GEOTECHNICAL DESIGN MANUAL
CHAPTER 5
SOIL AND ROCK CLASSIFICATION AND LOGGING
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Table of Contents
5.1 OVERVIEW ...... 5-4
5.2 SOIL CLASSIFICATION ...... 5-6 5.2.1 Brief Description of Terms ...... 5-6 5.2.2 Coarse-Grained Soils ...... 5-6 5.2.3 Fine-Grained Inorganic Soils ...... 5-7 5.2.4 Organic Fine-Grained Soils ...... 5-8 5.2.5 Highly Organic Soils ...... 5-9 5.2.6 Particle Shape ...... 5-9 5.2.7 Color ...... 5-10 5.2.8 Moisture ...... 5-10 5.2.9 Plasticity ...... 5-10 5.2.10 Soil Structure ...... 5-11 5.2.11 HCI Reaction...... 5-11 5.2.12 Subsurface Exploration Logging...... 5-12 5.2.12.1 NYSDOT Soil Description ...... 5-12 5.2.12.1.1 Use of NYSDOT Soil Identification System on Subsurface Exploration Log ...... 5-14 5.2.12.2 Unified Soil Classification System ...... 5-16 5.2.12.3 AASHTO Soil Classification System ...... 5-23
5.3 ROCK CLASSIFICATION ...... 5-26 5.3.1 Rock Type ...... 5-26 5.3.1.1 Igneous Rocks ...... 5-26 5.3.1.2 Sedimentary Rocks ...... 5-26 5.3.1.3 Metamorphic Rocks ...... 5-27 5.3.2 Rock Properties ...... 5-27 5.3.2.1 Rock Color ...... 5-27 5.3.2.2 Mineralogy ...... 5-27 5.3.2.3 Weathered State of Rock ...... 5-27 5.3.2.4 Relative Rock Strength and Hardness ...... 5-28 5.3.2.5 Fractures ...... 5-28 5.3.2.6 Core Recovery (CR) ...... 5-28 5.3.2.7 Rock Quality Designation (RQD) ...... 5-28 5.3.2.8 Additional Rock Properties ...... 5-28 5.3.3 Rock Core Logging ...... 5-28 5.3.3.1 NYSDOT Rock Core ...... 5-28
5.4 BRIEF GLOSSARY OF GEOLOGIC TERMS ...... 5-33
5.5 REFERENCES ...... 5-33
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5.1 OVERVIEW
The detailed description and classification of soil and rock are an essential part of the geologic interpretation process and the geotechnical information developed to support design and construction. The description and classification of soil and rock includes consideration of the physical characteristics and engineering properties of the material. The soil and rock descriptions that are contained on the field logs should be based on factual information. Interpretive information may be included on the field logs if a description is provided on the reason for the interpretive information. This chapter provides standards for describing and logging soil and rock.
The NYSDOT Soil Classification System is outlined in the Geotechnical Engineering Bureau’s An Engineering Description of Soils Visual-Manual Procedure (GTP-2). This manual presents a procedure for describing soil samples obtained for foundation engineering and earthwork purposes by the New York State Department of Transportation. The procedure involves visually and manually examining soil samples with respect to texture, plasticity and color. The classification method establishes a “word picture” on the moisture and relative percentages of types of soil particles which can be entered onto official documents such as a subsurface exploration log or other appropriate data sheet.
Another classification system, the Unified Soil Classification System (USCS) as outlined in ASTM 2487 – “Standard Practices for Classification of Soils for Engineering Purposes (Unified Soil Classification System)”, provides a conventional system for classifying soils which is a precise classification system based on laboratory determination of particle-size characteristics, liquid limit, and plasticity index. As an option, ASTM 2488 – “Standard Practices for Description and Identification of Soils (Visual – Manual Procedure)” identifies a procedure for the description of soils for engineering purposes. This practice describes a procedure for identifying soils based on visual examination and manual tests.
The NYSDOT Rock Classification System is outlined in the Geotechnical Engineering Bureau’s Rock Core Evaluation Manual (GEM-23). It presents a procedure for describing rock core samples obtained for the New York State Department of Transportation by State work forces, and/or private drilling companies, for foundation, rockslope, and other engineering purposes. The evaluation is intended to provide a comprehensive word description of the core samples to those involved in the planning, design, construction, and maintenance processes.
There are numerous other rock classification systems, but none of these is universally used. This chapter provides a composite of those classification systems that incorporates the significant descriptive terminology relevant to geotechnical design and construction.
An important facet of soil and rock classification is the determination of what constitutes “rock”, as opposed to extremely weathered, partially cemented, or altered material that approaches soil in its character and engineering characteristics. Extremely soft or decomposed rock that is friable (easily crumbled), and can be reduced to gravel size or smaller by normal hand pressure, should be classified as a soil.
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COARSE-GRAINED SOILS (Identify by Size of Particles)
BOULDERS COBBLES GRAVEL SAND Greater Than 12” 12” to 3” 3” to No.10 No.10 to No. 200 (305 mm) (305 mm to 75 COARSE COARSE mm) 3” to 1” No. 10 to No. 40 (75 mm to 25 mm) (2.0 mm to 0.425 mm) FINE FINE 1” to No. 10 No. 40 to No. 200 (25 mm to 2.0 mm) (0.425 mm to 0.075 mm)
FINE-GRAINED SOILS Identify by Behavior (Individual Particles Not Visible)
SILT CLAY 1. Nonplastic. 1. Plastic, acts like putty when moist and 2. Powders easily when dry. w et. 3. Dries rapidly. 2. High dry strength. 4. Free w ater appears w hen shaken. 3. Dries slow ly – sticky. 5. Wire cut surface – rough. 4. No free w ater w hen shaken. 5. Wire cut surface – smooth.
MIXED-GRAINED SOILS Significant proportions of coarse-grained and fine-grained sizes. Make judgment on whether fine-grained or coarse- grained dominates behavior.
ORGANIC SOILS (Identify by Appearance, Behavior, Color, etc.)
PEAT MUCK MARL ORGANIC ORGANIC SILT CLAY
Figure 5-1 NYSDOT Identification Procedure Chart NYSDOT Geotechnical Page 5-5 June 17, 2013 Design Manual
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5.2 SOIL CLASSIFICATION
Soils are divided into four broad categories. These soil categories are coarse-grained soils, fine- grained inorganic soils, organic soils, and peat. The first step in identifying soil is to register the soil into one of the four broad categories. The definitions for these broad categories are as follows: • Coarse-Grained Soils: Soils that contain 50 % or less of soil particles passing a 0.0030 in. (0.075 mm) opening. • Fine-Grained Inorganic Soils: Soils that contain more than 50 % of soil particles passing a 0.0030 in. (0.075 mm) opening. • Fine-Grained Organic Soils: Soils that contain enough organic particles to influence the soil properties. • Peat: Soils that are composed primarily of vegetative tissue in various stages of decomposition that have a fibrous to amorphous texture, usually dark brown to black, and an organic odor are designated as a highly organic soil called peat. Once a soil has been identified as a peat (group symbol PT), the soil should not be subjected to any further identification procedures.
5.2.1 Brief Definition of Terms
The Geotechnical Engineering Bureau’s An Engineering Description of Soils Visual-Manual Procedure (GTP-2) presents a brief glossary of terms in GTP-2, Part 2. Definition of Terms.
Figure 5-2 Particle Size
5.2.2 Coarse-Grained Soils
Coarse-grained soils consist predominately of cobbles, gravel or sand size particles and are non- plastic. Density of coarse-grained soils in the natural state is often based on the soil sampler blows per foot of penetration. The following general guide may be used for field description by Drillers and Inspectors:
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No. of Blows per foot with Density Automatic Hammer (SPT)1 Very Loose 0 – 4 Loose 5 – 10 Medium Compact 11 – 24 Compact 25 – 50 Very Compact Over 50 1 Standard Penetration Test – 30 in. drop of 140 lb. hammer, 2 in. OD sampler
Table 5-1 Density of Coarse-Grained Soil Based on Sampler Blows
5.2.3 Fine-Grained Inorganic Soils
Table 5-2 applies to fine-grained plastic soils consisting predominately of silt and clay particles. For non-plastic silt, use adjectives and blows in coarse-grained Table 5-1.
There is an approximate relationship between consistency of plastic fine-grained soil and blows on the sampler as indicated below. Also, the consistency can be estimated for a relatively undisturbed sample by molding in hand as indicated below. If hand rating shows greater consistency, use this rating on log.
No. of Blows per foot Field Test on Approximate Consistency with Relatively Shear Strength Automatic Hammer Undisturbed psf (SPT)1 Samples Very Soft 0 – 1 When squeezed in fist 0 to 250 will ooze between fingers. Soft 2 – 4 Easily molded in 250 to 500 fingers. Firm 5 – 8 Can by molded by 500 to 1000 strong pressure with fingers. Stiff 9 – 15 Readily indented with 1000 to 2000 thumb – difficult to mold in fingers. Hard Over 15 Difficult to indent 2000 and greater with thumb nail. 1 Standard Penetration Test – 30 in. drop of 140 lb. hammer, 2 in. OD sampler
Table 5-2 Consistency of Fine-Grained Soil Based on Sampler Blows
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The field list identified in Table 5-3 may be used to distinguish between the structural characteristics of a silt or clay soil.
Characteristic Silt Clay Plasticity in the moist state Blocks of soil cannot be Blocks of soil are sticky and reshaped without crumbling. can be reshaped without crumbling. Cohesiveness in dry state Will crumble with moderate Crumbles with great difficulty hand pressure and slake and slakes slowly in water. readily under water. Visual inspection and feel Coarse silt grains can be seen. Clay grains cannot be Silt feels gritty when rubbed observed by visual inspection. between fingers. Wire cut of Soil feels smooth and greasy soil leaves feathered surface. when rubbed between fingers. Wire cut of soil leaves smooth, shiny surface. Settlement in water Will settle out of suspension Will stay in suspension in within 10 hrs. water for several days. Movement of water in the When moist silt is shaken in When moist clay is shaken in voids the palm of the hand, a watery the palm of the hand, a watery sheen will appear on the sheen will not appear on the surface of the soil. When surface. shaking is stopped, the watery sheen will gradually disappear.
Table 5-3 Field Identification of Fine-Grained Soils
5.2.4 Organic Fine-Grained Soils
If the soil contains enough organic particles to influence the soil properties or performance, it should be identified as an organic fine-grained soil.
• Organic clay and organic silt have microscopic-size organic material dispersed throughout their matrices. This soil may contain shells and/or fibers and has a weak structure, which exhibits little resistance to kneading. • Marl is a white or gray calcium carbonate paste. It may contain granular spheres, shells, organic material or inorganic soils and reacts with weak hydrochloric acid. • Muck is a black or dark brown finely divided organic material mixed with various proportions of sand, silt, and clay. It may contain minor amounts of fibrous material such as roots, leaves, and sedges.
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5.2.5 Highly Organic Soils
Highly organic soils which have characteristics that are undesirable for construction materials and foundations are individually classified. No laboratory criteria are established for these soils, as they generally can be readily identified in the field by their distinctive color, odor (i.e. decaying vegetation), spongy feel, and, frequently, fibrous textures. Particles of leaves, grass, branches, or other fibrous vegetable matter are common components of these soils.
• Peat is comprised of black or dark brown plant remains. The visible plant remains range from coarse fibers to finely divided organic material.
5.2.6 Particle Shape
The field description of angularity of the coarse-sized particles of a soil (gravel, cobbles and sand) are provided in Table 5-4.
Figure 5-3 Particle Shape
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Description Criteria Angular Coarse-grained particles have sharp edges and relatively plane sides with unpolished surfaces. Subangular Coarse-grained particles are similar to angular description but have rounded edges. Subrounded Coarse-grained particles have nearly plane sides but have well rounded corners and edges. Rounded Coarse-grained particles have smoothly curved sides and no edges.
Table 5-4 Particle Shape Description
5.2.7 Color
Soil color is not in itself a specific engineering property, but may be an indicator of other significant geologic processes that may be occurring within the soil mass. Color may also aid in the subsurface correlation of soil units. Soil color should be determined in the field at their natural moisture content. The predominant color of the soil should be based on the Munsell Soil Color Charts.
5.2.8 Moisture
A visual estimation of the relative moisture content of the soil should be made during the field classification. The field moisture content of the soil is based on the criteria outlined in Table 5-5 and should be judged by appearance of the sample before manipulating.
Moisture Description Criteria Dry Absence of moisture; dusty; dry to the touch. Moist Damp but no visible water. Wet Visible free water.
Table 5-5 Moisture Condition
5.2.9 Plasticity
Plasticity is the propensity of a material to undergo permanent deformation under load or remolding. Significantly plastic soils can be transformed from a solid to a putty-like, and ultimately fluid-like, state by adding water to the matrix of the soil. Increasing the moisture content of a plastic soil reduces the soil’s shear resistance to sliding.
A fine-grained sample may be termed Plastic (PL), Low Plastic (LPL), or Non-plastic (NP). The sample must be in a moist or wet condition for the plasticity determination. For dry samples requiring wetting, make a note in the description. Example: “plastic (low or nonplastic) when wet”. Plasticity is not required for marl, muck and peat. NYSDOT Geotechnical Page 5-10 June 17, 2013 Design Manual
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5.2.10 Soil Structure
Soils often contain depositional or physical features that are referred to as soil structure. These features are described in Table 5-6. The description of layering for coarse-grained soils must be made from field observations before the sample is removed from the sampler.
Description Criteria Fissured Breaks along existing planes with little resistance to fracturing. Blocky Low to moderate cohesive soil that can be broken down into smaller angular clumps. Varved Varves form in a variety of marine and lacustrine depositional environments. The classic pattern is a light/dark colored couplet. Light layer usually comprised of coarser silt and fine sand deposited under high energy conditions, while the dark is the fine clay-sized sediment. Layered Alternating layers of varying material or color with layers at least ¼ in. thick, note thickness and inclination. Homogeneous Same color and appearance throughout.
Table 5-6 Soil Structure
5.2.11 HCL Reaction
Calcium carbonate is a common cementing agent in soils. To test for the presence of this cementing agent, the soil sample may be tested with dilute hydrochloric acid (HCL). The reaction of the soil sample with HCL is reported as outlined in Table 5-7.
HCL Reaction Description Criteria No HCL Reaction No visible reaction. Weak HCL Reaction Some reaction with bubbles forming slowly. Strong HCL Reaction Violent reaction with bubbles forming immediately.
Table 5-7 Soil Reaction to Hydrochloric Acid
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5.2.12 Subsurface Exploration Logging
5.2.12.1 NYSDOT Soil Description
The NYSDOT Soil Classification System is outlined in the Geotechnical Engineering Bureau’s An Engineering Description of Soils Visual-Manual Procedure (GTP-2). It presents a procedure for describing soil samples obtained for earth and foundation engineering purposes by the New York State Department of Transportation. The procedure involves visually and manually examining soil samples with respect to texture, plasticity and color. A method is presented for preparing a “word picture” of a sample for entering on a subsurface exploration log or other appropriate data sheet. The procedure applies to soil descriptions made in the field or laboratory.
It is the intent of this system to describe only the constituent soil sizes that have a significant influence on the visual appearance and behavior of the soil. This description system is intended to provide the best word description of the sample to those involved in the planning, design, construction, and maintenance processes.
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1st Decision Is sample coarse-grained, fine-grained, mix-grained, or organic?
If mix-grained, decide whether coarse- grained or fine-grained predominates.
2nd Decision What is the principal component? Use as a noun in the soil description. The principal component is to be written in all capital letters: (e.g. SAND).
3rd Decision What is the secondary component? Use as an adjective in the soil description. All additional descriptive components have only the first letter written in capitals: (e.g. Silty SAND).
4th Decision Are there additional components?
Use as an additional adjective in the soil description. All additional descriptive components have only the first letter written in capitals: (e.g. Silty SAND Gravelly).
The Word Picture The apparent moisture and plasticity abbreviations are written in all capital letters: e.g. Brown Silty SAND Gravelly, M-NPL
Figure 5-4 NYSDOT Soil Sample Identification Procedure
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5.2.12.1.1 Use of NYSDOT Soil Identification System on Subsurface Exploration Log
Field logs, for soil test borings, shall be prepared by the driller at the time of drilling. The field logs shall be reviewed by an experienced geotechnical engineer. In addition, the geotechnical engineer shall also review all samples to confirm the accuracy of the field logs. The Soil Test Boring Logs shall be prepared and forwarded to the geotechnical designer for selection of samples for laboratory testing. Figure 5-5 provides an example of a log used on a NYSDOT project.
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Figure 5-5 NYSDOT Subsurface Exploration Log
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5.2.12.2 Unified Soil Classification System (USCS)
Dr. A. Casagrande developed the USCS for the classification of soils used to support Army Air Corps bomber bases. This system incorporates textural (grain-size) characteristics into the engineering classification. The system has 15 different potential soil classifications with each classification having a two-letter designation. The basic letter designations are listed in Table 5- 8.
Letter Meaning Letter Meaning Designation Designation G Gravel O Organic S Sand W Well-graded M Non-plastic or P Poorly-graded low plasticity fines (Silt) C Plastic fines L Low liquid limit (Clay) Pt Peat H High liquid limit
Table 5-8 USCS Letter Designations
The classification of soil is divided into two general categories, coarse-grained and fine-grained soils. Coarse-grained soils (gravels and sands) have more than 50 percent (by weight) of the material retained on the No. 200 sieve, while fine-grained soils (silts and clays) have more than 50 percent of the material passing the No. 200 sieve. Gravels and sands are typically described in relation to the particle size of the grains (See NYSDOT GDM Section 5.2.2 Coarse-Grained Soils). Silts and clays are typically described in relation to plasticity (see NYSDOT GDM Section 5.2.3 Fine-Grained Inorganic Soils). Fine-grained soils are defined by the plasticity chart identified in Figure 5-6.
In many soils, two or more soil types are present. When the percentage of the minor soil type is equal to or greater than 30 percent and less than 50 percent of the total sample (by weight), the minor soil type is indicated by adding a “y” to its name; i.e. Sandy SILT, Silty SAND, Silty CLAY, etc.
Figures 5-7, 5-8, 5-9, 5-10, and 5-11 provide the flow charts for the classification of coarse- and fine-grained soils using the USCS.
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Figure 5-6 Plasticity Chart for Unified Soil Classification System (Mayne, et al. May 2002)
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Figure 5-7 USCS Group Symbol and Group Name for Coarse-Grained Soils (Gravel) (Mayne, et al. May 2002)
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Figure 5-8 USCS Group Symbol and Group Name for Coarse-Grained Soils (Sand) (Mayne, et al. May 2002)
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Figure 5-9 USCS Group Symbol and Group Name for Fine-Grained Soils (LL ≥ 50) (Mayne, et al. May 2002)
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Figure 5-10 USCS Group Symbol and Group Name for Fine-Grained Soils (LL < 50) (Mayne, et al. May 2002)
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Figure 5-11 USCS Group Symbol and Group Name for Organic Soils (Mayne, et al. May 2002)
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5.2.12.3 AASHTO Soil Classification System (AASHTO)
Terzaghi and Hogentogler originally developed this classification system for the U.S. Bureau of Public Roads in the late 1920s. This classification system divides all soils into eight major groups designated A-1 through A-8 (see Figure 5-13). In this classification system, the lower the number the better the soil is for subgrade materials. Coarse-grained soils are defined by groups A-1 through A-3, while groups A-4 through A-7 define the fine-grained soils. Group A-4 and A- 5 are predominantly silty soils and group A-6 and A-7 are predominantly clayey soils. Group A-8 refers to peat and muck soils.
Groups A-1 through A-3 have 35 percent or less passing the No. 200 sieve, while groups A-4 through A-7 have more than 35 percent passing the No. 200 sieve. The classification system is presented in Figures 5-12 and 5-13. Table 5-9 indicates the gradation requirements used in the AASHTO classification system.
Soil Component Grain Size Gravel between 3” to No. 10 Sand between No. 12 to No. 200 Silt and Clay less than No. 200
Table 5-9 AASHTO Gradation Requirements
For soils in Groups A-2, A-4, A-5, A-6 and A-7 the plasticity of the fines is defined in Table 5- 10.
Soil Component Plasticity Index Silty ≤ 10% Clayey ≥ 11%
Table 5-10 AASHTO Plasticity Requirements
To evaluate the quality of a soil as a highway subgrade material, a number called the Group Index (GI) is incorporated with the groups and subgroups of the soil. The GI is written in parenthesis after the group or subgroup designation. The GI is determined by the following equation:
Equation 5-1
GI = (F 35){0.2 (0.005)(LL 40)} 0.01(F 15)(PI 10)
Where: F = percent passing No. 200 sieve (in percent) LL = Liquid Limit PI = Plasticity Index
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Listed below are some rules for determining the GI:
If the equation yields a negative value for the GI, use zero; Round the GI to the nearest whole number, using proper rules of rounding; There is no upper limit to the GI; These groups, A-1-a, A-1-b, A-2-4, A-2-5, A-3, will always have a GI of zero; The GI for groups A-2-6 and A-2-7 is calculated using the following equation
Equation 5-2
GI = 0.01(F 15)(PI 10)
Figure 5-12 provides the range of liquid limit and plasticity index for group A-2 to A-7 soils.
Figure 5-12 AASHTO Range of LL and PI for Soils in Groups A-2 through A-7 (Mayne, et al. May 2002)
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Figure 5-13 AASHTO Soil Classification System (Mayne, et al. May 2002)
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5.3 ROCK CLASSIFICATION
Rock classification is based on the character of the intact rock (hand specimens and rock core) in terms of its genetic origin, mineralogical make-up, texture, and degree of chemical alteration and/or physical weathering, and the engineering characteristics (orientation, spacing, etc.) of the bounding discontinuities (bedding, joints, foliation planes, shear zones, faults etc.) within the rock mass.
5.3.1 Rock Type
Geologists divide rocks into three categories based on genetic origin, and then subdivide them into smaller groups on the basis of composition and texture. These categories are igneous rocks, sedimentary rocks, and metamorphic rocks.
5.3.1.1 Igneous Rocks
The Geotechnical Engineering Bureau’s Rock Core Evaluation Manual (GEM-23) presents a procedure for describing rock core samples for foundation, rockslope, and other engineering purposes. Table 1 of GEM-23 provides rock names, color groups, texture, minerals, and properties of common igneous rocks.
Igneous rocks formed from the solidification of molten material (magma or lava). They can be intrusive (solidifying below the surface from magma) or extrusive (solidifying at the surface from lava). In general, igneous rocks tend to have an interlocking, crystalline texture with little to no appearance of layering, banding, or bedding. Intrusive igneous rocks are generally composed of larger crystals/grains (coarser texture) while extrusive igneous rocks are generally composed of smaller crystals/grains (finer texture).
5.3.1.2 Sedimentary Rocks
Sedimentary rocks (Table 3 of GEM-23) are formed by lithification of accumulated sediments that were transported and deposited by water, wind, ice, biological activity and/or chemical precipitation. Lithification is a complex process whereby freshly deposited loose grains of sediment are converted into rock. Lithification may occur at the time sediment is deposited or later. Cementation is one of the main processes involved, particularly for sandstones and conglomerates. Most sedimentary rocks are composed of the weathering products of pre-existing rocks.
Sedimentary rocks usually, but not always, have a layered appearance due to the presence of bedding planes. The grains of sedimentary rocks can range from sharply angular to well-rounded and from very fine to very coarse. Sedimentary rocks may contain fossils.
Additional characteristics of Sedimentary rocks can be found in GEM-23, Part III, C, 3. Sedimentary Rocks.
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5.3.1.3 Metamorphic Rocks
Metamorphic rocks (Table 2 of GEM-23) are Igneous and/or Sedimentary rocks whose textures and /or mineral content have been changed by heat, pressure and/or chemically active solutions (invading gasses and/or liquids). In some instances, metamorphic rocks have been re- metamorphosed into other higher or lower grade metamorphic rocks.
Metamorphic rocks can appear banded or layered due to the alignment of platy, tabular, or elongated mineral grains, or the concentration of different minerals into distinct bands, or the relict structure of the original sedimentary rock.
5.3.2 Rock Properties
5.3.2.1 Rock Color
Rock color is not in itself a specific engineering property, but may be an indicator of the influence of other significant geologic processes that may be occurring in the rock mass (e.g. fracture flow of water, weathering, alteration, etc.). Color may also aid in the subsurface correlation of rock units. The color of the rock is determined by comparing the core pieces with color chips in the Geological Society of America (GSA) Rock Color Chart. Rock color should be determined as soon as the core has been recovered from the test hole.
5.3.2.2 Mineralogy
Mineralogy is a description of the major minerals composing the rock. If possible, the size of the individual grains/crystals and the texture of the rock (very fine to very coarse) are noted. General descriptions of mineralogy, grain size, and texture for the various rock types are included in the rock type tables in the Geotechnical Engineering Bureau’s Rock Core Evaluation Manual (GEM- 23).
5.3.2.3 Weathered State of Rock
Weathering is the process of mechanical and/or chemical degradation of the rock mass through exposure to the elements (e.g. rain, wind, ground water, ice, change in temperature etc.). In general, the strength of the rock tends to decrease as the degree of weathering increases. In the earliest stages of weathering only discoloration and slight change in texture occur. As the weathering of the rock advances significant changes occur in the physical properties of the rock mass until, ultimately, the rock is decomposed to soil.
Table 8 of GEM-23 provides weathering categories broken into degrees of weathering with a detailed description of each.
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5.3.2.4 Relative Rock Strength and Hardness
Rock strength is controlled by many factors including degree of induration, cementation, crystal bonding, degree of weathering or alteration, etc. Determination of relative rock strength can be estimated by simple field tests, which can be refined, if required, through laboratory testing.
Table 7 of GEM-23 provides a description of rock hardness.
5.3.2.5 Fractures
The spacing, orientation, filling, and degree of healing of the fractures can be important in determining the properties of the rock mass for structure foundation design.
Table 5 of GEM-23 provides fracture density categories with a description for the degree of fracturing, and Table 6 of GEM-23 provides fracture healing categories with a description for the degree of healing.
5.3.2.6 Core Recovery (CR)
Core recovery is defined as the ratio of core recovered to the run length expressed as a percentage. Detailed information on core recovery can be found in GEM-23, Part III, A. Recovery.
5.3.2.7 Rock Quality Designation (RQD)
The Rock Quality Designation (RQD) is a modified measure of recovery, calculated in order to estimate the quality of the intact rock mass. Detailed information on RQD can be found in GEM- 23, Part III, B. RQD.
5.3.2.8 Additional Rock Properties
Additional rock properties, such as color, mineralogy, grain size, texture, etc. can be found in the Geotechnical Engineering Bureau’s Rock Core Evaluation Manual (GEM-23).
5.3.3 Rock Core Logging
5.3.3.1 NYSDOT Rock Core
The Geotechnical Engineering Bureau’s Rock Core Evaluation Manual (GEM-23) presents a procedure for describing rock core samples for foundation, rockslope, and other engineering purposes. Rock core evaluation data is recorded on a Rock Core Evaluation Sheet (Figure 5-14 and 15), then entered into the Rock Core database.
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The Rock Core Log shall be prepared by an Engineering Geologist at the completion of inspection and laboratory testing of the rock core. Figure 5-16 provides an example of a log used on a NYSDOT project.
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Figure 5-14 NYSDOT Rock Core Evaluation Sheet
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CHAPTER 5 Soil and Rock Classification and Logging
Figure 5-15 NYSDOT Rock Core Evaluation Sheet (cont.)
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CHAPTER 5 Soil and Rock Classification and Logging
Figure 5-16 NYSDOT Rock Core Log NYSDOT Geotechnical Page 5-32 June 17, 2013 Design Manual
CHAPTER 5 Soil and Rock Classification and Logging
5.4 BRIEF GLOSSSARY OF GEOLOGIC TERMS
The Geotechnical Engineering Bureau’s Rock Core Evaluation Manual (GEM-23) presents a procedure for describing rock core samples for foundation, rockslope, and other engineering purposes. A brief glossary of terms can be found in GEM-23, Part II, Brief Glossary of Geologic Terms.
5.5 REFERENCES
ASTM International, American Society for Testing and Materials (ASTM), D2488, Standard Practices for Description and Identification of Soils (Visual – Manual Procedure), 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959
Geotechnical Engineering Bureau, An Engineering Description of Soils Visual-Manual Procedure, Geotechnical Test Procedure GTP-2, New York State Department of Transportation, Office of Technical Services, https://www.dot.ny.gov/divisions/engineering/technical- services/technical-services-repository/GTP-2b.pdf
Geotechnical Engineering Bureau, Rock Core Evaluation Manual, Geotechnical Engineering Manual GEM-23, New York State Department of Transportation, Office of Technical Services, https://www.dot.ny.gov/divisions/engineering/technical-services/technical-services- repository/GEM-23b.pdf
Mayne, P.W., Christopher, B.R., DeJong, J., Subsurface Investigations – Geotechnical Site Characterization Reference Manual, US Department of Transportation, Federal Highway Administration, Publication No. FHWA NHI-01-031, May, 2002: http://isddc.dot.gov/OLPFiles/FHWA/012546.pdf
Munsell Soil Color Charts, 2000, GretagMacbeth, 617 Little Britain Rd., New Windsor, NY 12553.
Rock Color Charts, Geological Society of America, The Rock-Color Chart Committee, Boulder, CO.
Sabatini, P.J., Bachus, R.C., Mayne, P.W., Schneider, J.A., Zettler, T.E., Evaluation of Soil and Rock Properties, Geotechnical Engineering Circular No. 5, FHWA-IF-02-034, April 2002.
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