1、Section 17 SOIL-THERMOPLASTIC PIPE INTERACTION SYSTEMS 17.1 GENERAL 17.1.1 Scope The specifications of this section are intended for the structural design of plastic pipes. It must be recognized that a buried plastic pipe is a composite structure made up of the plastic ring and the soil envelope, an
2、d that both ma- terials play a vital part in the structural design of plastic pipe. 17.1.2 Notations A = area of pipe wall in square inchedfoot (Articles B = water buoyancy factor (Articles 17.2.2 and c = distance from inside surface to neutral axis (Arti- De = effective diameter = ID + 2c E = modul
3、us of elasticity of pipe material (Articles FF = flexibility factor (Articles 17.2.3 and 17.3.3) fa = allowable stress-specified minimum tensile strength divided by safety factor (Article 17.2.1) f, = critical buckling stress (Articles 17.2.2 and 17.3.2) f, = specified minimum tensile strength (Arti
4、cles 17.2.1, 17.3.1, and 17.3.2) I = average moment of inertia, per unit length, of cross section of the pipe wall (Articles 17.2.2, 17.2.3, and 17.3.3) ID = inside diameter (Articles 17.2.2, 17.3.2, and 17.4.2) M, = soil modulus (Articles 17.2.2, 17.3.2) OD = outside diameter (Article 17.4.2) P = d
5、esign load (Article 17.1.4) SF = safety factor (Article 17.2.1) T = thrust (Article 17.1.4) TL = thrust, load factor (Article 17.3.1) T, = thrust, service load (Article 17.2.1) = capacity modification factor (Article 17.3.1) 17.2.1 and 17.3.1) 17.3.2) cles 17.2.2, 17.3.2, and 17.4.2) 17.2.2 and 17.3
6、.2) 17.1.3 Loads Design load, P, shall be the pressure acting on the stnic- ture. For earth pressures see Article 3.20. For live load see Articles 3.4 to 3.7, 3.11, 3.12, and 6.4, except that the words “When the depth of fill is 2 feet or more” in Article 6.4.1 need not be considered. For loading co
7、mbinations see Article 3.22. 17.1.4 Design 17.1.4.1 The thrust in the wall shall be checked by two criteria. Each considers the mutual function of the plastic wall and the soil envelope surrounding it. The cri- teria are: (a) Wall area (b) Buckling stress 17.1.4.2 The thrust in the wall is: D T=PX y
8、 L where: P = design load, in pounds per square foot; D = diameter in feet; T = thrust, in pounds per foot. (17-1) 17.1.4.3 Handling and installation strength shall be sufficient to withstand impact forces when shipping and placing the pipe. 17.1.5 Materials The materials shall conform to the AASHTO
9、 and ASTM specifications referenced herein. 17.1.6 Soil Design 17.1.6.1 Soil Parameters The performance of a flexible culvert is dependent on soil structure interaction and soil stiffness. 43 1 432 HIGHWAY BRIDGES 17.1.6.1 The following must be considered: (a) Soils: (1) The type and anticipated beh
10、avior of the founda- tion soil must be considered; Le., stability for bedding and settlement under load. (2) The type, compacted density, and strength proper- ties of the envelope immediately adjacent to the pipe must be established. Good side fill is obtained from a granular material with little or
11、 no plasticity and free of organic material, Le., AASHTO classification groups A-1, A-2, and A-3, compacted to a minimum 90% of standard density based on AASHTO T 99 (ASTM D 698). (3) The density of the embankment material above the pipe must be determined. See Article 6.2. (b) Dimensions of envelop
12、e The general recommended criteria for lateral limits of the culvert envelope are as follows: (1) Trench installations-the minimum trench width shall provide sufficient space between the pipe and the trench wall to ensure sufficient working room to prop- erly and safely place and compact backfill ma
13、terial. As a guide, the minimum trench width should not be less than the greater of the pipe diameter plus 16.0 inches, or the pipe diameter times 1.5 plus 12.0 inches. The use of specially designed equipment may enable satisfac- tory installation and embedment even in narrower trenches. (2) Embankm
14、ent installations-the minimum width of the soil envelope shall be sufficient to ensure lateral restraint for the buried structure. The combined width of the soil envelope and embankment beyond shall be adequate to support all the loads on the pipe. As a guide, the width of the soil envelope on each
15、side of the pipe should be the pipe diameter or 2.0 feet, whichever is less. (3) The minimum upper limit of the soil envelope is 1 foot above the culvert. 17.1.7 Abrasive or Corrosive Conditions Extra thickness may be required for resistance to abra- sion. For highly abrasive conditions, a special d
16、esign may be required. 17.1.8 Minimum Spacing When multiple lines of pipes greater than 48 inches in diameter are used, they shall be spaced so that the sides of the pipe shall be no closer than one-half diameter or 3 feet, whichever is less, to permit adequate compaction of backfill material. For d
17、iameters up to and including 48 inches, the minimum clear spacing shall not be less than 2 feet. 17.1.9 End Treatment Protection of end slopes may require special consider- ation where backwater conditions may occur, or where erosion and uplift could be a problem. Culvert ends con- stitute a major r
18、un-off-the road hazard if not properly de- signed. Safety treatment, such as structurally adequate grating that conforms to the embankment slope, extension of culvert length beyond the point of hazard, or provision of guardrails, is among the alternatives to be considered. End walls on skewed alignm
19、ent require a special design. 17.1.10 Construction and Installation The construction and installation shall conform to Sec- tion 26, Division II. 17.2 SERVICE LOAD DESIGN Service Load Design is a working stress method, as tra- ditionally used for culvert design. 17.2.1 Wall Area A = T,/fa where: A =
20、 required wall area in square inches per foot; T, = thrust, service load in pounds per foot; fa = allowable stress, specified minimum tensile strength, pounds per square inch, divided by safety factor, f,/SF. (For, SF, see Article 17.4.1.2.) 17.2.2 Buckling Walls within the required wall area, A, sh
21、all be checked for possible buckling. If the allowable buckling stress, fcJSF, is less than fa, the required area must be recalculated using fcJSF in lieu of fa. The formula for buckling is: f, = 9.24 (WA) VBM, EV0.149R3 where: B = water buoyancy factor or = 1 -0.33hWk; h, = height of water surface
22、above top of pipe; h = height of ground surface above top of pipe; E = Long term (50-year) modulus of elasticity of the plastic in pounds per square inch; M, = soil modulus in pounds per square inch; = 1700 for side fills meeting Article 17.1.6; f, = critical buckling stress in pounds per square inc
23、h; 17.2.2 DIVISION 1-DESIGN 433 R = effective radius in inches = c + ID/2; A = actual area of pipe wall in square inchedfoot. 17.2.3 Handling and Installation Strength Handling and installation rigidity is measured by a flexibility factor, FF, determined by the formula: FF = D:/EI where: FF = flexib
24、ility factor in inches per pound; De = effective diameter in inches; E = initial modulus of elasticity of the pipe material in pounds per square inch; I = average moment of inertia per unit length of cross section of the pipe wall in inches to the 4th power per inch. 17.3 LOAD FACTOR DESIGN Load Fac
25、tor Design is an alternative method of design based on ultimate strength principles. 17.3.1 Wail Area A = TJ+f, where: A = required area of pipe wall in square inches per TL = thrust, load factor in pounds per foot; f, = specified minimum tensile strength in pounds 4 = capacity modification factor.
26、foot; per square inch; 17.3.2 Buckling If f, is less than f, A must be recalculated using f, in lieu off,. The formula for buckling is: f, = 9.24 (RIA) VBM, EI/0.149R3 where: B = water buoyancy factor or = 1 - 0.33h,/h; h, = height of water surface above top of pipe; h = height of ground surface abo
27、ve top of pipe; E = Long term (50-year) modulus of elasticity of the plastic in pounds per square inch; M, = soil modulus in pounds per square inch = 1,700 for side fills meeting Article 12.1.6; f, = critical buckling stress in pounds per square R = effective radius in inches = c + ID/2; A = actual
28、area of pipe wall in square inches/foot. inch; 17.3.3 Handling and Installation Strength Handling rigidity is measured by a flexibility factor, FF, determined by the formula: FF = DmI where: FF = flexibility factor in inches per pound; De = effective diameter in inches; E = initial modulus of elasti
29、city of the pipe material in pounds per square inch; I = average moment of inertia per unit length of cross section of the pipe wall in inches to the 4th power per inch. 17.4 PLASTIC PIPE 17.4.1 General 17.4.1.1 Plastic pipe may be smooth wall, corrugated or externally ribbed and may be manufactured
30、 of poly- ethylene (PE) or poly (vinyl chloride) (PVC). The mater- ial specifications are: Polyethylene (PE) Smooth Wall -ASTM F 7 14 Polyethylene (PE) Plastic Pipe (SDR-PR) Based on Outside Diameter Corrugated -AASHTO M 294 Corrugated Polyethylene Pipe, 12 to 36 in. Diameter -ASTM F 894 Polyethylen
31、e (PE) Large-Diameter Profile Wall Sewer and Drain Pipe Ribbed Poly (Vinyl Chloride)(PVC) Smooth Wall -AASHTO M 278 Class PS 46 Polyvinyl Chloride (PVC) Pipe, ASTM F 679 Poly (Vinyl Chloride) (PVC) Large-Diame- ter Plastic Gravity Sewer Pipe and Fittings -AASHTO M 304 Poly (Vinyl Chloride) (PVC) Rib
32、bed Drain Pipe and Fittings and Based on Ribbed 17.4.1.1 434 HIGHWAY BRIDGES Controlled Inside Diameter ASTM F 794 Poly (Vinyl Chlo- ride) (PVC) Large-Diameter Ribbed Gravity Sewer Pipe and Fittings Based on Controlled In- side Diameter 17.4.1.2 Service Load Design-safety factor, SF: Wall area = 2.0
33、 Buckling = 2.0 17.4.1.3 Load Factor Design-capacity modifica- tion factor, +: 17.4.1.4 Flexibility Factor: PE, FF = 9.5 X PVC, FF = 9.5 x 10-2 Note: PE and PVC are thermoplastics and, therefore, subject to reduction in stiffness as temperature is in- creased. 17.4.1.5 Minimum Cover The minimum cove
34、r for design loads shall be ID/8 but not less than 12 inches. (The minimum cover shall be measured from the top of a rigid pavement or the bottom of a flexible pavement.) For construction requirements, see Article 26.5, Division II. 17.4.1.6 Maximum Strain The allowable deflection of installed plast
35、ic pipe may be limited by the extreme fiber tensile strain of the pipe wall. Calculation of the tension strain in a pipe signifi- cantly deflected after installment can be checked against the allowable long-term strain for the material in Article 17.4.3. Compression thrust is deducted from deflectio
36、n bending stress to obtain net tension action. The allowable long-term strains shown in Article 17.4.3 should not be reached in pipes designed and constructed in accordance with this specification. 17.4.1.7 Local Buckling The manufacturers of corrugated and ribbed pipe should demonstrate the adequac
37、y of their pipes against local buckling when designed and constructed in accor- dance with this specification. 17.4.2 Section Properties The values given in the following tables are limiting values and do not describe actual PE or PVC pipe products. Section properties for specific PE or PVC pipe pro
38、ducts are available from individual pipe manufacturers and can be compared against the following values for compliance. 17.4.2.1 PE Corrugated Pipes (AASHTO M 294, MPG-95) Nominal Size 12 15 18 24 30 36 42 * 48 * (in.) Min. Max. I.D. O.D. (in.) (in.) 11.8 14.7 14.8 18.0 17.7 21.5 23.6 28.7 29.5 36.4
39、 35.5 42.5 41.5 48.0 47.5 55.0 Min, A (in .Vft) 1.50 1.91 2.34 3.14 3.92 4.50 4.69 5.15 Min. C (in.) 0.35 0.45 0.50 0.65 0.75 0.90 1.11 1.15 Min. I (in./in.) 0.024 0.053 O I 062 0.116 O. 163 0.222 0.543 0.543 For 42“and 48“pipe, the wall thickness should be designed using the long term tensile stren
40、gth provision (900 psi) until new design cri- teria are established. 17.4.2.2 PE Ribbed Pipes (ASTM F 894) Min. I (in ./in.) Nominal Min. Max. Min. Min. Cell Cell Size I.D. O.D. A C Class Class (in.) (in.) (in.) (in.Vft) (in.) 334433C 335434C 18 17.8 21.0 2.96 0.344 0.052 0.038 21 20.8 24.2 4.15 0.4
41、09 0.070 0.051 24 23.8 27.2 4.66 0.429 0.081 0.059 27 26.75 30.3 5.91 0.520 0.125 0.091 30 29.75 33.5 5.91 0.520 0.125 0.091 33 32.75 37.2 6.99 0.594 0.161 0.132 36 35.75 40.3 8.08 0.640 0.202 0.165 42 41.75 47.1 7.81 0.714 0.277 0.227 48 47.75 53.1 8.82 0.786 0.338 0.277 17.4.2.3 Profile Wall PVC P
42、ipes (AASHTO M 304) Min. I (in. /in.) Nominal Min. Max. Min. Min. Cell Cell Size I.D. O.D. A C Class Class (in.) (in.) (in.) (in.%) (in.) 12454C 12364C 12 11.7 13.6 1.20 0.15 0.004 0.003 15 14.3 16.5 1.30 0.17 0.006 0.005 18 17.5 20.0 1.60 0.18 0.009 0.008 21 20.6 23.0 1.80 0.21 0.012 0.011 24 23.4
43、26.0 1.95 0.23 0.016 0.015 30 29.4 32.8 2.30 0.27 0.024 0.020 36 35.3 39.5 2.60 0.31 0.035 0.031 42 41.3 46.0 2.90 0.34 0.047 0.043 48 47.3 52.0 3.16 0.37 0.061 0.056 17.4.3 DIVISION I-DESIGN 435 17.4.3 Chemical and Mechanical Requirements The polyethylene (PE) and poly (vinyl chloride) (PVC) materi
44、als described herein have stresdstrain relationships that are nonlinear and time dependent. Minimum 50-year tensile strengths are derived from hydrostatic design bases and indicate a minimum 50-year life expectancy under continuous application of that tensile stress. Minimum 50- year moduli do not i
45、ndicate a softening of the pipe mater- ial but is an expression of the time dependent relation be- tween stress and strain. For each short-term increment of deflection, whenever it occurs, the response will reflect the initial modulus. Both short- and long-term properties are shown. Except for buckl
46、ing for which long-term properties are required, the judgment of the Engineer shall determine which is appropriate for the application. Initial and long term relate to conditions of loading, not age of the instal- lation. Response to live loads will reflect the initial modu- lus, regardless of the a
47、ge of the installation. 17.4.3.1 Polyethylene 17.4.3.1.1 Smooth wall PE pipe requirements- ASTM F 714 Mechanical Properties for Design Initial 50 -Year Minimum Minimum Minimum Minimum Tensile Mod. Tensile Mod. Strength of Elast. Strength of Elast. (psi) (psi) (Psi) (psi) 3,000 110,000 1,440 22,000 M
48、inimum cell class, ASTM D 3350, 335434C Allowable long-term strain = 5% 17.4.3.1.2 Corrugated PE pipe requirements- AASHTO M 294: Mechanical Properties for Design Initial 50 -Year Minimum Minimum Minimum Minimum Tensile Mod. Tensile Mod. Strength of Elast. Strength of Elast. (psi) (psi) (psi) (psi)
49、3,000 110,000 900 22,000 Minimum cell class, ASTM D 3350, 335400C, with additional environmental stress crack resistance evalua- tion according to SP-NCTL test as per recommendations in NCHRP Report 429. Allowable long-term strain = 5% 17.4.3.1.3 Ribbed PE pipe requirements-ASTM F 894 Mechanical Properties for Design Initial 50 -Year Minimum Minimum Minimum Minimum Tensile Mod. Tensile Mod. Strength of Elast. Strength of Elast. (psi) (psi) (psi) (psi) 3,000 80,000 1 , 125 20,000 Minimum cell class, ASTM D 3350, 334433C Allowable long-term strain = 5% OR: Initial 50 -Year
