January/February 2023 - 17

To accommodate the truck traffic to the loading dock, a
bridge was designed into the retention system. The bridge
utilized the retention beams drilled into the rock. These
beams were drilled and dropped because the owner
requested as much of the retention system be removed as
possible to limit the impact on future projects for both the
owner and the city.
A temporary pedestrian bridge and multiple emergency
egresses were constructed for pedestrian safety over the
tunnel excavation and through the job site, and a separate
construction bridge was constructed across the southeast
tunnel to allow construction crews' access.
The construction site had more than a dozen active
utilities that needed to be supported and remain in use
including gas, water main, sanitary, fiber and more. Many of
these utilities dated back to the early 1900s, creating a delicate
pathway for beams and tiebacks around the brittle and
unstable lines. Beams were then used to span the excavation
to hang and support the existing utilities. Based on the type of
utility encountered, the contractor adjusted the support type,
frequency and materials so as to not damage the utility.
In order to support the existing, 18 ft (5 m) wide by 11 ft (3m)
tall concrete pedestrian tunnel that connected the parking
ramp across the street with the Mayo campus, support beams
were installed, and w-sections were used to span the
excavation. From the w-sections, cables were extended below
the tunnel and tensioned. This allowed for the system to hang
the tunnel and allow excavation to take place below the
tunnel. To limit the extent of undermining the tunnel, soil
grouting was performed along the retaining wall alignment.
As excavation entered rock, a rock mill was utilized to
reduce vibration and limit the disturbance to the retention
beams as some were socketed into rock while others were
anchored on top of bedrock.
Loading Dock Work
While the excavation for the tunnel project started in the first
phase of construction, the second phase of construction for
the loading docks was put out to bid by the owner. This second
phase was located adjacent to phase one's tunnel system and
connected to the service tunnel. The project consisted of a new
26,000 sq ft (2,415 sq m) " atypical " loading dock facility, which
would replace the existing loading dock once construction of
the Proton building started. The new loading dock will service
all deliveries coming in and out of the Mayo Clinic.
To accommodate the new construction on the loading dock
project into the first phase of construction for the tunnels, the
original retention system needed to be modified. Since the
excavation for the loading dock project tied into the tunnel
excavation, the earth retention designs for both projects had
to be integrated to allow access. This required an additional
The pedestrian and service tunnel system during construction
pedestrian bridge to keep access to the existing facilities and
to the city streets, as well as a connecting corridor joining the
phase 1 and phase 2 construction.
As with most construction projects, unforeseen conditions
were encountered in numerous locations. As noted
previously, rock quality was variable across the site. Though
the predrilling gave good information about the rock
elevation, it did not always accurately portray the rock quality.
When rock quality was notably degraded or in question, rock
bolts and mesh were utilized to secure the rock. In some areas
the rock socket proved to be in question, this required a poured
anchor thrust block or an additional waler added.
With the limited soil borings, water elevations were not
clearly documented. During excavation, water was found to be
flowing over the top of bedrock in multiple locations. The
water appeared to be flowing in natural channels in the rock.
These channels intersected the excavation at varying
elevations, which made controlling the water a challenge.
Multiple sumps were installed to dewater the excavation for
the retention crews and subsequent concrete tunnel crews.
These sumps were connected by a drain tile excavated into the
rock outside the footing. This allowed for centralized pumping
of water and allowed water to flow along the excavation
without impacting the concrete placement or access.
DEEP FOUNDATIONS * JAN/FEB 2023 * 17
January/February 2023

Table of Contents for the Digital Edition of January/February 2023
