Proper Bedding for PVC Pressure Pipe
                     Astkh e  E n g i n e e r :
P R O P E R  B E D D I N G  F O R  P V C  P R E S S U R E  P I P E
By Mike Luckenbill Historically, a flexible pipe has been zontal diameter expands, it engages the
Southwestern Regional Engineer defined as a conduit that will deflect at least passive resistance of the soil support at the
two percent without any sign of structural sides of the pipe.  At the same time, the
arious parameters must be consid- distress, such as injurious cracking. compression of the vertical diameter
Vered when designing a buried piping However, for a conduit to truly behave as a relieves the pipe of the major portion of thesystem.  Two of the main considera- flexible pipe when buried, it is required that vertical soil load, which is then carried by
tions should be the pipe properties and the the pipe be more yielding than the embed- the surrounding soil through the mecha-
soil envelope around the pipe. ment soil surrounding it.  This is the source nism of an arching action over the pipe.
To the layman, the word “soil” can mean HISTORICALLY, A FLEXIBLE PIPE HAS BEEN DEFINED AS A CONDUIT
different things.  To engineers involved in
pipe burial, soil is any earthen material THAT WILL DEFLECT AT LEAST TWO PERCENT WITHOUT ANY SIGN OF
excluding bedrock.  STRUCTURAL DISTRESS, SUCH AS INJURIOUS CRACKING.  
Soil has been used as a construction mate-
rial throughout history.  Soil is important not
of flexible pipe’s external-load-carrying Superimposed loads on buried PVC pipe
only as a material upon which the structure
capacity.  Under soil load, the pipe tends to fall into two categories - earth loads and
rests, but also for support and load transfer.
deflect.  The vertical diameter is com- live loads.  In the design of any buried pip-
The soil envelope transfers surface and
pressed and the horizontal diameter ing system, both categories of superim-
gravity loads to, from, and around the struc-
expands by approximately the same posed loads must be considered.  In accor-
ture.
amount in both directions.  When the hori- dance with common design practice, earth
CONTINUED ON PAGE  14
Shearing Forces Over Rigid Pipe Shearing Forces Over Flexible Pipe
Shearing Forces Increase The Load Shearing Forces Decrease The Load
Figure A: Comparing Trench Loads for Flexible and Rigid Products
P V C  P I P E  N E W S •  S P R I N G 2 0 0 4 1 3
SIDE PRISM
CENTRAL PRISM
SIDE PRISM
SIDE PRISM
CENTRAL PRISM
SIDE PRISM
EXCAVATED TRENCH WIDTH
FINAL
BACKFILL
COVER PIPE WIDTH INITIAL
BACKFILL
PIPE SPRINGLINE PIPE
ZONE
HAUNCHING
BEDDING
FOUNDATION
(MAY NOT BE REQUIRED)
Figure B: Trench Embedment Terminology
CONTINUED FROM PAGE  13 on earth loading technology for buried con- buried pipe is modified by the response of
duits throughout the world is related, in the pipe and the relative movement of the
loads and live loads are treated as separate
design parameters.
ALMOST ANY REASONABLE SOIL STRUCTURE WILL WORK FOR PVC
The first solution to the problem of soil-
induced loads on buried pipe was pub- PRESSURE PIPE BURIAL. 
lished by Professor Anson Marston at Iowa
State University in 1913.  Since then, the part, to Marston's load theory.  The basic side columns of soil to the central column.
Marston Theory of Loads on Underground concept of the theory is that the load due When the side columns of soil between the
Conduits has been used in determining the to the weight of the column of soil above a pipe and the trench wall (pipe zone) are
loads on buried pipe.  Much of the research
Table 1
Average Values Of Modulus Of Soil Reaction, E'
(For Initial Flexible Pipe Deflection)*
E' for Degree of Compaction of Haunching, in psi
ASTM D2321
Embedment Material Classification Dumped Slight Moderate High
< 85% Proctor 85% - 95% Proctor > 95% Proctor
Manufactured Granular Angular Class I 1,000 3,000 3,000 3,000
Clean Sand & Gravel Class II 200 1,000 2,000 3,000
Sand & Gravel with Fines Class III 100 400 1,000 2,000
Silt & Clay Class IV 50 200 400 1,000
Organic Materials Class V No data available; consult a competent soils engineer; otherwise use E’ = 0
* A more detailed table is available for download from Uni-Bell’s website, www.uni-bell.org. The table is in Uni-Bell’s technical report “Deflection: The Pipe/Soil Mechanism,” UNI-TR-1.
1 4 P V C  P I P E  N E W S •  S P R I N G 2 0 0 4
PIPE EMBEDMENT
more compressible than the pipe, this causes the
BEDDING ANGLE pipe to assume load generated across the width of
the trench.  This is typically the case for rigid prod-
ucts like concrete and clay.  However, when pipe has
the ability to deflect without cracking, this produces
a situation that allows the central prism of soil
(directly over the pipe) to settle more in relation to
the adjacent soil columns (between the pipe and the
trench wall).  This settlement produces shearing
forces which reduce the load on a flexible pipe to an
BEDDING amount less than the weight of the prism directly
over it.  The two scenarios are shown in Figure A on
page 13.  Regardless of pipe stiffness, as soil in the
VALUES OF BEDDING CONSTANT ,K trench settles or moves downward compared to the
trench sidewall, friction forces are generated which
BEDDING ANGLE K act to reduce the weight of the trench-wide soil col-
umn.  Marston's Load Theory predicts and accounts
0° 0.110 for these frictional shearing forces. 
30° 0.108 Figure B on page 14 shows a typical trench cross-
45° 0.105 section denoting standard nomenclature used in the
plastic pipe industry.  The word “bedding” is gener-
60° 0.102 ally accepted as the soil structure around the pipe
and not necessarily the bedding upon which the
90° 0.096 pipe rests.  This would include the haunching and
120° 0.090 initial backfill areas.  The soil structure requirements
for pressure pipe are less stringent than for gravity
180° 0.083 sewer pipe.  This is primarily due to the fact that
pressure pipe is usually buried at shallow depths.
Figure C: Bedding Angle Defined Also, pressure pipes tend to have thicker walls than
CONTINUED ON PAGE  16
Table 2
Calculated Deflections Of Buried PVC Pressure Pipe; Deflection (percent) For Prism, Highway H20, or Railway E80 Loads
Height of Cover 2’ 4’ 6’ 8’ 10’
Live Load Prism H20 E80 Prism H20 E80 Prism H20 E80 Prism H20 E80 Prism H20 E80
E’ Value DR 14
50 0.13 0.58 2.25 0.27 0.49 1.75 0.40 0.51 1.66 0.54 0.59 1.43 0.67 0.67 1.28
200 0.12 0.54 2.10 0.25 0.46 1.63 0.37 0.48 1.54 0.50 0.55 1.33 0.62 0.62 1.20
400 0.11 0.50 1.92 0.23 0.42 1.49 0.34 0.44 1.42 0.46 0.50 1.22 0.57 0.57 1.10
1000 0.09 0.40 1.54 0.18 0.34 1.19 0.27 0.35 1.13 0.37 0.40 0.97 0.46 0.46 0.88
2000 0.07 0.30 1.15 0.14 0.25 0.89 0.21 0.26 0.85 0.27 0.30 0.73 0.34 0.34 0.66
E’ Value DR 18
50 0.29 1.26 4.89 0.58 1.07 3.79 0.87 1.11 3.60 1.16 1.28 3.10 1.45 1.45 2.79
200 0.25 1.09 4.22 0.50 0.92 3.27 0.75 0.96 3.10 1.00 1.11 2.67 1.25 1.25 2.40
400 0.21 0.92 3.57 0.42 0.78 2.76 0.64 0.81 2.62 0.85 0.94 2.26 1.06 1.06 2.03
1000 0.14 0.63 2.43 0.29 0.53 1.89 0.43 0.55 1.79 0.58 0.64 1.54 0.72 0.72 1.39
2000 0.09 0.41 1.59 0.19 0.35 1.23 0.28 0.36 1.17 0.38 0.42 1.01 0.47 0.47 0.91
E’ Value DR 21
50 0.46 1.99 7.71 0.92 1.68 5.97 1.37 1.76 5.67 1.83 2.02 4.89 2.29 2.29 4.39
200 0.37 1.59 6.16 0.73 1.34 4.77 1.10 1.40 4.53 1.46 1.62 3.90 1.83 1.83 3.51
400 0.29 1.25 4.86 0.58 1.06 3.76 0.87 1.11 3.57 1.15 1.27 3.08 1.44 1.44 2.77
1000 0.18 0.77 2.97 0.35 0.65 2.30 0.53 0.68 2.19 0.71 0.78 1.88 0.88 0.88 1.69
2000 0.11 0.47 1.81 0.21 0.39 1.40 0.32 0.41 1.33 0.43 0.47 1.14 0.54 0.54 1.03
E’ Value DR 25
50 0.75 3.23 12.56 1.49 2.74 9.73 2.24 2.86 9.23 2.98 3.29 7.96 3.73 3.73 7.15
200 0.53 2.29 8.91 1.06 1.94 6.90 1.59 2.03 6.55 2.12 2.34 5.65 2.65 2.65 5.07
400 0.38 1.65 6.42 0.76 1.40 4.97 1.14 1.46 4.72 1.53 1.68 4.07 1.91 1.91 3.66
1000 0.21 0.90 3.49 0.42 0.76 2.71 0.62 0.80 2.57 0.83 0.92 2.21 1.04 1.04 1.99
2000 0.12 0.51 1.99 0.24 0.43 1.54 0.35 0.45 1.46 0.47 0.52 1.26 0.59 0.59 1.13
E’ Value DR 26
50 0.83 3.59 13.95 1.66 3.04 10.80 2.49 3.18 10.26 3.31 3.66 8.84 4.14 4.14 7.94
200 0.57 2.47 9.59 1.14 2.09 7.43 1.71 2.18 7.05 2.28 2.51 6.07 2.85 2.85 5.46
400 0.40 1.74 6.77 0.80 1.47 5.24 1.21 1.54 4.98 1.61 1.77 4.29 2.01 2.01 3.85
1000 0.21 0.93 3.59 0.43 0.78 2.78 0.64 0.82 2.64 0.85 0.94 2.28 1.07 1.07 2.05
2000 0.12 0.52 2.02 0.24 0.44 1.56 0.36 0.46 1.48 0.48 0.53 1.28 0.60 0.60 1.15
P V C  P I P E  N E W S •  S P R I N G 2 0 0 4 1 5
CONTINUED FROM PAGE  15
a comparable gravity pipe in order to han- symbolically represented as E’.  The aver- becomes increasingly important as depth
dle the pressure capacity typically speci- age values are shown in Table 1 on page 14. of cover increases.  However, at relatively
fied.  This often results in external load This variable is very important in flexible shallow depths in the 3 to 10 foot range E’
capabilities that far exceed the design
requirement.  As a consequence, almost THE STRENGTH AND SUITABILITY OF THE PVC PIPE FOR BURIAL
any reasonable soil structure will work for
PVC pressure pipe burial. MADE IT EQUAL TO (OR BETTER THAN) OTHER TRADITIONAL PIPING
The “bedding factor” is also used in burial MATERIALS...
equations.  Precise values are shown in
Figure C on page 15. The bedding factor pipe burial calculations.  E’ in the haunch values of 700 to 2,000 psi will cover almost
has little effect on results of burial calcula- area is a measure of the ability of the soil to any burial situation as long as there is full
tions and is usually taken as 0.100. absorb live and dead loads transmitted support in the haunches of the pipe.
through the pipe as it deflects over time.  E’
What is the modulus of soil reaction?  It is CONTINUED ON PAGE  18
Table 2 (cont.)
Calculated Deflections Of Buried PVC Pressure Pipe; Deflection (percent) For Prism, Highway H20, or Railway E80 Loads
Height of Cover 2’ 4’ 6’ 8’ 10’
Live Load Prism H20 E80 Prism H20 E80 Prism H20 E80 Prism H20 E80 Prism H20 E80
E’ Value DR 32.5
50 1.44 6.24 24.22 2.88 5.28 18.77 4.32 5.52 17.81 5.76 6.35 15.35 7.20 7.20 13.79
200 0.80 3.49 13.53 1.61 2.95 10.48 2.41 3.08 9.95 3.22 3.55 8.57 4.02 4.02 7.70
400 0.51 2.19 8.52 1.01 1.86 6.60 1.52 1.94 6.26 2.02 2.23 5.40 2.53 2.53 4.85
1000 0.24 1.04 4.04 0.48 0.88 3.13 0.72 0.92 2.97 0.96 1.06 2.56 1.20 1.20 2.30
2000 0.13 0.55 2.15 0.26 0.47 1.66 0.38 0.49 1.58 0.51 0.56 1.36 0.64 0.64 1.22
E’ Value DR 41
50 2.31 10.01 38.88 4.62 8.47 30.12 6.93 8.85 28.59 9.24 10.19 24.63 11.55 11.55 22.13
200 1.02 4.42 17.14 2.04 3.74 13.28 3.05 3.90 12.60 4.07 4.49 10.86 5.09 5.09 9.76
400 0.58 2.53 9.82 1.17 2.14 7.61 1.75 2.24 7.22 2.33 2.58 6.22 2.92 2.92 5.59
1000 0.26 1.11 4.31 0.51 0.94 3.34 0.77 0.98 3.17 1.02 1.13 2.73 1.28 1.28 2.45
2000 0.13 0.57 2.22 0.28 0.48 1.72 0.40 0.51 1.64 0.53 0.58 1.41 0.66 0.66 1.27
E’ Value DR 51
50 3.22 13.94 54.13 6.43 11.79 41.93 9.65 12.33 39.80 12.86 14.19 34.30 16.08 16.08 30.82
200 1.16 5.04 19.57 2.33 4.27 15.16 3.49 4.46 14.39 4.65 5.13 12.40 5.81 5.81 11.14
400 0.63 2.72 10.57 1.26 2.30 8.19 1.88 2.41 7.78 2.51 2.77 6.70 3.14 3.14 6.02
1000 0.26 1.14 4.44 0.53 0.97 3.44 0.79 1.01 3.27 1.06 1.17 2.82 1.32 1.32 2.53
2000 0.13 0.58 2.26 0.27 0.49 1.75 0.40 0.51 1.66 0.54 0.59 1.43 0.67 0.67 1.29
Height of Cover 12’ 14’ 16’ 18’ 20’
Live Load Prism H20 E80 Prism H20 E80 Prism H20 E80 Prism H20 E80 Prism H20 E80
E’ Value DR 14
50 0.80 0.80 1.25 0.94 0.94 1.27 1.07 1.07 1.35 1.21 1.21 1.43 1.34 1.34 1.51
200 0.75 0.75 1.16 0.87 0.87 1.19 1.00 1.00 1.26 1.12 1.12 1.33 1.25 1.25 1.40
400 0.69 0.69 1.07 0.80 0.80 1.09 0.91 0.91 1.15 1.03 1.03 1.22 1.14 1.14 1.29
1000 0.55 0.55 0.85 0.64 0.64 0.87 0.73 0.73 0.92 0.82 0.82 0.97 0.91 0.91 1.03
2000 0.41 0.41 0.64 0.48 0.48 0.65 0.55 0.55 0.69 0.62 0.62 0.73 0.68 0.68 0.77
E’ Value DR 18
50 1.74 1.74 2.71 2.04 2.04 2.76 2.33 2.33 2.93 2.62 2.62 3.10 2.91 2.91 3.27
200 1.50 1.50 2.34 1.75 1.75 2.38 2.01 2.01 2.53 2.26 2.26 2.67 2.51 2.51 2.82
400 1.27 1.27 1.98 1.48 1.48 2.01 1.69 1.69 2.14 1.91 1.91 2.26 2.12 2.12 2.38
1000 0.87 0.87 1.35 1.01 1.01 1.37 1.16 1.16 1.46 1.30 1.30 1.54 1.45 1.45 1.63
2000 0.57 0.57 0.88 0.66 0.66 0.90 0.76 0.76 0.95 0.85 0.85 1.01 0.95 0.95 1.06
E’ Value DR 21
50 2.75 2.75 4.28 3.21 3.21 4.35 3.66 3.66 4.62 4.12 4.12 4.89 4.58 4.58 5.15
200 2.20 2.20 3.42 2.56 2.56 3.48 2.93 2.93 3.69 3.29 3.29 3.90 3.66 3.66 4.12
400 1.73 1.73 2.70 2.02 2.02 2.74 2.31 2.31 2.91 2.60 2.60 3.08 2.89 2.89 3.25
1000 1.06 1.06 1.65 1.24 1.24 1.68 1.41 1.41 1.78 1.59 1.59 1.88 1.77 1.77 1.99
2000 0.64 0.64 1.00 0.75 0.75 1.02 0.86 0.86 1.08 0.97 0.97 1.14 1.07 1.07 1.21
E’ Value DR 25
50 4.48 4.48 6.97 5.22 5.22 7.09 5.97 5.97 7.52 6.71 6.71 7.96 7.46 7.46 8.39
200 3.18 3.18 4.94 3.70 3.70 5.03 4.23 4.23 5.34 4.76 4.76 5.65 5.29 5.29 5.95
400 2.29 2.29 3.56 2.67 2.67 3.62 3.05 3.05 3.85 3.43 3.43 4.07 3.81 3.81 4.29
1000 1.25 1.25 1.94 1.45 1.45 1.97 1.66 1.66 2.09 1.87 1.87 2.21 2.08 2.08 2.33
2000 0.71 0.71 1.10 0.83 0.83 1.12 0.94 0.94 1.19 1.06 1.06 1.26 1.18 1.18 1.33
1 6 P V C  P I P E  N E W S •  S P R I N G 2 0 0 4
Type 1 Type 2 Type 3 Type 4
Flat-bottom trench.* Loose embedment. Flat-bottom trench.* Embedment lightly Pipe bedded on 4 in. (100 mm) minimum of Pipe bedded on sand, gravel, or crushed stone
consolidated to centerline of pipe.
loose soil.† Embedment lightly consolidated to depth of 1/8 pipe diameter, 4 in. (100mm)
E’ = 50 psi (340 kPa), K = 0.110
E’ = 200 psi (1,380 kPa), K = 0.110 to top of pipe. minimum. Embedment compacted to top of
pipe. (Approximately 80 percent Standard
E’ = 400 psi (2,760 kPa), K = 0.102
Proctor, AASHTO T-99 or ASTM D 698.)
E’ = 1,000 psi (6,900 kPa), K = 0.096
Figure D Notation
Type 5
NOTE: Required embedment type will depend on the pipe’s dimension ratio, internal operating pressure, and external load,
Pipe embedded in compacted granular material and shall be specified by the purchaser. (see Sec. 5.3)
to centerline of pipe. Compacted granular or
* “Flat-bottom is defined as undisturbed earth.
select material† to top of pipe. (Approximately
90 percent Standard Proctor, AASHTO T-99 or † “Loose soil” or “select material” is defined as native soil excavated from the trench, free of rocks, foreign materials, and
ASTM D 698) frozen earth. A soft “loose soil” bedding will contour to the pipe bottom. Caution must be excercised to ensure proper
E’ = 2,000 psi (13,800 kPa), K = 0.083 placement of embedment material under the haunches of the pipe.
Figure D: Typical Trench Types in the PVC Installation Standard, AWWA C605
Type 1 Type 2 Type 3 Type 4
Flat-bottom trench.† Backfill lightly consolidated
Flat-bottom trench.† Loose backfill. Pipe bedded on 4 in. (100 mm) minimum of Pipe bedded on sand, gravel, or crushed stone
to centerline of pipe.
loose soil.‡ Backfill lightly consolidated to depth of 1/8 pipe diameter, 4 in. (100mm)
to top of pipe. minimum. Backfill compacted to top of pipe.
(Approximately 80 percent Standard Proctor,
AASHTO T-99.)
Figure E Notation
Type 5 * For 14-in. (355-mm) and larger pipe, consideration should be given to the use of laying conditions other than type 1.
Pipe embedded in compacted granular material † “Flat-bottom is defined as undisturbed earth.
to centerline of pipe. Compacted granular or ‡ “Loose soil” or “select material” is defined as native soil excavated from the trench, free of rocks, foreign materials, and
select material‡ to top of pipe. (Approximately frozen earth. A soft “loose soil” bedding will contour to the pipe bottom. Caution must be excercised to ensure proper
90 percent Standard Proctor, AASHTO T-99.) placement of embedment material under the haunches of the pipe.
Figure E: Typical Trench Types in the Ductile Iron Installation Standard, AWWA C600
P V C  P I P E  N E W S •  S P R I N G 2 0 0 4 1 7