Civil Engineering Reference
In-Depth Information
Figure 5.78
One of the jacked box tunnels during the jacking process, note the
cover depth above the tunnel and rail road lines at the ground
surface (courtesy of Massachusetts Turnpike Authority)
Three concrete jacking pits were constructed as part of the jacking
operations. In each of these jacking pits tunnel box sections approximately
24 m (80 ft) wide and 12 m (40 ft) high were constructed. The lengths of
the three tunnels were approximately 50 m (150 ft), 79.2 m (260 ft) and
115 m (380 ft) respectively. The two shorter tunnels were constructed in
two section lengths and the longest in three sections. Intermediate jacking
stations were positioned between each box section, similar to pipe jacking
operations, with the main thrust jacks at the rear of the last box. This
allowed each of the boxes to be jacked forwards in sequence thus reducing
the thrust required by the main jacks (the intermediate jacks acted as
intermediate jacking stations as described in pipe jacking, section 5.11).
The sections were jacked at a rate of between one and two metres (three
and six feet) per day. Figure 5.78 shows one of the jacked box tunnels
during the jacking process.
The ground conditions consisted of Boston Blue Clay, sands, gravels and
organic material, and had to be stabilized prior to excavation. In order to
increase the stability of the ground during the tunnel jacking operation,
and hence reduce the resulting ground movements, ground freezing was
utilized on this project (see section 4.2.1 for more information on ground
freezing). The freezing process used a closed system of re-circulating brine
as the coolant. The brine was passed down vertical freeze tubes into the
ground from the ground surface (Figure 5.79).
