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Context Sensitive Rock Slope Design Solutions

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3. Rock Excavation Methods

Blasting Design

Blasting projects must be designed with several factors in mind, including the type of explosives used, borehole diameter, and loading levels. Each type of blasting-presplit, smooth, and cushion blasting-has its own formula.

Presplit Blasting

As discussed above, presplit blasting (or presplitting) is most often done before production blasting to create a secondary fracture plane that will protect the final slope face from damage in the main production blast. Table 4 lists recommended borehole diameters, burden, spacing, and explosive charges for presplit blasting.

Table 4. Parameters for drilling in a presplit blasting operation (modified from U.S. Department of the Interior 2001).

(mm) (in) (m) (ft) (kg/m) (lb/ft)
38-44 1.50-1.75 0.3-0.46 1.00-1.50 0.03-0.1 0.08-0.25
50-64 2.00-2.50 0.46-0.6 1.50-2.00 0.03-0.1 0.08-0.25
75-90 3.00-3.50 0.6-1.0 1.50-3.00 0.05-0.23 0.13-0.50
100 4.00 0.6-1.2 2.00-4.00 0.23-0.34 0.25-0.75

In order to reduce fracturing, presplit blasting holes are drilled with smaller diameters than production holes. Presplit-hole diameter will also be influenced by many other factors, as well. For example, large-diameter holes can hold more explosives and can be spaced further apart than small-diameter holes, but can cause more backbreak if the burden-to-spacing ratios are not properly designed. Large-diameter holes yield lower drilling and blasting costs because they are less expensive per unit volume to drill. Large-diameter holes are better suited for relatively homogeneous, easily fractured rock with few planes of weakness (discontinuities) and for deep rock cuts. Small-diameter holes use less explosives and require smaller spacing between holes, which allow for better distribution of explosives, more uniform rock breakage, less backbreak, and reduced ground vibrations. Although more holes must be drilled, small-diameter holes can be drilled quickly, resulting in a relatively low unit cost. However, this may be offset by higher explosives costs, as more explosives are required to fill the extra holes. In addition, drilling depths on small-diameter holes are limited because the small-diameter drill bits are more likely to wander at depth than larger bits.

Theoretically, the burden for presplit blasting is unlimited. But in reality, variations in geology that are not visible on the outer face of the slope can limit that burden. Thus, the engineer must core the interior of the slope to identify the condition of the rock before determining the blasting design. In any case, a minimum of 10 m (30 ft) of burden is recommended for any presplit blasting procedure.

Hole spacing in presplit blasting is typically 10 to 12 times the borehole diameter. In very favorable geologic conditions, spacing can be increased to 14 times the borehole diameter. Wider spacing is used for hard, competent material with relatively few discontinuities; in very soft and/or weathered materials, spacing is decreased. In weak and soft formations or where corners are blasted, unloaded guide holes are recommended to direct the cracking. These guide holes are drilled between the normally spaced presplit holes (thus, using guide holes prevents the contractor from spacing the presplit holes any further apart).

Holes used for presplit blasting are lightly loaded and range from 22.5 to 25 mm (7/8 to 1 in) in diameter. A heavier charge (2 to 3 times the normal load) is used at the bottom of the borehole to ensure shearing at the floor. A common charge density is approximately 0.45 kg per square meter (0.1 lbs per square foot) of face area in the main section of the hole and 0.9 to 1.3 kg per square meter (0.2 to 0.3 lbs per square foot) at the bottom. The loads may have an air annulus (ring) surrounding them to cushion the explosive blast and reduce the radial cracking around the borehole. Figure 20 indicates three configurations.

The authors of the DuPont Blaster's Handbook (1978) show that slurry or water gel (in the form of "Tovex T-1") can provide excellent presplitting results while permitting increased loading rates and reduced labor costs. Konya and Walter (2003) recommend ammonium nitrate for all controlled blasting. For small-scale blasting (such as sliver cuts) in presplitting operations, 50- grain detonation cord has proven effective.

Illustration. Three options using lightly loaded, distributed charges in presplit blasting (modified from U.S.
              Department of the Interior 2001).
Figure 20. Illustration. Three options using lightly loaded, distributed charges in presplit blasting (modified from U.S. Department of the Interior 2001).
Smooth Blasting

Smooth blasting is a type of controlled blasting that's done either before production blasting, as an alternative to presplitting, or afterwards, either as an entirely different event or as the last delay of the production blast. Table 5 shows recommended borehole diameters, burden, spacing, and explosive charges for smooth blasting.

Table 5. Parameters for drilling in a smooth blasting operation (modified from U.S. Department of the Interior 2001).

(mm) (in) (m) (ft) (m) (ft) (kg/m) (lb/ft)
38-44 1.50-1.75 0.6 2.00 1 3.00 0.05-0.55 0.12-0.25
50 2.0 0.75 2.50 1.06 3.50 0.05-0.55 0.12-0.25

As with presplit holes, smooth blasting holes are smaller than production holes in order to limit fracturing around the drill hole. The diameter of the hole is a function of geology, as discussed above.

The burden-to-spacing ratio for smooth blasting is approximately 1.5 to 1. Hole spacing for smooth blasting is slightly greater than presplit blasting, about 14 to 20 times the hole diameter, which means that holes are approximately 0.7 to 1.5 m (2.3 to 5 ft) apart. Wider spacing is used for hard rock and closer spacing is used for weak rock. Unloaded guide holes (drilled between the normally spaced blastholes) are recommended for weak and soft formations or for blasting corners.

The charge density, diameter, distribution, and explosive type used in smooth blasting are essentially the same as with presplit blasting. In smooth blasting, the borehole should be sealed with a tamping plug, clay plug, or other type of stemming to prevent the charge from being extruded from the hole by charges on earlier delays. Stemming also prevents excessive rifting (splitting) of the rock and permits the use of lighter charges because blast energy is better contained and therefore better distributed.

Cushion Blasting

Cushion blasting, another type of controlled blasting that's typically done after production blasting, uses a row of lightly loaded "buffer" holes filled with crushed stone (stemming), which reduces the impact on the surrounding rock as well as the finished slope face. Cushion blasting can be used with both vertical and angled holes, and good alignment is essential in both cases. The cushion holes are drilled along the final slope line and loaded with light, well-distributed charges and fired after the main production blast.

The required burden is established either by the last row of production boreholes or by a separate set of buffer holes (these buffer holes determine the burden so that the cushion blast holes produce enough backbreak to avoid borehole traces).

Table 6 shows the recommended borehole diameters, burden, spacing, and explosive charges for cushion blasting.

Table 6. Parameters for drilling in a cushion blasting operation (modified from U.S. Department of the Interior 2001).

Hole Diameter Spacing Burden Explosive Charge
(mm) (in) (m) (ft) (m) (ft) (kg/m) (lb/ft)
50-64 2.00-2.50 1.0 3.00 1.2 4.00 0.03-0.1 0.08-0.25
75-90 3.00-3.50 1.2 4.00 1.5 5.00 0.05-0.2 0.13-0.5
100-115 4.00-4.50 1.5 5.00 1.8 6.00 0.1-0.3 0.25-0.75
127-140 5.00-5.50 1.8 6.00 2.1 7.00 0.3-0.45 0.75-1.00
152-165 6.00-6.50 2.1 7.00 2.7 9.00 0.45-0.7 1.00-1.50

Diameters of buffer and cushion boreholes are smaller than those of production holes, and buffer boreholes have a slightly larger diameter than cushion boreholes. The diameter of a cushion hole depends on many factors, as discussed above.

The burden for cushion holes varies according to the rock characteristics. For example, the burden of hard, competent rock will be smaller than the burden of soft, easily fractured rock. It is important to conduct one or more test blasts and continually analyze and back-calculate results from production blasting to determine the proper burden, spacing, and charge density.

The spacing on cushion and buffer blastholes varies depending on the bedrock type and structural characteristics, but generally ranges from 15 to 24 times the borehole diameter. The burden-to-spacing relationship varies, but spacing on cushion holes should always be less than the width of the burden being removed (U.S. Department of the Interior 2005). When removing weak, heavily fractured material or when blasting corners, uncharged guide holes can be drilled between the normally spaced cushion holes to guide the blast-induced fracture.

Charge loading in cushion blasting is similar to that in smooth blasting in that lightly loaded, well-distributed charges are fired after the main production blast. Charges are typically 25 mm (1 in) in diameter and charge densities are 0.45 to 0.7 kg per square meter (0.1 to 0.15 lb per square foot) of face area for the main borehole and two to three times that for the bottom of the borehole. Historically, cushion holes are surrounded by some type of stemming (crushed rock or other loose inert material that helps cushion the blast) for the entire length of the borehole. According to the DuPont Blaster's Handbook (1978), an air annulus surrounding the charge produces similar results and reduces loading time.

Removing Drill Hole Traces and Blasting Scars

In weak rock, drill hole traces can be removed by chipping away at the corners of the traces or removing the outer layer of rock using a hoe ram, excavator bucket, or pneumatic hammer. For strong rock, it is nearly impossible to completely remove these traces.

In some cases, blasting is followed by other methods of excavation, which are used to get rid of remaining rock and soil to create a more aesthetically appealing final face. In cases where there is less rock to remove in the first place, these methods can be used on their own.

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