It is important for civil engineers to be somewhat familiar with sedimentary rocks because they cover roughly 75% of the continental surface. Sedimentary rocks provide most of the material directly applicable to civil engineers, in the form of limestone for concrete, sands and gravels for aggregates and glass, and shales for producing brick and tile. Furthermore, knowing what sedimentary rocks are on or near a site provides insight into the depositional environment, which may provide other important clues to conditions affecting site geology.
The criteria are somewhat different than igneous rocks, the methodology is the same.
clastic versus non-clastic
For the purposes of CE 70, sedimentary rocks may be broadly categorized into either clastic or non-clastic types. Clastic rocks are composed of materials transported, then deposited. Examples include sandstone and conglomerate. Non-clastic are deposited in-place. Examples include limestones built up from coral reefs, or gypsum precipitated from super-saturated water trapped in desert sabkhas.
Fabric
Fabric refers to linear or planar features, such as bedding planes, which separate the rock into distinct zone.
Grain size
Grain size in sedimentary rocks can be an excellent indicator of rock type. For example, silt size grains form siltstones and sand-sized grains form (surprise) sandstones.
Induration
Induration just means (for this class) how much like a rock is it. As an example, some of the sandstone specimens are very friable, they are easily destroyed using finger pressure. Others are very hard.
Color
Color is not as reliable an indicator as it is igneous rocks. Sandstones, limestones and shales may come in colors ranging from red to black. There will be a wide range of examples in the 481 tray.
Density
Not as reliable an indicator as with igneous rocks, but still should be noted.
Acid reaction
Critical test for limestone. If it reacts strongly with HCL, it is limestone. If not, scrape a bit of rock powder and test that. A weak reaction with rock flour indicates dolomite.
Limestone and dolomite are mainly carbonates of calcium and magnesium. Dolomite, also known as dolostone, consists of the mineral of the same name, and may be a competent foundation material. Limestone is primarily composed of calcium carbonate. It may be fine and crystalline, but its texture is variable and may even be brecciated. In some limestones calcite grains are held together with clay cements. The color varies: white, yellow, brown, and gray being the most common. The rock also may vary from impermeable to loose-textured and porous. Limestone and dolomite are liable to contain clay, silica and other impurities which might have a strong influence on their satisfactory use in construction. Structurally, they are of moderate resistance to abrasion and impact, and have relatively low hardness, but they vary in structural properties to a great extent. Some may be almost the equal of basalt while others are greatly inferior.
Limestone and dolomite may contain sufficient clay to be termed clayey or argillaceous rocks. Other units may contain shale or chert. The presence of shale may indicate an unsound rock, while chert-bearing rock should be viewed with suspicion as a material of high expansion or reactive with alkali in Portland cement. In limestone, extensive cavities and solution channels may form in fracture and joint systems. Dolomite, which may be recrystallized from limestone, is less soluble.
Generally, limestone and dolostones are good foundation materials unless they are cavernous. Considerable leakage may be expected if a cavernous formation occurs under a dam or reservoir. Chalk is a weak variety of limestone and is not regarded as a competent bearing material for heavy structures.
Sandstone, like limestone and dolomite, may be a rock of great variance in its physical properties. It is composed of sand-sized grains bound together with some cementing medium. If the cement is silica or iron, a hard rock of excellent quality should be found. Cementing with clay (and calcite) may produce a sandstone of inferior quality. The strength and permeability of sandstone depends on the type and degree of cementation. Fracturing, folding and jointing leads to deterioration of cementing materials (Why?).
The size of grains in sandstone varies. Accordingly, their texture may vary from fine to coarse grained. The structure of a sandstone may be massive, horizontally bedded, or cross-bedded. Generally, sandstones are assumed to be fairly competent bearing materials. They should be examined, however, for the presence of softer materials (such as siltstones) interspaced between harder, more resistant layers. Argillaceous (clayey) sandstones may be subject to air and water slaking. Slaking is a process of physical disintegration caused by expansion of the clay minerals, loss of cohesion, and compression of air by imbibing water.
Because of their granular nature, sandstones are often porous and permeable and serve as water-bearing formations. These conditions are highly desirable for developing ground water aquifers, but may be troublesome when dry conditions are required for excavation.
Sandstones, unless thinly bedded, crust to well-shaped fragments, and have an excellent bond with cementing agents. Varieties in which the sand grains are not firmly bonded are used for constructing skid-proof pavements.
Chert has no visible grains, being chiefly composed of cryptocrystalline quartz. Due to the silica content, chert is one of the most troublesome rocks the construction engineer has to handle. Although it is very hard and has an adequate resistance to abrasion and impact, it frequently shows a marked increase in volume upon absorption of water. This leads to disfiguring popouts in concrete construction. The lighter (in weight) varieties of chert are readily damaged by freezing and thawing. Cherts from many deposits contain alkali-reactive forms of silica and suitable for use in concrete only with adequate safeguards against the alkali-aggregate reaction. Unweathered chert has a smooth, almost glassy surface, and a good bond with cement paste may not be obtained.
Attention must be paid to the extreme range of quality shown in chert. Some cherts may be found of suitable quality for use in concrete, but it would be best to consider all as suspect until proven otherwise.
Shale is another material which may cause the engineer considerable concern. Like chert, shale may be good or bad, and may have a great range in physical properties. Shale is a laminated sedimentary rock. Often dark in color, it is composed predominantly of clay-sized particles, although a small percentage of sand or silt sizes also may occur. The degree of induration of shale is extremely variable and may range from rock soft enough to be scrtached with a fingernail to a rock that can be scratched only by a knife blade.
Siltstones are composed primarily of silt-sized grains (0.062-0.0039 mm in size) and may be similar to shale in appearance. Silstones are often distinguished by a gritty feel, while shales appear smooth, with little or no gritty feeling.
Clays and silts are changed to into shales and sandstones by the process of desiccation, compaction and cementation. There is a wide variation in engineering properties between shales formed by compaction alone or with cementation. Compaction shales may soften, slake and swell on exposure. When subjected to alternate wetting and drying, insufficiently indurated shales may revert to the original clayey, often sticky, mass from which they had been formed. Natural shale deposits can vary from soft weathered layers that can be excavated by rippers or rooters (equipment for removing roots and stumps) to indurated rocks requiring explosives for removal.
If shale of poor quality is used in construction, the resistance of the concrete to frost action may be greatly reduced. In addition, shales often contain clay minerals that tend to swell when wetted, resulting in is referred to as expansive shale that will crack foundations or high pavements. A problem of particular importance to engineers is the tendency of shale to slide along bedding planes which act as planes of weakness. If these planes dip toward the excavation, the removal of lateral support may lead to slope failure.
Breccia and comglomerate are formed by the bonding together of angular or rounded fragments of rock by a cementing agent such as silica, iron oxide, calcite or other materials.
Conglomerate is a term generally applied to rock composed predominantly of cemented, coarse grained, rather rounded rock fragments, sometimes well-graded down to very small sized particles. By definition, at least 10 percent of the rounded fragments must be over 2mm in diameter. If these fragments are not rounded, the rock is classified as a breccia. If there is a scattering of large rounded pebbles or boulders embedded in indurated clay, the resulting formation is a tillite. If the large fragments have resulted from volcanic ejection, and are cemented with small fragments, the resulting mass is known as a volcanic breccia. Unless they are well-cemented, any of these materials may weather severely in deep cuts. It is always advisable to peform a slaking test if the conglomerate is to be considered for engineering use.
Graywacke is a dark-colored or black sandstone, frequently cemented with silica. It may contain particles and flakes of slates and shales. The dark color and occasional extreme induration are its most distinctive characteristics and are the reason why graywacke often is mistaken for igneous rock. Perhaps one of the most important means of identification is the fact that graywackes commonly are found in association with slates and shales. Generally, graywackes are competent bearing rocks.