Diaphragm Walls Constructed by Hydromill Technology for the
Foundations of The Public Safety Answering Center II, (PSAC II) Bronx,
New York, U.S.A.
THE BRONX - This past fall of 2010, Bencor Corporation
installed the reinforced concrete diaphragm walls
for the Public Safety Answering Center II (PSAC II). The
PSAC II development, being undertaken by the NYC
Department of Design and Construction, consists of a
ten-story office building with one cellar level, an earthen
berm around the entire building perimeter, site retaining
walls, a myriad of site utilities to service the new building
including a buried storm water detention system all on an
8 acre site.
The diaphragm walls were designed by Langan Engineering
and Weidlinger Associates, Inc. They serve as
both a temporary excavation support system and a building
foundation wall. The diaphragm wall will support
the heavy perimeter line loads of the building, and will
provide lateral load carrying capacity. The building has
a 231-foot by 231-foot square shape in plan dimension,
and 10 stories in height. It consists of cast-in-place perimeter
walls, and an interior steel-framed structure, with
concrete on metal deck floors. The diaphragm wall will
also provide an effective groundwater cut-off. To achieve
these criteria, the walls were designed at 36 inches thick,
and the walls are keyed at least 3 feet into the bedrock.
The total square footage for the designed slurry walls is
88,000 square feet.
The site geology is underlain by glacial deposits formed
during the retreat of the Wisconsinian glaciations over
metamorphic bedrock. The bedrock at the site is represented
by the Precambrian-Cambrian gneiss and schist,
which are intermixed both vertically and horizontally.
Other surficial materials consist of unconsolidated silt
and peat at the bottom of the former glacial lakes, which
sometimes form the marsh deposits. The top of bedrock
at the building footprint varies greatly from -35 feet to
-75 feet below ground surface. Bedrock core strengths at
the site range from 7,790 to 19,260 psi, with an average
of close to 15,000 psi, indicating generally a very high
strength for this rock.
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Due to the very hard rock conditions at the site, coupled
with an aggressive installation schedule mandated by the
Owner, Bencor determined that the most effective and efficient
manner to install these diaphragm walls would be
by using hydromill technology. With the help of Bauer-
Pileco, Bencor was able to mobilize to the site a new cutter
system and carrier from Germany.
Hydromill technology for the excavation of slurry wall
trenches was developed in the 1970’s. It is a continuous
excavation procedure by the reverse circulation method.
The material is cut and loosened with two cutting wheels
mounted at the bottom of a steel frame and rotating in opposite
directions. The cutting wheels are equipped with
various types of teeth depending on the type of ground
being excavated. The loosened or cut material mixes into
the bentonite slurry suspension which is pumped from the
trench by a very powerful suction pump mounted on the
hydromill and conveyed to a processing plant for cleaning.
The processing plant consists of screens, sieves and
cyclones of various sizes which are able to screen out all
of the coarse and fine material (cuttings) from the slurry
and then subsequently pump the cleaned suspension back
to the trench.
The equipment utilized for the installation of
the diaphragm walls consists of the following:
1) Foundation crane fitted with Bauer BC-40
Cutter and Bauer HTS60.
2) Foundation crane fitted with Leffer
Diaphragm Wall Grab SWG 3,2-6/915.
3) Sotres 450-300 Desander.
4) MAT SKC-30-K Slurry Mixing Plant.
5) Service crane.
6) Manitowoc 777 – service crane.
Milling technology offers abundant advantages to conventional
slurry wall techniques by centralizing the plant
and disposal of excavated material at one location on the
site. Moreover, the hydromill offers a greater rate of penetration
for the excavation than most any other type of
slurry wall technique, vibration free, especially when the
wall must penetrate hard ground and rock. The hydromill
also offers the highest level of accuracy in verticality
through it’s on board telemetry as well as the fact that
the quality of the slurry is always maintained through the
beauty of reverse circulation through the desander plant.
Ultimately, when properly maintained, the hydromill can
provide scheduling advantages by out-producing any other
technique of slurry wall excavation.
Trench Cutter BC-40:
The trench cutter is an excavating machine that operates
on the principles of reverse circulation. It is made up of a
heavy steel frame (1) to the bottom of which are mounted
two gear boxes (2). Cutting wheel drums fitted with a series
of teeth are fixed to the gear boxes; they rotate in
opposite directions, break up the soil and mix it with the
bentonite suspension (3). As the cutter penetrates, soil,
rock and bentonite are conveyed towards the openings of
the suction box (4), from where they are pumped by a
powerful centrifugal pump (5), located right above the
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cutter wheels, through the slurry pipe incorporated in the
cutter’s frame and back to the desanding plant.
The torque output of the cutter wheels in combination
with the weight of the cutter, approximately 43 tons, is
sufficient to cut into any type of soil and to crush cobbles,
small boulders, rock, and high strength concrete. Depending
on the soil conditions, different types of cutting
teeth can be deployed, ranging from aggressive teeth for
cutting fine-grained soil to percussive teeth for crushing
boulders. In order to protect the cutter’s gear boxes from
excessive dynamic forces when cutting rock and stones,
elastic shock absorbers are located between the cutting
wheel drums and the gear boxes.
The verticality of the trench cutter and thus the trench
alignment are generally measured on two axis by means
of two independent inclinometers (6): the “x”-axis, normal
to the trench alignment and the perpendicular “y”-axis.
Data provided by these inclinometers is processed by
the on-board computer system and can then be displayed
and or transmitted in real-time on-line. This on-board telemetry
allows the operator to monitor the cutter’s progress
continuously and if needed make corrections to the
cutter’s verticality. If so needed, the operator can make
adjustments to the cutter’s verticality in both directions
by utilizing the steering plates built into the cutter’s frame
(7). Through the excavation process the cutter operator is
continually prompted by the machine’s software which
calculates the cutter’s status and indicates the most appropriate
action to take. All information can be downloaded
on a panel report that can be printed after completion of
each panel and used for QA/QC purposes.
Circulation And Desanding Equipment:
Bentonite slurry is required to stabilize the trench. In addition,
when working with the trench cutter, the slurry
is used to convey excavated materials out of the trench.
Slurry laden with cuttings is pumped back to the desanding
plant from the cutter, where the solid content of the
slurry is separated from the liquid and then pumped back
to the trench cutter.
At the PSAC site, the primary components consist of:
1) The mixing plant: The MAT SKC-30-K mixing plant
was utilized on this project, comprising of an efficient
mixing unit which is fed bentonite powder from a silo
storage tank and mixes it with water and then pumps it
into a holding/hydration pond where the slurry is kept
in motion and aerated in order to hydrate. After approximately
12 hours, the bentonite slurry has hydrated and
fully developed its properties of viscosity and thixotropy.
The hydrated bentonite slurry can then be transferred by
a pump to either the trench directly or to reservoir tanks
for future use.
2) The desanding unit: The Sotres 450-300 desanding
plant is comprised of three primary elements: (i) a coarse
screen separator (scalping unit) that removes all particles
larger than 5mm through a vibrating screen; (ii) two large
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hydrocyclones and vibrating drier screens which separate
from the slurry all particles down to 60 microns; and (iii)
a 10 cone desilter bank which removes all solids down to
35 microns. The desanded slurry is then stored, agitated,
and pumped in a 5 cell container system under the desander
unit from which the clean slurry is then pumped
back to the trench cutter.
3) The storage unit: On this particular project a series of
ponds was excavated and utilized for slurry storage. The
layout of the ponds is not as important as the total capacity
of the ponds in order to insure continuity in the work.
The volume of the storage ponds should be at least three
times the volume of one panel. On this project, there were
three storage ponds excavated, two large ponds, one for
fresh slurry and one for used slurry. These two ponds held
approximately 1000 cy each. A third smaller pond holding
approximately 100cy was utilized for fouled slurry
ready for disposal.
4) The conveying unit: The conveying unit is made up
of a series of pumps, pipes, valves and controls designed
to facilitate conveying bentonite to and from the trench.
Generally, the pipelines utilized are all 6 inch ID HDPE
pipe, which convey slurry to and from the field for the
hydromill, clamshell, and the concrete pours, and a fresh
water supply line.
It is important to note that one of the major advantages of
excavation by hydromill technology is that once the panel
excavation is complete and the bottom has been verified,
the bentonite slurry will also be clean and meet the rigorous
standards for concrete placement. This saves time
over conventional methods of airlifting and desanding.
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Due to the site being laden with man-made rubble fill, prior
to commencing slurry wall construction, the footprint
of the slurry wall was pre-cleared to a depth of 10 feet
and backfilled with a self-hardening flowable fill mix.
On top of this flow fill the cast in place guide walls were
constructed. The guide walls provide stability, protection,
and guidance for the slurry wall tools, as well as all lines
and grades are taken from the guide walls.
The typical panel excavation began with the clam shell
opening the panels through the overburden material. The
panel design on this project consisted of 25 foot primary
panels and 10 foot closing panels. Therefore, primary panels
comprised of two complete bites, and one half-bite, or
middle bite. Additionally, there were four corner panels
which were installed monolithically using the same threebite
methodology. Once the Leffer grab opened the panel
through the overburden and especially the peat and clay
layers, the clam shell would be moved to the next panel
and the hydromill would be moved into position. As a
general rule, some pre-excavation is always necessary
with hydromills, since the cutter’s mud pump is located
above the cutting wheels and in order to prime this pump
it must be fully submerged in the bentonite fluid. In addition
to pre-excavating for the hydromill, the grab also
served the critical role of pre-clearing large rubble and
boulders from the trench prior to the cutter’s introduction.
To ensure continuity of the diaphragm wall, joints between
successive primary panels are formed when excavating
the secondary panel trenches by overcutting into
the concreted primaries. The amount of overcut on this
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project was 6 inches into each primary panel, which is a
fairly typical distance. Therefore, the distance between
the edges of the adjacent primary panels is designed to
leave a clearance of 10’ 3” (3.2m) for excavation of the
secondary panel trench. This distance will include the 6
inch overcut into the concrete of the two adjacent primary
panels, resulting in grooved, roughened surface of
the primary panel concrete.
The verticality of the trench excavation is constantly measured
in the panel axis and perpendicular to the panel axis
by means of two independent inclinometer systems that
are mounted on the trench cutter. The B-Tronic system
records the inclination of the hydromill in the excavation
and correlates it with depth. Additionally, all of the vital
parameters of pressures and flows of oil and slurry are
measured, recorded and displayed. The onboard computer
then processes this information and displays the information
graphically on the monitor inside the operator’s
cabin. The information as displayed in real time on the
screen assists the operator in maintaining the verticality
of the trench excavation and making sure the cutter and
its systems are all functioning properly. Additionally, the
data is all recorded and stored and can be printed out as
a “verticality report” which will form part of the QA/QC
records for the operation. Also, the real time data which is
viewed in the operator’s cabin can be transmitted via the
internet and viewed in the office real time as the operation
progresses! Excavation tolerances with the hydromill adhered
to 0.5% verticality tolerances.
Installation of Reinforcement and Concreting:
The reinforcing steel cages were constructed on the jobsite
complete as one piece cages made of epoxy coated
rebar, including all blockouts for floor tie-ins with lenton
couplers, tieback sleeves, instrumentation pipes, and any
miscellaneous blockouts for future utility penetrations.
The largest cages weighed approximately 42 tons. Once
the panel excavation was completed, the reinforcing steel
cages would be lifted using two cranes in tandem. Once
vertical, the Manitowoc walked the cage to its panel and
slowly lowered the cage into the panel.
To ensure continuity of the diaphragm wall, joints
between successive primary panels are formed
when excavating the secondary panel trenches by
overcutting into the concreted primaries.
One crane would then follow and place the tremie pipes
in the panel to the bottom. On the primary panels we utilized
3 tremie pipes and on the closing panels 2 tremies
were utilized. The concrete redi-mix trucks would then
back up to the hoppers at the panel location and via gravity
the 5000 psi mix would be poured. Generally speaking
with normal delivery conditions, we were able to pour
between 80 to 100 cubic yards per hour. A total of ap-
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proximately 12,000 cy of tremie concrete was poured for
the slurry walls.
Quality Assurance/Quality Control:
A rigorous quality control program was employed by
Bencor to assure that all aspects of the diaphragm wall
installation were carried out at the highest quality and
care possible. From the onset of panel installation, Bencor
QC Engineers and Superintendents worked diligently
to ensure that excavation is carried forward to exact lines
and grades. It is instrumental in hydromill usage that the
panel jointing layout is closely followed and adhered to
so that the closing panels will be installed perfectly providing
the needed overlap for positive jointing.
As discussed previously, the hydromill is equipped with
state-of-the-art telemetry to insure a verticality of 0.5%
tolerance. Additionally, all panel excavations are verified
prior to reinforcing steel placement with the Koden ultrasonic
drilling monitor. The Koden is lowered through the
slurry bentonite and provides a print out reading of the
actual panel wall alignment. A Koden reading is taken of
each trench cutter bite as well as of the two end joints to
verify the panel’s exact layout.
The QA/QC Engineer’s tasks also include the monitoring
and regulating the preparation, maintenance and cleaning
of the slurry bentonite fluid so that it is continuously
kept at optimal working conditions during excavation and
concrete placement. This is critical to insure trench stability
and the quality of the finished wall as well as the panel
Finally, the fabrication of the reinforcing steel cages and
the pouring and monitoring of the concrete were critical
features also requiring the QA/QC Engineer’s attention.
The cages were constructed to very tight tolerances requiring
special attention to blockout placement and bar
layout. Additionally, close monitoring of the concrete
pours and the absorption rate of the concrete as the pour
progresses is critical to ensure continuity and quality of
the final wall.
The reinforcing steel cages were constructed on the jobsite complete as one piece cages made of epoxy
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The General Contractor, Urban Foundation and Engineering,
Elmhurst, NY, installed the 80 tiebacks on the
project. Prefabricated anchor blockouts were installed
in each slurry wall reinforcing steel cage to provide access
through the wall for drilling. Urban utilized its own
custom-built hydraulic drill rigs to drill and install the tiebacks.
The maximum design load for the anchors is 175
kips, and the anchor lengths are 85 feet. The type of anchors
being installed are 5 strand epoxy-coated anchors,
in lieu of the original design utilizing 1 7/8 inch bars.
Urban Foundation drilled 7 inch holes through the prefabricated
blockouts with a combination of rotary and
percussive techniques, advancing casing as the hole was
drilled, and flushing cuttings using air and water. Once
the hole was drilled to depth the steel strand tendons were
installed in the hole and then the hole was tremie grouted
using a 5000 psi grout mix. The holes would be regrouted
through the regrout tube on the tendon to insure strength
and grout continuity. The anchor head assembly was then
welded in place and the tendons were tension tested and
locked off at the design load.
The PSAC II Slurry Wall Project in the Bronx, New York
brought to light a number of challenging aspects of installing
a deep foundation retention system in tough soil
conditions under a very tight schedule. Bencor Corporation
of America has acquired considerable experience
with the hydromill excavation equipment and process in
the past fifteen to twenty years, and with its current fleet
of 10 such machines in its equipment inventory, Bencor
was able to tackle this very difficult project utilizing this
cutting edge technology. The Bauer BC-40 cutter provided
Bencor with definite advantages in excavating the
tough geology encountered at this site coupled with the
reliability and professional service provided by Bauer-
Editorial: Ty Weaver
Technical Writer: Lawrence Piccagli, Bencor
Corporation of America Foundation Specialist
111 Berry Road, Houston, TX 77022
A total of approximately 12,000 cy of tremie concrete was poured for the slurry walls.