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Keywords: underground voids, runways, fatigue life, finite elements, subgrade, camouflet, concrete.

Summary

This research considers the fatigue life a cement concrete runway when an explosive has detonated in the subgrade beneath the runway forming a camouflet and that the runway does not heave or crack. A camouflet is an almost spherical void surrounded by a shell of compacted subgrade. The subgrade between the void and the runway, the cone of disturbance, may or may not be affected, depending upon the depth of detonation. As the distance from the detonation point increases beyond the void, compaction of the subgrade due to the detonation reduces. This reduction may be modified by the proximity of the runway, the ground surface and the ground surface's ability to reflect the detonation induced compression waves. Thus the support provided to the cement concrete runway by the subgrade will change and alter the fatigue life of the runway.

To determine the serviceability limit-state for the runway, the equation developed by the authors is used [1,2,3]. The number of load repetitions, the runway can sustain is predicted from $\sigma$ the maximum principal concrete tensile stress and the modulus of rupture of the concrete through the equation, $N = 225,000[MR/\sigma]^4$ .

The depth of detonation of 8.35m is considered as depth 0. The depth of detonation is then increased by 1m, 2m, 3m, 4m, 7m and 10m to a depth of 18.35m. For each depth, the seventeen material property sets are modelled computationally using three-dimensional finite elements. For each depth an additional material property set comprising the same finite element model, but with the subgrade undisturbed is also run. This model is used as a base line and as a means of checking the accuracy of the fatigue values obtained. A downward point load of 100kN is applied to the upper surface of the runway directly over the centre point of the void. The value of 100kN was used as it produces a realistic fatigue life in the runway. The material property set encountered can be determined by carrying out a standard penetration test (SPT) or a cone-penetration test (CPT) and relating the results to the Young's moduli [4].

Reference [5] identifies seven of the seventeen material property sets considered in this paper as being not realistic. These seven material property sets are not considered further. For six of the ten realistic material property sets the cement concrete runway overlaying the cone of disturbance had their fatigue life either not changed or increased. For the four remaining material property sets the minimum fatigue life of 77.7% was found at the minimum detonation depth of 8.35m. As the detonation depth increased so did the fatigue life. For six of the ten realistic material property sets the cement concrete runway not over the cone of disturbance had their fatigue life for all depths of detonation either not changed or increased. For the four remaining material property sets the minimum fatigue life of 5.8% was found at the detonation depth of 10.35m. Further for all depths the fatigue life reduced as the detonation depth increased and then increased to a maximum of 20.0% at the maximum detonation depth of 18.35m.

References

1: Bull, J.W., "Precast concrete raft units", Blackie and Sons, Glasgow, 1991.
2: Bull, J.W., "An analytical solution to the design of precast concrete pavements", Int J Numer Anal Methods in Geomech, 10, 115-123, 1986. doi:10.1002/nag.1610100202
3: Bull, J.W., "The interaction between precast concrete raft unit pavements and their soil support", in "Soil-structure interaction, numerical analysis and modelling", Bull, J.W., [Editor], Chapman and Hall, London, 1994.
4: Bowles, J.E., "Foundation analysis and design", 4th ed, McGraw-Hill, New York, 1988.
5: Bull J.W., Woodford C.H., "Realistic subgrade strength variations in the cone of disturbance following an underground detonation", submitted to Computers and Structures, 20p, 2002.

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Front Page Browse CCP CSETS CTR IJRT Other Authors Search Purchase Guide FAQ Contact us	Civil-Comp Proceedings ISSN 1759-3433 CCP: 77 PROCEEDINGS OF THE NINTH INTERNATIONAL CONFERENCE ON CIVIL AND STRUCTURAL ENGINEERING COMPUTING Edited by: B.H.V. Topping Paper 25 The Fatigue Life Remaining in an Airfield Runway Following an Underground Explosion J.W. Bull+ and C.H. Woodford* +School of Civil Engineering and Geosciences, Computing Service The University of Newcastle upon Tyne, United Kingdom doi:10.4203/ccp.77.25 purchase the full-text of this paper Full Bibliographic Reference for this paper J.W. Bull, C.H. Woodford, "The Fatigue Life Remaining in an Airfield Runway Following an Underground Explosion", in B.H.V. Topping, (Editor), "Proceedings of the Ninth International Conference on Civil and Structural Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 25, 2003. doi:10.4203/ccp.77.25 Keywords:* underground voids, runways, fatigue life, finite elements, subgrade, camouflet, concrete. Summary This research considers the fatigue life a cement concrete runway when an explosive has detonated in the subgrade beneath the runway forming a camouflet and that the runway does not heave or crack. A camouflet is an almost spherical void surrounded by a shell of compacted subgrade. The subgrade between the void and the runway, the cone of disturbance, may or may not be affected, depending upon the depth of detonation. As the distance from the detonation point increases beyond the void, compaction of the subgrade due to the detonation reduces. This reduction may be modified by the proximity of the runway, the ground surface and the ground surface's ability to reflect the detonation induced compression waves. Thus the support provided to the cement concrete runway by the subgrade will change and alter the fatigue life of the runway. To determine the serviceability limit-state for the runway, the equation developed by the authors is used [1,2,3]. The number of load repetitions, the runway can sustain is predicted from $\sigma$ the maximum principal concrete tensile stress and the modulus of rupture of the concrete through the equation, $N = 225,000[MR/\sigma]^4$ . The depth of detonation of 8.35m is considered as depth 0. The depth of detonation is then increased by 1m, 2m, 3m, 4m, 7m and 10m to a depth of 18.35m. For each depth, the seventeen material property sets are modelled computationally using three-dimensional finite elements. For each depth an additional material property set comprising the same finite element model, but with the subgrade undisturbed is also run. This model is used as a base line and as a means of checking the accuracy of the fatigue values obtained. A downward point load of 100kN is applied to the upper surface of the runway directly over the centre point of the void. The value of 100kN was used as it produces a realistic fatigue life in the runway. The material property set encountered can be determined by carrying out a standard penetration test (SPT) or a cone-penetration test (CPT) and relating the results to the Young's moduli [4]. Reference [5] identifies seven of the seventeen material property sets considered in this paper as being not realistic. These seven material property sets are not considered further. For six of the ten realistic material property sets the cement concrete runway overlaying the cone of disturbance had their fatigue life either not changed or increased. For the four remaining material property sets the minimum fatigue life of 77.7% was found at the minimum detonation depth of 8.35m. As the detonation depth increased so did the fatigue life. For six of the ten realistic material property sets the cement concrete runway not over the cone of disturbance had their fatigue life for all depths of detonation either not changed or increased. For the four remaining material property sets the minimum fatigue life of 5.8% was found at the detonation depth of 10.35m. Further for all depths the fatigue life reduced as the detonation depth increased and then increased to a maximum of 20.0% at the maximum detonation depth of 18.35m. References 1 Bull, J.W., "Precast concrete raft units", Blackie and Sons, Glasgow, 1991. 2 Bull, J.W., "An analytical solution to the design of precast concrete pavements", Int J Numer Anal Methods in Geomech, 10, 115-123, 1986. doi:10.1002/nag.1610100202 3 Bull, J.W., "The interaction between precast concrete raft unit pavements and their soil support", in "Soil-structure interaction, numerical analysis and modelling", Bull, J.W., [Editor], Chapman and Hall, London, 1994. 4 Bowles, J.E., "Foundation analysis and design", 4th ed, McGraw-Hill, New York, 1988. 5 Bull J.W., Woodford C.H., "Realistic subgrade strength variations in the cone of disturbance following an underground detonation", submitted to Computers and Structures, 20p, 2002. purchase the full-text of this paper (price £20) go to the previous paper go to the next paper return to the table of contents return to the book description purchase this book (price £123 +P&P)
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