artetter
Structural
- Oct 20, 2008
- 13
Hoping someone on the forum may be able to provide some design guidance on buried HDPE welded pipe.
I'm working on a new water pipe design using PE 4710, 6" diameter HDPE welded pipe. Roughly 2000 feet of the pipe will be installed to replace the existing PVC pipe. The pipeline will only see partial flow, so very little if any static water pressure. The existing pipe must be replaced because longwall coal mining operations are projected below the buried water pipeline. Water flow can only be interrupted for brief periods of time so a new pipeline is required to be installed that can resist the subsidence affects.
My question does not require knowledge of longwall mining but a brief description is presented. The longwall will mine a 10' high face of coal 900' wide almost perpendicular to the pipeline. The mining will start well before, proceed under, and end well away from the pipeline. The coal is 1200' below the surface so the subsidence from the mine will present itself in the form of a wide trough roughly 5' deep at the bottom. The resulting change in the surface topography will not be visible with the naked eye and will require surveying to establish the final boundaries and curvature of the subsidence.
We have ample case history at the mine to accurately model the subsidence trough and have reasonably accurate subsidence modeling software that has been calibrated to the case history. We have been able determine what the final subsided curvature will look like along with the locations and magnitudes of both compression and tensile strains at the ground surface. The total strain on the pipe can be calculated based on the change in curvature of the ground, change in length of the pipe and ground strain transfer onto the pipe from the soil/pipe friction.
On to my question... I don't have much experience with HDPE pipe design but recently have become familiar with Plastics Pipe Institute's PE Handbook. The viscoelastic properties of HDPE are throwing me off track. If it was a steel pipe design, the subsidence induced strain and the elastic modulus could be used to determine the subsidence induced stress then the factor of safety could be calculated based on the allowable yield stress of the steel. Do you follow the same procedure for HDPE using the tensile strength at yield (3000-3500psi) and flexural modulus (80000-110000psi)? The modulus of elasticity for HDPE is significantly affected by the time duration of the load, which at 50 years reduces to 29,000psi. It's difficult to locate a value for yield strain for HDPE, I'm assuming it's in the range of 3 to 5%. Can I simply calculate the yield strain from the published values of tensile strength at yield and the flexural modulus (mentioned above) then compare it to the calculated subsidence induced strain on the pipe? Or does there need to be a stress based factor of safety determined from the subsidence induced strain and time dependent elastic modulus (50 year)?
Thanks in advance for your comments.
I'm working on a new water pipe design using PE 4710, 6" diameter HDPE welded pipe. Roughly 2000 feet of the pipe will be installed to replace the existing PVC pipe. The pipeline will only see partial flow, so very little if any static water pressure. The existing pipe must be replaced because longwall coal mining operations are projected below the buried water pipeline. Water flow can only be interrupted for brief periods of time so a new pipeline is required to be installed that can resist the subsidence affects.
My question does not require knowledge of longwall mining but a brief description is presented. The longwall will mine a 10' high face of coal 900' wide almost perpendicular to the pipeline. The mining will start well before, proceed under, and end well away from the pipeline. The coal is 1200' below the surface so the subsidence from the mine will present itself in the form of a wide trough roughly 5' deep at the bottom. The resulting change in the surface topography will not be visible with the naked eye and will require surveying to establish the final boundaries and curvature of the subsidence.
We have ample case history at the mine to accurately model the subsidence trough and have reasonably accurate subsidence modeling software that has been calibrated to the case history. We have been able determine what the final subsided curvature will look like along with the locations and magnitudes of both compression and tensile strains at the ground surface. The total strain on the pipe can be calculated based on the change in curvature of the ground, change in length of the pipe and ground strain transfer onto the pipe from the soil/pipe friction.
On to my question... I don't have much experience with HDPE pipe design but recently have become familiar with Plastics Pipe Institute's PE Handbook. The viscoelastic properties of HDPE are throwing me off track. If it was a steel pipe design, the subsidence induced strain and the elastic modulus could be used to determine the subsidence induced stress then the factor of safety could be calculated based on the allowable yield stress of the steel. Do you follow the same procedure for HDPE using the tensile strength at yield (3000-3500psi) and flexural modulus (80000-110000psi)? The modulus of elasticity for HDPE is significantly affected by the time duration of the load, which at 50 years reduces to 29,000psi. It's difficult to locate a value for yield strain for HDPE, I'm assuming it's in the range of 3 to 5%. Can I simply calculate the yield strain from the published values of tensile strength at yield and the flexural modulus (mentioned above) then compare it to the calculated subsidence induced strain on the pipe? Or does there need to be a stress based factor of safety determined from the subsidence induced strain and time dependent elastic modulus (50 year)?
Thanks in advance for your comments.
