Pile Layout Plan and Section Detail Autocad Drawing
Description And Uses Of Bituminous Binders
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Description And Uses Of Bituminous Binders
Bituminous binders can be classified into three general groups: asphalt cement, asphalt cutbacks, and emulsified asphalt.
Blown asphalt and road tars are also other types of bituminous material that now are not used commonly in highway construction.
Asphalt Cements
Asphalt cements are obtained after separation of the lubricating oils. They are semisolid hydrocarbons with certain physiochemical characteristics that make them good cementing agents. They are also very viscous, and when used as a binder for aggregates in pavement construction, it is necessary to heat both the aggregates and the asphalt cement prior to mixing the two materials.
For several decades, the particular grade of asphalt cement has been designated by its penetration and viscosity, both of which give an indication of the consistency of the material at a given temperature. The penetration is the distance in 0.1 mm that a standard needle will penetrate a given sample under specific conditions of loading, time, and temperature.
The softest grade used for highway pavement construction has a penetration value of 200 to 300, and the
hardest has a penetration value of 60 to 70. For some time now, however, viscosity has been used more often than penetration to grade asphalt cements.
Asphalt cements are used mainly in the manufacture of hot-mix, hot-laid asphalt concrete, which is described later in this chapter. Asphalt concrete can be used in a variety of ways, including the construction of highways and airport pavement surfaces and bases, parking areas, and industrial floors. The specific use of a given sample depends on its grade.
Asphalt Cutbacks
The asphalt cutbacks are slow-curing asphalts, medium-curing cutback asphalts, and rapid-curing cutback asphalts. They are used mainly in cold-laid plant mixes, road mixes (mixed-in-place), and as surface treatments.
Slow-Curing Asphalts
Slow-curing (SC) asphalts can be obtained directly as slow-curing straight run asphalts through the distillation of crude petroleum or as slow-curing cutback asphalts by “cutting back” asphalt cement with a heavy distillate, such as diesel oil.
They have lower viscosities than asphalt cement and are very slow to harden. Slow-curing asphalts usually are designated as SC-70, SC-250, SC-800, or SC-3000, where the numbers relate to the approximate kinematic viscosity in centistokes at 60°C (140°F). Specifications for the use of these asphalts are no longer included in The American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Transportation Materials.
Medium-Curing Cutback Asphalts
Medium-curing (MC) asphalts are produced by fluxing, or cutting back, the residual asphalt (usually 120 to 150 penetration) with light fuel oil or kerosene. The term medium refers to the medium volatility of the kerosene-type diluter used. Mediumcuring cutback asphalts harden faster than slow-curing liquid asphalts, although consistencies of the different grades are similar to those of the slow-curing asphalts.
However, the MC-30 is a unique grade in this series as it is very fluid and has no counterpart in the SC and RC series.
The fluidity of medium-curing asphalts depends on the amount of solvent in the material. MC-3000, for example, may have only 20 percent of the solvent by volume, whereas MC-70 may have up to 45 percent. These medium-curing asphalts can be used for the construction of pavement bases, surfaces, and surface treatments.
Rapid-Curing Cutback Asphalts
Rapid-curing (RC) cutback asphalts are produced by blending asphalt cement with a petroleum distillate that will evaporate easily, thereby facilitating a quick change from the liquid form at the time of application to the consistency of the original asphalt cement. Gasoline or naphtha generally is used as the solvent for this series of asphalts.
The grade of rapid-curing asphalt required dictates the amount of solvent to be added to the residual asphalt cement. For example, RC-3000 requires about 15 percent of distillate, whereas RC-70 requires about 40 percent. These grades of asphalt can be used for jobs similar to those for which the MC series is used. Specifications for the use of these asphalts are given in AASHTO’s Standard Specifications for Transportation Materials.
Emulsified Asphalts
Emulsified asphalts are produced by breaking asphalt cement, usually of 100 to 250 penetration range, into minute particles and dispersing them in water with an emulsifier.
These minute particles have like-electrical charges and therefore do not coalesce. They remain in suspension in the liquid phase as long as the water does not evaporate or the emulsifier does not break.
Asphalt emulsions therefore consist of asphalt, which makes up about 55 to 70 percent by weight, water, and an emulsifying agent, which in some cases also may contain a stabilizer.
Asphalt emulsions generally are classified as anionic, cationic, or nonionic. The first two types have electrical charges surrounding the particles, whereas the third type is neutral. Classification as anionic or cationic is based on the electrical charges that surround the asphalt particles.
Emulsions containing negatively charged particles of asphalt are classified as anionic, and those having positively charged particles of asphalt are classified as cationic.
The anionic and cationic asphalts generally are used
in highway maintenance and construction, although it is likely that the nonionics may be used more frequently in the future as emulsion technology advances.
Each of these categories is further divided into three subgroups based on how rapidly the asphalt emulsion returns to the state of the original asphalt cement. These subgroups are rapid-setting (RS), medium-setting (MS), and slow-setting (SS).
A cationic emulsion is identified by placing the letter “C” in front of the emulsion type; no letter is placed in front of anionic and nonionic emulsions. For example, CRS-2 denotes a cationic emulsion, and RS-2 denotes either an anionic or nonionic emulsion.
Emulsified asphalts are used in cold-laid plant mixes and road mixes (mixedin-place) for several purposes, including the construction of highway pavement surfaces and bases and in surface treatments.
Note, however, that since anionic emulsions contain negative charges, they are more effective in treating aggregates containing electropositive charges (such as limestone), whereas cationic emulsions are more effective with electronegative aggregates (such as those containing a high percentage of siliceous material).
Also note that ordinary emulsions must be protected during very cold spells because they will break down if frozen. Three grades of high-float, medium-setting anionic emulsions designated as HFMS have been developed and are
used mainly in cold and hot plant mixes and coarse aggregate seal coats. These highfloat emulsions have one significant property: They can be laid at relatively thicker films without a high probability of runoff.
Specifications for the use of emulsified asphalts are given in AASHTO M140 and
ASTM D977.
Blown Asphalts
Blown asphalt is obtained by blowing air through the semisolid residue obtained during the latter stages of the distillation process.
The process involves stopping the regular distillation while the residue is in the liquid form and then transferring it into a tank known as a converter. The material is maintained at a high temperature while
air is blown through it.
This is continued until the required properties are achieved.
Blown asphalts are relatively stiff compared to other types of asphalts and can maintain a firm consistency at the maximum temperature normally experienced when exposed to the environment.
Blown asphalt generally is not used as a paving material. However, it is very useful as a roofing material, for automobile undercoating, and as a joint filler for concrete pavements.
If a catalyst is added during the air-blowing process, the material obtained usually will maintain its plastic characteristics, even at temperatures much lower than that at which ordinary asphalt cement will become brittle. The elasticity of catalytically blown asphalt is similar to that of rubber, and it is used for canal lining.
Road Tars
Tars are obtained from the destructive distillation of such organic materials as coal.
Their properties are significantly different from petroleum asphalts. In general, they are more susceptible to weather conditions than similar grades of asphalts, and they set more quickly when exposed to the atmosphere. Because tars now are used rarely for highway pavements, this text includes only a brief discussion of the subject.
The American Society for Testing Materials (ASTM) has classified road tars into three general categories based on the method of production.
- Gashouse coal tars are produced as a by-product in gashouse retorts in the manufacture of illuminating gas from bituminous coals.
- Coke-oven tars are produced as a by-product in coke ovens in the manufacture of
coke from bituminous coal. - Water-gas tars are produced by cracking oil vapors at high temperatures in the
manufacture of carburated water gas.
Road tars also have been classified by AASHTO into 14 grades: RT-1 through RT-12, RTCB-5, and RTCB-6. RT-1 has the lightest consistency and can be used effectively at normal temperatures for prime or tack coat (described later in this
chapter).
The viscosity of each grade increases as the number designation increases to RT-12, which is the most viscous. RTCB-5 and RTCB-6 are suitable for application during cold weather, since they are produced by cutting back the specific grade of tar with easily evaporating solvent. Detailed specifications for the use of tars are given by AASHTO Designation M52-78.
What is Frost Action In Soils?
What is Frost Action In Soils?
When the ambient temperature falls below freezing for several days, it is quite likely that the water in soil pores will freeze. Since the volume of water increases by about 10 percent when it freezes, the first problem is the increase in volume of the soil.
The second problem is that the freezing can cause ice crystals and lenses that are several centimeters thick to form in the soil. These two problems can result in heaving of the subgrade (frost heave), which may result in significant structural damage to the pavement.
In addition, the ice lenses melt during the spring (spring thaw), resulting in a considerable increase in the water content of the soil. This increase in water significantly reduces the strength of the soil, causing structural damage of the highway pavement known as “spring break-up.”
In general, three conditions must exist for severe frost action to occur:
1. Ambient temperature must be lower than freezing for several days.
2. The shallow water table that provides capillary water to the frost line must be
available.
3. The soil must be susceptible to frost action.
The first condition is a natural phenomenon and cannot be controlled by humans. Frost action therefore will be more common in cold areas than in warm areas if all other conditions are the same.
The second condition requires that the groundwater table be within the height of the capillary rise, so that water will be continuously fed to the growing ice lenses.
The third condition requires that the soil material be of such quality that relatively high capillary pressures can be developed, but at the same time that the flow of water through its pores is restricted.
Granular soils are therefore not susceptible to frost action because they have a relatively high coefficient of permeability. Clay soils also are not highly susceptible to frost action because they have very low permeability, so not enough water can flow during a freezing period to allow the formulation of ice lenses.
Sandy or silty clays or cracked clay soils near the surface, however, may be susceptible to frost action. Silty soils are most susceptible to frost action. It has been determined that 0.02 mm is the critical grain size for frost susceptibility.
For example, gravels with 5 percent of 0.02 mm particles are in general susceptible to frost action, whereas well-graded soils with only 3 percent by weight of their material finer than 0.02 mm are susceptible, and fairly uniform soils must contain at least 10 percent of 0.02 mm particles to be frostsusceptible.
Soils with less than 1 percent of their material finer than the critical size are rarely affected by frost action.
Current measures taken to prevent frost action, include removing frost-susceptible soils to the depth of the frost line and replacing them with gravel material, lowering the water table by installing adequate drainage facilities, using impervious membranes or chemical additives, and restricting truck traffic on some roads during the spring thaw.
The truth about the 50 Lane Highway in China
The truth about the 50 Lane Highway in China
Yes, there exists a 50 lane highway in China and it merges to 4 lanes!!!
It is a 50-lane parking lot on the G4 Beijing-Hong Kong-Macau Expressway, one of the country’s busiest roads.
An aerial view from Google maps shows that the G4 Expressway is typically a 4-lane highway. The road expands to the width of approximately 50 cars when it approaches the Zhuozhou Toll Gate, but before and after this toll checkpoint it is only a 4-lane road.
Here’s an aerial view of the toll we stitched together from Google Maps. Note how both the the northbound and southbound portions of this highway are merely 4-lane roads after they leave the toll area:
Highway Functional Classification
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Highway Functional Classification
Highways are classified according to their functions in terms of the service they provide. The classification system facilitates a systematic development of highways and the logical assignment of highway responsibilities among different jurisdictions. Highways and streets are categorized as rural or urban roads, depending on the area in which they are located. This initial classification is necessary because urban and rural areas have significantly different characteristics with respect to the type of land use and population density, which in turn influences travel patterns. Within the classification of urban and rural, highways are categorized into the following groups:
• Principal arterials
• Minor arterials
• Major collectors
• Minor collectors
• Local roads and streets
Freeways are not listed as a separate functional class since they are generally classified as part of the principal arterial system. However, they have unique geometric criteria that require special design consideration.
Functional System of Urban Roads
Urban roads comprise highway facilities within urban areas as designated by responsible state and local officials to include communities with a population of at least 5000 people. Some states use other values, for example, the Virginia Department of Transportation uses a population of 3500 to define an urban area. Urban areas are further subdivided into urbanized areas with populations of 50,000 or more and small urban areas with populations between 5000 and 50,000. Urban roads are functionally classified into principal arterials, minor arterials, collectors, and local roads.
A schematic of urban functional classification is illustrated in Figure 1 for a suburban environment.
Urban Principal Arterial System
This system of highways serves the major activity centers of the urban area and consists mainly of the highest-traffic-volume corridors.
It carries a high proportion of the total vehicle-miles of travel within the urban area including most trips with an origin or destination within the urban area. The system also serves trips that bypass the central business districts (CBDs) of urbanized areas.
Fig 1 : Schematic Illustration of the Functional Classes for a Suburban Road Network
All controlled-access facilities are within this system, although controlled access is not necessarily a condition for a highway to be classified as an urban principal arterial.
Highways within this system are further divided into three subclasses based mainly on the type of access to the facility: (1) interstate, with fully-controlled access and gradeseparated interchanges; (2) expressways, which have controlled access but may also include at-grade intersections; and (3) other principal arterials (with partial or no controlled access).
Urban Minor Arterial System
Streets and highways that interconnect with and augment the urban primary arterials are classified as urban minor arterials. This system serves trips of moderate length and places more emphasis on land access than the primary arterial system. All arterials not classified as primary are included in this class.
Although highways within this system may serve as local bus routes and may connect communities within the urban areas, they do not normally go through identifiable neighborhoods. The spacing of minor arterial streets in fully developed areas is usually not less than 1 mile, but the spacing can be 2 to 3 miles in suburban fringes.
Urban Collector Street System
The main purpose of streets within this system is to collect traffic from local streets in residential areas or in CBDs and convey it to the arterial system. Thus, collector streets usually go through residential areas and facilitate traffic circulation within residential, commercial, and industrial areas.
Urban Local Street System
This system consists of all other streets within the urban area that are not included in the three systems described earlier. The primary purposes of these streets are to provide access to abutting land and to the collector streets. Through traffic is discouraged on these streets.
Functional System of Rural Roads
Highway facilities outside urban areas comprise the rural road system. These highways are categorized as principal arterials, minor arterials, major collectors, minor collectors, and locals. Figure 2 is a schematic illustration of a functionally classified rural highway network.
Fig 2 : Schematic Illustration of a Functionally Classified Rural Highway Network
Rural Principal Arterial System
This system consists of a network of highways that serves most of the interstate trips and a substantial amount of intrastate trips. Virtually all highway trips between urbanized areas and a high percentage of trips between small urban areas with populations of 25,000 or more are made on this system.
The system is further divided into freeways (which are divided highways with fully controlled access and no at-grade intersections) and other principal arterials not classified as freeways.
Rural Minor Arterial System
This system of roads augments the principal arterial system in the formation of a network of roads that connects cities, large towns, and other traffic generators, such as large resorts. Travel speeds on these roads are relatively high with minimum interference to through movement.
Rural Collector System
Highways within this system carry traffic primarily within individual counties, and trip distances are usually shorter than those on the arterial roads. This system of roads is subdivided into major collector roads and minor collector roads.
Rural Major Collector System
Routes under this system carry traffic primarily to and from county seats and large cities that are not directly served by the arterial system. The system also carries the main intracounty traffic.
Rural Minor Collector System
This system consists of routes that collect traffic from local roads and convey it to other facilities. One important function of minor collector roads is that they provide linkage between rural hinterland and locally important traffic generators such as small communities.
Rural Local Road System
This system consists of all roads within the rural area not classified within the other systems. These roads serve trips of relatively short distances and connect adjacent lands with the collector roads.
Materials for pavement construction
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Materials for pavement construction
Soil
Every pavement, other than those on bridges, self-evidently includes soil. The most basic design requirement of any pavement is that the underlying soil is adequately protected from applied loads. Thus, no pavement engineer can avoid the need to understand soil. The following list features some key facts.
- Soils vary from heavy clays, through silts and sands to high-strength rocky
materials. - Soils are not usually consistent along the length of a road or across any pavement
site. - Soils are sensitive to water content to differing degrees.
- Water content will vary during the life of a pavement, sometimes over quite short
timescales, in response to weather patterns. - Some soils are highly permeable; some clays are virtually impermeable.
All this leads to one thing – uncertainty. However clever one tries to be in understanding and characterising soils, it is quite impossible to be 100% sure of the properties at a given time or in a given location.
This uncertainty makes life considerably harder. Nevertheless, it is necessary to categorise each soil type encountered in as realistic a way as possible, and there are two fundamental areas in which soil behaviour affects pavement performance. These are :
- stiffness under transient (i.e. moving wheel) load
- resistance to accumulation of deformation under repeated load, likely to be
related to shear strength.
Granular material
Granular material is unbound material with relatively large particle sizes, and includes natural gravel, crushed rock and granulated industrial by-products such as slag from steel production. Soils are technically granular materials, albeit often with a very small particle size (2 mm or less for clay), but the key difference is that a soil is not, in general, ‘engineered’ in any way.
A granular pavement layer, on the other hand, will be selected and quite possibly deliberately blended to give a particular combination of particle sizes. It can also be mixed with a predetermined amount of water. One would
therefore naturally expect that much of the uncertainty inherent in soil properties is removed in the case of a granular material.
However, it may still be difficult to predict performance accurately, as different material sources, most commonly different rock types, might be expected to exhibit slightly different properties due to their different
responses to crushing or their differing frictional properties.
Nevertheless, a granular material will be a much more controlled and predictable component than the soil.
Even the water-content variation of a granular material will be a little more predictable, in both magnitude and effect, than in the case of soil.
However, the properties of granular material of interest to the pavement engineer are actually more or less the same as those of soil, namely
- stiffness under transient load
- resistance to accumulation of deformation under repeated load, related to shear
strength.
Hydraulically-bound material
Nowadays, the availability of Portland cement, and substitutes such as fly ash or ground granulated blast-furnace slag, means that it can be economical to use such a binding agent to strengthen a granular material. These binders are known as ‘hydraulic’ binders, as they require the presence of water for the cementing action to take place.
Cement technology is a vast subject in its own right and involves several different chemical reactions, the most important of which are the conversion of tricalcium silicate (c. 50% of Portland cement) and dicalcium silicate (c. 25%) into hydrates (forming strong solids) by reaction with water, also generating calcium hydroxide and heat. The
first reaction is rapid; the second is slower. The reader should refer to specialist literature for details.
Hydraulically-bound materials, including so-called pavement-quality concrete (hereafter referred to as PQC) at the upper end of the strength spectrum, introduce a quite different type of behaviour and totally different design requirements. They possess a key property that is lacking in soils and granular materials, namely the ability to withstand tension.
Individual particles are rigidly bonded together by the binding agent, and a definite tensile force is required to break that bond. In the case of a strong concrete, all the large particles are well bonded into a continuous matrix of fine aggregate and cement paste, and the whole material is solid and rigid. It has a stiffness that is still partly
governed by the contacts between the large particles, but which is also heavily dependent on the qualities of the surrounding cementitious matrix.
In the case of a weaker hydraulically-bound material, the binding effect may be less complete and there may be many particle contacts that remain unbound, giving a certain freedom of movement
within the material and a reduced stiffness and strength.
Nevertheless, even a weak hydraulically-bound material will remain as a solid, with negligible permanent deformation
until the bonds are fractured, that is until the tensile strength is overcome. The key properties for the pavement engineer are, therefore
- stiffness
- tensile strength.
One further property that could arguably be added is fatigue resistance, that is, the resistance of the material to failure under repeated load at a stress level less than its failure strength. However, the relationship with tensile strength is so close that it is hardly a separate property.
It would also have been possible to add curing rate (the rate of strength gain), as this certainly affects the construction process and economics significantly, and thermal expansion coefficient, as this property strongly influences the tendency of a hydraulically-bound material to crack under day–night temperature variation, requiring the introduction of movement joints in concrete pavements
Bitumen-bound material
This is a material almost unique to pavement engineering, amaterial whose beneficial properties were discovered almost by accident, but a material which is now very much at the centre of pavement technology. There are countless stories as to when bituminous products were first used on roads, such as the accidental spillage of tar outside Derby iron works in 1901.
Although mastics including natural asphalt had been used on footways since the 1830s, they were not stable enough for roads, and it was not until around 1900–1901 that the first usages of tar-bound stone occurred at approximately similar dates in the USA and Europe. Lay (1990) gives further information.
While proportions differ around the world, typically some 90% of paved highways have a bitumen-bound surface layer; whatever the make-up beneath the surface, bitumen and bitumen-bound materials (referred to hereafter as asphalts) currently play a major role.
And asphalt is quite different from concrete or any hydraulically-bound material. Bitumen is a binding agent, like Portland cement and the other hydraulic binders, but it has very different properties. Whereas hydraulic binders create a rigid material that cannot deform appreciably unless it first cracks, bitumen remains a viscous liquid at
normal in-service temperatures. It therefore has the ability to ‘flow’.
An ability to flow may seem a rather undesirable quality in a material that is aiming to bind rock particles together, and it does indeed lead to the possibility that an asphalt can deform – hence the phenomenon known as ‘rutting’ or ‘tracking’. However, it also overcomes some of the difficulties encountered with rigid hydraulically-bound materials.
For a start, the expansion and contraction with day–night temperature variation is accommodated simply by a small viscous strain within an asphalt, meaning that no movement joints are required, and that thermally-induced cracking will only occur under the most extreme temperature conditions (continental winters, deserts).
Asphalts are also able to accommodate any moderate movement within the foundation, for example, minor differential settlement in an embankment, movement that might lead to the fracture of a rigid concrete slab. Furthermore, the tendency of asphalt to flow can be controlled by proper mixture design such that rutting is avoided.
However, despite the flexible nature of asphalt, it can still crack. It is impossible to define a tensile strength, as this will vary with temperature and rate of loading; the relevant parameter is the ‘fatigue characteristic’, defining resistance to cracking under repeated load. The key properties required for design are therefore:
- stiffness
- resistance to deformation under repeated load
- fatigue characteristic.
Other materials
The fourmaterial types introduced so far represent the basic building blocks available to the pavement engineer. However, it is worth referring here to a couple of materials that do not fit so easily into any of the four categories.The first is block paving. Blocks are often made of concrete and so could have been introduced under ‘hydraulically-bound materials’.
On the other hand, they can be cut fromnatural stone or may comprise fired clay bricks.Moreover, the discontinuous nature of block paving means that the properties of the parent material are less important than the effects of the discontinuities. The blocks themselves may have the properties of concrete, for example, a stiffness modulus of some 40 GPa, but the effective layer modulus once the discontinuities are taken into account may be as little as 500MPa.
The second special case is a hybrid material, known in the UK as grouted macadam and in the USA as resin-modified pavement; it is also sometimes known as ‘semi-flexible’ material. This too does not fit neatly into any of the previous categories, as it combines an asphalt skeleton with a cementitious grout, filling the voids in the asphalt mixture.
It therefore utilises both bituminous and hydraulic binders. Having a two-stage production process the material tends to be expensive, and is used in particular heavyduty applications such as bus lanes and industrial pavements. As will be demonstrated later, it actually resembles an asphalt much more than a concrete, but it is nevertheless
distinct.
Block paving and grouted macadam are bulk-use materials at the expensive end of the range.
There are also specialist products that are only used in small quantities to strengthen, or in some way improve, a pavement layer.Here one could include steel reinforcement of concrete.
There are also reinforcing products designed for asphalt; some are steel, others polymeric or made of glass fibre.Asimilar range of products is available for the reinforcement of granular materials. Generically, these products are known as geogrids and their use is widespread in some areas, for example, as a means of stabilising roads over very soft ground.
Geotextiles comprise a closely related range of products, produced in various ways and forming continuous layers separating two different pavement materials (commonly the soil and a granular layer). These too can have a reinforcing function, but their most common use is simply as a separator, ensuring that fine soil particles do not migrate up into the pavement and that stones from a granular layer do not lose themselves in the soil.
The entire spectrum of geogrids and geotextiles is known under the collective name of geosynthetics and, although geosynthetics are specialist products, it is the responsibility of the pavement engineer to understand how (and whether) they work in particular applications rather than relying solely on the, sometimes not unbiased, opinion of a supplier.
The long history of the paved highway
The long history of the paved highway
It is impossible to know where or when the wheel was invented. It is hard to imagine that Stone Age humans failed to notice that circular objects such as sections of tree trunk rolled.
The great megalithic tombs of the third millennium BC bear witness to ancient humans’ ability to move massive stones, and most commentators assume that tree trunks were used as rollers; not quite a wheel but a similar principle!
However, it is known for certain that the domestication of the horse in southern Russia or the Ukraine in about 4000 BC was followed not long afterwards by the development of the cart.
It is also known that the great cities of Egypt and Iraq had, by the late third millennium BC, reached a stage where pavements were needed. Stone slabs on a rubble base made an excellent and long-lasting pavement surface suitable for both pedestrian usage and also traffic from donkeys, camels, horses, carts and, by the late second millennium BC, chariots.
Numerous examples survive from Roman times of such slabbed pavements, often showing the wear of tens of thousands of iron-rimmed wheels. Traffic levels could be such that the pavement had a finite life.
Even in such ancient times, engineers had the option to use more than simply stones if they so chose – but only if they could justify the cost! Concrete technology made significant strides during the centuries of Roman rule and was an important element in the structural engineer’s thinking.
Similarly, bitumen had been used for thousands of years in Iraq as asphalt mortar in building construction. Yet neither concrete nor asphalt was used by pavement engineers in ancient times, for the excellent reason that neither material came into the cheap, high-volume category. As far as the pavement engineer was concerned, economics dictated that the industry had to remain firmly in the Stone Age.
Even in the days of Thomas Telford and John Loudon Macadam – the fathers of modern road building in the UK – the art of pavement construction consisted purely of optimising stone placement and the size fractions used.
Times havemoved on; themassive exploitation of oil has meant that bitumen, a by-product from refining heavy crude oil, is now much more widely available. Cement technology has progressed to the stage where it is sufficiently cheaply available to be considered in pavement construction. However, there is no way that pavement engineers can contemplate using some of the twenty-first century’s more expensive materials – or, at least, they can be used only in very small amounts. Steel can only be afforded as reinforcement in concrete and, even in suchmodest quantities, it represents a significant proportion of the overall cost.
Plastics find a use in certain types of reinforcement product; polymers can be used to enhance bitumen properties; but always the driving force is cost, which means that, whether we like it or not, Stone Age materials still predominate.
Satellite positioning (GPS), advantages and disadvantages for site engineers
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Satellite positioning (GPS), advantages and disadvantages for site engineers
A 1960s surveying text book consulted in 1980 would reveal little change in twenty years. That is not true today, with modern technology, systems and software are being continually updated. Nowhere is this more obvious than with satellite positioning.
It has exploded onto the construction market changing some operations beyond all recognition. As technology improves, accuracy increases and costs come down it becomes more economical to employ it on smaller and smaller jobs.
What is it?
Satellite positioning is the determination of the position of a point using a satellite receiver. Satellite positioning is generally known as GPS or global positioning system after the American military system, which was fi rst available for public use.
Unlike most surveying and setting out tasks, the skill required of the operator is minimal. The skill with GPS is with the management of the system’s input and out put data. The satellite receiver does all the work in gathering the data and outputting or storing it as required. With setting out it can provide the operator with predetermined setting out coordinates.
How accurate is it?
Accuracy depends on the methods employed and the equipment used. For construction setting out centimetre level accuracy is achievable. This makes it suitable for many setting out tasks. Unlike traditional survey methods, each point is independent of the points around it, and therefore each point is of a similar accuracy.
Degradation of accuracy (due to creep) with distance from the main station is no longer a prob lem. If used in unsuitable conditions, accuracy may be compromised.
An error in one point is not passed on to adjacent points.
What are the advantages?
When used for setting out, a single engineer with a setting out pole equipped with a satellite receiver can set out points almost as fast as he can mark them. With a road centre-line for example, the operator can walk the route and mark centreline points at whatever frequency is required. The setting out information can be taken straight from the design on disk without the need to input a mass of figures.
Work is unaf fected by weather or daylight or a lack of it. Visibility between points is not required, so local obstructions (shrubbery, mechanical plant, low buildings, walls etc.) do not hinder the process. Productivity increases are considerable. As well as giving plan coordinates (Eastings and Northings), it will automatically provide heights as a mat ter of course.
Satellite systems can also be integrated into computer-controlled plant, in which, for example, a grader has the road design in its memory. The grader blade is automatically adjusted to give the correct earthwork
profile. This eliminates the need for a complete setting-out team along with their instruments, forest of timber-work, chainmen and their transport.
What are the disadvantages?
Cost is always an issue, but this has to be balanced against productivity. GPS is not suited to all locations. Due to the fact the position of the receiver is derived from observing a number of satellites, a clear view of the sky is necessary. This may make GPS unsuitable for city centre sites shielded by adjacent tall buildings.
A received signal may give inaccurate results if defl ected off the side of a building. GPS is not suitable for tunnelling work. However GPS can be used very effi ciently to establish a control either side of an obstruction under which tunnelling is required.
GPS does not work well in tree-covered areas, again due to the need for a clear line of sight to the sky.
The height element of the output is of a lower order of accuracy than the plan coordinates. Additionally, heights given are not above mean sea level (as with traditional levelling), but above the mathematical model of the Earth, WGS84 (World Geodetic System 1984).
Unfortunately, for Europe this does not run parallel to mean sea level. However the GPS output can be confi gured to give correct information. GPS is not suffi ciently accurate enough to obtain the 1 mm precision that can be
achieved with a theodolite.
The principal types of foundations
Table of Contents
The principal types of foundations
Isolated Footings
Footings are understood formed by a rigid rectangular base of stone or concrete of dimensions : width B and length L, in which the ratio of L/B will not exceed 1.5.
The foundation structure will support the column load. The bearing capacity of the footing may be stimated, and its dimensions selected ; thereafter, a forcast of the settlement is made.
To illustrate the case pf footing foundations, consider a building with nine columns (Fig 1) supported on isolated footings. In this case, the footings will work independently of each other. Therefore, it is required that the differential settlements between footings will not exceed the allowable total and differential settlement requirements.
Figure 1: Isolated Footings
The differential settlements may be reduced selecting properly the area of the footings, and at times, using the stiffness of the superstructure.
From the structural point of view, however, the superstructure should not be allowed to take high secondary stresses induced bu the differential settlements of the footinhs, except in very special casees.
Singles footing foundations, in general, will be user only in soils of low compressibility and in structures where the differential settlements between columns may be controlled by the superstructure flexibility, or including in the design og the building joints or hinges that will take the differential settlements and/or rotations, respectively, without damaging the construction.
Continuous Footings
When it is necessary to control within certain limits the magnitude of differential settlements between columns supported on footings, and when soil deposits of medium or low compressibility atre encountered, it is recommended to use continuous footings. They may be defined as resisting elements joining columns together by foundation beams.
Continious footings are arraged by joining two or more columns together with beams.
The vertical differential displacements may be controlled via beam stiffness (Fig 2). The selection of the foundation beams, either running in one direction or the other along columns rows, depends largely on the layout of the column loads, and othe functional requirements concerning the structural and architectural design of the project.
Figure 2: Continuous Footings
For heavier loads, and when the project calls foor stiffness in both directions (namely, along column rows A, B and C also rows 1,2 and 3), the foundations is given stiffness with beams in both directions (Fig 3).
Figure 3: Continuous Footings
In this case, it may be observed that the footing slabs will cover practically all the foundation. This type of foundation using continuous footings is advantageous in soils of medium compressibility, where it is necessary to control differential movements between colomns.
The foundation beams are designed with the necessary stiffness to fulfill the differential settlements requirements.
Raft Foundation
When the loads are so largee that continious footings will occupy close to 50% of projected area of the building, it is more economical to use a continuous mat covering the entire area, as shown in Fig 4. The total load in this case may be assumed uniformly distributed in the area covered by the building.
The soil reaction is determined on the basis of a safe bearing capacity. The total and differential settlements may be investigated considering the stiffness of tthe raft or foundation slab is a matter of economy, compatible with the allowable differential settlements.
Figure .4 : Mat Foundations
Flexibility is important to obtain economy ; however, restrictions in differential vertical displacements between colmuns may call for certain slab stiffness, either by making it thicker or by placing foundation beams joining columns rows.
The beams can be designed with the required stiffness to reduce differential displacements.
This type of foundation may be used generally in soil deposits of medium compressibility ; however, in certain instances, the surface raft foundation may be used in soils of high and very high compressibility, where large total settlements may be allowed.
This type of foundation may be used efficiently in reduucing differential settlement.
Compensated Foundations
In soil deposits of medium, high and very high compressibility and low bearing capacity, compensated foundations are indicated.
This type of foundation requires a monolithic box foundation, as shown in Fig 5. When the water table is close to the ground surface, water proofing is necessary to use the buoyancy effect in designing the foundation.
Figure .5 : Compensated Foundation
In the design of compensated foundations, it should be borne in mind that the soil should be consedered as a material of two phases, namely : a solid and liquid phase.
Therefore, in a compensated foundation, the compensation is made by adding two effects :
- Substitution of the submerged weight of solids.
- The buoyancy effect by weight of liquid displaced.
Both effects are used to equalize the total weight of the building. The volume of the concrete box forming the foundation strructure annd basements will displace a weight of liquid that, according to Archimedes’ principal, will contribute in floating the foundation up to this value, reducing the load applied to the solid phase.
The load taken by the solid phase will, however, deform the soil because of the change in effective stresses induced in the soil structure. It shoul be investigated from the point of view of bearing capacity of the soil and total and differentioal settlements, as previously discussed for other foundations.
Compensated Foundations with Friction Piles
When a compensated foundation as described is not sufficient to support the load with the allowable total settlement, in spite of desigin the foundation with sufficient stiffness to avoid detrimental differential settlements within the foundation itself, friction piles may be used in addition to the concept of compensation.
This case may be present in deposits of high or very high compressibility extending to great depth. The piles will reinforce the upper part of the soil where a higher comopressibility is encountered.
The applicability of this foundation calls for a soil that varies from very high compressibility at the upper part of the deposit, to medium or low compressibility at the bottom (Fig 6).
Figure 6: Compensated friction pile Foundation
Point Bearing Pile Foundations
When the loads to be supported are higher than those a compensated friction pile foindation can take, the nit will be required to find a deep-seated hard stratum of low to very low compressibility and high shear strenth, where piles can be driven to point bearing. One can distinguish two cases of point bearing foundations (Fig 7 and Fig 8).
The first case is recognized when the hard stratum of convenient thickness is found underlain by materials of medium compressibility. In these cases the piles should be evenly distributed as shown in Fig 7. After solving the problem of point bearing of the piles in the hard stratum, there still existas the problem of finding if the lower compressible soil stratum will have a safe bearing value, and also if the total and differential settlements will be within the allowable values specified for the foundation in question.
This type of foundation should be designed with sufficient stiffness to control differential settlements.
The second type of pile fooundation is recognized wwhen the point bearing piles rest in a firm deposit of low compressibility extending to great despth (Fig 8). In this case, it is economical to use groups of piles to solve the foundation problem.
Figure 7 : Point Bearing Piles
Figure 8 : Point Bearing Piles in groups
The columns will rest on single footings supported on the piles. The piles driven in the firm stratum develop laterad friction contributing to the mechanical properties on shear strengh of the depostigs in whttich the are driven, on the spacing of the piles, on the length of penetration into the bearing stratum, and on the state of density and confinement of such stratum.
Pier Foundations
Pier foundations are used to support very heavy loads in buried soil deposits of very low compressibility (Fir 9). Their load capacity is a function of the mechanical properties of the soil under the base of the pier, and of the confining stress of the bearing stratum. Actuelly, the bearing capacity of such an element is determined as a deep-seated isolated footing.
Figure 9 : Pier Foundation
The piers, columns-like elements cast in place, in most cases carry high loads of 500 ton or moore ; therefore, the compressibility of the deposit on which the are resting should be very low, in order that they may be recommended.
Pier shafts may be used from dimensiosn are also a function of the procedure used to perform the excavation, and of the way the hydraulic conditions are handled. The density of the material wherre these elements aare bearing may be altered during excavations if an upward water flow is produced. Specially important is the case when the material is a cohesionless fine sediment or when the cohesion is small, in which case it is necessary to perform the excavation using a pneumatic system, introducing air under sufficient high pressure to balance the flow of water toward the bottom of the excavation, preserving the natural confining and density connditions of the bearing stratum.
Usually, if precautions are taken in the installation of these elements, the settlements will be very small. The settlement, however, may be estimated knowing the stress-strain characterictics of the strata encouontered under the base of the piers. The nagative friction on these elements may take large proportions : hence, it shoul be estimated.
When these rigid elements are used in seismic regions to support loads through deposits of high and very high compressibility, it is necessary to investigate the effect ot the horizontal motion of the soil mass during earthquakes. The horizontal drift forces against the piears because of soil displacement should not be overlooked. In occasions, rigid elements have been damaged because of the strong horizontal motions produced bu the earthquakes.
Sand Pier Foundations
The solution of foundations using sand pier or sand piles is shown in Fig 10.
Figure 10 : Sand Piers
This type of foundation is used to increase the load capacity of the soil by reducing its compressibility and increasing its shear strength capacity properties. This type of pile may be used in loose or medium dense sand deposits.
The improvement of the subsoil is a function of the volume of sand introduced at the time thses elements are installed. Usually first a hole is driven in the ground, then sand is introduced and highly compacted in layers, using a heavy ram.
The sand element will take the load because of the lateral confinement given by the subsoil. The deformation of these elements may be estimated by means of the stress-strain properties of the sand ussed, considering the pier as a long sand cylinder laterally confined bu the soil. This type of foundation is only recommended in places where the cost of cement is very high, and good aggregates to fabricate concrete are difficult to obtain.
Reinforced Rectangular Concentric Concrete Footing Design Spreadsheet
Reinforced Rectangular Concentric Concrete Footing Design Spreadsheet
General Description: Calculates the reinforcement required for a reinforced rectangular or square concrete footing with a rectangular or square columns at the centre of the footing, for flexure and checks 1 way and 2 way shear at the concrete column, using ductility class N reinforcement.
Limitations: For rectangular or square pads only, with no applied moment or horizontal forces. Does not design for shear reinforcement.
Codes/Theoretical Basis:
AS3600 – 2009 (Incorporating Amendment 1, 2010 )
Warner, Rangan, Hall & Faulkes, Concrete Structures, Longman, Melbourne, 1999
Foster, Kilpatrick and Warner, Reinforced Concrete Basics 2E, Pearson, 2010
Nomenclature: Symbols and notation as generally used in AS3600.
Input:
Yellow cells require data input by the designer
Geometry for the pad footing , etc
Concrete strength etc
Geometry of column etc
Applied actions and allowable bearing capacity
Area of tension reinforcement in both directions based on initial calculations and minimum reinforcement
Iterate if does not meet minimum design values
Where pink fill is used it alerts designer to options or information
Output:
Boxed cells with green background calculated automatically using formulae.
Footing weight, working load, total ultimate load, load factor and actual bearing pressure under the footing
Ultimate moment in each direction and initial area of reinforcing along with minimum reinforcement
Number, size and spacing of reinforcing bars
Maximum bending capacity in both directions Muo
Moment capacity fMu in both directions for the chosen reinforcement
Checks for minimum reinforcement
Checks one way and two way shear
Provides summary of the results
Feedback: For comments, corrections, suggestions or other feedback regarding this spreadsheet, please contact the CCAA
Calculation Reference
AS3600