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.