# Chapter 8: Geology

Geology is a dominant factor affecting blast performance. This chapter discusses and illustrates geology and the rock structures that influence blasting results. Blasters and blasting engineers should understand rock conditions and their potential impact on blasting results. Rock content and structural integrity influence blast performance in different ways. Rocks contain inherent strength based on their composition. Structural imperfections separate the mass from its original homogeneous condition. Structures create barriers that limit fragmentation. The blaster must have or develop the ability to "read the rock" and locate zones of varying strength and structure.

![Figure 8.1: Geologic profile showing complex geology in bench faces on two levels (masked by arrow)](images/074.png)

*Figure 8.1 – Geologic profile showing complex geology in bench faces on two levels (masked by arrow). (Courtesy: D. Ramsey)*

Blasters are challenged to use just enough energy to break the rock and modify designs to address gradual or abrupt rock mass changes at the blast site. Experience and knowledge help the blaster-in-charge evaluate results and improve future blast designs. In addition, local geologic conditions may directly influence project plans and affect blast designs.

The amount of energy in an explosive may need to be distributed or protected by stemming. At the rock drill, from blastability tests and data from the load per unit of volume and powder factor distribution methods rock properties can be determined. The more structured the mass is, the better the energy distribution must be to produce the desired fragmentation. Unless these factors are considered, blasting results will vary from expected. The focus of this chapter is rock blastability features and geotechnical characteristics. Blastability is as important as blast design parameters but they are two different topics. Rock factors such as borehole diameter and pattern layout, can be determined. Blastability is determined by a thorough characterization of the rock mass. The rock mass is characterized by its engineering (geotechnical and geological) properties, and structure. The engineering properties dictate the energy factor required to break the rock, while the structures determine the energy distribution required.

## Rock

In the 18th Edition, rock refers to the material to be blasted. The material is defined by its engineering properties.

## Rock Formation and Classification

Rocks form in different ways. The rock cycle illustrated in figure 8.2 depicts the earth's crust as a rock-recycling machine. As a result, rocks can be transformed from one class to another.

Initially, most rock masses are formed as homogeneous unions of minerals defined by their crystal structure, or particle composition. Rock masses are continually subjected to forces of nature that break, change, and deform them. This evolution or transformation to a complex non-homogeneous nature creates a challenge to blasters.

Rock is classified on the basis of the geologic process under which it was formed. Rocks are divided into three classes: (1) Sedimentary, (2) Igneous, and (3) Metamorphic. Each class has its own unique characteristics. Rocks are aggregates of: (1) one or more minerals; (2) undifferentiated mineral matter; or (3) solid organic materials.

![Figure 8.2: The rock cycle illustrating rock formation](images/075.png)

*Figure 8.2 – The rock cycle illustrating rock formation.*

### Sedimentary Rock

Sedimentary rock is formed by one of the three processes: (1) particle deposition into beds, (2) evaporation, or (3) accumulation of organic material. Drillers can usually identify where bedding planes intersect the borehole. Sedimentary rocks are very common and make up approximately 70% to 75% of rocks exposed at the earth's surface.

There are many types of sedimentary rock. The following discussions and illustrations are offered to illustrate general sedimentary rock qualities that affect blasting.

![Figure 8.3: Bench face in a sedimentary formation showing bedding](images/076.png)

*Figure 8.3 – Bench face in a sedimentary formation showing bedding. (Courtesy: B. Wingfield)*

#### Clastic Sedimentary Rocks

Clastic (fragmental or detrital) sedimentary rocks are depositional in nature and composed primarily of rock particles detached by the erosion of preexisting rocks. Particle cohesion is by a cementing material, or compaction. Erosion occurs when rocks are exposed to moving water, wind, or ice over long periods of time.

For example, once the weathered rock particles are detached, moving water, wind, or ice deposit particles as they lose the energy necessary to keep them suspended in motion. The larger and heavier particles are deposited first, followed by the smaller lighter particles, resulting in a sorted gradation.

Sedimentary rocks are composed of these rock fragments and loose aggregates. These rock fragments or sediments become rock by a process called lithification, where compaction and/or ceite=ation joins together the particles.

Clastic sedimentary rocks are often described by their component particle's size. It is helpful to be familiar with the two common systems used to define the names and sizes of soil and rock particles (*See* table 8.1).

### Rock Particle Size Grading Systems

| **Particle Name** | **Wentworth System Metric** | **Unified System U.S.** |
|---|---|---|
| Boulders | 256 mm or greater | 12 in. or greater |
| Cobbles | 256 mm – 64 mm | 12 in. – 3 in. |
| Gravel | 64mm – 2 mm | 3 in. – 0.75 in. |
| Sand | 2mm – 0.062 mm | 0.25 in. – 0.06 in. |
| Silt | 0.02 mm – 0.002 mm | Not available |
| Clay | 0.002 mm or less | Moldable |

*Table 8.1 – Rock particle size grading systems.*

#### Conglomerate, Breccia

Conglomerate and sedimentary breccia consist of large fragments of various rocks and minerals cemented together. Conglomerate is composed of smooth-rounded gravels or rocks (*See* figure 8.4), where sedimentary breccia is composed of rough-angular shaped gravels and rocks (*See* figure 8.5). Fragments forming these rocks may be of a single rock type or a combination of rock types.

![Figure 8.4: Conglomerate](images/077.png)

*Figure 8.4 – Conglomerate. (Courtesy: D. Ramsey)*

![Figure 8.5: Breccia](images/077.png)

*Figure 8.5 – Breccia. (Courtesy: R. Elliott)*

#### Sandstone

Sandstone is composed of rock or mineral fragments measuring approximately 2 millimeters (< inches) that have been cemented or bonded together by lithification. Normally the sand grains are predominantly quartz grains (see figure 8.6). Sandstone is often identified by the nature of the cement binding the grains together. For example, calcareous sandstone has calcite cement and siliceous sandstone has a silica (quartz) cement. If the grains are not predominantly quartz, the rock may be given a different name, such as arkose (feldspathic grains) or greywacke (quartz, feldspar and other mineral grains).

![Figure 8.6: Purple sandstone highwall (bottom right) and Sandstone specimen showing breakage across its grainy texture (top left)](images/078.png)

*Figure 8.6 – Purple sandstone highwall (bottom right) (Courtesy J. Wigand), and Sandstone specimen showing breakage across its grainy texture (top left). (Courtesy: D. Ramsey)*

#### Mud Rocks – Shales, Siltstones, Mudstones

These fine-grain clastic sedimentary rocks are composed of clay and silt sized particles. Mud rocks are typically much weaker than coarser grained varieties. Their bedding planes are typically more closely spaced than those found in coarser-grained varieties (*See* figure 8.6). They are relatively soft and have a characteristic platy structure (closely spaced parallel planes) (*See* figure 8.7) and can be easily broken into fine fragments. The presence of clays in these rocks can make them weak when exposed to moisture. Some clays such as bentonite can expand significantly when wetted. Rocks containing clay or silt sized particles can occur as beds of their own or thin parting beds between larger beds of other rocks such as limestone as shown in figure 8.8. In this case they present significant planes of weakness.

![Figure 8.7: Weathered mudstone](images/079.png)

*Figure 8.7 – Weathered mudstone. (Courtesy: J. Wigand)*

![Figure 8.8: Blasting breakage across limestone with shale (gray bands) bench face (right) and a sample of shale's platy texture (above)](images/079.png)

*Figure 8.8 – Blasting breakage across limestone with shale (gray bands) bench face (right) and a sample of shale's platy texture (above). (Courtesy: D. Ramsey)*

#### Evaporites

Evaporites are non-clastic sedimentary rock composed primarily of mineral salts precipitated from evaporating water. They are formed as a result of extensive or total evaporation of the solution's water. Rock salt and gypsum are the most common rocks that are characterized as evaporites. Limestone and dolomite are carbonate rocks that can be formed by the evaporation process. They can also be formed by the deposition and lithification of small macroscopic organisms and shells. Depending on the structure and cementation, limestone and dolomite can be notoriously blasted to produce desired fragmentation. Some evaporite deposits contain jointing and bedding planes that can pose a challenge under certain blasting conditions. It may be difficult to obtain acceptable pre-split results when blasting heavily-jointed limestones and dolomites. Evaporites generally have fine crystalline textures like that shown in figure 8.9.

![Figure 8.9: Blasting breakage across limestone bench face (left) and a sample of its fine crystalline texture (above)](images/080.png)

*Figure 8.9 – Blasting breakage across limestone bench face (left) (courtesy: R. Elliott) and a sample of its fine crystalline texture (above). (Courtesy: D. Ramsey)*

Under certain conditions, limestone can be dissolved by acidic ground and surface water, and, thus some deposits are honeycombed with solution features such as caves and voids. Sometimes the voids and caves reach the surface of the ground and create an irregular surface topography, commonly referred to as Karst topography.

#### Other Biochemical Sedimentary Rocks

Coal is a rock formed from the remains of plants and vegetation. The remains were typically deposited under conditions similar to our present day swamps. The carbonaceous remains are altered by pressure of overlying sediments into a peat composition. An additional accumulation of sediments are deposited over the peat, the peat is transformed into a soft, dull coal called lignite. Additional pressure, heat, and biochemical alteration to the lignite, produce a harder, more lustrous black coal referred to as bituminous (*See* figure 8.10). A further transformation of the bituminous coal by heat, pressure, and biochemical alteration, yields a metamorphic type of coal called anthracite (*See* figure 8.11). Anthracite is the highest grade of coal, and contains up to 98% carbon. Anthracite (*See* figure 8.11) is considerably harder than bituminous and, when burned, produces more heat than any of the other coals.

![Figure 8.10: Bituminous coal showing breakage across its texture](images/081.png)

*Figure 8.10 – Bituminous coal showing breakage across its texture. (Courtesy: D. Ramsey)*

Coal deposits can range from a few inches in thickness to tens of feet in thickness (*See* figures 8.12 through 8.14). Thick coal deposits like those mined in the Powder River Basin in the western United States, are sometimes blasted in benches. Coal can be easily damaged and diluted by overburden and interburden blasting activities.

![Figure 8.11: Anthracite coal showing breakage across its texture](images/081.png)

*Figure 8.11– Anthracite coal showing breakage across its texture. (Courtesy: D. Ramsey)*

![Figure 8.12: Coal stringer](images/082.png)

*Figure 8.12 – Coal stringer. (Courtesy: D. Ramsey)*

![Figure 8.13: Multi-seam coal](images/082.png)

*Figure 8.13 – Multi-seam coal. (Courtesy: J. Wigand)*

![Figure 8.14: Thick coal bed (75 feet)](images/083.png)

*Figure 8.14 – Thick coal bed (75 feet) (Courtesy: J. Wigand)*

### Igneous Rock

Igneous rocks form when molten or semi-molten material crystallizes or solidifies. Igneous rock can be formed by either solidifying within the earth, or solidifying on the earth's surface. Magma forms deep within the earth and flows to the surface and into openings within the earth's crust as illustrated in the figure 8.15. As molten rock cools, minerals form rocks with interlocking mineral grains. Different minerals crystallize at different temperatures. The cooling rate of the hot fluid controls the size of mineral crystals.

![Figure 8.15: Igneous rock formation process](images/083.png)

*Figure 8.15 – Igneous rock formation process.*

Igneous rocks are classified by crystal size, texture, and mineral content. They tend to be of low porosity. The interlocked crystal texture of igneous rocks tends to make them stronger than sedimentary rock types. They strength can be reduced by the preliminary fractures (discussed later) formed by cooling.

There are many types of igneous rock. The following discussions and illustrations are offered to illustrate general igneous rock qualities that affect blasting efficiency.

#### Intrusive Igneous Rock

Intrusive igneous rocks can be found within rocks of other classes as illustrated by the intrusions in figure 8.15 (e.g. sills and dykes). Intrusive igneous rocks cool more slowly than extrusive ones. Intrusive rocks often exhibit texture with interlocking crystals. The confined cooling of intrusive igneous rocks results in larger crystal sizes than found in extrusive deposits formed in lava flows and by pyroclastic ash. Figure 8.15 suggests how variable the thickness of intrusive igneous can be. The size of the formation usually controls the homogeneity and strength of the rock mass.

Common examples of intrusive igneous rocks (those that cool below the surface of the earth) include granites (intrusives), basalts (extrusive) (trap rocks), and gabbro (intrusive). Notice the characteristic interlocking crystal texture of intrusive rocks shown in figure 8.16.

![Figure 8.16: Quartz diorite showing breakage across interlocking crystal texture (above) and intrusive igneous rock face (quartz diorite) (right)](images/084.png)

*Figure 8.16 – Quartz diorite showing breakage across interlocking crystal texture (above) and intrusive igneous rock face (quartz diorite) (right). (Courtesy: R. Elliott)*

#### Extrusive Igneous Rocks

Molten rock solidifying above the earth's surface is called extrusive (its cooling rate is much faster than intrusive types). Rapid cooling by air, water, or escaping hot gasses during crystallization does not allow for large crystal growth. These cooling conditions create a broad range of igneous rock textures and breakages characteristics as illustrated in figures 8.17 through 8.20.

![Figure 8.17: Glassy Texture – Obsidian](images/085.png)

*Figure 8.17 – Glassy Texture – Obsidian. (Courtesy P. Konen)*

![Figure 8.18: Fine crystal texture-basalt](images/085.png)

*Figure 8.18 – Fine crystal texture-basalt. (Courtesy: R. Elliott)*

![Figure 8.19: Coarse crystal texture – quartz Diorite](images/085.png)

*Figure 8.19 – Coarse crystal texture – quartz Diorite. (Courtesy R. Elliott)*

![Figure 8.20: Porous texture – Vesicular basalt](images/086.png)

*Figure 8.20 – Porous texture – Vesicular basalt. (Courtesy: R. Elliott)*

### Metamorphic Rock

Metamorphic rock forms when existing igneous, sedimentary or other metamorphic rock is altered by subjection to intense pressure, heat, and fluid activity that changes their texture or composition. Intense heat and pressure occur at the boundary interface (*See* figure 8.21) with cooling magma or near edges of moving tectonic plates.

Metamorphism can also be associated with fault zones. Metamorphic rocks fall into one of the two categories: (1) Foliated (a platy texture where the arrangement of minerals resembles bedding), and (2) non-foliated. Foliation is the parallel alignment of altered mineral crystals.

Metamorphic rocks are described by their (1) foliation or lack of foliation, (2) mineralogical composition, and (3) grain size. When mineral grains are interlocked the rock gains strength.

Common examples of metamorphic rocks include slate, schist, gneiss, quartzite and marble. Metamorphic rocks are formed by the alteration of other rock types.

There are many types of metamorphic rock. The following discussions and illustrations are offered to illustrate general metamorphic rock qualities that affect blasting.

![Figure 8.21: Metamorphic rock formation process](images/086.png)

*Figure 8.21 – Metamorphic rock formation process.*

#### Foliated Metamorphic Rock

Two examples of common foliated metamorphic rocks are slate and gneiss. These two rocks illustrate very fine-grained to coarse-grained textures. Gneiss is shown in figure 8.22.

![Figure 8.22: Metamorphic specimen showing breakage across foliated texture (Granite gneiss) (left) and Granite gneiss face (below)](images/087.png)

*Figure 8.22 – Metamorphic specimen showing breakage across foliated texture (Granite gneiss) (left) and Granite gneiss face (below). (Courtesy: R. Elliott)*

#### Non-Foliated Metamorphic Rock

Some metamorphic rocks do not exhibit the foliation described above. These rocks do not have the inherent planes of weakness as in foliated types. Typically foliated rocks are more massive in nature than non-foliated rocks. Marble is illustrated in figure 8.23.

![Figure 8.23: Non-foliated metamorphic specimen showing breakage across the crystal grains (Marble) (left) and non-foliated metamorphic bench face (marble) (below)](images/088.png)

*Figure 8.23 – Non-foliated metamorphic specimen showing breakage across the crystal grains (Marble) (left) and non-foliated metamorphic bench face (marble) (below). (Courtesy: D. Ramsey)*

## Rock Mass Features

This section discusses the planar breaks ("structures") that sub-divide the rock and create free surfaces that influence the fragmentation process. Rock structure is the single most important geologic factor that affects blast performance. There are three basic types of structures: (1) bedding planes, (2) faults, and (3) joints. Bedding planes occur only in sedimentary rocks, but faults and joints can occur in all rock formations. Parallel and intersecting structure planes divide the rock into blocks of preexisting size.

The strength of these structures can vary significantly. Geologists and geotechnical engineers can help identify the properties of these structures. The identification and classification of structural extremes is extremely important when designing blasting programs. This is particularly important when blasting underground, where the stability of the roof and pillar is paramount. Structures can decrease drill penetration rates and cause drill bits and steel to break or bind when they intersect dipping bedding planes. Rock faces are highwalls can fail because of these faults and bedding planes. Flyrock and air overpressure can occur along these structures.

Open structures may be filled with sediments or chemical precipitates. Poor blasting results may be avoided when the blaster properly identifies and addresses these structures. When rock masses have a combination of intersecting structures, fragmentation size may be limited by the preexisting block sizes. These structures may limit fragmentation size.

### Non-Structure Rock Mass Features

| **Structure** | **Occurrence** |
|---|---|
| Geologic discontinuities | Blasting induced |
| Subterranean excavations | Man made |
| Mud pockets | Natural |
| Mud filled seams | Natural or Man made |

*Table 8.2 – Non-structure rock mass features.*

The following definitions describe the naturally occurring breaks in the rock mass.

### Discontinuity

A geologic discontinuity marks an abrupt change in the physical properties of adjacent parallel rock strata.

### Rock Structure

In this chapter, structure refers to a planar break in the rock mass where the physical properties of the adjacent rock materials are the same.

### Rock Folds

The complete spatial and geometrical configuration of all features that make up a deformed rock mass.

### Rock Structure Mapping

Rock structure mapping determines and records the location and trending direction of structures. Mapping allows the blast designer to anticipate their location for future blasts. Mapping describes the structure's trend direction in two ways: (1) Strike (bearing of the line of intersection of the inclined surface and an imaginary horizontal surface); and (2) Dip (slope angle from the earth's surface). Dip is always measured in a plane perpendicular to the strike. Strike and dip are illustrated in figure 8.24.

Structures are discovered in three ways: (1) Outcropping, (2) Drill logs, and (3) Local geologic maps. The driller can alert the blaster of an intersecting borehole and structure or discontinuity. The driller should identify this type of structure in the drill log. Once blasters have been notified of the anomaly, they can adjust borehole loading and initiation delay sequencing, if desired. The lateral extent of structures can be determined from area core drillings, additional outcrop sightings, or old mine and excavation records.

![Figure 8.24: Strike and dip](images/089.png)

*Figure 8.24 – Strike and dip.*

### Bedding Planes

Bedding planes are surfaces that separate sedimentary rock layers that differ considerably from other beds. A bedding plane marks an abrupt change in depositional conditions and possibly significant changes in rock properties. Bedding planes are usually recognized by changes in sediment particle size, composition, or color. Initially beds are formed in horizontal layers. Bedding planes can be zones of potential ore dilution when they are boundaries between mineable ore and waste rock. Tectonic forces can fold bedding as illustrated in figure 8.25.

The presence of bedding can be a positive rock blasting property, when: (1) it is strong enough to ensure confinement, (2) its thickness approaches the desired fragmentation size, or (3) it is located at a planned floor, roof, or wall location.

![Figure 8.25: Illustration showing regional folding features](images/090.png)

*Figure 8.25 – Illustration showing regional folding features.*

When beds are subjected to lateral or vertical forces, they can buckle and fold. In figure 8.25, the fold axis is parallel to the strike. When the beds are folded in an up-arched fold and strata dip away from the fold axis, the structure is called an anticline. In a syncline, the structure has a down-arched fold and the strata dip toward the axis of the fold. Folding can be quite exaggerated, producing dip angles ranging from 0° to 90°. Unfolded horizontal beds have a 0° dip angle. Blast site strike and dip changes with the blast site's orientation relative to the fold axis. In large projects the blaster often has some latitude to change face orientation to take advantage of strike and dip. Whereas, in smaller projects or in trenching, this opportunity does not exist.

As beds dip deeper into the earth, stripping costs to uncover the deposit may increase to the point where mining from the surface is not profitable or physically possible, and underground mining becomes the only mining option.

When inclined structures are present, proper blast design and face orientation can minimize negative blasting effects (e.g. highwall instability, backbreak, high bottom, misfires, and cutoffs). Figure 8.26 shows a face with dipping beds where the face is oriented perpendicular to the strike.

![Figure 8.26: Face showing dipping limestone beds (left) and illustration showing projection of dipping bedding planes beyond the blast site (right)](images/091.png)

*Figure 8.26 – Face showing dipping limestone beds (left) (Courtesy: D. Ramsey) and illustration showing projection of dipping bedding planes beyond the blast site (right).*

### Faults

Faults are fractures in rocks where there has been relative displacement of the two sides. Actual displacement is always measured in a vertical plane perpendicular to the strike of the fracture (like measuring the dip angle).

Faulted areas may be very extensive and recognized by surface features as illustrated in figure 8.27.

![Figure 8.27: Illustration showing faulting over a large area with associated surface features](images/091.png)

*Figure 8.27– Illustration showing faulting over a large area with associated surface features.*

There are three basic types of faults: (1) Normal, (2) Reverse, and (3) Lateral (Strike-Slip). They are illustrated in figures 8.28 through 8.30. The range magnitude of displacement is illustrated in figures 8.31 and 8.32.

![Figure 8.28: Normal fault](images/092.png)

*Figure 8.28 Normal fault.*

![Figure 8.29: Reverse fault](images/092.png)

*Figure 8.29 – Reverse fault.*

![Figure 8.30: Lateral fault (strike-slip)](images/092.png)

*Figure 8.30 – Lateral fault. (strike-slip)*

![Figure 8.31: Normal fault](images/093.png)

*Figure 8.31 – Normal fault. (Courtesy: J. Wigand)*

![Figure 8.32: Vertical strike slip fault](images/093.png)

*Figure 8.32– Vertical strike slip fault. (Courtesy: J. Wigand)*

### Joints

Joints can be the result of (1) tectonic forces sufficiently strong to fracture rock in place, or (2) the contraction of cooling magma. Therefore, when a fracture is identified as a joint, others should be suspected. Joints can have movement that is perpendicular to the fracture (open or widths), these faults show movement parallel to the fracture surface. When joints are formed by tectonic forces, there may be two to three distinct joint sets, intersecting at nearly mutually perpendicular angles as shown in figure 8.33.

![Figure 8.33: (Left) Bench face showing three intersecting joint sets and (Right) illustration of joint formation process resulting in up to three intersecting joint sets](images/094.png)

*Figure 8.33 – (Left) Bench face showing three intersecting joint sets (Courtesy: J. Floyd) and (Right) illustration of joint formation process resulting in up to three intersecting joint sets.*

The dominant joint set is called the primary set followed by the secondary and tertiary sets. This type of complex jointing occurrence alone divides the rock mass into preexisting blocks. Joints within a set usually occur in a series of parallel fractures dividing the rock into slab shaped masses as shown in figure 8.33.

Joint spacing, like bed thickness, (*See* figure 8.34) is typically a good predictor of their fragmentation size, especially in areas where there is poor explosives energy distribution. Poor explosives energy distribution occurs when borehole patterns are larger than joint spacing. See Overriding Geologic Factors discussed later in this chapter.

Joints sometimes separate or widen allowing the joint to fill with fine-grained materials such as mud or clay that serve to weaken the joint. These weakened joints could lead to one of four problems such as (1) poor fragmentation, (2) air overpressure, (3) flyrock, and (4) highwall failure.

When cooling magma contracts, intersecting joints can form a honeycomb pattern as illustrated in figure 8.35. These joints are commonly referred to as columnar jointing. In these formations final grade can be difficult to pull.

![Figure 8.34: Neatly vertical joint set (right) and illustration of vertical joint pattern (above left)](images/095.png)

*Figure 8.34 – Neatly vertical joint set (right) (Courtesy: D. Ramsey), and illustration of vertical joint pattern (above left).*

![Figure 8.35: Illustration of hexagonal columnar jointing process caused by cooling magma (above) and columnar joints in basalt (right)](images/095.png)

*Figure 8.35 – Illustration of hexagonal columnar jointing process caused by cooling magma (above) and columnar joints in basalt (right). (Courtesy: R. Elliott)*

### Voids and Caverns

Voids and caverns are formed in evaporite sedimentary deposits. If voids form close enough to the surface, sinkholes might form on the surface, as overlying rock strata collapses and subsides. It is not uncommon to see this type of condition where massive limestone deposits are present. Once again this kind of topography is called Karst topography. Karst topography can be easily recognizable on the surface.

The rock can be exposed to water for long periods of time. Voids and caverns pose blasting problems when boreholes get too close or intersect them. Problems include: (1) voids can be accidentally filled with explosives, (2) poor blasts can result from inadequate confinement, and (3) boreholes too close may have inadequate burden. If the void is an abandoned underground mine tunnel, air overpressure may be broadcast out of an old portal. Underground voids and caverns might also contribute to unusual off-site vibration characteristics.

![Figure 8.36: Solution cavity above coal seam](images/096.png)

*Figure 8.36 – Solution cavity above coal seam. (Courtesy: J. Wigand)*

Voids and caverns are formed in evaporite sedimentary deposits. If voids form close enough to the surface, sinkholes might form on the surface, as overlying rock strata collapses and subsides. It is not uncommon to see this type of condition where massive limestone deposits are present. Once again this kind of topography is called Karst topography. Karst topography can be easily recognizable on the surface.

## Blasting Damage

Blasting itself can damage the rock mass beyond the intended blast area, and cause cracks that weaken the rock. Cracks extending beyond the blast site are called "overbreak." When overbreak is not controlled from blasting results may be adversely affected. Overbreak damage is limited by both the (1) blast design control measures, and (2) degree of relief provided. Methods to control overbreak are discussed in chapter 36 and other chapters throughout this book.

Blast designs should always be engineered to optimize breakage and minimize overbreak. Overbreak manifests itself in three common ways: (1) backbreak, (2) preconditioning, and (3) ore dilution.

### Backbreak

Backbreak is rock broken or displaced beyond the intended blast boundary (back, sides and bottom (*See* figures 8.37 through 8.39). Operators sometimes consider rock from overbreak "free rock", since the rock is blasted beyond the designed blast pattern. Rarely does this type of overbreak create stable and true free faces desired for subsequent blasts, or create a safe and stable environment. Many times the consequences and costs to deal with backbreak blasting damage exceed the value of the so-called "free rock."

![Figure 8.37: Backbreak](images/097.png)

*Figure 8.37– Backbreak. (Courtesy: J. Wigand)*

Backbreak can make it difficult to collar boreholes and position them with the desired burden. Boreholes may have to be angled on subsequent blasts to achieve the desired burdens. Backbreak can occur when dipping or plunging rock strata is in close proximity to the blast column load is too close to the surface, or when inadequate lateral relief is present.

![Figure 8.38: Backbreak along the side](images/097.png)

*Figure 8.38 – Backbreak along the side. (Courtesy: J. Wigand)*

### Preconditioning

Preconditioning is a term used for describing cracking damage to free faces (*See* figure 8.39). Loose rocks in free faces can become pieces of flyrock. Shattering can occur in bench walls, floors, and in roofs and pillars in underground mines. Poor relief along final surfaces and overburdening in previous blasts can cause preconditioning and damage by blasting near a geologic structure.

Excessive subdrilling can contribute to the preconditioning of future bench floors (*See* figure 8.38). Preconditioned floors make collaring boreholes very difficult. Fractured rock around borehole collars increase the potential of borehole collapse, flyrock, and stemming ejection. The presence of preconditioning in the drilling surface may create the need for casings to be added but accessory drilling apparel.

An increase in air-blast vibration is a possibility in any blast when relief is insufficient. An increase in airblast is possible when the rock mass is weak. Overbreak can contribute to poor fragmentation (oversize), and increases the potential for flyrock, excessive blast noise, and inadequate confinement.

![Figure 8.39: Preconditioned bench face and top surface](images/098.png)

*Figure 8.39 – Preconditioned bench face and top surface. (Courtesy: D. Ramsey)*

When patterns are drilled in advance, prior blasts sometimes produce cracks that extend into the pre-drilled pattern as shown in figure 8.40. If such cracks exist before drilling begins, boreholes should not be located too close to the crack.

![Figure 8.40: Borehole too close to a blasting damage crack](images/099.png)

*Figure 8.40 – Borehole too close to a blasting damage crack. (Courtesy: D. Ramsey)*

### Other Potential Rock Weakness Areas

Mining and other excavations create other potentially weak rock features that must be treated with caution. Blasting near them can result in reduced burdens. One example is when surface mining operations approach abandoned or active underground mine workings. Over time the rock strata above an abandoned mine may subside. This may or may not be visible from the surface. Drilling may also intersect the old mine workings. If drilling intersects or occurs in close proximity to these structures, explosive confinement may be lost. Figure 8.41 shows overburden subsidence above an abandoned mine.

![Figure 8.41: Bed subsidence (arrow) over abandoned underground coal mine](images/099.png)

*Figure 8.41 – Bed subsidence (arrow) over abandoned underground coal mine. (Courtesy: D. Ramsey)*

If mining is conducted over abandoned underground mine workings, blasting operations may encroach on the abandoned workings causing the blasts to experience severe energy loss. A surface operation that passed through abandoned underground ore chutes is shown in figure 8.42.

![Figure 8.42: Exposed ore chutes (indicated by arrows) when mining through a discontinued underground mine](images/100.png)

*Figure 8.42 – Exposed ore chutes (indicated by arrows) when mining through a discontinued underground mine. (Courtesy: D. Ramsey)*

## Rock Geotechnical Properties

Specific rock types are characterized by their engineering properties. Rock properties are usually specified for drillers and blasters by six common properties: (1) density (specific gravity), (2) hardness, (3) porosity, (4) strength (compressive, tensile and shear), (5) rock velocity, and (6) resilience (elasticity). It is important to know and understand these properties for blast design and modeling purposes. The following definitions are offered for the understanding of these properties.

### Stress

Stress is the force per unit area on a rock.

### Strain

Strain is the change in shape of a rock due to stress. Strain is a measure of a rock's elastic property and is measured with strain gauges or extensometers.

### Rock Density

Rock density is its mass per unit volume, where the ratio of its density to the density of water is called its specific gravity.

Blast designers use rock density to design proper energy or powder factors. For blasters, accurate densities are important when converting rock volumes to weights for blast log reporting, billing requirements, or adherence to contractual obligations.

### Rock Resilience

Resilience is the rock's ability to store the elastic energy of strain. The two constant measures most commonly used are (1) modulus of elasticity and (2) Poisson's ratio.

### Modulus of Elasticity

The modulus of elasticity is the ratio of applied stress to its corresponding strain in elastic materials. Common synonyms are Young's modulus of elasticity and coefficient of elasticity.

### Poisson's Ratio

Poisson's ratio is the ratio of the lateral unit strain to the longitudinal unit strain of a rock that has been stressed longitudinally within its elastic limit.

### Rock Porosity

Rock porosity is a measure of the void space within a rock. A highly porous rock has a high percentage of voids or pore spaces. These voids or open spaces can increase the potential for a rock to take in and possibly hold water. Extreme porosity, as previously illustrated with vesicular basalt, can effectively reduce explosive energy confinement.

### Rock Strength

Rock strength is measured as the force under which rock fails or breaks. Rock can fail in three ways: (1) compression, (2) tension and, (3) shear. Rock is generally strongest in compression, so blast designs should strive to place the rock in tension for breakage and in shear for creating smooth surfaces, such as presplitting.

### Rock Velocity

Rock velocity is the velocity at which rock transmits a shock wave. Some theories recommend the explosive velocity of detonation (*See* chapter 14) exceed the rock velocity. Typically, most dense and low porosity formations have velocities that will transfer load effectively with medium to higher VOD products.

The properties for select rock types are usually based on a limited number of samples. Values can vary substantially from sample to sample. The only way to know precise values for a rock, is to measure them.

## Rock-Soil Interface

Rock near the earth's surface is subjected to weathering processes such as wind, water, ice, heat, and agents in the atmosphere. Soil is formed by the accumulation of weathered rock particles and organic materials above the bedrock. Over time this weathering zone becomes thicker and varied in nature. Knowledge about the localized rock-soil interface is important to blasters. Surface shipping operations and surface construction blasting often occur at this interface. Soils can be used as matting or as a "natural stemming" material in small-scale blasting operations. In large scale blasting operations caution should be exercised when loading explosives in or near this rock-soil interface. Typically, the overburden profile contains more rocks and rock fragments with depth, or near to the actual bedrock (*See* figure 8.43).

![Figure 8.43: View of a weathered rock-soil interface profile (left) and illustration of weathered rock-soil interface profile (right)](images/101.png)

*Figure 8.43 – View of a weathered rock-soil interface profile (left), (Courtesy: K. Eltschlager) and illustration of weathered rock-soil interface profile (right).*

Drillers and blasters constantly experience problems when they work at the rock-soil interface. Boreholes and collars tend to collapse or allow rocks to fall from fractured borehole walls into unloaded holes. Material can also fall into the hole when loading and contaminate or dilute the explosive product.

Irregularities and weaknesses in borehole walls sometimes allow mud to squeeze into the borehole and obstruct or plug the borehole. This may cause packaged explosives to hang, loading hoses to get snagged, and detonators to get damaged.

## Ground Water

Ground water can have an influence on five important blasting factors: (1) explosive type (e.g. water resistance, rheology, and density), (2) loading techniques (e.g. conventional, pumpable, or bulk loading), (3) potential vibration levels, (4) potential explosive performance problems (e.g. dead pressing and sympathetic detonation), and (5) potential detonator damage (water hammer). The water table is the surface below which rock is saturated. When drilling penetrates the water table, water can freely flow to the surface (*See* figure 8.44).

![Figure 8.44: Water flowing through borehole from the water table](images/102.png)

*Figure 8.44 – Water flowing through borehole from the water table. (Courtesy: D. Ramsey)*

Water can flow and migrate through rock layers and through rock fractures. A permeable rock medium that allows the flow of water is called an aquifer. The presence of water in a porous rock can change its engineering properties. Water can also slowly migrate through small fractures. Water can also be released out of solution in certain rock types. Over time, a sufficient amount of water may enter the borehole and adversely affect even water resistant explosive products.

During the preblast check of unloaded boreholes, water may be discovered in a borehole that was deemed dry when the drill log was originally completed. Moisture on borehole walls can deteriorate the periphery of ANFO or low water resistance products. For example, if the outer 3 millimeters (⅛ inch) of an ANFO column load is desensitized in a 102 millimeter (4 inch) diameter borehole, then approximately 12% of the column load is directly affected. If the same depth outer surface of an ANFO column load 406 millimeters (16 inch) diameter borehole is desensitized, then only about 3% of the column load is affected. Thus, the performance of water-degradable products in small diameter boreholes is more efficient than the same desensitized depth of larger diameter boreholes.

In cold climates ground water often freezes and thaws (*See* figure 8.45). When water freezes it expands by approximately 9% its original liquid volume. Repeated freeze-thaw cycles severely weaken exposed rock surfaces and can make stable highwalls weak and dangerous.

![Figure 8.45: Freezing water flow from wall cracks and surface](images/103.png)

*Figure 8.45 – Freezing water flow from wall cracks and surface. (Courtesy: J. Stokes)*

## Geologic Information in a Drill Log

The driller is a critically important member of the drilling and blasting team. The driller records as much geologic information as possible in the drill log. This information can help the blaster understand the borehole conditions which allows the blaster to load the borehole properly. Many times, geologic features are visible on the bench face and surface. At other times features like small cracks, fissures, joints, mud seams, cavities, and changes in rock hardness, are not easily recognizable. Drilling strategies to deal with these conditions are discussed in chapter 18. Documenting local geologic conditions, as accurately as possible, is necessary for a successful blast.

## Overriding Geologic Factors

A common mistake made in mine and quarry blasting operations is to repeat previous blasting patterns by laying out boreholes in nice evenly spaced geometric patterns, with rows parallel to the free face. In a changing geologic environment this may produce unexpected and undesirable blasting results.

Unfortunately, the location of geologic features and structures can change rapidly within a project area or blast site. Failure to locate, identify, and analyze can lead to understandable, and possibly dangerous blasting results.

## Contract Issues Due to Geology

Successful contracts often depend on clear agreement of what is considered rock and what is considered soil. For example, some projects define rock as "any material requiring blasting" and define soil as "any material that can be excavated without blasting." Simplistic descriptions can create contractual disputes. In contractual situations the blaster should reach agreement with the owner to clarify what is considered soil and rock before blasting begins (*See* figure 8.46).

![Figure 8.46: Illustration of a project transitioning from the rock-soil interface to rock](images/104.png)

*Figure 8.46. Illustration of a project transitioning from the rock-soil interface to rock.*

Sometimes, it can be difficult to distinguish between soil and rock when an excavation project begins in soil and continues through a weathered zone, and eventually encounters rock that cannot be excavated without drilling and blasting.

It is important to know the geologic identity of the material to be blasted and its specific engineering properties. At some project sites, the properties may vary considerably with changing depth and horizontal position. If the practice to record total tons blasted, but production by volume is required, an accurate rock conversion factor from volume to weight is necessary.

## Summary

Geology is the main factor that affects blasting results. Explosives should be chosen for their compatibility and explosive properties relative to a specific rock's geologic and geotechnical properties. Blast designers must determine proper borehole placement and detonation delay sequencing in order to compensate for changing rock features. Therefore blast designers and blasters-in-charge must always be aware of changing rock conditions and be willing to modify blasting operations to deal with these changes.

## Additional Resources

International Society of Explosives Engineers (ISEE). 1998. ISEE Blasters' Handbook TM 17th Edition. ISEE, Cleveland, OH.

American Geological Institute. 1997. Dictionary of Mining, Mineral, and Related Terms. 2nd Edition. American Geological Institute, Alexandria, VA.

Atlas Powder Company. 1987. Explosives and Rock Blasting. Atlas Powder company, Dallas, TX.

Bealky, W.C. 1978. Geology and its effect on blasting, p. 105. International Society of Explosives Engineers (ISEE) Proceedings of the 9th Annual Conference on Explosives and Blasting Technique, February 7 - 9. St. Louis, MO. ISEE, Cleveland, OH.

Goodman, Richard E. 1993. Engineering Geology. John Wiley & Sons, New York, NY.

Johnson, Robert B. and Jerome V. DeGraph. 2003. Principles of Engineering Geology, John Wiley and Sons, New York, NY.

Mine Safety and Health Administration (MSHA). Highwall Safety Presentation. MSHA, Washington, D.C.

Monroe, James S. and Reed Wicander. 2001. Physical Geology, Brooks/Cole, Pacific Grove, CA.