Diaphragm Walls in Deep Excavations
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.
Figure 1: Excavation with a small grab for a T-Panel in Central Artery Project, Boston MA
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.
Figure 2: Diaphragm wall grab in action
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.
Figure 3: Reinforcement cage lifting for diaphragm wall panel construction
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.
Figure 4: Diaphragm wall excavation modelled with our DeepEX software program
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