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

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Plastic Flexible Pipe Products 489

pressure equivalent to 52 ft of cover and about 12 percent vertical

deflection, a dimpling pattern on the inside wall became noticeable to

the eye. This pattern, which is the beginning of localized buckling,

started at the 3 o’clock position. The center distance between dimples

was about the same as the internal rib spacing. This pattern was some-

what like a wavy checkerboard in appearance and, of course, just the

beginning of localized instability of the inner wall. However, this dim-

pling was small and would in no way impair the structural perfor-

mance of the pipe.

As the soil load increased, these dimples became slightly more pro-

nounced, but did not cause a performance limit. At a soil pressure

equivalent to 58 ft of cover, a flattening was noted at the invert about

1 ft away from the joint. At a soil pressure equivalent to 65 ft of cover,

a dimpling pattern was very apparent in the zones around the 3 and

9 o’clock positions. As the load was increased from 65 ft, the dimpling

pattern become more pronounced and two cracks formed near the cen-

ter of the test section and on the horizontal diameter. These small

cracks followed the helix joint and were longitudinally about 2 ft apart.

Loading was terminated at a soil pressure equivalent to 72 ft of cover.

Data for this test are given in Fig. 7.61.

Test 2 results. In test 2, the pipe was installed in soil compacted to

95 percent of standard Proctor density and was loaded to a vertical soil

load of 17,167 lb/ft

which is equivalent to 143 feet of cover. At a soil

pressure equivalent to about 108 ft of cover and about 3.5 percent

deflection, small dimples began forming near the 3 and 9 o’clock posi-

tions. This dimpling was extremely small and would in no way impair

the structural performance of the pipe. As the soil load was increased,

these dimples became more pronounced and were concentrated in the

3 and 9 o’clock positions but did not cause a performance limit. The

test was terminated at 143 ft of cover. Data for this test are given in

Fig. 7.62.

Test 3 results. The pipe was placed in soil compacted to only 75 per-

cent of standard Proctor density. The vertical soil load was increased

to 7340 lb/ft

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

At 44 ft of cover and about 13 percent vertical deflection, a slight dim-

pling pattern began. This pattern started at about the 3 and 9 o’clock

positions and spread as the load was increased. The center distance

between dimples was about the same as the internal rib spacing. This

pattern was somewhat like a wavy checkerboard in appearance and of

course just the beginning of localized instability. However, this dim-

pling was extremely small and in no way would impair the structural

performance of the pipe.

490 Chapter Seven

Cover (Feet)

Deflection (Percent)

More Dimpling

Start of Dimpling

2 Cracks Near Center of Pipe on Horizontal

Dia. about 10 ft. apart (Cracks at Helix Joint)

Figure 7.61 Load-deflection curves for 60-in-diameter HDPE pipe. Soil is silty sand

compacted to 83 percent of standard Proctor density. Measurements made at center

of pipe.

Cover (Feet)

Deflection (Percent)

More Dimpling

Start of Dimpling

Figure 7.62 Load-deflection curves for 60-in-diameter HDPE pipe. Soil is silty sand

compacted to 95 percent of standard Proctor density. Measurements are made at cen-

ter of pipe.

Plastic Flexible Pipe Products 491

As the soil load increased, these dimples became more pronounced

but still were not judged to be a performance limit of the pipe. At

61 ft of cover, the test was terminated. Data for this test are shown in

Fig. 7.63.

Figure 7.64 gives the vertical deflection curves for the pipes tested in

three soil densities in the height of cover range typically encountered

in actual projects. These curves in this height-of-cover range (0 to 40 ft)

show graphically the importance of soil density in controlling the pipe

deflection in typical installations. This figure also shows approximated

vertical deflection curves for intermediate soil densities (dashed lines).

Test 4 results. The pipe was placed in soil compacted to 83 percent of

standard Proctor density, and the vertical soil load was increased to

9972 lb/ft

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

). At 55 ft of

cover and about 12 percent vertical deflection, a dimpling pattern on the

inside wall became noticeable to the eye. This pattern started at the 3

and 9 o’clock positions. The center distance between dimples was about

the same as the internal rib spacing. This pattern was somewhat like a

wavy checkerboard in appearance. However, this dimpling was small

and would in no way impair the structural performance of the pipe.

As the soil load increased, these dimples became more pronounced

but did not cause a performance limit. At 69 ft of cover, the dimpling

Cover (Feet)

Deflection (Percent)

More Dimpling

Start of Dimpling

Figure 7.63 Load deflection curves for 60-in-diameter HDPE pipe. Soil is silty sand

compacted to 75 percent of standard Proctor density. Measurements are made at cen-

ter of pipe.

492 Chapter Seven

pattern was very apparent in the zones around the 3 and 9 o’clock posi-

tions and hinging began at the 3 and 9 o’clock positions. As the load was

increased from 69 ft, the helix seams showed distress. At 76 ft of cover,

cracks formed near the center of the test section and on the horizontal

diameter. These small cracks followed the helix joint. Loading was ter-

minated at 83 ft of cover. Data for this test are given in Fig. 7.65.

Test 5 results. In test 5, the pipe was installed in soil compacted to

95 percent of standard Proctor density and was loaded to a vertical soil

load of 20,196 lb/ft

, which is equivalent to 168 ft of cover. At about

126 ft of cover and about 6 percent deflection, small dimples began

forming near the 3 and 9 o’clock positions. This dimpling was

extremely small and would in no way impair the structural perfor-

mance of the pipe. These dimples became more pronounced as the soil

load was increased and were concentrated in the 3 and 9 o’clock posi-

tions but did not cause a performance limit.

At about 168 ft of cover, circumferential cracks were noted on the

horizontal diameter. These cracks were at the helix weld. At this point,

the test was terminated. Data for this test are given in Fig. 7.66.

Test 6 results. The pipe was placed in soil compacted to only 75 per-

cent of standard Proctor density. The vertical soil load was increased

incrementally to 8268 lb/ft

(about 69 ft of cover based on a soil weight

of 120 lb/ft

). At 48 ft of cover and about 15 percent vertical deflection,

a slight dimpling pattern began. This pattern started at about the

Depth of Cover (Feet)

Vertical Deflection (Percent)

Figure 7.64 Vertical deflection curves for 60-in HDPE pipe at various soil den-

sities. The dashed lines are approximated curves for intermediate densities.

Plastic Flexible Pipe Products 493

Cover (Feet)

Deflection (Percent)

More Dimpling

Start of Dimpling

Circumferential Cracks Near

Center of Pipe on Horizontal

Diameter (Cracks at Helix Joint)

Figure 7.66 Load deflection curves for 42-in-diameter HDPE pipe. Soil is silty sand

compacted to 95 percent of standard Proctor density. Measurements are made at cen-

ter of pipe.

Figure 7.65 Load deflection curves for 42-in-diameter HDPE pipe. Soil is silty sand

compacted to 83 percent of standard Proctor density. Measurements are made at cen-

ter of pipe.

Cover (Feet)

Deflection (Percent)

More Dimpling

Start of Dimpling

Beginning of hinging

2 Circumferential Cracks Near Center of Pipe

on Horizontal Diameter (Cracks at Helix Joint)

494 Chapter Seven

3 and 9 o’clock positions and spread as the load was increased. The

center distance between dimples was about the same as the internal

rib spacing. This dimpling was extremely small and in no way would

impair the structural performance of the pipe.

As the soil load increased, these dimples became more pronounced,

but still were not judged to be a performance limit of the pipe. At 69 ft

of cover, wall hinging was noted at the 3 and 9 o’clock positions, and

the test was terminated. Data for this test are shown in Fig. 7.67.

Figure 7.68 provides the vertical deflection curves for the pipes tested

in three soil densities in the height-of-cover range typically encountered

in actual projects. These curves in this cover range (0 to 40 ft) show

graphically the importance of soil density in controlling the pipe deflec-

tion in typical installations. This figure also shows approximated verti-

cal deflection curves for intermediate soil densities (dashed lines).

Test 7 results. The pipe was placed in soil compacted to 77 percent of

standard Proctor density, and the vertical soil load was increased to

6918 lb/ft

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

). A flatten-

ing at the 5 o’clock position started at 46 ft of cover (see Fig. 7.69).

Excavation after the test showed this to be buckling of the ribs at that

position.

At 52 ft of cover and about 18 percent vertical deflection, a dimpling

pattern on the inside wall became noticeable to the eye. This pattern,

Cover (Feet)

Deflection (Percent)

More Dimpling

Start of Dimpling

Beginning of hinging

Figure 7.67 Load deflection curves for 42-in-diameter HDPE pipe. Soil is silty sand

compacted to 75 percent of standard Proctor density. Measurements are made at

center of pipe.

Plastic Flexible Pipe Products 495

Figure 7.69 Local buckling at 5 o’clock. Density  77 percent, and load is 46 ft of cover.

Depth of Cover (Feet)

Vertical Deflection (Percent)

Figure 7.68 Vertical deflection curves for 42-in HDPE pipe at various soil densities.

The dashed lines are approximated curves for intermediate densities.

496 Chapter Seven

which is the beginning of localized buckling, started at the 3 and

9 o’clock positions. The center distance between dimples was about the

same as the external rib spacing. This pattern was somewhat like a

wavy checkerboard in appearance and, of course, just the beginning of

localized instability of the inner wall. Also, the joint began opening at

the 3 and 9 o’clock positions.

As the soil load increased, the dimples became more pronounced. At

58 ft of cover, the pipe buckled at the 11 and 1 o’clock positions (see Fig.

7.70), and the joint failed at 3 and 9 o’clock. Buckling of a pipe in soil

is not like classical buckling. In a buried pipe it takes another incre-

ment of load to produce another increment in the buckling phenome-

non. Loading was terminated at 58 ft of cover. Data for this test are

given in Fig. 7.69.

Test 8 results. In test 2, the pipe was installed in soil compacted to 95

percent of standard Proctor density and was loaded to a vertical soil

load of 18,228 lb/ft

which is equivalent to 152 ft of cover.

At 87 ft of cover, flattening started at a location between the 5 and 6

o’clock positions. After excavation of the test pipe, a visual inspection

of the external ribs revealed that the flattening was caused by rib

buckling.

Figure 7.70 General buckling at 11 o’clock. Density  77 percent, and load is 58 ft of

cover.

Plastic Flexible Pipe Products 497

At about 113 ft of cover and about 4.9 percent deflection, dimples

began forming near the 3 and 9 o’clock positions. This dimpling was

small and would in no way impair the structural performance of the

pipe. As the soil load was increased, these dimples became more pro-

nounced and were concentrated in the 3 and 9 o’clock positions. The

dimples spread to the 2 and 4 o’clock positions as the cover was

increased to 126 ft.

Wall crushing started at the 9:30 and 2:30 o’clock positions at a cover

of 139 ft. As the load increased past 139 ft, the crushing and dimpling

became more pronounced. The test was terminated at 152 ft of cover.

Data for this test are given in Fig. 7.70.

Figure 7.71 gives the vertical deflection curves for the pipes tested

in the two soil densities. This shows graphically the importance of soil

density in controlling the pipe deflection. This figure also shows

approximated vertical deflection curves for intermediate soil densities

(solid lines).

Dimpling. The term dimpling as used in this book refers to the wavy

pattern that occurred in the inner wall of the pipe due to local insta-

bility of the wall. This is not general buckling and is not a structural

performance limit.

Cover (Feet)

Deflection (Percent)

Flattening at 5 o’clock position

Pipe Buckled at 11 & 1 o’clock Position, Joint Failure at 3 & 9 o’clock,

Dimpling more Pronounced

Joint Opening at 3 & 9 o’clock positions, more Flattening

Near Invert and Start of Dimpling Pattern

Figure 7.71 Load deflection curves for 48-in-diameter HDPE pipe. Soil is silty sand

compacted to 77 percent of standard Proctor density. Measurements are made at

center of pipe.

498 Chapter Seven

Deflection (Percent)

Cover (Feet)

Vertical

Horizontal

160

140

120

100

02 46810

Test Terminated

Start of Wall Crushing at 2:30 & 9:30

Now Dimpling at 2 & 4 o’clock

Start of Dimpling at 3 & 9 o’clock

Flattening at 5 o’clock Position

Figure 7.72 Load deflection curves for 48-in-diameter HDPE pipe. Soil is silty

sand compacted to 95 percent of standard Proctor density. Measurements are

made at center of pipe.

Cover (Feet)

Vertical Deflection (Percent)

77 Percent

95 Percent

Figure 7.73 Vertical deflection curves for 48-in HDPE pipe at various soil den-

sities. The solid lines are approximated curves for intermediate densities.