Moser A.P., Folkman S. Buried Pipe Design

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Plastic Flexible Pipe Products 499

Hinging.

The term hinging is used to describe yielding of the mater-

ial due to an excessive bending moment in the wall. These hinges usu-

ally take place at the 3 and 9 o’clock positions. These plastic hinges,

although primarily due to bending, can be influenced by a combination

of localized buckling and wall yielding caused by thrust in the wall of

the pipe. Hinging is usually considered to be a structural performance

limit.

Wall crushing. The term wall crushing is used to describe yielding in

the wall produced by excessive compressive stresses resulting from

thrust in the wall. These large compressive stresses produce local

yielding and/or local buckling. Crushing is usually considered to be a

structural performance limit (see Figs. 7.74 and 7.75).

Summary and conclusions. The pipe cross section started out circular

and became elliptical as the height of cover increased. For the 75 and

77 percent dense soils, this deviation from a circle to an elliptical

shape was quite pronounced, and for the 83 percent dense soil the

deflected shape was elliptical, but less pronounced. The shape of the

pipe in the 95 percent dense soil remained closer to being circular even

Figure 7.74 Wall crushing and dimpling pattern on left side. Density  95 percent, and

load is 152 ft of cover.

500 Chapter Seven

for extremely high heights of cover. None of the test pipes ever exhib-

ited a so-called squaring or a square shape at any load. This result is

just what one would expect. The ratio of ring compression stress to

bending stress for the 75 percent dense soil is very low (much less

than 1). For the 83 percent dense soil, this ratio is low, but may

approach a value of 1. The ratio of ring compression stress to bending

stress, for the pipe tested in soil compacted to 95 percent of standard

Proctor density, is much greater than unity.

For polyethylene, which has a fairly low modulus, ring compression

stresses cause circumferential ring shortening. This ring shortening is

small for pipes installed with low heights of cover and in low to mod-

erately compacted soils. For high-density soils, at high earth covers,

Figure 7.75 Wall crushing and dimpling pattern on

right side. Density  95 percent and load is 152 ft

of cover.

Plastic Flexible Pipe Products 501

this circumferential ring shortening is very significant and is the pri-

mary deformation that takes place. This circumferential shortening is

extremely beneficial in the performance of the pipe. The decrease in

circumference relieves the pipe ring of some of the soil pressure and

causes the surrounding granular pipe zone material to carry a higher

percentage of the load. This works on exactly the same principle as the

slotted bolt hole in corrugated metal pipe. In a very large measure, the

pipe in test 3 was able to withstand extremely high loads because of

the substantial circumferential shortening that took place

Noteworthy is the high load that can be applied without distress to

the pipe ring. Clearly, the pipes deflect more in loose soil than in dense

soil because loose soil compresses more. The pipes do not collapse, even

in loose soil.

The soil should be granular and carefully compacted if the pipe is

buried under high soil cover or under heavy surface loads. Granular

pipe-zone backfill material at moderate to high densities ensures that

the pipes will perform well at high earth covers.

Incipient dimpling occurred at equivalent depths of cover in the

range of 44 to 126 ft (see Table 7.21). For the pipes tested, this incipi-

ent dimpling load is primarily a function of soil density. Dimpling is

not a structural performance limit.

The load at which a structural performance limit takes place is also

a function of the soil density. For a relatively poor installation (75 per-

cent standard Proctor), the performance limit is at 55 ft of cover. For a

good installation (83 percent standard Proctor density), hinging or

cracking begins at about 70 ft of cover. For an excellent installation

(95 percent standard Proctor), the lowest performance limit was 143 ft

of cover (see Table 7.22).

Performance limits and preliminary design

recommendations for profile-wall HDPE

pipes

A performance limit for a pipe is reached when the pipe no longer per-

forms in an acceptable manner. For a polyethylene pipe, overdeflection,

wall buckling, and wall crushing are usually considered unacceptable.

Deflection is usually controlled by proper installation. Wall buckling

can be controlled by controlling strains and by maintaining proper wall

thicknesses. Wall crushing is controlled by maintaining an adequate

area per unit length. Thus, area per unit length is the most important

parameter since wall thickness is directly related to the area.

Pipe stiffness is directly related to moment of inertia which, in turn,

is a function of area, shape, and corrugation height. It is important to

meet minimum requirements for pipe stiffness. Increasing the pipe stiff-

ness above the minimum will give some added performance benefits.

TABLE 7.21 Dimpling of HDPE Pipes Tested

Load at start of Deflection

Manufacturer, Diameter, Percent of dimpling, ft at start of

Test no. & type in Proctor density of cover dimpling, percent

1 Y, type 1 60 75 44 13

2 Y, type 1 60 83 51 12

3 Y, type 1 60 95 108 3.5

4 Y, type 2 42 75 48 13

5 Y, type 2 42 83 55 12

6 Y, type 2 42 95 126 6

7 Z, type 3 48 77 52 18

8 Z, type 3 48 95 113 4.9

502

TABLE 7.22 Performance Limit for HDPE Pipes Tested

Load at Deflection

Manufacturer, Diameter, Percent of performance at performance

Test no. & type in Proctor density limit limit, percent

1 Y, type 1 60 75 Excessive deflection 17

and dimpling at

55 ft of cover

2 Y, type 1 60 83 Cracks at 72 ft 17

of cover

3 Y, type 1 60 95 Excessive dimpling 5.7

at 143 ft of cover

4 Y, type 2 42 75 Excessive deflection at 16, 20

55 ft of cover, hinging

at 69 ft of cover

5 Y, type 2 42 83 Hinging at 69 ft of 15.6, 18

cover, cracks at 76 ft

6 Y, type 2 42 95 Cracks at 168 ft of cover 9.3

7 Z, type 3 48 77 Excessive deflection 15

and rib buckling at 46 ft

8 Z, type 3 48 95 Rib buckling at 87 ft 3.6

of cover

503

504 Chapter Seven

However, a higher pipe stiffness without a sufficient wall area can lead

to premature pipe failure. The proper design sequence for a specific

installation condition is as follows:

1. Choose area to prevent wall crushing.

2. Choose wall thickness to prevent buckling.

3. Corrugation height will be fairly well defined by the first two

items if pitch and shape are predetermined.

Test data were analyzed, and numerous finite element analysis

(FEA) runs were made using the computer program PIPE5 for HDPE

pipe buried in a silty-sand soil. Various combinations of wall area, wall

thickness, and corrugation height were run to determine the minimum

wall area for various depths of cover. Because of space limitations in

this book, FEA data are not presented. Also, the theoretical basis for

much of the following discussion is the field of dimensional analysis,

sometimes called similitude, which is well documented in engineering

texts. The conditions under which one may use results from one sys-

tem (test setup) to predict the behavior of another similar system are

determined by dimensional analysis and the use of dimensionless

numbers. One major advantage of such a theory is that a solution that

works for one diameter can be immediately extrapolated to other

diameters.

Area and thickness. The most important parameters in controlling

performance are area per unit length and wall thickness. Corrugation

height is a direct function of the area and thickness and can be calcu-

lated if the area per unit length, thickness, and shape are known.

It is recommended that all thicknesses, including the thickness of

the liner, be as close to equal as possible. The reason for this is that the

liner is strained close to the same level as the crown of the corrugation

(rib top). This is due to high ring compressive stresses at the spring

line. The thinner the wall, the more likely it is that localized buckling

will occur. It may be a detriment to make one part of the profile thick

and another thin. The one area that will be thicker is where the liner

joins the valley of the corrugation. The strains across the cross section

are fairly uniform because a major contributor to strain is thrust in the

wall which is uniform through the cross section. The above discussion

should not be construed to mean that thickness alone will control

buckling, since it is well known that controlling the unsupported

length of a section is of equal importance.

Corrugation height. If the corrugation height is too large, bending

strains become significant; and if one increases corrugation height

Plastic Flexible Pipe Products 505

while holding the area constant, then thickness has to decrease, which

leads to early wall buckling.

Pitch. Geometrically, pitch should be a function of diameter. That is, if

a pitch of 4 in works well for a 24-in-diameter pipe, a pitch of 8 in will

work well for a 48-in-diameter pipe.

Nondimensional parameters. Test data and finite element data suggest

that, for a profile-wall pipe, certain dimensionless parameters be set

within limits to ensure satisfactory performance of pipes subjected to

earth loadings. Table 7.23 is a list of possible parameters that arise

from the data, along with suggested limiting values. These values

appear to provide adequate structural stability. The pipes tested gen-

erally meet these requirements. However, these studies are still in

progress, and these values are provided as guidelines only.

Acrylonitrile-Butadiene-Styrene Pipes

ABS plastic for pipe manufacture is available in several types and

grades per ASTM D 1788. The physical properties of the various ABS

materials vary quite widely, as is indicated by Table 7.24. Most ABS

TABLE 7.23 Profile Wall Pipe Dimensionless Geometric

Parameters to Control

Dimensionless Possible value for

parameters HDPE

min

/r 0.005

min

uns

0.02

I/r

4  10

5

A/r 0.02

/r 0.3

A  area per unit length of cross section of profile

r  effective radius of pipe

min

 minimum thickness of any particular section of profile

uns

 unsupported length (or width) of any particular

section of profile

I  moment of inertia per unit length of profile section

 length of profile section

TABLE 7.24 ABS Design Properties

Hydrostatic-design basis, lb/in

1600–3200

Hydrostatic-design stress, lb/in

800–1600

Elastic modulus, lb/in

200,000–400,000

Tensile stress, lb/in

2500–7000

Hazen-Williams coefficient C 150

Manning’s coefficient n 0.009

506 Chapter Seven

pipes, especially pressure pipes, are manufactured from grades with

the higher tensile properties.

Solid-wall ABS is used widely for drain, waste, and vent piping. It is

also used for smaller-diameter sanitary sewers. It is used to a very lim-

ited extent for smaller-diameter pressure piping.

The design methods and procedures are essentially the same as

those for PVC pipes with the appropriate elastic modulus for calculat-

ing pipe stiffness and the appropriate hydrostatic-design stress for

pressure pipe design.

A list of selected ASTM standards for ABS plastic pipes is given in

Table 7.25.

Example 7.15—An 8-in ABS pipe An 8-in solid-wall ABS pipe has been

selected for a sewer installation. The native soil is clay, and the water table

is about 8 ft deep. Most of the line will be installed about 10 ft deep, but one

section has depths up to 20 ft. The long-term deflection is not to exceed 5

percent. What pipe-zone soil and soil density should be specified?

From ASTM D 2751, SDR  42 and PS  F/y  20 lb/in

. Use Spangler’s

equation. See Eq. (7.4) of Example 7.1.

Required E′ 

H  20 ft PS  20 lb/in

 0.05

y



0.56H/(y/D)  PS



0.41

TABLE 7.25 Selected Standards for ABS Plastic Pipe

ASTM D 1788 Rigid Acrylonitrile-Butadiene-Styrene (ABS) Plastics

ASTM D 2680 Acrylonitrile-Butadiene-Styrene Composite Sewer Piping

ASTM D 2661 Acrylonitrile-Butadiene-Styrene Plastic Drain, Waste and Vent Pipe,

and Fittings

ASTM D 628 Acrylonitrile-Butadiene-Styrene Plastic Drain, Waste and Vent Pipe

Having a Foam Core

ASTM D 2468 Acrylonitrile-Butadiene-Styrene Plastic Pipe Fittings, Schedule 40

ASTM D 1527 Acrylonitrile-Butadiene-Styrene Plastic Pipe, Schedules 40 and 80

ASTM D 2282 Acrylonitrile-Butadiene-Styrene Plastic Pipe (SDR-PR)

ASTM D 2750 Acrylonitrile-Butadiene-Styrene Plastic Utilities Conduit and Fittings

ASTM D 2751 Acrylonitrile-Butadiene-Styrene Sewer Pipe and Fittings

ASTM D 2469 Socket-type Acrylonitrile-Butadiene-Styrene Plastic Pipe Fittings,

Schedule 80

ASTM D 2235 Solvent Cement for Acrylonitrile-Butadiene-Styrene Plastic Pipe and

Fittings

ASTM D 3138 Solvent Cements for Transition Joints between Acrylonitrile-Butadiener

Styrene and Poly(vinyl Chloride) (PVC) Nonpressure Piping

Components

ASTM D 2465 Threaded Acrylonitrile-Butadiene-Styrene Plastic Pipe Fittings,

Schedule 80

Plastic Flexible Pipe Products 507

E′  498 lb/in

 500 lb/in

The pipe-zone material should be either a granular material compacted

to 90 percent Proctor density or a crushed angular stone. Because of the

high water table, the crushed stone should be specified since little or no com-

paction will be required for an angular stone. See Table 3.4 for E′ values for

various soils.

Other Thermoplastic Pipes

In addition to the thermoplastic piping materials discussed previously,

there are other types of thermoplastic piping materials which are used

to a lesser degree. These materials include polybutylene (PB), cellulose

acetate butyrate (CAB), and styrene-rubber (SR). A selected list of

standards for these materials is given in Tables 7.26, 7.27, and 7.28.

The design techniques which are used for thermoplastics such as PVC

can also be applied to these thermoplastic materials. The design engi-

neer should obtain necessary design parameters such as the hydrosta-

tic-design stress and pipe stiffness for the particular pipe and

material. These parameters may be used in design equations discussed

previously.

Example 7.16—Brittle behavior A strain-sensitive plastic sewer pipe has been

installed in an area where expansive soils are known to exist. The pipe

deflects as a flexible pipe, has a high pipe stiffness, and has a somewhat

brittle behavior. A TV inspection made 2 years after installation indicates

vertically elongated pipe with many pipe sections showing longitudinal

cracks along the 3 and 9 o’clock positions. The pipe was installed with a com-

pacted granular material around the pipe and 10 ft of cover. The expansive

soil is to the sides and under the pipe but not over the pipe. The TV pho-

tographs indicate the pipe to be vertically elongated in the 3 to 8 percent

range. Estimate the horizontal swell pressure exerted by the soil. (Assume

E′  1000 lb/in

The actual buried pipe may be used as a transducer to obtain a fair estimate

of the in situ horizontal swell pressures. This is accomplished by use of the

Iowa formula and the actual deflection behavior of the pipe. In short, this

formula may be used by providing pipe properties, soil properties, and pipe

deflection and then back-calculating the pressure necessary to produce that

deflection. [See Eqs. (7.1) and (7.2).]

Soil modulus E′  1000 lb/in

Pressure  (deflection ratio) (10)



 0.061(soil modulus)



P 



(10)



 0.061E′



F/y



6.7

y



pipe stiffness



6.7

508 Chapter Seven

Table 7.29 indicates probable swell pressures in the range of 23 to 60 lb/in

for deflections of 3 to 8 percent.

Thermoset Plastic Pipe

Thermosetting resins give off heat during the curing process

(exotherm). Such resins cannot be melted and reformed as thermo-

plastics can. Epoxy, polyester, and phenolic resins are part of the ther-

mosetting resin family. Pipes made from such resins are usually

fiber-reinforced, and the fiber is normally E-type glass. The glass may

be continuous strands or rovings placed in a winding process, or it may

be chopped and placed in a centrifugal casting process. Glass fabric

and glass mats may also be used.

TABLE 7.26 Selected Standards for Polybutylene

ASTM F 809 Large-Diameter Polybutylene

Plastic Pipe

ASTM F 809M Large-Diameter Polybutylene

Plastic Pipe (Metric)

ASTM F 845 Plastic Insert Fittings for

Polybutylene (PB) Tubing

ASTM D 2662 Polybutylene Plastic Pipe

(SDR-PR)

ASTM D 3000 Polybutylene Plastic Pipe (SDR-

PR) Based on Outside Diameter

ASTM D 2666 Polybutylene Plastic Tubing

AWWA C902 Polybutylene Pressure Pipe,

Tubing, and Fitting,



through 3 in, for Water

TABLE 7.27 Selected Standards for CAB

ASTM D 2446 Cellulose-Acetate-

Butyrate-Plastic Pipe

(SDR-PR) and Tubing

ASTM D 1503 Cellulose-Acetate-

Butyrate Plastic Pipe,

Schedule 40

ASTM D 2560 Solvent, Cements for

Cellulose-Acetate-

Butyrate Plastic Pipe,

Tubing, and Fittings

TABLE 7.28 Selected Standards for Styrene-Rubber (SR) Pipe

ASTM D 3122 Solvent Cements for Styrene-Rubber Plastic

Pipe and Fittings

ASTM D 3298 Styrene-Rubber Plastic Drain Pipe, Perforated

ASTM D 2852 Styrene-Rubber Plastic Drain Pipe and Fittings