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Context Sensitive Rock Slope Design Solutions

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3. Rock Excavation Methods

Blasting Methods

Blasting is used for rock excavation on both small- and large-scale projects. There are two general types: production blasting and controlled blasting.

Production Blasting

Production blasting uses large explosive charges, widely spaced, that are designed to fragment a large amount of burden (the rock that lies between the existing slope face and the blasthole).

Production blasting is the most efficient way to remove large rock burdens, but it typically creates radial fractures around the blasthole and backbreak (fractures that extend into the final slope face), which reduce the strength of the remaining rock mass and increase its susceptibility to slope raveling and rockfall.

Controlled Blasting

Controlled blasting is used for removing material along the final slope face. In some cases, it's also used before production blasting to create an artificial fracture along the final cut slope, which will prevent the radial cracks caused by production blasting from penetrating back into the finished face.

Controlled blasting can also be used alone, without production blasting. Controlled blasting creates less backbreak than production blasting because it removes less burden and uses more tightly spaced drill holes with lighter charges.

There are several types of controlled blasting; they vary most importantly in the amount of burden they remove and the type of powder they use. The discussion below will focus on controlled blasting techniques that best minimize the visual impacts of the blasting process, thus meeting the objectives of CSS design. These techniques are presplit blasting, smooth blasting, and cushion blasting.

Presplit Blasting:

Presplit blasting, or presplitting, is used before production blasting to protect the final rock face from damage caused by the production blasting. Presplitting creates a fracture plane along the final slope face, which prevents the radial cracks created by production blasting from penetrating into the finished face; without presplitting, production blasting damage can extend up to 15 m (50 ft) into the final slope face. Presplitting also allows for steeper and more stable cuts than any other blasting procedure. In massively bedded, competent rock, a properly charged presplit blast will contain drill hole half cast (the hole trace is split in half, axially) for almost the entire length of the blast line and will have no backbreak because the energy from the blast will travel uniformly, thus creating a continuous fracture between holes.

However, presplitting creates abundant visible drill traces, which makes it unsuitable in some areas (such as national parks). In some cases, these half casts can be chipped away with a pneumatic hammer, but it's very difficult to eliminate them without completely removing the outer layer of rock. In areas where such scars are not acceptable, presplit blasting will not be a suitable option. Figure 12 shows an example of a cut slope that used presplit basting methods which left visible half casts in the slope face.

Presplit blasting requires relatively small drill holes, between 5 to 10 cm (2 to 4 in) in diameter because its goal is to create discrete fractures, not massive breaking. However, because the small hole diameters allow the drill bit to deviate from the anticipated line more readily than larger drill diameters; the maximum depth of presplitting is usually about 15 m (50 ft). For this reason presplitting is used only for relatively small blasting operations.

Because of these limitations, presplitting is most often used on slopes steeper than 1H:1V (45°), which helps the drillers to maintain adequate hole alignment at depth. Presplitting performs best in competent, hard to extremely hard rock; it is the best method for minimizing backbreak, as the induced fracture plane prevents the shockwave from the main blast from being effectively propagated behind the final face of the rock mass. Presplitting is most difficult in highly fractured, weathered, and/or soft rock, where it requires the use of closely spaced drill holes and/or uncharged guide holes (see below).

Photo. Example of a presplit slope in massive sandstone. Note the abundant drill hole traces (half casts).
Figure 12. Photo. Example of a presplit slope in massive sandstone. Note the abundant drill hole traces (half casts).

Smooth Blasting:

Smooth blasting, also called contour blasting or perimeter blasting, can be used before production blasting as an alternative to presplitting. It's also used after production blasting, either as an entirely different event or as the last delay of the production blast. Smooth blasting uses drill holes with roughly the same diameter and depth as those used in presplitting, spaced slightly further apart and loaded with a slightly larger charge density. If the burden is adequately reduced, smooth blasting produces a more ragged slope face with minimal backbreak.

Smooth-blasted slopes may require more maintenance than presplit slopes due to increased radial fractures from the controlled blasting and overall fracturing from production blasting. Although smooth blasting creates abundant drill hole traces, they're generally less noticeable than the half casts left by presplitting. If drill hole traces are not acceptable, smooth blasting may be suitable only if the cut slope height is small and the drill traces can be easily removed with a pneumatic hammer or other device (see below).

Smooth blasting is best preformed in hard, competent rock, although it can be used in soft or highly fractured rock by increasing the spacing of the drill holes and/or adding uncharged guide holes to the pattern. Smooth slope blasting can be used on a variety of cut slope angles and is effective in developing contoured slopes with benches or other slope variations.

Cushion Blasting:

Cushion blasting, sometimes referred to as trim blasting, uses a row of lightly loaded "buffer" holes filled with crushed stone over the entire depth of the hole, which reduce the impact on the blasting holes and protect the surrounding rock mass from the shock caused by the blast, thus minimizing the stress and fractures in the finished slope face as shown in Figure 13.

Figure 13 also illustrates other blast hole drilling techniques (breaker, production, and looker drill holes), which can be used in conjunction with cushion blasting to fragment and mobilize the rock mass in the production zone. The application of these drilling methods is contingent on the structural characteristics of the rock, the existing and final slope geometry, and access via pioneering to the production zone.

Illustration. Cross section of a cushion blasting design using buffer holes to control the burden on the cushion
            holes (modified from Cummings 2002).
Figure 13. Illustration. Cross section of a cushion blasting design using buffer holes to control the burden on the cushion holes (modified from Cummings 2002).

The maximum diameter for cushion holes used in transportation is typically 75 mm (3 in). The drill steel used to advance these smaller holes tends to drift at depth, meaning the maximum depth is usually held to 12 m (40 ft). Cushion blasting creates some backbreak, which can make a slope more prone to raveling. Because cushion blasting can increase the danger of rockfall, the catchment area may need to be enlarged.

Figure 14 shows a recently constructed slope created using cushion blasting in fractured granitic rock. Note the wider ditch width in relation to the cut height.

Cushion blasting is more demanding than presplit or smooth blasting for the explosives engineer because hole spacing, burden, and charge density must be carefully chosen and continually reassessed in order to minimize backbreak. It also can be more time consuming because more drilling is required and charges take more time to load.

In poorly lithified, moderately to highly fractured and weathered formations, cushion blasting produces better results than smooth or presplit slope blasting. However, even cushion blasting may can leave drill hole traces in massive, homogeneous formations with few fractures.

Figure 14. Photo. Final configuration of a cushion-blasted slope in granitic rock.
Figure 14. Photo. Final configuration of a cushion-blasted slope in granitic rock.

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