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Diaphragm Walls Constructed by Hydromill Technology for the Foundations of The Public Safety Answering Center II, (PSAC II) Bronx, New York, U.S.A.

Bencor Jobsite 021THE BRONX, NY - 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.

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:

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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.

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 cutter wheels, through the slurry pipe incorporated in the cutter’s frame and back to the desanding plant.

Bencor 6948The 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:

Bencor 6714Bentonite 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 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.

Construction Sequence:

Excavation:

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 three-bite 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 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:

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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.

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 approximately 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 printout 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 joints.

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.

Tiebacks:

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.

Conclusion:

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-Pileco.

PUBLISHER’S DETAILS
Editorial: Ty Weaver
Technical Writer: Lawrence Piccagli, Bencor
Corporation of America Foundation Specialist
Bauer-Pileco
111 Berry Road, Houston, TX 77022
(713) 691-3000
www.bauerpileco.com
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