TECHNICAL | MECHANISED TUNNELLING


Right top: Figure 1. Lining and mechanised tunnelling methods in the small- diameter range


Right bottom: Figure 2. Influence of hard rock, loose soil and groundwater on the TBM


IMAGES COURTESY OF HERRENKNECHT


Classic soft soil geologies consist of gravel, sand,


silt or clay in varying proportions. The following four parameters are central to the analysis of their exact geotechnical properties: 1. The grain size distribution is determined by sieving (for coarser grain sizes) and by slurry sedimentation analysis (for finer grain sizes) and is represented as a distribution curve.


of the subsoil, high strength, high abrasiveness, the


occurrence of boulders, mixed ground conditions and aggressive groundwater. Further requirements may arise from the tunnel alignment itself, such as in case of long tunnel sections, extremely low or very high overburden, small curve radii or steep gradients. In addition, small tunnel diameters with limited or


no accessibility, e.g., for replacement of worn cutting tools, present specific challenges to the smooth execution of tunnelling projects. Examples of relevant MTBM design parameters directly influenced by ground conditions are face support, cutterhead design, anti-clogging measures, excavation tools, muck transport, performance and wear, appropriate ground support measures, and measures for mitigating risks associated with ground instability or dangerous gases along the tunnel alignment (see Figure 3).


APPLICATION RANGES AND GEOTECHNICAL PARAMETERS In practice, most of the underground proves to be a mixture or, especially in the case of long sections, a sequence of different geological conditions. The geotechnical parameters and the respective investigation methods are theoretically as diverse as the geological conditions worldwide. Each ground naturally has its own requirements in


terms of the most suitable machine specifications. Specifically for the preliminary geotechnical investigation of pipejacking projects, eight indices have emerged as recommendations over the course of many successful projects – four for soft soil and four for hard rock. These should provide a pragmatic and reasonably cost-effective geotechnical planning basis for every pipejacking project.


12 | January 2026


2. The hydraulic conductivity or permeability of a soil are important hydrological parameters. The hydraulic conductivity, known as the kf value, depends on the grain size, porosity, bulk density and pore structure.


3. Plasticity is a decisive factor in determining how prone a material is to clogging and lumping. It is determined in the laboratory and described using the Atterberg limits (shrinkage limit WS, plastic limit WP and liquid limit WL).


4. Soil strength (of cohesive soils) and compactness (of non-cohesive soils) are determined by standard penetration testing (SPT).


Hard rock, in the sense of this classification, typically


includes rock types such as sandstone, claystone, limestone, granite, basalt and gneiss. Four indices are also particularly relevant to this area in relation to tunnel construction: 1. The degree of weathering describes and classifies the extent to which a rock has already been altered or decomposed by physical, chemical or biological processes.


2. The strength and quality of the rock are quantified using the Rock Quality Designation (RQD) method and other parameters such as the joint set number or spacing. In the RQD, the proportion consisting of contiguous core pieces of a certain length is measured by visual inspection.


3. Compressive and tensile strength - Unconfined Compressive Strength (UCS), Brazilian Tensile Strength (BTS) and Point Load Index (PLI) are determined by laboratory tests. The greater the number of rock samples used for this purpose, the more reliable the statistical statements on the strength of the material.


4. And, not least, the abrasiveness of the rock must be determined. The Cerchar Abrasivity Index (CAI)


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45