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Geotechnics and consulting

The ground conditions as starting point

A professional evaluation of geological profiles is the basis for choosing the optimum tunnelling technology.

________________ The most important basis for planning and construction of a tunnelling project is knowledge about the characteristics of the subsoil, that is, the geotechnical and hydrogeological conditions. The first information on the ground conditions is achieved by evaluating general and detailed geotechnical maps.

Each individual tunnel route has special geological and hydrogeological conditions. The machine type appropriate in each individual case is chosen after all available project parameters have been considered. To make product allocation easier, the variety of ground conditions have been divided into three fundamental categories.

 

Cohesive soils such as clay, silt or loam with low water permeability and a smaller range of grain sizes

Properties
Mixture of minerals and/or fractions of rock and/or organic materials without mineral cohesion. Usually unstable, cohesive loose soils with a high silt or clay content and pulpy consistency. Classical area of application for earth pressure balanced tunnelling methods.

 

Incohesive soils such as sand, gravel or stones with high water permeability, or mixed geologies of solid and loose rock

Properties
Alternate layers of loose soils and solid rock or alternate layers and interlocking of cohesive and incohesive loose soils. Often in combination with geological deposits such as boulders, when crossing beneath bodies of water, for example. Classical area of application for slurry-supported tunnelling methods

 

Rock such as sandstone, limestone, basalt or granite with high compressive strengths in some parts. The main feature is that components have contact or are joined in most parts

Properties
Mixture of minerals and /or fractions of rock with mineral cohesion. The term rock includes a wide variety of rock types, ranging from soft formations to gneiss or granite with rock strengths of up to 400 MPa. Classical area of application for hard rock machines

Probing methods, for example, using surface exploration sites and sounding borings, make it possible to describe the ground quality and characteristics in the required detail to assure optimum TBM design. In the following, the most important geotechnical information to evaluate what machine type should be used are summarized in brief:

  • Grain size distribution curves
  • Bulk density / buoyant bulk density
  • Friction angle
  • Cohesion
  • Stiffness modulus
  • Permeability
  • Groundwater level
  • Atterberg limits
  • Uniaxial compressive strength / rockmass strength / tensile rock strength
  • Quartz content
  • Abrasiveness

However, not only is the choice of method important, but special focus is also placed on the cutting wheel design. The ground condition parameters have a great influence on the design of the cutting wheel, its opening ratio and the required tools. Taking the appropriate measures can minimize wear, tear and clogging, while optimizing soil and rock excavation.

The grain size distribution curves are among the most important information for machine selection in soft ground.

 

Planning success

The success of a mechanized tunnelling project depends to a great extent on knowledge about the subsurface conditions to be expected. The earlier the interactions between ground conditions, site constraints and tunnelling method in the entire system are considered in the planning, the more effective and safe the construction and completion of the project will be. At Herrenknecht, mechanical engineers, civil engineers, geologists and process engineers work together closely right from the planning phase. Comprehensive project expertise in different and varying ground conditions allow them to make reliable recommendations about the appropriate tunnel boring machine. As technology leader, Herrenknecht is able to offer consultancy and special solutions as required, even when faced with extraordinary challenges.

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