Study On The Behavior And Application Of Stiffened Micro-pile

Traditional micro-pile with reinforcement cage in the core, due to its low ratio of reinforcement and the low strength of materials as well as its small stiffness, is not conducive to the exertion of bearing capacity of pile and to the improvement of ultimate bearing capacity of pile. Stiffened micro-pile (SMP) with pipe steel or I-shaped steel in the core has higher rigidity, which can be embedded in the rock to acquire higher capacity, hence has higher applicable values.The subject of research is SMP. Currently, also SMP are widely range application in engineering, but SMP's load-bearing mechanism and design method are not yet involved in, so study on the bearing properties of SMP when embedded in rock under vertical load has higher practical significance.In-situ test is a direct means of measuring the pile bearing capacity, is also a fairly accurate method. In this paper testing study is made on SMP embedded in Limestone, the mechanism of pile under vertical load is studied; the test results are compared with those calculated by using empirical formula, and the function of lateral friction of miscellaneous fill after grouting is discussed. The test results indicate that the behavior of this kind of piles under vertical loading is of small settlement, the velocity of load-settlement curve changes slowly, small pile-soil relative displacement, end-bearing characteristic.FLAC3D is an explicit finite differential method program based on Lagrange analysis, which can easily achieve a variety of constitutive models and structural units, is very convenient for rock and soil mechanics analysis. Through the establishment of the numerical analysis model and applied it to the ultimate bearing capacity, just to make up In-situ test, FLAC software is used to setup the model; the design factors for the mechanism of SMP, such as:the strength of the rock under pile tip, the embedded depth of pile and other factors are taken into consideration.Both of the results of In-situ test and numerical analysis show that:at the early stages of loading, that skin friction of soil on upper segment of pile take up a key extent, that skin friction of soil on bottom segment of pile take up a limited extent. With the increase of load of pile, the shaft resistance gradually develop from up to bottom. The result of numerical analysis also shows, the value of skin friction of the soil are constant, the total resistance of the rock-socketed segment is increased significantly. At the failure stages of loading, the total resistance of the rock-socketed segment provides a principal reaction under vertical load, it shows that singlepile bearing characteristics are of end-bearing, finally the material of the pile reached its ultimate strength of the pile material then failure. For the SMP, its ultimate capacity are rest with the strength of the material of the pile.The factors to the ultimate bearing capacity of SMP are complex, in this thesis, in the condition of pile diameter as a constant value, the variable of ultimate bearing capacity of SMP under different ratio of socketed length to pile diameter and different ratio of pile length to diameter and different rock strength are studied. The results indicate that:when ratio of socketed length to pile diameter up to 6, the ultimate bearing capacity changes no more; when ratio of pile length to diameter up to 50, the ultimate bearing capacity changes no more. The ultimate bearing capacity change approximately linear increases with the increase of rock strength.The load-settlement curve of pile groups changes graded, non-linear characteristics is not so obvious, the attenuation of the axial force of the center in pile groups are of tardiness, it shows that the total resistance of the rock-socketed segment provides a principal reaction under vertical load. Pile groups bearing characteristics are of end-bearing.