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The concept of satellite position fixing commenced with the launch of the first Sputnik satellite by the USSR in October 1957. This was rapidly followed by the development of the Navy Navigation Satellite System (NNSS) by the US Navy. This system, commonly referred to as the Transit system, was created to provide a worldwide navigation capability for the US Polaris submarine fleet. The Transit system was made available for civilian use in 1967 but ceased operation in 1996. However, as the determination of position required very long observation periods and relative positions determined over short distances were of low accuracy, its application was limited to geodetic and low dynamic navigation uses.

In 1973, the US Department of Defense (DoD) commenced the development of NAVSTAR (Navigation System with Time and Ranging) Global Positioning System (GPS), and the first satellites were launched
in 1978.

The system is funded and controlled by the DoD but is partially available for civilian and foreign users. The accuracies that may be obtained from the system depend on the degree of access available to the user, the sophistication of his/her receiver hardware and data processing software, and degree of mobility during signal reception.

Global Positioning System logo
Global Positioning System logo
In very broad terms, the geodetic user in a static location may obtain ‘absolute’ accuracy (with respect to the mass centre of the Earth within the satellite datum) to better than ±1 metre and position relative to another known point, to a few centimetres over a range of tens of kilometres, with data post-processing. At the other end of the scale, a technically unsophisticated, low dynamic (ship or land vehicle) user, with limited access to the system, might achieve real time ‘absolute’ accuracy of 10–20 metres.

The GPS navigation system relies on satellites that continuously broadcast their own position in space and in this the satellites may be thought of as no more than control stations in space. Theoretically, a user who has a clock, perfectly synchronized to the GPS time system, is able to observe the time delay of a GPS signal from its own time of transmission at the satellite, to its time of detection at the user’s equipment. The time delay, multiplied by the mean speed of light, along the path of the transmission from the satellite to the user equipment, will give the range from the satellite at its known position, to the user. If three such ranges are observed simultaneously, there is sufficient information to compute the user’s position in three-dimensional space, rather in the manner of a three-dimensional trilateration. The false assumption in all this is that the user’s receiver clock is perfectly synchronized with the satellite clocks.

1. GPS Observing Methods

The use of GPS for positioning to varying degrees of accuracy, in situations ranging from dynamic (navigation) to static (control networks), has resulted in a wide variety of different field procedures using one or other of the basic observables. Generally pseudo-range measurements are used for navigation, whilst the higher precision necessary in engineering surveys requires carrier frequency phase measurements.

The basic point positioning method used in navigation gives the X, Y, Z position to an accuracy of better than 20 m by observation to four satellites. However, the introduction of Selective Availability (SA), see below, degraded this accuracy to 100 m or more and so led to the development of the more accurate differential technique. In this technique the vector between two receivers (baseline) is obtained, i.e. the difference in coordinates (ΔX, ΔY, ΔZ). If one of the receivers is set up over a fixed station whose coordinates are known, then comparison with the observed coordinates enables the differences to be transmitted as corrections to the second receiver (rover). In this way, all the various GPS errors are lumped together in a single correction. At its simplest the corrections transmitted could be in a simple coordinate format, i.e. δX, δY, δZ, which are easy to apply. Alternatively, the difference in coordinate position of the fixed station may be used to derive corrections to the ranges to the various satellites used. The rover then applies those corrections to its own observations before computing its position.

The fundamental assumption in Differential GPS (DGPS) is that the errors within the area of survey would be identical. This assumption is acceptable for most engineering surveying where the areas involved are small compared with the distance to the satellites.

Where the area of survey becomes extensive this argument may not hold and a slightly different approach is used called Wide Area Differential GPS.

It can now be seen that, using DGPS, the position of a roving receiver can be found relative to a fixed master or base station without significant errors from satellite and receiver clocks, ionospheric and tropospheric refraction and even ephemeris error. This idea has been expanded to the concept of having permanent base stations established throughout a wide area or even a whole country.

As GPS is essentially a military product, the US Department of Defense has retained the facility to reduce the accuracy of the system by interfering with the satellite clocks and the ephemeris of the satellite. This is known as Selective Availability (SA) of the Standard Positioning Service (SPS). This form of degradation has been switched off since May 2000 and it is unlikely, though possible, that it will be reintroduced as there are other ways that access to the system can be denied to a hostile power. The P can also be altered to a Y code, to prevent imitation of the PPS by hostile forces, and made unavailable to civilian users. This is known as Anti-Spoofing (AS). However, the carrier wave is not affected and differential methods should correct for most SA effects.

Using the carrier phase observable in the differential mode produces accuracies of 1 ppm of the baseline length. Post-processing is needed to resolve for the integer ambiguity if the highest quality results are to be achieved. Whilst this, depending on the software, can result in even greater accuracies than 1 ppm (up to 0.01 ppm), it precludes real-time positioning. However, the development of Kinematic GPS and ‘on-the-fly’ ambiguity resolution makes real-time positioning possible and greatly reduces the observing times.

The following methods are based on the use of carrier phase measurement for relative positioning using two receivers.

1.1 Static positioning

This method is used to give high precision over long baselines such as are used in geodetic control surveys. At its simplest, one receiver is set up over a station of known X, Y, Z coordinates, preferably in the WGS84 reference system, whilst a second receiver occupies the station whose coordinates are required.

Observation times may vary from 45 min to several hours. This long observational time is necessary to allow a change in the relative receiver/satellite geometry in order to calculate the initial integer ambiguity terms.

More usually baselines are observed when the precise coordinates of neither station are known. The approximate coordinates of one station can be found by averaging the pseudo-range solution at that station.

Artist's impression of GPS Block IIR satellite in Earth orbit
Artist's impression of GPS Block IIR satellite in Earth orbit
Provided that those station coordinates are known to within 10 m it will not significantly affect the computed difference in coordinates between the two stations. The coordinates of a collection of baselines, provided they are interconnected, can then be estimated by a least squares free network adjustment. Provided that at least one, and preferably more, stations are known in WGS84 or the local datum then the coordinates of all the stations can be found in WGS84 or the local datum.

Accuracies in the order of 5 mm ±1 ppm of the baseline are achievable as the majority of errors in GPS, such as clock, orbital and atmospheric errors, are eliminated or substantially reduced by the differential process. The use of permanent active GPS networks established by a government agency or private company results in a further increase in accuracy for static positioning.

Apart from establishing high precision control networks, it is used in control densification, measuring plate movement in crustal dynamics and oil rig monitoring.

1.2 Rapid static

Rapid static surveying is ideal for many engineering surveys and is halfway between static and kinematic procedures. The ‘master’receiver is set up on a reference point and continuously tracks all visible satellites throughout the duration of the survey. The ‘roving’ receiver visits each of the remaining points to be surveyed, but stays for just a few minutes, typically 2–10 min.

Using difference algorithms, the integer ambiguity terms are quickly resolved and position, relative to the reference point, obtained to sub-centimetre accuracy. Each point is treated independently and as it is not necessary to maintain lock on the satellites, the roving receiver may be switched off whilst travelling between stations. Apart from a saving in power, the necessity to maintain lock, which is very onerous in urban surveys, is removed.

This method is accurate and economic where there are many points to be surveyed. It is ideally suited for short baselines where systematic errors such as atmospheric, orbital, etc., may be regarded as equal at all points and so differenced out. It can be used on large lines (>10 km) but may require longer observing periods due to the erratic behaviour of the ionosphere. If the observations are carried out at night when the ionosphere is more stable observing times may be reduced.

1.3 Reoccupation

This technique is regarded as a third form of static surveying or as a pseudo-kinematic procedure. It is based on repeating the survey after a time gap of one or two hours in order to make use of the change in receiver/satellite geometry to resolve the integer ambiguities.

The master receiver is once again positioned over a known point, whilst the roving receiver visits the unknown points for a few minutes only. After one or two hours, the roving receiver returns to the first unknown point and repeats the survey. There is no need to track the satellites whilst moving from point to point. This technique therefore makes use of the first few epochs of data and the last few epochs that reflect the relative change in receiver/satellite geometry and so permit the ambiguities and coordinate differences to be resolved.

Using dual frequency data gives values comparable with the rapid static technique. Due to the method of
changing the receiver/satellite geometry, it can be used with cheaper single-frequency receivers (although extended measuring times are recommended) and a poorer satellite constellation.

1.4 Kinematic positioning

The major problem with static GPS is the time required for an appreciable change in the satellite/receiver geometry so that the initial integer ambiguities can be resolved. However, if the integer ambiguities could be resolved (and constrained in a least squares solution) prior to the survey, then a single epoch of data would be sufficient to obtain relative positioning to sub-centimetre accuracy. This concept is the basis of kinematic surveying. It can be seen from this that, if the integer ambiguities are resolved initially and quickly, it will be necessary to keep lock on these satellites whilst moving the antenna.

1.4.1 Resolving the integer ambiguities

The process of resolving the integer ambiguities is called initialization and may be done by setting up both receivers at each end of a baseline whose coordinates are accurately known. In subsequent data processing, the coordinates are held fixed and the integers determined using only a single epoch of data. 

These values are now held fixed throughout the duration of the survey and coordinates estimated every epoch, provided there are no cycle slips.

The initial baseline may comprise points of known coordinates fixed from previous surveys, by static GPS just prior to the survey, or by transformation of points in a local coordinate system to WGS84. An alternative approach is called the ‘antenna swap’ method. An antenna is placed at each end of a short base (5–10 m) and observations taken over a short period of time. The antennae are interchanged, lock maintained, and observations continued. This results in a big change in the relative receiver/satellite geometry and, consequently, rapid determination of the integers. The antennae are returned to their original position prior to the surveys.

It should be realized that the whole survey will be invalidated if a cycle slip occurs. Thus, reconnaissance of the area is still of vital importance, otherwise reinitialization will be necessary. A further help in this matter is to observe to many more satellites than the minimum four required.

1.4.2 Traditional kinematic surveying

Assuming the ambiguities have been resolved, a master receiver is positioned over a reference point of known coordinates and the roving receiver commences its movement along the route required. As the movement is continuous, the observations take place at pre-set time intervals, often less than 1 s. Lock must be maintained to at least four satellites, or re-established when lost. In this technique it is the trajectory of the rover that is surveyed and points are surveyed by time rather than position, hence linear detail such as roads, rivers, railways, etc., can be rapidly surveyed. Antennae can be fitted to fast moving vehicles, or even bicycles, which can be driven along a road or path to obtain a three-dimensional profile.

1.4.3 Stop and go surveying

As the name implies, this kinematic technique is practically identical to the previous one, only in this case the rover stops at the point of detail or position required (Figure 9.17). The accent is therefore on individual points rather than a trajectory route, so data is collected only at those points. Lock must be maintained, though the data observed when moving is not necessarily recorded. This method is ideal for engineering and topographic surveys.

1.4.4 Real-time kinematic (RTK)

The previous methods that have been described all require post-processing of the results. However, RTK provides the relative position to be determined instantaneously as the roving receiver occupies a position.

The essential difference is in the use of mobile data communication to transmit information from the reference point to the rover. Indeed, it is this procedure that imposes limitation due to the range over which the communication system can operate.

The system requires two receivers with only one positioned over a known point. A static period of initialization will be required before work can commence. If lock to the minimum number of satellites is lost then a further period of initialization will be required. Therefore the surveyor should try to avoid working close to major obstructions to line of sight to the satellites. The base station transmits code and carrier phase data to the rover. On-board data processing resolves the ambiguities and solves for a change in coordinate differences between roving and reference receivers. This technique can use single or dual frequency receivers. Loss of lock can be regained by remaining static for a short time over a point of known position.

The great advantage of this method for the engineering surveyor is that GPS can be used for setting-out on site. The setting-out coordinates can be entered into the roving receiver, and a graphical output indicates the direction and distance through which the pole-antenna must be moved. The positions of the point to be set-out and the antenna are shown. When the two coincide, the centre of the antenna is over the setting-out position.

1.4.5 Real-time kinematic on the fly

Throughout all the procedures described above, it can be seen that initialization or reinitialization can only be done with the receiver static. This may be impossible in high accuracy hydrographic surveys or road profiling in a moving vehicle. Ambiguity Resolution On the Fly (AROF) enables ambiguity resolution whilst the receiver is moving. The techniques require L1 and L2 observations from at least five satellites with a good geometry between the observer and the satellites. There are also restrictions on the minimum periods of data collection and the presence of cycle slips. Both these limitations restrict this method of surveying to GPS friendly environments. Depending on the level of ionospheric disturbances, the maximum range from the reference receiver to the rover for resolving ambiguities whilst the rover is in motion is about 10 km, with an achievable accuracy of 10–20 mm.

For both RTK and AROF the quality of data link between the reference and roving receiver is important. Usually this is by radio but it may also be by mobile phone. When using a radio the following issues should be considered:
  • In many countries the maximum power of the radio is legally restricted and/or a radio licence may be required. This in turn restricts the practical range between the receivers.
  • The radio will work best where there is a direct line of sight between the receivers. This may not always be possible to achieve so for best performance the reference receiver should always be sited with the radio antenna as high as possible.
  • Cable lengths should be kept as short as possible to reduce signal losses.

Freezing reduces strength of concrete by 20 to 40 % when fresh concrete is subjected to freezing. Antifreeze admixtures of concrete, its properties and uses in cold weather concreting are discussed.

The resistance of the fresh concrete against the freeze and thaw cycle is given by the durability factor which is also lowered by 40 to 60%. There is 70% decrease in the bond between the reinforcement and the concrete that is normally cured.

 Cold Weather Concreting_engineersdaily.com
 Cold Weather Concreting

Hence it is very essential during the concreting in cold weather conditions to ensure that the concrete will not undergo freezing in its plastic state.

There are two methods for carrying out cold weather concreting:
  1. Provision of normal ambient temperatures for the concrete. This can be done through the heating of the concrete ingredients or bley providing heating enclosures.
  2. The addition of chemical admixtures.

Conventional Chemical Admixtures in Cold Weather Concrete

 

Conventional concrete used calcium chloride as accelerating admixtures to offset the retarding effects of slow hydration of concrete in low temperatures. This admixture is not effective below the freezing temperatures.

This is found to be a drawback in the conventional form of admixtures. Hence, for arctic weather conditions, special admixtures are necessary. One such is antifreeze admixtures.
Antifreeze Admixtures for Concrete

The antifreeze admixtures affect the physical condition of the mix water used in the concreting. These can depress the freezing point of the water to a large extent and can be used in temperatures lesser than -30 degrees Celsius. This can enable the extension of the period of the construction activity.

Chemical Composition and Action of Antifreeze Admixtures


There are two groups of antifreeze admixtures that provide the characteristics of antifreeze and the accelerated setting and hardening properties.

They are:

1. First Group


This includes chemicals, weak electrolytes, sodium nitrite, sodium chloride and non-electrolytic organic compounds which lower the freezing point of the water used in the concrete. But these group acts as weak accelerators to promote the setting and hardening.

2. Second Group


These include binary as well as ternary admixtures which contains potash and additives based on calcium chloride, sodium nitrite, calcium chloride with sodium nitrite, calcium nitrite -nitrate-urea and other chemicals.

These have effective antifreeze properties and accelerating property to promote the setting and hardening. These are used in larger dosages compared to that of conventional admixtures.

One such example is the use of 8% of sodium nitrite to keep the liquid at a temperature of -15-degree Celsius.

These admixtures function by lowering the liquid phase freezing point and by accelerating the cement hydration at the freezing temperatures.

Based on the dosage in the mixture, the non-chloride admixture enables the mix (concrete or the mortar) to be placed at sub-freezing temperatures. This hence reduces the need of protective measures required during the cold weathering works.

The method improves the quality of the concrete and as it facilitates early setting, early stripping of formworks can also be carried out. This helps in the reuse of the form within a small duration and hence speed up the construction.

The table-1 shows the significant difference is strength gain at 3, 7 and 28 days for plain concrete and antifreeze admixture used concrete.

Table.1: Concrete Compressive strength with and without antifreeze admixture

 

(As per Ratinov and Rosenburg)


PropertyPlain ConcreteFreeze-protection Admixture
Set time (-4 degree Celsius)
Compressive strength (MPa)

-4 degree Celsius (3 days)3.49.24
-10 degree Celsius (7 days)8.339.3
-10 degree Celsius (28 days)18.149.9


It is possible for the incorporation of other admixtures that contains superplasticizers to be incorporated with the antifreeze admixtures. The main advantage of such combination is that in totality there will be a reduction of water.

The water reduction will reduce the freezable free water content in the mix. This freezable water content is the one that serves as the heat sink for the heat liberated by the initial hydration reactions. This will hence reduce the number of antifreeze admixtures.

Selection of Antifreeze admixtures


The factors based on which the selection of antifreeze admixtures is carried out are:
  1. The type of structure
  2. The operating Conditions
  3. Protecting methods employed in winter concreting
  4. Cement brand and aggregate types

A laboratory test must be carried out with the operating materials and the dosage of antifreeze admixtures that are intended to be used in the field.

The incorporation of other admixtures like retarders, superplasticizers with antifreeze admixtures is not restricted in cold weather concreting. The dosage of all the admixture that are used must be established experimentally.

Application and Advantages of Antifreeze admixtures


The antifreeze admixtures are technologically simple and beneficial for cold weather concreting. The admixture helps in improving the cohesiveness, cold joint minimizations, sand streaking, and plasticity. These are estimated to provide large cost saving than other methods of steam curing or concrete enclosures.

The combination of antifreeze agents with water reducing agents or air-entraining agents will help in increasing the resistance of concrete towards the frost action and corrosion.

Recently the global construction industry has witnessed great technological evolution. Several useful applications emerges out which can make your construction project management process superior from project planning to achievement. The following are some newest applications useful for effective construction project management.

Useful Project Management Applications for Competent Construction Management
PlanGrid: This construction application will be useful if anyone directly performs with architect in future construction projects. The construction software enables the users to distribute plans, markups, photos, specifications, reports with the whole project team members effortlessly. So the possibilities for all types of mis-communications are reduced to a great extent and all the information can be easily allocated throughout planning stage to all the parties involved with the construction project.

Material Estimator: With this handy construction tool the contractors, designers, remodelers, engineers, architects as well as other building professionals will be able to calculate feet inch fraction construction math and building materials estimating. This construction application deals with square footage concerning any project and offers a instant estimate of the total materials (drywall, decks, fencing, gravel, concrete, flooring and paint etc.) utilized in a construction project.

Build It Live: It is (SaaS) construction project management software program useful for owners, architects, engineers, general contractors, vendors, laborers, who are associated with the project, to get all the updated information concerning the latest drawings, schedules, and changes online on the cloud. So the concerned parties in the planning & building of the projects will get access to the above information. If any modification is occurred to a project, all the project stakeholders will be notified about the modification automatically through the email.

Control Center 7: Control Center 7 allows the users to keep a track of their jobsite from any remote location. This construction application performs in tandem with cameras already set up in the jobsite and provides you visual access to the job ceaselessly. Get crucial visual information long after your project is finished as well as a complete online database of recorded images from your camera system.

Take photograph & post detailed views to be conveyed to the team members. Trim down lag time while finding any complication on site and resolving it. An immediate photo is used to provide change orders, a recently completed milestone or what materials are required for giving order.

SmartBidNet: This commercial construction bidding software can allow you to keep track on the financial information of vendors and subcontractors as well as bid project data, documents etc. associated with the project through a web-based and mobile platform. Just put the bid information into the app to have some ideas on whom to select. Besides, measurements and other statistics are also affixed to each bid for taking effective decision.

With bid invitations, the subcontractors will be able to get customized access to the online plan room. One can keep track of what plan files they view and download and collaborate on estimating through takeoff integrations.

Project Quote Estimate: It is a handy construction field estimator in the app store. It consists of 19 diversified calculators and tools for materials and costs that can generate an on-site estimate efficiently. This application is only compatible with residential properties at present.

This application lets building contractors to produce professional quotes instantly for any client in the construction site as well as estimate costs and quantities quickly for any renovation project. Just create a pdf quote and sent it to the clients through email.

iQuick Contract Maker: Quick Contract Maker can be applied to generate a legal contract instantly and effortlessly through any phone or handheld appliance. The application includes pre-written information useful for creating legal contract. It can be customized for any contract agreement. All stages of the contract are contained with the application, just select the section according to your choice and add it to the device and send it to the clients through email.

One can apply it for diverse projects which range from Construction, Landscape, Real Estate, Movers, Home Repair, etc.

Reference : http://tech.co/

3D printing has emerged in the recent past as a smart and quick alternative to many fields of production. Its applications in the field of construction are no exception. It is however relatively easier to handle 3D printing of small or medium sized products, ensuring stability, strength and durability in case of a structure for the built environment pose serious challenges.

The cycle bridge is part of the Noord-Om project which is a new section on the ring road around the village of Gemert in Netherlands. Although the team claims it to be World's 1st 3D printed bridge, there was already a 3D printed pedestrian bridge built this year in Madrid, Spain which in our opinion is the rightful deserver of the title '1st 3D printed bridge'. Another project was to be initiated by a company named MX3D to print a steel bridge with huge robotic arms but the project appears to be still.


Dutch inaugurate the 3D Printed Reinforced Concrete Bridge Designed by Technical University of Eindhoven
Inauguration of the 3D printed bridge in Netherlands (Image courtesy: BAM Infra)
This Dutch bridge is a practical bridge for cyclists which was printed using a custom built cement printer at the Technical Universty of Eindhoven. Individual layers each having a thickness of 1cm are printed by injecting liquid mortar into the printhead through the storage. Steel reinforcing wire is fed down before a layer dries and the process goes on. The printer builds up sections layer by layer which are approximately 1m high.
Dutch inaugurate the 3D Printed Reinforced Concrete Bridge Designed by Technical University of Eindhoven
(Images courtesy: BAM Infra)

3D-printed concrete bridge from Royal BAM Group nv

Six of the printed sections were transported to the site to be glued together to form a bridge that is 8 m long, 3.5 m wide and is 0.9 m thick. During the testing phase, the bridge was found to support an imposed load of 5 Tons which is far greater than that of the cyclists. The expected life of the bridge is 30 years. The bridge was built by the company BAM Infra and the team claims to have gained valuable experience from this versatile project which will allow the members to print much larger sections for much bigger structures.

3D printing in construction has endless possibilities and benefits. A very important aspect is the economical use of cement and other materials. Another aspect is not requiring a formwork or minimum if at all needed which also saves costs and material wastage.

Groundwater causes extreme geotechnical problems in excavations such as sand running for most of construction projects such as tunneling. So, issues caused by ground water would increase construction budget and extend construction time unless the ground water is properly controlled which is the root cause of the problems.

There are two major methods for controlling ground water including exclusion technique (Figure 1a) and pumping technique (Figure 1b).

Figure 1a: Ground water control in excavations by exclusion
Figure 1a: Ground water control in excavations by exclusion
Figure 1b: Ground water control in excavations by pumping
Figure 1b: Ground water control in excavations by pumping

Figure 2: Controlling Ground Water in Shafts
Figure 2: Controlling Ground Water in Shafts

Why to use Exclusion Methods to Control Ground Water?


There are cases in which the application of pumping techniques to control ground water is not recommended, for example, in water bearing rock formation and high permeability ground.

This is because utilization of large capacity pumps, which are required for high permeability ground and formation of well points in rock formation would be considerably costly.

So, it would be economical to consider exclusion methods in the aforementioned situations and alike cases.

Exclusion Methods to Control Ground Water in Excavations


There are number of techniques by which ground water exclusion are obtained:
  • Forming impervious barriers by grouting with cement, clay suspension
  • Chemical consolidation for controlling ground water in excavation
  • Ground water control by compressed air
  • Freezing ground water control

Forming Impervious Barriers by Grouting with Cement, Clay Suspension or Bitumen


This strategy is considered in water bearing rock formation or high permeability ground where the use of high pump capacity or digging well point is expensive.

In this technique, the permeability is reduced by creating an impervious barrier by injecting suspension material or fluids into the fissures of rocks or pore spaces. Fineness of fissures in rocks or soil particle size distribution would control types of materials used for grouting.

This means that the grout material particle size should be considerably smaller than the pore spaces. Figure 3 illustrates limiting particles sizes of materials which may be grouted by different types of grout.

Figure 3: Various Types of Soils Which can be Grouted with Different Types of Grouting Material
Figure 3: Various Types of Soils Which can be Grouted with Different Types of Grouting Material
Additionally, groutability ratio, which is the ration between D.15 size of soil to the D.85 size of grouting material particle, is also used to determine suitability of suspension grouts.
So, suspension grouts would not be appropriate choice for the soil under consideration unless the groutability ratio is higher than 5:15 for clay grouts and 11:25 for cement grouts.

Furthermore, it is necessary to pay attention to the quantity of materials used for grouting since it could be costly if the excessive amount is employed. This concern might be dealt with by considering chemical grouts even though its cost is higher than clay and cement for the same quantity.

As far as fluid gout is concerned, it is more effective than suspension grout since it fills all pores and spaces in soil whereas small size pores would be left empty in the case of suspension grout.

When grouting technique is considered, it is required to practice great care regarding structures and facilities such as sewer sanitary system around the grouting area. This is because grouting is conducted under great pressure, so it might impair considerable damage to these facilities.

Figure 4: Distribution of Grouting Pipes Around Excavation Area
Figure 4: Distribution of Grouting Pipes Around Excavation Area

Finally, there are three major methods for injecting grouts which are provided in Table-1 along their application conditions and procedures.

Table-1: Principle Methods for Grout Injection

Grouting Methods
Suitable Conditions
Grouting Procedure


Open hole
Very coarsely graded soils or rocks with broad fissures
Firstly, grouting pipe, which its lower end closed by an expandable plug and upper end is sealed on the surface, is driven into the soil. Secondly, grout is forced into the pipe and driven out the plug and wide fissures will be filled with grout.

Stage grouting

Not specified
In this method, a hole is drilled in advance then a lance is inserted after that grouting is carried out by either bottom up method or top down method. In the former, the hole is dug and a lance is inserted into the hole then grouting is proceeded. The latter approach, the upper part of the hole is grouted and after its setting, the lower part would be grouted.


Sleeve grouting
Suitable for grouting soils
It makes use of Tube-a-Manchette as shown in Figure 5. After a hole is drilled to a determined depth and cased a sleeve tube is inserted and surrounded by partially plastic grout. Then, the case is pulled up and perforated injecting pipe is inserted into the sleeve pipe. finally, the grouting material is injected and plastic grout would be broken and the grout material will spread through the ground.

Figure 5: Tube-a-Manchette Used for Grouting in Soils
Figure 5: Tube-a-Manchette Used for Grouting in Soils
Open hole very coarsely graded soils or rocks with broad fissures Firstly, grouting pipe, which its lower end closed by an expandable plug and upper end is sealed on the surface, is driven into the soil. Secondly, grout is forced into the pipe and driven out the plug and wide fissures will be filled with grout.

Stage grouting Not specified In this method, a hole is drilled in advance then a lance is inserted after that grouting is carried out by either bottom up method or top down method. In the former, the hole is dug and a lance is inserted into the hole then grouting is proceeded. The latter approach, the upper part of the hole is grouted and after its setting, the lower part would be grouted.

Sleeve grouting Suitable for grouting soils It makes use of Tube-a-Manchette as shown in Figure-5. After a hole is drilled to a determined depth and cased a sleeve tube is inserted and surrounded by partially plastic grout. Then, the case is pulled up and perforated injecting pipe is inserted into the sleeve pipe. finally, the grouting material is injected and plastic grout would be broken and the grout material will spread through the ground.

Chemical Consolidation for Controlling Ground Water in Excavation


Figure 6: Chemical Grout Formation prior to Injection
Figure 6: Chemical Grout Formation prior to Injection
Chemical consolidation method is suitable for sandy gravels and fine grading sands. The most usual chemical material used for chemical consolidation is the sodium silicate. If the sodium silicate is mixed with other chemicals, moderately strong and insoluble silica gel can be produced.

Two approaches have been practiced to conduct chemical consolidation, namely, two shot process and one-shot process. By and large, the latter process which is the most common one has replaced the former process.

In two shot process, two pipes with spacing of 50cm are forced into the ground, then sodium silicate are driven to one pipe and calcium silicate injected into the other while they are pulled up gradually.

Alternatively, one chemical is injected while the pipe is driven into the ground, the other chemical material is driven though the pipe as it is withdrawn.

Figure 7: Acrylic Polymers
Figure 7: Acrylic Polymers
As far as one-shot process is concerned, chemical grouts are usually created prior the injection process. So, the most important consideration in this technique is to postpone the formation of grout gel. This is because grout penetration would be easier and more efficient when its viscosity is low.

Therefore, it is desired to have low viscosity grout during injection and the increase in grout viscosity occurred after the completion of injection process.

Finally, several attempts have been made to achieve gouts with such favored property, for example, resins and lignins and acrylic polymers.

Control Ground Water in Excavation by Compressed Air Method


There are several factors that motivate the application of compressed air to control ground water in excavations. For example, the use of other ground water control methods is not possible due to hydrological conditions.

The use of compressed air is advised in the case where environmental concerns are encountered specifically when ground water employed as a reservoir for drinkable water, consequently the use of solid materials like cement is prevented.
Figure 8: Preparation for Compressed Air Technique to Control Groundwater, Diaphragm Wall Construction
Figure 8: Preparation for Compressed Air Technique to Control Groundwater, Diaphragm Wall Construction
Compressed air technique is commonly employed for controlling ground water in excavations of tunnels and shafts.
Figure 9: Removing Waste Materials from Excavations
Figure 9: Removing Waste Materials from Excavations
Controlling ground water by compressed air cannot be carried out unless certain conditions are met. Firstly, the side walls and lid of the structure in which air is kept should be nearly impermeable. Secondly, compressed air static pressure shall be equal to the hydrostatic pressure of ground water at the lowest point of the to be maintained dry. Thirdly, air static pressure throughout the entire dry hollow space of the structure is should be constant.
In order to achieve the above conditions, the following construction procedures should be considered.
  • Construct diaphragm wall as shown in Figure-8 along the side of the planned structure such as tunnel. The depth of the wall should extend below the final bottom slab of the structure.
  • After that, construct a lid for the tunnel and the joint between the lid and diaphragm wall should be compressed airtight. In this stage, preliminary drainage may be employed provided that the conditions do not pose obstacles. It should be bore in mind that both diaphragm wall and the lid should be airtight as well.
  • Construct a dividing wall or bulkhead with locks for workers and materials at one portal of the tunnel to avoid the escape of air toward the other end of the tunnel. Complementary momentary diaphragm wall might be placed at specified spacing along longitudinal axis of the tunnel and at its far end portal.
  • Configuring mechanical and electrical machineries for waste material disposal as shown in Figure-9, compressed air provision and supporting plant.
  • Finally, the excavation processes and compressed air utilization will be started under the lid at the tunnel portal.
In this technique, substantial care should be practiced to prevent undesired events since compressed air techniques involves high level of risk that could lead to human loss.
Figure 10: Excavation and compressed air application, PL: air pressure height in the tunnel, Dtt: distance between artesian ground water and invert WK: artesian Tithonian water pressure

Control of Ground Water in Excavations by Freezing


Controlling excavation ground water by freezing is not recommended to use unless all other methods fails to provide desired result or inappropriate to choose due to certain factors. This is because the cost of controlling ground water by freezing is significantly high due to large number of boreholes required to be drilled around the excavation area.

However, there cases in which freezing is the only practical method to control ground water for example in extremely deep shaft excavation where the pressure of ground water is seriously high.
Figure 11: Controlling Ground Water in Excavation by Freezing Method
Figure 11: Controlling Ground Water in Excavation by Freezing Method
To prevent the formation of unfrozen spaces in the frozen area, boreholes shall be exactly vertical and errors must be kept as minimum as possible in addition to provide small spacing between boreholes.

Regarding disadvantages of freezing method, considerable time needed for the completion of drilling boreholes, installing plants, freezing grounds and certain types of soils might experience heaving.

Added to that, compressed air operation is possible to hinder due to low temperature of excavation and construction activities such as concreting will face difficulties.

Nonetheless, it should be known that the most outstanding benefits of freezing technique is the effective controlling of ground water which other methods are lacking.

Freezing procedure involves drilling boreholes around excavation area, then inserting an outer plastic or steel tube with diameter of 10-15cm and an inner tube of 3.8-7.5cm into the boreholes, the outer tube end is closed whereas the inner tube end is opened.

The upper end of inner tube is connected to refrigeration plant from which cooled brine is pushed into the inner tube and after that returns to the refrigerator plant. The time during which the ground is frozen ranges between 1 to 4 months.

Finally, it is recommended to use liquid nitrogen rather than brine because freezing time would be reduced considerably. One might argue that the liquid nitrogen is expensive but its low construction cost may offset that and it freezes the ground five times faster than case where brine is used.

Nov 22, 2017

Colliers International has reported that there has been an increase of almost 127% of visitors to the most popular museums in Dubai. The number of visitors jumped to 1.75 million visitors in only five of the most popular museums in the year 2015.

It is expected that these figures will rise dramatically in the coming 2 years with the opening of  Museum of The Future and the Mohammed Bin Rashid Library.

Louvre Abu Dhabi

Top Museums in United Arab Emirates (UAE)
Louvre Abu Dhabi
(Image courtesy: archdaily)
The landmark project of Louvre Abu Dhabi which took 10 years in its development process, opened to public on November 11, 2017 is also expected to boost the number of visitors to United Arab Emirates.
"We had some pains and some pleasures in the project," said Manuel Rabaté, director of Louvre Abu Dhabi during a recent speech at the Louvre Abu Dhabi.

"I remember, in 2009, for the ground breaking ceremony, we were a full delegation in the sand of Saadiyat Island and we were looking for the mathematical centre of the dome before [works] started.

"I remember when this whole place was filled with scaffolding and now the dome is floating above us," he added.

Museum of the Future

The project of 'Museum of the future' as perceived by the ruler of UAE as the incubator of ideas and innovations and a global magnet for enthusiasts was revealed two years ago in 2015. It will be located near the Emirates Towers along Shaikh Zayed road. The main contract costs $212.4m and has been awarded to BAM International. The project is expected to be completed by mid 2019.


Top Museums in United Arab Emirates (UAE) Museum of the Future
Museum of the future, Dubai
(Image courtesy: http://aasarchitecture.com)
Geophysical logging, a multi-channel analysis of surface waves (MASW), as well as down-hole and cross-hole tests is to be performed by ACES Dubai. It has also carried out former geotechnical investigations including bore hole testing with depths ranging from 40m to 80m, cone penetration tests (CPTs), trial pit excavation, installation of piezometers and conduction of packer permeability and pressure-metre testing.

Etihad Museum

Top Museums in United Arab Emirates (UAE) Etihad Museum
Etihad museum, Dubai
(Image courtesy: https://www.thenational.ae)
Etihad Museum costing $133m was opened to public on January 7, 2017 by the country's leadership. Etihad Museum was designed by Moriyana & Teshima Architects has been regarded as a symbol of country's unity and marks the signing of the document that led to its formation in 1971. The visitors to this splendid museum have the opportunity to go through country's history, its founding and growth with the help of documents, videos, images and other informative media.

Whether the working equipment moves on tracks or tires has a major influence on productivity (how much dirt can be moved or excavated in a certain amount of time or how fast material can be transported). Both types of movements offer advantages and disadvantages based on working and surface conditions.

Usable force available to perform work depends on the coefficient of traction of the work surface and the weight (lbs) carried by the running gear or wheels. The amount of tractive force necessary to push or pull a load is important for sizing the right machine. Manufacturers provide rimpull or drawbar pull tables for most of their equipment models showing tractive power that can be delivered at specified operating speeds. This information can be used to verify a machine’s ability or capacity to work in specified job conditions (primarily rolling or surface resistance and grade resistance) and achieve the desired production.

Coefficients of traction vary based upon the travel surface. They measure the degree of traction between the wheel or track and travel surface. Slick surfaces have lower coefficients of traction than rougher surfaces (assuming both surfaces are relatively level and flat). Coefficients of traction for rubber-tired vehicles range from 0.90 for a concrete surface, 0.20 for dry sand to 0.12 for ice. Typically, coefficients of traction tables are available in equipment performance handbooks. The better the traction generated by the piece of equipment on the travel surface, the shorter the travel time and less wear and tear on the piece of equipment. Simply stated, maximum tractive effort (drawbar or rimpull in pounds) equals the equipment weight multiplied by the coefficient of traction of the travel or work surface.

This formula calculates the maximum amount of force that can be generated for a load on a surface. Excess tractive effort generated by the equipment will cause the tires or tracks to spin. Overloading will cause this result. The machine’s engine provides the power to overcome the resistances and move the machine. The engine must be sized or matched to meet the tractive effort required to the capabilities of the machine. The model selected would be appropriate if it can generate enough tractive effort to perform the specific task without overburdening the machine.
Tracked equipment is designed for work activities requiring high tractive effort (drawbar) or the ability to move and remain stable on uneven or unstable surfaces. Tasks such as pushing over trees, removing tree stumps, or removing broken concrete flatwork require a very high pushing force. The tracked bulldozer is ideal for this type of work. Tractive effort results from the track cleats or grousers gripping the ground to create force necessary to push or pull dirt, material, or any other piece of equipment. Tracked equipment is most efficient when used for short travel distances less than 500 ft. Figure 1 shows a typical piece of heavy construction equipment running on tracks. Most loaders on construction sites run on tires.
Consideration of Tracked or Tired Machinery in Excavating and Earthmoving
Figure 1 Tracked loader at work

Tracks can be metal or rubber. Metal tracks are more durable and can withstand much greater abuse than rubber tracks. Heavy-duty dirt moving equipment will almost always run on metal tracks. Rubber tracks are lighter and best for smaller equipment working in organic matter and surfaces requiring minimal disturbance. Tracks come in varying widths and thicknesses. The width of the track shoe determines the ground pressure. The wider the track the more surface area covered and the wider the load distribution. Wider track shoes have greater flotation on the work surface. The heavier the track, the more power required to make it move. Narrow track shoes are better for harsh irregular hard work surfaces. Shoes are typically designed with single or double grousers. Single grouser shoes are better for developing traction and double grouser shoes typically are less damaging to travel or work surfaces.

It should be noted that tracked equipment typically marks or gouges the surface on which it is operating. Skid-steer types of equipment (bulldozers and loaders) will gouge the surface with the track cleats when they turn. To avoid ‘‘customizing’’ a parking lot surface, plywood can be laid, on which the tracked equipment can maneuver, and rubber or padded tracks or use a tired piece of equipment could be used. A hot asphalt surface typically will mark or rip with tires or tracks unless the surface is protected.
Tired equipment is more mobile and maneuverable than tracked equipment. Machines can achieve greater speed and therefore are better for hauling. However, pulling ability is reduced to reach a higher speed. Tired equipment is more efficient than tracked equipment when the distance is greater than 500 ft. The tire diameter and width, tread design, and inflation pressure influence the ability to roll. The larger the tire, the more power required to make it roll. Tread and track design influence the ability to grip the travel surface. A more pronounced deeper tread grips better. The inflation pressure also influences how much resistance the tire has on the travel surface. The less the inflation pressure, the greater the surface area covered by the tire, the harder it is to roll and more buoyant the equipment.

Rolling resistance is the resistance of a level surface to a uniform velocity motion across it. It is the force required to shear through or over a surface and is also termed wheel resistance (e.g., a truck tire developing friction on the road surface as it turns). Rolling resistance has two components: surface resistance and penetration resistance.

Surface resistance results from the equipment trying to rollover the travel surface material. Penetration resistance results from the equipment tires sinking into the surface. Obviously, this resistance will vary greatly with the type and condition of the surface over which the equipment is moving. Simply put, soft surfaces have higher resistance than hard surfaces.
Consideration of Tracked or Tired Machinery in Excavating and Earthmoving

On a hard surface, a highly inflated tire has less rolling resistance than a less inflated tire, primarily because of less tire surface area coming in contact with the road surface. A highly inflated tire has greater rolling resistance in sand than a less inflated tire because it will sink deeper into the rolling surface. The rolling resistances shown in Table 1 are adapted from John Schaufelberger’s book, Construction Equipment Management. The table shows several surfaces and their rolling resistances. Rolling resistance is expressed in pounds of resistance per ton of vehicle or equipment weight. The rolling resistance is greater for a loaded piece of equipment than when it is unloaded. Use the loaded weight of the equipment (equipment including fuel and lubricants plus load) in tons when calculating resistance.

When there is no real penetration into the travel or operating surface, the rolling resistance is about 40 lbs/ton. The weight of the equipment should include the load. When a tire sinks in the mud until it is stable, the rolling resistance as it tries to climb out of the rut increases about 30 lbs/ton (2000 lbs) for each inch of penetration.

Example

92,000 lbs/2000 lbs/ton = 46 tons.
Rolling resistance = 46 tons (50 lbs/ton) = 2300 lbs.
Penetration resistance = 200(46 tons) (30 lbs/ton/inch) = 2760 lbs.
Total tractive effort = 2300 lbs + 2760 lbs = 5060 lbs.

With this number, the equipment manager can refer to the manufacturer’s performance specs to select a piece of equipment that can generate enough power (in this case rimpull) to overcome this resistance.

Grade resistance is the force-opposing movement of a vehicle up a frictionless slope (does not include rolling resistance). The effort required to move a vehicle up a sloping surface increases approximately in proportion to the slope of the surface. The effort required to move a vehicle down a sloping surface decreases approximately in proportion to the slope of the surface. For slopes less than 10%, the effect of grade increases for a plus slope and decreases for a minus slope. The required tractive effort increases or decreases 20 lbs per gross ton of weight for each 1% of grade.

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