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)
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 t