Engineering Soil on a Road Project
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Engineering Soil on a Road Project Names:
Paddy Mc Cormack ack (A00133543 543)
Highway De Design
Lect Lectur urer er::
Micha ichael el McLo McLoug ughl hlin in
BEng (H (Hons) in in Ci Civil En Engineering
Contents 1. Introduc Introduction.. tion....... .......... .......... ........... ............ ............ ............ ............ ............ ............ ............ ............ ............ ............... ..........3 .3 2. Soil Classificati Classification on and Composition Composition..... .......... .......... .......... .......... ........... ............ ............ .............. ........3 3 3. The Origin Origin of Irelands Irelands Soil........ Soil............. .......... ........... ........... .......... .......... ........... .................... ....................5 ......5 4. The Engineerin Engineering g Characterist Characteristics ics of Soil........ Soil............. .......... .......... ........... ............ ................ ..........9 9 5. Importan Importantt Soil Parameters Parameters and Testing Testing...... ............ ............ ............ ............ ............. .............11 ......11 6. Soil Stabilizat Stabilization.. ion....... .......... .......... ........... ............ ............ ............ ............ ............ ............ ............ .............. .............1 .....16 6 7. Conclusio Conclusion.... n......... .......... .......... ........... ............ ............ ............ ............ ............ ............ ............ ............ ............ .............. ........18 18 8. Biliograp Biliography... hy........ ........... ............ ............ ............ ............ ............ ............ ............ ............ ............... ....................... ...............19 .19 Biliography
It’s safe to say that soil is the foundation for all roads projects, be it in the shape of the original insitu subgrade material or replacement reworked material, and thus should be considered the primary engineering material with respect to road design, construction and maintenance. As a result of this the soil types tend to have an important impact with respect to both selecting an appropriate route and the design of that route. Most authorities or developers are therefore required to appropriately identify and manage the physical properties of the soil when undertaking a roads project. However unlike steel and many other materials incorporated into design by engineers, soil deposits are not homogeneous and their characteristics and properties may vary significantly over short distances. Basically, soils must be dealt with in an efficient manner as they are encountered. Modifications may be incorporated or procedures undertaken to enhance its properties but with respect to the economical aspect of a project, appropriate soil would be an important factor. It is essential therefore that a highway engineer have a thorough knowledge of soils and their associated properties.
2. Soil Classification and Composition
According to CA O’Flaherty, soil is any naturally occurring loose or soft deposit resulting from the weathering or breakdown of parent rock formations or from the decay of vegetation. In general it can be taken as consisting of five main components which are as follows: 1. Mineral Matter: This is the predominant characteristic in soil and can generally define the soil’s physical and chemical properties. It is normally dependent on the parent rock and consists of finite pieces of rocks broken down through weathering. 2. Air: In less consolidated soil air becomes a more influential component, filling the voids between soil particles. This air is 3
essential for the biological aspect of the soil as it contains both the oxygen and nitrate to facilitate organisms in the soil. 3. Water: Depending on the location, soil can have a varying water content. This water content can have a major influence with respect to the engineering capabilities of the soil and its physical properties as will be outlined in a following section. 4. Living Organisms: These help break down the dead plants and organic material and consist of many creatures ranging from earthworms and slugs to millions of micro-organisms. 5. Humus: This consists of decaying organic matter and is the remains of plants, leaves, grass and dead creatures. A large amount of this material is undesirable in terms of engineering soils. If it were possible to excavate to the bedrock and see a cross-section of the soil profile the soil could most often be clearly separated into three main horizons, usually defined by the letters “A”, “B” and “C” (see Figure 1). The First horizon A is the uppermost layer of soil and is often referred to as topsoil. Most organisms live in this horizon and thus it normally has a darker colour due to its high humus content. In general this layer of soil is excavated due to its higher proportion of organic matter which is undesirable in engineering applications since the topsoil supports growth of trees and other vegetation and often does not have appropriate physical properties. When removed, topsoil usually is stockpiled and later restored onsite for landscaping or to support growth of vegetation to control erosion. Horizon B lies immediately beneath the A horizon and corresponds closely to the so-called “sub-soil”. Lying between both A and C it possesses some of the properties of both. It is lighter in colour due to lower humus content, contains more parent rock particles and is protected from weathering. The process of leaching, that is, the washing of minerals, nutrients and humus through the topsoil, often results in the accumulation of relatively high content of iron and aluminium oxides, humus or clay particles in this layer. Often this 4
subdivided with respect to the accumulation of these leached products. The C horizon however consists of a more geological material such as the parent rock. It is in effect the upper section of the loose and partially decayed rock or another material such as glacial deposits similar to that from which the soil has developed.
Figure 1 - Soil Profile (www.saburchill.com)
1. The Origin of Irelands Soil
It is evident that a soil’s composition and characteristics are strongly related to the local geology. It thus could be said that in general the combination of parent material and soil forming process defines a soil type. With this in mind, to understand the origin of Ireland’s soils, one must first examine the geological aspect.
carboniferous limestones, which vary from very pure to impure shaley varieties (see Figure 3). Within this large area there are two types of rock giving rise to pronounced topographic relief. The first and more frequent consist of lower palaeozoic shales and sandstones and old red sandstones (Note Figure 2 for Timeline). The second consist of younger carboniferous rocks, predominantly shales, siltstones and sandstones. In the northwest, most of the area is underlain by a complex series of rocks comprising schists (metamorphosed from mudstones and muddy sandstones), and quartzites (from sandstones). These have been intruded by a series of granites during the caledonian era. The south and south West are dominated by old red sandstone and some shales in the west, whilst the easternmost part is more variable with sandstones, shales and limestones during the hercynianera. West Galway and west Mayo are characterized by granite in the south and a combination of lower palaeozoic and late pre-cambrian metamorphic rocks north of this.
Figure 2 -Simplified figure of the major geological eras, periods and epochs in their time frames (EPA)
During the Pleistocene era, Ireland experienced at two glacial episodes which are important with respect to soil formation. The older of the two was the munster general glaciation (200,000 to 130,000 years ago) and enveloped the whole country. The later glacial period, known as the midlandian general glaciation (75,000 to 10,000 years ago), intruded into Ireland from the North down an ‘east-west’ line running from the Shannon estuary to Arklow and also into the south eastern coastal area. Most evidence of the earlier glacial period north of this line was removed or covered by that of the second. (D Boys) In lowland areas the main effect was the deposition of sheets of drift material. When the glaciers retreated, deposits composed of boulders, rock fragments, gravel, sand, silt and clay were left behind. This material was laid down unevenly varying in type and size, such as particles of dust, large boulders, erratics and drumlins. Other features of glacial deposition include the outwash plains, eskers and corries. 7
Deposits of boulder clay and old glacial lake beds have thus resulted in good quality, well drained soil in some areas and badly drained soil in others. The boulder clay consists of a rich soil layer covering a rolling landscape. In some areas, this layer can be very thin in which case the underlying rocks can have a strong local impact on the composition of the soil. Also eskers are quite common in counties such as Kildare, Offaly, Galway and Roscommon and there are very widespread thick deposits of glacial sands in the Curragh often greater than 70m. With respect to advantages in terms of highway engineering, eskers are commonly used for route construction and quarrying.
Figure 3 – Geological Maps of Ireland (EPA)
2. The Engineering Characteristics of Soil
The inorganic soil particles or that most commonly found in the sub-soil or horizon B (see previous) are normally composed of primary mineral fragments often related to the underlying parent rock and the secondary minerals being products of weathering. Nonetheless, soils are normally described with respect to the relative proportion of various size particles in the mineral fraction of a soil. More specifically, it refers to the relative proportions of sand, silt and clay in the mineral fraction less than 2mm in diameter. Sand is mostly comprised of quartz particles and as a result relatively inactive chemically providing very little bond on this front, thus its physical properties define its characteristics. Unlike clay its particles are normally bulky in shape and can be either angular or rounded depending on the degree of abrasion received before final deposition. With respect to road design the stability potential of a sandy soil is significantly influenced by compaction, gradation and particle shape. Moisture content also has an effect upon the strength of a sandy soil. Also an important note is that in design clean particles of sand are often considered cohesionless and thus are undesirable with respect to embankments. As well as this due to the presence of large pores, sand is very permeable and susceptible to ground water contamination. Silt particles are often considered as the transitional between clays and sands. With respect to their performance they are similar to sands deriving most of their strength from the mechanical interaction among the particles. Essentially these particles are finite sand particles with similar properties but an important change is that they also possess a small amount of cohesion due to the chemical action at the interparticle water films between the particles. Detailed investigation may be required when working with silt materials as they can often be an unpredictable material. The process of liquefaction may sometimes occur where slight vibration could result in a “quicksand” like material undesirable in road design.
On a microscopic level particles in the clay fraction differ from those of sand and silt. Physically they are commonly flat and elongated or “lamellar” and thus have a much larger surface area per mass then sand particles. Also, most importantly, clay differs in its chemical makeup. A clay fraction is essentially controlled by its microscopic or colloidal constitutes. Therefore these colloidal particles are primarily responsible for the cohesiveness of a plastic soil, its shrinking and swelling characteristics, and its ability to solidify into a hard mass upon drying. Also this affects the permeability or hydraulic conductivity of the soil or clay. These colloids are in turn controlled by the electrical charges on their surface. The exchange of positively charged ions, or “cation exchange” is at the basis of the stabilisation of soils with certain chemical components. An important note here is that the intensity of this physic-chemical phenomena is also associated with surface area thus clay minerals such as montmorillionite for example with expandable lattice structures may have a high exchange capacity. A common form of defining soil is through the use of a texture classification chart. As shown in figure 4, a soil sample may be defined as appropriate by determining each of its sand, silt and clay percentages and using the chart.
Figure 4 – Textural Classification Chart
3. Important Soil Parameters and Testing
With respect to the design of highways, be it in foundation design or embankment construction, there are many properties which must be investigated to ensure appropriate design and construction. Proper analysis of the soil is often undertaken through various tests and procedures to gain an in depth knowledge on the parameters involved in design. These quantitative results from laboratory tests on the soil samples are necessary to analyze the soil conditions and thus affect an appropriate design on factual data. The importance of securing sufficient and accurate data cannot be overemphasised. Important engineering parameters with respect to a road project or highway engineering include: 1. Soil Shear Strength(τ): Shear strength is a fundamental property of undisturbed cohesive soils and knowledge of this is necessary for the solving of many soil related problems. It is normally defined in terms of unconfined compressive stress. Shear 12
strength is often considered negligible with respect to dry sands or
performance also. On the other hand it is a major factor in terms of a clay type soil where its main characteristics are formed through its cohesion and chemical binding action (refer to Section 4). This is most commonly calculated through either the triaxial or direct shear lab tests and a value obtained for the aid of design. 2. Cohesion(c): This characteristic is majorly related to strength and is the main characteristic in defining cohesive and cohesionless soils. In effect it is the ability of a soil to maintain its strength when unconfined, that is, to cling together and maintain its form through changes in moisture content. 3. Moisture Content (w): The Moisture content of a soil is the amount of liquid (water) per volume of mass of soil. This has a major effect on the properties of a soil as outlined throughout this report. 4. Density (γ): In effect density is the measure of mass per volume of a given soil. Density is as important for cohesionless soils as strength is for cohesive soils. It can be found by comparing the soils actual void ratio with the range in void ratio from loose to dense for that soil. Its importance intertwines with that of compaction and the concept of a maximum dry density and both water content air voids and material characteristics effect this parameter. This is further outlined in section 5.2. 5. Permeability (k): This is concerned with a soils ability to allow the passage of liquid through its composition. Also referred to as the “hydraulic conductivity” (k), this can have an influential effect when it comes to drainage etc. with respect to road design. 1.1Moisture Content The moisture content of a soil (sometimes referred to as the water content) is an indicator of the amount of water present in the soil. By definition, moisture content is the ratio (expressed as a percentage) of 13
the weight of water in the sample to the weight of solids in the sample. In simple terms this is calculated by first weighing a sample of the soil in its insitu state and again weighing the sample after it has been oven dried to remove any moisture. It is written as follows: w= WwWs×100 Where, w is the moisture content, Ww is the weight of water and Ws is the weight of the soil solids after being dried in an oven. The moisture content of a soil mass will have a major impact on its engineering capabilities, for example different percentages of water may affect the strength, shear stress and plasticity of a given soil 1.2The Proctor Test (Moisture-Density Relationship) Compaction is a very important factor when it comes to the design of highways. It is nearly almost always necessary to compact a soil when working with it as a foundation material and this is generally done by mechanically increasing the density of the soil. If performed improperly on a roads project settlement could result in unnecessary maintenance costs or structural failure. As a general rule, dry soils can be best compacted if for each soil there is a specified amount of water added to it. This water effectively acts as a lubricant and allows the soil particles to be packed together better. However, if too much water is added a lesser density will result. Therefore, for a given compactive effort, there is a particular moisture content at which dry density is the greatest and compaction is best and this is called the “optimum moisture content”. Also the associated dry density is referred to as the” maximum dry density”. In the lab these parameters are most commonly calculated through the “standard proctor test”. When a series of samples of soil are compacted at various moisture content values the plot usually shows a distinct peak. Thus the maximum dry density occurs at an optimum moisture content shown by the peak. The curve is drawn with axes of dry density and water content and the controlling values of γd max (maximum dry density)
and wopt. (optimum water content) may be simply read off (see Figure 5).
Figure 5 – Moisture-Density Relationship
1.3The Moisture Condition Value (MCV) In simple terms The MCV is a measure of the minimum compactive effort required to produce near full compaction of a soil. It is used in the design and construction of roads to assess the suitability of earthwork material in relation to the specified upper limits of moisture content. The test is based on the determination of the minimum number of blows of a standard rammer required to result in a state of full compaction in a soil sample. After calibration over a range of moisture contents the suitability of earthwork material can be assessed without the measurement of the moisture content of the soil with its associated delays. The soil placed in a mould and compacted by blows from a rammer dropping from a height of 250 mm, and then the penetration after each blow is recorded. The change in penetration (∆p) is recorded between that for a given number of blows (n) and that for 4n blows. A graph is plotted of ∆p/n and a line drawn through the steepest section. Therefore the moisture condition value (MCV) is given by the intercept of this line and a special scale as shown below.
Figure 6 – Example Plot of MCV
1.4The California Bearing Ratio Test (CBR)
Arguably one of the most useful tests in relation to testing of a foundation soil in highway design is the California bearing ratio test. This test is effectively a measure of the strength of the subsoil on which a road is to be constructed and a measure of it’s suitability for the intended purpose. Although the method makes no attempt to incorporate any of the standard soil properties such as density, the value is an integral part of road design and is referenced in many design charts and calculations related to road foundations. Its main function is to determine an appropriate thickness of capping layer required with respect to a flexible pavement. The CBR for a soil is the ratio obtained by dividing the unit load required to cause a given piston to penetrate a standard distance into the soil by a standard unit load of 1000psi. The CBR may be thought of as an indicator to the strength relative to that of crushed rock and expressed in the following form:
Figure 7 –Typical CBR Design Chart
2. Soil Stabilization
A common problem with respect to road design and construction is the discovery of a soil which may not meet the requirements needed for use in either embankments or as a subgrade, be it due to undesirable strength, incompressibility or other characteristics. The process of improving the soil in order to meet the requirements is most commonly referred to as “stabilization”. In its broadest meaning, stabilisation includes compaction, drainage, preconsolidation, and protection of the surface from erosion and moisture infiltration. However, nowadays the term stabilization is not restricted to simply the aspect of alteration of the soil itself but more specifically the alteration by either chemical or mechanical means to result in an improved soil possessing the desired engineering properties. Dependent
requirements of a project, the mode of alteration or stabilization will vary. In most cases the objective is to provide additional strength, for example where the soil is cohesionless, this could be provided in many fashions including confinement, adding a cementing or binding material or by some mechanical feature. Regardless of the purpose for stabilization or its form, the ultimate result is the creation of a material which will fulfil its purpose with respect to the end use conditions for the design life of the project. The following are some common stabilization techniques undertaken with regard to roads projects. 2.1Chimerical Stabilisation A common way of improving the engineering properties of a soil is by adding chemicals or other materials to improve the exsisting soil. This technique is often cost effective because the price of transportation and processing of a stabilizating agent or additive outlined below to treat an insitu soil will most probably be more economical than importing aggregate for the same thickness of support. Additives include Portland cement, lime, flyash, calcium chloride and bitumen.
These additives are introduced by various machines and can be mixed or grouted into the soil depending on the required outcome. ○
Moisture Holding Admixtures: As previously discussed moisture in the soil provides some cohesion in sands and silts. Also it prevents shrinkage and cracking of cohesive soils thus reducing surface degradation. To
commonly used in the soil. Salt is often used and is applied at a rate of approximately 15kg/m 3 Calcium chloride is also used at a rate of 915kg/m3 is also very effective as it is able to take moisture from the air as well. ○
Cementing: Wide varieties of cementing or binging agents are used in cementing and are arguably the most used form of soil stabilization. The most predominant advantage is the increase in strength through cohesion but also there is a reduction in permeability of most soils by filling the voids with cementing agents. Compressibility is also decreased respectively forming a more desirable subgrade when used in road design. Many additives are used in this process including traditional Portland cement, bitumous binders, flyash and sodium silicate.
6.2 Mechanical Stabilisation
This refers to either the compaction of the soil or the introduction of fibrous and other non-biodegradable reinforcement to the soil. This practise does not affect the soil in a sense but more influences the soil mass as a whole. It is also common practise to incorporate this with some form of chemical stabilisation. Some mechanical stabilisation techniques are as follows: ○
Compaction: This method, as outlined previously, affects the density, reduces air voids and is a common form of stabilization. Typically this employs a heavy weight to increase soil density by applying pressure from above. Machines usually form this purpose such as rollers and vibrating plates.
Soil Reinforcement: This often incorporates specially engineered mechanical solutions to enhance the soil. Geo-textiles and engineering plastic mesh or “geogrids” are designed to trap soils and help control moisture conditions erosion and permeability. These
In conclusion, the importance of a thorough understanding of the properties, characteristics and behaviour of influential soil formations is clear.
embankment, drainage or any other influential area, this is a predominant factor with respect to road planning, design and construction. Often undesirable soil types may be encountered and have a huge influence with respect to not only a projects progress but also, arguably most importantly, a projects budget. It is thus detrimental to ensure detail investigation and analysis in both the planning and design stage. Physical properties, formations and engineering capabilities which are required at this stage must be appropriately determined and incorporated to result in a safer yet efficient outcome. Also with regard to the construction phase it is best practise to both monitor conditions on-site and ensure best practise to protect the well being of all involved and result in an appropriate finished project.
1. Townsend, W.N, (1973), An Introduction to the Scientific Study of the Soil, 5th Edition, London, Edward Arnold. 2. Whitlow, R, (2001), Basic Soil Mechanics, Harlow, England. 3. Caterpillar Engineering
Basics of Soil Stabilization: An Overview of Materials and Techniques. 4. The Environmental Protection Agency (Ireland), (2007), Soil Geochemical Atlas of Ireland. 5. Soil Science Society of Ireland, http://www.ucd.ie/sssi/Home.html, Accessed 19/1/’12. 6. Teagasc, Soil Associations of Ireland and their Land Use Potential: Explanatory Bulletin to the Soil Map of Ireland 1980. 7. O’Flaherty, C.A, (1988), Highways, Highway Engineering, Volume 2, Third Edition, Edward Arnold. 8. Liu, C, & Evett, J.B, (1984), Soil Properties: Testing, Measurement and Evaluation, McGraw-Hill. 9. Smith, G.N, (2006), Smith’s Elements of Soil Mechanics. 10.Berry, P.L, (1987), An Introduction to Soil Mechanics. 11.Jones, C.J.F.P, (1985) Earth Reinforcement and Soil Structures.