September 21 2005: Removing Stonehenge
The Problem: There are a number of supporting columns linking the
Town Creek and Cooper River segments of the Pearman Bridge. How does
one remove these?>
The problem can be readily seen here: a group of columns and caps that
have to disappear.
Rather imposing when viewed from Drum Island: looking east (left)
and looking west (right)
The idea dropping one of these concrete support structures
is similar to dropping a tree. The structural integrity of the lower
section of each vertical column is first compromised by chipping
away at the base (which has been previously marked -
Holes drilled into the inner face of each column that accepts
an explosive charge that, when ignited, will push the structure in the
direction of the compromised base.
Practice makes perfect. This particular support will serve
two purposes. Dropping it vertically will be a test of the strategy used
to drop supports in the river. In addition, dropping it vertically
will provide a way to minimize the domino effect -
if all the segments fall in the same direction there will be
a possible domino effect which each structure leaning up against
its neighbor. To reduce the risk the middle structures is dropped
vertically by placing the explosive charges and then timing the
ignition to simply collapse the H-shaped concrete support.
The explosive charges for the east (to the right) and west (to the left)
H-supports supports are placed so that the strucures will fall toward this
center support. Here you can note the holes drilled up the entire length of
each vertical column of the center H-support as shown here from
Drum Island and from the Grace Bridge
To get to the cap on the top of the columns,
a looooooong man basket is used
Here is a closer view of holes being drilled into the top cap of the
central support (that will be vertically dropped)
