Diaphragm wall or slurry wall is one of the methods of earth retention. As the name implies, a concrete wall is installed to protect the excavation wall from collapsing. diaphragm walls or slurry walls is executed by special drilling machines called grab or hydrofraise. Bentonite slurry is used to prevent a local collapse of the drilled wall. Drilling continues to the floor level of the diaphragm wall or slurry wall. After drilling the diaphragm wall, bar is done and finally the whole drilled part is concreted and a concrete wall is created. Concreting the diaphragm wall is done using a thermal tube. A tremie tube is a tube by which concrete is poured at the lower level of the excavation. By doing this, during concreting, the higher specific gravity of concrete compared to bentonite, causes bentonite to rise and bentonite to be collected for reusable.
This method is also used to prevent leakage in the subway and under the core of earthen dams. The thickness of the diaphragm walls varies between 0.6 to 1.1 m. The wall is made of panels with a depth proportional to the depth of the excavation. The width of the panels is between 2.5 to 6 m. Small widths of 2.5 m are used for soils with low stability, high overhead and deep excavations. The cross section of such walls is generally rectangular, but other shapes such as T and L can be used for specific purposes.
Diaphragm Wall Design
Like many other simulations, the 1 m specifications of the wall section are given to the 2D Geo-Slope and Plaxis 2D finite element software, and then the reliability and wall displacement are obtained accordingly. In 3D software such as Plaxis 3D, the whole wall is simulated and only the specifications of the materials are given.
Diaphragm wall execution method
1) Drilling
Before starting the drilling, the ground surface is prepared for drilling operations by leveling and eliminating the surface protrusions. In order to accurately execute the diaphragm wall at the desired location and its verticality in the early stages of drilling and its continuation, as well as to prevent the walls from collapsing during drilling and concreting operations, the guide wall is used. The loose soil in the area of the guide wall is removed and after identifying and diverting the facilities and underground things interfering with the wall, the guide wall is executed.
To facilitate the execution of the diaphragm wall, this wall is executed in smaller parts called panels. Dimensions and shape of the panels depending on:
- Ground conditions of the area
- Depth of wall execution
- Execution position of the panel in the structure
- Type of drilling equipment
They are designed and executed in different forms such as U, T, L, I, so on.
Drilling of panels is done in 1, 2 or 3 stages depending on the length of the panel. For this purpose, each panel is divided into smaller pieces called bart. The number of barts depend on the length and dimensions of the clamshell grab, the maximum size of each bart is equal to the length of the drilling clamshell. For example, in the execution of Ahwaz city train stations, each panel was usually divided into three barts. Drill the barts at both ends with the full length of the clamshell (270 cm for the grab C800 and 250 cm for the grab B250) and one middle bart is less than the length of the clamshell.
For example, a 7-meter panel is divided into 3 parts as follows:
As shown in the figure above, to drill a 7-meter panel, the first bart is drilled by a grab and then the second and third barts are drilled. After drilling this panel, it is time for the next panel. To prevent the bart walls from collapsing, the bentonite solution is continuously directed into the excavation during drilling. The main role of drilling mud is to maintain the stability of the drilled trench against lateral pressures caused by groundwater, soil and slag on the platform, which is done by forming a bentonite cake.
In general, diaphragm wall panels are executed in 2 ways:
1. Execution of the panels one by one
In this method, the panels are executed one by one. For this purpose, first one panel is executed and then the next panel is not executed and the third panel is executed and continues in the same way until the end. After the completion of these panels, the remaining panels are executed.
2) Execution of the panels adjacent to each other
In this method, first the panel is drilled and concreted, then the next panel is drilled and concreted next to it.
Depending on the type of soil, different devices are used to drill the diaphragm wall:
- Hydraulic Cutter: Suitable for hard and rocky grounds
- Hydraulic Grab: Suitable for relatively hard grounds
- Cable Grab: Suitable for soft grounds
- Kelly Grab: Suitable for muddy grounds
2) Reinforcement
To reinforce the concrete, the diaphragm walls or slurry walls of the reinforcement baskets is placed in the drilled panels. Reinforcement baskets are designed according to the capacity of the cranes, executive considerations and the length of the panels. There may be more than one basket in a panel. Due to the impossibility of placing the waiting bar with the required length on the reinforcement basket, the usual method of connecting the bars cannot be used. Therefore, mechanical connections are used to connect the foundation and floor reinforcements to the diaphragm wall.
3) Dewatering the executive joint
If necessary, various methods such as plastic concrete, June tube, precast beam (concrete or metal) or CWS method are used to dewater the joint between the panels. In the CWS method, after drilling the panel, the metal sheath (Stop end) containing the water stop is placed on both sides in such a way that the metal sheath is installed at the end of the drilling and then concreting is done.
4) Concreting
After installing metal sheaths on both sides of the panel and placing reinforcement baskets, concreting operation is performed. If concreting is done in depth, it is necessary to use tremie tubes to maintain the quality of concrete. To prevent the penetration of bentonite slurry in the concrete poured into the panel, the penetration depth of the tremie tubes in the concrete should be adjusted so that at least 2 m of it is always in the concrete. Concreting should continue up to the level of the guide wall or the height of the diaphragm wall so that the contaminated concrete is at a higher level and then demolished to connect the first roof reinforcement.
5) Excavating and bracing walls
After finishing the diaphragm wall, excavation is done step by step. Due to the depth of excavation and soil specifications, at some levels it may be necessary to combine this method with other stabilization methods such as soil anchorage.
Different methods of executing diaphragm wall (slurry wall)
- Drilling using hydrofraise
It is used for drilling non-stick soils and hard rocks. Hydrofraise is a drilling machine that works using three motors (descending) at the tip of the hydrofraise frame (guide frame) and using the drilling mud rotation system.
2) Drilling using a grab
Another method of constructing a diaphragm wall is to use a grab, in which the wall created is the main wall of the building. Dewatering of executive joints in this type of drilling is done by the following different methods:
Groove and tongue
- Plastic piles
- Precast concrete beams
- June Tube
- CWS (Coffrage With Waterstop)
Diaphragm wall Advantages
- Appropriate degree of safety
- Dual purpose use of diaphragm wall: 1) Retaining structure, 2) Retaining wall
- Suitable for excavations with long walls
Diaphragm Wall Disadvantages
- Requiring special equipment
- Requiring specialized personnel
- High cost in small projects
- Need for space for drilling equipment (more than other stabilization methods)
Related contents:
Leave a Reply