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

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At 18 m of cover, a hinge (crease) began to form in the wall at the 9 o’clock

position (west side). Loading was terminated at 19.8 m of cover. Data

for this test are given in Fig. 7.52.

Test Y-3. This 48-in-diameter pipe was placed in soil at 96.5 percent

of standard Proctor density. The weld/ribbing became visually notice-

able at 18.9 m of cover and about 1.35 percent vertical deflection. A

slight dimpling pattern began at 36.6 m of cover. General localized

buckling of the wall began at the 3 and 9 o’clock positions at 55 m of

cover (see Fig. 7.52). This shows graphically the importance of soil den-

sity in controlling the pipe deflection.

Comments on test results. Noteworthy is the high load that can be

applied without distress to the pipe ring. Clearly the pipes deflect more

(for the same load) in loose soil than in dense soil because loose soil

compresses more. From a structural point of view, there are no reasons

why high-density polyethylene pipes cannot perform well. 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 back-

fill material at moderate to high densities ensures that the pipes will

perform well even at high earth covers.

The load at which localized buckling occurs is primarily due to ring

compression stress and is a function of soil density. At this point it is

not totally clear what the exact role of soil density is in preventing

buckling. It is clear that pipes installed in soils at high densities will

support higher loads without buckling. However, in the range of 75 to

85 percent standard Proctor, the effect of soil density is not clear.

General wall buckling in these tests was considered the upper per-

formance limit. The height of cover at which general wall buckling

takes place is as low as 10.4 m for lower-density soils. However, the

height of cover for generalized wall buckling can be as high as 55 m for

well-compacted soils.

The pipe cross sections started out circular and became elliptical as

the height of cover increased. None of the test pipes exhibited a so-

called squaring or a square shape at any load. For polyethylene, which

has a fairly low modulus, ring compression stresses cause circumfer-

ential ring shortening. This ring shortening is small for pipes installed

with low heights of cover and in low to moderately compacted soils. For

high-density soils at high earth covers, this circumferential ring short-

ening is very significant and is the primary 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 gran-

ular pipe-zone material to carry a higher percentage of the load. This

works on exactly the same principle as the slotted bolthole in corrugated

Plastic Flexible Pipe Products 479

480 Chapter Seven

metal pipe. In a very large measure, the pipes in these tests were able

to withstand high loads because of substantial circumferential short-

ening that took place.

HDPE pipes with a steel rib. In another design, a polyethylene profile

section is wound helically to form the pipe; then a steel rib is also

wound helically, interlocking mechanically with the profile section of

the polyethylene. The result is a polyethylene pipe with an external

steel rib. In this design, the steel rib is much stiffer than the plastic.

Thus, in ring deflection, the steel rib carries most of the load. When

buried in soil, polyethylene relaxes with time if the ring configuration

is held constant. In good backfill, for a given height of cover, the soil

does hold the pipe in a constant cross section; so the polyethylene expe-

riences stress relaxation, and the steel rib carries essentially all the

load on the pipe. In addition, the composite pipe (steel and HDPE) is

flexible so the soil takes a large share of the vertical load. The stati-

cally indeterminate soil-structure interaction is mutually beneficial.

The pipe serves as a form for the soil arch, and the soil supports and

protects the pipe against vertical loads by arching action of the soil.

The steel rib stiffens the pipe and helps to maintain the cross-sectional

shape during backfilling. However, for a HDPE pipe with a steel rib,

catastrophic failure is possible if the pipe is subjected to a load suffi-

cient to cause either yielding or buckling in the steel rib.

Test C-1. A 1900-mm steel-ribbed HDPE pipe was installed in soil

compacted to 87 percent of standard Proctor density. Audible sounds

were heard, indicating a slipping of the steel with respect to the plas-

tic at 8.5 m of cover. Yielding of the steel may have been taking place.

A slight bulging was noted near the 3 and 9 o’clock positions at about

10.4 m of cover. As the vertical load was increased, this bulging

increased. Localized buckling occurred at 15.5 m of cover. At 18 m of

cover, general wall buckling was evident, the pipe began to collapse,

and the test was terminated. For results, see Fig. 7.53.

Test C-2. A 2000-mm pipe was installed in soil compacted to 86 per-

cent of standard Proctor density. Audible sounds were heard, indicat-

ing a slipping of the steel with respect to the plastic at 10.4 m of cover.

Yielding of the steel may have been taking place. Local wall buckling

was noted, and the joint liner buckled at about 14 m of cover. General

buckling occurred at the top of the pipe at 15.8 m of cover. At 17.7 m of

cover, the pipe could no longer sustain the load, and the test was ter-

minated (see data in Fig. 7.53).

Test C-3. A 2000-mm pipe was installed in soil compacted to 91 percent

of standard Proctor density. Localized buckling began near the

5 and 7 o’clock positions at 12.2 m of cover. At about 14 m of cover, local

Plastic Flexible Pipe Products 481

wall buckling at the 5 and 7 o’clock positions became more pronounced.

This buckling became more prominent as the vertical load was increased.

General buckling began near the 2, 3, 9, and 10 o’clock positions at 15.9 m

of cover. At 17.4 m of cover, the pipe could no longer sustain the load, and

the test was terminated (see data in Fig. 7.53 and Table 7.20).

The steel-ribbed pipe behaves essentially the same as a low-stiffness

corrugated metal pipe. This is because the steel rib is much stiffer than

the polyethylene material. The higher-stiffness steel essentially car-

ries all of the load. Thus, the behavior of the steel rib is essentially the

behavior of the pipe. The load-deflection curves for the steel-ribbed

pipes do not resemble curves for other plastic pipe; rather, they resem-

ble curves for low-stiffness corrugated metal pipes. For example, for

polyethylene pipe, the horizontal deflection is substantially less than

the vertical deflection (see Fig. 7.50). On the other hand, for steel-

ribbed pipe, the horizontal and vertical deflections are close to being

equal up to the point where the pipes begin to fail (see Fig. 7.54). This

is the way a metal pipe behaves. Also, in the tests of plastic pipe, signs

of distress at the 5 to 7 o’clock positions do not occur. However, on the

steel-ribbed pipe, localized buckling took place on the invert section of

the pipe. Tests of corrugated metal pipe installed in highly compacted

soil show this same type of behavior. Also, failure is much more cata-

strophic in the steel-ribbed polyethylene than in either corrugated

steel or HDPE (i.e., collapse can progress without an increase in load).

Height of Cover (Meters)

Vertical Deflection (Percent)

Height of Cover (Feet)

Figure 7.53 Data for tests C-1, C-2, and C-3; vertical deflections and buckling.

TABLE 7.20 Overall Test Results for Steel-Ribbed HDPE Pipe

Test data Local buckling General buckling

Test Manu- Diameter, % Proctor Deflection, Deflection,

no facturer mm density Cover, m percent Cover, m percent

C-1* C 1900 87 12.0 2.8 18.0 6.7

C-2* C 2000 86 12.2 3.5 15.8 5.0

C-3* C 2000 91 12.2 0.9 15.9 1.4

*Steel-ribbed polyethylene pipe.

482

Plastic Flexible Pipe Products 483

The behavior of this pipe shows that pipe stiffness alone will not control

localized buckling.

Test results for HDPE profile-wall pipe. Part of the data included in the

previous section were published in Transportation Research Record No.

1624, 1998. Data reported here are from a follow-on study. These data are

for 42-, 48-, and 60-in-diameter pipes. Profile design parameters have sig-

nificant influence on the structural performance of a pipe. Stable wall

designs will exhibit higher performance limits than less stable designs,

in terms of both dimpling of the interior and ultimate failure.

Procedure. During the summer of 1999, tests were performed on profile-

wall polyethylene pipes. These tests permitted an investigation of the

performance limits of the pipes subjected to external soil pressures.

Tests were performed in the USU large soil cell into which the sample

pipe is buried and onto which a vertical soil load is applied by means

of 50 hydraulic cylinders (see Figs. 7.55 through 7.60 for test cell and

testing procedure). The basic soil type was silty sand and is designated

as a class III soil by ASTM D 2321. This soil is classified as SM accord-

ing to the Unified Soil Classification System. See Figs. 6.11 and 6.12

for soil gradation and Proctor data.

Depth of Cover (Feet)

Deflection (Percent)

Horizontal

Vertical

0123456

Buckling at 5 & 7 o'clock

Continued Buckling at 5 & 7 o'clock

Buckling at 2, 3, 9, & 10 o'clock

Incipient Collapse

Figure 7.54 Performance of 2000-mm steel-ribbed HDPE

pipe tested in silty-sand soil compacted to 91 percent of stan-

dard Proctor density shows that horizontal and vertical

deflections are almost equal.

484 Chapter Seven

Figure 7.55 shows the test pipe being placed in the large soil test

cell. Figure 7.56 is a photo of the loading rams used to apply the verti-

cal load simulating a soil embankment. Figure 7.57 shows the embed-

ment soil being compacted to the required density. Figures 7.58 and

7.59 show part of the process used to fill the test cell. Figure 7.60

shows the test cell full with the vertical load being applied.

Test details

Test 1

Pipe: 60-in diameter. Profile 1, HDPE

Embedment soil: silty sand

Compaction: 83% of standard Proctor

Test date: 6/24/99

Figure 7.55 A 60-in HDPE pipe being placed in the test cell.

Plastic Flexible Pipe Products 485

Test 2

Pipe: 60-in diameter. Profile 1, HDPE

Embedment soil: silty sand

Compaction: 95% of standard Proctor

Test date: 7/07/99

Test 3

Pipe: 60-in diameter. Profile 1, HDPE

Embedment soil: silty sand

Compaction: 75% of standard Proctor

Test date: 7/15/99

Figure 7.56 Photograph showing the 50 hydraulic cylinders used

for loading.

486 Chapter Seven

Test 4

Pipe: 42-in diameter. Profile 1, HDPE

Embedment soil: silty sand

Compaction: 83% of standard Proctor

Test date: 7/30/99

Test 5

Pipe: 42-in diameter. Profile 1, HDPE

Embedment soil: silty sand

Compaction: 95% of standard Proctor

Test date: 8/09/99

Figure 7.57 Soil placement, compaction, and density measurement.

Plastic Flexible Pipe Products 487

Figure 7.59 Soil placement process.

Figure 7.58 Backhoe loading soil in test cell.

488 Chapter Seven

Test 6

Pipe: 42-in diameter. Profile 1, HDPE

Embedment soil: silty sand

Compaction: 75% of standard Proctor

Test date: 8/15/99

Test 7

Pipe: 48-in diameter. Profile 2, HDPE

Embedment soil: silty sand

Compaction: 77% of standard Proctor

Test date: 9/14/99

Test 8

Pipe: 48-in diameter. Profile 2, HDPE

Embedment soil: silty sand

Compaction: 95% of standard Proctor

Test date: 9/22/99

Test 1 results. The pipe was placed in soil compacted to 83 percent of

standard Proctor density and the vertical soil load was increased to

8664 lb/ft

(72.2 ft of cover based on a soil weight of 120 lb/ft

). At a soil

Figure 7.60 Soil cell full, loading beams down, and load being applied to soil surface.