Click for next page ( 16


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 15
16 GEOLOGIC AND INDEX PROPERTIES OF ROCK TABLE 3 ROCK GROUPS AND TYPES The most basic characterization of rock for engineering Igneous purposes is a description of rock core based on visual and Intrusive Extrusive physical examination. The International Society of Rock (coarse-grained) (fine-grained) Pyroclastic Granite Rhyolite Obsidian Mechanics (ISRM) proposed a standardized method for Syenite Trachyte Pumice descriptions of rock masses from mapping and core logging Diorite Andesite Tuff ("Basic Geotechnical Description of Rock Masses" 1981). A Diabase Basalt Gabbro summary of the ISRM method as given by Wyllie (1999) is Peridotite adopted in the FHWA manuals on subsurface investigations Pegmatite and soil and rock properties (Mayne et al. 2001; Sabatini Sedimentary et al. 2002) and is summarized here. Clastic (sediment) (chemically formed) (organic remains) Shale Limestone Chalk Mudstone Dolomite Coquina A rock mass is described in terms of five categories of Claystone Gypsum Lignite properties, as follows: Siltstone Halite Coal Conglomerate Limestone, oolitic 1. Rock Material Description--a. Rock type, b. Wall Metamorphic strength, c. Weathering Foliated Nonfoliated 2. Discontinuity Description--d. Type, e. Orientation, f. Slate Quartzite Roughness, g. Aperture Phyllite Amphibolite Schist Marble 3. Infilling--h. Infilling type and width Gneiss Hornfels 4. Rock Mass Description--i. Spacing, j. Persistence, k. Number of sets, l. Block size/shape 5. Groundwater--m. Seepage. rock types listed in Table 3 based on lithologic characteristics that include color, fabric (microstructural and textural fea- Each of the 13 parameters listed (a through m) is assigned tures), grain size and shape (Tables 4 and 5), and mineralogy. a description using standardized terminology. Descriptive Sedimentary rock descriptions should include bedding thick- terms are given in Tables 3 through 6 and in Figure 11, which ness (Table 6). The rock unit name, which may be a formal is an example of a Key used for entering rock descriptions on name of a formation or an informal local name, should be a coring log and includes details of several categories. identified; for example, Bearpaw Shale or Sherman Granite. Rock Material Descriptors Compressive strength of rock core can be evaluated us- ing simple field tests with equipment commonly available Rock type is defined in terms of origin (igneous, sedimentary, (knife, rock hammer, etc.) and summarized in the Key of or metamorphic) and then further classified into one of the Figure 11 ("Rock Strength") or evaluated from point load TABLE 4 TERMS TO DESCRIBE GRAIN SIZE OF SEDIMENTARY ROCK Description Diameter (mm) Characteristic Very coarse grained >4.75 Grain sizes are greater than popcorn kernels Coarse grained 2.004.75 Individual grains can be easily distinguished by eye Medium grained 0.4252.00 Individual grains can be distinguished by eye Fine grained 0.0750.425 Individual grains can be distinguished with difficulty Very fine grained <0.075 Individual grains cannot be distinguished by unaided eye TABLE 5 TERMS TO DESCRIBE GRAIN SHAPE (for sedimentary rocks) Description Characteristic Angular Showing very little evidence of wear. Grain edges and corners are sharp. Secondary corners are numerous and sharp. Subangular Showing definite effects of wear. Grain edges and corners are slightly rounded off. Secondary corners are slightly less numerous and slightly less sharp than in angular grains. Subrounded Showing considerable wear. Grain edges and corners are rounded to smooth curves. Secondary corners are reduced greatly in number and highly rounded. Rounded Showing extreme wear. Grain edges and corners are smoothed off to broad curves. Secondary corners are few in number and rounded. Well-rounded Completely worn. Grain edges and corners are not present. No secondary edges or corners are present.

OCR for page 15
17 TABLE 6 tests or uniaxial compression tests conducted on specimens. TERMS TO DESCRIBE STRATUM The rock strength descriptions given at the bottom of the THICKNESS second page of the Key correspond to the seven categories Descriptive Term Stratum Thickness of rock strength, R0 through R6, of the ISRM ("Basic Geo- Very thickly bedded >1 m Thickly bedded 0.5 to 1.0 m technical Description of Rock Masses" 1981), with R0 cor- Thinly bedded 50 mm to 500 mm responding to extremely weak rock and R6 corresponding Very thinly bedded 10 mm to 50 mm to extremely strong rock. The degree of physical disinte- Laminated 2.5 mm to 10 mm Thinly laminated <2.5 mm gration or chemical alteration of rock can be described by the terms and abbreviations given in the Key. Weathering and alteration reduces shear strength of both intact rock and discontinuities. FIGURE 11 Key for rock core description (sheet 1).

OCR for page 15
18 FIGURE 11 (continued ) (sheet 2). Discontinuity Descriptors to the core. Roughness and surface shape of joint surfaces is best measured in the field on exposed surfaces at least 2 m in A discontinuity is defined as any surface across which any me- length and can be described using the terms in the Key or chanical property of a rock mass is discontinuous. Discon- quantified in terms of a Joint Roughness Coefficient (Barton tinuity descriptors are summarized in Figure 11 (Key), items a 1973). Aperture is the width of a discontinuity with no infill- through g. Types of discontinuities include faults, joints, shear ing and can be classified according to Box c of the Key. planes, foliation, veins, and bedding. Orientation refers to the measured dip and dip direction of the surface (or dip and strike). Dip is defined as the maximum angle of the plane to Infilling the horizontal and dip direction (strike) is the direction of the horizontal trace of the line of dip measured clockwise from Infilling is the term for material separating adjacent rock north, in degrees. Determination of dip and dip direction from walls of discontinuities. Infilling is described in terms of its core samples is possible using oriented coring techniques, type, amount, and width (Key). Additional laboratory testing borehole televiewers, downhole cameras, or other devices may be conducted to determine soil classification and shear capable of establishing orientation of the discontinuity relative strength of infilling materials. Direct shear tests provide a

OCR for page 15
19 means to measure shear strength of joints with infilling, as described by Wyllie and Norrish (1996). Infilling properties LENGTH OF SOUND L = 250 mm CORE > 100 mm vary widely and can have a significant influence on rock RQD = PIECES TOTAL CORE RUN LENGTH mass strength (RMS), compressibility, and permeability. L=0 250 + 190 + 200 HIGHLY WEATHERED RQD = 100% 1200 DOES NOT MEET SOUNDNESS REQUIREMENT Rock Mass Descriptors RQD = 53% (FAIR) L=0 Spacing is the perpendicular distance between adjacent dis- CENTER LINE CORE RUN TOTAL LENGTH = 1 2 0 0 m m PIECES < 4" & HIGHLY WEATHERED continuities. Spacing has a major influence on seepage and mechanical behavior and can be described using the terms in Figure 11 (Key). Persistence refers to the continuous length L = 190 mm or area of a discontinuity and requires field exposures for its determination. L=0 < 4" MECHANICAL BREAK CAUSED The number of sets of intersecting discontinuities has a BY DRILLING L = 200 mm PROCESS major effect on RMS and compressibility. As the number of sets increases, the extent to which the rock mass can deform without failure of intact rock also increases. Field mapping L=0 NO RECOVERY or observations made in exploratory pits or large excavations provide the best opportunity to map multiple sets of discon- FIGURE 12 RQD determination of rock core (after Deere and tinuities. Block size and shape is determined by spacing, per- Deere 1989). sistence, and number of intersecting sets of discontinuities. Descriptive terms include blocky, tabular, shattered, and measured along the centerline or axis of the core, as shown columnar, while size ranges from small (<0.0002 m3) to very in Figure 12. large (>8 m3). Only natural fractures such as joints or shear planes should be considered when calculating RQD. Core breaks Seepage caused by drilling or handling should be fitted together and Field observations of seepage from discontinuities should the pieces counted as intact lengths. Drilling breaks may be be described whenever it can be observed. The presence identified by fresh surfaces. For some laminated rocks it may and type of infilling controls joint permeability and should be difficult to distinguish natural fractures from those caused be described wherever seepage is observed. Seepage can by drilling. For characterization of rock mass behavior rele- range from dry to continuous flow under high pore water vant to foundation design it is conservative to not count the pressure length near horizontal breaks. RQD should be performed as soon as possible after the core is retrieved to avoid the effects of deterioration, which may include slaking and separation of Rock Quality Designation core along bedding planes, especially in moisture-sensitive rocks like some shales. It is also desirable because RQD is a A simple and widely used measure of rock mass quality is quantitative measure of core quality at the time of drilling provided by the RQD (rock quality designation, ASTM when the rock core is "fresh" and most representative of in D6032). RQD is equal to the sum of the lengths of sound situ conditions. pieces of core recovered, greater than 100 mm (4 in.) in length, expressed as a percentage of the length of the core Rock assigned a weathering classification of "highly weath- run. Originally introduced by Deere (1964), the RQD was ered" or above should not be included in the determination of evaluated by Deere and Deere (1989), who recommended RQD. RQD measurements assume that core recovery is at or modifications to the original procedure after evaluating its near 100%. As core recovery varies from 100%, explanatory field use. Figure 12 illustrates the recommended procedure. notes may be required to describe the reason for the variation Several factors must be evaluated properly for RQD to pro- and the effect on RQD. In some cases, RQD will have to be vide reliable results. determined on the basis of total length of core recovered, rather than on the length of rock cored. One state (Florida) uses per- RQD was originally recommended for NX size core, but cent core recovery as an index of rock quality in limestone. can also be used with the somewhat smaller NQ wireline sizes and with larger wire line sizes and other core sizes A general description of rock mass quality based on RQD up to 150 mm (6 in.). RQD based on the smaller BQ and is given here. Its wide use and ease of measurement make it BX cores or with single-tube core barrels is discouraged an important piece of information to be gathered on all core because of core breakage. Core segment lengths should be holes. Taken alone, RQD should be considered only as an