Tunnel-Infrastructure Convergences/monitoring and Engineering role.
Monitoring provides information for ground movements and disturbances, preventing accidents, providing evidence of
the real risk on underground infrastructure.
NATM (new Austrian tunnelling method) used in Albania, Tirana-Elbasan project.
SEM (sequential excavation method) used in Canada, Eglinton LRT project.
Convergence/monitoring of tunnels is a basic tool for understanding and recording of the rock mass behaviour during
excavation phase.
This information and study publication provide technical and engineering directives/solution on how convergences and
monitoring can be captured and reported for each project you may be involved including Survey Department, Geological
Department and Structural Department. The explanation will provide theoretical and practical solution of convergence
monitoring process in two similar methodologies of opening with different techniques on tunnel projects located in two different
types of rock. Methodologies studied and used are NATM and SEM for these convergences. On a tunnel/mine project, during
construction areas that are excavated and will deform or “converge” as we have disturbed the stability of the rock mas itself.
That’s why prior to project activities begin we should have geological drillings and analysing of samples to define type of rock,
strength, and its characteristics so we can have approximate information about the expectations of the movements of the rock
mas on tunnel surrounding. The mine boring/excavation/drilling or blasting always creates instability and as result of it we may
expect large quantities of soil fall from above, rock, debris or whole segment to be collapsed/displaced. This means the
surrounding environment of the tunnel/mine it is reacting to the change of the original condition to settle and find the balanced
position where all forces are equal.
Detecting in a very short period any anomalous displacement of preliminary support lining is partially a matter of safety condition
and provides a clear overview of the rock mass behaviour giving the opportunity of taking immediate actions toward
stabilisation. Providing a plot of absolute displacement with an accuracy of -/+1mm, the obtained data are appropriate for use
into the mathematical model for tunnel design during the excavation phase. An optical system used on monitoring displacement
in tunnelling is based on the use of survey Network equipment as Total Station (0.5’-1”) and installation of convergence stations,
composed by reflective targets on the preliminary support, having range of measurements as needed during 24 hr. Measured
data cannot always be a realistic overview of displacement and is necessary to perform an error source analysis before data are
being inserted to the database and diagrams. During the extraction of a mining body with underground methods, an
underutilized rock block with a surface A, at a depth h, is created. This causes the under-exploited rock block, whose gravity
was born just after the ore/cavern/dome began to be extracted, to lose its stability.
NATM: In this case methodology and information provided it is related on using NATM (New Austrian Tunnelling Method) which
is effective and sustainable as the mass of the rock it is extracted through drill and blast processes and the ground can be
managed through opening and blasting calculations to not disturb more rock than the template used for the tunnel design.
SEM: In other cases, the same method of convergences and monitoring it is used and called SEM (Sequential Excavation
Method). The same diameter tunnel being excavated in sequences, the opening time varies based on soft ground conditions
and active cities above surface. SEM objective is for infrastructures under the cities for Railway, Subway, Metro, Transportation
tunnels, storm and sewer, pumping stations under the cities without disturbing the life of the city above the tunnel.
Mine and Geology Engineering: the vertical gravity force G of this underutilized rock body is the product of its mass “m” in kg.
and the action of gravity g = 9.81 m/s2; so, we can write G = m g (in N) (1) Giving the mass of the rock per 1 m3 of its volume,
thus giving its density ρ = m / V (in kg/m3) (2) The pressure exerted by a vertical rock column of height h (in m), on a surface of
A = 1 m2 can be determined by the equation: p = hρg (in Pa) (3) From the estimate γ = ρg = 25000 kg/m3 x 9.81 m/s2 ≈ 2500
kg/m2s2, we can get in SI units for force 1 Newton = 1N = 1 kg m/s2 and for pressure – 1 Pascal = 1Pa = 1 N/m2. An
approximate calculation for the normal pressure exerted by the underutilized rock mass at a depth h (in m) can be done with the
equation: ρ ≈ γh ≈ h/40 (in MPa) (4) where the force of the specific weight (gravity) ρ = G/V = 0.025 MN/m.3 For example, we
accept the pressure of the undercut rock, derived only from the weight of the rock mass, without considering the mining
influence, approximately 20 MPa in a depth 800 m, which means 20 N/mm2 = 2000 N/cm2, or in the previous units – 200
kp/cm2 (200 bar).
Geological Engineering: classification of the rock will be used only theoretical (not real values), and it is only for study
purposes of this bulletin of convergences and do not represent real value of the project.
Lower Heading W A
CROWN 40mm 70mm
SIDEWALLS 20mm 30mm
At Bench 15mm 20mm 25mm
The classification can be extracted from the department of Geological Engineering and the geological studies of the project you
are working on, which usually take place prior project construction. Based on those specification engineers who deal with
convergences/monitoring have their expectations of the real movements and deformation of the rock mass from Geological
Engineers and the detailed structures of the type of the rock mass on the portion of the project that will be underground.
Surveying Engineering: surveying network on tunnels it is a complex process, and it is managed in two phases which are
critical for the project progress/completion. In both phases survey data/measurements are the information/coordinates will be
used to calculate convergences on the Mine/Tunnel project.
First phase: it is the surface network that cover whole project including tunnel. This takes place on the surface, above the
tunnel. Moving forward with excavation of the tunnel, this network it is taking place on both sides of the tunnel, on its portals,
inside cavern itself and it is called Attached Network and the minimum required CP (stations) used per each side of the tunnel
are two to make the inside portion of the network attached to the project network. Attaching the tunnel network to general
network of the project has its benefits as we know the accuracy and we can reproduce the same network in case of losing them
during contraction of the tunnel from the beginning phases to completion of the project. Four GCP are shown on the graphic
Second phase: Attached network take place once we start the excavation of the rock mas inside the tunnel. According to
activities inside the cavern this network must be checked often or can be recalculated again as it is critical to the opening of the
project and both sides of excavation can meet each other as per design/specification and on approved accuracy. This is the
network used also for the convergences, portal and entrance of tunnel can be captured even with surface network in case portal
has to be monitored.
Network calculation.
Diagrams and reports of the measurements. Design that Surveying department creates for the Tunnel is related to structural
design itself and values are added on specific software as TCP, Amberg, etc, to complete alignment, longitudinal section (lines-
curbs-radius-vertex), section are created including multiple templates as umbrella, excavation line, iron arch line, shotcrete line,
final line, inverts, sidewalks, layers for aggregate, asphalt etc.
This design created may be used in other software’s as Alignment Solver for defining change of the target, distance from centre
line etc, when it is first measured and created a baseline for monitoring. In this practical of convergences Alignment Solver has
been used and the values are extracted from its results.
The diagram and chart: represented on the below graphic contain values for each target measured on the section of the tunnel
and they represent date and time of the measurement, northing and easting of the target, elevation, chainage of the target in the
tunnel alignment, distance from centre line of the centre of the tunnel. The diagram include also Station (CP) where the
instrument has been set and the station of orientation(closure error to), measured vectors(distances) created between targets
D1 to D7, excavation of the chainage(front line) to know the distance of it as we have two parallel tunnels and during blasting
front lines of both of them has to be known as per safety standards, teams have to be evacuated on the other tunnel during
blasting. Also, on this diagram are reported convergences dD which are the real movements of the measured section for the two
phases of the tunnel. Measurements are captured with base instrument fare from the active area of the construction to
represent only the movement of the rock mass. It is important to set up instruments always on decommissioned area where we
have stabilized underground, 0.00 values of the movement of the rock mass.
Displacements. Reporting and representing displacement are required to create and maintain diagram and schemes as they
will visualize values and results of the measured/data captured. Below are represented displacements and they are shown on
the section of tunnel (full cavern) for their direction and visualizing differences d-h vertical displacement provide the differences
in elevation certain target had during process or monitoring from first moment of the baseline, for the completion of the project
and d-cl horizontal displacement provide the difference in location considering the distance from target to centre lime of the
tunnel, d-ch displacement along the tunnel (through chainage) provide the difference in location towards the alignment of the
Cavern d-h and d-cl Plan d-cl and d-ch
dch-displacement through chainage dcl- horizontal displacement.
dh- vertical displacements dD-convergences
Conclusion: Convergences and monitoring process as the one described in this brochure/study material it is one of the
methods used in today’s industry and it is efficient, functional and brings accurate information, real data, precision and control
during tunnel/mine excavation. Conditionally require management, manpower, qualified personnel to measure, evaluate,
prepare and report the convergences as based on the information provided will follow all the structural activity of the opening of
the tunnel/mine, structural design, final lining and existence of the project during years.
This study brings to light the importance of the monitoring and convergences in the infrastructure industry today, mining,
building, port lands etc.
Formulas and other details for the calculation of the network, displacement, graphics and final convergences are not shared as
are subject of the projects where have been used/applied.
I would like to express my very great appreciation to Eng. Ferdinand Kullolli for his support and collaboration, to all my
colleagues and those working closely to monitoring department during those projects, including IRD engineers on Tirana-
Elbasan project (Albania), Dr, Sauer and Partners team for consulting the theoretical and practical method for monitoring and
convergences of the Eglinton LRT project (Canada).
In conclusion it is important to express my appreciation to those companies that used means and methods for infrastructure
tunnel projects, dedicating a monitoring department on the project for the safety, measurements, analysis, collecting data, and
making risk analysis to prevent collapse during construction, making each single member of their project return healthy and safe
home each day.
Engineering is amazing, it is the change and progress through knowledge and science for good in our life.
Engineer, Sali BEDAJ.
Gordie Howe International Bridge
Detroit, Michigan, USA.