TYPES OF CENTERS AND MOLDS EMPLOYED IN CONSTRUCTING TUNNEL LININGS OF
MASONRY.
The masonry lining of a tunnel may be described as consisting of two or
more segments of circular arches combined so as to form a continuous
solid ring of masonry. To direct the operations of the masons in
constructing this masonry ring, templates or patterns are provided which
show the exact dimensions and form of the sectional profile which it is
desired to secure. These patterns or templates will vary in number and
construction with the form of lining and the method of excavation
adopted. Where the excavation is fully lined on all four sides, the
masonry work is usually divided into three parts,--the invert or floor
masonry, the side-wall masonry, and the roof-arch masonry. At least one
separate pattern has to be employed in constructing each of these parts
of the lining; and they are known respectively as ground molds, leading
frames, and arch centers, or simply centers. In the following paragraphs
the form and construction usually employed for each of these three kinds
of patterns is described.
=Ground Molds.=--Ground molds are employed in building the tunnel
invert. They are generally constructed of 3-inch plank cut exactly to
the form and dimensions of the invert masonry as shown in Fig. 38. To
permit of convenience of handling in a restricted space, they are
generally made in two parts, which are joined at the middle by means of
iron fish-plates and bolts. Either one or two ground molds may be
employed. Where two molds are used they are set up a short distance
apart, and cords are stretched from one to the other parallel to the
axis of the tunnel, by which the masons are guided in their work.
Extreme care has to be taken in setting the molds to ensure that they
are fixed at the proper grade, and are in a plane normal to the axis of
the tunnel. Where only one ground mold is employed, the finished masonry
is depended upon to supply the place of the second mold, cords being
stretched from it to the single mold placed the requisite distance
ahead. The leveling and centering of the molds is accomplished by means
of transit and level.
[Illustration: FIG. 38.--Ground Mold for Constructing Tunnel Invert
Masonry.]
[Illustration: FIG. 39.--Combined Ground Mold and Leading Frame for
Invert and Side Wall Masonry.]
Two modifications of the form of ground mold shown by Fig. 39 are
employed. The first modification is peculiar to the English method of
excavation, and consists in combining the ground mold with the leading
frame for the lower part of the side walls, as shown by Fig. 39. The
second modification is employed where the two halves or sides of the
invert are built separately, and it consists simply in using one-half of
the mold shown by Fig. 38. When the last method of constructing the
invert masonry is resorted to, extreme care has to be observed in
setting the half-mold in order to avoid error.
[Illustration: FIG. 40.--Leading Frame for Constructing Side Wall
Masonry.]
=Leading Frames.=--As already stated, leading frames are the patterns,
or molds, used in building the side walls of the lining. Like the ground
mold they are usually built of plank; one side being cut to the curve of
the profile, and the other being made parallel to the vertical axis of
the tunnel section. The vertical side usually has some arrangement by
which a plumb bob can be attached, as shown by Fig. 40, to guide the
workmen in erecting the frame. The combined leading frame and ground
mold shown in Fig. 39 has already been described. The use of this frame
is possible only where the masonry is begun at the invert and carried up
on each side at the same time. This mode of construction is peculiar to
the English method of tunneling; in all other methods the form of
separate ground frame shown by Fig. 40 is employed. The ground frames
are lined in and leveled up by transit and level; and, as in setting the
ground frames, care must be taken to secure accuracy in both direction
and elevation.
=Arch Centers.=--The template or form upon which the roof arch is built
is called a center. Unlike the ground molds and leading frames, the arch
centers have to support the weight of the masonry and the roof pressures
during the construction of the lining, and they, therefore, require to
be made strong. Owing to the fact that the pressures are indeterminate,
it is impossible to design a rational center, and resort is had to those
constructions which past experience has shown to work satisfactorily
under similar conditions. In a general way it can always be assumed that
the construction should be as simple as possible, that the center should
be so designed that it can be set up and removed with the least possible
labor, and that the different pieces of the framework and lagging should
be as short as possible, for convenience in handling.
Tunnel centers are usually composed of two parts,--a mold or curved
surface upon which the masonry rests, and a framework which supports the
mold. The curved surface or mold consists of a lagging of narrow boards
running parallel to the tunnel axis, which rests upon the arched top
members of two or more consecutive supporting frames. The supporting
frame is built in the form of a truss, and must be made strong enough to
withstand the heavy superimposed loads, consisting of the arch masonry
during construction, and of the roof pressures which are transferred to
the center when the strutting is removed to allow the masonry to be
placed. The framework of the center is supported either by posts resting
upon the floor of the excavation, or upon the invert masonry when this
is built first, as in the English and Austrian methods, or it may be
supported directly upon the ground where the arch masonry is built
first, as in the Belgian method of tunneling.
In describing the various methods of tunneling in succeeding chapters,
the center construction and method of supporting the center peculiar to
each will be fully explained, and only a few general remarks are
necessary here. Centers may be classified according to their
construction and composition into plank centers, truss centers, and iron
centers.
[Illustration: FIG. 41.--Plank Center for Constructing the Roof Arch.]
One of the most common forms of plank centers is shown by Fig. 41. It
consists of two half-polygons whose sides consist of 15 in. × 4 ft.
planks having radial end-joints. These two half-polygons are laid one
upon the other so that they break joints, as shown by the figure, and
the extrados of the frame is cut to the true curve of the roof arch. The
planks commonly used for making these centers are 4 ins. thick, making
the total thickness of the center 8 ins. Plank centers of the
construction described are suitable only for work where the pressures to
be resisted are small, as in tunnels through a fairly firm rock,
although there have been instances of their successful use in
soft-ground tunnels.
Where heavy loads have to be carried, trussed centers are generally
employed, the trusses being composed of heavy square beams with scarfed
and tenoned joints, reinforced by iron plates. Different forms of
trusses are employed in each of the different methods of tunneling, and
each of these is described in succeeding chapters; but they are
generally either of the king-post or queen-post type, or some
modification of them. The king-post truss may be used alone, with or
without the tie-beam, as shown by Fig. 42; but generally a queen-post
truss is made to form the base of support for a smaller king-post truss
mounted on its top. This arrangement gives a trapezoidal form to the
center, which approaches closely to the arch profile. Owing to the
character of the pressures transmitted to the center, the usual tension
members can be made very light.
[Illustration: FIG. 42.--Trussed Center for Constructing the Roof Arch.]
The combined center and strutting system devised by Mr. Rziha has
already been described in a previous chapter. In recent European tunnel
work quite extensive use has also been made of iron centers consisting
of several segments of curved I-beams, connected by fish-plate joints so
as to form a semi-circular arch rib. The ends or feet of these I-beam
ribs have bearing-plates or shoes made by riveting angles to the
I-beams. Centers constructed in a similar manner, but made of sections
of old railway rail, were used in carrying out the tunnel work on the
Rhine River Railroad in Germany. The advantages claimed for iron centers
are that they take up less room, and that they can be used over and over
again.
_Setting Up Centers._--According to the method of excavation followed in
building the tunnel, the centers for building the roof arch may have to
be supported by posts resting on the tunnel floor; or where the arch is
built first, as in the Belgian and Italian methods, they may be carried
on blocking resting on the unexcavated earth below. Whichever method is
employed, an unyielding support is essential, and care must be taken
that the centers are erected and maintained in a plane normal to the
tunnel axis. To prevent deflection and twisting, the consecutive centers
are usually braced together by longitudinal struts or by braces running
to the adjacent strutting. Only skilled and experienced workmen should
be employed in erecting the centers; and they should work under the
immediate direction of the engineer, who must establish the axis and
level of each center by transit and level.
_Lagging._--By the lagging is meant the covering of narrow longitudinal
boards resting upon the upper curved chords of the centers, and spanning
the opening between consecutive centers. This lagging forms the curved
surface or mold upon which the arch masonry is laid. When the roof arch
is of ashlar masonry the strips of lagging are seldom placed nearer
together than the joints of the consecutive ring stones, but in brick
arches they are laid close together. Besides the weight of the arch
masonry, the lagging timbers support the short props which keep the
poling-boards in place after the strutting is removed and until the arch
masonry is completed.
_Striking the Centers._--The centers are usually brought to the proper
elevation by means of wooden wedges inserted between the sill of the
center and its support, or between the bottom of the posts carrying the
center and the tunnel floor. These wedges are usually made of hard wood,
and are about 6 ins. wide by 4 ins. thick by 18 ins. long. To strike the
center after the arch masonry is completed, these wedges are withdrawn,
thus allowing the center to fall clear of the masonry. Usually the
center is not removed immediately after striking, so that if the arch
masonry fails the ruins will remain upon the center. The method of
striking the iron center devised by Mr. Rziha has been described in the
previous chapter on strutting.