DIAPHRAGM WALLS

DIAPHRAGM WALLS
TECHNOLOGY OVERVIEW

 

INTRODUCTION

Diaphragm walls are underground structural elements commonly used for retention systems and permanent foundation walls. They can also be used as deep groundwater barriers.

Diaphragm walls are constructed using the slurry trench technique, which was developed in Europe and has been used in the United States since the 1940's. The technique involves excavating a narrow trench that is kept full of an engineered fluid or slurry. The slurry exerts hydraulic pressure against the trench walls and acts as shoring to prevent collapse. Slurry trench excavations can be constructed in all types of soil, even below the ground water table.

 

 

METHODS

Cast-in-place diaphragm walls are usually excavated under bentonite slurry. Various types of excavation equipment can be used depending on project conditions, including hydraulic excavators and kelly-mounted or cable-hung clam buckets. Depths in excess 150 feet are possible. (The Hydrofraise, a highly specialized excavation tool, can reach depths of 500 feet.)

Diaphragm wall construction begins with the trench being excavated in discontinuous sections or "panels". Stop-end pipes are placed vertically at each end of the primary panel to form joints for adjacent secondary panels. Panels are usually 8 to 20 feet long, with widths varying from 2 to 5 feet.

Once the excavation of a panel is complete, a steel reinforcement cage is placed in the center of the panel. Concrete is poured in one continuous operation through one or more tremie pipes that extend to the bottom of the trench. The tremie pipes are extracted as the concrete rises; however, the discharge end of the tremie pipe always remains embedded in the fresh concrete.

The slurry that is displaced by the concrete is saved and reused for subsequent panel excavations. As the concrete sets, the end pipes are withdrawn. Similarly, secondary panels are constructed between the primary panels to create a continuous wall. The finished wall may be cantilever or require anchors or props for lateral support.

A variation of the technique is the precast diaphragm wall. With this method, a continuous trench, or longer panel is excavated under self-hardening cement-bentonite slurry. The slurry is retarded to remain fluid during construction. After a sufficent length of excavation is complete, a crane lifts the precast concrete wall section into the trench. The cement bentonite slurry sets to form the final composite wall. Alternately, the trench is excavated under bentonite slurry, which is then displaced with cement bentonite slurry.

 

 

APPLICATIONS

Diaphragm walls are commonly used in congested areas. They can be installed in close proximity to existing structures with minimal loss of support to existing foundations. In addition, construction dewatering is not required, so there is no associated subsidence.

The cut and cover method is used to construct tunnels. Two parallel diaphragm walls are installed and the area between the walls is excavated. Floor and roof slabs are poured and area above the roof is backfilled.

 

CONCLUSION

The diaphragm wall technique is a tried-and-true construction method, which provides unequaled support of existing foundations during adjacent construction operations. The method has many civil applications.

 

 

 

 

DIAPHRAGM WALLS
SPECIFICATIONS

TABLE OF CONTENTS

PART 1 GENERAL

            1.01 REFERENCES
            1.02 SCOPE
            1.03 WORKING UNDER OTHER SECTIONS
            1.04 QUALIFICATIONS
            1.05 QUALITY CONTROL
            1.06 STANDARDS
            1.07 SUBMITTALS
            1.08 SITE CONDITIONS

PART 2 - MATERIALS

PART 3 - EXECUTION

            3.01 INSPECTION
            3.02 PREPARATION
            3.03 INSTALLATION
            3.04 TOLERANCES
            3.05 PLACING CONCRETE
            3.06 SLURRY WALL CLEANING
            3.07 FIELD QUALITY CONTROL
            3.08 DISPOSAL OF SPOIL

 

PART 1 – GENERAL

1.01 REFERENCES

  1. Examine all Drawings for requirements therein affecting the work of this trade.


1.02 SCOPE

  1. In accordance with the Contract Documents, provide all equipment, material, labor, and supervision to install a Diaphragm Slurry Wall foundation system.
  2. Construction of solid reinforced concrete slurry wall including, but not limited to, the following items:
    1. Pre-excavation of existing obstructions along line of slurry wall and backfilling with appropriate materials. Excavation of starter trenches and installation of guide walls.
    2. Excavation of slurry filled slots.
    3. Circulation and maintenance of bentonite slurry.
    4. Placing all keyways and dowels as required for framing and additional sleeves and anchorage assemblies as required for tiebacks or temporary bracing.
    5. Placing concrete and reinforcing in the slurry filled slots as shown on the Drawings.
    6. Cleaning of interior face of wall by raking, brushing, and water jetting, as excavation proceeds.
    7. Cleaning and patching of all areas of voids, honeycombs, aggregate runs, and pockets.
    8. Clean up and removal of excavated soil, including removal of bentonite slurry from site.
    9. Grouting and sealing, as required, to render the wall watertight at construction joints and other potential locations of leakage.
    10. Cutting of all concrete bulges on the inside face beyond the specified tolerances.
  3. The slurry wall shall be founded on undisturbed soil strata capable of developing the bearing capacity indicated on the Drawings. The depth of the slurry wall has been estimated from the soil borings and some variation in the depth is to be anticipated.
  4. During construction, the slurry wall is intended as a bulkhead to prevent the ground, streets, utilities and other structures from damage due to loss of ground and subsequent movements. The wall's reinforcement and strength shown on the Drawings, are provided for the conditions imposed after completion of the slurry wall and all connecting structural framing and slabs of the completed building. Temporary loading conditions shall be verified and compensated for with increased wall reinforcing and/or wall thickness by the Contractor.
  5. In the completed building, the slurry wall is intended as a basement wall spanning between floors to retain the surrounding earth, as well as to transmit the loads to a suitable bearing strata. The inside face of the slurry wall shall be straight and free from flaws and shall be watertight. The term “watertight” means that no running water from the wall or formation of droplets on the wall surface shall be permitted and that only formation of moist patches -- allowing water to evaporate from the surface will be permitted. 


1.03 WORKING UNDER OTHER SECTIONS

  1. Work in other Section related to Slurry Wall Construction:
    1. Section ______ Temporary Tieback Construction
    2. Section ______ Tieback Testing and Stressing
    3. Section ______ Excavation
    4. Section ______ Cast-In-Place Concrete
    5. Section ______ Structural Steel
  2. Coordinate access to and egress from site with Contractor.


1.04 QUALIFICATIONS

  1. Contractor: Experienced specialty firm having a minimum of 5 years successful experience installing Work of the same type required for this project. Employ only skilled tradesmen who are thoroughly experienced with the materials and equipment to be used.
  2. Contractor’s Soil Consultant: The Contractor shall retain, at his own expense, the services of a qualified Soils Consultant to advise him on all slurry wall construction techniques involved in this Work, including the design, checking and approvingl of slurry wall items pertinent to the Work, and construction methods for solution of problems that may be encountered during the prosecution of the Work. The Consultant shall be primarily concerned with construction methods that will result in a finished slurry wall of the required quality and strength, and with methods that will prevent water leakage, settlement and/or damage to surrounding structures, roads, utilities, embankments, both within the property lines and on adjoining properties.
  3. Owner’s Soils Consultant: The Owner shall retain, at his own expense, a Consultant to conduct tests and inspections of soil conditions related to the slurry wall. The Owner’s Soil Consultant shall be responsible for conducting and interpreting tests. The inspections shall take the form of a full-time, continuous inspection of all excavation and panel installation operations. In addition, the Consultant shall review the construction methods, equipment, and other pertinent construction details as furnished by the Contractor.

1.05 QUALITY CONTROL

  1. Contractor''s Responsibility to Facility Quality Control Testing and Inspection.
    1. Furnish Owner's Soils Consultant with a complete set of shop drawings for slurry wall panels and a complete set of procedures/sequences for slurry wall installation.
    2. Furnish specific notification, with at least one day's advance notice, for all slurry wall fabrication and installation operations.
    3. Furnish material samples and access as required for testing.
    4. Furnish access and proper facilities, including scaffolding, temporary work platforms, hoisting facilities, as required for inspection of all slurry wall operations.
    5. Furnish labor as required to facilitate testing and inspection of the Work.
    6. Provide storage facilities for concrete test cylinders.
    7. The Contractor will be responsible for all additional slurry wall inspection and testing as a consequence of the Work a) not evidencing compliance with this specification, b) performed with improper supervision, c) performed without prior notice, or d) performed contrary to standard construction practice.
  2. Owner's Concrete Testing Agency: The Owner will retain, at his own expense, a testing agency to perform concrete quality control inspection and testing for the slurry wall as required.
  3. Regulatory Requirements: Comply with applicable requirements of the laws, codes, ordinances and regulations of Federal, State and Municipal authorities having jurisdiction. Obtain necessary approvals form all such authorities.
  4. Surveying:
    1. Contractor shall engage a registered surveyor to perform surveys, layouts, and measurements for Work. Conduct layout work to lines and levels required and record actual measurements of horizontal location, bottom and top elevations, deviations from required tolerances and other data required for Work.
    2. Contractor shall record and maintain information pertinent to work and cooperate with Owner’s Soil Consultant to provide data for required slurry wall reports.
    3. Contractor shall install survey point at 50 ft. maximum spacing along top of wall. Install at least 1 inclinometer along north, east and west sides of excavation. Coordinate this work with Owner’s Soils Consultant to meet regulatory requirements.
    4. Contractor shall monitor survey points and inclinometers weekly during excavation and submit results to Owner, Architect, Structural Engineer, Owner’s Soils Consultant, and others designated by Owner.


1.06 STANDARDS

American Society for Testing and Materials (ASTM):

  1.  
    1. ASTM A615, “Specification for Deformed and Plain Billet-Steel Bars for Concrete Reinforcement”.
    2. ASTM C33, “Specification for Concrete Aggregate”.
    3. ASTM C150, “Specification for Portland Cement”.
    4. ASTM D2166, “Test Methods for Unconfined Compressive Strength of Cohesive Soil”.
    5. ASTM D2216, “Method for laboratory Determination of Water (Moisture) Content of Soil, Rock and Soil-Aggregate Mixtures.”


1.07 SUBMITTALS

  1. Prepare submittals as specified in this Section for review and approval by Owner, Architect, Structural Engineer and Owner’s Soils Consultant.
  2. Satisfactory proof of installer qualifications.
  3. Concrete mix design: In accordance with Concrete Section ________.
  4. Shop Drawings: Submit shop drawings for the following:
    1. Reinforcing steel.
    2. Beam and slab anchorage plates, details of keyways, and pockets.
  5. Bentonite slurry mix proportions.
  6. Engineering calculations for slurry wall and tiebacks for conditions during construction; prepared and sealed by a qualified structural engineer registered in the State of ______________. Calculations must consider intermediate excavation levels related to tieback construction.
  7. Description of installation procedures, sequences, panel layouts and provisions for temporary conditions.


1.08 SITE CONDITIONS

  1. Test Boring Report: Examine the site, the drawings, the record of test borings, and the subsurface exploration reports prepared by ________________________________, which will be made available by the Owner to determine all conditions under which the slurry wall will be installed. The results of test borings are for information only and are not guaranteed to represent all conditions that will be encountered.
  2. Site Survey: The survey of the site, existing utilities, and existing construction prepared by ____________________________________, drawings of any existing structure that are available through the Owner, which represent all conditions known to the Owner. Contractor shall formulate his own conclusions as to the extent of such construction and shall remove all material of any nature to the design subgrades indicated or hereinafter specified.

 

PART 2 – PRODUCTS

2.01 MATERIALS

  1. Concrete:
    1. Proportion, mix and placed concrete materials in accordance with Concrete Section ______, except as modified in this Section.
    2. Cement: ASTM C150, Type II or I.
    3. Aggregate: Hard rock, ASTM C33.
    4. Weight: 150 pounds per cubic foot.
    5. Strength at 28 days: See Drawings.
  2. Mild Steel Reinforcement:
    1. ASTM A615.
    2. Grade: See Drawings.
    3. Fabricate and place in accordance with Section ______, except as modified in this Section.
  3. Stone Concrete for Placement by “Tremie Method” in Slurry: ASTM C94, having coarse aggregates with a maximum size of ฝ-inch conforming to ASTM C33 Size 7, with a minimum of 7 ฝ bags of cement per cubic yard. Slump between 7-inches and 9-inches. Minimum design strength of 4,000 psi at 28 days.

 

 

PART 3 – EXECUTION

3.01 INSPECTION

  1. Examine areas and conditions under which Work is to occur. Notify Owner immediately, in writing, of all condition detrimental to proper and timely completion of Work.
  2. Proceed with Work only after unsatisfactory conditions have been acceptably remedied.


3.02 PREPARATION

  1. Contractor shall establish:
    1. Building layout lines.
    2. Temporary bench marks.
    3. Line and level of slurry wall panels.
  2. Contractor shall be entirely responsible for, and shall bear the entire cost of correcting all mislocated slurry wall panels or incorrect cutoff levels at top of slurry walls.
  3. Locate Utilities:
    1. Cooperate with others to locate utility lines, which might be cut or disturbed during slurry wall construction.
    2. Excavate pits near ends of lines to determine exact utility locations.
    3. Review proposed Work with all utilities and arrange for removal, relocation, or disconnecting and capping of all lines.
    4. Contractor has entire responsibility for cost incurred because of damage to utilities by its operations.
  4. Bracing and Shoring: Take every precaution and guard against movement or settlement of the Work, adjacent property, utilities or improvements. Design and provide bracing tiebacks or shoring necessary to guard against movement or settlement. The Contractor shall be entirely responsible for the strength and adequacy of all tiebacks, bracing and shoring and for the safety and support of such construction.
  5. Before commencement of operations, and after final Shop Drawings have been reviewed by the Architect, Structural Engineer and Owner’s Soils Consultant, arrange for representatives responsible for field procedures to meet with Architect, Structural Engineer, Contractor and Owner’s Soils Consultant and Concrete Testing Agency to finalize all procedures relating to slurry wall construction.


3.03 INSTALLATION

  1. Panel excavation equipment shall be capable of excavating slots to the required widths and depths. Equipment shall provide for continuous excavation through all materials encountered.
  2. Provide a stable suspension of powdered bentonite in water that is adequate to support the trench sides during excavation. The slurry shall not produce adverse effects on the tremie concrete in setting time, density or strength. Density of the slurry shall prevent the formation of excessive caking.
  3. The excavation shall be filled with a stable suspension of bentonite slurry at all times and excavating operations shall proceed through the slurry. Slurry shall be added, as required, to maintain its top level as excavation operations proceed. The slurry shall be continuously circulated and cleaned to remove excavated material. The slurry shall be contained at all times to prevent leakage and seepage into vaults, basements, utilities and pits. Desanding equipment shall be required in the slurry cleaning operations.
  4. The panel width of the wall shall result in a stable excavation condition and be such as to produce a monolithic wall with continuous concrete pouring.
  5. The vertical joints formed in the wall panels shall be interlocking and watertight. The joints shall be completely cleaned to establish good contact bond between adjacent surfaces. 


3.04 TOLERANCES

  1. Concrete Surfaces: The plumb, alignment, and building of the inside face of the wall shall be limited to a lateral dimension of 2 ฝ-inches from the theoretical face at any point. All excess material shall be chipped at the Contractor’s expense. The total out-of-plumb from the top of the wall to the bottom shall not exceed 1% of the height at any point in the wall.
  2. Reinforcing Steel: Accurately locate and secure in place before, and during, placement of concrete. Add steel and spacers necessary to maintain the shape and location of reinforcing cages. Concrete skid spacers shall be provided on either side of the cage in order to hold the reinforcing away from the sides of the trench. The tolerance in placing the reinforcing steel shall be limited to +/- 1-inch in a direction normal to the wall.
  3. Steel Plates, Keyways and Dowels: Provide for framing members a shown on the Drawings. The location and tolerances for plates, keyways and dowels shall not exceed +/- 1 inch in both vertical and horizontal directions.


3.05 PLACING CONCRETE

  1. Cleaning: Before placing concrete in the slurry filled trench, the joints of the next panels shall be thoroughly cleaned to obtain watertight bond. All loose debris shall be removed by a large capacity airlift system. Concreting shall not start until the Owner’s Soil Consultant has verified the required bearing capacity, cleanliness, and levelness of the bottom.
  2. Placement: Place concrete in the slurry-filled trench by the tremie method, in such a manner that the concrete displaces the slurry from bottom and rises in such a manner that mixing of concrete with the slurry does not occur. The concrete shall be placed through a top metal hopper and into a rigid leak proof tremie pipe, sufficiently large enough to permit free flow of concrete. Initially, there shall be a suitable plug at the bottom of the tremie pipe, which will not discharge concrete until the concrete head has at least reached the top level of the slurry.
    Alternatively, a “pig” or “rabbit” can be placed in the tremie pipe, on top of the slurry, after the pipe has been set on the trench bottom. The “pig” separates the concrete from the slurry. The pipe should be filled with enough concrete before lifting off the bottom to release the “pig”. Thereafter, a positive concrete head will be maintained throughout. The bottom of the tremie pipe shall remain in the concrete at least 5 feet and this depth shall be maintained throughout.
    The number of tremie pipes in each trench shall be determined from the requirement that the concrete level shall be kept horizontal during the pouring operations. No horizontal movement of the tremie shall be permitted. When the concrete approaches the top of the slurry wall, dowels shall be set for the required construction above the wall. The concreting of the panels shall proceed continuously to the top of the guide walls to produce a monolithic wall. Cold joints, should they occur, will have to be rendered watertight by grouting or some other means.
  3. Joint Pipe Removal: The withdrawal of panel joint pipes used at the ends of the trench, shall be accomplished before the initial setting of concrete and before any damage to the concrete is likely to occur. The concrete mix at the time of design shall be checked for setting time, and retarders added if required to delay the setting time. Unless retarders are added, the panel joint pipes shall be withdrawn no later than 45 minutes from the time of placing of concrete. Controlled, hydraulic jacking of end pipes may be required.


3.06 SLURRY WALL CLEANING

  1. During earthwork excavation and after the placement of the slurry wall, clean the interior wall face using rakes, brushes, or air and water jetting to remove all bentonite caking and film materials. During this operation, chip out all interior face wall bulges to within the stated tolerances. Structurally patch all voids, honeycombed areas, and areas of exposed reinforcing bars, to provide the required coverage for reinforcing, and full wall thickness as indicated. Clean all keyways and embedment items, and bring wall dowels to their final conditions. Provide internal and external methods of sealing of the wall to the watertight condition, as previously indicated for all areas indicated wall leakage including but not limited to: panel joints, wall penetrations, locations of temporary tieback system.


3.07 FIELD QUALITY CONTROL

  1. Representative of slurry wall foundation system installer, qualified to control field practices, shall be available at all times for detailed control of procedures and quality.
  2. Field Quality Control Testing by Owner’s Soils Consultant and Concrete Testing Agency:
    1. Concrete: Sample and test concrete during placement in accordance with Section_________.
    2. Panel Installation: Provide full time visual inspection of all panel excavation operations including verification of slurry densities, slurry cleaning operations, final panel bottom elevation, final panel cleaning at bottom and at joints, final panel top elevation, variations in wall location, and variation from plumb.
    3. Soil Bearing Capacity: At the completion of the panel excavation, test and verify the required soil bearing capacity at the panel bottom. Owner’s Soils Consultant will interpret the tests and determine if additional excavation depth is required to achieve the required soils bearing capacity.


3.08 DISPOSAL OF SPOIL

  1. Clean and remove water, excavated soil, and bentonite slurry from site. Dispose of these materials in a manner to avoid endangering public health, property and portions of work under construction or completed. Comply with applicable requirements of the laws, codes, ordinances and regulations of Federal, State and Municipal authorities having jurisdiction. Obtain necessary permits and approvals from all such authorities.