Central project of the researchers group FOR 1136 with demonstrator experiments

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In project phase II of the central project laboratory tests will be performed as a benchmark for numerical modelling of geotechnical boundary value problems. These tests are adapted for each subproject (TP) of the collaborating partners. In project phase II important preliminary work concerning the test methods has been performed, especially dealing with the sealing concept, the stress measurement and the Digital Image Correlation (DIC).The main part of the interface-test-box is an instrumented wall, divided into four segments, which can be moved relative to an adjacent sand body. In this test configuration the contact zone between sand and structures (piles, walls) with different surface roughnesses can be observed under plane strain conditions. Various configurations of the wall segments allow the investigation of the development of skin friction and tip resistance of pile-like structures. On both sides of the testbox the displacements of soil and wall are observable through glass windows for the evaluation of the soil deformations with the Digital Image Correlation technique (DIC). Several improvements of the post-processing software allow now a quantitative and not only a qualitative interpretation of the test results. Also the construction and the instrumentation of the existing test-box was extended and upgraded. Cyclic tests can now be performed and in addition to the tangential stresses on the wall segments the normal pressures can be captured. This turned out to be an important requirement for quantitative evaluation of the tests and comparisons with simulations.Due to these improvements, further, mainly cyclic, tests in the interface-test-box are planned for the second phase. Master and bachelor theses will be used to give students the opportunity to work on this subject after being introduced in the topic as student assistants.The major task of the central project in the second phase will be the tests in saturated soil. The results of shake-box tests showing the behaviour of saturated soil under sinusoidal base excitation have been prepared for simulations of the project partners. In the second phase additional shakebox tests are planned, but the majority of the tests will focus on the pile penetration in saturated soils. Therefore a new test-box for quasi-static (monotonic/cyclic) and dynamic tests, following the ideas of the rotationally symmetric cavity expansion, is under construction. Quasi-static (monotonic/cyclic) and dynamic tests in this new test-box will be performed. In the soil body the evolution of the pore water pressure and the total stresses will be captured. The pile forces resulting from base and skin will be investigated. For the saturation the dry pluviated sand body will be evacuated to a very low pressure level. This procedure was tested on small samples and will be transferred to the larger scale.Particular attention is paid to the determination of the initial state (density distribution and initial stresses) of the soil mass. During the sand pluviation the stresses will be captured and DIC will be applied to evaluate the soil deformations. Another topic of the central project is the parameter estimation for the constitutive laws developed in TP1 and TP2. These parameters will be used in the other subprojects for numerical simulations of the performed laboratory tests and boundary value problems.