How To Choose The Right Tooling For Your Crossing
Tooling choice is a key decision when planning an upcoming auger boring crossing. With a plethora of varying ground conditions, and multiple different styles of cutting heads; how does one make the right choice? Let’s break it down!
Dirt
“Dirt” can be broken down into many different things. As true as it is, we generally describe dirt as “ground most commonly dug with a backhoe tooth bucket”. When encountering dirt, you must make one of two distinctions; is the ground stable or unstable? Stable dirt, like “dense clay”, is generally dealt with by using a backhoe style tooth, cutting head. This style head will cut, and pull the “cuttings” back into the auger the fastest. This slight pull effect significantly helps reduce head bawling, in which the ground material is sticky enough to retain itself onto the face of the cutting head. Head bawling increases down hole torque, and can significantly reduce overall production.
Unstable dirt can mean that there is high sand content present in the ground formation, which resists the ground from being fully bonded. When encountering unstable formations, it is important to evaluate the ground water content in the bore path. If there is no ground water, a good choice of head is a 3 or 4-bank dirt head. With the standard head having two banks, an additional bank or two, creates extra surface area on the cutting face of the head, and decreases the size of the entry points for the cuttings. If low groundwater or controlled groundwater (dewatering in place) is encountered while in unstable ground, a sand head is the most limiting cutting head for auger boring. A sand head has the smallest entry point to cutting surface area ratio, of all cutting heads available.
Glacial Till
Glacial Till can be a very challenging ground condition to encounter. Glacial till is generally described as; dirt, sand, cobble, and boulder, combined in varying amounts. The vast possibilities and inconsistencies of this ground condition is what makes glacial till such a challenge. Considering all the different mixtures of this ground, using a versatile cutting head is the answer. An appropriate choice of cutting head in this ground condition is a 3-bank DT87, or as we call them, Shark-teeth, style cutting head. The reason this style of head is a good choice is due to its adaptability. A shark-tooth head can handle consistent dirt, sand, or other displaceable ground just fine, all while being able to cut rock. Additionally the shark-tooth head provides a stronger “tooth support”, which allows these heads to handle sudden impact, like intermittent cobbles, better than any of its standard counterparts.
Although these heads are very versatile, this comfortability should never supersede the decision to stop, pull augers, and address an ever evolving down hole environment, when casing size and permits allow. This decision can be the difference between a successful or unsuccessful trenchless crossing.
Rock
Rock is a ground condition that strikes many people as being difficult or something to shy away from. Fortunately, by asking ourselves some simple questions, we can deal with rock easier than you think. Some main rock attributes to asses are; density (psi), and RQD (Rock Quality Designation). The best way to analyze rock is to collect a sample and send it to a geological bioengineer. But if time is pressing, the big question is; can you dig it? If a backhoe bucket can scratch, fracture, and dig the formation, then most standard rock heads will perform sufficiently in this type of softer formation.
If the rock formation can not be scratched or fractured by a bucket; can a hammer attachment handle the formation or is blasting required? In the case of a hammer attachment being able to fracture the rock formation sufficiently, and the rock is solid and consistent throughout the bore path, there are two options in the more serious class of rock. Retractable roller-cone heads and disc-cutter heads. Disc-cutter heads have been a staple in the industry when it comes to some of the hardest rock bores accomplished over time. While roller-cone heads have not been known to handle the hardest of rock as effectively as a disc-cutters, the roller-cone head is able to be retracted out of the casing, unlike a disc-cutter head which is welded to the front or lead casing. This major design difference plays a big role in the decision making process, especially in an industry where resourcefulness is key.
Non-Consistent Rock
Non-consistent rock can be encountered due to depth of bore, where the bore path is not fully engaged into the solid rock formation. We call this type of rock formation a “split-face”, or mixed ground condition. Another non-consistent rock formation is created by weathering. “Weathered rock” is generally softer than its original state, and can be dealt with by utilizing a variety of standard rock heads. However, generally a roller-cone head will be most productive in this ground condition.
If the split-face ground condition is encountered, it’s typically the most challenging of all ground conditions. When the cutting face is split with rock and dirt, the chances of torque up’s from sudden head impact is heightened. In this type of ground condition, an ample choice of cutting head is the roller-cone head. Being the most versatile head, the roller-cones can handle rock, cobble, boulder, and dirt split-faces. Additionally, the roller-cone heads are based on bearing supported cones. The bearings allow this head to roll through the sudden impacts that a standard head could hang up on. Most important to note, the ability to stop, pull augers, and address the down hole conditions, are not forfeited with a retractable roller-cone head.
Trenchless crossings are an ever evolving environment where sometimes there is no perfect tool for the job. Ground formations can vary so aggressively that it is impossible to have one specific cutting head for any ground condition. Due to this reality, the best way to approach these choices is to narrow down which cutting head provides you with the most versatility in your specific ground condition, and always be prepared to pull auger, before you put yourself and your equipment into an unexpected situation.