Research On The Mechanism Analysis And Improvement Of Osterberg Load Test Technique

The Osterberg Cell method is a pile loading test technique for pile's bearing capacity that the Osterberg Load Cell is placed at the bottom or some distance up from the bottom when pressure is applied to the device. An equal upward and downward force applied to the pile makes the upper part of the pile upward and the lower part of the pile downward until bearing capacity of the pile is reached. The pile's bearing capacity is obtained from an accurate transformation based on the original load-upward movement curve and load-downward settlement curve.As a new pile loading test technique, the Osterberg Cell method has the traits that the downward force is resisted by the side shear and therefore no overhead load frame or dead weight reaction is needed. As a rusult, it has been applied at home and abroad widespreadly especially in high capacity load tests and bridge pile load tests when the conventional static load test method has disadvantages in those situations.Since the equal force is applied at the bottom or some distance up from the bottom in an Osterberg load test, the situation a pile experiences is different from its actual working conditon. As a result, the premiss for the new load test's widespread application is to make the pile-soil interaction mechanism known clearly and obtain the bearing capacity reasonabley and accurately according to the transformation of the test results.Firstly, the thesis which is based on the former study on Osterberg theory and engineering applications summarizes some problems such as the pile-soil interaction mechanism, device position technique and bearing capacity obtaining. At the same time, the thesis analyses the pile-soil interaction mechanism by means of Elastic Theory and numerical simulation. Analytical solutions for piles loaded from the bottom or some distance up from the bottom are deduced during the Osterberg load test and the factors affecting the distribution of the shaft friction and displacement along the pile such as the ratio of pile's length and diameter, the stiffness ratio of pile and soil and passion's ratio of soil are all analysed. In addition, the influence on pile's bearing capacity generated from the interaction between the upper part and the lower part of the pile during the Osterberg load test is also analysed.Secondly, the thesis gives some improvements on the device position technique and bearing capacity obtaining technique as a result of the above theoretical work and analysis. A complete set of technique of the Osterberg Cell method is proposed and itsapplicability in engineering practice is checked. The results indicate that the new device position technique is fit for the local engineering practice and the transformation curve derived form the improved formula in this thesis is in accordance with the conventional static load test curve.Finally, a kind of new Osterberg load test technique which proper load is applied on top of the pile is proposed. The technique which gives the calculation method including the magnitude of the proper load applied on top of the pile and the bearing capacity of the entire pile shows great simple and convenient in its application. The effort for the technique's verification will result in its brilliant prospect.