TECHNICAL | MECHANISED TUNNELLING
Table 2: Structure of the TBM performance database TBM field data
Thrust force (Tf )
Penetration per revolution (p) Revolution per minute (ROP)
TBM utilisation factor (U) Advance rate (AR)
Field Penetration Index (kN/cutter/mm/rev)
Geological/geotechnical data Rock mass fracturing Intact rock properties Rock mass classification
Joint spacing, Jv, RQD UCS, BTS, CAI RMR, Q, GSI
DATA COMPILATION
The database contains different levels of information that defines the tunnel, rock mass conditions, and TBM field performance parameters over the full length of tunnel drives. The database contains more than 666 data sets where ground conditions and machine performance were reliable input parameters, and the data were available and could be verified. The data sets comprised two main categories -
machine performance and some geological parameters, respectively. Machine performance parameters included net boring time, length of mined section, and also the average of machine operational parameters (thrust, cutterhead rotational speed, power and applied torque) throughout the section. Geological parameters included intact rock properties (compressive and tensile strength, quartz content, porosity); discontinuity characteristics, such as spacing, weathering, surface condition, and the calculation of some rock mass parameters (like RQD, rock mass rating (RMR)) in selected tunnel sections. Also included were important performance
parameters, such as average penetration rate (ROP), penetration per revolution (P), average cutter load Fn
ROP = , P = 1
Lb tb
FPI = , Fn
Fn P
ROP×1000 RPM×60
= (Th – Ff , ) / Ncutters’ where, ROP is rate of penetration (m/h), Lb is boring length (m), tb is boring time (h), P is cutter penetration
per revolution (mm/rev), RPM is cutterhead rotational speed (rev/ mm), FPI is Field Penetration Index expressed (kN/ cutter/mm/rev), Fn normal force, Th (kN), Ff
is cutter load or is the applied thrust of the machine is the estimated friction force between the machine and the ground (kN), and Ncutters is number of
disc cutters installed on the TBM cutterhead. To estimate the frictional force, machines were
placed in two groups (see Table 1) – gripper/open and double shield TBMs, respectively. In open type TBMs, the friction force is much lower
than shielded machines. In some cases, the front shoes of the machine are pressed against the walls and can impose a high pressure on the walls and thus high friction. However, for the most part, friction of the machine can be included in the calculations by subtracting 20% machine weight from the total thrust force applied by the thrust cylinders (Delisio and Zhao 2014; Salimi et al. 2019a).
18 | July 2024 , and field penetration index (FPI), estimated as follows:
For shielded TBMs the friction force builds-up
between shield and surrounding ground, and hence is significantly higher than for open machines, especially for double shield TBMs. Previous studies have used 20% weight of the machine in non-squeezing grounds, or 20% of the rock load against the shield in low- to medium-level squeezing conditions. For highly squeezing conditions the value of friction forces could be higher than the thrust, leading to jamming. In such conditions, the use of an arbitrary percentage of weight of the machine is misleading. Further investigations are needed when shield TBMs are being utilised to assess the friction between shield and respected ground conditions (Salimi et al. 2019a; Salimi 2021). The general database structure is presented in Table 2. An important issue of note is missing data for
different parameters in different project records. Due to the difficulty of dealing with volumes of detailed data, in several separate databases for different projects it was necessary to reduce the number of data sets to a manageable number. Heterogeneity of the data was also an issue, caused by differing protocols for recording TBM performance data on different job sites. UCS is a commonly-used representative of rock
strength in almost all of the TBM tunnel projects. Increasing UCS causes a decrease in PR (Rostami 2013; Gong and Zhao 2009; Salimi et al. 2016). Rock mass behaviour is a function of rock material, and both joint frequency and conditions, and also influence rock cutting by TBM. Joint spacing can be represented by RQD, joint frequency and volumetric joint count. In this study, based on availability of the database information, both RQD and Jv were taken into account to represent joint frequency. As shown in Table 1, the database covers three main rock types: igneous (38%); metamorphic (31%); and, sedimentary (31%). The TBM boring diameter varied from 3.6m-10.5m. Various TBM performance indices have been proposed
based on FPI, specific penetration (SP, the inverse of FPI) and boreability index (BI, which is similar to FPI). FPI has had more attention and, considering it as representative of TBM performance, it is commonly used to present ‘boreability’ of rock with changing geological/ geotechnical circumstances. The main advantage of FPI is allowing penetration
to be normalised for cutterload and thus it automatically takes care of machine thrust variations. It can also be used across different TBM diameters as it accounts for cutterhead RPM and the number of disc cutters.
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