Underground Installation

Underground piping must be installed in accordance with any applicable regulations, ordinances and codes. Since piping is installed in a wide range of sub soils attention should be given to local pipe laying techniques which may provide a solution to a particular pipe bedding issue. The following information is applicable to PVC and CPVC piping joined via the solvent cementing method and may be considered as a general guide. Refer to Gasketed Pipe section for additional information pertaining to installation of gasketed pipe.

Storage & Handling: Thermoplastic pipe must not be exposed to elevated temperatures during shipping and/or storage. Exposure to excessive temperatures will result in distortion/deformation of the pipe.  PVC and CPVC pipe should not be dropped, have objects dropped on them, nor subjected to external loads. Thermoplastics can be damaged by abrasion and gouging. Pipe must not be dragged across the ground or over obstacles. Impacts such as dropping from sizable heights and/or rough handling should be avoided, particularly in cold weather. The product shall be inspected for any scratches, splits or gouges that may have occurred from improper handling or storage. If found, these sections must be cut out and discarded. Refer to the “Storage & Handling” section of this 112/401 publication for additional information. 
 
Inspection: Before installation, PVC and CPVC piping products should be thoroughly inspected for cuts, scratches, gouges or split ends which may have occurred to the products during shipping and handling. Do not use damaged sections. Damaged sections found must be cut-out and discarded.
 
Trench Construction: For buried non-pressure applications trench construction, bedding, haunching, initial backfill, compaction, and final backfill shall be conducted as required by the project engineer or by following the Standard Practice for Underground Installation of Thermoplastic Pipe Sewers and Other Gravity-Flow Applications (ASTM D2321). For pressure applications, the Standard Practice for Underground Installation of Thermoplastic Pressure Piping (ASTM D2774) shall be followed in conjunction with this information when details are not provided by the project engineer. 
 
The trench should be of adequate width to allow convenient installation, while at the same time be as narrow as possible. Minimum trench widths may be utilized by joining pipe outside the trench and lowering it into the trench after adequate joint strength has been achieved. Trench widths will have to be wider where pipe is joined in the trench or where thermal expansion and contraction is a factor.
 
Refer to manufacturer’s instructions for recommended set and cure times for solvent cemented joints – do not lower into trench until adequate joint strength is achieved.   
 
Trench depth is determined by intended service and local conditions. Pipe for conveying liquids susceptible to freezing should be buried no less than 12" below the maximum frost level. Permanent lines subjected to heavy traffic should have a minimum cover of 24". For light traffic 12" to 18" is normally sufficient for small diameter pipe (typically <3" diameter). With larger sizes, bearing stresses should be calculated to determine cover required. Reliability and safety should always be considered, as well as local, state, and national codes.
 
Water filled pipe should be buried at least 12" below the maximum expected frost line.
It is recommended that thermoplastic piping be run within a metal or concrete casing when it is installed beneath surfaces that are subject to heavyweight or constant traffic such as roadways and railroad tracks. Piping systems must be designed and installed to ensure they can handle anticipated loads. Refer to Critical Collapse Pressure Ratings under Engineering & Design Data for additional information.
 
The trench bottom should be continuous, relatively smooth and free of rocks. Where ledge rock, hardpan or boulders are encountered, it is necessary to pad the trench bottom using a minimum of four (4) inches of tamped earth or sand beneath the pipe as a cushion and for protection of the pipe from damage.
Sufficient cover must be maintained to keep external stress levels below acceptable design stress. Reliability and safety of service is of major importance in determining minimum cover. Local, state and national codes may also govern.
 
Snaking of Pipe: For small diameter piping systems (typically <3" diameter), snaking of pipe is particularly important to compensate for thermal expansion and contraction of the piping when installing pipe in hot weather. This may also apply to larger diameter piping under specific applications and site conditions. After the pipe has been solvent welded and allowed to set properly, it is advisable to snake the pipe according to the following recommendations beside the trench during its required drying time (cure time). BE ESPECIALLY CAREFUL NOT TO APPLY ANY STRESS THAT WILL DISTURB THE UNDRIED JOINT. This snaking is necessary in order to allow for any anticipated thermal contraction that will take place in the newly joined pipeline. Refer to the section on Thermal Expansion & Contraction for additional information.
 
Snaking is particularly necessary on the lengths that have been 
solvent welded during the late afternoon or a hot summer’s day, because their drying time will extend through the cool of the night when thermal contraction of the pipe could stress the joints to the point of pull out. This snaking is also especially necessary with pipe that is laid in its trench (necessitating wider trenches than 
recommended) and is backfilled with cool earth before the joints are thoroughly dry.
 
For Pipe Diameters <3" diameter
 

 

Maximum Temperature Variation, °F, Between Time of Solvent Welding and Final Use
Loop Length 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°
  LOOP OFFSET
20 Feet 3" 4" 5" 5" 6" 6" 7" 7" 8" 8"
50 Feet 7" 9" 11" 13" 14" 16" 17" 18" 19" 20"
100 Feet 13" 18" 22" 26" 29" 32" 35" 37" 40" 42"

 

NOTE: Expansion and contraction could become excessive in systems operating at near or at the maximum allowable temperature ranges with intermittent flow and buried lines. In these cases the lines should not be snaked. The use of properly installed expansion joints installed within suitable concrete pit is recommended for PVC and CPVC systems operating at or near upper temperature limits. A section of larger diameter PVC pipe or other suitable sleeve should be used over the carrier pipe to pass through the wall of the concrete. This will minimize the potential for damage (scratching & scarring) to the carrier pipe as the result of movement caused by thermal expansion/contraction. Expansion joints should be suitably anchored independently of the carrier line. Axial guides should be used to direct movement into the expansion joint.

Bedding and Haunching: The pipe must be uniformly and continuously supported over its entire length on firm, stable material. Proper bedding and haunching materials are dependent on local soil conditions and type. Follow classes of embedment and backfill materials called-out in Table 1 “Pipe Stiffness Values for PVC Pipe” ASTM D2321. The trench bottom should be continuous, relatively smooth and free of rocks. Where ledge rock, hardpan or boulders are encountered, it is necessary to pad the trench bottom with proper bedding using a minimum of six (6) inches of suitable bedding beneath the pipe as a cushion and for protection of the pipe from damage. For belled-end pipe, provide bell holes in bedding no larger than necessary to ensure uniform pipe support. Embedment materials (initial backfill) shall be placed by methods that will not disturb or damage the pipe. The haunching material placed in the area between the bedding and the underside of the pipe shall be worked-in and hand tamped prior to placing and compacting the remainder of the embedment material in the pipe zone. Install and compact bedding materials in 6-inch maximum layers within the pipe zone. Refer to the diagrams on the following page for clarification. Compaction techniques and equipment used must not contact or damage the pipe.
 
Backfilling: Where possible underground pipe should be thoroughly inspected and tested for leaks prior to backfilling. 
The pipe should be uniformly and continuously supported over its entire length on firm, stable material. Blocking should not be used to change pipe grade or to intermittently support pipe across excavated sections. Pipe is installed in a wide range of subsoils. These soils should not only be stable, but applied in such a manner so as to physically shield the pipe from damage. Attention should be given to local pipe laying experience that may indicate particular pipe bedding problems. Initial backfill materials free of rocks with particle sizes 1/2" or less should be used to surround the pipe, and should be placed and compacted in layers. Each layer should be sufficiently compacted to uniformly develop lateral passive soil forces during the backfill operations. It may be advisable to have the pipe under water pressure (15 to 25 psi) during backfilling.  Final backfill should be placed and spread in uniform layers in such a manor to fill the trench completely so that there will be no unfilled spaces under or about rocks or lumps of earth in the backfill.  Large or sharp rocks, frozen clods and other debris greater than 3” diameter should be removed.  
 
Sufficient cover must be maintained to keep external stress levels below acceptable design stress. Reliability and safety of service is of major importance in determining minimum cover. Rolling equipment or heavy tampers should only be used to consolidate the final backfill. Attention should be given to local pipe laying experience that may indicate particular pipe bedding problems.  Local, state and national codes may also govern.  
 
Cold Temperature Underground Installation of PVC and CPVC Piping: PVC and CPVC are rigid thermoplastic materials, as such, pipe stiffness increases and impact resistance decreases in colder temperature environments.  PVC and CPVC can become more susceptible to physical damage when exposed to cold temperatures. Following the guidelines below will minimize the potential for damage. Impact resistance and ductility decrease at colder temperatures. In addition, a drop in temperature will cause the piping to contract, which must be addressed with proper system design. Due to PVC and CPVC's coefficient of thermal expansion, a 20-foot length of pipe will contract approximately 3/4" and 7/8", respectively, when cooled from 95F to -5F.  Since pressure bearing capacity is not reduced with a decrease in temperature, PVC and CPVC piping are suitable for use at colder temperatures provided the fluid medium is protected from freezing, consideration is given to the effects of expansion and contraction, and additional care and attention are given during handling, installation and operation of the system to prevent physical damage caused by impact or other mechanical forces.
 
 
NOTE: Use of threaded connections should be avoided in underground applications. Where transition to alternate materials is required the use of a flange component with suitable gasket is recommended. At vertical transitions from below ground systems to connections above ground, follow above ground installation procedures with regard to compensating for thermal expansion/contraction, weatherability, and proper support recommendations. Valves and other concentrated weight loads should be independently supported. Avoid excessive bending of pipe; excessive deflection of pipe and joints can reduce pressure bearing capability and cause failure.
 
Additional information on underground installations is contained in ASTM D2774 "Underground Installation of Thermoplastic Pressure Piping",  ASTM F645, Standard Guide For “Selection Design and Installation of Thermoplastic Water Pressure Piping Systems”, and ASTM D2321 "Underground Installation of Flexible Thermoplastic Sewer Pipe."
 

Underground Installation Trench Cross Section

Depth of Burial for GF Harvel PVC Pipe: When installed underground an external load is placed on a PVC Pipe, its diameter will begin to deflect, meaning its sides will move outward and slightly downward. If GF Harvel PVC pipe is buried in supportive soil, the stiffness of the soil will help support the pipe. This action and reaction is the key to how a PVC pipe carries external loads while buried.
 
The support from the embedded soil and the pipe stiffness form a combination to resist deflection from external loads. PVC Pipe’s resistance to deflection in an unburied state is measured by its pipe stiffness. Due to the excellent quality of GF Harvel PVC Piping, it has a high pipe stiffness value. In general, the greater the pipe stiffness values the higher the load capacity. 
 

Underground Installation

Calculating Burial Depth Limitations
Due to the ability of GF Harvel PVC to flex before it breaks, a limit is placed on pipe diametric deflection.  This limit is expressed in terms of percentage reduction in diameter due to external loading.  The maximum allowable diametric deflection for GF Harvel PVC Piping is 5%. Any deflection greater then 5% could lead to the failure of a piping system.
 
One method that is commonly used to estimate pipe deflection based on its burial depth is the Modified Iowa Equation. A simplified, version of the equation is presented below where 5% deflection is the limiting factor:
 
Modified Iowa Equation
 
                             0.1 (P + L) 100
% Deflection =  _____________________
                            0.149 (PS) + 0.061E’
 
Where:
 
% Deflection = Predicated percentage of deflection of the buried pipe’s outside diameter (5% is the maximum allowable deflection per ASTM D2665)
 
P = Prism Load Soil Pressure in lbs/in2 = Pressure on the buried pipe from the weight of the soil column above it (Prism Loads values can be found in Table 2).
 
L = Live Load on Buried Pipe in lb/in2 = Pressure transferred to the buried pipe from traffic on the surface above it (Live Load values can be found in Table 3).
 
PS = Pipe Stiffness in lb/in2 = The inherent strength of pipe to resist deflection in an unburied state, per ASTM D2412 (Pipe Stiffness values for GF Harvel PVC Pipe can be found in Table 1).
 
E’ = Modulus of Soil Reaction in lb/in2 = Stiffness of soil column on top of buried pipe. (Average Values of Modulus of Soil Reaction can be found in Table 4).
 
Table 1 - Pipe Stiffness Values for PVC Pipe in lb/in2
Sch 40 PVC Pipe
Sch 80 PVC Pipe
Size
(in.)
Pipe
Stiffness (PS)
Size
(in.)
Pipe
Stiffness (PS)
1/8"
15424
1/8"
54031
1/4"
13854
1/4"
42399
3/8"
7103
3/8"
22696
1/2"
6224
1/2"
17919
3/4"
3293
3/4"
9532
1"
2675
1"
7345
1-1/4"
1467
1-1/4"
4127
1-1/2"
1059
1-1/2"
3057
2"
626
2"
1938
2-1/2"
823
2-1/2"
2248
3"
534
3"
1547
3-1/2"
403
3-1/2"
1209
4"
323
4"
996
5"
216
5"
709
6"
161
6"
637
8"
110
8"
438
10"
82
10"
374
12"
67
12"
347
14"
63
14"
340
16"
63
16"
323
18"
63
18"
311
20"
54
20"
301
24"
48
24"
287
SDR Series PVC Pipe
Sch 120 PVC Pipe
Size
(in.)
Pipe
Stiffness (PS)
Size
(in.)
Pipe
Stiffness (PS)
SDR 13.5
950
1/2"
30668
SDR 21
235
3/4"
13535
SDR 26
120
1"
10835
SDR 41
29
1-1/4"
6184
 
1-1/2"
 
4551
 
2"
 
3057
 
2-1/2"
 
2969
 
3"
 
2575
 
4"
 
2336
 
6"
 
1495
 
8"
 
1406
In order to determine the allowable pipe burial depth, the pipe dimension, soil density, traffic load, soil type, and compaction density of embedment soil will be obtained from the tables provided. The values obtained would then be used the Modified Iowa Equation in order to determine the predicated percentage of pipe deflection. Georg Fischer Harvel LLC does not recommend the use of GF Harvel PVC Piping when the pipe diameter is deflected more then 5% due to the possibility of pipe failure. Therefore, it would not be recommended to use GF Harvel PVC Piping when the percentage of deflection, obtained through the Modified Iowa Equation, is greater than 5%. See the examples provided below.
 
Example 1:
 
4" Schedule 80 GF Harvel PVC Pipe is to be buried 10 feet under E80 railway traffic. The soil is coarse grained with little to no fines with a high proctor and 110 pounds per cubic foot soil density. Will this be an appropriate application for 4" Schedule 80 GF Harvel PVC Pipe?
 
Using the Modified Iowa Equation:
 
                             0.1 (P + L) 100
% Deflection = ________________________
                             0.149 (PS) + 0.061E’
 
                             0.1 (7.64 + 18.4) 100
% Deflection = ___________________________
                             0.149 (996) + 0.061(3000)
 
% Deflection = 0.78 ± 1%
 
The maximum predicted pipe deflection is 0.78 ± 1%, this is below the maximum recommended deflection for GF Harvel PVC pipe of 5%. Therefore, with proper trench construction the pipe would be able to withstand the external load when buried in this application.                        
 
Example 2:
 
GF Harvel SDR 26 PVC Pipe is to be buried 30 feet underground with negligible foot traffic. The soil is crushed rock with a slight proctor and 140 pounds per cubic foot soil density. Will this be an appropriate application for GF Harvel SDR 26 PVC Pipe?
 
Using the Modified Iowa Equation:
 
                             0.1 (P + L) 100
% Deflection = _______________________
                             0.149 (PS) + 0.061E’
 
 
                             0.1 (29.17 + 0) 100
% Deflection = _____________________________
                            0.149 (120) + 0.061(3000)
 
% Deflection = 1.45 ± 1%
The maximum predicted pipe deflection is 1.45 ± 1%, this is below the maximum recommended deflection for GF Harvel PVC pipe of 5%. Therefore, with proper trench construction the pipe would be able to withstand the external load when buried in this application.
 
Table 2 - Prism Load Soil Pressure in lbs/in2 (Soil Density)
Height of Soil Cover Soil Unit Weight in lbs/ft3
(ft) 100 110 120 130 140 150
1 0.69 0.76 0.83 0.90 0.97 1.04
2 1.39 1.53 1.67 1.81 1.94 2.08
3 2.08 2.29 2.50 2.71 2.92 3.13
4 2.78 3.06 3.33 3.61 3.89 4.17
5 3.47 3.82 4.17 4.51 4.86 5.21
6 4.17 4.58 5.00 5.42 5.83 6.25
7 4.86 5.35 5.83 6.32 6.81 7.29
8 5.56 6.11 6.67 7.22 7.78 8.33
9 6.25 6.88 7.50 8.13 8.75 9.38
10 6.94 7.64 8.33 9.03 9.72 10.42
11 7.64 8.40 9.17 9.93 10.69 11.46
12 8.33 9.17 10.00 10.83 11.67 12.50
13 9.03 9.93 10.83 11.74 12.64 13.54
14 9.72 10.69 11.67 12.64 13.61 14.58
15 10.42 11.46 12.50 13.54 14.58 15.63
16 11.11 12.22 13.33 14.44 15.56 16.67
17 11.81 12.99 14.17 15.35 16.53 17.71
18 12.50 13.75 15.00 16.25 17.50 18.75
19 13.19 14.51 15.83 17.15 18.47 19.79
20 13.89 15.28 16.67 18.06 19.44 20.83
21 14.58 16.04 17.50 18.96 20.42 21.88
22 15.28 16.81 18.33 19.86 21.39 22.92
23 15.97 17.57 19.17 20.76 22.36 23.96
24 16.67 18.33 20.00 21.67 23.33 25.00
25 17.36 19.10 20.83 22.57 24.31 26.04
26 18.06 19.86 21.67 23.47 25.28 27.08
27 18.75 20.63 22.50 24.38 26.25 28.13
28 19.44 21.39 23.33 25.28 27.22 29.17
29 20.14 22.15 24.17 26.18 28.19 30.21
30 20.83 22.92 25.00 27.08 29.17 31.25
31 21.53 23.68 25.83 27.99 30.14 32.29
32 22.22 24.44 26.67 28.89 31.11 33.33
33 22.92 25.21 27.50 29.79 32.08 34.38
34 23.61 25.97 28.33 30.69 33.06 35.42
35 24.31 26.74 29.17 31.60 34.03 36.46
36 25.00 27.50 30.00 32.50 35.00 37.50
37 25.69 28.26 30.83 33.40 35.97 38.54
38 26.39 29.03 31.67 34.31 36.94 39.58
39 27.08 29.79 32.50 35.21 37.92 40.63
40 27.78 30.56 33.33 36.11 38.89 41.67
41 28.47 31.32 34.17 37.01 39.86 42.71
42 29.17 32.08 35.00 37.92 40.83 43.75
43 29.86 32.85 35.83 38.82 41.81 44.79
44 30.56 33.61 36.67 39.72 42.78 45.83
45 31.25 34.38 37.50 40.63 43.75 46.88
46 31.94 35.14 38.33 41.53 44.72 47.92
47 32.64 35.90 39.17 42.43 45.69 48.96
48 33.33 36.67 40.00 43.33 46.67 50.00
49 34.03 37.43 40.83 44.24 47.64 51.04
50 34.72 38.19 41.67 45.14 48.61 52.08

 

Table 3 - Live Load on Buried Pipe in lb/in2 (Traffic Load)

Height of Cover in ft. Highway H201 Railway E802 Airport3
1 12.5    
2 5.56 26.39 13.14
3 4.17 23.61 12.28
4 2.78 18.4 11.27
5 1.74 16.67 10.09
6 1.39 15.63 8.79
7 1.22 12.15 7.85
8 0.69 11.11 6.93
10 N 7.64 6.09
12 N 5.56 4.76
14 N 4.17 3.06
16 N 3.47 2.29
18 N 2.78 1.91
20 N 2.08 1.53
22 N 1.91 1.14
24 N 1.74 1.05
26 N 1.39 N
28 N 1.04 N
30 N 0.69 N
35 N N N
40 N N N

1: Simulates 20 ton truck traffic plus impact.
2: Simulates 80,000 lb/ft railway load plus impact.
3: 180,000 lbs. dual tandem gear assembly; 26-inch spacing between tires and 66-inch center-to-center spacing between fore and aft tires under a rigid pavement 12 inches thick + impact.
N= Negligible live load influence. L = 0