CONSTRUCTION METHODS & TECHNOLOGY CIVL462 Lecture No: 6

Rock Excavation

Rock characteristics Rock may be classified as igneous, sedimentary, or metamorphic. Igneous rock formed when the earth’s molten material cooled. Because of its origin, it is quite homogeneous and is therefore the most difficult type of rock to excavate. Examples of igneous rock are granite and basalt. Sedimentary rock was formed by the precipitation of material from water or air. As a result, it is highly stratified and has many planes of weakness. Thus it is the most easily excavated type of rock. Examples include sandstone, shale, and limestone. Metamorphic rock originated as igneous or sedimentary rock but has been changed by heat, pressure, or chemical action into a different type of rock. Metamorphic rock is intermediate between igneous rock and sedimentary rock in its difficulty of excavation. Examples of metamorphic rock include slate, marble, and schist.

Rock characteristics

Rock characteristics

Rock characteristics

Rock characteristics

Rock Investigation Relative hardness is measured on Moh’s scale from 1 (talc) to 10 (diamond). As a rule, any rock that can be scratched by a knife blade (hardness about 5) can be easily excavated by ripping or other mechanical methods. For harder rock, additional investigation is required to evaluate the rock characteristics described above. The principal methods for investigating subsurface conditions include drilling, excavating test pits, and making seismic measurements.

Rock Investigation Drilling may be used to remove core samples from the rock or to permit visual observation of rock conditions. Core samples maybe visually inspected as well as tested in the laboratory. Observation in a test pit or inspection by TV cameras placed into drilled holes will reveal layer thickness, the extent of fracturing and weathering, and the presence of water. Use of the refraction seismograph permits a rapid determination of rock soundness by measuring the velocity at which sound travels through the rock.

Rock Investigation In performing a seismicre fraction test, a sound source and an umber of receivers (geophones) are set up, as illustrated in Figure 1. The time required for a sound wave to travel from the sound source to each receiveris measured and plotted against the distance from the sound source, as illustrated in Figure2.In this plot the slope of each segment of the curve represents the sound velocity in the corresponding subsurface layer.

Rock Investigation The methods depend on the fact that seismic waves have differing velocities in different types of soil (or rock): in addition, the waves are refracted when they cross the boundary between different types (or conditions) of soil or rock. The methods enable the general soil types and the approximate depth to strata boundaries, or to bedrock, to be determined.

1 xc = 15.0 m 1 Depth to Rock: zc = 5.65 m Vp2 = 4880 m/s Seismic Refraction Vp2 = 4880 m/s 1 xc = 15.0 m Vp1 = 1350 m/s 1 Depth to Rock: zc = 5.65 m t values x values

Rock Investigation the thickness of rock layers. Equation may be used to determine the thickness of the upper layer when the sound velocity increases with layer depth, that is, when the velocity in the top layer is less than the velocity in the second layer, which is the usual case in the field.

Rock Investigation Find the seismic wave velocity and depth of the upper soil layer based on the following refraction seismograph data:

Rock Investigation

Rock Investigation

Rock-Handling Systems The process of rock moving may be considered in four phases: loosening, loading, hauling, and compacting Table 6-1. Principal rock-handling systems

DRILLING Drilling Equipment Common types of drilling equipment include percussion drills, rotary drills, and rotary percussion drills, as listed in Table. Percussion drills penetrate rock by impact action alone. While the bits of these drills rotate to assist in cleaning the hole and to provide a fresh surface for each impact, rotation takes place on the upstroke.

DRILLING Rotary blast hole drill.

Drilling Patterns and Rock Yield The choice of hole size, depth, and spacing, as well as the amount of explosive used for each hole, depends on the degree of rock break desired, rock type and soundness, and the type of explosive utilized.

Drilling Patterns and Rock Yield

example Trial blasting indicates that a rectangular pattern of drilling using 3-in. (7.6-cm) holes spaced on 9-ft (2.75-m) centers, 20 ft (6.1 m) deep will produce a satisfactory rock break with a particular explosive loading. The effective hole depth resulting from the blast is 18 ft (5.5 m). Determine the rock volume produced per foot (meter) of drilling.

example

BLASTING The principal explosives used for rock excavation include dynamite, ammonium nitrate, ammonium nitrate in fuel oil (ANFO), and slurries. For construction use dynamite has largely been replaced by ammonium nitrate, ANFO, and slurries because these explosives are lower in cost and easier to handle than dynamite. Ammonium nitrate and ANFO are the least expensive of the explosives listed. ANFO is particularly easy to handle because it is a liquid that may simply be poured into the blast hole. However, ammonium nitrate explosives are not water resistant, and they require an auxiliary explosive (primer) for detonation. Slurries are mixtures of gels, explosives, and water.

BLASTING They may also contain powdered metals(metalized slurries)to increase blast energy. Slurries are cheaper than dynamite but are more expensive than the ammonium nitrate explosives. Water resistance and greater power are their principal advantages over ammonium nitrate explosives. Slurries are available as liquids or packaged in plastic bags. Slurries also require a primer for detonation.

Dynamite A. Sawdust (or any other type of absorbent material) soaked in nitroglycerin. B. Protective coating surrounding the explosive material. C. Blasting cap. D. Electrical cable (or fuse) connected to the blasting cap.

BLASTING Detonators used to initiate an explosion include both electric and nonelectric caps. Electric blasting (EB) caps are most widely used and are available as instantaneous caps or with delay times from a few milliseconds up to several seconds. For less- sensitive explosives such as ammonium nitrates and slurries, caps are used to initiate primers, which in turn initiate the main explosive. Primers may be small charges of high explosives or primacord, which is a high explosive in cord form.

Detonator

BLASTING The amount of explosive required to produce the desired rock fracture is usually expressed as a powder factor. The powder factor represents the weight of explosive used per unit volume of rock produced (lb/BCY or kg/BCM). Except in specialized applications, blastholes are usually loaded with a continuous column of explosive to with in a few feet of the surface. Stemming (an inert material used to confine and increase the effectiveness of the blast) is placed in the top portion of the hole above the explosive. A primed charge is placed near the bottom of the hole for blast initiation.

BLASTING FIGURE 6-12. Types of electric blasting circuits.