Geology 229 Engineering Geology. Lecture 6. Basic Rock Classification and Engineering Considerations (West, Chs. 2, 3, 4, 5)

Transcription

1 Geology 229 Engineering Geology Lecture 6 Basic Rock Classification and Engineering Considerations (West, Chs. 2, 3, 4, 5)

2 Outline of this Lecture 1. Rock types and rock cycle 2. Geological and engineering definitions of rocks 3. Basic Mineralogy 4. Engineering Geology considerations

3 Geology is the study of the earth, from different perspectives, it studies Surface expression Geomorphology Composition Mineralogy Structure Structural geology Internal activities Global geophysics Formation process Stratigraphy, geochronology The physical nature of the earth and its interaction with engineering construction - Engineering Geology.

4 What is a rock? In Geology, Rock is defined as the solid material forming the outer rocky shell or crust of the earth. There are three major groups of rocks by its origin: (1) Igneous rocks: cooled from a molten state; (2) Sedimentary rocks: deposited from fluid medium; e.g., products of weathering of other rocks in water; (3) Metamorphic rocks: formed from pre-existing rocks by the action of heat and pressure.

5 Rock Cycles

6 Thus, in pure geological sense rock is defined as the essential part of the earth s crust. Geologists concern about the origin, classification, history, and the spatial aspects of rocks. So, geologically speaking, ice, sand, marble, coal, basalt, can be simply regarded as rocks. However, the Engineering Geologists have a different, and relatively narrower view of rocks. The Engineering Definition of Rocks Rock is the hard and durable material.

7 The Engineering Definition of Rocks (cont.) By an excavation point of view, Rocks are the earth materials that cannot be excavated without blasting. This definition clearly excludes other kinds of earth materials such as soils, and glacial tills, etc. Here is another engineering definition of rocks: The earth materials that do not slake when soaked into water. For example, a thick loess deposit is regarded as rock geologically and regarded as soil in engineering.

8 Basic Mineralogy of Rocks Rocks are formed with minerals. What is a mineral? 1) a naturally occurring chemical element or compound; 2) formed by inorganic processes; 3) with an ordered arrangement or pattern for its atoms crystalline structure; 4) possesses a definite chemical composition or range of compositions. The opposite of mineral property is amorphous, i.e., the property of non-crystal, order-less property possessed by glass, volcanic glass, etc.; oil or coal can neither be regarded as minerals by their organic involvement.

9 Basic Mineralogy of Rocks (cont.) So we can simply express the mineral as mineral = composition + crystalline structure For two minerals if the composition are the same but the structures are different, they can be called a pair of polymorphs. The common examples for polymorphs include 1) pyrite/marcasite (FeS 2, isotropic vs anisotropic iron atom spacing); 2) diamond/graphite (C, the same composition of carbon but different structure); 3) Calcite/aragonite (CaCO 3 ); 4) quartz/cristobolite (SiO 2 ).

10 Basic Mineralogy of Rocks (cont.) There are more than 2000 naturally occurred minerals have been discovered; only a bit more than 100 are common and used in college mineralogy. However, of the 100 common minerals only about 25 are abundant rock-forming minerals. The main types of minerals are: metallic minerals; nonmetallic minerals; carbonate minerals; sulfate minerals; sulfide minerals; silicate minerals; oxide minerals; clay minerals.

11 Mineralogy Identification for Engineering Purposes From an engineering point of view, certain properties of minerals, especially when they are introduced into or encountered with another mineral, are of special concern to engineers. For example, gypsum in a limestone can become swelling when water presents; pyrite (the fool s gold) in shale can be deteriorated by acid water; swelling clays in shale can become wetting and cause instability problem of a slope. Thus, fundamental mineralogical acknowledge is needed when identifying engineering material is needed.

12 Mineralogy Identification Minerals can be identified by its color; streak (strip); luster; hardness; specific weight; cleavage; fracture; crystal form; magnetism; tenacity; diaphaneity; striation; chemical reaction.

13 Rock Identification Rocks are identified mostly by its texture; mineral composition; field relationships; color; hardness; specific weight; crystal form; magnetism; Apparently, some techniques used in identifying minerals can also be used to classify the rock type.

14 Rock Properties for Engineering Rock are significant for two major reasons in engineering: (1) As building materials for constructions; (2) As foundations on which the constructions are setting; For the consideration of rocks as construction material the engineers concern about: (a) Density to some extent (for calculating the weight, load to the foundation, etc.); (b) Strength; (c) Durability;

15 For the consideration of rocks as the construction foundation the geological engineers concern about the rock s: (a) Density; (b) Strength; (c) Compressibility;

16 Engineering concerns of different rocks: (a) Igneous; (b) Sedimentary; (c) Metamorphic;

17 The Identification Chart of the Igneous Rocks

18 Engineering Considerations of Igneous Rocks (1) Fine-grained igneous rocks cannot be used as aggregates in Portland cement due to volume expansion caused by the Alkalisilica reaction. Solutions include: (a) Can be used in low alkali cement; (b) Non-reactive aggregates go with the high alkali cement; (c) Add pozzolans, coal-ashes, etc. in the aggregate-cement mixture to minimize the reaction. (2) Coarse-grained igneous rocks (e.g., granite, syenite, etc.) are not for aggregates for constructions because its low abrasion resistance; but fine-grained igneous rocks (e.g., basalt) are good fro aggregates (e.g., basalt as paving aggregates goes with asphalt. (3) Siting of foundations needs to avoid weathered rocks (e.g., dams, bridge piers, etc.); (4) Igneous rocks are good for dimension stone (tombstone etc.) because their resistance to weathering but need avoid fractures.

19 The Nathan Hale Monument in Coventry, Connecticut. Built in 1851 with granite blocks quarried from Quincy, MA.

20 Sediments and Sedimentary Rocks Sediments are pieces of loose debris that have not been lithified. Sediments are soil, with the engineering definition. Sediments are the combination of gravels, sands, silts, and clays; Sedimentary rocks are lithified sediments that held together by various types of cementing agents, such as calcite, quarts, and iron oxide; or by compaction of the mineral grains into an indurate mass. There are three ways in lithification: compaction cementation crystallization - reduction of pore size; - pores filling by binding agents; - new minerals crystallized in pores.

21 Sedimentary Rocks (1) 95% of the volume of the crust (the first 10 miles in depth of the earth) are igneous and metamorphic rocks; (2) However, it is about 75% of the surface of the earth are covered by sedimentary rocks; (3) Consequently, engineers are most likely end up with working more often on sedimentary rocks. Sedimentary rocks composed of mineral grains or crystals that have been deposited in a fluid medium, and subsequently lithified to form rocks. Of the sedimentary rocks on the earth s surface, 46% are shale, 32% are sandstone and 22% are limestone.

22 Conglomerate is a clastic sedimentary rock that forms from the cementing of rounded cobble and pebble sized rock fragments. Conglomerate is formed by river movement or ocean wave action. The cementing agents that fill the spaces to form the solid rock conglomerate are silica, calcite, or iron oxides. Chert is a very hard sedimentary rock that is usually found in nodules in limestone. Chert is light gray to dark gray in color. It probably formed from the remains of ancient sea sponges or other ocean animals that have been fossilized. Silica has replaced the tissue forming the sedimentary rock. Limestone is the most abundant of the non-clastic sedimentary rocks. Limestone is produced from the mineral calcite (calcium carbonate) and sediment. The main source of limestone is the limy ooze formed in the ocean. Sandstone is a clastic sedimentary rock that forms from the cementing together of sand sized grains forming a solid rock. Quartz is the most abundant mineral that forms sandstone. Calcium carbonate, silica, or iron has been added to the water that is in contact with the sand grains.

23 Engineering Considerations of Sedimentary Rocks (1) The sedimentary rocks also have the Alkali-silica reaction problem when used as aggregates with Portland cement. The sedimentary rocks with this problem are chert and graywacke. (2) Fine-grained sedimentary rocks like limestone and dolomite are the best for being used as aggregates; siltstone, shale, conglomerate, and quartz sandstone are not acceptable; (3) Stream and terrace gravel contains weak pieces, they are not good for aggregates in concrete. Weathered chert, shale, and siltstone can cause pop-outs at the concrete surface after freeze-thaw cycles; (4) Coarse-grained limestone is not good for aggregates by reducing particle size; (5) Sinkhole problem in carbonate terrains due to the high dissolvability of limestone and dolomite.

24 Engineering Considerations of Metamorphic Rocks (1) The metamorphic rocks also have the Alkali-silica reaction problem when used as aggregates with Portland cement. The metamorphic rocks with this problem are argillite, phyllite, impure quartzite, and granite gneiss; (2) Coarse-grained gneiss can be abraded severely when used as aggregates; (3) For metamorphic rocks the stability of rock mass greatly affected by the foliation orientation; (4) Marble as a metamorphic rock from carbonate sedimentary rocks can cause similar problems, eg., leakage of reservoirs, sinkhile collapse, solution cavities, and channels.

25 Readings: Ch. 2, 3, 4, 5 Homework: Chapter 3: Problem 9 Chapter 4: Problem 6