January/February 2023 - 15

To properly select and design a retention system for both
projects, the soil borings and the geology of the region were
first examined. Rochester is in Olmsted County and is unique
as it is one of the only areas in the state of Minnesota that did
not experience glaciation. The soil in the area is primarily
granular alluvium and terrace deposits overlying shallow
limestone bedrock 10-40 ft (3-12 m) from the surface. The
irregular bedrock surface is primarily due to erosion
associated with glacial melt and the Zumbro River, which
flows through the city approximately three blocks from the
site. The rock strength ranges from 10-14 ksi (69-97 MPa) and
is extremely weathered (residual rock) to slightly weathered.
The bedrock posed a challenge for the construction project
as excavation was required to extend into the bedrock, but the
competence and elevation could not be well defined.
Excavation for the project required a 100% shored design as
the excavation limits were not allowed to disturb adjacent
walkways, roadways or buildings. The variability of the rock
elevation required an agile design, and the presence of
numerous existing tunnels and utilities required the design to
minimize the number of tiebacks extending into the adjacent
right of way.
Tunnel Design Improvements
The first phase of the project included three new tunnel
systems for the expansion of the Mayo campus and was
awarded as a design-build construction project. The rectangular
tunnels varied in diameter with the largest at 21 ft (6 m)
wide and 25 ft (8m) tall. Their construction required
approximately 1,000 lft (305 m) of tunnel, but the joint venture
construction teams proposed a stacked tunnel approach,
which limited excavation requirements and minimized
retention. To complete this excavation, a retention system
Since January 2022, Veit
& Company has been
working on the
construction of the Mayo
1st Ave Tunnel, which
comprises three tunnels
(pedestrian, utility and
services), and the RMC
Loading Dock projects in
Rochester.
DEEP FOUNDATIONS * JAN/FEB 2023 * 15
needed to be designed to allow up to a 30 ft (9 m) deep
excavation along the operating hospital, road and existing
tunnels, and to allow all vehicle and pedestrian traffic access
to the Mayo facilities while handling the geotechnical and
geometric constraints.
To accomplish this feat, the contractor designed a shoring
system that was comprised of H-beam and lagging, soil
anchors, rock bolts, micropiles, helical piles, soil nails and
shotcrete, conventional underpinning, and soil solidification.
As the project went from conceptual to construction
design, the initial design was optimized to accommodate
differing site conditions, removal of a building vestibule and a
storage building, and the owner's schedule. In this process
the contractor was able to eliminate soil nail shotcrete to
reduce schedule impacts due to cure time and cold weather
conditions during installation. This was replaced with
conventional underpinning and H-Beam and lagging. By
eliminating the vestibule, micropiles were no longer
necessary to support the footings during construction, which
resulted in schedule and time savings for the owner.
Advantages of Predrilling
H-beam and lagging, both cantilevered and tied back, were
the primary retention systems designed for the project. Due to
the variability of rock and the limited number of borings
available, the contractor elected to predrill each location and
document the elevation of the rock. This allowed for vibration
reduction and design modifications as needed due to soft rock
or rock elevation changes. In order to accomplish the
predrilling into the rock as well as expedite construction, a
CZM200 with a rock auger and continuous flight auger kit
(CFA) was used. The drill rig was capable of drilling through
the bedrock up to 15 ft (5 m) in CFA mode, which proved to be
A temporary bridge providing pedestrian access through the campus
January/February 2023

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