Dr. DEVANSHU PANDIT & RIDDHI GEDIA

A diaphragm wall (D-wall) is a reinforced concrete continuous wall constructed below ground which is a proven structural element and form of construction technology for retaining walls. This referred to as slurry walls in the USA as the excavated trench is supported by heavy liquid such as bentonite or polymer-based slurry. When the slurry is replaced by the concrete, it serves as the primary structural element for supporting excavation and in many cases it becomes a part of the permanent structure. A diaphragm wall is suitable for the urban infrastructure projects where the structure is more than 25 m deep (Structural Engineering Institute, 2000). Diaphragm walls are popular in Metro underground stations and basement walls, particularly where soil has less cohesion.

The first diaphragm walls were constructed in 1950 for the Milan Metro, Italy. Slurry walls were introduced by European contractors (Puller, 2003). Most diaphragm walls applications in 1960s were for 20-30 m deep trenches. In UK, diaphragm walls used in projects such as Jubilee Line station, Channel Tunnel Rail Link, and Stratford Box have the depths of 30 to 55 m range (Puller, 2003).

CLASSIFICATION OF DIAPHRAGM WALL

Based on the use of construction material D-walls are classified as under.

1. Rigid Type: This type of D-walls is constructed using RCC.
2. Flexible Type: Earth, cement bentonite slurry, or plastic concrete is used to fill the trench.

Depending on the function, the following types of D-walls are used.

1. Structural Walls: It is provided as retaining walls for the perimeter of underground structures such as subways, underpasses, metro stations, and deep basements.
2. Load Bearing Elements: D-walls are used in place of drilled piers in the foundation of buildings, bridges piers, etc.
3. Cutoff Walls: D-walls are used as impermeable cutoffs in hydraulic structures such as earth dams, weirs, and levees to prevent seepage.

CONSTRUCTION METHODOLOGY FOR D-WALLS

Typical construction sequence for the D-walls starting from hydraulic grabbing to concreting is illustrated in Figure 1.

Site Establishment and Setting Out

Figure 1: Methodology Flow-chart

The ground is cleaned and leveled properly to provide firm ground and allow safe movement of plant and equipment. Safety barriers are provided to prevent unauthorized access to the construction area. Site office, polymer tanks, muck bins, DG’s, workshops, stores, and security cabin are established on-site through a detailed site utilization plan. Generally, batching plant, reinforcement yard, panel formation beds, testing lab, and stacking area are allocated in the casting yard. The general arrangement of the site is represented in Figure 2.

Utility Diversion

If any underground/underlying utilities pass through the excavation area, it should be shifted marginally during the pre-excavation process. Investigation for utilities is done through existing maps and verification of underground utilities is done by manual excavation. In case of any utilities found, the mapping is done through a survey and the local diversion is provided by consultation with utility owner.

Guide Wall

As name suggests reinforced concrete guide walls are constructed for initial depth to prevent the soil collapse and to guide the grabbing equipment, and upholds the verticality of the D-wall. It also aids the positioning of the steel cages during concreting. Standard guide walls are constructed in an inverted ‘L’ shape with a depth of 1500mm. Figure 3 illustrates the general guide wall section.

Polymer Slurry Mixing Plant

To prevent soil from collapsing in the D-wall excavation Bentonite slurry was used for more than 60 years which is now replaced with polymer slurry. A plant is established on the adequate impermeable slab with mixing tank having a capacity of 1.5 times the excavation’s volume, a storage tank, and a sedimentation tank (Figure-2). The polymer slurry continuously flows into the trench from the storage tank to stabilize the trench throughout its depth. As the slurry gets mixed with soil and loses its properties, return slurry is pumped into the sedimentation tank for de-sanding. The mixing plant is erected with proper arrangement of pumping, compressed air, routing valves, and cascade before the excavation.  

Panel Construction Sequence

The D-walls are constructed in panels each of generally having 5m width. Panels are classified into primary and secondary panels. All primary panels are constructed first with stop ends at both sides and then secondary panels are taken up. Waterproofing of joints between the panels is one of the most crucial items. Steel pipes, disposable PVC pipes, stop-end piling, and the Milan joints are the most commonly used design to achieve waterproof joint. Stop-end piling between panels is appropriate for hydraulic grab equipment. Plastic water stopper inserted in stop end rests into concrete and it increases the water tightness. Figure 4 illustrates panel construction sequence and joint details.

Panel Excavation by Grabbing

The diaphragm wall panel is excavated by a hydraulic diaphragm wall grab or the rotary depending on the soil strata. The hydraulic grab is used for soft soil strata. Each panel is excavated in two stages. The first bite grabbing of 2.5m width (depending on the size of grab) is done up to the required founding level (the lower end) of the diaphragm wall and then the second bite is done. The grab is lowered, and excavation is started. After grab is completely filled with soil it is moved up and shifted and stroked for soil to remove away from the grab. Each 2.5m wide grab excavates about 1 cum soil from the trench at a time. See Figure 5.

Before the commencement of excavation, the verticality of the grab is checked by the spirit level. The panel profile is maintained by the operator through an inclinometer mounted on the grab by monitoring the measurement of deviations displayed on a monitor inside the operator’s cabin. The acceptable verticality tolerance of the diaphragm wall is within 1:200. Also, the trench verticality is checked at every 5-7m interval by the Ultrasonic Drilling Monitor (Coden brand) to avoid undercut or overcut of the diaphragm wall excavation.

Polymer slurry is introduced into the trench after 1 meter of excavation. The specific weight of polymer slurry is usually ranging from 1.00 to 1.04 gm/cm³. Polymer slurry must be monitored continuously and checked to ensure the stability of the open trench. As excavation continues, polymer slurry inflows into the trench is continued to maintain 2-3m of slurry head pressure above the groundwater table and 500-700mm below the guide wall top.

Installation of Stop Ends

After completing excavation, the stop-ends are installed to the full height and on both sides of the panel using two mobile cranes. Water stopper is fixed in the stop end groove with no waving or undulation to increase the water tightness. Before the use, the stop-end is cleaned, and shuttering oil is applied to the surface. At the time of stop-end lowering, the verticality is checked by the spirit level every 1 m.

Reinforcement Cage Preparation and Lowering

After the installation of stop-ends, the reinforcement cage is lowered. Lifting hooks and resting hooks are welded to the main reinforcement as per the requirement and the length of hooks is calculated based on the guide wall top-level and diaphragm cutoff level. The circular cast covers having 150mm dia. are fixed to the cage typically at 1.5m interval. The lifting and lowering of the cage are done using one main and one support crane depending on the weight of the cage (Figure-5).

Tremie Lowering

After lowering of reinforcement cage, tremie having minimum 250 mm dia. and 1.2m to 2m in length is lowered in pieces and connected while lowering (Figure-5). Generally, a tremie hopper of 0.75 Cum capacity is attached to the top of the tremie pipes with the air vent pipe.

Concreting

After pour approval, ready-mix concrete is transported to the site via transit mixers. M50 or higher grade of concrete is directly poured into the hopper as shown in Figure 5. The tremie pipe is embedded 1.5m into concrete to prevent polymer inclusion in the concrete. Ideally, the temperature at the pouring point should be less than 32 degrees and the minimum slump should be 150-200mm. The compaction of concrete is done by poking the tremie up and down with the help of a crane. Also, tremie pipes are shortened depending on the rise of the level of the concrete.

Stop-end Removal

Stop-end stripping and lifting is done within 3-5 hours of concreting using crane, 50T capacity hydraulic jacks, and the extraction frame. The steel extraction frame made is placed over the guide wall and the stop-end is gripped by frame using bolts. Two hydraulic jacks are placed over the guide wall at the center of the extraction frame. The jacks push the frame. As such, a stop-end gripped by frame is also moves up. Then the stop-end is lifted by the crane.

Slurry Disposal

After re-using slurry, the contaminated slurry cannot be directly discharge to the ground as it is harmful to the environment.  The most commonly used method is treatment with calcium hypochlorite and muriatic acid. The 60-70% concentration of calcium hypochlorite is added to the slurry that neutralizes the PH level of slurry and transfers the slurry in residual water. The resultant residual water can be directly discharge to the sewage system.

Muck Removal

The bore muck generated from the excavation is initially dumped at the available space near the working area and then it is dumped to dumping yard via covered and leak-proof truck.

CHALLENGES DURING EXECUTION

• Tremie is choked
• Abrupt Change in soil strata
• Heavy Collapse of sides into the trench
• Polymer loss
• Stop-end is choked
• Tilting of reinforcement cage

CONCLUSION

Diaphragm walls construction entails a accurate sequence of works to follow, mainly in four major construction steps: guide wall construction, panel excavation (grabbing–stabilization–stop-end installation), reinforcement, and concreting (tremie lowering – casting – stop-end removal). D-walls provides quiet construction with minimum noise and vibration level which enhances the suitability for work carried out in congested sites and densely populated urban areas. D-walls also facilitate watertight joints and floor slab connections into the walls. It offers tangible benefits and optimizes the sequence of operations in the top-down construction. D-walls can be designed to take very heavy structural loads too and are suitable for a wide range of soil types and rocks. It is widely used for retaining deep excavations. The higher cost of diaphragm wall construction and use of heavy machinery can be economical only if they can be a part of a permanent structure such as rail station, deep basement, underpasses, tunnel approaches, and pumping station where the structure depth is more than 25m.

Bibliography

Puller, M. (2003). Deep Excavations: A practical Manual (Vol. 0727731505. 9780727731500). Thomas Telford.

Structural Engineering Institute. (2000). Effective Analysis of Diaphragm Walls. American Society of Civil Engineers, 2000.

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