GDS Triaxial Automated System (GDSTAS) Technical University of Munchen (TUM)

Since its inception in 1868, TUM has established its reputation as a foremost academic institution with 6 Nobel prizes and many other prestigious awards, making it repeatedly the number one German university in various rankings. The university covers a large spectrum of fundamental and applied research with studies ranging from engineering, natural sciences, including life and medical sciences, to economics. Today TUM comprises 13 faculties with more than 23,300 students (about 20 percent of whom come from abroad), 420 professors, and roughly 6,500 academic and non-academic staff.

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When investigating the bearing capacity of pile foundations in danger of buckling, information on the time-dependent resistance of the surrounding soft viscous soils is essential. The aim of TUM’s research project was the examination of the lateral support of the viscous soft soil against a pile foundation. To take into account the effects of time, TUM are examining the implications of the flow of pore water and soil viscosity on the consolidation of the soil.
Oedometric compression and shearing tests under triaxial compression were used to measure the soil strain. To produce the most accurate results, tests for phases of creep under constant deviatoric stress and relaxation, as well as scaled loading tests were undertaken. The triaxial tests also included shearing under partially drained conditions, to recreate the complex interaction between the effects of viscous strain and pore water on consolidation of the soil.


A fundamental prerequisite of the research project was the development, construction and operation of a test device, specially made to examine the behaviour of soft soils under low effective stresses in oedometric compression and triaxial shearing tests. 

It was important to prepare accurate samples to recreate the pile conditions. The soft soil had to be prepared so it could hold its shape, without any lateral support. With the construction of two new triaxial cells in which a mechanism of half shells is mounted, using load cells as a horizontal guiding rod. TUM were able to measure the behaviour of soft soils under low effective stresses in oedometric compression and triaxial shearing tests.

The investigations were carried out on soft samples over several weeks. TUM had to choose accurate measuring apparatus that could automatically log results and was compatible with their newly developed cell. TUM chose GDS testing equipment to fit their requirements. The cell was fitted into a GDS loadframe and the sample pressurised by GDS controllers. GDS displacement transducers were fitted to measure the sample strain, all of which was recorded and controlled by GDSLAB software.

Fig 1. TUM’s specially adapted cell allowing the examination of very soft cohesive soil

Fig 1b. Computer controlled triaxial testing apparatus by GDS


The first stage of evaluating the critical state of the used Kaolinit soil, was the examination of triaxial compression tests. Fig. 2: shows the undrained shear strength against the consolidation pressure before shearing. Fig. 3: Shows the deviatoric stress at the critical state against the mean effective stress. The results show that even under very low stresses at the border to the liquid consistency, the soil is showing a measurable undrained strength.

Results from the angle of friction successfully support the critical state theory that is successfully applied in the mechanics of stiffer soils for decades.

The soil viscosity is tested during undrained shearing, by evaluating the deviatoric stress response during a stepped axial strain test (jump test). Results from several tests can be seen in Fig. 4. The viscosity index measured during the undrained shearing is increasing for low consolidation pressures. When carrying out drained tests under low effective stress, the soft soil was contracting stronger and the number of particle contacts was increasing while shearing. Therefore the soil viscosity expressed by the viscosity index is well below that of the values measured in the undrained shearing tests.

During the triaxial compression test, a phase of stress relaxation was included. While the axial strain is held constant, the deviatoric stress was reduced 20% over a 40 hour period, see Fig 5. The graph emphasises the importance of taking the viscosity of soft soils into account when assessing foundations. The results show that soil resistance after stress relaxation is higher than in the shearing phase before the relaxation phase. This is interesting because the rate of deformation is twice as slow in the shearing phase after relaxation, compared to the shearing phase before relaxation. The structure is capable of carrying higher deviatoric stresses during further shearing. However, the structure is somehow destroyed when enough shear strains are applied on the soil sample and the deviatoric stress is decreased between jump 4 and 5 in the left diagram of Fig. 5: At high shear strains the additional strength gained during soil relaxation vanishes as given in Fig. 5: where the curve (6,8 ∙ 10-4 %/min) after jump 5 is on the same path as the one that may be extended from the curve between jump 2 and 3 (6,6 ∙ 10-4 %/min).

Fig 2: Undrained Shear Strength Vs Consolidation before Shearing

Fig 3: Deviatoric stress at the critical state against the mean effective stress

Fig. 4 Drained and undrained viscosity consolidation results

Fig 5: Deviatoric Stree was reduced 20% over a 40 hour period


Stefan Vogt from TUM, said …”Due to the low forces and pressures measured on the samples during the undrained shear strengths (as low as 5 kPa) the test equipment had to be chosen with special care. The equipment including the sensors, pressure controllers, software and load frames provided by GDS are capable to work reliably during the tests, each of which lasts up to 4 weeks. In the next step of the research effects of soil structure and anisotropy on the time dependent lateral pile resistance will be examined. Therefore tests that last over 2 months are necessary to monitor the

strength gain due structural resistance developing in the soil during long phases of creep or relaxation.

We will again use the GDS equipment to measure and accurately log our results. From the results obtained in our shearing and model scaled loading tests, it was possible to gain information about the time dependent interaction between a pile shaft and the surrounding soft soil which was one of the main aims of the investigation.”

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