DIGITAL/BIM | TECHNICAL
Longitudinal section
row A row B row C row D
Geotechnical synthesis model attributes BIM
A1 A2 A3 B1 B1 B2 C1 C1
C1 D1 D1 D1
A3 B2 C1 D2
A3 B3 C1 D3
A3 B4 C1
D4
A3 B5 C1
D4
A3 B5 C1
D5
Left, figure 5: The concept of the Geotechnical Synthesis Model as the BIM-based version of a ‘classical’ tunnel longitudinal section. The rows A-D in the longitudinal section stand for real properties, such as rock mass types, permeability, lithology, etc.
during tests. In this context, the limit is usually set according to data exchange points, like information submitted from a laboratory, contractor for drilling and in-situ testing, or a field geologist.
4.3. Geotechnical Model In contrast to the Factual Data Model, the Geotechnical Model is an ‘interpreted’ sub-discipline model as its content is fully based on processed information from the former and also interpolation between points of confirmed information. The Geotechnical Model is derived from an
engineering geological conceptual model for a specific purpose, like e.g., tunnel design in a certain project phase. It represents volumes of homogeneous properties which are called geotechnical units that can be separated from each other with volumes or with boundary surfaces only. In the latter case, however, it has to be specified exactly if the boundary surfaces represent upper or lower boundaries for the respective geotechnical unit (see also buildingSMART (2020, p.67)). In cases of high investigation densities, well-known
ground conditions and/ or high-resolution models, the attributes of the geotechnical units can comprise specific information concerning, for example, the general geology (geological formation, lithological characterisation, etc.,) or mechanical properties of the ground (e.g., density, friction angle, cohesion etc.). In cases of deep tunnels, where an exact localisation
of geotechnical properties is often not possible (ÖGG, 2021), a high degree of uncertainty and/ or a generally low resolution model has to be produced. The geotechnical units’ properties then can be made up of distributions of ground classifications (e.g., rock mass types, Q-classes, etc.). In Figure 4, the concept of the geotechnical model
with multiple geotechnical units is visualised and also two examples of attributes are given.
Uncertainty related to geological investigations or
interpretations can be represented within the model either via attributes (i.e., qualitative or quantitative description) or also via the geometry itself by, e.g., modelling different likelihoods of boundary surfaces between geological units. Dealing with the topic of uncertainty itself is, however, in principle not different for BIM ground modelling in comparison to conventional engineering geological modelling (Weil, 2020).
4.4. Geotechnical Synthesis Model The Geotechnical Synthesis Model contains all the 1D information along the tunnel that is usually communicated with longitudinal sections (Figure 5). The geometry of it can be as simple as a tube along the tunnel axis, that is split into multiple sections of homogeneous properties. The name Geotechnical Synthesis Model was
developed in the IFC Tunnel working group as a term used for a sub-model that contains the essence of the other sub-models in relation to certain buildings or design structures, like e.g., a tunnel tube and its alignment. Developed during the design phase of a tunnel, it is used as an interface to the model of the planned building, especially the planned excavation and support structures that interact with the ground. One important function is to consider uncertainty in
prediction of geological structures and geotechnical conditions to be expected along the alignment. It follows a common approach in tunnelling: the definition of ‘homogeneous sections’ along the alignment with characteristic, similar geotechnical conditions and expected ground behaviour, usually defined for certain chainage intervals and documented in a longitudinal section (Figure 5). Such planning documents are commonly used as the basis for defining and/selecting the choice of excavation methods, support types, ground improvement and other measures, as required, and
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