Diaphragm Wall Construction Methodology and Safe Work Method

Diaphragm wall

Diaphragm wall is a technique used to construct reinforced concrete walls in the ground after deep trench excavation. The walls act as cut off wall or serve as a structural member. Excavated trench creates a form for the wall. The trench is filled with bentonite slurry continuously circulated at all times. The slurry provides outward pressure to balance the inward hydraulic pressure. This prevents the trench from collapsing and prevents water flow into the trench. Mechanical excavation techniques like rotary or percussion or grabbing or a combination are utilized for trenching. The reinforcement cage is then lowered and the trench is filled with the concrete that displaces the slurry. Usually, the width of the wall varies from 450 to 1200 mm and depths anywhere between 20 to 50 meters. Inclined rock anchors are used to anchorage the diaphragm wall structure to withstand the lateral thrust on the wall. Continue reading to find out detailed Diaphragm wall construction procedure.

Diaphragm Wall Applications

• Underground stations
• Multi-level car parks
• Open cut tunnels
• Bays in the water for shipbuilding and ship repair
• Quay Walls
• Tunnel ventilation shafts
• Support for open or top down excavations
• Groundwater flow barrier
• Retaining wall
• Cut-off provision to support deep excavation
• Final wall for basement or other underground structure (e.g. tunnel and shaft)
• Separating structure between major underground facilities
• As a form of foundation (barrette pile –rectangular pile)
• Used in congested areas
• Practically suited for deep basements.

Diaphragm Wall types

Diaphragm walls are of two types. T-Shape Panel and L-Shape Panel

Diaphragm Wall Advantages

• Faster Execution. Considerable reduction in basement construction time.
• Designable to take high horizontal and vertical loads and bending moments.
• An economic and favorable solution for a large deep basement in saturated and unstable soil profiles.
• Savings in construction costs as a single element can be used as a supporting wall, a cut-off wall, and a deep foundation element.
• Construction is possible near existing buildings. A clearance of only 30 cm from the outer walls of buildings is adequate.
• Low environmental impact throughout the works. Construction causes only moderate noise.
  No vibration during installation.
• For inner-city areas, the top-down method will help to optimize the operation sequence.

Equipment

Diaphragm Wall Construction Procedure

• Stage 1: Construction of Guide Walls
• Stage 2: Preparation of the Supporting Slurry
• Stage 3: Excavation of Diaphragm-wall and Stop ends fixing
• Stage 4: Lowering of Reinforcement Cage
• Stage 5: Concreting  Operations & Stop Ends Removal

Guide wall construction

Figure 4: Diaphragm wall Guide wall construction

Guide walls are two parallel concrete walls along the face of the wall. Excavate and construct 250 mm thick and 2 meters deep RCC guide walls on each side of the diaphragm wall center line. The guide walls direct the clamshell or grabbing equipment to maintain the desired width, keeps vertical alignment and retains bentonite slurry. Typically, a vertical tolerance up to 1:200 is permissible for diaphragm wall construction.

Supporting slurry

Figure 5: Diaphragm wall supporting slurry arrangement

Make bentonite tank and circulation arrangements for desanding and centrifuge. Typical capacity required is 100 to 500 cum per hour. Circulate the bentonite fluid continuously with reverse mud circulation to prevent side collapse. Always keep bentonite slurry level at minimum 1 meter higher than the groundwater table. Clean the slurry fluid to achieve the desired limits of density, content, viscosity, and PH. Use flocculants if the demand to remove fine silty solids and clay solids from the slurry.

Typical parameters for slurry

Excavation

Excavation is done by digging technique using clamshell or grabs to the desired depth. For sand and soft rock strata, excavation by cutting technique using reverse circulation trench cutters can be used. Excavation is accomplished with the Hydrofraise or comparable drilling methods. A heavy metal frame, serving as a guide, is mounted at its base with two cutting drums carrying tungsten carbide tipped cutters. A pump is placed just over the drums and evacuates the loose soil, which is carried up to its surface by its drilling mud. The mud with cuttings is continuously filtered and then poured back into the trench.

Diaphragm wall Panels

Constructing long sections of wall continuously is not possible due to limitation and size of the plant. The wall is usually constructed in alternative section leaving an intermediate section in between. Two stop end tubes will be placed at the ends of the excavated trench before concreting. Withdraw the tubes at the same time of concreting in order that a semi-circular end section is shaped.
The in-between segments are built similarly subsequently, but without the end tube. By the end, a continual wall has been built with the panel sections closely joined by the semi-circular groove.

Stop Ends Fixing

Stop ends fixing with water stops

Reinforcement

Lower the reinforcement cage in the excavated trench. Reinforcement cage must have a sufficient transverse and diagonal reinforcement. This is to withstand lifting and lowering handling forces. Good space must be available inside the reinforcement cage for tremie pipes to pour concrete.
Concreting: Concrete is placed with tremie pipes to prevent the segregation of concrete. The concrete progressively replaces the slurry. Lift the tremies progressively as the concrete level raises. Maintain a minimum depth of 0.6 meters of tremie pipe into the concrete.

Concreting and Stop ends removal

Concrete is placed with tremie pipes to prevent the segregation of concrete. The concrete progressively replaces the slurry. Lift the tremies progressively as the concrete level raises. Maintain a minimum depth of 0.6 meters of tremie pipe into the concrete.

Quality Assurance and Quality control

• Submit a detailed inspection and test procedure
• Maintain concrete pour cards and checklists during pre-pouring, pouring and after-pouring.
• Cast the concrete cubes at the specified frequency.

Standards

IS 456 : 2000 – Code of Practice for Plain and Reinforced Concrete

IS 9556 : 1980 – Code for practice for design and construction of diaphragm wall

Health and Safety

Safety is essential and imperative part of the construction process. Provide personal protective equipment (PPE) to all personnel involved in the Diaphragm wall activity and provide the required safety equipment. Below are the suggested PPE:

Safety Procedure of Diaphragm wall Construction

• Inspect the rig/service crane by the lifting supervisor and operator with the provision of the checklist.
• Cordon off the working area with red-white safety tape when the boring rig/service crane is in operation.
• Allow no person other than the crane operator, safety supervisor, and lifting supervisor to engage in the lifting works.
• Keep unauthorized personnel out of the lifting works area.

Safety Procedure of night work

• Provide sufficient lighting for the night shift at the working area.
• All night shift employees shall wear reflective jackets.
• All activities will be under close supervision.

Safety Supervisor activities during execution

• Conduct toolbox meeting once or twice a week prior to the commencement of work.
• Conduct safety inspection and safety meetings.
• Arrange safety induction for new workers who enter first time into the site.
• Issue PPE for all workers and maintain a record for the same.
• Keep appropriate equipment and machinery certificates on site.
• Record accidents in “Accident Report Form”.
• Ensure good housekeeping on site

Risk Identifications

• Risk of damage to “live utilities”.
• Risk of public wandering into the work area.
• Risk of site personnel falling into the excavated trench.
• Risk of traffic accidents within the site.

Risk Management

• Mark and display clearly the type of utilities.
• Allow only authorized persons with permission.
• Use appropriate PPE before entering into the work site.
• Cordon off the area around the openings.

This article on Diaphragm wall construction procedure and all the images were contributed by Keller Ground Engineering India Pvt. Ltd.

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