“Salt and Pepper” Tower Reinstallation Process Explained
August 2015 Update
After careful historical and engineering review, the
Massachusetts Department of Transportation (MassDOT)
developed a plan to dismantle, clean, restore, and reset the
granite blocks of the iconic “Salt and Pepper” towers of the
Longfellow Bridge. Over the past century, the towers have
settled, requiring the dismantling and re-setting. The plan will
safely maintain the blocks’ integrity during the process. Each
58-foot tower has 515 pieces of granite, which vary in size and
can weigh as much as three tons.
The restoration process began in March 2014 with the
dismantling of the upstream towers, followed by the downstream
towers in July 2015. The process involved dismantling the
Removal of the Granite Top of
towers, demolishing the interior concrete lining, and creating a
the Pier 5 Tower – May 2014
new support structure to hold up the new, level concrete slab on
which the towers will be reinstalled. With the completion of the preparatory work and new
concrete road decks placed to provide access to this
part of the bridge, the rebuild of the towers can begin.
Before the granite can be reinstalled, epoxy coated
reinforcing steel is formed into a cage that will
become part of the new concrete liner walls (see
photo at left). This will define the interior of the tower.
Reinforcing Steel Cage Cambridge (Pier 6)
Tower – August 2015
The restored granite blocks will then be loaded onto a
barge at the storage location and floated to the
jobsite. Several sets of blocks for both upstream pier
towers will be brought to the bridge at one time.
A separate barge will carry a crane for lifting the
blocks into place. It will also provide an area to mix
sand, cement, lime, and miscellaneous materials,
so that mortar can be made close to the work area.
After many measurements are taken and locations
for the blocks are confirmed, the first row is lifted
from the barge to the pier deck using Lewis Pins
(see photo at right). They will be set in a mortar bed
with shims to ensure they are the proper height.
This first step is critical to ensure the proper
alignment and placement of all the blocks above.
Granite Blocks with Lewis Pins – July 2015
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There are 17 blocks in the first row and 19 in the second. Each row has a different number of
pieces – as many as 22 or as few as eight. When the location has been confirmed, the Lewis
Pins are removed and the block is in its final location. Any excess mortar is removed before it
sets, so that it does not adhere to the exterior face of the granite.
During the restoration of the granite,
a hole was drilled in the back of
each block for a dowel to be
installed to meet seismic
requirements. After the piece is set
in its final placement, the dowel is
installed in the hole. Records on the
setting sequence will be kept by the
masons doing the installation.
For this first part of the
reinstallation, the masons will install
three rows of blocks at the
Cambridge (Pier 6) upstream tower.
While the masons are working at
Pier 6, the iron workers will build the
Before and after images of the granite door posts of the
reinforcing steel cage at the Boston
Pier 6 tower.
(Pier 5) upstream tower. Once the
masons complete the three rows at the Pier 6 tower, they will then repeat the process on the
Pier 5 tower.
Meanwhile, back at Pier 6, wood will be installed on the interior side of the rebar cage of the
newly installed granite to help form the liner wall when the concrete is poured. A pump truck
will deliver the concrete to the bridge and will pour the concrete to a predetermined height
below the top of the granite. The concrete will tie together the granite with the newly installed
dowels and the steel to make a reinforced wall that can withstand seismic shifts.
The masons, iron workers, carpenters, and laborers will continue to go back and forth
between the two piers to maintain progress. This also allows the mortar and concrete time to
cure before starting the row above. The process is repeated 14 times to install 34 rows of
blocks at each tower, two or three rows at a time. The granite is always higher than the
concrete, because it acts as the outside wall of the concrete form.
Completion of the upstream tower rebuild is anticipated for the end of 2015, but is weather
dependent. The downstream towers will be rebuilt in a later phase of the project.
If you have any questions or would like to know more, please contact the project team at
longfellowbridge@state.ma.us or 617-519-9892.
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