Split Set vs Swellex Friction Bolts: Load Capacity, Cost and Where Each Fails

Split Set vs Swellex Friction Bolts: What Is the Real Difference?

They are both friction bolts used to strengthen rock mass underground. However they operate differently.

The Split Set style friction stabiliser works by pushing the stabiliser down into the undersized hole. Once the tube is pushed in it will create a force outwards and create friction over its installed length. The official split set website states that the anchorage capacity is dependant on stabiliser size and hole diameter. As well as ground conditions.

The swellex bolt is a folded up steel tube. This is placed into the hole and water under high pressure is used to expand the tube. It will create contact with the borehole and create friction with the surrounding rock mass. The current Epiroc website has published the following breaking loads for there swellex products Pm12, Mn12, Pm16 and Pm24 are 120kn, 120kn, 160kn and 240kn.

To put it simply.

Split Set you push the tube in to the hole.

Swellex you place the tube in then use water to expand it.

There is friction created on the wall of the borehole for both types. But as mentioned above there are differences in each one.

If you want to learn more about Friction bolts in an Underground Support system check out Wanfeng’s page on Mining Ground Support Systems.

Split Set and Swellex Are Both Friction Bolts — But Not the Same System

Friction Bolt.

Many people use the term friction bolt to describe many types of friction bolts.

Which can cause confusion when we start discussing friction bolts.

Both the Split Set and Swellex work by gripping the wall of the borehole. The difference being that the Split Set is designed to create radial forces when driven into the rock. While the Swellex will expand hydraulically once inserted into the borehole.

Due to this we have different factors that affect how well these products are installed.

Split Set will depend on the following:

• Size of the stabiliser
• Borehole size (actual)
• Bit condition
• Hardness of the rock
• Time spent driving the bolt
• Insertion device

Whereas the Swellex will depend on:

• Correct borehole size
• Model of Swellex
• Expansion Pressure
• Pump
• Full expansion of the bolt

So there you have it. Comparing nominal sizes of the bolts isn’t as beneficial as understanding how they each work.

How does a split set friction stabiliser work?

A Split Set friction stabiliser is basically a tube of steel with one slot running along the length of it.

You would drill a hole just under the outside diameter of the stabiliser. When you drive the stabiliser into the hole it will contract. Creating friction between the stabiliser and the wall of the borehole.

From the official Split Set website “installation is normally as simple as drill & drive, no mixing, injecting resins or torquing or pressurising.”

One of the benefits of using this type of friction support is its simplicity.

But this can also be it’s downfall. Because the diameter of your borehole will play a huge role in how well your anchor is installed.

Split Set Friction Stabiliser and borehole diameter

As you can see from the diagram below from Split Sets website. You can change the anchorage by changing your borehole diameter.

For the SS-33 stabiliser. They recommend you complete some pull tests on site to determine the correct bit size to use. Around 30-33mm holes will give you an initial anchorage of approx. 3-6 tons (depending on the conditions your drilling).

With the SS-39. From there official website they recommend using trial holes between 35-38mm to obtain the desired initial anchorage of 3-6 tons.

For the SS-46 stabiliser. There recommended initial anchorage is between 6-9 tons with trial holes of approx. 41-45mm.

Again this isn’t saying that this is what your pull out capacity is going to be.

Its simply telling you what to expect when establishing your initial anchorage.

How does a swellex bolt work?

Swellex operates on the principle below.

A folded closed steel tube.

This is inserted into a drilled hole and expanded by the use of water under pressure.

Once expanded the tube conforms to the shape of the bore hole.

According to Epiroc this is an easy to install bolt with good bolt/rock mass interaction.

Current published Swellex data include:

These are manufacturer-published values for specific Swellex products, not general figures for all inflatable rock bolts.

Split Set vs Swellex: Load Capacity Comparison

This is where most people will make wrong comparisons.

You cannot state that.

“Split set = xKN”

Because as previously stated your going to get varying results with split set depending on how much interference you have between stabiliser and hole size.

So from manufacturer information we will test on site and determine what size drill bit we need to achieve desired result.

With swellex we can use the information provided by epiroc and look at there minimum breaking loads.

Which would be a more accurate statement of.

This table is not a direct capacity ranking because the published numbers refer to different test concepts.

That is exactly why procurement teams should avoid comparing only one kN figure from each supplier.

Advantages Of Split Set

There can be benefits when choosing a Split Set type stabiliser. These include:

Rapid installation

Can be installed with basic mechanical procedures:

Drill → Install → Stabilise

As little as under a minute per stabiliser according to manufacturers and without mixing resins. No pumping or torquing required.

Less equipment required

Drilling and driving equipment is all that is needed to install a Split Set stabiliser.

Provided you have the correct driving equipment this can reduce installation operations.

Frictional support achieved

As soon as a Split Set is installed correctly it will start to support ground.

Yieldable support

Split Set claims to offer a yieldable support as well as frictional support if ground movement occurs.

Disadvantages Of Split Set

One downside of the Split Set stabiliser is the method it operates on.

1. Hole diameter too large

If the hole diameter is too large the stabiliser will not achieve full compression.

Therefore not generating enough radial pressure to obtain frictional support.

Split Set suggests doing site trials and using the correct bit to achieve this.

2. Hole diameter too small

If the hole is too small it can lead to greater driving forces being applied to install the stabiliser.

Which could result in the stabiliser not installing to the desired depth.

Manufacturers have stated that when drill bits dull down and the hole decreases in diameter more friction will occur and you may not be able to fully install the stabiliser.

3. Different diameters along the borehole

If the ground conditions cause the hole diameter to fluctuate greatly along the borehole the support may not be as efficient.

When splitting through soft rock you can get hole breakouts and larger diameter holes than your bit size. In harder rock you can have a hole that is closer to your bit diameter. Split Set mentions this in there installation instructions.

4. Corrosive conditions

Like all other stabilisers made of carbon steel they can corrode if exposed to wet and corrosive conditions.

Split Set come in standard, galvanised and stainless steel versions which shows that corrosion should be taken into account.

Advantages Of Swellex

Some advantages that Swellex can offer are:

1. More tolerant of hole shape

Since the Swellex expands it can mould to any variations in the hole instead of just being inserted into a small hole.

2. Easier to control installation

There is a pressure used when installing Swellex.

According to Epiroc most swellex bolts have an installation pressure of around 24-30MPa.

3. Load capacity

There are load capacities listed for each Swellex bolt.

Which can help when specifying.

4. Quick installation

Swellex is also known for it’s quick installation time. There is no need for resin to be mixed and cured before the bolt can be expanded and inserted into rock.

Disadvantages Of Swellex

There are also some disadvantages when it comes to using Swellex bolts.

1. Need High Pressure Equipment to Install

As we know with Split Set you are able to just hammer it in to the hole.

You need to have the correct equipment that can install Swellex.

Which will most likely be Swellex Pumps as listed on Epirocs website.

More consideration will need to be given to:

• Having the pump available
• Maintaining the pump
• Water
• Hose’s and training of operators

2. Will need to be installed correctly

The bolt needs to reach its installed condition if not it will not have the desired rock contact.

So having the pump and knowledge of how to install is important.

3. More Equipment to consider.

As mentioned above Split Set is fairly simple drill a hole and hammer it in. You will need more equipment to install Swellex.

Capital costs, maintenance and availability of equipment.

4. Will Corrode

Swellex is still a steel product that can corrode.

Depends on your groundwater and how long you want your support to last.

Should consider corrosion protection.

Split Set and Swellex what’s cheaper?

Many people think they know the answer to this question but its actually very difficult to give you an answer.

You can’t say

Split Set is cheaper

Swellex is more expensive

As there are many variables when it comes to installed cost of bolting.

What we can do is look at the different factors that play a role.

Split Set will advertise no need for pressure, mixing resins and using specialised equipment which can help decrease your bolting costs.

This shouldn’t be translated into a certain % less.

If you have too much variation with your hole diameter. Failed installations or not achieving required pull out values you could end up with a cheaper bolt that isn’t cheaper when supporting ground.

Swellex may require more equipment but can give you better installation in certain conditions which can offset some of those costs.

Always compare cost of a installed support system.

The Real Difference: Hole Control vs Expansion Control

A good analogy would be.

Split Set

Dependant on hole size → amount of compression on tube → Friction → Anchorability

Swellex

Dependant on hole size → insertion of bolt → Expansion via hydraulic → contact with wall.

So you have 2 systems that will fail at different points.

If your hole size falls outside the criteria for an interference fit split set will become an issue.

If your expansion kit or pressure or installation technique isn’t sufficient Swellex will become an issue.

Which System Handles Variable Ground Better?

Its hard to say.

If you are working in good consistent ground and can control your hole size Split Set can be very efficient and easy to install.

If you have varying hole sizes that fall within the criteria for installation the inflatable system can be beneficial when expanded against the wall.

Neither should be used as a default in poor ground. Considerations would need to be made if.

• Your hole can stay open during installation.
• There is excessive void space.
• Support needs to be grouted.
• Ground is subject to dynamic loading.
• Corrosion is a factor.

There may be better options such as a grouted bolt, yielding bolt or even cable or self drilling anchors.

More info on bolt types here Wanfeng Rock Bolt Types.

What About Dynamic or Squeezing Ground?

Although they can both deform to some degree they should not be compared to an energy absorbing bolt.

Split Set state on their website that their system is able to deform and maintain ground support when the rock moves.

If you are in an environment with rock bursts or large deformation then these are some things you should take into consideration when looking at your support.

• Deformation
• Energy absorbing
• Load displacement curve
• Mesh & Surface Support

As you can see there are many other factors that you should look at when picking your support. A purpose designed Yielding/Dynamic bolt may be better suited for this application rather than picking from 2 Quasi-Static Friction bolts.

Split Set vs Swellex for Mesh Installation

Yes they can.

The split set info mentions using their product with mesh and screen. It will make installing mesh easier in some applications.

Swellex also has bearing plates to assist with surface support applications.

What we need to know is.

Is the

Bolt and Plate and Mesh and Ground condition

Designed to work together?

See our other Surface Support Products from Wanfeng here.

Selection Framework: Split Set or Swellex?

Here are some guidelines to follow when initially selecting a bolt system.

If you want to use a Split Set type of Friction Stabilizer then;

• You are able to quickly install via drilling and driving;
• The bore hole will allow you to maintain a consistent diameter;
• Installation can be done with basic equipment;
• Frictional support is needed immediately after installation;
• Design criteria for ground support can utilize this form of frictional support.

Swellex should be used if;

• Installation via hydraulic expansion works for your situation;
• The mine has the capability to provide equipment for installation at higher pressures;
• Ability to conform to the bore hole wall is beneficial;
• Capacity Class for bolt is available to use during design stage;
• Bore hole diameters can be maintained.

Use other types of bolts when;

• The hole will not stand on its own;
• Full column grout is needed;
• Need deeper support into rock;
• More energy is needed to be absorbed due to ground stress;
• Corrosion needs to be accounted for.

These are just some general guidelines to use when selecting what bolt to use.

Split Set vs Swellex: Things to consider when purchasing

You will need to have the following information ready when requesting a quote;

• Application i.e mine/tunnel;
• Rock and ground type;
• Length of bolt required;
• Borehole nominal diameter;
• How you will be drilling;
• What bit you will be using;
• How much your hole will vary in diameter;
• Anchorage/structural strength required;
• What you require from surface support;
• Corrosion environment;
• Equipment available for installation;
• Quantity required;
• Standards/drawings.

It is also important to do a pull test when installing Split Set products. All manufacturers require pull testing to determine what size bit to use and anchorage.

Preguntas frecuentes

1. Is Swellex higher in strength then Split Set?

There is no definite yes or no to this question. As there are many variables that can effect how these numbers are published.

When Epiroc state the minimum break load they provide a range of 120-240kN for certain products. When looking at Split Set information they will mention initial anchorage achieved through pull tests. An example of this would be the manufacturer recommending between 3-6tons for SS-33/SS-39 and 6-9tons for the SS-46 when installed to there guidelines.

As you can see the information is not comparable to each other.

2. Does Split Set need to be grouted/resin?

Split Set as a drill and drive system does not call for the use of resin or grout.

3. Does Swellex need to be grouted?

No Swellex is expanded hydraulically. No need for resin or cement grout.

4. What system installs quicker?

Both systems have quick installation times. Split Set is driven into the ground. Swellex needs to be inserted and expanded. Time will depend on how long it takes to drill and what equipment is being used and site workflow.

5. Which is cheaper?

There is no set answer to this question. Split Set generally uses less installation equipment then Swellex. Which needs to be inflated with a high pressure pump. Price should also factor in drilling costs, installation costs, maintenance, failures and quality control. Not just the cost of the bolt.

6. What is the largest issue when installing Split Set?

Not using the correct hole size. The information from the manufacturer states that hole size will determine initial anchorage and installation.

7. What is the largest issue when installing Swellex?

Not getting proper expansion and contact with rock due to installation equipment.

8. Can I use both in the same mine?

Yes. There will be different areas within your mine that call for different types of support.

Conclusion: Split Set vs Swellex

Both systems are similar in the fact they are both friction bolts. However there are many differences between them.

Split Set will gain anchorage when driven into a smaller diameter hole then the bolt itself.

Swellex gains support by being inflated against the walls of the hole it’s drilled in.

So when comparing the two systems keep in mind.

• Loadings;
• Hole size;
• Installation equipment;
• Quality Control;
• Maintenance;
• Price once installed.

If you are able to control your hole size and want an easy install method. Then a Split Set may be a good option.

Swellex may be better for your application if you need a hydraulically expanded product and know what product you want to use.

Always make sure your using the correct diameter and don’t just compare price of the bolt.

Ground Condition → Required Support Mechanism → Borehole Quality → Capacity Requirement → Installation Equipment → Installed Cost

For a wider comparison of mining reinforcement methods, see:

Mining Ground Support Systems

Rock Bolt Types

Ground Support Products

Servicios de ingeniería y soporte en tierra para fabricantes de equipos originales (OEM).