Use observations in the field in conjunction with results of lab testing to develop a soil and bedrock profile for use in the reporting. Keep the number of strata to a minimum. Remember that every small variation in a soil—such as a change in clay from “slightly sandy” to “sandy” does not necessarily warrant a stratigraphy change. The logger must define strata that have significance to designers and contractors who will use the core log information. Capture the primary and secondary soil or rock constituent and whether ground water is present.

This soil system is based on the recognition of the type and predominance of the constituents considering grain size, gradation, plasticity index, and liquid limit. General soil description is determined in the field based on visual observations and is confirmed or revised once laboratory testing data is available. USCS contains three major divisions of soil: coarse-grained, fine-grained, and highly organic. See ASTM D2487, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), for the procedure for determining soil classification. TxDOT test procedures, Tex-141-E, Manual Procedure for Description and Identification of Soils and Tex-142-E, Laboratory Classification of Soil for Engineering Purposes may also prove useful in the determination of soil type.

Use the following charts to determine the density or consistency and hardness of material encountered.

Table 4-1: Soil Density, Cohesionless Soils

Relative Density/Density (Cohesionless)	Uncorrected SPT N-values	Legacy TCP Blowcounts	Field Identification
Very loose	Less than 4	0 to 8	Easily penetrated by rebar many inches (> 12) ½ inch rebar, pushed by hand
Loose	4 - 10	8 to 20	Easily penetrated with ½ inch rebar several inches, pushed by hand
Medium Dense	10 - 30	NA	Easily to moderately penetrated using ½ inch rebar driven by 5-pound hammer
Slightly compact	NA	20 to 40	Sample can be imprinted with considerable pressure
Compact	NA	40 to 80	Sample can be imprinted only slightly with fingers
Dense	~30 - 50	80 to 5in / 100	Penetrated 1 foot with difficulty using ½ inch rebar driven by 5-pound hammer Sample cannot be imprinted with fingers but can be penetrated with pencil
Very dense	> 50	5in / 100 to 0in / 100	Penetrated a few inches with ½ inch rebar driven by 5- pound hammer

Table 4-2: Soil Consistency, Cohesive / Clay Soils

Consistency (Cohesive)	Uncorrected SPT N-values	Legacy TCP Blowcounts	Approx. Undrained Shear Strength, Su (Tons per Square Foot)*	Field Identification
Very soft	< 2	0 to 8	< 0.125	Sample (height twice diameter) sags under own weight. Squeezes between fingers when fist is closed. Easily penetrated several inches by palm or fist.
Soft	2 - 4	8 to 20	0.125 – 0.25	Sample can be pinched or imprinted easily with finger. Easily penetrated several inches by thumb.
Medium Stiff	4 - 8	NA	0.25 – 0.50	Molded by strong pressure of fingers. Can penetrated several inches by thumb with moderate effort.
Stiff	8 - 15	20 to 40	0.50 – 1.0	Sample can be imprinted with considerable pressure.
Very stiff	15-30	40 to 80	1.0 – 2.0	Sample can be imprinted only slightly with fingers.
Hard	30-60	80 to 5in/100	>2.0	Sample cannot be imprinted with fingers but can be penetrated with pencil.
Very hard	>60	5in/100 to 0in/100		Sample cannot be penetrated with pencil.

*Pocket Penetrometer and unconfined compression tests yield qu, within clay, Su = qu / 2

Table 4-3: Field Bedrock Hardness

Hardness (Relative Rock Hardness)	Mohs’ Hardness Scale	Examples	Approx. SPT Values	Legacy TCP Blowcounts	Field Identification
Very hard	5.5 to 10	Sandstone, chert, schist, granite, gneiss, some limestone	SPT Refusal > 100 blows	0 in./100 to 2 in./100	Rock will scratch knife. Core requires many blow of hammer to fracture or chip. Hammer rebounds after impact
Hard	3 to 5.5	Siltstone, shale, iron deposits, most limestone	SPT Refusal > 100 blows	1 in./100 to 5 in./100	Rock can be scratched with knife blade or pick with difficulty
Moderate Hard	N/A	Shale, some limestone	SPT Refusal > 100 blows	2 in./100 to 5 in./100	Cannot scratch with fingernail but can be peeled with knife. Fracturing with single blow of hammer.
Soft	1 to 3	Gypsum, calcite, evaporites, chalk, some shale	80 blows to 100 blows	4 in./100 to 6 in./100	Rock can be scratched with fingernail or knife. Crumbles under firm blow with hammer. Grains from sandstones and mudstones/shales can be rubbed off with fingers.
Very Soft	~1	shale	< 80 blows	< 100 blows	Can be indented with fingers or crushed with fingers. Can be excavated easily with point of geologic hammer.

Estimate the relative strength of intact rock in the field with a use of a geological hammer or pocket-knife and record in the boring logs. Field identification methods should be confirmed by laboratory uniaxial compressive strength tests performed on representative rock core sample(s) within the stratum and presented in the boring logs. Perform uniaxial compressive strength tests in accordance with ASTM D7012, Method C.

Table 4-4 provides relative strength descriptions of intact rock based on field identification methods

and laboratory uniaxial compressive strength tests of rock

. Use Table 4-4 in combination with the field observations from Table 4-3 for boring log strength classification of rock.

Table 4-4: Criteria and Description for Relative Rock Strength

Grade Designation	Strength Description	Field Identification	Approximate Compressive Strength (psi)
R0	Extremely weak rock	Specimen can be indented by thumbnail	35 – 150
R1	Very weak rock	Specimen crumbles under sharp blow with point of geological hammer and can be peeled by a pocketknife	150 – 725
R2	Weak rock	Shallow cuts or scrapes can be made in a specimen with a pocketknife. A firm blow with a geological hammer creates shallow dents.	725 – 3,500
R3	Medium strong rock	Specimen cannot be scraped or cut with a pocketknife. Specimen can be fractured with a single firm blow with a geological hammer point.	3,500 – 7,250
R4	Strong rock	Specimen requires more than one firm blow of the point of a geological hammer to fracture.	7250 – 14,500
R5	Very strong rock	Specimen requires many blows of geological hammer to cause fracture	14,500 – 36,250
R6	Extremely strong rock	Specimen can only be chipped with firm blows from the hammer end of a geological hammer.	> 36,250

If any moisture exists, note the extent present. The samples will be assumed dry if the degree of moisture is not indicated. If free water is present, describe the soil as wet or water-bearing.

Describe the primary color and restrict description to one color. If one main color does not exist in a sample, call it multicolored.

Identify the degree of cementation if any is present. Use Table 4-5 for the classification:

Table 4-5: Cementation Status

Description	Field Identification	Approximate SPT
Cemented Sand / Soil	Difficult drilling or SPT layer(s) comprised of sand and fines.	30 to 99
Highly cemented sand / soil, weathered sandstone / bedrock	1” or longer in-situ specimen can still be collected in split-spoon often designated as top of bedrock layer if no weaker strata is encountered below.	SPT Refusal, > 100 blows
Bedrock	1” or shorter in-situ specimen collected in split-spoon and coring efforts should commence. Use weathering, strength and rock descriptions located in this section.	SPT Refusal, > 100 blows

Use any descriptive adjectives that might further aid in the description. This is especially important for core material recovered.

Weathering is the process of chemical and/or mechanical degradation of the rock mass over the course of time through exposure to the elements such as rain, wind, ground water, ice, changing temperature, etc. In general, the strength, stiffness, and general quality of intact rock tends to decrease with increase in the degree of weathering. As weathering advances significant changes occur in the physical properties and general quality of the intact rock, until ultimately the rock is decomposed to soil. Therefore, weathering is an important component of classification for engineering purposes.

Identify and record the weathering grades of the rock mass in accordance with the weathering grade shown in Table 4-6 below.

Table 4-6: Descriptive Terms for Weathering State of Rock

Term	Description	Grade
Fresh (F)	No visible sign of rock material weathering; slight discoloration on major discontinuity surfaces is possible.	I
Slightly weathered (WS)	Discoloration indicates weathering of rock material and discontinuity surfaces. All rock material may be discolored by weathering and the external surface may be somewhat weaker than in its fresh condition.	II
Moderately weathered (WM)	Less than half of the rock material is decomposed and/or disintegrated to a soil. Fresh or discolored rock is present either as a discontinuous framework or as corestones. A minimum 2 in. diameter sample cannot be broken readily by hand across the rock fabric.	III
Highly weathered (WH)	More than half of the rock is decomposed and/or disintegrated to soil. Fresh or discolored rock is present either as a discontinuous framework or as corestones. A minimum 2 in. diameter sample can be broken readily by hand across the rock fabric.	IV
Completely weathered (WC)	All rock material is decomposed and/or disintegrated to soil. The original mass structure is largely still intact. Material can by granulated by hand.	V
Residual soil (RS)	All rock material is converted to soil. Material can be easily broken apart by hand.	VI

Depending on if evident in visual observations of the core sample (intact rock), indicate grain sizing according to Table 4-7. Grain size refers to the sizes of individual particles or mineral crystals that comprise the intact rock. Unlike soils, where grain size is generally characterized based on sieve or hydrometer tests, the grain size for intact rock is generally characterized from visual observation.

Table 4-7: Criteria for Defining Rock Grain Size

Grain Size	Description	Criteria
< 0.003 in. (< 0.075 mm)	Very Fine-Grained	Cannot be distinguished by unaided eye. Few to no mineral grains are visible with a hand lens
0.003 – 0.02 in. (0.075 – 0.425 mm)	Fine-Grained	Few crystal boundaries are visible; grains can be distinguished with difficulty by the unaided eye but can be somewhat distinguished by hand lens
0.02 – 0.8 in. (0.425 – 2 mm)	Medium-Grained	Most crystal boundaries are visible; grains distinguishable by eye and with hand lens
0.8 – 2 in. (2 – 4.75 mm)	Coarse-Grained	Crystal boundaries are visible; grains distinguishable with naked eye
2 in. (> 4.75 mm)	Very Coarse-Grained	Crystal boundaries are clearly visible; grains are distinguishable with the naked eye

Spacing refers to the distance between fractures or thickness of beds visible in the core. In the case of fractures, spacing does not represent the thickness of the open space produced by a fracture, but rather the amount of rock material between two distinct fractures. For bedding thickness, this represents the amount of rock material between two distinct bedding planes. Discontinuities, such as joints and fractures, are often found in crystalline rock that has undergone deformations. Whereas bedding terms are typically used for sedimentary rocks such as sandstones and limestones.

Table 4-8: Joint and Bedding Terms

Joint Term	Bedding Term	Spacing (inch)
Very Close	Laminated	< 0.5
Close	Very Thin	0.5 – 2
Moderately Close	Thin	2 – 12
Wide	Medium	12 - 36
Very Wide	Thick	> 36

Discontinuity spacing is the distance between natural discontinuities as measured along the borehole core. Evaluate the discontinuity spacing within each core run, and report on the boring logs in accordance with the criteria provided in Table 4-9 below. Do not include mechanical breaks due handling or drilling in the measurement of discontinuity spacing.

Table 4-9: Discontinuity Spacing

Description	Discontinuity Spacing
Very widely spaced	>10 feet
Widely spaced	3 feet to 10 feet
Moderately Spaced	1 feet to 3 feet
Closely Spaced	2 inches to 12 inches
Very Closely Spaced	Less than 2 inches

The surface properties of discontinuities, in terms of roughness, wall hardness, and/or gouge thickness, affects the shear strength of the discontinuity. As the discontinuities within each core run, and report in the boring logs in accordance with the descriptions and conditions provided in Table 4-10 below.

Table 4-10: Discontinuity Condition

Condition	Discontinuity Spacing (feet)
Excellent Condition	Very rough surfaces, no separation, hard discontinuity wall
Good Condition	Slightly rough surfaces, separation less than 0.05 inches, hard discontinuity wall
Fair Condition	Slightly rough surfaces, separation greater than 0.05 inches, soft discontinuity wall
Poor Condition	Slickensided surfaces, or soft gouge less than 0.2 inches thick, or open discontinuities 0.05 to 0.2 inches
Very Poor Condition	Soft gouge greater than 0.2 inches thick, or open discontinuities greater than 0.2 inches

Design and construction of engineering structures on rock or rock deposits heavily depend on proper characterization of both the “intact rock” as well as the “rock mass” with discontinuities. For the purposes of this manual “intact rock” is defined as an intact piece of rock containing no discontinuities. “Rock mass” is defined as rock as it occurs in-situ, including its system of discontinuities, and weathering profile.

The extent of characterization of intact rock properties and rock mass properties shall be determined in accordance with data needs for the design and construction of the proposed structure, the type of proposed structure, and criticality of the proposed structures

Establish and report the properties of both the intact rock as well as the rock masses in the boring logs and the geotechnical report.

Intact rock is generally classified based on qualitative observations and simple measurements as described in the sections in this chapter. Laboratory tests using uniaxial compressive strength tests (Table 4-4) shall also be used to supplement qualitative observations and classify the relative strength of intact rock.

The primary basis for classification of intact rock is rock type. Establish rock type by first identifying the origin, whether the intact rock is igneous, sedimentary, or metamorphic in origin. Establish the specific rock type from consideration of additional characteristics such as mineralogy, texture, and experience with local geology. Tables 4-10 to 4-12 show the three rock origins, and rock types found depending on their origin. Texas Geology contains mostly sedimentary rocks and a few exposures of Precambrian igneous and metamorphic that are less common. The Geologic Atlas of Texas is primary resource that investigation should use ahead of drilling to anticipate rock type:

Texas geology contains a variety of rock types and investigation should be aware of rock type to expect in any unique region or project location. Should anticipated bedrock not be observed during the drilling, indicate what rock type and characteristics are present in the investigation.

Table 4-11: Common igneous rocks

Intrusive	Extrusive	Primary Minerals	Common Secondary Minerals
Granite	Rhyolite	Quartz, K-Feldspar	Plagioclase, Mica, Amphibole, Pyroxene
Quartz Diorite	Dacite	Quartz, Plagioclase	Hornblende, Pyroxene, Mica
Diorite	Andesite	Plagioclase	Mica, Amphibole, Pyroxene
Gabbro	Basalt	Plagioclase, Pyroxene	Amphibole Olivine

Table 4-12: Common Sedimentary Rocks

Clastic		Non-Clastic
Rock Type	Original Sediment	Rock Type	Primary Mineral	HCl Reaction
Conglomerate	Sand, gravel, cobbles	Limestone	Calcite	Strong
Sandstone	Sand	Dolomite	Dolomite	Weak
Siltstone	Silt	Chert	Quartz	None
Claystone	Clay
Shale	Laminated clay & silt

In addition to rock type, classify intact rock according to relative strength or hardness, degree of weathering, grain size or texture. Color and grain size are often key characteristics that facilitate identification of rock type.

In ASTM D5878 several systems of rock mass classifications are described. Certain design methodologies in AASHTO require rock mass classification using Geological Strength Index (GSI). Classify the strength of a jointed rock mass using GSI in accordance with AASHTO LRFD Bridge Design Specifications Article 10.4.6.4.

Percent Recovery is defined as the ratio of core recovered to the run length expressed as a percentage:

$\text{Percent Recovery}\left(\%\right)=\frac{\text{Length of core recovered}*\hspace{0.17em}100}{\mathit{\text{Length of core run or interval}}}$

Determine the RQD for rock core samples following ASTM Test Procedure D6032, Standard Test Method for Determining Rock Quality Designation (RQD) of Rock Core.

$\text{RQD}\left(\%\right)=\frac{(\sum \text{Length of sound core segements > or = 4inch}es)\hspace{0.17em}*100}{\mathit{\text{Lenght of core run or interval}}}$

As illustrated by the example:

$\text{Percent Recovery =}\frac{\text{(10 in. + 8 in. + 10in. + 7.6 in. + 3.2in. + 4.8 in)}}{\text{48}}=96\%$

$\text{RQD (\%) =}\frac{\text{(10inches+7.5inches+8.0inches*\hspace{0.17em}100)}}{\mathrm{48}}=\text{53\%Fair}$

Mechanical breaks in the core (perpendicular to the length of the core) should not be counted towards RQD reduction. Use segments of 4” or above only by breaks identified as natural fractures or joints within the rock mass. Record the rock type (limestone, shale, sandstone, etc.), degree of weathering (highly, moderate, minimal, unweathered), natural fracture frequency (number of visible joints or natural discontinuities within a typical 12” segment of recovered core) and jointing condition (closed or open), and size of the jointing or discontinuities to use classification criteria as specified in this chapter.

Always note the percent recovery and RQD on boring logs where rock is encountered.

Fracture frequency is defined as the number of natural fractures per unit length of core recovered.

$\text{FF=}\frac{\text{number of natural fractures}}{\text{total length of core recovered}}$

The fracture frequency can be determined for the entire length of a core run, or for a smaller segment of core. As is the case with

RQD

, artificial fractures or mechanical breaks created during drilling or core handling should be neglected when calculating fracture frequency.

Classify the strength of a jointed rock mass using GSI in accordance with AASHTO LRFD Bridge Design Specifications Article 10.4.6.4. Present GSI values in the geotechnical data or design report to aid in foundation design or when used as a basis for foundation recommendations.

Geotechnical Data Reports are required to contain the following minimum information:

See Chapter 5 for Geotechnical Design Report requirements for both drilled shaft and driven pile foundation design. See Chapter 7 for Geotechnical Design Report requirements for retaining wall design.

Standard log forms are available in various software packages to display all required information and description within each borehole. TxDOT Wincore was developed for and can only be used with legacy TCP data and is not sufficient for use in LRFD geotechnical design. Group the materials encountered into strata consisting of the same or similar constituents. Pay close attention to the classification descriptions within this Chapter.

Currently PDF export of logs, or inclusion of logs within a PDF of the geotechnical data report is acceptable in contact plans. See the TxDOT Bridge Detailing Guide for boring display requirements.