistance with the construction costs discussed in this paper.
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