Temporary casing is a critical tool in drilled shaft construction, ensuring borehole stability, preventing soil collapse, management of overconsumption of concrete, and managing groundwater intrusion.
Whether working in loose soils, water-bearing formations, or challenging rock conditions, selecting and installing the right casing can make the difference between a smooth operation and costly delays.
In this guide, we’ll explore everything contractors need to know about temporary casing — from selecting the right type and understanding the best installation practices to ensuring safe removal and maintaining borehole integrity.
We’ll also cover the challenges of working with irregular rock surfaces, the role of water management in casing stability, and the economic advantages of avoiding using telescoping casing. By following best practices, contractors can optimize their drilling operations while ensuring safety, efficiency, and long-term structural integrity.
What is temporary casing?
We use temporary casing to stabilize drilled shafts during construction, helping to maintain borehole integrity until the concrete is in place. These casings support the walls of the borehole for only long enough for fluid concrete to be poured and then extracted.
Depending on the project, you may remove the casing during or after concrete placement. Since it’s only used temporarily, this type of casing is sometimes called a “temporary top casing.”
The casing prevents soil and groundwater from collapsing the borehole before the concrete can provide enough pressure to hold its shape. As the casing is extracted, it’s important to carefully add more concrete to maintain stability. Once fully removed, the pressure from the fresh concrete keeps the borehole intact.
In some cases, an engineer may decide that the casing should stay in place. Before making this call, they will evaluate how it affects the shaft’s ability to handle weight and pressure. If left in place, the casing becomes a permanent part of the foundation and may influence the structure’s overall performance.
It’s important to keep the casing clean to avoid problems when removing it. Dirt, lubricants, or debris inside the casing can create unnecessary friction, making extraction more difficult.
A rough interior surface can also increase resistance, pulling the concrete upward as you remove the casing. This can cause voids or narrowing in the shaft, weakening the foundation.
Before installing a temporary casing, inspect it to ensure it’s free of soil, lubricants, or other contaminants that could interfere with the concrete or removal process. Proper handling and maintenance of casings help prevent structural defects and ensure the success of the drilled shaft construction.
Different types of temporary casing
Drilling contractors typically maintain an extensive inventory of temporary casings in various diameters and lengths, allowing them to adapt to different project needs. These casings can be modified by welding or cutting to meet specific site conditions.
When temporary casings must be set into solid foundations such as rock, they can function as a tool by applying rotational or driving forces. In such cases, contractors may reinforce the casing’s lower edge or equip it with cutting teeth to help penetrate the material effectively.
The International Association of Foundation Drilling (ADSC-IAFD) has standardized the use of outside diameter (O.D.) measurements for casing.
O.D. measurements use widely available, cost-effective casing rather than custom-fabricated pipes with specific inside diameters, which tend to be significantly more expensive and require longer lead times.
Standard casing sizes are typically available in six-inch increments, ranging from 18 inches to 120 inches in diameter.
If project specifications do not require a particular casing size, contractors generally select one with an O.D. that is at least six inches larger than the drilled shaft diameter. This extra space allows drilling tools to pass through freely during excavation.
However, in conditions involving boulder layers or where telescoping casings are needed, the first casing installed may be significantly larger to accommodate shifting ground conditions.
Selecting the appropriate casing requires careful consideration of the forces acting on it. The casing must be strong enough to withstand external soil and rock pressures, as well as the forces from drilling fluids inside and outside the borehole.
Most steel casings used in foundation construction have a minimum wall thickness of 0.500 inches, though larger casings — those 48 inches or more in diameter — often require thicker walls to resist deformation.
Casings installed with vibratory or impact hammers typically need even greater structural strength. If workers must enter the excavation, the casing — whether temporary or permanent — must be designed with a sufficient safety factor to prevent collapse.
Determining how much lateral pressure a temporary casing can withstand is a complex process. Contractors must evaluate factors such as diameter, wall thickness, out-of-roundness, corrosion, small defects, seismic activity, unstable slopes, and variations in lateral pressure at different depths.
In some cases, semi-rigid liners can be used as an alternative to traditional temporary casing. These liners may be constructed from corrugated sheet metal, plain sheet metal, or pressed fiber. Liners are commonly used as surface casing to prevent loose soil from falling into freshly poured concrete, which could compromise the structure’s integrity.
This is especially important when the concrete cutoff elevation is below the working grade. In some instances, precast concrete pipe sections can also serve as rigid liners.
The use of rotators and oscillators with segmental casing is becoming more widespread in the construction of large-diameter, deep-drilled shafts. These systems allow the casing to be advanced ahead of excavation, stabilizing the borehole without relying on slurry for sidewall support.
However, slurry or water may still be necessary to prevent base heave. Soil inside the casing can be removed using clamshell buckets, hammer grabs, or rotary drilling tools.
These casings are typically constructed from high-strength, double-wall steel with flush-fitting joints that enable the transmission of torque, compression, and tension between casing segments, ensuring structural integrity throughout the drilling process.
How to install casings and liners
There are two primary ways to install and extract temporary casing: casing seated through the drilled hole and casing advanced ahead of the excavation.
Seated through the drilled hole
This involves placing the temporary casing through a pre-drilled hole, which is usually made using a wet method like drilling slurry. It is possible to drill the hole without drilling fluid, but it may not stay open as long.
Holes made without fluid are generally used when the shaft can be drilled relatively quickly through a residual soil-to-rock and where the temporary casing prevents a soil cave-in during rock excavation. If water-bearing sands are in the shallow strata, a drilling slurry is likely necessary.
Polymer slurry for casing lubrication and easier removal is also a good idea.
When is additional stabilization necessary?
If the casing is securely sealed into a low-permeability layer such as clay, chalk, or rock, excavation beneath it can proceed as a dry hole without the need for additional stabilization.
Sealing the casing
The next step is sealing the casing. Here, a twister bar attachment to the Kelly bar is a good idea so that the drill rig can apply torque and crowd to the casing and get it into the soil or rock.
It might also be a good idea to put cutting teeth on the end of the casing. There are several types to choose from, including pointed rock teeth or welded-on carbide chips.
The rock surface
If the rock surface is highly irregular, steeply sloped, or has seams and joints that allow water to seep in below the casing, achieving a secure seal can be difficult. Additionally, uneven or hard rock formations can cause the casing to shift off alignment, damage cutting teeth, or even bend under pressure.
Deeper excavations
In the case of super deep excavations, the telescope casing process with more than one piece of casing is often used. This method is often more cost-effective since it allows the use of smaller cranes and support equipment, which are sufficient for installing and removing telescoping casings instead of handling a single large casing.
Telescope casing is done with two diameters of the drilled shafts – one at the surface and below that, which is then supported with another section of smaller-diameter casing. The outside diameter of the second borehole is a minimum of six inches smaller than the first one above it.
Advanced ahead of excavation
If the hole won’t stand open for a shorter time frame, or if slurry drilling isn’t ideal, advance temporary casings ahead of excavation with a vibratory hammer or oscillator/rotator equipment.
Vibro-driven temporary casings
Vibro-driven temporary casings are best for driven casing. Clamp the hammer to the top of the casing; reinforce it with extra thickness at the end to resist transmitted forces. The weight of the casing and hammer combined will allow for penetration.
However, vibro-driven casings won’t work well if there’s an underlying hard layer because the soil creates side resistance against the casing after removing the vibration.
It’s important to plan for vibro-driven temporary casings since the vibrations can affect nearby structures or cause settlements in loose sands.
If using the vibro-driven method, remove the temporary casing while the concrete is still fluid.
Oscillator/rotator method
Another choice is the oscillator/rotator method. Clamp the casing with hydraulic jaws, then use hydraulic pistons to twist and push it down. Cutting teeth slightly larger than the outside dimension is necessary to advance the casing and overcome the soil shearing resistance to twisting.
Fit a cutting shoe to the bottom section of the casing for optimal penetration and stress relief on the casing sides.
As the casing is placed, the soil inside is excavated simultaneously to minimize resistance during installation. However, a portion of the soil must remain inside the casing to help stabilize the bottom of the borehole throughout the process.
In water-bearing soils, maintaining a balanced water head within the casing will help prevent base instability or upward soil movement. One way to achieve this is by using a slurry inside the casing to reinforce stability. Alternatively, keeping a soil plug in place can eliminate the need for slurry while still maintaining the integrity of the excavation.
Get high-quality Leffer Casing from Western Equipment Solutions
When it comes to foundation drilling, having the right casing is essential for maintaining borehole stability, preventing soil collapse, and ensuring a smooth and efficient operation.
At Western Equipment Solutions, we provide high-quality Leffer casing designed to meet the demands of any drilling environment. Whether you’re working with loose or unstable soils, deep foundation drilling, or groundwater challenges, our durable and reliable casing solutions will help you do the job right.
Our team of experts is here to guide you in selecting the best casing for your specific needs, ensuring that your project stays on schedule while meeting all safety and regulatory requirements. With a wide range of casing options, we offer solutions that provide strength, durability, and ease of installation and removal.
Don’t let ground instability slow your progress or compromise your foundation work. Trust Western Equipment Solutions to equip your job site with the highest-quality Leffer casing backed by industry expertise and superior customer support.
