I.
Introduction
1. Pipe roof as alternatives to open cut and other method.Pipe
roof method of construction can generally be suitable when a
relatively short tunnel orculvert/underpass need to be installation
in heavy traffic or where road /rail services cannot be interrupted
and where settlement due to work are to be minimized. 2. Such
method involve the construction of temporary works using
interlocked steel pipes,installed by microtunnelling or pipe
jacking ,to form the temporary ground support inside which the
permanent structure is constructed. Considerations 3. The cases for
the need to consider pipe roof will include: a) Installing sections
across roads, railway for underpass or drainage/sewer /other
culvert b) Relatively large sections, sometimes as long as 19 m x
4.5m x 293 m c) Martha Railway, Atlanta Fig 1-pipe roof in Martha
Railway, Atlanta
e) Sensitive settlement issues, where cost of
interruptions/repair are probative, or as mandated by Authorities
such as near MRT or other services ( Singapores LTA project 6 m x 4
x 53 m - undercrossing Major road, and above MRT Lines) 4. The
general soil parameters used in the design of the shaft and tunnel(
pipe jacking ) and portal frame will include soil investigation,
study of the project areas , and discussions with the client and
authority 5. Generally the pipe roof is undertaken on a design and
build basis, where by the appointed main contractor will appoint
either specialize sub contractor for such task or have its own
specialist design team, and assemble specialist trade such as pipe
jacking to carry out the works,
6. Additional teams of specialist will include soil
improvements, geotechnical measurement and monitoring teams 7. The
client will appoint its own supervising team, and sometimes
independent check. 8. The performance criteria will follow pipe
jacking tolerances for jacking works, and structural tolerances for
others sections .The monitoring /suspension. Alert levels for
instrumentation works will be decided by site and design
considerations, and agreed upon prior to work commencement. II.
Planning and Design
The soil parameters are mainly used to design the shafts for
which to launch the pipe roof pipe jacking machine .typically: Fig
5 Soil profile (typical) -General soil profile of the pipe jacking
/ mined tunnel zone
The design parameters is sampled as shown Fig 6 Design soil
parameters.
The key considerations.The key design considerations can include
factors such as: i. Geotechnical aspects, including soilstructure
interaction between pipe-roof and surrounding ground, stability of
the excavated face and base stability during excavation; ii.
Structural aspects such as the design of the pipe-roof and the
structural steel frames supporting the piped-roof; iii. Soil
improvements and types and extent of protection to existing
services; iv. Other pertinent points related to method and
operation included: the adoption of single or multiple sided( 3, 4)
pipe roof , type and capabilities of jacking machine used, their
maintenance of face pressure, excavation speed especially when
encountered with obstructions such as sheet piles, timber pieces,
skill of operators, details of pipe clutching and damages expected
to be caused by excessive settlement , mitigation measures and
others. v. Risk assessment and hazard analysis including alert and
suspension levels for instrumentations such as inclinometers and
settlement markers, tilt meter etc during construction stage Other
calculations: a) Thrust wall design b) Jacking force calculations;
c) Final drawing of culvert Fig 7-sample design drawings of culvert
section
Design modelling: Use 2 D Plaxis -2D Plan strain model Fig 8- 2
D Plaxis Modelling
Typical section of the pipe roof is as shown Fig 9 - -typical
section of pipe roof (3 sided)
III.1.
Construction Construction methodology The temporary earth
retaining structure to support the ground during the excavation of
the mined tunnel consists of a series of 780mm diameter, 16mm thick
circular steel pipes interconnected by clutch to form an inverted U
pipe-roof structure around the footprint of the proposed drain.
This method known as pipe roof/ arch, have been deployed in USA and
Singapore in the last twenty years, as reported in Martha Railway
Underpass ,USA (Atalah, A.L. and D.T. Iseley, 1991)Orchard-C.K
.Tang Underpass Singapore (Darling, P.,1993). The pipe roof formed
by 22 rows of steel pipes , comprised of 10 at the top and 6 on
either side .They were installed in predetermined sequence,
starting from the lead central row on top, and consecutively on
left and right before the vertical rows were done.
- The interlocking sections are sampled below Fig 10-
-interlocking steel pipe sample
-
Pipe jacking of individual pipe .typical work
-
The following pictures show the pipejacking and pipe roof
construction i. Slurry shield jacking layout Fig 10- typical layout
for pipe jacking
ii. Constrction of jacking and receiving shaft Fig 11- shaft
construction
iii. View of jacking machine Fig 12- jacking machine
iv. Preparing guard Rail for jacking
v. Setting TBM Fig 13-preapring and setting up Jacking
Machine
vi. Welding steel sections Fig 14- welding joint
Vii. ready for jacking Fig 15 ready for jacking
viii. Break through at receiving shaft Fig 16- Break through
ix. Completed row Fig 17- completed row
The pipe roof (Parsons B 2009) is in-turn supported by
structural steel frames at every 1.5ms interval. Each excavation
step is also 1.5m with the width of the excavated slope also
limited to 1.5m. Three sections of soil improvements were provided
and these are located at the interfaces with the jacking shaft, the
receiving shaft, and a 10m zone at mid-point of the mined tunnel.
Drainage holes (2 nos.) were also provided to release any built-up
of excess pore-water pressure during the excavation. Soil
reinforcement in the form of reinforcement bar arranged in a square
grid pattern at 0.65 m spacing both ways, would be provided to
stabilize the face of the excavation in the F2 layer. The face of
the excavation would also be protected temporarily during each
stage excavation with a layer of short-crete of 15mm thick. After
the pipe roof and the last portal frame( at the tunnel opening into
the receiving shaft) were installed, the soldier piles and the
sheet piles around the shaft were cut.
The permanent drain section inside the mined tunnel between the
jacking shaft and the receiving shaft were then cast. Some of the
piped-roof pipes were filled with approved cement-based backfill
material before the casting of the final drain section, whereas
others were done after that to ensure that the steel pipes do not
pose any corrosion problems in the long run. IV. Construction
problems
1. Pipe Jacking Pipe jacking force for each pipe was calculated
to be 50 tonne for the 53 m of jacking. This was mostly achieved
with the jacking force ranging from 50 to 80 tonne for all except
three rows of pipe that encountered obstruction, which were
unforeseen. These obstructions were in the form of sheet pile,
galvanised steel pieces and timber residues, which were left in
place by earlier contracts. These had not identified during the
tender stage. The jacking force encountered for these cases went up
to over 150 tonne and additional time was needed to complete these
rows. However with skilled operators and proper design and site
management, no significant damages were encountered in the works or
utilities during the process. The particular case of encountering
of sheet pile during jacking proved to be very challenging indeed.
Jacking for this particular row progressed smoothly until the
cutter head of the machine hit very hard object, later found to be
sheet pile. A thorough investigation was carried, and it was
decided that a series of steel caisson of diameter 1.8 m were to be
driven and the soil excavated , so as to enable the cutting and the
extraction of the sheet pile for a number of locations. The works
were carried out overnight with partial lane closures to ensure
minimal traffic interruption along the North Buona Vista Road. Luck
does seem to play a big part as the sheet piles were encountered
only in one row of jacking. The pipe jacking works were planned on
a 24 hours basis, alternating between actual pipe jacking and butt
welding of the steel pieces and testing of the welds, which took
over 4 hours for each joint. This resulted in a cycle of 9 days in
between each row of jacking. Two jacking machines were utilised and
the teams work to jack the pipes in set predetermined designed
sequence to enable proper pipe roof to be formed after all pipes
have been jacked. The three rows which hit obstructions - took much
longer (over 30 days each). In all, the jacking 22 pipes was
completed from Aug 2009 to Jul 2010, a total of 11 months. This is
within the overall construction period planned for the pipe jacking
phase. 2. Settlement Control and Monitoring The CIAR Report
(Parsons B., 2010) helped to design and adopt a comprehensive
monitoring system as shown in Fig 10 . This provided the systematic
monitoring of the ground settlement of the surrounding areas and
other changes such as in ground water table and other as a result
of construction of the pipe jacking and the mined tunnel
Fig 10. Layout of instrumentation and monitoring system The
allowable limits for settlement as set in CIAR are asfollows: Alert
level =0.7 x deisgn values Suspension level =deisgn value The
maximum settlement at the ground level was expected to be along the
centre of the mined tunnel axis, at some 20 m from the jacking
shaft. Fig 11 shows the predicted settlement (Wong K S, 2009 )
along the traverse section of 51mm, or maximum gradient of 1/250 or
0.4%. This was well within the LTA allowable limit of 1/100 (LTA
2009e).
Fig 11 The predicted settlement along the traverse section
during various stages of Excavation
During construction, instrumentation monitoring meetings with
the contractor, specialists, sub contractors, the instrumentation
personnel, authorities (such as LTA, Power Grid, SingTel, PUB) and
consultants were held daily / weekly accordingly to the needs. In
the meetings, the participants evaluated and decided on
rectification measures and adjustments to the procedures to ensure
working within the alert and suspensions levels and in a safe and
acceptable manner. Plots in Fig 12 depict the readings taken with
the prisms installed to measure the combined effect of pipe roof
and mined tunnel. The prisms were installed along pipe L1 next to
the lead pipe (The lead pipe is in the centre of the top rows of
pipes) which was expected to produce the maximum settlements. The
maximum settlement at this pipe was 30 mm at about 20 m from the
jacking shaft, after the completion of the tunnel mining . This is
below the allowable of 125 mm , and predicated of 51 mm. At two
other prisms locations in the centre of the left and right vertical
pipes, maximum settlements were muchless than this figure. V.
Conclusion
Pipe roof/arch method of construction by pipe jacking generally
are useful and effective solution to situations where the
constraints of settlement , crossings and client's requirements
cannot be met by other cheaper method such as open cut or other
trenchless method. There are numerous issue to consider and as the
work are normally contracted out on a design and build basis , the
contractor will need to weight the risk and allow for specialist
inputs to be always on the site, and during planning/design stage.
With well coordinated efforts from all stakeholders, the pipe roof
method can perform as planned, and has been successfully adopted
since the 90s.
