July/August 2023 - 104
Project Specifics
The original project specifications
Over the years, the city has spent more
than $20 million in flood control work,
including a $2.9 million contract to
construct the Harbor Brook Overflow
Culvert, a more economical alternative
than attempting to raise the bridge. The
culvert system was designed and
constructed to carry water overflow
from Harbor Brook under the Amtrak
rails and discharge it back into Harbor
Brook downstream. All project phases
included two high-density polyethylene
(HDPE) pipe overflow culverts from
upstream of the Amtrak bridge, to
downstream of the bridge; steel casing
pipes jacked under the Amtrak railroad
and grouting in place; HDPE water
carrier pipes sleeved within the steel
casing pipes and grouting in place; a concrete
junction chamber and construction
of headwalls plus reconstruction of
the parking lot in which the pipe culvert
and jacking pit are located. The focus of
this paper is on the steel casing pipes
jacked under the Amtrak rails.
To install the 73.5 in (1.8 m) outside
diameter steel casings under the rail
tracks, the pipe jacking method was
selected. Pipe jacking is a trenchless
technique for installing underground
pipelines such as utility tunnels or
culverts using hydraulic jacks that push
the pipes through the ground behind a
shield, excavating the face of the pipe
back through the pipe being installed.
Pipe jacking would allow the
continued use of the active Amtrak rail
lines during construction. The twin
steel casings would be installed at a
60-degree skew regarding the rail
configuration, in 10 ft (3 m) lengths, and
would be welded together for a total of
50 ft (15 m) casing. There would be 3 ft
(1 m) spacing between the twin steel
casings. However, before pipe jacking
could occur, site conditions and soil
reports dictated the loose, sandy soils
beneath the rails had to be stabilized
prior to installation.
called for injection of sodium silicates to
stabilize the soil ahead of the steel
casings. However, as a portion of the pipe
system would be located below the
water table, and in situ soil fines content
was high, utilizing sodium caused
concern. Sodium silicates will often
Pipe jacking is a trenchless
technique for installing
underground pipelines
such as utility tunnels or
culverts using hydraulic
jacks that push the pipes
through the ground behind
a shield, excavating the
face of the pipe back
through the pipe being
installed.
expunge water after gelling by a process
called syneresis. Sodium silicates can be
sensitive, bordering on unstable, when
injected into any groundwater condition.
With relatively short gel times, sodium
silicates are commonly used as a
temporary solution for water control or
structural support with an estimated life
span of a few years. Site conditions
called for a product that was used for
groundwater control and soil stabilization,
extremely thin so it could penetrate
the higher soil fines content and would
not cure or harden too stiff that there
would be difficulty hand mining ahead
of the steel casing. After further assessment
of product alternatives, AV-100
acrylamide grout was chosen for its
long-term design life, low viscosity, soil
stabilization design and the ability to
easily control material set times.
Stabilizing Soils with
Injection Grout
Acrylamide injection grout is a chemically
activated, ultra-low viscosity
(1-2 cP at 72°F [22°C] in solution)
system designed to stabilize soils to
create a long-lasting water barrier
when combined with soils. Acrylamide
grout is a dual-component
system: first, acrylamide is mixed in
one tank with an activator (triethanolamine)
and water, then an initiator
(ammonium persulfate) for the radical
polymerization reaction of acrylamide
monomers is mixed with water in a
separate 30 gal (115 L) tank. When the
components of the two tanks are
pumped and mixed at a one-to-one
ratio, the reaction begins immediately,
turning from a viscous liquid to a cured
gel form in seconds to hours if
necessary. In its neat form (cured
acrylamide mix only) the outcome is a
firm but soft gel that can be broken
apart. When injected into external soil
substrate, the cured grout will produce
a strong, impermeable gel/soil matrix
obtained via a copolymerization
reaction. Acrylamide grout has no
suspended solids, higher strength
compared to acylates and greater
longevity compared to silicates.
Acrylamide grout at a 10% concentration
has a life span of 25-50 years or
more when properly injected, whereas
sodium silicate's lifecycle typically
spans 2-8 years. Generally, a 10% concentration
grout mix design is used
for injection grout, however, mix
design can be altered based on gel
times needed, tank sizes, concentration
desired and additional additive
options.
104 * DEEP FOUNDATIONS * JULY/AUG 2023
July/August 2023
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