Steel bridges, as well as other steel structures, are built of steel memberssuch as beams, columns, and truss members by connections or joints. Theuse of connections can affect the fabrication method, serviceability, safety,and the cost, thus they are particularly important in the steel bridge construction.
In general, the connection design should follow the principle thatshould be safe, reliable, simply in design and fabrication, easy installation, and should be able to save the materials and costs.
In steel bridges, the often used connecting methods include rivet connection, bolt connections, and welding connections, as shown in Fig.1.
Bolt connection is used earliest since the mid-18th century and stillis being used as one of the most important connections.
The rivet connection has been used since the early 19th century; there after the welding connection was also created and used in the end of 19th century.
The welding joint became very popular and gradually replaced the rivet connection in thesteel bridge construction. With the development of high-strength bolted connection at the mid-20th century, they are also widely used in the steel bridge construction.
Bolted connection is more frequently used than other connection methods.They are very easy to operate and no special equipment is required. This is in particular due to the development of higher strength bolts, the easy to use and strong structural steel connections become possible.
In the bolt design, two kinds of forces including tension and shear forces should be considered.Bolted connection can be divided into ordinary bolted connection or high-strength bolted connection. Both of them are easy in installation, particularlysuitable for connection in the construction site.
Ordinary bolts are easy todisassemble and are generally used in temporary connections or those needto be disassembled. High-strength bolts are easy to disassemble, and theyhave higher strength and stiffness. However, the bolted connections alsohave some disadvantages because it is necessary to drill holes and adjustthe holes during the installation.
The cutting of the holes may weakenthe steel members and increase the use steel materials due to the memberoverlapping, and also this will increase the workload in the construction.There are many reasons that may result in the failure of the bolted connec-tions, such as overloading, over torquing, or damage due to corrosion.
2. Rivet Connection
From the mechanical behavior and design points of view, the rivet connectionis very similar to ordinary bolt connection. A rivet is a permanent mechanicalfastener, which was very popular for the early steel bridges due to their good performance in plasticity, toughness, integrity under statistic load, and fatigue performance under dynamic load.
Also, quality inspection of welded connection is also relatively easy than other connection methods.However, the rivet connection is rarely used in nowadays due to disad-vantages like complex in structure, high consumption of steel, high noiseduring the construction, etc., and gradually replaced by the bolted connec-tion and welded connection.
3. Welded Connection
Welding is another connecting method used to connect steel components inthe fabrication factory and on bridge construction site. Common types ofwelds are butt welds, fillet welds, and plug welds, as shown in Fig.2.
The work place (in a factory or on site) is an important criterion fordeciding whether to choose a bolted or a welded connection. If the connec-tion is performed in a factory, it is generally most economically achievedthrough welding. Although it is technically possible for site welding, theadditional cost for setting up welding and testing facilities as well as theincreased erection time usually makes bolted connections become moreefficient.
Properly applied shotcrete is a structurally sound and durable construction material which exhibits excellent bonding characteristics to existing concrete, rock, steel, and many other materials. It can have high strength, low
absorption, good resistance to weathering, and resistance to some forms of chemical attack. Many of the physical
properties of sound shotcrete are comparable or superior to those of conventional concrete or mortar having the
same composition. Improperly applied shotcrete may create conditions much worse than the untreated
condition.
2. Advantages of shotcrete
Shotcrete is used in lieu of conventional concrete, in most instances, for reasons of cost or convenience. Shotcrete is advantageous in situations when formwork is cost prohibitive or impractical and where forms can be reduced or eliminated, access to the work area is difficult, thin layers or variable thicknesses are required, or normal casting techniques cannot be employed. Additional savings are possible because shotcrete requires only a small, portable plant for manufacture and placement.
Shotcreting operations can often be accomplished in areas of limited access to make repairs to structures.
3. Strength of bonding.
The excellent bonding of shotcrete to other materials is often an important design consideration. The force of the impact of this pneumatically propelled material on the surface causes compaction of the shotcrete paste matrix into the fine surface irregularities and results in good adhesion to the surface. Within limits, the material is capable of supporting itself in vertical or overhead applications.
Applications
The selection of shotcrete for a particular application should be based on knowledge, experience, and a careful
study of required and achievable material performance.
The success of the shotcrete for that application is contingent upon proper planning and supervision, plus the skill and continuous attention provided by the shotcrete applicator.
The following paragraphs discuss the use of shotcrete in several of the more common applications.
1. Repair
Shotcrete can be used to repair the damaged surface of concrete, wood, or steel structures provided there is access to the surface needing repair.
The following examples indicate a few ways in which shotcrete can be used in repairs:
Bridges:
Shotcrete repair can be used for bridge deck rehabilitation, but it has generally been uneconomical for major full-thickness repairs. It is very useful, however, for beam repairs of variable depths, caps, columns, abutments, wingwalls, and underdecks from the standpoint of technique and cost.
Building:
In building repairs, shotcrete is commonly used for repair of fire and earthquake damage and deterioration, strengthening walls, and encasing structural steel for fireproofing. The repair of structural members such as beams, columns, and connections is common for structures damaged by an earthquake.
Marine Structures:
Damage to marine structures can result from deterioration of the concrete and of the reinforcement. Damaging conditions are corrosion of the steel, freezing and thawing action, impact loading, structural distress, physical abrasion from the action of waves, sand, gravel, and floating ice, and chemical attack due to sulfates. These problems can occur in most,marine structures such as bridge decks, piles, pile caps, beams, piers, navigation locks, guide walls, dams, powerhouses, and discharge tunnels. In many cases, shotcrete can be used to repair the deteriorated surfaces of these structures.
Spillway surfaces:
Surfaces subject to highvelocity flows may be damaged by cavitation erosion or abrasion erosion. Shotcrete repairs are advantageous because of the relatively short outage necessary to complete the repairs.
2. Underground excavations.
For the most part, shotcrete is used in underground excavations in rock; but on occasion, it has been successfully used in the advancement of tunnels through altered, cohesionless, and loose soils. Typical underground shotcrete applications range from supplementing or replacing conventional support materials such as lagging and steel sets, sealing rock surfaces, channeling water flows, and installing temporary support and permanent linings.
3. Slope and surface protection.
Shotcrete is often used for temporary protection of exposed rock surfaces that will deteriorate when exposed to air. Shotcrete is also used to permanently cover slopes or cuts that may erode in time or otherwise deteriorate. Slope protection should be properly drained to prevent damage from excessive uplift pressure.
Application of shotcrete to the surface of landfills and other waste areas is beneficial to prevent surface water infiltration.
4. New structures.
Shotcrete is not necessarily the fastest method of placing concrete on all jobs, but where thin sections and large areas are involved, shotcreting can be used effectively to save time. The following paragraphs describe some of the applications involved with construction of new structures.
Pools and tanks. Shotcrete has been used extensively to construct concrete swimming pools. More recently, large aquariums have been constructed using shotcrete.
Shotcrete floors and walls. Shotcrete floors in tanks and pools on well compacted subbase or on undisturbed earth have generally given excellent service. Vertical and overhead construction for walls, slabs, columns, and other structural members has been frequently shotcreted.
Shotcrete domes. Construction techniques using inflatable air-forming systems have made the construction of shotcrete shells or domes practical. These large structures have been used for residential housing, warehousing, bridge, and culvert applications.
Stone has been used for many years to protect embankments, levees, river banks, and engineered features against erosion and in the construction of dams, breakwaters, and other large structures. Advantages over other materials and designs are often contingent on low cost of large-scale production and processing of stone and placement on the structure.
1. Slope Protection
a. Slope protection generally means the engineered feature composed of large-stone material constructed as a relatively thin overlay on a slope otherwise vulnerable to erosion. A bedding layer is usually included. The large-stone material is commonly called riprap. At the heart of some riprap design is the characteristic of physical flexibility. Riprap adjusts to minor flank erosion or undercutting and continues to function in its protective role.
b. One key consideration in slope protection is stone size, and the specifications for riprap should be detailed in regard to median size, gradation, and allowable tolerances. The cost advantage may be even greater where gradation requirements allow quarry-run material to be used. Even here, the importance of well defined specifications must be made clear since there are still limitations on oversize or undersize components that may require at least some separation processing.
At the other extreme from quarry-run material is riprap of narrow size range for manual placement in a keyed or fitted-stone arrangement. This labor-intensive and costly method of protecting slopes is practiced only rarely today but may be encountered in maintenance of features constructed years ago.
2. Training Structures
River training structures are relatively short, linear features constructed near the bank of a channel to control the pattern and velocity of flow. Examples are spur dikes and groins perpendicular to the bank and vane dikes more or less parallel. Rock training structures may be unzoned or earth-cored but are always designed for low cost, sometimes at the sacrifice of longevity. Groins are also used along coastlines to control sea drift currents and sand deposition. Coastal jetties train the flow of river outlets and may secondarily function as groins and breakwaters.
3. Retention Dikes
Retention dikes are designed to impound saturated materials such as from dredging. Elaborate, zoned designs are sometimes used where the waste is contaminated and the stability and the control of leachates are of high priority. Where wave attack is predictable, two or more zones of large stone are commonly incorporated, with the most critical usually being the outer armor.
As designs prescribe larger and larger stones, the problems of quality and cost increase dramatically. Smaller sized stone materials are usually more than adequate to remain stable and support the superimposed layers. Emphasis may then be redirected from high-quality stone to quantity and the need to provide large volumes of core stone.
4. Breakwaters and Jetties
a. Large breakwaters and jetties provide the outstanding examples of construction with large stone. The term rubble mound, though somewhat entrenched in the technical literature, seems inappropriate to describe these engineered structures which are commonly designed with several massive zones of different materials.
The special demands for protection against ocean wave attack lead to the ultimate among numerous possible requirements, that is very large, dense, and durable individual stones. At some large size, different for each project, the cost of constructing armor with stones exceeds that of construction with man-made concrete units such as tetrapods and dolosse. For projects near this cutoff, an especially thorough investigation of stone sources is needed. Quarries at great distance are often included, and the cost of transportation and handling must be estimated carefully.
b. Structures in deep water present special construction problems. Placement is partly remote and obscure so that quality control, quality assurance, and measurement for payment are comparably more difficult than in construction above water.
5. Zoned Embankments
a. Construction of zoned rockfill dams has constituted the largest use of large stone in some CE districts. The demands regarding material quality are usually different than for rubble mounds or even for retention dikes, and
weak rock can be used. Instead, the focus is on the immense volumes required.
b. Much of the stone material can come from excavations required as part of the project, particularly, excavations for spillways and outlet works. Quarries or stone borrow areas may be needed at least as supplementary sources. An accurate estimate of processing requirements is essential to assure sufficient volumes where suitable deposits are thin or otherwise in short supply or the material quality is marginal.
The attendant blasting, handling, and placement sometimes degrade some of the material and can cause shortfall in acceptable stone.
6. Other Uses
Other uses of large-stone material are subordinate as a group to those identified above. Nevertheless, special uses can be significant.
a. Energy Dissipators. Energy dissipators may be constructed from derrick stone placed in plunge pools to reduce erosion of foundation soil or bedrock. The high-energy environment may preclude long service life without
additions of fresh stone.
b. Structure Protection. Stone placed around the upstream end of a bridge pier is an example of protection of structures. Stone placed to stabilize and protect dikes or breakwaters primarily composed of concretefilled caissons is another example.
c. Masonry. Masonry construction is included in this manual for completeness. The high cost of this labor-intensive method largely disfavors its use today, but stone masonry still exists in CE structures and facilities and occasionally can be repaired with essentially new masonry construction.
Dimensional and cut stone for masonry are produced in a relatively few, specialized quarries.
d. Landscaping Stone. Landscaping stone is not part of an engineered structure or feature as such, but it essentially completes some CE projects. Guidance in selecting stone and in estimating or inspecting stone work for landscaping is potentially useful on a broad scale.
Project functions and their overall social, environmental, and economic effects may influence the hydraulic design of the spillway. Optimization of the hydraulic design and operation requires an awareness by the designer of the reliability, accuracy, sensitivity, and possible variances of the data used.
The ever-increasing importance of environmental considerations requires that the designer maintain close liaison with other disciplines to assure environmental and other objectives are satisfied in the design.
Spillway Function
The basic purpose of the spillway is to provide a means of controlling the flow and providing conveyance from reservoir to tailwater for all flood discharges up to the spillway design flood (SDF). The spillway can be used to provide flood-control regulation for floods either in combination with flood-control sluices or outlet works, or in some cases, as
the only flood-control facility.
A powerhouse should not be considered as a reliable discharge facility when considering the safe conveyance of the spillway design flood past the dam. A terminal structure to provide energy dissipation is usually provided at the downstream end of the spillway. The degree of energy dissipation provided is dependent upon the anticipated use of the spillway and the extent of damage that will occur if the terminal structure capacity is exceeded.
The standard project flood is a minimum value used for terminal structure design discharge. The designer must keep in mind that damage to the dam structure that compromises the structural integrity of the dam is not acceptable. Acceptance of other damages should be based on an economic evaluation of the extent of damage considering the extremely infrequent flood causing the damage.
Spillway Classification
Spillways are classified into four separate categories, each of which will serve satisfactorily for specific site conditions when designed for the anticipated function and discharge.
1. Overflow Spillway
This type of spillway is normally used in conjunction with a concrete gravity dam. The overflow spillway is either gated
or ungated and is an integral part of the concrete dam structure.
Figure 1 : Chief Joseph Dam overflow spillway
2. Chute Spillway
This type of spillway is usually used in conjunction with an earth- or rock-filled dam; however, concrete gravity dams also employ chute spillways. In these cases the dam is usually located in a narrow canyon with insufficient room for an overflow spillway. The chute spillway is generally located through the abutment adjacent to the dam; however, it couldbe located in a saddle away from the dam structure.
Figure 2 : Mud Mountain Dam
Figure 3 : Wynoochee Dam
3. Side Channel Spillway
This type of spillway is used in circumstances similar to those of the chute spillway. Due to its unique shape, a
side channel spillway can be sited on a narrow dam abutment. Side channel spillways generally are ungated; however, there is no reason that gates cannot be employed.
Figure 4 : Townshend Dam side channel spillway
4. Limited Service Spillway
The limited service spillway is designed with the knowledge that spillway operation will be extremely infrequent, and when operation occurs, damage may well result. Damage cannot be to the extent that it would cause a catastrophic release of reservoir water.
Gravity dams are solid concrete structures that maintain their stability against design loads from the geometric shape and the mass and strength of the concrete. Generally, they are constructed on a straight axis,
but may be slightly curved or angled to accommodate the specific site conditions.
Gravity dams typically consist of a nonoverflow section(s) and an overflow section or spillway. The two general concrete construction methods for concrete gravity dams are conventional placed mass concrete and RCC.
1. Conventional concrete dams
Conventionally placed mass concrete dams are characterized by construction using materials and techniques employed in the proportioning, mixing, placing, curing, and temperature control of mass concrete (American Concrete Institute (ACI) 207.1 R-87). Typical overflow and nonoverflow sections are shown on Figures 1 and 2.
Figure 1 : Typical dam overflow section
Figure 2 : Nonoverflow section
Construction incorporates methods that have been developed and perfected over many years of designing and building mass concrete dams. The cement hydration process of conventional concrete limits the size and rate of concrete placement and necessitates building in monoliths to meet crack control requirements.
Generally using large-size coarse aggregates, mix proportions are selected to produce a low-slump concrete that gives economy, maintains good workability during placement, develops minimum temperature rise during hydration, and produces important properties such as strength, impermeability, and durability. Dam construction with conventional concrete readily facilitates installation of conduits, penstocks, galleries, etc., within the structure.
Construction procedures include batching and mixing, and transportation, placement, vibration, cooling, curing, and preparation of horizontal construction joints between lifts.
The large volume of concrete in a gravity dam normally justifies an onsite batch plant, and requires an aggregate source of adequate quality and quantity, located at or within an economical distance of the project.
Transportation from the batch plant to the dam is generally performed in buckets ranging in size from 4 to 12 cubic yards carried by truck, rail, cranes, cableways, or a combination of these methods. The maximum bucket size is usually restricted by the capability of effectively spreading and vibrating the concrete pile after it is dumped from the bucket. The concrete is placed in lifts of 5- to 10-foot depths. Each lift consists of successive layers not exceeding 18 to 20 inches. Vibration is generally performed by large one-man, air-driven, spud-type vibrators.
Methods of cleaning horizontal construction joints to remove the weak laitance film on the surface during curing include green cutting, wet sand-blasting, and high-pressure air-water jet. Additional details of conventional concrete placements are covered in EM 1110-2-2000.
The heat generated as cement hydrates requires careful temperature control during placement of mass concrete and for several days after placement. Uncontrolled heat generation could result in excessive tensile stresses due to extreme gradients within the mass concrete or due to temperature reductions as the concrete approaches its annual temperature cycle.
Control measures involve precooling and postcooling techniques to limit the peak temperatures and control the temperature drop. Reduction in the cement content and cement replacement with pozzolans have reduced the temperature-rise potential. Crack control is achieved by constructing the conventional concrete gravity dam in a series of individually stable monoliths separated by transverse contraction joints.
2. Roller-compacted concrete (RCC) gravity dams
The design of RCC gravity dams is similar to conventional concrete structures. The differences lie in the construction methods, concrete mix design, and details of the appurtenant structures. Construction of an RCC dam is a relatively new and economical concept.
Economic advantages are achieved with rapid placement using construction techniques that are similar to those employed for embankment dams. RCC is a relatively dry, lean, zero slump concrete material containing coarse and fine aggregate that is consolidated by external vibration using vibratory rollers, dozer, and other heavy equipment.
In the hardened condition, RCC has similar properties to conventional concrete. For effective consolidation, RCC must be dry enough to support the weight of the construction equipment, but have a consistency wet enough to permit
adequate distribution of the past binder throughout the mass during the mixing and vibration process and, thus,
achieve the necessary compaction of the RCC and prevention of undesirable segregation and voids.
Compass surveying is the branch of surveying in which directions of survey lines are determined with compass and the lengths of the lines are measured with a tape or a chain. In compass surveying the direction of survey line called the bearing of line is defined as the angle made by the line with the magnetic meridian. In practice compass is generally employed to run a traverse. Traverse consists of series of straight lines connected together to form an open or closed traverse.
COMPASS
The commonly used instrument for compass surveying is Compass. A compass is small instrument which consists essentially of magnetic needle, a graduated circle and a line of sight. When the line of sight is directed towards a line, the magnetic needle points towards magnetic meridian and the angle which the line makes with the magnetic meridian is read at the graduated circle.
Compass consists of cylindrical metal box of about 8-12 cm diameter in the center of which is a pivot carrying a magnetic needle which is already attached to the graduated aluminum ring. The ring is graduated and is read by reflecting prism. Diametrically opposite to the prism is the object vane hinged to the box side carrying a horse hair with which the object in the field is bisected.
TYPES OF COMPASS
There are two forms of compass as under
Prismatic compass
Surveyors compass
The two compass are almost same except few differences so far as their construction is concerned. Prismatic compass uses WCB (0⁰-360⁰) circular ring while as the surveyors compass uses Quadrantal Bearing (0⁰-90⁰) circular ring system. Besides bearing system, the former has graduated ring attached to magnetic needle as the result of which when compass box and sight vane is rotated the needle remains stationary on the other hand in surveyors compass graduated ring being attached to compass box moves with it as the box is rotated
ADJUSTMENT OF COMPASS (WORKING)
Working of compass involves three steps:
CENTERING
LEVELLING
OBSERVING THE BEARING
Centering involves to align the compass is such a way the Centre is placed vertically over the station point. It is done with the help of tripod stand and is checked by dropping a small pebble below the Centre of compass.
Levelling is done so that the graduated ring swings quite freely. It is done with the help of ball and socket arrangement and can be checked by rolling a round-pencil on the compass box
Observing the bearing once centering and levelling has been done, raise or lower the prism until the graduations on the ring are clearly visible when looked through the prism. Afterwards turn compass-box until the ranging rod at the station is bisected by horse-hair of objective vane. At this position note down the reading.
COMPASS TRAVERSING
Whenever in traversing compass is used for making angular measurements, it is known as compass traversing or compass surveying. In compass traversing, the compass is used to determine the direction of survey lines of the framework of the traverse for measuring the angles which these lines make with the magnetic meridian.
The process of chaining and offsetting is the same as in chain surveying and running the check lines is not necessary. Compass traverse may be closed or open. Close traverse starts from one traverse station and closes either on same or on another traverse station whose location is already known. On the other hand an open traverse starts from one station and closes at other station whose location is neither known nor established.
EQUIPMENT USED IN COMPASS SURVEYING
PRISMATIC COMPASS
MEASURING TAPE
RANGING RODS
PLUMB BOB
CHAIN
CROSS STAFF
MEASUREMENT OF INCLUDED ANGLE
In a compass (prismatic) which uses Whole Circle Bearing (0⁰ to 360⁰) system the included angle is given by
Included angle = F.B of next line — B.B of previous line at same station
PRECAUTIONS
Compass surveying is used in case of rough surveys where speed and not accuracy is main consideration. One of the biggest disadvantages of compass is that magnetic needle which gives bearing of lines is disturbed from its normal position in presence of materials such as iron-pipes , current carrying wires , proximity of steel structures , transmission lines etc. called sources of local attraction.
The value of LiDAR lies in the fact that it virtually places you on your target site without having to leave the office. Combining imagery with LiDAR point cloud data is the next best thing to being there. Plus, it’s fast, accurate, and affordable.
2. Can I Collect Other Information While I’m Gathering LiDAR Data?
You name it — cameras, video imaging systems, multi-spectral and hyper-spectral imaging systems can all be mounted on and operated with most current LiDAR systems. Because you can operate these multiple systems using the same components, you’ll save time and money, a big benefit to corridor mapping projects such as transportation, pipeline, and transmission mapping.
Mobile mapping systems typically include two or more cameras within the system. One drawback: These passive systems require a light source, which means your collection times are lim-ited. Unlike LiDAR, they can’t see in the dark.
3. Do I Need Breaklines If I’m Using LiDAR?
Not necessarily. It all depends on the requirements of the product you’re generating. Typically, LiDAR is very good at defining the surface, as long as the sample spacing is ade-quate and there isn’t too much vegetation. Most features and terrain are very well defined in LiDAR data.
The rule of thumb relative to breakline usage comes down to edge recognition needs in the surface. If you have key elements, such as a back of curb line or a lip of gutter line, you will want to collect breaklines. If you’re only producing large-scale contour maps, you may not need the accuracy that breaklines provide.
4. Where Can I Find an End-to-End Solution for LiDAR Data?
Try Autodesk. Specifically, you’ll find an end-to-end solution in AutoCAD 2011, AutoCAD Labs, AutoCAD Civil 3D, Map 3D, and Navisworks products. Another robust data extraction solution for feeding all these applications with classified LAS and featurized GIS is the Virtual Geomatics solution.
5. Classical Photogrammetric Data Collection Works for Me — How Does LiDAR Compare?
LiDAR is about 40 percent less expensive than classical pho-togrammetric collection. And it takes less time to collect, process, and extract the needed information from LiDAR com-pared with traditional methods.
6. Is LiDAR Data Accurate Enough to Use on Road Overlay Projects?
You bet, as long as you use the appropriate collection method with the sufficient survey control. Just be careful when you pick your method and plan the control.
7. What Is Corn-Rowing?
You can’t eat it. The term corn-rowing refers to an artifact of LiDAR sampling that typically occurs at the edges of scans and overlapping data areas. It’s caused when LiDAR points are sampled close together and the difference in the sampled points is greater than the relative accuracy. With proper col-lection and filter processes that remove the points causing the trouble, you can minimize corn-rowing.
8. What Type of LiDAR Data Do I Really Need?
The best way to determine what LiDAR products you need is to really understand the application for the data. Call a qualified LiDAR collection agency for a recommendation on the appropriate collection methods based on your specific requirements. Here’s the info you’ll need to pass along:
✓ Accuracy requirements for data.
✓ Extraction requirements — do you just need points, or linear features, too?
✓ End products you’ll require. Do you need a triangulated surface, a classified LAS, and/or GIS features?
9. What Is an Intensity Image?
An intensity image is a monochromatic (shades of gray) image of the illumination (energy) returns from the LiDAR system. These images can be used for generating planemetric and breaklines by using LiDARgrammetry. The intensity image is typically a Geotiff, and the accuracy of the image is a function of the horizontal accuracy of the LiDAR, along with the inter-polation of the point to a raster image.
10. You Just Said LiDARgrammetry—What’s That?
LiDARgrammetry is the process of using intensity images to generate synthetic stereo pairs, much like the stereo pairs used in photogrammetry. The data generated from LiDARgrammetry tends to be only as accurate as the LiDAR from which it’s generated. There are varying opinions regard-ing the usefulness of this information and how accurate it is. Still, it’s a good byproduct of LiDAR, and whether it’s useful to you just depends on the scope of your project.
If you want to feel a lot more fluent in the language of LiDAR, you must know theses terms:
Repetition rate: This is the rate at which the laser is pulsing, and it’ll be measured in kilohertz (KHz). Fortunately, you don’t have to count it yourself, because these are extremely quick pulses. If a vendor sells you a sensor operating at 200 KHz, this means the LiDAR will pulse at 200,000 times per second. Not only does the laser transceiver put out 200,000 pulses, the receiver is speedy enough to receive information from these 200,000 pulses.
Scan frequency: While the laser is pulsing, the scanner is oscillating, or moving back and forth. The scan frequency tells you how fast the scanner is oscillating. A mobile system has a scanner that rotates continuously in a 360 degree fashion, but most airborne scanners move back and forth.
Scan angle: This is measured in degrees and is the dis-tance that the scanner moves from one end to the other. You’ll adjust the angle depending on the application and the accuracy of the desired data product.
Flying attitude: It’s no surprise that the farther the plat-form is from the target, the lower the accuracy of the data and the less dense the points will be that define the target area. That’s why for airborne systems, the flying attitude is so important.
Flight line spacing: This is another important measure for airborne systems, and it depends on the application, vegetation, and terrain of the area of interest.
Nominal point spacing (NPS): The rule is simple enough — the more points that are hit in your collection, the better you’ll define the targets. The point sample spac-ing varies depending on the application. Keep in mind that LiDAR systems are random sampling systems. Although you can’t determine exactly where the points are going to hit on the target area, you can decide how many times the target areas are going to be hit, so you can choose a higher frequency of points to better define the targets.
Cross track resolution: This is the spacing of the pulses from the LiDAR system in the scanning direction, or per-pendicular to the direction that the platform is moving, in the case of airborne and mobile systems.
Along track resolution: This, on the other hand, is the spacing of the pulses that are in the flight direction or driving direction of the platform.
Swath: This is the actual distance of the area of coverage for the LiDAR system. It can vary depending on the scan angle and flying height. If you’re flying higher, you’ll have a larger swath distance, and you’ll also get a larger swath distance if you increase the scan angle. Mobile LiDAR has a swath, too, but it is usually fixed and depends on the particular sensor. For these systems, though, you might not hear the word “swath;” it may instead be referred to as the “area of coverage,” and will vary depending on the repetition rate of the sensor.
Overlap: Just like it sounds. It’s the amount of redundant area that is covered between flight lines or swaths within an area of interest. Overlap isn’t a wasted effort, though — sometimes it provides more accuracy.
Pile load test are usually carried out that one or some of the following reasons are fulfilled:
To obtain back-figured soil data that will enable other piles to be designed.
To confirm pile lengths and hence contract costs before the client is committed to over all job costs.
To counter-check results from geotechnical and pile driving formulae
To determine the load-settlement behaviour of a pile, especially in the region of the anticipated working load that the data can be used in prediction of group settlement.
To verify structural soundness of the pile.
Test loading: There are four types of test loading:
compression test
uplift test
lateral-load test
torsion-load test
the most common types of test loading procedures are Constant rate of penetration (CRP) test and the maintained load test (MLT).
CRP (constant rate of penetration)
In the CRP (constant rate of penetration) method, test pile is jacked into the soil, the load being adjusted to give constant rate of downward movement to the pile. This is maintained until point of failure is reached.
Failure of the pile is defined in to two ways that as the load at which the pile continues to move downward without further increase in load, or according to the BS, the load which the penetration reaches a value equal to one-tenth of the diameter of the pile at the base.
Fig.1 – Test being carried out
Fig.1, In the cases of where compression tests are being carried out, the following methods are usually employed to apply the load or downward force on the pile:
A platform is constructed on the head of the pile on which a mass of heavy material, termed “kentledge” is placed. Or a bridge, carried on temporary supports, is constructed over the test pile and loaded with kentledge. The ram of a hydraulic jack, placed on the pile head, bears on a cross-head beneath the bridge beams, so that a total reaction equal to the weight of the bridge and its load may be obtained.
MLT (the maintained increment load test)
Fig.2, the maintained increment load test, kentledge or adjacent tension piles or soil anchors are used to provide a reaction for the test load applied by jacking(s) placed over the pile being tested. The load is increased in definite steps, and is sustained at each level of loading until all settlements has either stop or does not exceed a specified amount of in a certain given period of time.
Classification of pile with respect to load transmission and functional behaviour
End bearing piles (point bearing piles)
Friction piles (cohesion piles )
Combination of friction and cohesion piles
End bearing piles
These piles transfer their load on to a firm stratum located at a considerable depth below the base of the structure and they derive most of their carrying capacity from the penetration resistance of the soil at the toe of the pile (see figure 1.1).
The pile behaves as an ordinary column and should be designed as such. Even in weak soil a pile will not fail by buckling and this effect need only be considered if part of the pile is unsupported, i.e. if it is in either air or water. Load is transmitted to the soil through friction or cohesion. But sometimes, the soil surrounding the pile may adhere to the surface of the pile and causes “Negative Skin Friction” on the pile.
This, sometimes have considerable effect on the capacity of the pile. Negative skin friction is caused by the drainage of the ground water and consolidation of the soil. The founding depth of the pile is influenced by the results of the site investigate on and soil test.
Fig.1.1 – End Bearing Piles
Friction or cohesion piles
Carrying capacity is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile (see fig 1.2).
Fig.1.2 – Friction Pile
Cohesion piles
These piles transmit most of their load to the soil through skin friction. This process of driving such piles close to each other in groups greatly reduces the porosity and compressibility of the soil within and around the groups. Therefore piles of this category are some times called compaction piles.
During the process of driving the pile into the ground, the soil becomes moulded and, as a result loses some of its strength. Therefore the pile is not able to transfer the exact amount of load which it is intended to immediately after it has been driven. Usually, the soil regains some of its strength three to five months after it has been driven.
Friction piles
These piles also transfer their load to the ground through skin friction. The process of driving such piles does not compact the soil appreciably. These types of pile foundations are commonly known as floating pile foundations.
Combination of friction piles and cohesion piles
An extension of the end bearing pile when the bearing stratum is not hard, such as a firm clay. The pile is driven far enough into the lower material to develop adequate frictional resistance.
A farther variation of the end bearing pile is piles with enlarged bearing areas. This is achieved by forcing a bulb of concrete into the soft stratum immediately above the firm layer to give an enlarged base.
A similar effect is produced with bored piles by forming a large cone or bell at the bottom with a special reaming tool. Bored piles which are provided with a bell have a high tensile strength and can be used as tension piles (see fig.1.3)
Fig 1.3. Under-reamed base enlargement to a bore-and-cast-in-situ pile
Classification of pile with respect to type of material
Timber
Concrete
Steel
Composite piles
Timber piles
Used from earliest record time and still used for permanent works in regions where timber is plentiful. Timber is most suitable for long cohesion piling and piling beneath embankments. The timber should be in a good condition and should not have been attacked by insects.
For timber piles of length less than 14 meters, the diameter of the tip should be greater than 150 mm. If the length is greater than 18 meters a tip with a diameter of 125 mm is acceptable. It is essential that the timber is driven in the right direction and should not be driven into firm ground.
As this can easily damage the pile. Keeping the timber below the ground water level will protect the timber against decay and putrefaction. To protect and strengthen the tip of the pile, timber piles can be provided with toe cover. Pressure creosoting is the usual method of protecting timber piles.
Concrete pile
Precast concrete Piles or Pre fabricated concrete piles : Usually of square (see fig 1.4 b), triangle, circle or octagonal section, they are produced in short length in one metre intervals between 3 and 13 meters. They are pre-caste so that they can be easily connected together in order to reach to the required length (fig 1.4 a) .
This will not decrease the design load capacity. Reinforcement is necessary within the pile to help withstand both handling and driving stresses. Pre stressed concrete piles are also used and are becoming more popular than the ordinary pre cast as less reinforcement is require.
Fig 1.4a – Concrete pile connecting detail
Fig 1.4b – Squared pre-cast concert pile
The Hercules type of pile joint (Figure 1.5) is easily and accurately cast into the pile and is quickly and safely joined on site. They are made to accurate dimensional tolerances from high grade steels.
Fig 1.5 – Hercules type of pile joint
Driven and cast in place Concrete piles
Two of the main types used in the UK are: West’s shell pile : Pre cast, reinforced concrete tubes, about 1 m long, are threaded on to a steel mandrel and driven into the ground after a concrete shoe has been placed at the front of the shells. Once the shells have been driven to specified depth the mandrel is withdrawn and reinforced concrete inserted in the core. Diameters vary from 325 to 600 mm.
Franki Pile: A steel tube is erected vertically over the place where the pile is to be driven, and about a metre depth of gravel is placed at the end of the tube. A drop hammer, 1500 to 4000kg mass, compacts the aggregate into a solid plug which then penetrates the soil and takes the steel tube down with it. When the required depth has been achieved the tube is raised slightly and the aggregate broken out.
Dry concrete is now added and hammered until a bulb is formed. Reinforcement is placed in position and more dry concrete is placed and rammed until the pile top comes up to ground level.
Steel piles
Steel piles: steel/ Iron piles are suitable for handling and driving in long lengths. Their relatively small cross-sectional area combined with their high strength makes penetration easier in firm soil. They can be easily cut off or joined by welding. If the pile is driven into a soil with low pH value, then there is a risk of corrosion, but risk of corrosion is not as great as one might think. Although tar coating or cathodic protection can be employed in permanent works.
Fig 1.6 – Steel piles cross-sections
It is common to allow for an amount of corrosion in design by simply over dimensioning the cross-sectional area of the steel pile. In this way the corrosion process can be prolonged up to 50 years. Normally the speed of corrosion is 0.2-0.5 mm/year and, in design, this value can be taken as 1mm/year.
Composite piles
Combination of different materials in the same of pile. As indicated earlier, part of a timber pile which is installed above ground water could be vulnerable to insect attack and decay. To avoid this, concrete or steel pile is used above the ground water level, whilst wood pile is installed under the ground water level (see figure 1.7).
Fig 1.7 – Protecting timber piles from decay: a) by pre-cast concrete upper section above water level. b) by extending pile cap below water level
Classification of pile with respect to effect on the soil
A simplified division into driven or bored piles is often employed.
Driven piles
Driven piles are considered to be displacement piles. In the process of driving the pile into the ground, soil is moved radially as the pile shaft enters the ground. There may also be a component of movement of the soil in the vertical direction.
Fig 1.8 – Driven Piles
Bored piles
Bored piles(Replacement piles) are generally considered to be non-displacement piles a void is formed by boring or excavation before piles is produced. Piles can be produced by casting concrete in the void.
Some soils such as stiff clays are particularly amenable to the formation of piles in this way, since the bore hole walls do not requires temporary support except cloth to the ground surface. In unstable ground, such as gravel the ground requires temporary support from casing or bentonite slurry. Alternatively the casing may be permanent, but driven into a hole which is bored as casing is advanced.
A different technique, which is still essentially non-displacement, is to intrude, a grout or a concrete from an auger which is rotated into the granular soil, and hence produced a grouted column of soil.
There are three non-displacement methods: bored cast- in – place piles, particularly pre-formed piles and grout or concrete intruded piles.
