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Abstract:The effective analysis of the nonlinear behavior of cement-based engineering structures not only demands physically-reliable models, but also computationally-efficient algorithms. Based on a continuum interface element formulation that is suitable to capture complex cracking phenomena in concrete materials and structures, an adaptive mesh processing technique is proposed for computational simulations of plain and fiber-reinforced concrete structures to progressively disintegrate the initial finite element mesh and to add degenerated solid elements into the interfacial gaps. In comparison with the implementation where the entire mesh is processed prior to the computation, the proposed adaptive cracking model allows simulating the failure behavior of plain and fiber-reinforced concrete structures with remarkably reduced computational expense.Keywords: fiber-reinforced concrete; crack model; interface solid element; finite element method; mesh adaptation; computational efficiency
Compared to the exclusively calcitic reef limestone encountered during the drilling of the access shaft, the red-brown, yellow and white facies at the cave walls also contained kaolinites and smectites as typical tropical weathering products. Whether the clayey fillings in open cracks and voids were products of in situ weathering or of subsequent sedimentation could not be decided.
Drilling of boreholes was accompanied by endoscopic analyses from the beginning of the planning and implementation process. This technology is a recommended practice to identify cracks, joints and cavities in boreholes on a qualitative level. Initially, the observation depth was limited to 4 m using a bar endoscope. A later utilized flexible endoscope extended the reach to 20 m.
In cooperation with the industrial partner GIF (Geotechnisches Ingenieurbüro Prof. Fecker & Partner GmbH, Ettlingen, Germany, sub-project 15 of the IWRM joint research project), a borehole scan had been carried out in Gua Bribin and in another karst cave, using a borehole camera combining a rotatable optical sensor with a camera. It produces a video stream of a borehole wall, which enables to measure gap openings and crack orientations and to survey the geometry of cavities.
Theoretically, the same information content should be given by the drill cores. In practise, this is not the case, however. Breaks in cores may have occurred due to pre-existing cracks or due to mechanical overstraining. The drill cores in the core box may not still have their original orientation or sequence, particularly when the geological situation caused a poor core recovery. Anyway, the video records proved much more reliable, but still the evaluation was rather demanding.
Based on the hydrochemistry, it is also possible to calculate the saturation of both types of water with regard to calcite. The results clearly show that river and seepage waters are intermittently subsaturated. This means that the water periodically has the potential to dissolve calcite and, consequently, that active karstification takes place. Hydrochemical modelling using the code PhreeqC indicates that, in an extreme case, up to 95 mg calcite per litre could be dissolved by the seepage water during passage of the limestone assuming that full saturation takes place. This, however, is not expectable. Furthermore, most of the samples had a much lower subsaturation leading to lower amounts of calcite dissolution.
First indications on the flow dynamics of seepage water were gained by means of tracer tests using fluorescence dyes. The results indicate the existence of different flow regimes: (1) a quick component with residence times in the limestone in the range of hours to days. In this context, larger cracks and fissures are probably decisive; (2) a very slow matrix flow component with retention times of several months.
To estimate the extent of karstification, the results were combined in a 1D hydro-geochemical transport model using PhreeqC. The model was based on a potential crack with a diametre of 1 cm and a length of 50 m. Two scenarios were considered differing mainly in the flow velocity and the discharge flowing through the karst. In case (A), the flow velocity was set to 10 m/h and the discharge to 0.8 l/h. The model predicts that along this crack the amount of limestone which will be dissolved is 0.64 kg/a. Increasing the flow velocity in case (B) to 50 m/h leads to an increase of the amount of potentially dissolved calcite to 2.3 kg/a. This model, however, assumes that subsaturated Bribin river water is constantly available throughout the year which is not the case.
Due to levelled bedding joints, the identification of the crack inventory was not so much a focus of the Bribin investigations. In different conditions, the use of Terrestrial Laser Scanning has proved its worth (Mutschler et al. 2014). 2ff7e9595c
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