Vertical Deflection Limit of Buried Profile-Wall HDPE Drain Pipes (abstract No. 118)

Transcription

1 Presentation at OTEC Session 72 Buried Conduit Mechanics Vertical Deflection Limit of Buried Profile-Wall HDPE Drain Pipes (abstract No. 118) PI: Dr. Teruhisa (Terry) Masada, Faculty Advisor Co-PI: Xiao Han, Ph.D. Student Civil Engineering Department Russ College of Engineering & Technology Ohio University, Athens, OH 10:30-12:00 October 28 (Wed.), 2015

2 Introduction Thermoplastic pipelines have been utilized extensively below the ground surface to: 1) Provide utility service (water, natural gas, electricity, ) to customers; 2) Serve as alternatives to more conventional concrete and metal culverts; and 3) Serve as cheaper alternatives to short-span bridges. 2

3 Introduction In transportation related projects, HDPE pipe culverts are becoming more common than PVC pipe culverts. Structural health of the underground plastic pipes is vital to the overall infrastructure asset management. Structural conditions of buried HDPE pipes must be evaluated during and after construction. HDPE pipes are soft and flexible. They can deform much more easily than pipes made from other materials. 3

4 Introduction HDPE pipes come with profiled wall sections. Straight wall Profile wall (a) Straight Wall (b) Corrugated Wall (c) Profile Wall Corrugated Wall + Liner 4

5 Introduction Because they are flexible, HDPE pipes interact with surrounding soil to resist dead and live loads. Soil-Pipe Interaction Plastic pipes require high-quality structural soil fill (backfill) to be able to perform well. 5

6 Introduction Special Terms Used Cover Crown Soil Envelope Springline Soil Envelope Invert Bedding Foundation Soil 6

7 Introduction Potentially Critical Regions Located in Pipe Cross-Section: Crown Compressive Strain (outside wall); Tensile Strain (inside wall) Springline Compressive Strain (inside wall); Tensile Strain (outside wall) Cracking; Buckling Invert Stresses/Strains are significant when pipe sits directly on rigid media (ex. bedrock.). 7

8 Introduction Limiting the maximum vertical deflection usually takes care of all concerns about the HDPE pipe s structural performance. What will be the largest vertical deflection which can assure sound structural performance in varied field installation/service conditions for HDPE pipe culverts? 8

9 Objective The main goal of the OU study was to establish the threshold vertical deflection values for thermoplastic pipes installed in a variety of installation conditions 9

10 Methodology Task 1: Literature Review Summarize documented field performance cases for buried HDPE pipes Task 2: ASSHTO LRFD Design Calculations Apply AASHTO LRFD method calculations to correlate vertical deflections to performance limiting conditions for HDPE pipes Task 3: Computer Simulations Use industry standard computer software CANDE to also correlate vertical deflections to performance limit conditions for HDPE pipes Task 4: Development of Charts Combine the results from all three tasks to draw conclusions and develop charts concerning threshold vertical deflections for HDPE pipes 10

11 Task 1: Literature Review Hurd, J. O. (1985). "Field Performance of Corrugated Polyethylene Pipe Culverts in Ohio." Flattening occurs at vertical deflections exceeding15%. Wall buckling occurs at vertical deflections exceeding 25%. The vertical deflections of HDPE pipes without apparent flattening, buckling, and dent are less than 10%. The vertical deflections at the pipe joints are general slightly larger than those throughout the rest of the culvert. 11

12 Task 1: Literature Review There is no increase in vertical deflection after 2 to 4 years within pipes less than 10% deflection. Shallow cover and heavy truck traffic do not appear to be detrimental to the structural performance of pipes. Deflection appears to be built into the culverts instead of caused by highway loading. 12

13 Task 1: Literature Review Hashash, N., Selig, E. T. (1989). "Results of Field Measurements on Polyethylene Pipe Taken in August 1988." A 24-in profile-wall HDPE pipe was buried in the embankment and only loaded by soil dead load. The deflection, stress and strain are recorded with the rise of cover thickness. The maximum soil cover height was 100 ft. 24-in corrugated and profile HDPE pipes are installed in a trench with 5-ft depth and 6-ft width. The backfill is wellgraded crushed limestone under 100% compaction. The load is soil dead load only. 13

14 Task 1: Literature Review Height of Fill (ft) Avg. Vertical Stress (psi) Avg. Horizontal Stress (psi) Avg. Strain at Crest (%) Avg. Strain at Centroid (%) Avg. Circumfere ntial Strain (%) Avg. Vertical Deflection (%) Avg. Horizontal Deflection (%) N/A N/A N/A N/A N/A N/A

15 Task 1: Literature Review At 95-ft soil cover, the average deflection of corrugated pipe is 3.4%, and the average deflection of profile pipe is 4.4%. The max. vertical deflection is 7.2%, and no local distress or structural damage observed. 15

16 Task 1: Literature Review Hsuan, Y.G., McGrath, T. J. (1999). NCHRP Report 429: HDPE Pipe: Recommended Material Specifications and Design Requirements. National Academy Press, Washington, D.C. Field performances of buried HDPE pipes in various regions of the U.S. are surveyed and presented in this report. 16

17 Task 1: Literature Review Year Installed Pipe Type and Diameter Backfill Type and Depth Pavement Type and Thickness Daily Traffic Vertical Deflection Conditions <1985 Corrugated 15 in granular and native soil 3 ft asphalt / light 13.3% circumferential crack appears slip at outlet 1981 Corrugated 24 in gravel bank run ODOT ft asphalt 15 in light 14.6% circumferential and longitudinal crack due to vehicle impact and mower damage at pipe ends 1983 Corrugated 24 in ODOT ft asphalt / mediu m max 8.3% all good 1984 Corrugated 12 in ODOT 411 limestone 5 ft asphalt / light 8.3% all good 1982 Corrugated 15 in gravel 4.5 ~ 6 ft asphalt 15 in light 10% all good 17

18 Task 1: Literature Review Year Installed Pipe Type and Diameter Backfill Type and Depth Pavement Type and Thickness Daily Traffic Vertical Deflection Conditions 1983 Corrugated 15 in mixed 1 ft asphalt / light 13.3% all good 1995 Profile 30 in sand 8 ft unpaved / none 10% circumferential crack; deflection at joints 1994 Profile 42 in unknown 3 ft asphalt 6 in mediu m average 9.5% circumferential crack growth from 1 to 4.5 in in one year 18

19 Task 1: Literature Review Moser A. P. (2000). "Structural Performance of 42-inch (1050mm) Corrugated Smooth Interior HDPE Pipe." Dr. Moser conducted three tests on 42-inch diameter profilewall HDPE pipes. The pipes were buried in the embankment constructed using a silty-sand (SM) backfill material. The compaction level was different among the three tests. The load was applied using hydraulic cylinders to simulate dead load. 19

20 Task 1: Literature Review Test 1. The relative compaction level on backfill was 75%. At 40 ft, deflection 10% and a dimpling pattern started at 3 and 9 o'clock. The center distance between dimples was about the same as the rib spacing. At 55 ft, 14.5% vertical deflection, hinging was noted at 3 and 9 o'clock. At 60 ft, 16.5% deflection, hinging was very apparent and become more pronounced at 3 and 9 o'clock. Test ended at 69 ft, 20% deflection. 20

21 Task 1: Literature Review Test 2. The relative compaction level on backfill was 85%. At 70-ft soil cover, 8.7% deflection, small dimples began to form near 3 and 9 o'clock. At 82 ft, 11% deflection, waffling pattern formed. At 110 ft, 16.5% deflection, hinging started at 4 and 9 o'clock. Test ended at 110 ft. 21

22 Task 1: Literature Review Test 3. The relative compaction level on backfill was 95%. At 110 ft, 3.5% deflection, dimpling pattern began at 3 and 9 o'clock and spread toward the top of the pipe as load increased. This pattern was like a wavy checkerboard in appearance and the beginning of localized instability. The dimpling was extremely small and would not affect the structural performance of the pipe. At 140 ft, 5.2% deflection, dimples became pronounced but were not judged to be a performance limit of the pipe. At 170 ft, 6.7% deflection, first signs of wall crushing were noted at 10 o'clock. At 225 ft, 9.4% deflection, waffling and wall crushing became more pronounced but the pipe did not fail. Test ends at 225 ft. 22

23 Task 1: Literature Review Sargand, S. M. and Masada, T. (2011). Field Service Conditions of the Oldest Corrugated HDPE Pipe Culvert Under Ohio's Roadway." In this ASCE journal paper, the ORITE researchers reported the field conditions of a 24" diameter corrugated HDPE pipe culvert that had served under SR 145 (Noble County) under a shallow soil cover and low ph (4-5) drainage flow. The pipe was installed in 1981 under a cover of 1.0 to 1.3 ft. 23

24 Task 1: Literature Review Vertical Deflection Data Location Along Pipeline Data Collected in 1982 Data Collected in 2004 Outlet End Middle of Section 1-1.5% -1.0% End of Section 1-5.2% -5.2% Joint -7.3% -8.3% Middle of Section 2 0.5% 1.0% Inlet End AVERAGE -2.3% -2.3% The pipe was observed to be functioning well, with no signs of structural distress (wall crushing, wall buckling, cracking, ). 24

25 Task 2: AASHTO Calculations The AASHTO LRFD Bridge Design Specifications (2013 Interim Revisions to the Six Edition 2012). The vertical deflection, combined tensile strain, combined compressive strain, and buckling strain are computed through the formulas presented in Section 12: Buried Structures and Tunnel Liners. The live load and load combination are set up according to Section 3: Loads and Load Factors. The complete assumptions of parameters and formulas are listed in "AASHTO Equations". 25

26 Task 2: AASHTO Calculations Scanned Wall Profile of Each HDPE Pipe Diameter 26

27 Task 2: AASHTO Calculations General Dimensions of Each HDPE Pipe Diameter Nominal Diameter (in) Inside Diameter (in) Outside Diameter (in) Inner Liner Thickness (in) Wall Area (in 2 /in) Moment of Inertia I (in 4 /in) Dist. to Centroid c (in) [Comment] Above dimensions based on ADS Product Note

28 Task 2: AASHTO Calculations Detailed Dimensions of Each HDPE Pipe Diameter Nominal Diameter S (in) w-crest (in) w-liner (in) w-valley (in) w-web (in) t-crest (in) t-liner (in) t-valley (in) t-web (in) [Note] w = width; and t = thickness. 28

29 Task 2: AASHTO Calculations The constrained soil modulus is set at 1 ksi and 3 ksi. The unit weight of backfill with various constrained modulus is assumed according to the recommended values listed in user`s manual of CANDE. Unit Weight (pcf) Constrained Modulus (ksi) The modulus of HDPE pipe material is considered in terms of short term and long term. The short-term modulus is 110 ksi, and long-term (50-years) modulus is 22 ksi. 29

30 Task 2: AASHTO Calculations AASHTO calculations are based on pipe wall strains, not on pipe wall stresses. The critical strains for HDPE pipe wall are 5% (tensile) and 6.15% (compressive). Calculations show that compressive strains in the pipe wall tend to reach the compressive strain limit well before tensile strains reach the tensile strain limit. Thus, a focus is placed on compressive strains in pipe wall. The dead load on the pipe is continuously increased to the failure of the pipe. The loading cases have "dead load only" as well as "dead and live loads". 30

31 Task 2: AASHTO Calculations Short-Term & Long-Term Strain vs. Ver. Def., Ms = 1 & 3 ksi, Dead Load Only 31

32 Task 2: AASHTO Calculations Short-Term Buckling Ratio vs. Ver. Def., Ms = 1 & 3 ksi, Dead Load Only 32

33 Task 2: AASHTO Calculations When the vertical deflection is held constant, higher constrained modulus induce higher strains in the pipe wall. The critical vertical deflection in the long-term service condition is less than the critical vertical deflection in the short-term service condition, especially for the HDPE pipes buried in the lower modulus backfill. If the vertical deflections are smaller than 7.5%, the combined compressive strain will not surpass 6.15% limit in most cases, except when the pipes are buried in the 3-ksi modulus backfill material and under the long-term service condition. Wall buckling failure is not a concern when vertical deflections are held below 12%. 33

34 Task 2: AASHTO Calculations Short-Term Strain and Buckling Ratio vs. Ver. Def., Ms = 1 & 3 ksi, Dead and Live Load 34

35 Task 2: AASHTO Calculations Combined compressive strain in the pipe wall never reach the strain limit even when the cover thickness is as low as 1 ft. When the soil cover thickness is held constant, the combined compressive strain decreases as the soil modulus increases. The portion of combined compressive strains caused by the live load diminishes with the soil cover. At the cover thickness of 15 ft, combined compressive strains occurred under the dead and live loads become the same to those under the dead load only. Under the dead and live loads, wall buckling failure mode is not a concern even at a cover thickness of 1 ft. 35

36 Task 2: Validations Sargand, S. M. and Masada, T. (2011). Field Service Conditions of the Oldest Corrugated HDPE Pipe Culvert Under Ohio's Roadway." A 24-in diameter corrugated HDPE pipe had served State Route 145 (Noble County) from The pipe was installed under a cover of 1 to 1.3 ft, and the constrained modulus of soil is estimated to be around 1 ksi. The pipe was subjected to the dead and live loads. The vertical deflection was 8.3% in No overstraining/no buckling was observed. Based on AASHTO calculation`s results, the vertical deflection of 24-in diameter HDPE pipe under live load is 8.16%, and max. combined compressive strain is 3.48% (<6.15%), and buckling ratio is 0.62 (<1.0). 36

37 Task 3: CANDE Simulations Culvert Analysis & Design (CANDE) computer software Originally developed by Dr. Michael Katona & his team Latest version is CANDE 2007 Pipe represented by a series of column/beam elements Soils represented by a number of quadratic elements Requires a detailed mesh with material properties, boundary condition statements, construction sequences, and loading instructions 37

38 Task 3: CANDE Simulations Model Half-Mesh 38

39 Task 3: CANDE Simulations The computer simulations were made for 12-in, 24-in, 36-in, 48-in, and 60-in diameter HDPE pipes. The installation modes included embankment (positive projection) mode, trench mode, and negative projection mode. The increments of soil cover thickness were the same as those applied in the AASHTO calculations. For the dead and live loads analysis, the cover thickness was increased from 1 to 15 ft. The analysis also addressed both short-term and long-term service conditions. 39

40 Task 3: CANDE Simulations Pipe Type Pipe ID Installation Mode Backfill Soil Cover & Loading Corrugated HDPE Short-term Long-term Embankment (or Positive Projection Mode) 90, 95% Compaction 90, 95% Compaction 85, 90, 95% Compaction Shallow; (D + L) Intermediate; (D + L) Deep; (D only) Shallow; (D + L) Intermediate; (D + L) Deep; (D only) Shallow; (D + L) Intermediate; (D + L) Deep; (D only) [Note] * There are 42 different cases illustrated above for each pipe size. * Shallow (< 7 ft), Intermediate (7-15 ft), Deep (> 15 ft). 40

41 Task 3: CANDE Simulations Pipe Type Pipe ID Installation Mode Corrugated HDPE Short-term Long-term Trench (Trench Width is OD + 1 ft at each side of pipe. This value is much greater than AASHTO Min. 12" Cover above pipe top) Backfill 90, 95% Compaction 90, 95% Compaction 85, 90, 95% Compaction [Notes] * There are 42 different cases illustrated above per pipe size. * Shallow (< 7 ft), Intermediate (7-15 ft), Deep (> 15 ft). Soil Cover & Loading Shallow; (D + L) Intermediate; (D + L) Deep; (D only) Shallow; (D + L) Intermediate; (D + L) Deep; (D only) Shallow; (D + L) Intermediate; (D + L) Deep; (D only) 41

42 Task 3: CANDE Simulations The in-situ soil in the embankment mode and trench mode were both specified as SW-100. The backfill and bedding materials were the same during each analysis. In accordance with AASHTO M145 and ASTM 2487, CA-90 and CA-95 corresponded to A-1, SM-85 and SM-90 correspond to A-2, and SW-85, SW-90 and SW95 correspond to A-3. CA-90, CA-95, SM-85 and SM-90 were only applicable to Duncan formation. SW-85, SW-90 and SW95 were applicable to Duncan/Selig formulation 42

43 Task 3: CANDE Simulations Pipe wall strain limits were set at 6.15% (compressive). Pipe wall buckling strain limits were calculated for each pipe diameter size. 43

44 Task 3: CANDE Simulations Combined Compressive Strain vs. Ver. Def., Dead Load, Embankment Mode & Trench Mode 44

45 Task 3: CANDE Simulations Short-Term Buckling Ratio vs. Ver. Def., Dead Load, Embankment Mode & Trench Mode 45

46 Task 3: CANDE Simulations If the vertical deflections are smaller than 7.5%, the combined compressive strain will not surpass 6.15% limit in most cases, except when the pipes are buried in the SW-95 backfill material and/or under the long-term service condition. Wall buckling failure is not a concern as long as the vertical deflection is kept below 7.5% (only exception occurs with the SW-95 backfill). Compared to the pipes in the embankment mode, trench mode can help the pipes raise critical vertical deflection. The higher modulus backfill can cause higher strains in the pipe wall when the vertical deflection is constant. 46

47 Task 3: CANDE Simulations Short-Term Strain vs. Ver. Def., Dead & Live Load, Embankment Mode & Trench Mode 47

48 Task 3: CANDE Simulations As shallow as 1-ft soil cover thickness, the combined compressive strain in 24-in diameter HDPE pipe wall is greater than 6.15% stain limit when backfill is SM-85 and CA-90 in embankment mode and CA-90 in trench mode. Combined compressive strain of other diameter pipes buried in any backfill types are all smaller than the strain limit, even when the soil cover thickness is only 1 ft. Under 2-ft cover thickness, 24-in diameter HDPE pipe wall never buckles regardless of which when backfill material is surrounding the pipe. The effect of live load on combined compressive strain disappeared when soil cover thickness approaches 15 ft. 48

49 Task 3: Validation Vertical Deflections of HDPE Pipes after the installations Hashash, N., Selig, E. T. (1989). "Results of Field Measurements on Polyethylene Pipe Taken in August 1988." Height of Fill (ft) Dia. (in) Field Test Data Backfill Ver. Def. (%) Backfill CANDE Simulation Height of Fill (ft) Ver. Def. (%) CA CA CA CA (estimated) The estimated vertical deflection of 24-in dia. HDPE diameter pipe is higher than the measured value. In CANDE simulation, combined compressive strain and buckling ratio are respectively smaller than their limits when the backfill is CA-95. Since the strains and buckling ratio will decrease with the rise of compaction level, the 24-in dia. HDPE pipe has no failure under 99-ft cover thickness, and the actual pipe in the field test exhibited no signs of failure. 49

50 Task 4: Comparisons Maximum Vertical Deflections (%), AASHTO Calculations Dia. (in) M s = 1 ksi M s = 3 ksi ST LT ST LT Average [Note]: Dia. = diameter of the pipe; M s = constrained modulus of the backfill; ST = short-term service condition; LT = long-term service condition. 50

51 Task 4: Comparisons Maximum Vertical Deflections (%), CANDE Simulation Backfill Type 12-in 24-in 36-in 48-in 60-in ST LT ST LT ST LT ST LT ST LT CA CA SM SM SW SW SW [Note]: ST = short-term service condition; LT = long-term service condition. 51

52 Conclusion Although conservative, the AASHTO LRFD method largely pointed out that 7.5% vertical deflection limit can protect HDPE pipes from developing structural failures/major distress. CANDE is even more conservative than the AASHTO method, as its stiffness based formulation makes the pipe stiffer and also it suffers from convergence problems in some cases. Performing two separate analyses using the short-term and long-term pipe material moduli does not really make sense for HDPE pipes servicing in the field. 52

53 Conclusion * Actual pipe-soil system in the field acts as a viscoelastic system. Long-Term Vertical Deflection Actual Short-Term Time 53

54 Conclusion Documented field performance data uncovered during the literature review are littered with cases where the HDPE pipes have performed fine at vertical deflections larger than 7.5% (less than 10%). Also, a limited volume of strain gage readings available from OU s Thermoplastic Pipe Deep Burial project indicate the strains in the HDPE pipe walls are less than half of what are estimated by the AASHTO method. [Main Conclusion] For the HDPE pipes buried in 1-ksi and 3- ksi backfill materials, a 7.5% vertical deflection limit is sufficient for both short-term and long-term service conditions. 54

55 Conclusion [Additional Note] The HDPE pipe s vertical deflection window can be extended if the pipe is allowed to peak during the initial backfilling stage. Peaking 1 to 2% 7.5% Deflection Limit 8.5 to 9.5% Deflection Limit 55

56 Acknowledgement We would like to thank ODOT for supporting this research study. 56

57 Thank you! PI: Xiao Han, Graduate Student Co-PI: Teruhisa (Terry) Masada, Faculty Advisor Civil Engineering Department Russ College of Engineering & Technology Ohio University, Athens, OH Tel: (740) Fax: (740) s: