Figure: Diaphragm wall designed with DeepEX software
The continuous diaphragm wall (also referred to as slurry wall) is a structure formed and cast in a slurry trench (Xanthakos, 1994). The trench is initially supported by either Bentonite polymer based slurries. The term "diaphragm walls" refers to the final condition when the slurry is replaced by tremied concrete that acts as a structural system either for temporary excavation support or as part of the permanent structure. This construction sequence is illustrated in Figure 1.The term slurry wall is also applied to walls that are used as flow barriers (mainly in waste containment), by providing a low permeability barrier to contaminant transport.
Figure: Typical Diaphragm Wall Section.
Slurry wall technology hinges on specialized equipment for excavating slurry trenches. The simplest type of trenching equipment is the mechanical clamshell attached on a kelly bar. Individual contractors have developed their own specialized trenching equipment like hydraulic clamshells, fraise or hydromills (sample manufacturers: Icos, Bauer, Casagrande, Case Foundation, Rodio etc.).
The first diaphragm walls were tested in 1948 and the first full scale slurry wall was built by Icos in Italy in 1950 (Puller, 1996) with Bentonite slurry support as a cut-off wall. Icos constructed the first structural slurry wall in the late 1950s for the Milan Metro (Puller, 1996). Slurry walls were introduced in the US in the mid-1960s by European contractors. The first application in the US was in New York City [1962] for a 7m diameter by 24m deep shaft (Tamaro, 1990), that was followed by the Bank of California in San Francisco (Clough and Buchignani, 1980), the CNA building in Chicago (Cunningham and Fernandez, 1972), and the World Trade Center in New York (Kapp, 1969, Saxena, 1974). The majority of diaphragm wall projects in the US are located in six cities Boston, Chicago, Washington DC, San Francisco and New York.
Diaphragm walls are extensively used in the Central Artery/Tunnel project (CA/T) in Boston, Massachusetts. Work in the CA/T involves many cut and cover tunnels constructed under the existing artery. Some of the deepest T-slurry walls, extending 120' below the surface have been constructed for the Central Artery (Lambrechts et al., 1998).
Diaphragm (Structural) Wall Applications
Earth retention walls for deep excavations, basements, and tunnels.
High capacity vertical foundation elements.
Retaining wall-foundations
Retaining wall-water control
Used in top-down construction method as permanent basement walls
Slurry Wall for Cut-Off Wall Applications
Water and seepage control for deep excavations
Cut-off curtains
Contaminated groundwater / seepage control
Gas barriers for landfills
LIMITATIONS OF SLURRY WALLS
Slurry wall construction requires the use of heavy construction equipment that requires reasonable headroom, site area, and considerable mobilization costs. In limited headroom conditions smaller cranes can be used and the technique can be altered to “remote backfill mixing”, where the excavated soil is transported and mixed to a remote location, and then is returned as backfill.
Cement-bentonite slurry walls also provide another alternative. In this method, the trenches are excavated under a slurry that later solidifies and create the permanent barrier/backfill.
Also, one should check that used bentonite slurry and soil-bentonite slurries are able to withstand chemical attacks from the insitu soils. In such a case, alternate slurry materials such as attapulgite and treated bentonites can be used. Other backfill compositions may be used when deemed appropriate (soil-attapulgite and soil-bentonite with geomembrane inserts). When required, cement-bentonite and soil-cement-bentonite can provide greater strengths.
SLURRY WALL COSTS
Slurry wall construction cost for cut-off barries is considerably cheeper than diaphragm wall construction for deep excavations. The differences arise mainly from construction method differences. In cut-off walls construction is much quicker as a continuous trench is excavated and backfilled and reinforcement cages are seldomly used. In contrast, in diaphragm walls the wall perimeter is constructed panel by panel and reinforcement cages are almost always used.
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DIAPHRAGM WALL CONSTRUCTION METHODS
Diaphragm wall construction requires that a proper sequence of works is followed. Specialized excavating equipment has to be used. This equipment requires more available space when compared to other construction methods.
1. Guide wall installation for diaphragm walls
Guide walls are constructed in-situ typically as lighly reinforced concrete elements. Guide walls maintain the horizontal allignment and wall continuity of a diaphragm wall while the provide support for the upper soils depth during panel excavation. This temporary support is important as the slurry levels vary during construction and the upper few feet or one meter of the wall tends to be unstable. Equally important, guide walls help guide the diaphragm wall grabs vertically and aid in the positioning of the final structure.
2. Pre-excavation for diaphragm wall installation
Prior to the diaphragm wall grabs starting excavation, the slurry pump must be fully submerged in bentonite slurry. To achieve this, a small initial excavation by the grab is carried out that is filled with slurry. Occassionally, some preexcavation might be required before guide walls are installed to remove certain obstructions.
3. Primary panel excavation for diaphragm wall construction
The primary panels are excavated first. The minimum length of a panel depends on the grab equipment size and is generally in the order of 3.0m (15ft). If soils are stable, the primary panels can be constructed in multiple bites. In such a case, a panel can be subdivided into three bites with the left and right panels excavated first while the middle bite is excavated last. With this approach, diaphragm wall panels in the order of 6.5m to 8.0m are achieved. Multiple bites are also required when corner or T panels are constructed.
4. Slurry cleaning and desanding for diaphragm wall construction
Prior to tremieying the concrete, and while the panel is excavated, the supporting slurry fluid must be cleaned so that it's properties are within acceptable levels (density, sand content, viscocity, PH). Slurry is circulated at regular intervals throughout the construction period through the regeneration plant. Otherwise, fresh slurry fluid can also be used although this approach is not the most economical.
5. Joint constuction methods for diaphragm wall construction
Diaphragm wall joints need to receive special attention do detail. Various joint types are available for diaphragm walls. Joint selection depends on the excavating equipment as much as contractor preference Joints can be flat, circular, with steel beams, or special grooved type with water stops. Grooved type joints with water stops are typically preferred while in the US it is also very common to use steel I beams for water stops. Flat panels and circular joints are generally avoided.
6. Reinforcement cage lowering and concrete tremieing
Once the bottom of the panel is reached (and cleaned), the reinforcement cage can be lowered into position. The reinforcement cage is typically suspended from the guide wall panels, and must have enough transverse and diagonal reinforcement to permit it to be properly lifted and lowered into place. Sufficient space must be left for at least two or three tremie pipes so that tremieing can take place.
Concrete tremieing refers to the process of replacing the supporting slurry with the permanent concrete with the use of vertical pipes called tremies. With the tremies, concreting of a diaphragm wall starts from the bottom and the tremies are lifted progressively as the concrete level rises. During this process the tremies are maintained within the freshly poured concrete for a minimum depth of 2ft or (0.6m). Overpouring might be required to make sure that all slurry is displaced from the panel by concrete. Poor tremieing can result in slurry pockets getting entraped within the diaphragm wall concrete. These pockets can then lead to excessive and costly groundwater leaks or even blowouts. This has been the case in certain portions of the Central Artery Project in Boston, MA (Big dig) and has led to costly repairs and delays.
7. Secondary panel excavation for diaphragm wall construction
Secondary panels are constructed between primary diaphragm wall panels. When trench cutters are used, the primary panel is formed with a single bite excavation. With trench cutters a flat panel joint is typically used, but the trench cutter eats into unreinforced concrete of the adjacent primary panels. After the specified depth is reached, the reinforcement cage is lowered into position and concrete is tremied with tremie pipes from the bottom up.
Diaphragm (Slurry) Wall Braced with Struts - 2D Section Design and Optimization
Diaphragm (Slurry) Wall Braced with Struts - 3D Model Design
Top/Down Excavation Example - Diaphragm (slurry) Walls with Slabs
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