Introduction to Soil Mechanics

AMRC 2011 MODULE 1 Introduction to Soil Mechanics CONTENTS Overview... 1-1 Objectives... 1-1 Procedures... 1-1 1.1 Introduction... 1-3 1.2 Erosion... 1-5 1.3 Importance of Identification of Soil Problems... 1-13 1.4 Nature and Origin of Rocks and Soils... 1-15 1.5 Soil Properties... 1-17 1.6 Soil Strength... 1-19 1.7 Soil Drainage... 1-21 1.8 Soil Compressibility... 1-23 1.9 Self-Test... 1-25 WPC #27373 07/09

Module 1 Introduction to Soil Mechanics Overview Soil and rock are materials which are infinitely diverse in composition. The properties of soil and rock are therefore also diverse. This module introduces the subjects of geology and soil mechanics and describes the key properties of soils applicable to the use of soil in construction. Note that the terms geotechnics or geotechnology are replacing the term soil mechanics. Objectives Upon successful completion of this module, you will be able to: discuss the basis of the soil erosion problems and concerns associated with road construction define the terms geology and soil mechanics and their usage in the design and construction of roads and minor structures discuss the concept of soil as a material made up of individual particles and describe how this particulate nature of soil affects all structures involving soil describe the properties of soil and hence the use of soil mechanics in the design and construction of roads and minor structures. Procedures Study the module materials and make notes as required. Perform the self-test on these principles and review the course materials in such a manner as to be able to successfully complete similar questions upon examination. WPC #27373 07/09 AMRC 2011 1-1

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SECTION 1.1 Introduction The study of the earth s crust, rocks and soils, their origin and formation is referred to as the science of geology. The effect of the geology of a particular region on engineering structures is usually referred to as engineering geology. Directly, or indirectly, geology has an influence on almost all civil engineering structures; foundations for roads, bridges, buildings, slopes for dams and roads and fills are some of the more significant. A thorough understanding of the geological processes is a prime requisite when attempting to solve problems of design and construction in soil and rock. WPC #27373 07/09 AMRC 2011 1-3

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SECTION 1.2 Erosion Prevention of road-related erosion is of great concern to the road designer and builder. Erosion includes surface soil erosion and mass wasting processes. In the context of roads: Mass wasting includes several dominant groups of landslide processes including: debris falls debris slides debris avalanches debris flows debris torrents rockfalls rockslides slumps and earth flows small scale slumping collapse ravelling of road cuts and fills. Surface soil erosion includes shallow erosion and gullying by running water across the road surface or cuts and fills. A well-designed and well-built road can sometimes improve an existing soil defect. On the other hand, a poorly planned and executed road construction job may cause deterioration of the soil mass in an area and threaten downslope/downstream (or upstream) social, economic and forest resource values. When constructing a road, the road builder alters the existing situation and the changes in the soil s pattern which may lead to problems. Examples of these are as follows roads cut across existing drainage patterns which have to be diverted or otherwise accommodated; roads cut into existing stable slopes normally leaving the soil at steeper and therefore less stable angles; roads often create dust and sediment problems. Soil Surface Erosion Factors which influence surface soil erosion are as follows: rainfall duration and intensity how long, how many mm/hour (i.e., how hard is it raining?) WPC #27373 07/09 AMRC 2011 1-5

soil erodibility density and size of soil particles; is it clay? sand? gravel? top soil? permeability of soil texture, density and size; will water run off or soak in? e.g., clay vs gravel topography slope position slope gradient and length; the steeper and longer the slope, the more the rainwater will run off and the faster it will do so catchment area above the point of concern; the larger the area, the more water can pass through the point plant and litter cover type and density; the more durable and dense the ground cover, the less likely erosion will occur after construction; also the plant root system will have an effect As mentioned previously, road construction physically changes the face of the earth that it passes over and through. Trees and ground cover are removed at an early stage in construction; bare soil is left open to erosion and damage by the elements. Compaction of the exposed surface soil is another concern. During construction the texture of the surface soil is changed as heavy equipment moves over it and compacts it. This makes the soil less permeable, leads to less infiltration of precipitation and hence an increase in surface runoff and the possibility of erosion downstream. In some construction techniques, earth moving equipment may also cause loosening of the natural soils (e.g., building of earth embankments on steep slopes where compaction of the side cast fill material is not possible). Loosening makes the soil more porous. The designer/builder should therefore try to keep the area disturbed by construction (i.e., the area of exposed bare soil) to a minimum so that the impact of erosion is as low as possible. In doing so, the designer or builder must bear in mind that restricting one item of design might lead to problems with another. For instance, a reasonable self-supporting slope for a cut through silty gravel may be 2 horizontal:1 vertical. However, you might consider steepening this to 1.5 H:I V to reduce excavation costs and to reduce the area exposed. After reviewing the slope you may have to protect the face with hydroseeding or other means to minimize erosion. 1-6 AMRC 2011 WPC #27373 07/09

Mass Wasting Downhill movement of rock and soil caused by the force of gravity is the most universal of all processes of erosion. Mass movement mechanisms such as landslides, slumps, earth flows, sheet wash, soil creep and subsidence in combination with transportation by running water, glaciers, wave action, wind, ground water and sea currents are responsible for most erosion. No matter where you are, you do not have to look far to find evidence of mass movement. Principles governing it are simple, but the variety of combinations of types of movement, of materials moved and of geomorphic forms assumed by these masses is great. The driving force behind all mass movement is the force of gravity. This force is directed toward the centre of the earth, but components of it act along any inclined plane. The steeper the inclination, the greater will be the component of force acting down the slope. This force is most effective in moving materials that are unstable in their existing position, such as on surfaces over which they might slide, for example, fractures or bedding planes. Downslope movements can be of major importance, locally and in combination with streams that are responsible for much long distance transportation. Downslope movements occur under all climatic conditions in the air and under the waters of oceans. As use of land is intensified, it becomes increasingly difficult to avoid areas where potential natural mass instability exists areas where floods, wave action or earthquakes may trigger disastrous mass movements. Engineers must take great care to circumvent costly mass movements in planning and constructing building foundations, dams, reservoirs, bridge abutments, tunnels, and in designing cuts and fills along highways and canals. Costly and sometimes even disastrous results have followed where the dangers of potential mass movement have not been fully recognized or efforts to meet the danger have been too limited. Numerous classifications of mass movements have been proposed. These are based on the type of materials moved, the rate of movement, the presence or absence of water or the mechanisms of movement. Road construction activities can accelerate surface erosion and influence slope stability for these reasons: interception and concentration of slope drainage bank exposure and slope undercutting slope scaling. WPC #27373 07/09 AMRC 2011 1-7

By identifying areas that are potentially unstable and understanding the reasons why the instability exists, the road builder can locate, design and construct a road which will minimize the likelihood of slope failures. Also, through a carefully designed maintenance program (particularly cleaning out of culverts) we can be more sure of continued success at reducing the impacts in the future. The following three pages (taken from BC MoF Land Management Handbook No. 18 authored by Chatwin, S.C., Howes, D.E., Schwab, J.W. and Swanston, D.N.) show the steps for identifying both potential and existing landslide areas. After following these steps, the user would be expected to recognize whether or not an instability problem exists within an area. Having recognized the problem, the user should seek the expertise of a specialist and recommendation for control and correction. 1-8 AMRC 2011 WPC #27373 07/09

Figure 1.1 WPC #27373 07/09 AMRC 2011 1-9

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You are not at this stage expected to recognize and understand all that is contained in the preceding pages. They have been included for overall reference purposes. WPC #27373 07/09 AMRC 2011 1-11

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SECTION 1.3 Importance of Identification of Soil Problems The importance of constructing well-engineered roads cannot be emphasized too strongly. Detrimental impacts related to road-related erosion may include: loss of fish and wildlife habitat through siltation, slide activity or from the actual construction of a road loss of, or damage to, bridges, culverts and road surfaces with an increase in maintenance and inspection costs adverse effect on drinking water supplies damage to high landscape values long-term loss of productive forest land. In extreme cases, road-related landslides may threaten life, property or equipment. Loss of topsoil/overburden by erosion is important. Not only because the soil is virtually irreplaceable, but also because of the effects on the waters into which the soil is washed. Fish spawning areas can be washed away by a torrent or simply covered with debris. WPC #27373 07/09 AMRC 2011 1-13

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SECTION 1.4 Nature and Origin of Rocks and Soils In order to be able to counteract the soil problems, such as erosion, one must be knowledgeable about how a soil/rock is formed and how it behaves under a variety of circumstances. The rest of this manual will introduce you to the nature, origins and properties of rocks and soils. It is estimated that the world is approximately four and a half billion years old. Since that time, the earth has been in a constant state of change. Bedrock has been weathered (eroded), transported, new material has been manufactured through volcanic action, mountains have risen up, seas have receded, ice has swept over the continental areas and through these geological conditions, natural aggregate deposits have been formed. This constant change is still in progress today. Even though it is slow by human standards, the rate of change today is no different than it was three or four million years ago. We still have earthquakes, volcano eruptions, glacial activity, wind storms and major tidal movements. All these activities are either producing new material or eroding older material. The geologist distinguishes among three basic types of rocks: igneous rocks sedimentary rocks metamorphic rocks. The igneous rocks were formed when molten magma from the interior of the earth erupted and was forced toward the surface. On nearing the earth s surface some of the magma cooled down to form a solid mass of crystals (e.g., granite), while some magma reached the surface in the form of volcanoes and lava (e.g., basalt). The sedimentary rocks were formed mainly by the weathering and erosion of older mountains, with the eroded material being deposited under water in lakes and seas (e.g., sandstone) followed by compression due to significant amounts of overlying sediments. The metamorphic rocks are rocks of either sedimentary or igneous origin, but whose properties have been changed by intense pressure and chemical change. Slate is a metamorphic rock. WPC #27373 07/09 AMRC 2011 1-15

During the formation of these rocks, stresses are created within the rock mass. Uneven cooling of the molten magma and distortions that the rock undergoes through the movement of adjacent rocks are significant causes of stress in rocks. Where the mass of jointed and fissured rock is exposed at the earth s surface, it is subject to the physical action of water, ice, wind and gravity. For instance, water may freeze and expand in a joint in the rock. As a result of this a fragment of the rock may become detached and fall to a new position. This movement will generally be accompanied by further fracturing and fissuring of the rock fragment when it comes into contact with other rocks or rock fragments. The process of continual breaking down into smaller and smaller fragments of rock is called weathering. The more these fragments come into contact with one another as the result of rolling down slopes, the more they are moved along streams and riverbeds or by glaciers, or are blown by wind, the more rounded and generally smaller the fragments become. This process will result in particles of rock varying from a size so small that they cannot be seen without the aid of a microscope to very large particles, metres in diameter, commonly called boulders. This mixture of particles is referred to as soil, and the study of how these materials behave when subject to loads and water pressures is called soil mechanics. The soil mechanics engineer is also concerned with the behaviour of other materials of a similar nature, such as peat, and rock which has not broken down sufficiently to be referred to as a soil. Soil mechanics engineering is also commonly called geotechnical engineering. 1-16 AMRC 2011 WPC #27373 07/09

SECTION 1.5 Soil Properties A soil is a mixture of individual particles. These particles may vary enormously in size, and each individual particle may vary from a very rounded, smooth shape to a sharp sided, angular shape. The natural process of wind or water action tends to sort the particles in such a way that the particles of one range of sizes will tend to congregate in a particular location, and the finer or coarser particles will be moved and deposited elsewhere. Deposits of soils are given names on the basis of the range of particle sizes on the deposit. For instance, the term sand has a very definitive meaning in soil mechanics. Only soils whose particle sizes fall within a specified range may be called a sand. The process of categorizing soils in this way is called classification. The particle structure of soil has a great influence on the soil properties. Therefore, soils of similar classification have similar properties. Among the more common properties of soil are strength, drainage and compressibility. The following discussion is an attempt to visualize how the soil shape, size and structure has a profound influence on these properties, and in turn, how these properties affect engineering structures, such as roads. WPC #27373 07/09 AMRC 2011 1-17

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SECTION 1.6 Soil Strength Soil develops strength in a rather complex manner; but it can be shown quite easily, how important the influence of size and shape of the soil grains is on this property. Figure 1.2 demonstrates how increased angularity of the grains increases the resistance of one layer of soil grains which has another layer being pushed across it. It is in this way that soil can develop resistance (strength) to a force pushing on it. Figure 1.2 Soil Resistance (Friction) Resistance (strength) opposing the pushing force is due to friction between the grains. Since smooth particles (A) cannot interlock like the angular particles (B), they develop less friction. This situation exists in a soil slope, Figure 1.3. Part of the weight of the slope is attempting to cause a collapse by rotation outward along a failure plane. At this failure plane the friction between the grains (i.e., the soil strength) acts in the opposite direction to prevent the collapse. If the soil is not strong enough to create sufficient friction to oppose the outward motion, then the slope will indeed collapse. WPC #27373 07/09 AMRC 2011 1-19

Figure 1.3 Forces Acting in a Soil Slope 1-20 AMRC 2011 WPC #27373 07/09

SECTION 1.7 Soil Drainage The ease or difficulty with which water passes through soil is referred to as the permeability of the soil. The fact that the size and shape of the grains influence this property is readily demonstrated. Figure 1.4 shows that as the soil grains become smaller, so the path of flow for water becomes much more difficult; the water then has to move through the soil much more slowly and hence the permeability is decreased. Figure 1.4 Soil Permeability In both (a) and (b), points l 2 are the same distance apart. In (a), however, the route which the water has to take is much more difficult and takes more time than in the case of the larger particles in (b). This is why coarse-grained soils such as gravels (with a little silt or clay) are used to build bases underneath wearing surfaces in highway construction. The high permeability allows any water to flow away quickly. This prevents any damage which might occur to the wearing surface if the water were trapped below it. WPC #27373 07/09 AMRC 2011 1-21

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SECTION 1.8 Soil Compressibility The amount by which a soil will settle and the time it takes to settle is, like soil strength, a complex matter. However, here again, it is not too difficult to show how grain size, shape and structure influence the settlement characteristics of the soil. Figure 1.5 illustrates how, under the influence of a load, the grains will become reoriented and will settle together more closely. Figure 1.5 Soil Reorientation under Load This property of the soil can lead to large settlements when a foundation load is placed on a soil which has high compressibility. A typical example of this is illustrated in Figure 1.6. WPC #27373 07/09 AMRC 2011 1-23

Figure 1.6 Differential Settlement The different soil structure of the clay, which makes it more compressible than the dense sand, causes it to settle (compress) more. This results in a differential settlement across the building, which can cause severe damage to it. Note: Clays vary in their compressibility and some sands will compress. 1-24 AMRC 2011 WPC #27373 07/09

SECTION 1.9 Self-Test 1. What is the purpose of soil mechanics when designing a road? 2. Describe two types of slope movement. 3. How are soil erosion and slope stability linked? 4. What factors influence surface erosion? Are they linked to each other? 5. Name the three basic classifications of rock and describe each. 6. What is meant by the term soil mechanics and how is it related to geotechnical engineering? 7. What is meant by the term weathering? 8. If a soil has high permeability, what does this mean? 9. Why is the compressibility of a soil important in engineering? WPC #27373 07/09 AMRC 2011 1-25

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