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A diaphragm wall may be described as an artificial membrane made of reinforced concrete constructed in the ground by means of a process of trenching with the aid of a fluid support. The most popular use of a diaphragm wall is for the construction of multilevel basements.

The Origin of the diaphragm wall can be traced to the post war years in Italy (early 1950's) where adjacent reinforced concrete piles (known as secant piles) were used to construct a barrage or a cut-off wall for hydroelectric dams. To answer the need for increased water-tightness which could be provided by minimizing the number of vertical joints and due to the fact that the secant pile technique involved the partial destruction of a completed pile by the boring of an overlapping pile (a seemingly wasteful technique)a more elegant solution of excavating a trench in sections in which to cast a series of in- situ adjacent reinforced concrete panels was developed.

The application of the diaphragm wall technique was gradually extended to solve problems involving the building of underground tunnels for rapid transit systems along busy streets, construction of deep basements without endangering the stability of the foundations of the nearby buildings, and the construction of closed-type docks and wharfs

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.