Model Studies Of Rapid Driving Of Piles Into Permafrost

Abstract

A comprehensive study was made of rapid pile driving into permafrost. This paper is limited to a discussion of flat-tipped model piles penetrating artificial permafrost under the influence of their kinetic energies alone. The first part of the paper is a study of the penetrations of thin, hollow, circular piles as a function of their masses and impact velocities. Experimental results show that, within the range of velocities explored [40 to 500 ft/sec], three types of resisting forces are predominant during penetration. These forces are: crushing, drag and frictional. A model of penetration was developed on the basis of these three forces. The law of penetration, expressed by an empirical equation derived from experimental results, was explainable by the simultaneous existence of these three forces. The second part of the paper is a study of the penetrations of cross-shaped, solid- and hollow-circular, rectangular, and square piles as a function of their shapes. An effective cross section was defined which permitted reduction to a common level of the penetrations of piles having the same mass and impact velocity. The effective cross section was defined as a function of the perimeter and a reduced thickness of the cross section of the piles. Distinctions were made between thick and thin piles. As a corollary of this investigation, the results of a series of field tests conducted in Alaska are presented. Introduction As a result of freezing and thawing of the ground's surface in the regions of Alaska and Northern Canada, problems relative to building construction are frequently encountered. This handicap to conventional building construction car. be overcome by driving piles into the permanently frozen layer of ground [called permafrost] and using them as foundation supports. An unusual pile-driving technique was proposed for rapid pile driving into permafrost. A fast-burning rocket could be affixed to the upper end of a pile, a steel tube for example. This assembly could then be raised a few feet in the air and launched vertically in the direction of the ground, letting the pile-rocket assembly penetrate permafrost under the influence of its impact kinetic energy alone. After an open literature survey it was believed that studies of this subject were never made before, despite the fact that much work has been done in connection with conventional pile driving. Therefore, this proposed technique of rapid pile driving was approached by conducting laboratory and field tests. Because of complications of using a rocket as the driving agent in the laboratory, piles were accelerated into permafrost with a gun. This paper, consisting of two parts, discusses only the results of the laboratory investigation where model flat-tipped piles were driven into a given type of artificial permafrost. Additional experiments conducted in the laboratory with other types of permafrost and larger model piles showed results similar to those presented here. The first part of the paper presents results obtained with thin, hollow., circular piles from which an empirical law of penetration was derived. Further experiments were performed in order to determine the mechanics of penetration and to give a physical meaning to the empirical law derived from the experimental results. A thin, hollow, circular pile is defined as a pile for which the ratio of wall thickness to external diameter is approximately equal to or smaller than 0.2. The second part presents results obtained with piles of the same mass having cross-shaped, solid- and hollow-circular, rectangular, and square cross sections. These experiments were performed in order to determine the influence of pile shape upon penetration. Analysis of results presented here has been done largely by graphical solutions, since the scatter of experimental data was relatively small.

Systematic Section

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