interpretation of epb tbm graphical data
TRANSCRIPT
Interpretation of EPB TBM Graphical Data
Keivan Rafie & Steve Skelhorn
McNally International
Abstract
Tunnel construction using a tunnel boring machine (TBM) involves a highly complex process
chain. Such processes generate large amounts of data that can be used for monitoring,
reporting and analysis. Major TBM manufacturers have developed software systems to support
tunnel contractors and their site teams in both data management and analysis. These programs
are mostly web‐based and have many advantages.
Data acquisition cannot prevent breakdowns from occurring but can facilitate investigations to
quickly determine the root cause of a breakdown and implement corrective actions. This paper
analyzes these data acquisition tools and presents case studies, primarily involving earth
pressure balance (EPB) TBMs, to illustrate how the formation of critical interpretations can be
made from user‐defined charts and diagrams to diagnose issues and optimize TBM operational
parameters.
RÉSUMÉ La construction des tunnels à l'aide de ‘Tunnel Boring Machine’ (TBM) est un processus
complexe. Ce type de procédure génère un grand nombre de données qui peuvent être
utilisées pour la surveillance, le signalement et l'analyse. La plupart des fabricants de TBM
développent des logiciels qui aident les ingénieurs et leur équipe dans la gestion et l’analyse de
données. La plupart de ces logiciels sont accessibles sur le web et offrent de nombreux
avantages. Cet exposé se penche sur l’utilisation de cet outil et présente des études de cas qui
démontrent l’importance que les interprétations des graphiques et diagrammes défini par
l’utilisateur peuvent avoir pour diagnostiquer et optimiser les paramètres opérationnels du
TBM.
Introduction The storage and visualization of measured values acquired by sensors and recorders is a crucial
element of TBM tunneling. All of the work being performed by the machine is documented in
terms of the recorded data to allow the complete or partial tracing of the tunnel construction in
real‐time or after completion.
This information could help engineers and operators examine a very large and complex set of
data related to TBM operation that cannot be ascertained in the field by the TBM engineers or
work crew, particularly when visualized in a graphic format. The examples of measured data
and sample graphs presented in this paper are mainly taken from Earth Pressure Balanced (EPB)
TBMs, but the logic behind the interpretation of these examples can also be applied to data
from hard rock or slurry TBMs.
TBM Data Acquisition and Visualization System The purpose of a TBM data and acquisition system can be summarized as the "acquisition,
processing, storage, display and evaluation of all data connected to the tunneling machine." A
TBM data acquisition system continuously records and visualizes all measured data in a
predefined cycle. Logging, however, occurs only at specific times. The average time period
between logs can be individually selected for each measuring point but is set to 10 s for most
parameters.
The operating phases of the tunneling machine are generally classified according to three
periods: advance, ring building and standstill. These three phases form a unit called a ring. The
data for each ring are usually stored in separate consecutively numbered files. An immediate
correlation to the respective construction phase can be made based on the ring number, file
date and file time. The measured data acquisition program automatically opens after each
system restart and loads all required program components into its memory. It then acquires,
stores and visualizes the currently available measured data.
TBM PLC and Data Acquisition System
Interpretation of TBM Operational Graphs in Case Studies
Some of the most common graphs representing the general status of TBM operations are ram
extension, rate of advance (ROA), thrust force, cutter head torque, EPB/slurry pressures,
weight/volume of excavated material, and grout volume. Of course, illustrating too many
parameters on one chart makes interpretation more difficult, so there must always be a
compromise between amount of information given and the clarity of the graphics.
EPB Pressure Graphs Case Study 1:
In successful EPB operation, face pressures should be maintained at all times and monitored
with the data acquisition system. Pressure of material in the chamber could be assessed by
information available from EPB cells. TBM operator closely monitors excavated material and
adjusts the type and amount of water, bentonite, polymers, and foam to ensure that the
material is properly forming a plug to resist piezometric and ground pressures.
Below graphs from pressure cell data are among the most used graphs in EPB tunneling and
demonstrate the difference between correct (A) and incorrect (B) operation. Excavations similar
to graph (A) result in safe and steady progress while performances similar to graph (B) are
usually linked with significant loss of ground and surface settlement.
(A) Maintaining face pressures in proper TBM operation
(B) Pressure drop during excavation
Case Study 2:
The EPB pressures for the top, middle and bottom sensors used in this case study are presented
in the graph below, which shows that the bottom sensors record higher pressures due to the
higher density and greater compaction of the excavated material. The top sensors record the
least pressure and fluctuation because they have less direct contact with the soil and mud in
the chamber.
EPB pressures for the top, middle and bottom sensors
EPB (Top) EPB (Middle) EPB (Bottom) bar bar bar
The proper estimation of material contact and density in an excavation chamber is important in
multiple stages of a project. It is common practice for the operator to perform and complete an
excavation with full level of material in chamber when using EPB TBMs. However, TBMs must
sometimes be operated in semi‐open, in which only portion of the face is balanced by
excavated material. These operating conditions are generally determined by engineers based
on the ground conditions and stoppage time. Compared to the semi‐open mode, full material
contact in a chamber requires more thrust and torque from the TBM and increases the
equipment wear and the cost of replacing excavation tools on the cutter head. Working in a
semi‐open mode could alleviate these issues but is not advisable if there is a high risk of ground
collapse and overexcavation. Other scenarios, such as preparation for cutter head maintenance
or leaving the TBM unused for long periods of time, could also influence decisions regarding the
level of material that should be present in a chamber during tunneling operations.
Case Study 3:
Smooth rise (or drop) in EPB graph lines indicates the passage of gaseous or liquid material into
(or out of) the excavation chamber which occurs mostly during the TBM ring build phase. Soft
rising curves may be the result of ground water filling the chamber or the injection of ground
conditioning material (foam, water or compressed air). A smooth drop in cell pressure suggests
the leakage of air or water through porous ground, a tail shield, purge line or screw conveyor.
Smooth rise (or drop) in EPB graph lines
Thrust, Cutterhead Torque, RPM and Rate of Advance Higher advance rates in TBMs are generally achieved in two ways.
A) A higher cutterhead rotation speed, which increases the distance that cutters or
rippers travel and thus their work per unit time (mm per min). In this case, the
cutterhead torque will increase. An increase in cutterhead torque can also result from
other factors such as poor ground conditioning or high material density in the
excavation chamber.
B) Higher forward forces in TBM cylinders to make cutters and rippers excavate more
intensively, thereby increasing the cut depth per cutterhead rotation. In this case, both
the TBM thrust force and its torque will increase. The TBM thrust can also be increased
due to shield friction with the ground or the TBM’s pulling force due to its weight (a
factor discussed later in conjunction with contact force).
Case Study 4:
Scenario (B) is illustrated in the graph below. The cutterhead rotation speed is set at
approximately 2 rpm, so the occasional increase in torque is due to higher thrust forces exerted
by the propulsion cylinders at that moment and increases the rate of advance.
Higher thrust forces and increased rate of advance
C/H Torque ROA Thrust
RPM ton‐m mm/min tons
It should be noted that higher efficiency is usually achieved in soft ground with a lower RPM
and higher thrust forces for deeper excavations, whereas cutters break into hard rock by rolling
on it. Therefore, better advance rates occur with higher RPMs.
Cutter Head Contact and TBM Thrust Forces The graphical representation of the relationship between thrust and contact force is mainly
used to identify any opposing forces to the TBM other than the excavation face. In general, the
TBM thrust is used to maintain EPB pressure, push the material in the chamber, and pull the
gantries and frictional forces of the shield.
TBM contact and thrust forces synchronized in their fluctuations
The thrust left over from propulsion energy is consumed by the cutterhead in the form of the
contact force required to cut through the ground. Because the parameters other than contact
force are relatively constant during normal TBM operation, TBM contact and thrust forces are
typically synchronized in their fluctuations. Therefore, any mismatch in the graphical patterns
between these two forces suggests a status change in other parameters and usually indicates
an obstacle during operation.
Case Study 5:
The theoretical graphs below show a sudden drop in contact force despite a constant increase
in the thrust force (Graph A). These data could indicate collapsed ground around the TBM
shield or an entrapped gantry back in the tunnel. Variations between the contact and thrust
force that are more gradual could result from a change in tunnel slope or the accumulation of
heavy, dense material in the chamber (Graph B).
Graph A: Sudden drop in contact force Graph B: Gradual drop in contact force
Sudden drops in contact force while TBM thrust force is increasing
Identifying Overexcavation
Most EPB machines today are equipped with weight sensors and laser scanners to estimate the
weight and volume of excavated ground. The theoretical weight and volume that a TBM data
acquisition system is expected to show is usually calculated manually based on the TBM
dimensions, ground properties and advancing distances. These figures are compared with the
quantities shown on TBM graphs to check for overexcavation. This information is also useful in
the analysis of excessive volume loss and settlement.
Typical weight scaling system on TBM conveyor
Case Study 6:
TBM advance with overexcavation can generally be recognized on TBM data graphs by a higher‐
than‐normal grade in the excavation weight or volume line. For example, the following
theoretical graph illustrates three sets of data from different advances. Line A, which is the
typical advance at a constant rate of excavation, is usually the preferred scenario and ensures
the consistency of other parameters, such as ground conditioning and screw conveyor speed
during the push.
3 scenarios for excavation weight/volumes
Compared to Line A, Line B has a steeper increase at the beginning and end of excavation
period and ends at a higher value. This condition can be interpreted as general overexcavation.
Line C represents a normal extraction scenario, except for a very sudden increase over short
period of time that could indicate overexcavation with a sudden rush of material through the
screw conveyor and out of the chamber. However, other aspects, such as those stated below,
must be considered to achieve a realistic understanding of the data.
1‐ The higher grades in the graph lines based on material weight and volume can be a
result of higher advance rates. Thus, the final excavation values must be checked. The
screw conveyor rotation speed in those time periods can provide insight as to whether
the high extraction rates were intentional or due to ground conditions.
Weight scale data showing higher quantity of excavated material for last 2 advances
2‐ The theoretical material weight depends on the advance distance of the TBM and the
density of the intact rock, but other factors, such as added water or ground conditioning
agents, should also be considered. In regard to ground conditioning material, only the
liquid portion will affect the material weight, so the foam expansion ratio (FER) should
not be considered in calculations. The FER of the ground conditioning material added to
the chamber has no effect on the weight calculations but should be considered with
regard to the material bulking factor when scanned by a laser on a conveyor belt for
volumetric data.
3‐ Comparing the weight and volume data/graphs of different advances makes sense only
if the level of material in the chamber after each advance remains full or relatively
constant. For example, if an operator has started an advance with a half‐full chamber
and decides to fill the chamber to its maximum level, less material will be extracted and
shown in the data, even though the same amount of material has been excavated from
the ground. On the other hand, when a TBM chamber must be emptied during an
advance (e.g., the last advance before cutterhead maintenance), the TBM data will show
a higher amount of extracted material than average. To eliminate this problem,
engineers also look at the rolling average of values for several consecutive rings, which
eliminates the effect of chamber space and gives a more realistic picture of the scenario
to identify possible overexcavation.
TBM parameters to be checked by operator to assess theoretical weight/volume
4‐ Added water should be considered in theoretical calculations. Occasionally, depending on ground conditions, most of the injected water is absorbed by the ground, and
sometimes added water only replaces the water in saturated soils.
5‐ The calibration of weight scales and laser scanners must be part of a contractor’s regular
maintenance program. Some weight sensors are very sensitive to misalignment and
curves in the TBM conveyor belt, while laser scanners could have inaccurate readings
depending on their position and air/light interference, such as dust. Utilizing two belt
scales and observing their averages can also aid in identifying errors and obtaining more
realistic results.
Grouting System Two‐component grouting (A+B) systems through the tail shield have been one of the most
problematic areas in TBM tunneling. Proper grouting is important to prevent ground movement
and surface settlement due to volume loss at the tail void. Grouting also stabilizes segmental
lining in the ground and improves a tunnel’s watertightness.
Information available in TBM data acquisition systems can show early signs of system
malfunctions and indicate which components require attention or which control settings need
to be modified.
TBM data loggers typically record flow, pressure and volume parameters for each grout line. To
check the quality of grouting behind segments and ensure that the correct dosage of
accelerator (B) is mixed with part (A), TBM data for injected volumes should be checked against
the theoretical volume of voids behind the segments. Gauge cutter wear should be considered
in theoretical calculations, particularly for larger TBMs. Understanding the bore and cut
diameters in hard and soft ground types can also lead to more accurate calculations.
If the grouting volumes are lower than their theoretical values, other data must be checked to
identify and solve the problem. The simultaneous spike in grout pressure and halt of grout flow
in the diagram below is commonly an indication of blockage in the line. If grout volumes cannot
be achieved when all lines are in operation, then the pre‐sets and cutoff levels should be
checked. Generally, grout pressures must overcome hydrostatic pressures by 1‐2 bar behind
segments.
Case Study 7:
Grout pressure without flow, showing the blockage in lines
If grouting volumes are higher than their theoretical values, assessments must be performed to
identify any channeling of grout to the surrounding environment or leakage through the tail
shield. In some instances, high‐pressure grout finds its way to the excavation chamber, mixes
with the excavated material and exits through the screw.
Grout lines 5 and 6 showing spikes in pressure which is usually associated with temporary blockage
Propulsion Cylinders and Ring Build
Information and graphs derived from TBM data on propulsion cylinders can be used to analyze
several aspects of their operation, including ring build and steering. These data can also explain
damage to segments that occurs after installation. TBM data acquisition systems generally
display the pressures and extension of ram groups using the sensors on a representative ram
from that group. The figure below shows information collected on 19 rams in 6 groups (A‐F).
The location of each group’s representative ram is shown in black.
19 rams collected in 6 groups (A‐F);
the location of a representative ram from each group is shown in black
Case Study 8:
The graph below shows the pressures applied to group of segments during ring erection. The
lines representing each group show a sudden jump from zero, indicating that the rams have
been extracted to hold each segment after its erection. The lines also indicate common slow
pressure loss due to micro‐movement of the TBM and ring compression in the tunnel. However,
an excessive loss of pressure in any group could loosen the adjacent segment and cause vertical
(step) and horizontal (gap) misalignments.
Ram pressures applied to segments during ring erection
Pressure (bar)
Time (min)
On the other hand, excessive pressures on cylinders can cause damage, particularly around the
circumferential joints of the segments in front of the cylinders. Ring build reports from the
guidance system (as shown below) must be studied in conjunction with ram pressure graphs to
confirm the location of segments in relation to the propulsion cylinders and explain the damage
incurred.
Sample ring build report from the guidance system
Conclusion
The graphical representation and measured values of TBM data can assist contractors by
providing information that helps TBM crews increase the reliability and productivity of TBM
operations. Such an advantage would ultimately lead to fewer breakdowns and lower tunneling
expenses. Data acquisition and visualization alone does not benefit the contractor unless the
data is accurately interpreted. The utilization and correct interpretation of data acquisition
systems’ outputs could greatly enhance the control of the excavation and operation of various
tunnel boring machine systems. As TBMs grow in size and complexity, advances in data
monitoring and presentation to optimize TBM parameters will likely continue as well. The
correct interpretation of these data is essential for the effective utilization of these tools and to
ensure efficient and productive tunneling operations.
The key to success in EPB tunneling is proper engineering combined with experienced
operators. Data acquisition cannot prevent breakdowns from happening but allows the rapid
identification of the root cause of a breakdown and the timely implementation of corrective
actions.