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MISCELLANEOUS PAPER GL-92-1

THICKNESS CRITERIA OF FAILURE FLEXIBLE PAVEMENT DEVELOPMENT REQUIREMENTS FOR MILITARY ROADS AND STREETS, ELASTIC LAYERED METHOD by Yu T. Chou Geotechnical Laboratory

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DEPARTMENT OF THE ARMY Waterways Experiment Station, Corps of Engineers 3909 Halls Ferry Road, Vicksburg, Mississippi 39180-6199

DTIC I'LEC

FEB a ?A I99?-

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January 1992 Final Report Approved For Public Release; Distribution Unlimited

92-02971

kBORATORY

Prepared for DEPARTMENT OF THE ARMY US Army Corps of Engineers Washington, DC 20314-1000

When this report is no longer needed return it to the originator.

The findings in this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.

The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products.

Form Approved

REPORT DOCUMENTATION PAGE

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43 percent of the maximum.

The current criterion of determining the ESWL is

based on the deflection basin, but the layered elastic method for the design of flexible pavement is based on the strain basin.

Discrepancy in results can

be expected when two procedures are used together. 23.

The representative curve for each subgrade modulus value is drawn

near the single-axle single wheel loads shown in Figure 4, the resultant curves for various subgrade modulus values are plotted in Figure 5 which is the subgrade strain criteria for flexible pavements for military roads and streets. and

E. -

For design purpose, a single curve drawn near the

E, - 10,000 psi

15,000 psi curves is used which may be approximated by the equation

Allowable coverage =

10A

where A - -(2,408 + log ev)/0.1408 c, - vertical strain at subgrade surface, in./in.

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PART V:

24.

DISCREPANCY BETWEEN THE CURRENT PROCEDURE AND THE ELASTIC LAYERED METHOD

In the current design procedure, the magnitude and compositions of

traffic are accounted for by the design index together with the concept of equivalent 18,000-lb basic loading, and the thickness design is completely based on the CBR design equation for flexible pavements.

Design index is not

used in the elastic layered method, and the thickness design is completely based on the computed subgrade strains induced by the traffic loads using the BISAR program.

In general, thickness designed by the two procedures are very

close except in certain conditions where the elastic layered method is more reasonable.

These conditions are explained as follows: a.

When traffic is characterized by design index numbers, the pavement thickness may vary greatly when the traffic is in the neighborhood of changing from one index number to the other. This is not the case for the elastic layered method since the traffic is directly input into the computation and the result varies smoothly with number of coverages.

b.

The design index method has another drawback. When the pavement is designed for two different types of vehicles, the heavier vehicle is the governing one as it requires the highest design index and the effects of other lighter vehicles are not considered. In the case of I ladered elastic design, the vehicles at a lower desig, index are not canceled in determining the pavement thickness. Each group of traffic is input into the analysis, and the design is based on the sum of the effects of all the traffic, regardless of the weights or types.

24

PART VI:

25.

CONCLUSIONS

The current CBR based design method for flexible pavements for

roads, streets, and open storage areas was reviewed.

The development of a design procedure using the elastic layered methods is presented, and the discrepancies between the two procedures are discussed.

25

REFERENCES

"ELSYM Computer Program for Determining Stresses and Ahlborn, C. 1972. Deformations in Five Layer Elastic System" University of California, Berkeley, CA. "Theory of Stresses and Displacements in Layered SysBurmister, D. M. 1943. tems and Application to the Design of Airport Runways," Proceedings. Highway Research Board, Vol 23, pp 126-144. "The General Theory of Stresses and Displacements in Lay. 1945. ered Soil Systems," Journal of Applied Physics. Vol 16, pp 89-94, 126-127, 296-302. "Development of a Structural Barker, W. R., and Brabston, W. N. 1975 (Sep). Design Procedure for Flexible Airport Pavements," Report No. FAA-RD-74-199 (Also designated TR S-75-17, US Army Engineer Waterways Experiment Station), Federal Aviation Administration, Washington, DC. "Development 1975 (Jul). Brabston, W. N., Barker, W. R., and Harvey, G. G. of a Structural Design Procedure for All-Bituminous Concrete Pavements for Military Roads," Technical Report S-75-10, US Army Engineer Waterways Experiment Station, Vicksburg, MS. "An Iterative Layered Elastic Computer Program for Chou, Y. T. 1976. Rational Pavement Design," Report No. FAA-RD-75-226 (Also published as Technical Report S-76-3, US Army Engineer Waterways Experiment Station, Vicksburg, MS), Federal Aviation Administration, Washington, DC. "Flexi1980 (Oct). Headquarters, Departments of the Army and the Air Force. ble Pavement for Roads, Streets, Walks, and Open Storage Areas", Technical Manual TM 5-822-5/AFM 88-7, Chapter 3. 1962. "Road Design and Dynamic Loading," Heukelom, W., and Klomp, A. J. G. Proceedings., Association of Asphalt Paving Technologists, Vol 33, p 499. "BISAR Users Manual; Koninklijke/Shell Laboratorium. 1972 (Jul). System Under Normal and Tangential Loads," Amsterdam, Holland.

Layered

"Stresses and Displacements in LayMehta, M. R., and Veletsos, A. S. 1959. ered Systems," Civil Engineering Studies, Structural Research Series No. 178, University of Illinois, Chicago, IL. 1963. "Analysis of Stresses and Displacements in an N-Layered Michelow, J. Elastic System Under a Load Uniformly Distributed on a Circular Area," California Research Corporation, Richmond, CA. Parker, F, Jr., Barker, W. R., Gunkel, R. C., and Odom, E. C. 1979 (Apr). "Development of a Structural Design Procedure for Rigid Airport Pavements," Technical Report TR-CL-79-4, US Army Engineer Waterways Experiment Station, Vicksburg, MS. Computer Program for Layered Systems Under "BISTRO: Peutz, M. G. F. 1968. Normal Surface Loads," Koninklijke/Shell Laboratorium, Amsterdam, Holland. "Mathematical Expression of the CBR Turnbull, W. J., and Ahlvin, R. G. 1947. (California Bearing Ratio) Relations," Proceedings. 4th International Conference on Soil Mechanics and Foundation Engineering.

26

"Revised Method US Army Engineer Waterways Experiment Station. 1961 (Aug). Installations," Military at Pavements Highway of Thickness Design for Flexible MS. Technical Report No. 3-582, Vicksburg,

27

APPENDIX A:

COMPARISON OF ESWL COMPUTED WITH DEFLECTION AND VERTICAL STRAIN

Vertical strains and deflections are computed in an elastic homogeneous The wheels are 13.5-in. apart and have

soil under a 9,000-lb dual wheel load. a constant pressure of 70 psi.

The maximum strains and deflections computed

at various depths are presented in Table Al.

The strains and deflections com-

puted under a 4,500-lb single wheel load are also presented.

The computed

ESWLs with respect to deflection and vertical strain are thus computed.

It is

seen that the ESWL based on deflection is much greater than that based on vertical strain.

It indicates that if the ESWL based on vertical strain is

used in the Corps of Engineers design procedure (Equation 1) the lines shown in Figure 4 will be closer to each other.

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