Are Diaphragm Walls And Retaining Walls Different?

A common question our civil engineers encounter in conversations with people within and related to the construction industry, is the difference between a retaining wall and a diaphragm wall.

The idea of retaining walls is simple, a solid wall used to restrain masses of soil from falling off a slope or the edge of an excavation. And when it comes to deep basement construction, it is required to resist the lateral pressure of the soil from crashing into the excavated cavity where people and machinery are building the underground space.

While diaphragm retaining wall construction helps build multi-level basements for residential and commercial projects and underground metro and subway stations.

Let’s dig deeper to understand the difference better.

How do Retaining Walls work?

A retaining wall is used to retain the backfill against excavation. The whole excavation is made for the structure and then the retaining wall is constructed. There are 4 common types of retaining walls that do this.

1.) Gravity retaining walls: Gravity retaining walls prevent sliding and toppling of soil and resist the lateral pressure behind them with the use of gravitational force of their own weight. They are usually built of bricks, concrete blocks or cast-in-situ concrete with a wider base and sloped faces.

2.) Cantilever retaining walls: Cantilever walls are L-shaped or T-shaped foundational walls, built with the footing or base slab beneath the backfill. The vertical pressure from the soil is transferred onto the base of the structure and the wall in the centre, forming the T shape and preventing the soil from crashing into the excavated area.

3.) Embedded retaining walls: Extending deeper than the site excavation, the embedded retaining walls take advantage, at least partly, of counteracting the active earth pressure being exerted on the wall above. They are used to construct near-surface underground structures, such as basements, car parks and metro stations.

4.) Reinforced retaining walls: Reinforced soil retaining walls, sometimes referred to as mechanically stabilised earth walls, are constructed using layers of geo-grid to reinforce the soil into a stabilised mass. This increases the bearing capacity of the wall, along with its resistance to differential settlement.

Are retaining walls and diaphragm wall construction the same?

A diaphragm wall is a type of retaining wall. As a matter of fact, a diaphragm wall construction can be a type of cantilever, embedded, or even a reinforced soil retaining wall.

Diaphragm walls are reinforced concrete walls built in the ground to provide support to the structure to be built above and act as walls for the deep basement space to be built later on.

They act as the cut-off walls for the excavated area of the site, providing a strong foundation to the surface and enclosing the structure while resisting the soil pressure from the other side.

In construction sites with collapsible soil or where the feasibility of deep excavations is as good as none, developers must opt to build a diaphragm wall. Diaphragm walls can be constructed up to the required depth and then excavation can be started so that soil would not collapse.

So basically, what diaphragm walls do is ‘retain’ the soil from collapsing, while the required excavation and construction of deep basements take place.

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Bentonite to Polymer Slurry, the Sustainable Route to Building Reinforced Concrete Diaphragm Walls

Bentonite has been a crucial construction element in building reinforced concrete diaphragm walls for deep basement structures. But over the years, civil construction companies have started using polymer in their excavation fluids and slurry.

Read on to understand the importance of both the materials in constructing concrete diaphragm walls, the differences between them, and how polymer is growing as a more sustainable option in excavation activity while bentonite remains the foolproof back plan for civil engineers.

What are Bentonite and Polymer Slurries?

Bentonite clay is formed by the alteration and weathering of volcanic ash and is abundant in the clay mineral montmorillonite, which swells when mixed with water and possesses exchangeable irons on the surface that produce thixotropic suspensions. Typically mined in Rajasthan India, it is extensively used in diaphragm wall construction as an excavation fluid. Bentonite clay (in powder form) is combined with water in a colloidal mixer where the clay particles bond with one another to form a gel-like texture.

The mixing process continues till the standard quality is achieved for the slurry.

Polymer slurry is prepared with the combination of water and hydrophilic superabsorbent polymers like polyethylene glycol, polyvinyl alcohol, or carboxymethylcellulose to form a thick, viscous mix. These water-soluble polymers have been used to stabilise deep excavations for reinforced concrete diaphragm wall trenches. When compared with conventional bentonite slurries,this engineered fluid is known to offer a variety of advantages including smaller site footprint and lesser environmental impact.

How is a bentonite slurry or polymer slurry prepared?

The slurry is prepared with the aim of attaining maximum hydration of bentonite clay or polymers. Hence, it must be slowly added into the mixing water to ensure all particles are irrigated and do not get lumped. Potable fresh water should be used in the mixing process to obtain the best results. Alongside this a chemical analysis, as listed out by the Indian Standard Code of Practice for Construction of Diaphragm Wall, must be conducted to ascertain the suitability of water. If needed, chemical treatment is given to the water before its usage.

The preparation of the slurry is done either in batches or in a continuous process, depending on the type of equipment used. The components are added in a high shear mixer through a simple venturi-hopper. To assure the dispersion of all the individual particles in the mixing water, the mixing equipment must generate sufficient shear. Once prepared, it is moved to storage tanks, and from there they are supplied to the excavation point, to be filled into the cavity of the soil. This entire process, including the testing is done on-site by the civil construction company assigned to build the diaphragm wall.

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