Problems of open pit mining
In mining, in addition to the extracted raw materials, increased amounts of overburden are produced which are usually backfilled near the mining area. There are particularly large amounts of overburden in open pit mines, in which the material is removed on one and backfilled again on the opposite side (see picture). Calculations regarding the slope stability are required for the dumps and dump systems consisting mainly of wet soils. However, for the calculations soil characteristics are required, which change over time due to the consolidation of the dump soils. A temporal change of the soil properties cannot be predicted with the classical theory of soil mechanics. Therefore, numerous research projects regarding this topic have been or are still being worked on at the Institute of Geotechnical Engineering. Emphasis is e.g. placed on the targeted dumping of backfill materials to take advantage of the soil properties. The investigations focus on the geometry of the dumps and dump systems as well as the mixture of different soils for optimum use of the soil properties. Furthermore, cone penetration tests have been developed to verify the predicted shear strengths. Also, the development of new constitutive model was carried out in order to depict the soil behaviour of mixed soils with different consistencies or soil types.
![Blick in den Tagebau](https://tu-dresden.de/bu/bauingenieurwesen/geotechnik/ressourcen/bilder/forschung/forschungsfelder/tagebau/DSC_0040.JPG/@@images/372becd4-fbd7-442f-bcdc-9c67f410f7a6.jpeg)
View over a lignite open pit mine (left: extraction, right: deposition)
In order to assess the properties of dump soils in open pit mines, numerous samples of freshly dumped soils were taken on-site and examined in the soil mechanics laboratory of the TU Dresden. By means of statistical evaluation methods, state variables and soil parameters were determined in the initial state and subsequently used for e.g. predictions of the shear strength.
A further emphasis was put on performed and documented large-scale tests, e.g. settlement measurements, which were evaluated and interpreted at the Institute of Geotechnical Engineering.
![Häufigkeits-Wassergahtls-Diagramm (mit Histogramm und dem Fit einer normalverteilung)](https://tu-dresden.de/bu/bauingenieurwesen/geotechnik/ressourcen/bilder/forschung/forschungsfelder/tagebau/M2fn_TUD_WN.jpg/@@images/fff74bd8-ae06-4c0a-8521-31d90b3166ee.jpeg)
Example for a normal distribution of determined water contents
From shearing and compression parameters as well as the current soil state, described by the void ratio and the effective stress state, the undrained shear strength can be determined by applying a suitable theory of soil mechanics. The “Critical State Soil Mechanics” is one of them. Based on this theory, the shear strengths for a fast and undrained loading can be calculated for different constitutive models. Constitutive models are used for the numerical depiction of the soil behaviour.
![Triaxialprobe](https://tu-dresden.de/bu/bauingenieurwesen/geotechnik/ressourcen/bilder/forschung/forschungsfelder/tagebau/Triax_UU.jpg/@@images/18fcc04e-3e6c-4e35-ac55-de0ffb20f2a9.jpeg)
Soil sample after shearing in a triaxial compression test (UU)
By coupling the mechanical and time-dependent effects (consolidation) in the soil, the time-dependent, undrained shear strength can be determined by means of analytical and numerical calculations. To improve the accuracy of the shear strength predictions, deterministic and statistical methods were designed. With these and using the distributions of the initial state (classified by the plasticity index and state variables such as the water content), an initial distribution of soil consistency can be established. Knowing the distribution of the soil consistency, the void ratio can be calculated. With numerical calculations and a suitable constitutive model, simplified consolidation calculations can be carried out and consequently the undrained shear strength can be derived.
The figure below depicts an example of the temporal development of the undrained shear strength for different depths. The starting point of this calculation was a soil column with a height of approx. 23m, with drainage possibilities on the left and right side.
![Scherfestigkeit-Zeit-Diagramm (Anstieg der Scherfestigkeit mit der Zeit)](https://tu-dresden.de/bu/bauingenieurwesen/geotechnik/ressourcen/bilder/forschung/forschungsfelder/tagebau/cu_teife.jpg/@@images/f9387d19-1464-4df2-bb83-700a54c034ba.png)
Temporal development of the shear strength
The figure shows the development of the pore water pressure in a soil column based on an analytical evaluation; first due to dead load (blue triangle) and subsequently due to overlay (from day 6).
![Porenwasserdruck-Tiefen-Diagramm (Verlauf des Porenwasserdruckes mit der Zeit)](https://tu-dresden.de/bu/bauingenieurwesen/geotechnik/ressourcen/bilder/forschung/forschungsfelder/tagebau/PWD_Tiefe.jpg/@@images/4d6a4287-40d3-4c07-a422-1003dc1f81e2.png)
Development of poren water pressure
If different soil types (e.g. sand and clay) or soils with different consistencies are mixed, soils with new properties are created. In an open pit mine, soils can be obtained selectively. However, there is always a blend at layer boundaries and thus a soil mixture. In order to examine the mixtures of fine- and coarse-grained soils, laboratory tests were carried out and the newly obtained soil properties and soil states of the mixtures were derived. In addition, various fine-grained soils were mixed and examined regarding their stiffness and shear behaviour. Furthermore, fine-grained soil samples were prepared which had different consistencies. Again for the mixtures, the soil properties were determined. In addition to the laboratory work, an extension of an existing constitutive model to a theory for mixed soils was carried out.
![Tonklumpen in Sand eingebettet](https://tu-dresden.de/bu/bauingenieurwesen/geotechnik/ressourcen/bilder/forschung/forschungsfelder/tagebau/Imhomo_1.jpg/@@images/fe70a71f-080d-4fdb-8dbe-d6c96eb33683.jpeg)
Clay lumps (diameter approx. 50 cm) in a sand layer