There are a number of stages in segment handling. With a segmental lining arrangement finalised and the moulds supplied, the first stage would be de-moulding when the concrete strength is low. To free up the mould for casting of the next segment, the precast segment is taken out of the mould at the earliest possible time.
Trials are carried out using the moulds proposed for segment production to establish the minimum stripping time, such that removal of the mould will not damage the surface of the concrete.
The de-moulding strength is often not just dependent on the concrete segment handling but on a number of other factors:
Can the pins be removed from fittings and holes?
Holes in the segment may slump and cause problems when bolts are inserted at a later date.
Cast in plastic sockets may turn if the threaded mould pins are taken out too early at a low concrete strength.
A minimum strength is required around the circle and radial joints; so when the side panels of the mould are released they do not cause the gasket groove and the fitting recesses to spall.
The designer will specify the minimum strength at which the segment can be taken out of the mould based on the length and thickness of the segments and the quantity of reinforcement in it. The minimum de-moulding strength is usually around 15Mpa. There are no codes or standards which give appropriate bending and shear capacity of concrete for strengths less than 15Mpa.
Vacuum pad lifting is the gentlest form of lifting where the stresses induced in the segment are more uniformly spread. If vacuum pads or scissor lifts with side grabs are not used, lifting anchors may be located in the circle joints. In this instance when the segments are lifted by their thinnest side there will be a reduction in the lifting capacity, especially if they are only reinforced with fibres, and the concrete would need to reach a higher strength at lift.
It is assumed that the edges projecting beyond the vacuum lifter act as a cantilever. The segments are checked for shear and bending capacity.
The segments are at their most vulnerable stage during de-moulding and need handling with care to prevent hairline cracks which can open up under loading.
The grabs extend along the circumferential edge of the segment. Bearing stress and shear stresses induced around the perimeter of the grabs have to be checked assuming no reinforcement acts to enhance the shear strength of the segment due to the proximity of the free edge of the segment.
Segments stored in a different orientation to that cast will require a turning frame.
Temporary storage for curing or for installation of gasket During storage, segments are checked for sagging moment at the centre, hogging moment at the support and also for shear at the supports.
It is common practice to specify that a load factor of five be applied to segments during handling. This is based on a dead load factor of 1.4 and a dynamic amplification factor of three, which gives a combined effect of 4.2 which is then rounded up to five. This factor of safety frame are smaller and support the selfweight of a single segment during the lifting operation. Bending moments for this case are the same as those for the temporary frame above. However, there is the additional bending moment induced across the width of the segment owing to the nature of the grab.
Segment stacking
When segments are stored in the yard prior to transportation to the launch shaft site it is common practice to stack each ring on its own which simplifies the transportation stage.
Segments can be stacked supported either on their back, front or side.
With a full stack of six segments forming a ring, it is appropriate to assume that the load factor of five is not applied to all six segments. We would assume that the top three segments are stacked onto the bottom three, and therefore a load factor of five is only applied to the self weight of the top three segments whilst 1.4 is applied to the lower three segments which would already have been placed in position.
The timber batten positions are determined such that the hogging moments induced at the supports are equal to the sagging moment in the middle of the segment. This makes the most economic use of the hoop reinforcement provided equally along the intrados andextrados of the segment. This is often approximated by contractors to ¼ points.
The battens should always be in line. However this could be difficult on site and therefore usually a positioning tolerance of +/-100mm is allowed for in design.
The height of the segment stacks has to be limited to prevent instability caused by mis-aligned battens. With vertical stacks, the number of segments in a stack is limited to two or three as the gaskets may be crushed under the timber battens.
Segment erection
Before installation the segments will have reached the 28 day strength. They will therefore have the strength to be lifted by a central lifting point if the segments are reinforced with steel bars. But if the segments are reinforced with steel fibres, the stresses have to be more uniformly distributed and will therefore be lifted using vacuum pads.
By doing so the risk of fibre reinforced segments cracking in the tunnel and causing injury to the miners can also be eliminated. Once underground, the segments will be lifted off the segment transporter in the backup, transported to the ring build area by a carousel, and positioned by the TBM erector which will either use a mechanically fixed lifting point which is screwed onto the segment cast-in lifting socket located on the centre of gravity point of the segment or lifted using vacuum pads with safety shear pins.
Ring geometry
Ring geometry greatly influences the segment installation process. Tapered rings allow for changes in the alignment from a straight to a curve and a clothoid without any packers. Rolling the ring, the varying segment positions provide the directional corrections required to achieve the desired alignment.
A balance has to be struck between speed of erection and an economic design. For many projects one of the key costs is the overall duration of tunnelling. The larger the segment size, the fewer the segments and number of joints which need to be bolted together and made watertight. The minimum number of segments that can be accommodated in a ring is five plus key. However, to reduce the risk of flexural failure with steel fibre reinforced segments, a minimum number of segments in a 6m internal diameter tunnel would be seven plus key.
In the past most tunnel linings have been rectangular in plan with the top and key segments having radial joints which are inclined to the long axis. But now trapezoidal segments for smaller tunnels and rhomboidal/trapezoidal segments for larger tunnels are being more commonly used.
As the name implies, trapezoidal tunnel linings have trapezoidal shaped segments. There is some evidence that trapezoidal segments are easier to erect and assist in ensuring that the ring is built circular prior to grouting and prevent cruciform joints even without ring roll. However, the geometry of the segments requires greater space for erection and longer jacks.
Both radial and circumferential joints have a chamfer/rebate along the edges and corners of the segments. The chamfer and the rebate prevent edge contact due to manufacturing and build tolerances, which could result in spalling during shoving, ring erection and subsequent loading of the lining. A thin self-adhesive packing is used between segments on the circumferential joints which assists in smoothing out any irregularities caused by lack of fit of segments and to spread and assist in absorbing propulsion ram loads.
Bolts, dowels and gaskets
The purpose of bolts in a segmental lining is only to provide temporary support until the grout annulus hardens. The bolts are required in the short term to keep the radial and circumferential joint gasket compressed when the TBM ram is released. They also assist in supporting the segment weight whilst a TBM ram is removed to install the next segment.
In the 1980s curved bolts were favoured as they required smaller pockets in the concrete segments. But with the reliability of plastic cast in sockets spear bolts have become more commonly used in linings of 200mm thickness and greater. The advantage of spear bolts is that they only require a single pocket although the pocket size is larger than for a curved bolt.
The bolts are placed in oversized bolt holes allowing for certain amount of construction tolerance and movement at the joints. The design of bolt fittings in enlarged holes is a balance between allowing enough movement and at the same time providing sufficient fixity to achieve a circular tunnel within given tolerances with the help of TBM thrust rams and the surrounding grout annulus.
Dowels are very often proposed for circumferential joint connections. Unlike bolts, dowels require no pockets. Dowels act as locating points when segments are brought together and they also prevent lipping between circumferential joints. However, the dowels require the thrust force from the TBM to push the segment into position as they snugly fit within the segment to provide shear and tensile resistance to keep the gaskets compressed when the TBM rams are retracted to install the next segment. However, dowels cannot be re-used like bolts.
To keep the tunnel watertight in tunnels constructed below the water table and in permeable ground, gaskets are provided in grooves close to the extrados of the tunnel lining. Gaskets girdle the segments and are installed in the segment manufacturing facility before the segments are taken underground. One of the most recent developments in segment manufacture is to include the installation of the gasket by incorporating it in the segment moulds.
Conclusion
These requirements will also have to be weighed against other factors such as local skills and materials and the possible re-use of equipment and plant which may limit options for the segment design. In future, the use of fibre reinforcement for tunnel lining manufacture is likely to become more widespread as is the use of multiple lifting points to support larger segments.
It is assumed that the edges projecting beyond the vacuum lifter act as a cantilever. The segments are checked for shear and bending capacity Figure 1, options of lifting by grabs and vacuum pads. Vacuum lifting is the most gentle, as with grab lifting loads are less uniformly spread Figure 2, battens in line for segment stacking Segments supported on frames while curing to full strength Segment transportation in the tunnel showing cars and locomotive Figure 3, segment dimensions and mould
