Imagine using the best rock excavating
technology on your next project.

Own or rent the Robbins Small Boring Unit (SBU-A) and experience the most time and cost effective way to bore through hard rock. The Robbins SBU-A has been proven on hundreds of jobsites across the country. Beat your competition on the bid, impress the job owner, and walk away more profitable when you use the Robbins SBU-A on your next job.

No other company exists that knows more about excavating hard rock. And no other company makes a product like the Small Boring Unit. Over one hundred contractors in cities and locations all over the North America have proven the SBU-A works better than any other method of excavating small diameter hard rock bores.

Many contractors around the U.S. have successfully used this equipment. Some contractors own nearly every size SBU-A that Robbins manufactures.

What is the Robbins SBU-A?

The SBU-A is a small diameter cutterhead and thrust bearing assembly (from 24 in / 600 mm to 72 in /1.8 m) that can be used with any auger boring machine (ABM) from 36 in (900 mm) to 72 in (1.8 m). The SBU-A is welded to the lead casing. The contractor-supplied auger pilots directly onto a taper hex shaft in the thrust bearing assembly. The SBU-A contains a high-thrust bearing and is equipped with mini disc cutters available in three sizes depending on your project. It can be steered to maintain line and grade.

The right machine for your job: Diameters, Lengths and Strengths

Diameter available in sizes ranging from 24″ (600 mm) to 72″ (1.8 m)

The SBU-A and all component parts are readily available in any bore diameter with standard English (inch) or metric (mm) casing sizes in stock.

The SBU-A can install drives over 500 ft (150 m) in length

The majority of the crossings are 100 to 300 ft (30 to 90 m) long; however, there have been several contractors that have utilized the SBU on drives over 500 ft (150 m) in length. The length of the drive depends on the ground conditions. Talk to a Robbins rep to discuss what the SBU is capable of on your specific job.

The SBU-A can bore through rock 4,000 – 25,000 psi UCS (25 – 170 MPa)

That means you can excavate rock without the hassle of pneumatic hammers or other less effective tools. You will be able to excavate rock faster in small diameter drives than any other method available. You will be able to excavate rock at a lower price due to the efficiency of the disc cutter.

The SBU-A can bore in more than just hard rock

If your job is in mixed ground containing cobbles and boulders, an alternative cutterhead is available. Depending on the conditions, mixed ground cutterheads can feature a combination single disc cutters or two-row tungsten carbide insert cutters and carbide cutter bits. Specialized larger muck openings allow the cobbles and boulders to pass through without stopping excavation.

Features and Benefits

The SBU-A has a rugged cutterhead with cutter saddles

The cutterhead is a smaller version of Robbins’ innovative hard rock tunnel boring machines (TBMs). The cutterhead is specifically configured to cut hard rock.

The SBU-A has patented mini disc gage cutters that will last the entire bore

With the SBU-A you can easily cut rock. The cutters allow the rock to be efficiently cut in the same method that TBMs excavate rock. They are available in 6.5″, 9.5″, and 11.5″ sizes, depending on your project requirements. There is so much force placed from the tip of the cutter onto the rock that it displaces the force by creating small fractures in the rock. Those fractures break the rock into chips.

Muck is removed either by an auger, muck car, or small conveyor.

There are two simple methods to take out the excavated material from the drive. For shorter drives, the auger can be used to remove spoil. Muck cars can be an economic option on shorter drives also. On longer drives, a small conveyor system can be installed. The best decision is highly dependent on your project requirements.

Steer the SBU-A with integral stabilizer/steering shoes

Since you will be able to control line and grade, it is possible to install gravity sewers. The machine is not steered through the entire bore. Instead the stabilizer shoes are adjusted through the first 20 to 40 feet (6 to 12 m) of the bore to get the machine in the right direction. The stabilizer shoes come standard on SBU-As 30″ (762 mm) and larger.

Your Project — From beginning to end

Call a Robbins representative to discuss your project today. Since Robbins has had years of experience with underground projects, we are capable of discussing the details of your project and recommending the right machine for your job.

Purchase or Lease?

Robbins offers SBU-A machine diameters ranging from 24″ (600 mm) to 72″ (1.8 m). All of these products are available in our Solon, Ohio or Kent, Washington facilities. They are available for purchase or lease. The SBU-A can be rented by the month with a minimum of a two-week rental. If you decide to purchase the machine after leasing, most of the rental paid will be applied to the purchase at project completion.


One of Robbins’ experienced technical staff will set up your SBU-A like they have set up hundreds before. We will come to your jobsite and teach you and your crew how to use the SBU-A so that there is no apprehension on your part. We will be there by your side, from selecting the correct machine for your job until the bore is completed.