Geology and Soil Mechanics
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Summary
This paper describes Igneous, sedimentary and metamorphic rock. Origins, properties, composition of rocks are also discussed. Engineering classification of rocks depend on its Uniaxial compressive strength. They have properties like porosity, transverse strength and crushing strength. Classification of rocks is done on the basis or their origin also.. Different types of minerals and their diagnostic properties are mentioned in this paper. Soil classification and description are part of ground investigation. It is an important engineering material. Besides soil texture and soil structure, physical, chemical and biological soil properties are mentioned. Measurement of various geotechnical design parameters are explained in this paper. Besides laboratory methods, methods of Ground investigation, sample acquisition and testing are described in detail. Laboratory measurements on soil, One-dimensional consolidation tests, Shear Strength parameters of soil and Soil Permeability Test are also discussed.
Table of content
Classification of Rocks 4
Identifying the common minerals 5
Engineering classification of Rocks 6
Some properties of Rocks 7
Soil Description 7
Soil as an engineering material 8
Classification of soil 9
Basic Soil Properties 10
Soil Structure 10
Soil Colloids 11
Measurement of parameters concerning geotechnical design 12
Laboratory methods 15
Methods of testing, sample acquisition and Ground investigation 15
Laboratory measurements on soil 16
One-dimensional consolidation tests 17
Shear Strength parameters of soil 18
Soil Permeability Test 18
References 23
Classification of Rocks
Rocks have been broadly classified into many categories on the basis of their origin and formation but after summing up all the factors three broad categories have been derived and all the rocks have been classified into three categories. The reason of this three way classification is because the processes that are involved in forming these types of rocks usually result in characteristic relationship between mineral grains. This relationship is diagnostic.
They are:
- Sedimentary Rocks
- Metamorphic Rocks
- Igneous Rocks
Sedimentary Rocks
Air or water bring sediments with them that are deposited and so sedimentary rocks are formed.. Every grain of the rock was separate from each other, when these grains stopped moving it started to settle down and thus touch many other grains but leaving pores and spaces between the grains. Soft grains are distorted by the compaction of sediments. It may have reduced the pores and increased the level of cohesion.
Igneous Rocks
Igneous rocks originate form cooling molten rocks called magma. There is crystallization of minerals then they stick together and as the melted magma cools down, it forms solid rocks that have crystals and no open spaces. These rocks may be entirely composed of one mineral or may be a combination of many minerals. Very rapid cooling may also result in non crystallize glass that may form a part of rock or the complete rock.
Metamorphic Rocks
Metamorphic rocks include many rocks, some may be actually metamorphic or some may be not. When deeply buried rocks are intruded by magma or influenced by large tectonic forces they go through great changes of pressure and temperature and go through a process known as metamorphosis. Metamorphic rocks formed from magmatic Intrusions are called thermal or contact metamorphic rocks.
Identifying the common minerals
It is minerals that basically form a rock, so it very important to identify as well as distinguish which rock is made up of what mineral and how on the basis of minerals all rocks shall be classified. Geologists have suggested many ways and have used the physical properties of minerals to identify them. Features like crystal hardness, lusture and color are carefully studied and then the minerals are accordingly classified. More detailed explanations are always available-
The most common minerals are:
Quartz- It is a hard yet a very glassy looking rock that has a white kind of color and a lustrous shine. The color may be white, grey or smoky, individual crystals are generally clear and quartz can easily strike a knife blade.
Feldspar- It is an easily available rock forming mineral that is dull and opaque. The color of feldspar may vary from red to pink and white; it has a porcelain kind of structure.
Mica- Mica is a very popular and a distinguished stone because of its quality of getting peeled into pieces .It has properties somehow similar to feldspar except in its cleavage properties.
Engineering classification of Rocks
A building may be defined as a rock that can be safely used as a rough unit over as a properly cut and shaped (dressed) block or slab or column or sheet in different situation in engineering construction.
According to this definition rocks used in foundations and floors and roofs and walls of building are all building stones and so are the rocks used in the construction of dams and retaining walls along rods and also those used in the erection of lining and tunnels. Taj Mahal of Agra, Red Fort of Delhi and temples of Jagannathpuri are a few of thousands of such marvelous examples of stones construction found in every big country of the world.
ENGINEERING CLASSIFICATION OF ROCKS
CLASS DESCRIPTION UNIAXIAL COMPRESSIVE STRENGTH
E very low strength less than 280
D low strength 280-560
C medium strength 560-1120
B high strength 1120-2240
A very high strength More than 2240
Some properties of Rocks
Crushing Strength- It is also termed as compressive strength of building stones and may be defined as max force expressed per unit area which a stone can withstand.
Transverse strength- It is defined as the capacity of the stones to withstand bending loads. Such loads are only rarely involved in situations where stones are commonly used. But when a stone is intended for use as a beam or a lintel, its transverse strength
Porosity- it is generally expressed in terms of percentage. It refers to the quality of a rock and the arrangement of the grains and pores and the size of pores and grains. Numerically ratio is also used.
Soil Description
During ground investigation various types of materials are observed. To explain them, it is important to identify a good system of soil classification and description. This system should be illustrative, have some meaning in context of engineering (so that it can be understood and interpret by engineers). It should be easy to summarize. To differentiate between classifying and describing is essential.
Soils consist of water air and grains. In case there is change of locations and conditions then there is change in the water and air contents: there may be no air content in the soil i.e. saturated to full extent, there may be presence of both water and air content i.e partial saturation, there may be no water content i.e. completely dry soil. At a given point, change in shape and size of the granular/solid content is rare. But from one location to another location there may be significant variation. Soil has to be referred as an engineering material. It is different then other solid and coherent material i.e. concrete and steel. Soil material is referred as particulate. There is a need to know about soil’s internal fabric or structure, particle composition, shape and size.
An engineering material – soil
For different persons “soil” refers to different things: Considering geological perspective, it is outcome of surface processes. Considering pedological perspective, it shows present chemical and physical processed.
Help for identification of size
In the field, to identify the size there are many physical characteristics concerning soil, those may serve the purpose. Following information may be generated if fingers are used to rub the soil:
With the help of naked eye coarser particles/sand are easily visible.
Dry and dusty silt particles are swiped off boots and hands.
When hard and wet particles of clay stick and greasy. When dry they are hard. They need to be removed very properly from boots.
Since its deposition time, soil develops in parent material. Biota, topography and local climate plays significant role in this regard. Soil weathering is a term used to describe the process of soil development. The process of soil development is often referred to as soil weathering. Over the time, distinct horizons or layers are created by act of many environmental forces. Soil surface is parallel to these horizons and layers. In such cases materials i.e. clay particles or organic matter move downwards. This movement is differential. Soil color, structure and texture are influenced by the accumulation and movement of material. Horizons are distinguished on the basis of these three properties. For instance, soil color (A, Bh etc.) is influenced by organic matter accumulation, structure and texture (e.g. Bt) is influenced by clay accumulation. To distinguish horizons there are other diagnostic properties also.
For instance, to estimate carbonates presence testing is done. Within calcareous till, it helps to know about parent material (C) depth.
Classification of soil
Within provided physical conditions set, soils are grouped as per performance order. Soils are grouped with the help of classification systems. In case physical conditions differ soil groups may vary in performance order. Different systems have different indented purpose. Hence, there are various classification systems as per the system requirement. For soil engineer, this is an important tool. Soil classification has proved to be a very useful tool to the soil engineer. Field experience of different workers/experts may be utlised with the help of soil classification system. These guidelines are general. Manner of providing guideline is empirical.
Classification of Grain Size
Here, grain size is the basis for classification system. Here, the terms gravel, clay, sand and silt are particle size.
Texture style classification
Here, percentage distribution and particle size are the basis for soil classification. It is referred as textural classification system. Percentage of clay, silt and sand is used to specify soil names in this system.
Soil Texture
There are various other properties, influenced by the soil texture. These physical properties are very important. Within soil there are three mineral particles i.e clay, sand and silt. Proportion of these particles is known as texture. Size is the diagnostic property for these three mineral particles. Texture is the proportion of three mineral particles, sand, silt and clay in a soil.
Basic Soil Properties
Physical properties
These include properties like moisture, texture, bulk, structure, bulk, infiltration and porosity.
Chemical properties
These include properties like nutrient content, salinity, pH, Organic matter, mineral content.
Biological properties
These include activities of microbes like biodiversity, biological activity, biomass etc.
Soil Structure
Binding and arranging soil particles is known as soil structure. This happens in big clusters. They are known as aggregates. Aggregation plays significant role in for improvement of carbon sequestration and soil fertility, stability enhancement, maintaining movement of soil water and porosity etc. Peds that are loosely packed form ‘Granular’ structure. Organic substances act as glue here. Many horizons are characterized by Granular structure. For instance those with biological characteristics and with high content of SOM. There are larger peds. They occur in form of prisms and plates. Normally they are found with B horizon. Process of their formation is adhesive substance and shrink-swell (Gardiner and Miller, 2004).
Soil Colloids
Within soil, SOM particles and finest clay are known as ‘Soil colloids’. They are significant in context of soil fraction. This is because they serve as location of most chemical and physical activity within soil. One such property is their large surface area is an important property for ‘Soil colloids’. For a given particle mass/volume larger particles have less surface area as compare to smaller one.
Measurement of geotechnical design parameters
For geotechnical engineers, soil parameters determination is a tough task. It is important to have values for different soil parameters, although some of them are constant i.e. in Barron’s or Terzaghi’s consolidation theory, coefficient of consolidation is referred to be constant. In practice for soft soil, coefficient of consolidation is not considered as constant. Many factors influence its value, i.e. the determination method, over consolidation ratio, the soil fabric, the stress state etc. (Chu et al., 2002, Holtz and Kovacs, 1981). For estimation of appropriate value, an engineering judgment is required.
Past experience and understanding of behavior of soil helps in making appropriate engineering judgment. Experience in solving with different geotechnical and soil problems helps in this regard. For design of vertical drain, coefficient of permeability is other parameter that is different. It is very difficult to determine this parameter. Considering different soil parameters, coefficient of permeability varies widely. For soft clay, variation is from 10-11 m/s to 10-3 m/s. For gravel and sand this variation is up to 108 times. For permeability estimation error may range from 10 to 100 times. In case of soil treatment by vertical drain, usually, soil permeability is low. Incase of error in permeability estimation, there may be error in getting consolidation of specific degree. Using Terghazi’s consolidation theory, consolidation of specific degree is obtained.
Hence, soil parameters should be estimated accurately. Proper site investigation should be conducted. This is important from economic point of view. Considering soft clay vertical drains, there is requirement of specific soil parameters. These parameters are need for soil improvement work design. These parameters are as follows:
- The undrained shear strength and the undrained Young’s modulus EU
- The over consolidation ratio (OCR) and the preconsolidation stress
- The secondary compression index Ca , the coefficient of compresibilty CC, the coefficient of recompressibilty CR. For settlement estimation, these parameters are needed.
- For both vertical and horizontal direction, the coefficient of permeability i.e. CV and CH
- When vertical drains are installed, mandreal disturbs surrounding soil. For installation of vertical drain, within soil, mandreal has to be inserted. Then there is a need to determine smeared soil permeability, ks, and smeared zone extent ds.
(Source: http://www.encyclopedia.com/article-1G2-3293400010/geotechnical-properties-soft-soil.html)
If there is repository operation and construction, then geological mapping, surveillance, testing and measurement should be conduced continuously. This is to be done to ensure that design and geotechnical parameters are confirmed. In order to meet actual field conditions, design changes are done. In context of design assumptions, surface conditions must be evaluated. Specific design and geotechnical parameters must be observed/measured. If there is any interaction between engineered and natural components/system then it should be mentioned in plan concerning performance confirmation. In context of these observations/measurements there should be comparative evaluation considering original design assumptions and bases. If there is significant difference between the original design assumptions/bases and observation/measurements, then there is a need to estimate the modifications in construction methods or in design as well as their importance to performance concerning repository. Modifications will have to be recommended and reported to concerned authorities. In case of underground facility, thermo mechanical response should be monitored in situ. This will help to know whether performances of engineering and geologic features are within limits of design or not (Legal Information Institute, no date).
Laboratory methods
Consolidation tests are important when there is a need to estimate rate of settlement. For soil improvement projects that is essential requirement. Laboratory consolidation test helps to estimate soil compressibility. In context of consolidation process and stress level, soil parameters variation may be evaluated. In order to estimate soft clay consolidation property and compressibility, reliable and accurate measurements are provided by Oedometer test (Jardine and Smith, 1991).
Methods of Ground investigation, sample acquisition and testing
There is preliminary evaluation of past geotechnical investigations. It may be related to nearby sites. Published literature may also be considered. Number of project phases can be identified. Performance information of existing projects and visible data are collected from site. Natures of ground and size and settlement sensivity are important here. Observations regarding ground water and surface drainage can be made. There may be excavation of test pits. For foundation design, grading and excavation plan, upper soil zone is important. Drill equipment concerning test hole will have to be mobilized. Collection of geotechnical information depends on size of project. It helps to determine type of foundation. Drill hole should be located on critical points. By using test holes, samples may be taken at specific intervals. Then they are put into plastic bags and transported to laboratories. There sample testing is done. Index properties are obtained with the help of laboratory test. They are obtained with the help of plastic limit tests, water content estimation and particle size distribution. Tests for thermal properties and Creep strength are also conducted. Then as per the classification system, soil is classified. Different soil group are established with the help of laboratory tests. They are described by different symbols. Photographs of soil and rock core should also be taken and documented.
Laboratory measurements on soil
Within soil, relative amount of sand, silt and clay may be determined with particle size analysis (PSA). Mineral components of soil are represented by these size fractions. Field texturing can be a substituted by PSA that is laboratory alternative. Reliable particle size distribution may be obtained with the help of PSA. It is also correlated with hand texturing (McDonald et al., 1998). Soil texture influences other properties i.e. drainage and water storage (Brown & Wherrett, no date).). In Sandy soil, there is high proportion of water as compare to clay soil. Fractionation is important part of PSA (Bowman and Hutka, 2002). After that, soil textural triangle is utilized to identify soil texture (Hunt and Gilkes, 1992). Magnesium carbonate, Calcium carbonate and Iron oxide are the cementing agents of soil in many cases. To study the site response it is important to evaluate dynamic properties concerning soil. Sometimes actual data is poorly related with homogeneous soil. Hence, it becomes risky to use curves concerning modulus degradation (Sun et al., 1988, Seed and Idriss, 1970).
One-dimensional consolidation tests
In this test, for each load increment, duration has to be selected. It helps in 100 percent consolidation of the sample. If plasticity of silt and clay is low to moderate, then load duration may be three to twelve hours. In case of organic material that fibrous, consolidation may be achieved within 15 minutes. In case of high plasticity material, twenty four hours may be needed for primary consolidation. Settlement analysis that is time dependent are performed with the help of consolidation properties. Time dependent reduction in soil volume is referred as consolidation. Within soil mass there is dissipation of pore pressure. It leads to consolidation. One-dimensional consolidation tests are utilized to estimate consolidation properties of soil (Fattah et al., 2006). There are different loading systems i.e. electro mechanical, pneumatic or hydraulic. Time period can be optimized for these different systems. Square root of time, time measurements versus continuous deformation may be used in context of test. In this test, there is a lateral restrain on soil specimen. Soil specimen is loaded axially. Then there is increment of total stress. It is done till dissipation of excess pore water pressure. Measurements are taken during consolidation process. This data is used to find the relationship between void strain/ratio and effective stress and the consolidation rate. Primary laboratory equipment used for this purpose is Oedometer (One dimensional consolidometer).
Shear Strength parameters of soil
Internal resistance per unit area is referred as shear strength concerning soil mass. Failure and sliding are resisted by soil mass using Shear Strength Parameter. Shear strength plays significant role in stability problems. Combination of shearing stress and normal stress that is critical causes failure of material. Theory of rupture was presented by Mohr (1900). There are many laboratory tests for estimation of shear strength parameters i.e. Direct simple shear test, Triaxial test, Direct Shear test, Plain strain triaxial test and torsional ring shear test. In case of Direct Shear test, a metal shear box is used to put soil specimen in it. This test can be either strain controlled or stress controlled. This test is simple to perform. Triaxial test is a reliable method. It is used for conventional and research testing.
Soil Permeability Test
The term permeability refers to capacity of passage of fluid through soil. There are void among solid particles of soil. Normally, they are interconnected. It allows water to flow through them. It shows that for water, soil is permeable. Hydraulic pressure difference is related to the degree of permeability. Rate of water flow is important in context of permeability. Velocity is used to express coefficient of permeability. There are different soil types i.e. ‘impervious clays’, sands, free-draining gravels etc. For all these, flow of water follows the same physical law. As compare to sand, clay is clay is 10 million times less permeable. Darcy was the first one to explain the passage of water through soil. For saturated soil, he provided experimental explanation for flow condition that is laminar. Dimension of velocity and coefficient of permeability are the same. In order to estimate coefficient of permeability, constant head method is utilized.. Apparatus used are Spare perspexcylinder, Constant-head permeability apparatus, Caliper, a 1 liter glass measuring cylinder, Thermometer, a stop clock, a Measuring tape etc. Time taken by the water to flow through the sample is measured. Steady head difference has to be there. There is a measuring cylinder. Test may be repeated many times.
(Source:
http://www.bc.cityu.edu.hk/~soillab/exp/Download/EXPeriment-5a.pdf)
References
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Bowman GM & Hutka J (2002) Particle Size Analysis. In Soil Physical Measurement and Interpretation for Land Evaluation (Eds N McKenzie, K Coughlan, H Cresswell) pp 224-239. CSIRO Publishing: Victoria.
Brown, K. & Wherrett, A. (no date). Measuring Soil Texture in the Laboratory. http://soilquality.org.au/factsheets/soil-texture-measuring-in-the-lab
Chu, J., Bo, M.W, Chang, M.F. & Choa, V. (2002). Consolidation and permeability properties of Singapore marine clay. J. Geotech. Geoenviron. Eng. 128 (9), 724–732.
Fattah, M. Y., Al-Nneami, M.A. L. & Rahil, F. H. (2006). Comparative Experiments between Conventional and Quick One-Dimensional Consolidation Tests. Proceedings of the 4th Jordanian Civil Engineering Conference, 28-30 March 2006, Amman – Jordan
Gardiner, D. T. & Miller., R.W. (2004). Soils in our environment, 10th Edition. Pearson Education, Inc. Upper Saddle River, New Jersey. 641 p.
Holtz, R. D. & Kovacs, W. D. (1981). An Introduction to Geotechnical Engineering. Prentice-Hall.
Hunt N and Gilkes R (1992) Farm Monitoring Handbook. The University of Western Australia: Nedlands, WA.
Jardine, R.J. & Smith, P.R. (1991). Evaluating design parameters for multi-stage construction. Proc. of International Conference on Geotechnical Engineering for Coastal Development, Yokosuka: 197–202.
Legal Information Institute, no date. 10 CFR 63.132 – Confirmation of geotechnical and design parameters. https://www.law.cornell.edu/cfr/text/10/63.132
Mohr, O. (1900). “Welche Umstiinde Bedingen die Elastizititsgrcnze und den Bruch eines Materiales?” Zeitschri.f’tles Vereines Deutscher IngenieLtre, Vol. ’14. 1524-1530, 1572-1577.
