Tests of piles in melted and frozen soils in creep mode: relaxation conditions

Abstract

A new method for the experimental determination of bearing capacity of piles is considered, which makes it possible to determine separately within one test the resistance under the lower end and along the lateral surface of piles in melted soils, the similar parameters for piles frozen in permafrost soils inclusive. The purpose of work is the development of a new test method, which allows to increase the accuracy and informative nature of tests and significantly shorten their time and cost. The proposed method is based on the dynamometric method for determining the long-term strength of soils in creep-relaxation regime proposed by Professor S.S. Vyalov. The essence of the proposed method for testing the immersed pile loaded with static load in a thawed ground is that the pile tested is immersed statically loaded at some depth, and after that it is tested in the creep-relaxation regime. After stabilizing the relaxing load, a step-up pulling load is applied to the pile, where each stage of the load is tested in the creep-relaxation regime. Maximum value of the stabilized pulling load can be considered as the sum of frictional forces along the lateral surface of the pile immersed to a given depth. The difference in the stabilized load of indentation and pulling out shall be equal to the resistance of pile under its lower end. By immersing the pile at different depths and testing it at each point in the manner described above, it seems possible to obtain the separate resistance values along the lateral surface and under the lower end of the pile at different depths. For frozen soils, the pile frozen into the ground is tested in the similar way, i.e. by indenting and pulling out thereof the creep-relaxation regime. To determine the bearing capacity of piles frozen in frozen ground at different negative temperatures during one test, the stock pile is first tested in creep-relaxation mode at a fixed maximum negative temperature value. After the stabilized value of the indentation and then pulling load is reached, the temperature around the pile is stepwise changed, and at each temperature value the stabilized values of the compressive and pulling loads are determined. The theoretical justification and results of field and laboratory testing of the proposed test method are outlined. The description of results of comparative experiments and the equipment used is also given. The comparison of the proposed test method is in good agreement with the results of experiments performed by traditional methods. This method can be successfully used with significant cost and time savings in course of construction and reconstruction of structures on pile foundations both for thawed and frozen soils.