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Context Sensitive Rock Slope Design Solutions

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3. Rock Excavation Methods


Ripping is a process of breaking up rock and soil with a large tooth or teeth attached to the back of a bulldozer.

Where feasible, ripping is generally preferred over blasting because it is considerably less expensive; ripping costs are typically 50 to 65% less than blasting. Ripping is also significantly less dangerous than blasting and requires fewer permits and special precautions. Ripping can be done in close proximity to populated areas or other places where blasting noise and vibrations are restricted. However, ripping is limited to soft to moderately firm, fractured rock and construction of low-angle cut slopes and shallow, near vertical cuts. In dense rock formations, light blasting is sometimes performed before ripping.

Once the material is loosened by ripping, an excavator can be used to remove it and perform slope sculpting (see below). Ripping gives the contractor a lot of freedom to aesthetically enhance a slope by adding additional contour and allowing for revegetation in certain areas.

The teeth on rippers can leave scar marks on the rock after excavation. In most cases, these can removed using the same procedure used to remove drill hole traces. In soft and/or massive rock, the contractor may use a jet of high-pressure water or a vibratory compactor, known as a plate bucket, which is attached to an excavator or loader to remove the scars.

Ripping is usually done in one direction, but in very tough materials ripping in a grid pattern will increase excavation efficiency. The pass spacing is determined by the end use of the material (fill, aggregate, waste, etc.) and the capacity of the excavating equipment. In most cases, it is best to maximize ripping depth, but in stratified formations it may be best to rip along the natural layers.

Ripping is generally done in the same direction that loading will take place (i.e., parallel to the plane). However, when the material exhibits a foliation or bedding plane, ripping perpendicular to the plane produces much better fracturing.

Ripping Equipment

There are basically two types of rippers: the pull- (or tow-) type ripper and the integral bulldozer- mounted ripper. In rock excavation, a bulldozer-mounted ripper as shown in Figure 21 works better than a pull-type ripper because it can exert greater downward pressure.

Rippers also come in single- and multi-toothed configurations. Single-toothed rippers are used for difficult ripping work, where maximum ripping depth is required and/or the material is dense. Multi-toothed rippers, which can use up to five teeth, are used for softer ground or for secondary purposes such as breaking up already ripped ground.

Photo. Bulldozer with an integral ripping attachment.
Figure 21. Photo. Bulldozer with an integral ripping attachment.

Choosing the proper bulldozer-mounted ripper depends of four factors: downward pressure on tip, horsepower of the bulldozer, weight of the bulldozer, and angle of tooth penetration (Kelly 1970). The first three factors will influence the size and type of the bulldozer used, and the last is a function of the style of ripper (see below). Bulldozer style and size is not discussed in this document. Most ripper manufacturers' websites includes information on the bulldozers that are compatible with their products; ripper mounting brackets and hydraulic control mechanisms vary widely.)

Here three main styles of rippers:

Hinge-Style Ripper:

The hinge-style ripper, also known as the radial-type ripper shown in Figure 22, is fixed to the bulldozer with a pin, around which the ripper arm rotates. Because of its maneuverability, this type of ripper is best at creating sculpted and natural-appearing rock cuts.

Parallelogram-Style Ripper:

The parallelogram-style ripper features two hinged arms, which keep the shank (the tang of the tooth) vertical and hold the tooth at a constant angle as it is lowered into the material. This provides excellent penetration in many types of rock. The parallelogram-style ripper works best in easy to moderately rippable materials. In more difficult conditions, contractors prefer to have the option of selecting different tooth angles, which provides better penetration, and so would use an adjustable parallelogram-style ripper.

Photo. Typical hinge- or radial-style ripper (Nichols and Day 2005).
Figure 22. Photo. Typical hinge- or radial-style ripper (Nichols and Day 2005).

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