Soil and Bedrock
Geologists divide bedrock into three classes.
1. Igneous Rocks
Igneous rocks are found in approximately 20 counties of the Llano Uplift, south-central Texas, and the Trans-Pecos areas. These rocks are derived from cooled and solidified molten rock material, called magma, that was pushed up from the interior of the earth.
Magma that cools beneath the surface forms intrusive rocks and magma that reaches the surface forms extrusive rocks. The rate of cooling, mineral composition, and mode of placement controls the type, texture, and shape of rocks.
These variables complicate identification. A background in mineralogy and petrology is necessary to identify each properly. The igneous rocks that outcrop in Texas are generally described as intrusive (such as granite) or extrusive (such as basalt):
2. Metamorphic Rocks
Metamorphic rocks are formed by the alteration of preexisting rocks (igneous, sedimentary, and other metamorphic rocks) by heat, pressure, or both. These alterations develop new textures, structures, and minerals.
Some metamorphic rocks are characterized by a banded or layered appearance and identified as: foliated-gneiss (irregular banding), schist (regular banding), and slate (layered), while others are massive or granular and are called non-foliated-marble.
3. Sedimentary rocks
Sedimentary rocks are clastic or non-clastic.
A. Clastic Rocks
Clastic sedimentary rocks are classified according to size. The unified soil size classification chart shows particle sizes in millimeters and inches in relation to the standard U.S. sieve sizes for clastic materials. Clastic sedimentary rocks are formed by the cementation of individual grains of respective particle sizes, and they include the following:
B. Non-Clastic Rocks
Evaporites are a group of water-soluble salts that have been precipitated upon the evaporation of water. They are similar in physical characteristics in that they are white or light colored, generally soft, and do not react with hydrochloric acid (except calcite). Halite and potash salts can be detected by their saline taste and are most commonly found in west and northwest Texas. Gypsum occurs extensively in west Texas.
Soil varies with parent material (bedrock), climate, plant and animal life, slope of the land, and time. These factors transform an original geologic deposit into a soil profile. The depth of soil ranges from a few inches to hundreds of feet based on these factors. Some sections of the state have no soil at all.
Residual and Sedimentary Soil
According to its geologic origin, soil is either residual or sedimentary. Residual soil is formed in place. That is, it is a result of the weathering, disintegration, and decomposition of the parent material. Sedimentary soil is formed from materials that have been moved from where they originated by either wind or water. These are commonly found in river flood plains and in arid wind-blown areas.
Soil is identified in the field by visual and mechanical tests. The criteria for these are grain size, color, density or consistency, and moisture content. For grain size, soil is either cohesive-clay, or cohesionless-silt, sand, or gravel. Most soil consists of a mixture of these grains and organic material.
1. Cohesive Soil
Cohesive soil (clay) is composed of extremely small mineral grains shaped like plates. Water is attracted between the plates by electrostatic forces to varying degrees based on the chemical composition of the clay. Clay exhibits a wide range of properties based on water content and chemical composition. When dry, clay is hard and rigid due to the close attraction between the grains. When clay is very wet, it exhibits an almost soupy consistency.
Clay occurs as both residual and sedimentary soil. Clay of a sedimentary origin is initially deposited in a soup-like state. In upland areas, water evaporation rapidly removes fresh clay deposits to produce fairly firm soil. In coastal areas, this usually does not occur due to high ground-water levels. In such an environment, the water is slowly squeezed from the clay by the weight of subsequently deposited overlying soil. The result is typically very soft surface clay that gradually increases in strength with depth.
2. Cohesionless Soil.
Cohesionless soil is composed of larger, more rounded particles than clay and is subdivided based on grain size. The most commonly encountered cohesionless soil is:
Cobbles (3 to 12 inches) and boulders (greater than 12 inches) are less commonly encountered. The larger sizes of the particles cause them to interact by mechanical means. Silt is fine enough that it exhibits some clay-like properties, but it is still considered cohesionless.
Pure cohesionless soil is free flowing when dry or completely saturated. Moist silt and sand often exhibit an apparent cohesion due to negative pore water pressures. This apparent cohesion is quite low but can still allow an excavation face to stand unsupported for some time before collapsing.
Cohesionless soil is usually mainly composed of siliceous materials with minor constituents of micas, feldspars, and carbonates. The most common siliceous materials are quartz and chert. The table below offers classifications of unified soil sizes.
Unified Soil Size Classification
Soil and Clay Characteristics
The core drill operation obtains subsurface data. To obtain accurate data, the logger must work closely with the driller, consulting on changes in materials and coring operations while drilling. The logger must recognize the reasons for adding extra water, drilling mud, or casing and should note obstacles to drilling, such as caving, boulders, caverns, and any ground water.
In some cases, a core sample cannot be recovered but the logger can watch the color of the circulation water to see if any change takes place and analyze the cuttings to see if the material correlates with the previous and subsequent core samples.
Logging Procedure: Before Drilling
Logging Procedure: During Drilling
Logging Procedure: After Drilling
Occasionally, core holes may need to be grouted or filled with bentonite pellets if the possibility exists for contaminates to enter from the surface or from subsurface aquifers. This is especially common in urban areas with petroleum-contaminated soil.
Logging Field Equipment
The logger needs the following equipment as aids to description of the materials: