Presentation on theme: "Design Parameters. Classification in Design Stage and Assessment of Design Parameter All the efforts of the engineering geological investigation steps."— Presentation transcript:
Classification in Design Stage and Assessment of Design Parameter All the efforts of the engineering geological investigation steps already performed in previous stages of the project are the basis for the establishment of geologıcal models and later on to the assesment of design criteria. The establishment of the geological model has the scope to define where and which geotechnical rock mass properties to be used as design parameters to be applied for the coming design steps. The values obtained by site and laboratory investigations represent characteristics for the special point where they were taken from.
The rock mass is affected by a number of impacts converting the physical and rock mechanical properties to the worst. These may be for example: Weathering Volcanic or postvolcanic processes Tectonic processes Influences of pore- and groundwater Slope instabilities After collecting all data by desk studies, field survey, subsurface investigations, laboratory and in situ testing, the geological model and geotechnical rock mass properties have to be established.
The empirical methods help to establish geological models by the use of statistical and probabilistic analysis of the collected data (structural data, data on mechanical properties and mineralogical analysis of samples, hydrogeological data as well as data on seismicity etc.). After the collection of the data is finished, an evaluation of the different impacts of these parameters (as mechanical behavior of discontiniuities, pore water pressure, activity of fault zones, distribution of swelling clayminerals, tectonic stress fields etc.) on the concerned project has to be performed. The empirical approach with the use of matrices for the introduction of the relative importance of the individual parameter sets requires an appropriate geological model and further on leads to the classification of the design stage.
How to achieve this geological model The first step is the evaluation of all available investigation results being elaborated in the previous working steps, such as Desk study reports concerning published papers, geological maps, seismicity maps, other geological or geotechnical project reports Analysis of aerial photos Results of detailed geological mapping Results of subsurface investigations as trial pits and trenches, drill holes, adits etc. Results of geophysical studies Results of insitu tests Hydrogeological studies Results of laboratory tests on physical properties and mechanical parameter of soil and rock samples Results of laboratory tests concerning minerology and petrography of soil and rock samples Results on slope monitoring by survey
Next step is the preparation of the; Geological longitudinal sections and Geological cross sections. Based on the results of the detailed geological mapping, the subsurface investigation results and the knowledge on the geological situation. The products of this step must give a clear information on, which different rock (soil) types occur in the project, their physical properties and mechanical parameters and locations.
Assesment of Design Parameters and Classification: The next step will be converting this knowledge into geotechnical terms applicable for the assesment of design parameters. The emphasis lies on gaining clear understanding concerning the geomechanical conditions before the design object is implemented, -changes that may arise during the excavation and construction and -the state after the completion of the project
Since geological conditions vary from site to site an examination of their impacts on the project must be done. The most advanced way to do this is to set up a matrix which highlights the individual geological parameters such as distribution of discontinuities, shape and position of keyblocks, mechanical behavior along discontinuities, ground or pore water conditions, tectonic stresses, expected deterioration of rock mass properties during excavationdue to minerological conditions like swelling etc.
For each project the influence of each geology related parameter must be rated indivudually as it depends on the size of the opening, the construction method, the orientation of the opening towards discontinuities. After the preparation of this matrix the geotechnical longitudinal profile has to be prepared. Parts of the tunnel with similar geotechnical behavior are now recognizable in this longitudinal profile. Change points and classification for the assesment of design parameters at the design stage are more clear at that point.
Selection of Design Zones (Longitudinal Section): By the selection of design zones, the tunnel is divided, along its alignment, in longitudinal sections. Within one zone a similar behavior of the ground and the structure is expected. In the evaluatiıon of design zones following steps are followed: -estimate of the size of the influence zone of construction -evaluation of the dominating construction specific parameters (required information!) -selection of the design zones
In relation to the surface and sub-ground condition and with respect to the structural layout “required information”: topography and overburden surface structures surface hydrological conditions stratification classification of soil layers (classification parameters, density, stiffness) assessment of the groundwater condition (GW flow, water pressure distribution, aquifer system) Tunnel geometry Conditions at the portal zones of the tunnel Zones of enlargement of the standard tunnel section Cross passages Connection with ventilation shafts, tunnels or similar structural constraints
Soil Parameters: Using the results of field and laboratory investigations, soil properties and soil parameters are evaluated according to the defined design zones and stratification system. This includes direct use of factual data and the direct assesment on the basis of correlation to other soil parameters. A sound local experience of the ground and engineering judgement may be needed for the verification of the data.
Design sections and Design parameters: The number of design sections will be defined on the basis of changes of the surface and subground condition and with respect to the structural layout. Design parameters are design specific soil parameters and are determined for the particular design section according to the input requirements of the calculation method. They comprise the following information: thickness of the individual layer unit weight Water table and water pressure distribution Modulus of the ground Poisson’s ratio Shear parameters (friction, cohesion) Coefficient of the earth pressure at rest Subgrade reaction Design section represents an average section, so there is no need to assume the lowest value for all design data. In general the average soil parameters within the respectve design zone and the average layering system will be adopted for the design.
For the definition of different load cases short and long term conditions as well as a possible range of design parameters must be considered. Such as drained or undrained conditions, low and high groundwater table. For the determination of the shear parameters, it will be necessary to consider both, short term and long term ground behavior with respect to the time of consolidation in relation to stress changes. The permanent design should be based on the less favourable condition. As in most of the soils fully undrained conditions will be for a very short period only. Undrained shear strength might be used for the estimate of the stand up time of the unprotected soil face during excavation, while the actual temporary support system will be determined also by the less favourable condition
Design methods and engineering classification of rock masses for the design of tunnel supports Basically there are four different groups of design methods: Empirical Observational Analytical numerical
Rock classifications: Rock classifications were developed to provide the design aids forming only a part of the tunnel engineer’s “bag of tools”. Describing the rock through which a tunnel has an importance to choose the support system and the excavation method. Different classification system are used commonly in practical applications. In general there are 3 applications: -Geologic, geotechnical descriptions where one or several parameters are standardized -Quantitative descriptions of significant parameters of the rock mass which can directly or indirectly be used as design support -Description of the qualitative behavior of the rock mass during and after excavation, including the influence of the support