Civil Engineering Reference
In-Depth Information
Dowel shear keys
Dowels cast into the second stage infill concrete placed above the immer-
sion joint can offer a suitable shear connection. This is a solution that
was originally used in tunnels built in the Netherlands. The dowels are
commonly steel circular hollow sections filled with concrete that are fully
embedded into one side of the immersion joint, but sit within a sleeve on
the opposite side, which allows movement and joint rotation to occur. The
arrangement is shown in Figure 9.18.
This type of shear connection has a limited shear capacity compared to
discrete shear keys in the tunnel walls. The reason for this is the limited
depth of concrete in which the dowels can be located. It is normal for
the ballast concrete to be replaced by structural concrete over the joint to
provide a greater depth, but even with this, there is limited room above
and below the dowel. Dowel diameters are typically 200-250 mm and
the depth of concrete will be in the order of 800-1500, depending on the
depth of ballast concrete required in the tunnel. At the lower end of this
range, there remains only 250-300 mm of concrete above and below the
shear dowels in which to provide the shear resistance. As the shear trans-
fer is concentrated at the dowels, loads can be high and the shear stresses
can demand high levels of reinforcement to prevent shear failure of the
concrete. Because the dowels need a reasonable depth of concrete in which
to be fixed, the only place they can be located is in the base of the tunnel
within the ballast concrete depth. This limits the shear capacity of the joint
as a whole.
Joint infill concrete
Joint expansion gap
Ballast concrete
Socket
Base slab
Shear dowel
Immersion joint
Figure 9.18
Dowel type of shear key.
