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SIMULATION OF TUNNELING OPERATIONS
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ABSTRACT:
The objective of this paper is to predict the tunnel advance
rate in the construction of a several-mile-long, small-diameter (3-3.5 m)
tunnel in soft rock. For this purpose, the CYCLONE simulation system
is used. Several simulation models are developed to investigate the
effect of different variables on the tunnel advance rate. At first the
suitable tunneling method, including the tunnel support and material
handling system, is selected by reviewing similar projects completed in
recent years. Various cycle times and durations, e.g., TBM penetration
rate and train travel time, are carefully calculated, based on previous
tunneling jobs. After specifying the construction method and duration
times, the developed simulation models are described. The impact of
each major variable on the tunnel advance rate is studied by sensitivity
analysis. These variables include the TBM penetration rate, the train
travel time, the number of muck trains, the type of rock, and the rock
standup time. Simulation models quantify the effect of each of these
variables on the tunnel advance rate.
INTRODUCTION
Simulation can be used in planning and scheduling of highly repetitive
tasks in a construction project. Traditional scheduling techniques like
CPM and Barchart are not suitable for this type of projects. For example,
the classical method of driving a tunnel is composed of drilling, loading the
drill holes with explosives, shooting, and muck handling. This cycle can be
repeated up to eight times per day depending on the tunnel diameter and
number of shifts and can go on for more than one year. An accurate model
of construction process will basically consist of hundreds of repetitive
cycles. On the other hand, one of the most important activities in a tunnel
job is the actual advance rate of tunnel. On most of the jobs, this activity
is either critical or very close to critical. Project duration, equipment
capacity, power requirements, and the total costs are all directly related to
the tunnel advance rate. Simulating the process of tunnel advancement can
help plan and control the project more efficiently.
GENERAL ASSUMPTIONS ABOUT TUNNEL
As mentioned before, the objective of the project is to predict the
advance rate in a several-mile long, small-diameter tunnel. The finished
diameter is assumed to be 10 ft (3 m). The excavated diameter before
placing the tunnel lining is assumed to be 12 ft (3.6 m). The material is
assumed to be uniform soft rock (shale or claystone) with a compressive
strength of less than 10,000 psi (68,900 kPa). Because of the possibility of
swelling and slaking, a support system is used. It is assumed that
groundwater does not cause major complications. Access to the tunneling
face is provided via a vertical shaft. It is felt that material handling via a
shaft is more problematical than a portal. In a long tunnel, using vertical
shafts for expediting the project is quite conceivable. The model considers
a vertical shaft and two tunneling faces (Fig. 1).
SELECTION OF TUNNELING SYSTEM
A comprehensive study is conducted to select the most suitable tunneling
system for this project.
Tunnel Boring Machine
As mentioned previously, TBM is the clear choice over the conventional
tunneling method (i.e., drill and blast) in this project. Since its first use in
the U.S. in 1954 (Thon 1982), TBM's capability has improved significantly.
Diameters of less than 5 ft (1.5 m) to more than 40 ft (12.2 m) can be driven
by TBM; some of the TBMs have variable diameters and can work on
inclined shafts. Advance rates have improved in different types of soil
(David and Bradford 1975; Thon 1982). Also, tunnel driving with TBM has
the advantage that it does not disturb the ground adjacent to the tunnel.
Material Handling
Material handling is divided into two parts: vertical and horizontal.
1. The conventional method for handling material and personnel inside
a tunnel is to use either train, rubber-tired equipment like trucks, or belt
conveyors. In special cases, pipeline has been used to haul the muck.
Pneumatic muck handling (Faddick and Martin 1974) has sometimes been
used in projects where advance rate had to be extremely high. Hydraulic
or slurry system is another alternative in which a mixture of water and
muck is transported by pipeline (David and Bradford 1975).
In this project, belt conveyors and pipeline system do not present
attractive alternatives because these are not capable of transporting
laborers or tunnel support segments. The writers have found it impossible
to place a belt conveyor or pipeline along with a railroad system (for
personnel transportation) in the confined cross section of the tunnel [10 ft
(3 m) finished diameter]. Rubber-tired equipment cause pollution problems;
also, there is not enough space in the tunnel heading area for turning.
Tunnel Support System
Precast concrete segments are chosen for tunnel support and lining. The
nature of assumed rock (shale or claystone) and the possibility of swelling
and slaking dictates a rather rapid protection of tunnel wall. Precast
segments can be installed rapidly. Using these segments will eliminate the
need for temporary support such as steel ribs or shotcrete. The New
Austrian Tunneling Method (NATM) has not been considered in this
study, because it is not used in the U.S. frequently. Also, the concretesegments
method seems to be faster. The first precast concrete segments
in the U.S. were used in Buckskin Mountain Tunnel, Arizona by the
Bureau of Reclamation (1976). It is interesting to note that bidding
documents for this 10.7-km long tunnel permitted the bidders to choose
either the conventional drill-blast method with cast-in-place concrete
lining, TBM with cast-in-place concrete lining, or TBM with precast
concrete segments. The three lowest bidders all chose the TBM and
precast segments alternative (Deere undated).
Muck Train and Material Handling Data
It is economical to use as large a muck car as possible because it reduces
the time required for dumping the material. It also reduces the overall
length of train, thus reducing the length of California switches. The
switching sections in the tunnel should be kept unlined to increase the
diameter to 12 ft (3.6 m). The rock can be supported by shotcrete (and wire
mesh if required) temporarily. The maximum size of the muck car that can
be accommodated at this tunnel diameter is 3.5 ft (1.1 m) in width and 3.0
ft (.9 m) in height (Fig. 2). The minimum requirements for clearances
between cars, car and tunnel walls, ventilation system, and man-way have
been considered in this figure (Parsons et al. 1984). Fig. 2(a) shows how the
unlined diameter of the tunnel can be utilized to accommodate a California
switch. In Fig. 2(b), the finished diameter of the tunnel is shown. A 3.5 x
3-ft. cross section is the largest cross section for the muck car that can be
used in the tunnel considering the switching stations [Fig. 2©], Each TBM
stroke of 5 ft (1.5 m) in a 12-ft (3.6-m) diameter tunnel shall produce about
32 yd3 (23.3 m3) of muck assuming a swell factor of 50% (Parsons et al.
1984; Nunnally 1977). Given the tunnel dimensions, it would be difficult to
use a 32-yd3 (23.3 m3) train. The length of the train would be too long,
requiring very long switching stations. Each train is assumed to have a
capacity of 16 yd3 (11.65 m3), consisting of eight 3 x 3.5 x 7.0-ft (0.9 x 1.05
x 2.14-m) muck cars. Therefore two trains are required to handle the muck
produced from each TBM stroke. For detailed discussion of the train
configuration, please refer to Asai (1986).
SUMMARY AND CONCLUSIONS
Application of computer simulation in a hypothetical tunneling project
has been described. The objective has been to estimate the advance rate of
a long, small-diameter tunnel. It was assumed that the rock was shale or
claystone and the tunnel located at least 150 ft (50 m) below the ground
level. The diameter of the tunnel was assumed to be 12 ft (3.6 m) before
lining. The finished diameter was assumed to be 10 ft (3 m).
A suitable tunneling system was selected based on the actual projects
completed in the past. TBM was selected as the main equipment for boring
the tunnel. A railroad system was selected for horizontal material handling.
A skip system was chosen for vertical material movement.