Download - BAUER-Pileco


North America

January 2011

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.


January 2011

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.

Milling Technology:

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


January 2011

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


January 2011

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.


January 2011

Construction Sequence:


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


January 2011

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.

Verticality Control:

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-


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

January 2011

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

coated rebar.



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

January 2011

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

(713) 691-3000

A total of approximately 12,000 cy of tremie concrete was poured for the slurry walls.

More magazines by this user
Similar magazines