208 Rock Blasting

Specification Changes

New item (formally Supplemental Specification 878).
The minimum length of the holes for the test section was changed to 20 feet.
The maximum hole diameter increased from 6 to 6 ¼ inches.
The insurance requirements were clarified and referenced in 107.12.
Sulfur and iron were added to the list of contaminates evaluated by the Hydrologist.
The detailed plan requirements for each cut was dropped.
Waiting to look at the back slopes prior to drilling ahead was changed.
The video recordings may be done electronically.
Payment requirements throughout the specification were moved to the Basis of Payment Section.

General Information

This specification, in more detail, is taught in a 5 day NHI Course, “Rock Blasting and Overbreak Control”. This section only covers the basic concepts.

Many of the figures and specification concepts originated from the Rock Blasting and Overbreak Control Manual.

Rock Blasting Basics

Rock Blasting consists of drilling holes in the rock at depths, in diameters, and at spacing so that the ANFO (ammonium nitrate [fertilizer] and fuel oil [diesel fuel]) can fracture the rock in a controlled manner. The rock must fracture enough to displace it and break it down to the size of the intended use.

The specifications have limited the way Blasting Contractors can blast so that rock or blast vibrations do not harm people or adjacent property.

Blasting Free Body Diagram

The basic geometry of Rock Blasting is shown in Figure 208.A.

Figure 208.A - Rock Blasting Free Body Diagram

Holes are drilled to the required depth to remove the rock, and filled with ANFO (Charge length). The charge is topped off with stemming that helps hold the blast down. The free body diagram in the right hand side of Figure 208.A shows a free body diagram illustrating the explosive pressure P and moment M from the blast.

The blaster and blasting consultant can arrange the geometry of the blast for optimal breakage. This is done so that P and M do not exceed the amount needed to break the rock. Excessive P and M causes ‘Fly Rock’ and excessive ‘Air Blast’ and ‘Vibration’ that can cause damage and injury.

Blasting Geometry and Symbols

Figure 208.B further defines the rock blasting geometry.

Figure 208.B - Rock Blasting Geometry and Symbols

Figure 208.B illustrates the following blast geometry parameters:

B (Burden) is the distance between the free face and the first hole.
T is the stemming (the inert material in the hole).
L is the length of the bench height.
H is the hole depth.
PC is the Powder Column Length. (ANFO).

Two main parameters to remember here are the L/B ratio and the stemming height.

Hole Spacing and Timing

The top view of the rock blasting geometry is shown in Figure 208.C. Notice the distance B is still the distance to the free face. The distance S or spacing of the holes is a function of the burden.

Figure 208.C - Rock Blasting (top view)

The spacing of the holes and the timing (or delay) of the holes are part of the blasting design. The bottom illustration in Figure 208.C shows how the blast is delayed by the sequencing numbers. Each hole may be blasted milliseconds apart to control the blast. The row-to-row shots are certainly time delayed.

An initiation system transfers the detonation signal from hole to hole at precise times. Plastic shock tubes or electric caps using a timing system are generally employed. A shock tube is non-electric, instantaneous, and has a thin reactive powder that propagates the shock wave signal.

The timing or delay minimizes the pounds of explosive per delay period. This can significantly control noise and vibration effects. It would be a disaster if all the holes went off at the same time.

The design variables of burden, stemming, subdrill, spacing, and timing are selected to maximize fragmentation and to minimize excessive vibration, airblast, and fly rock.

Affects of L/B Ratio

Figure 208.D shows what happens when the ratio between the distance L (Bench Height) and the Burden (B) is changed. Potential blasting problems are decreased as the ratio is increased. As this ratio is decreased, these problems are increased.

Stiffness Ratio (L/B)	1	2	3	4
Fragmentation	Poor	Fair	Good	Excellent
Air Blast	Severe	Fair	Good	Excellent
Flyrock	Severe	Fair	Good	Excellent
Ground Vibration	Severe	Fair	Good	Excellent
Comments	Severe backbreak & toe problems. Do not shoot. REDESIGN!	Redesign if possible.	Good control and fragmentation	No increased benefit by increasing stiffness ratio above 4.

Figure 208.D – Potential Problems as it Relates to Stiffness Ratio L/B

The specifications in 208.06.C require this ratio to be greater than one. ODOT blasters design the correct timing, hole spacing and stemming, and have not had problems with designs having a L/B ratio near one. Local blasters are also very familiar with local geology.

Generally, a ratio near one maximizes the rock blasting production. The main problem with designing a ratio that is near one is that the rock generally fractures in large chunks. This can pose problems for the Contractors when trying to use the material for fill.

When the ratio is increased, it can decrease the particle size of the rock. This allows the material to be used as fill easier.

Proper Burden

A rule of thumb in order to ensure that the Blaster is using the proper burden is as follows: Burden is usually 24 to 30 times the production hole diameter. For example:

If the production holes are 0.5 feet (6 inches) then the burden should be

24 x 0.5’ = 12’ or 30 x 0.5’ = 15’

The burden for the shot should be between 12 and 15 feet.

Affects of Stemming

The specifications in 208.06.E require that the stemming Depth (T) of inert material be at least 0.7 times the burden (B). This helps control the airblast.

Figure 208.E depicts the effects of stemming. If effective, the blast direction is lateral. If the stemming is ineffective, the blast can blow upward and cause excessive airblast. Notice that in the example, the blast cuts back into the cut slope. This is an obvious problem.

Figure 208.E - Stemming Effects

Drill cuttings are normally used for stemming. However, when blasting in water filled production holes, or when blasting within 200 feet of a structure, the stemming material is changed to prevent problems. For holes less than 4 inches, crushed number 8’s are required. For holes 4 inches or more, number 57’s are required. This helps hold the blast down better.

Affects of Timing

Timing the blast is another important parameter. Figure 208.F depicts the effects of poor and good timing.

Figure 208.F - Timing Effects

With correct timing, the blast has a distinct lateral movement. With poor timing, the movement is more upright and has potential problems.

Vibration and Airblast Monitoring

The blaster is required to design the burden, stemming, subdrill, spacing, and timing to minimize excessive vibration, airblast, and fly rock. The blaster must monitor the airblast and vibration for every shot at the nearest structure. Seismographs are used to monitor the vibration.

Specialized equipment is used to monitor the airblast. The maximum airblast, in 208.16.A, is required to be under 134 dB or lower if it causes damage.

A typical vibration criterion is given in Figure 208.G. This is from the US Bureau of Mines.

To lower the air blast check the stemming height and type of material used for the stemming. Thin or thick areas of the burden may create excess air blast and even fly rock. Read the burden of the free face to ensure of a uniform burden face.

To lower the vibration everything needs checked. This would include the blast design and layout of the blast holes.

Figure 208.G - Typical Vibration Blasting Criteria

Each blast has a particle velocity and frequency. The project can plot these values on the chart in Figure 208.G. If the plot is lower than the plotted line, the blast is considered not to cause damage.

Presplitting (208.09)

Presplitting is a very effective method of controlling the final appearance of steep slopes; it can result in a clean sheared face. Presplitting is required when the slope is steeper than 1:1 and deeper than 5 feet.

Specialized presplit blasting explosives are used. Hole diameters are about 3”, and the presplit holes are blasted prior to the production blast. The presplit hole spacing is started at 36 inches. This is adjusted to obtain a good shear face of the rock.