For structural performance reinforcement and concrete are used in a slurry wall to deliver a firm foundation or retaining capability. A slurry wall is a civil engineering technique for constructing an impervious barrier to prevent water or contaminants from leaching into adjacent ground.
A bentonite and water-based mix is pumped into the excavation to support the sides of the trench and enable the removal of material. The slurry method means that deeper, accurate, low-vibration continuous walls and foundations can be constructed quickly.
The slurry mix is typically 5% solid bentonite material suspended in a 95% water solution. A tanking slurry external wall may be required to protect ground conditions on either side.
How do you Build a Slurry Wall?
There are two main techniques for building a slurry wall, depending upon the depth of the wall, and the final function of the structure.
The Slurry Trench Method
The slurry trench method is a rapid-build technique that delivers a continuous wall. A hydraulic excavator is used, either attached to a 360, backhoe, or crawler rig.
- The excavation is supported by a liquid slurry, which is in turn replaced by a denser cementitious concrete mix at the opposite end of the trench to the excavator.
- A trench usually measures between 400mm and 600mm depending upon ground conditions and level of permeability.
- Steel reinforcement is sometimes specified when ground conditions are less stable.
- Slurry trenches are used when lower depths are required, from 25m up to a maximum of 50m.
- Some slurry walls use soil arisings from the excavation, mixed with cement and the bentonite slurry mix to form a protective, impermeable wall. It can reduce the amount of spoil that needs to be removed from a construction site
A slurry trench may be used as enabling works prior to the construction of piling or secant walls ahead of a basement, or other excavations.
The Slurry Panel Wall Method
The slurry panel method of construction is specified when the wall is also required to be structural. The panel system is used for retaining walls, when there are significant changes in elevation, or when there is a risk of failure due to associated surcharges.
- The first task is the construction of guide walls to delimit the extent of the slurry wall and control the direction of the excavation machinery.
- Guide walls are a minimum of 1m deep and 0.5m wide under normal ground conditions.
- Large excavating or lifting rigs may require designed temporary support mats in order to carry out the works.
- The slurry is prepped and stored in silos if the excavation is deep or extensive.
- The individual panels are excavated, supported by the slurry, and an end-stop or key panel is set.
- The reinforcement cage is installed by a crane. The concrete is then pumped into the excavation using watertight tremie pipes.
- The denser concrete displaces the slurry, which is pumped out, fines removed, and recycled ready for use on the next panel excavation.
- Adjacent panels can usually be excavated after five days when the concrete has reached a nominal hardness.
When the concrete has reached optimum hardness levels after 28 days, the panel wall can be excavated and exposed to design-limited depths.
What is a Slurry Wall Used For?
Slurry walls have a wide range of applications. As a low-vibration excavation method, it is a preferred option in some cases instead of sheet piling.
When a non-permeable below-ground barrier is required adjacent to existing residential or commercial buildings, or rail or road infrastructure, a slurry wall may deliver less vibration and noise.
Non-Structural Slurry Walls
A slurry wall may be required when a construction project is close to a waterway, or other identified risk or hazard, and to control ground conditions.
Groundwater Cut-Off Wall
A groundwater cut-off wall is designed to contain a landfill site with water levels lower within the contained area. Cut-off walls for landfill are commonly set 8m apart in 70 to 80m sections with the water level monitored to ensure any localized leaks are flagged up.
Dewatering Control Structure
A slurry dewatering wall is specified to control water ingress as part of the enabling works for a large construction site. In larger excavations, a secant piled wall may be required.
Groundwater Containment
The movement of groundwater through the subsoils above bedrock can cause all sorts of issues for engineering works. To control the flow of groundwater, to redirect it away from an area under construction, a slurry wall may be required.
Bunding an Industrial Site
Where there is a requirement to store or transfer a large amount of hydrocarbon or other chemical material, a slurry wall is often specified to bund the area. A refinery, or oil transit station next to a waterway will typically be protected by a series of impermeable tanking slurry external walls.
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Structural Slurry Walls
The slurry wall technique can also be employed as part of the foundation structural works of any size building, especially where the soil is soft, and the founds are required to extend to a greater depth.
A structural slurry wall is sometimes referred to as a diaphragm wall and can be up to a metre thick depending upon the structural engineer’s stress analysis.
Retaining Walls
Using the slurry method to construct a paneled retaining wall means that there are fewer construction joints than required if using a bored pile system. It is especially good for depths of up to 100m.
Slurry panel walls have a typical vertical tolerance level of 1:200, which is several times better than the average for column piles.
Deep Founds and Basements
The additional depth that can be achieved with slurry wall construction makes it an ideal method for constructing deeper foundations and basement levels, for underground car parking, or for taller buildings.
A new hospital building in the USA has reinforced concrete panel foundations over 30m deep. This short video gives an idea of the scale of such a structure, including the size of the reinforced concrete cages required.
Deep Shafts and Cut-and-Cover Tunnelling
Slurry walls have been used for decades to deliver cut-and-cover tunnels, relocating roads underground. They are also excellent for creating deep shafts for access to traditional deep tunnelling excavation works, such as Crossrail in London.
What Excavation Machinery is Required?
It all depends on how low you need to go to achieve what’s required. Sometimes the bedrock is the limit, and it is not too far below the surface. At other times, a slurry wall may have to go beyond the geology to form a shaft or basement at much lower levels.
Deep
For a continuous trench that is less than 20 meters in depth, there are specialist hydraulic excavators available that can be operated via a backhoe. This excavation method is great for shallow slurry walls and is a quick way to achieve what is required.
However, depth control is less accurate than with other excavation methods, and the trench will have to begin dry in order to confirm the initial target depth has been achieved.
Deeper
Deeper constructions are achieved with a hydraulic clam, which is two buckets that close on each other at the bottom of the trench. The depth of the rig can be controlled to the exact requirements of the project.
Deepest
For harder, deeper ground, a hydromill makes short work of rock, shale, or other tough ground conditions. It grinds the material, and the head of the rig is lubricated with slurry, which removes the fines through a screening process topside.
Experimental hydromill technology is currently taking slurry wall depths down to 250m (800 feet). That means a lot of reinforcement.
