Thus, it is confirmed that the design standard not only specifies the behavior of buried flexible pipes utterly different from that obtained in the tests, but also it generates definitely unsafe design
194 BURIED PLASTIC PIPE TECHNOLOGY
values for the Ditch-S type, so that it may cause the pipe damages in actual construction. The other Japanese design standards were confirmed to generate similar results to the ones described here.
CONCLUSIONS
A series of centrifuge model tests yielded accurate measured results to quantify successfully the effects of the following factors on earth pressure distribution, bending moment and pipe deflection of buried flexible pipes: pipe flexibility, type of pipe installation, ground condition, and three burial dimensions. The test results confirmed that flexible pipes deflect drastically at the sheet-pile extraction when the pipes are buried by open excavation method using sheet-piling (Ditch-S type of pipe installation). It was found that Japanese current design standards based on Marston-Spangler theory do not conform to the test results by ignoring the effects of almost all the investigated factors on the test results, and as a result, they specify definitely unsafe design values for the Ditch-S type to cause damages of flexible pipes in actual construction. In order to avoid the pipe damages due to the sheet-pile extraction, therefore, the current design standards should be revised on the basis of the test results represented in this paper as immediately as possible, and until this revision is performed, the countermeasures reported in the literature [9] should be employed in the actual design and construction works.
REFERENCES
[!] Spangler, M. G., "The Structural Design of Flexible Pipe Culverts,"
Bull.153, Iowa Engineering Experiment Station, Iowa State University, 1941.
[2] Tohda, J. et al., "Earth Pressure on Underground Concrete Pipe in A Field Test," Proceedings of ASCE International Conference on Advances in Underground Pipeline Engineering, 1985.
[~] Tohda, J., "Earth Pressure Acting on Buried Pipes," Developments in Geotechnical Aspects of Embankments, Excavations and Buried
Structures, Balkema, Rotterdam, 1991.
[4] Tohda, J. et al., "A Study of Earth Pressure on Underground Pipes Based on Theory of Elasticity", Proceedings of JSCE, Voi.376, 1986.
[~] Tohda, J., Mikasa, M., and Hachiya, M., "Earth Pressure on
Underground Rigid Pipes: Centrifuge Model Tests and FEM Analysis", Centrifuge 88, Balkema, Rotterdam, 1988.
[6] Tohda, J. et al., "FE Elastic Analysis of Measured Earth Pressure on Buried Rigid Pipes in Centrifuged Models," Proceedings of ASCE International Conference on Pipeline Design and Installation, 1990.
[!] Tohda, J. et al., "Earth Pressure Acting on Buried Flexible Pipes in Centrifuged Models," Proceedings of ASCE International
Conference on Pipeline Design and Installation, 1990.
[8] Ministry of Agriculture, Forestry and Fisheries in Japan, Design Standard of Pipelines, 1988.
[~] Tohda, J., Yoshimura, H. et al., "Centrifuge Model Tests on Several Problems of Buried Pipes", Centrifuge 91, Balkema, Rotterdam, 1991.
Timothy J McGrath', Ernest T. Selig' and Leonard C. DiFrancesco'
STIFFNESS OF HDPE PIPE IN RING BENDING
REFERENCE: McGrath, Timothy J., Selig, Ernest T., and DiFrancesco, Leonard C., "Stiffness of HDPE Pipe in Ring Bending," Buried Plastic Pipe Technoloqy: 2nd Volume, ASTM STP 1222, Dave Eckstein,Ed., American Society for Testing and Materials, Philadelphia, 1994.
ABSTRACT: A ring bending test was devised to evaluate the behavior of plastic pipes that have been held under constant deformation and then subjected to intermittent loads. The apparatus consists of a frame capable of locking the test pipe at a fixed level of deflection in ring bending. A load cell monitors the change in force with passing time required to hold the deflection level. At preselected time intervals an additional deflection increment is briefly applied to the pipe and then released. The load required to produce this deflection increment is recorded. The pipe is then returned to the original level of
deformation. Test results show that the long term decrease in load, or stress relaxation, is unaffected by the short term load increments. The short term modulus is much greater than the long term relaxation modulus and does not decrease with time. Test results also show that short term load increments may be treated as a new load on the pipe, with a
stiffness response governed by the short term modulus.
KEY WORDS: long-term modulus, parallel plate, plastic pipe,relaxation, short-term modulus, viscoelastic, Young's modulus
High density polyethylene plastic (HDPE), from which buried pipe are made, is a viscoelastic material. As such, the material stiffness, as characterized by Young's modulus, is time-dependent. Under constant load or constant deformation the modulus appears to decrease with increasing time; however, this is an equivalent elastic modulus resulting from forcing the material to fit a linear elastic model.
There has been some disagreement among engineers on the
appropriate value of elastic modulus to use in calculations that involve both short and long-term loading. One design situation where the
appropriate modulus must be known is when buried pipe, under long-term
'Senior Associate, Simpson, Gumpertz and Heger, Inc., Arlington, MA 02174, and Doctoral Student, University of Massachusetts, Amherst, MA 01003.
'Professor of Civil Engineering, University of Massachusetts, Amherst, MA 01003.
'Graduate Student, Department of Civil Engineering, University of Massachusetts, Amherst, MA 01003.
195