Bridges are marvels of engineering that stand inconspicuously amongst us. We don’t think of them much even when we are passing over them. Nowhere are these structures more impressive then when the are built over water, which brings us to the question how are bridges built over water?
When the water is shallow, construction is easy. A temporary foundation is made on which piers are built to support the upper structure and the bridge is then built! It’s when the water is deep that other techniques are needed.
There are many methods to complete such as task in deep water but here we will explore the main three. These three methods of bridge building are called battered piles, cofferdams and caissons.
Battered Piles:
These are poles that are driven into the soil underneath the water. Piles are hammered into the water until the turn outward or inward at an angle. This makes the piles firm and increases their ability to carry lateral loads.
Piles are inserted in the ground using pile drivers. These are mechanical devices that may be transported to a location on a floating pile driving plant.
Battered Piles at a bridge project in Sweden
Pile drivers may also be cantilevered out over the water from piles that have been installed in advance. With the use of pile frames, pile hammers and winches, pile drivers hammer the piles into the soil until the turn outward or inward at an angle. The pile are now ready to carry lateral loads and can provide the foundation of support for the bridge.
The next step is to construct the pile caps above the piles. Once this is done, the bridge is ready to be built.
Cofferdams:
These are temporary enclosures made be driving sheet piling into the bed of a body of water to form a watertight fence. This is called the cofferdam. There is more to this bridge technique. Once the sheet piles have been inserted in the water to create a cofferdam, the water is pumped out of the enclosure.
Now, the construction workers can built the bridge as if the are working on dry land. The process then becomes relatively easy.
Cofferdam
Caissons:
There are two types of caissons, open and pneumatic.
An open caisson is a structure that is usually shaped like a box. It is open, at the top and bottom. The caisson is usually constructed on land then floated into position and sunk, so that the upper edge is above water level.
The caisson has a cutting bottom edge so that it sinks through soft silt on the bed. Inside is a series of large pipes or dredging wells. These are used to dredge up the bed material. As more material is dredged up, the caisson sinks and more sections are added to the shaft to keep it above water.
Once the caisson reaches the correct depth, concrete is laid to seal the bottom and then more concrete is poured into the caisson to form a solid post.
Steel Open Caisson
A pneumatic caisson is similar to an open caisson but it has an airtight bulkhead above the bottom edge. This is fitted with air locks. The space between the cutting edge and the bulkhead is called the working chamber. In this space, the water is removed using air pressure. Construction workers can then enter the chamber and excavate the soil.
It is important that the air pressure in the chamber be carefully monitored so the workers do not get the bends.
Pneumatic Caisson
But how do engineers pick which technique to use?
This all depends on the condition of the site and the technology available. These are important decisions to make that only exports can fully handle.
Concrete is as much a part of the urban landscape as trees are to a forest. It’s so ubiquitous that we rarely even give it any regard at all. But underneath that drab grey exterior is a hidden world of compexity.
Concerete is one of the most versatile and widely-used construction materials on earth. It’s strong, durable, low maintenance, fire resistant, simple to use, and can be made to fit any size or shape from the unfathomably massive to the humble stepping stone.
However, none of those other advantages would matter without this : it’s cheap. Compared to oter materials, concrete is a bargain and it is easy to see why if we look at what’s made of.
Concrete has four primary ingredients : Water, sand (also called fine agregate), gravel (aka coarse aggregate) and cement.
A recipe that is not quite a paragon of sophistication, one ingredient falls from the sky and the rest essentially straight out of the ground. But, from these humble beginnings are born essentially the basis of the entire world’s infrastructure.
Actually, of the four, cement is the only ingredient in conrete with anay complexity at all. The most common type used in conrete is know as Portland cement. It’s made by quarried materials (mainly limestone) into a kiln, then grinding them into a fine powder with a few extra herbs and spices.
Cement role :
Cement is a key constituent in a whole host of construction materials, insluding grout, mortar, stucco and of course concrete. A lot of people don’t know this, but every time you say cement when you were actually talking about concrete, a civil engineer’s calculator runs out of batteries.
The cement key role es to turn concrete from liquide to a solid. Portland cement cures not through drying or evaporation of the water, but through a chemical reaction called hydration.
The water actually becomes a part of cured concrete, this is why you shouldn’t let concrete dry out while it’s curing. Lack of water can prematurely sop the hydration process, preventing the concrete from reaching its full strenght.
In fact, as long as you avoid washing out the cement, concrete made with Portland cement can be placed and cured completely under water. It will set and harden just as well (and maybe even better) as if it were placed in the dry.
Aggregate role :
But, you may be wondering « If water plus cement equals hard, what’s the need for the aggregate ? ».
To answer that question, let’s take a closer look by cutting this sample through with a diamond blade. Under a macro lense, is tarts to become obvious how the individual constituents contribute to the concrete.
Aggregates for Concrete
Notice how the cement paste filled the gaps between the fine and coarse aggregate. It serves as a blinder, holding the other ingredients together.
You don’t build structures from pure cement the same way you don’t build furniture exclusively out of wood glue.
Instead we use cheaper filler materials – gravel and sand – to make up the bulk of concrete’s volume. This saves cost, but the agregates also improve the structural properties of the concrete by increasing the strenght and reducing the amount of shrinkage as the concrete cures.
The reason that civil engineers and concrete professionales need to be pedantic about the difference between cement and concrete is this : even though the fundamental recipe for concrete is fairly simple with its four ingredients, there is a trmendous amount of complexity involved in selecting the exact quatities and characteristics of those ingredients.
In fact, the process of developing a specific concrete formula is called mix design. One of the most obvious knobs that you can turn on a mix design is how much water is inluded. Obviously, the more water you add to your concrete, the easier if flows into the forms. This can make a big difference to the people who are placing it. But, this added workability comes at a cost to the concret’s strenght.
Water towers are used as a local source of water at times of peak demand where it would not be economical to increase the size of the supply pipeline and add a booster pump installation.
In undu-lating terrain ground-level storage can provide the pressure needed but in areas of flat topography the storage must be elevated. Many shapes and design features are possible but the designer should aim to produce a structure that meets the requirements of both water supply and planning authorities, bearing in mind that it will become a landmark in the community which it serves.
Ancillary equipment including pipework, valves, ladders, instrumentation and booster pumps, if required, can all be hidden in the cylindrical shaft.
The optimum depth/diameter ratios should be determined taking into account the most efficient shape and the needs of the distribution system. It is usually advisable to avoid large pressure fluctuations in distribution that may be caused by draw down or filling in excessively deep tanks.
The main types of water towers are:
1- Concrete water towers:
Concrete water towers are built with capacities up to about 5000 m3. They are usually circular in plan although rectangular concrete towers have been built. The diameter of circular water towers is not usually sufficient to warrant the use of prestressing since cracks can be controlled by applying normal water retaining concrete criteria.
Concrete water towers allow some scope for architectural statement so that the result can be regarded as a visual asset.
Reinforced concrete water tower
Rectangular water towers are designed as small monolithic service reservoirs with the floor slab supported on some form of open column and beam framework or on a hollow vertical shaft, it self founded on a base slab, piled if necessary.
Wind and seismic loads should be taken into account in the design of tank, supports and foundations. Circular concrete water towers allow more scope for different styling from a simple cylinder with a flat base to a sophisticated form such as the hyperbolic-paraboloid of the 39 m high Sillogue tower near Dublin airport built in 2006.
Sillogue Reservoir
In this case the vase shape resembles an inverted version of the nearby control tower. The Intze type water tower (Rajagopalan, 1990) is designed so that bending moments are as near zero as possible at all sections.
Reinforced concrete water tower (Intze type)
2- Welded Steel Water Towers
Relatively small welded tanks have been used for over 100 years for industry and rail transport.These were usually small radius cylinders supported on a framework of steel columns with braces or ties.
Welded steel water towers of capacities up to 15 000 m3are now available and have been widely used all over the world, particularly in North America, the Middle East and the Far East.
These are now constructed of butt welded steel plate in several configurations:spheroids or ellipsoids on tubular columns belled out at the base; cylindrical or spherical shapes with conical bases and supported on wide steel columns which help resist seismic loads and provide space for plant rooms or offices or on a reinforced concrete frame.
Whilst the forms available for welded steel water towers do not offer much scope for architectural treatment, the coatings provide an opportunity for decoration and can be attractive.
Welded Steel Water Towers
3- Segmental Plate Tanks
The type of steel or GRP panel construction can also be used for elevated storage. However, it is unlikely that segmental plate tanks would be used for anything other than industrial or emergency water storage since their poor visual appearance is exaggerated by height.
Where they are used, the bases are placed on a series of beams which are supported on a framework of braced columns.
GPS is a product of the Cold War. Developed by the US military during the President Reagan years, this system consists of a series of 24 satellites in geosynchronous orbit. That is, these satellites remain in the same fixed location in the sky. Out of 360 degrees of longitude, each satellite covers a 15-degree sweep of the globe. They orbit the earth twice each day, broadcasting a timing signal. These signals can be intercepted by ground antennae mounted on ships, tanks, planes—or the buckets and blades of heavy earthmoving equipment.
Though, like all broadcast signals travelling at the speed of light, there is a small-but-measurable time lag between the signals from adjacent satellites. The difference, measured in milliseconds, allows for the triangulation between the ground antennae and the satellites emitting the signal. This triangulation measurement allows for the measurement of the precise location (measured in latitude, longitude, and elevation above sea level) on the surface of the earth where the receiving antennae is currently located. As is true so often in the history of technology, a technical advancement meant for use in war has been modified and adopted for peaceful civilian applications. A system intended to track the movements of men, weapons, ships, and war planes is now used to follow the movements of commercial shipments, find lost hikers, and guide construction equipment.
GPS guides equipment operations through their Automated Positioning Report System (APRS). Using triangulation between the several of the system’s broadcasting satellites allows for positional measurements with accuracies up to 30 centimeters (1 foot). The use of APRS increases this accuracy to 1 centimeter when used in conjunction with GPS. What APRS does is integrate GPS with the equipment’s controls. An APRS replaces the manually operated hydraulic drive cylinders traditionally used to control the movements of an excavator’s arm or dozer’s blade with electronically controlled servo-type valves. These servos send an electric current that creates magnetic field that rotate suspended armatures, that are further connected to fixed flapper arms. These flapper arms provide the linkage to the rotary spools that increase and decrease hydraulic pressure to the hydraulic systems. These are part of the closed-loop hydraulic system that controls the direction, flow rate, and applied pressure of the hydraulic fluid.
Going from individual pieces to equipment to an entire fleet of equipment or trucks is also easily managed by GPS applications. This allows a fleet owner to coordinate and choreograph the movements and activities of an entire fleet of earthmoving equipment and also track the locations of trucks delivering material to the site or hauling dirt away for disposal. Like pieces on a chessboard (or, more accurately, objects in a video game) these activities are tied to an onsite digital terrain model (DTM) of the project site.
This is a three-dimensional (3D) model created by an AutoCAD program, which utilizes a patchwork of connected triangles. The corners of each triangle is defined mathematically by three special coordinates (northing, easting, and elevation). While not a perfect match (no model ever is) to the actual terrain, this geometric surface comes the closes to matching actual surfaces. This is especially true for post construction or excavation surfaces, which tend to be regular and smooth.
Leica excavator machine control solution
The software interacts with the model and the equipment hardware via sensors attached to the business end of the machine (the edge of the dozer blade, the teeth of an excavator bucket, etc.). These sensors continuously record and update the movements of the equipment using the same 3D location system as the DTM. The sensors relay their current location back through the system to the operating controls, which in turn direct the movement of the equipment in accordance with the programmed DTM for the proposed construction surface or excavation grades.
Coordinating all of the mobile GPS sensors on each piece of equipment is a stationary GPS sensor combined with an antenna with a receiver called the base station that is set up adjacent to the operating area. The base station is permanently located over a pre-surveyed reference point, such as a third order benchmark that has been established by ground survey. If necessary, some relative location (manhole rims, street curbs, and building corners, etc.) whose elevation is not exactly known but can be treated as a local datum for the project area can also be utilized for ground antennae setup.
Grader GPS Control
The hardware for these control signals consists of a control box connected to the servo valves via electrical cable, which, in turn, connects to the hydraulic control system that physically moves the equipment. It is the incoming satellite data from the GPS system that tells the equipment where it is. The DTM design files are stored in a compact-flash memory card, memory stick, or accessed externally from data broadcast by the site’s controlled area network (CAN). The database, CAN, and GPS operate in real time to place a blade or bucket exactly where it needs to go and move it so that it accomplishes its task with the need for rework or wasted effort. All three elements, mobile sensors, base sensors, and equipment operator are in continuous communication with each other. The blades and buckets simultaneously move back and forth, up, and down in combination to achieve the desired movement.
Where and When Is GPS Best Used?
The advantages of using GPS guidance systems are legion. Using advanced systems, an operator can increase productivity by greater than 50% compared to purely manual operations. The increase in productivity comes indirectly by the avoidance of having to perform rework at the site. Guided by GPS, an equipment operator can get his cuts and placements right the first time. Furthermore, material wastage is minimized. GPS doesn’t necessarily increase the number of productive hours per day so much as it makes every hour a machine is active much more productive.
This has all sorts of secondary benefits and cost savings. Getting the most out his earthmoving equipment allows a contractor to get the most out of his workforce as well. This reduces labor costs, another costs savings, while reducing the impact of local labor shortages—always an issue in a booming economy with a considerable amount of construction activity. Furthermore, grades and elevations can be checked in real time as the work is being performed. The operator can check his elevation from within his cab as he is working. There is no more need to stop work at regular intervals and have a manual survey performed of the work zone to check its accuracy. In the past, the accuracy-checking task has traditionally relied on a crewmember inside the trenches to manually check depths and slopes. Now with these systems, the need for that task is greatly reduced—thereby cutting job site costs and improving crew safety. This traditional cause of equipment downtime is mostly avoided with the use of GPS.
Giving operators the tools to perform accuracy checks also gives greater responsibility, initiative, and job satisfaction. The additional training that the operators receive has the effect of empowering them, increasing senses of purpose and self-worth. This alone has a marked, if indirect, impact on productivity. By eliminating rework and improving employee morale, any money spent training operators on GPS is a wise investment that pays for itself very quickly. Human factors also show improvement in the area of site safety. The precision guidance and ability to choreograph equipment movement across a busy site improves safety by maintaining safe work zones, avoiding known utility locations, preserving the foundations of existing structures, and maintaining a safe flow of traffic.
GPS systems are often augmented by laser guidance for precise finish work. Lasers can compensate for some of GPS limitations. GPS works best with an open sky and no significant overhead blockage. (For example, GPS is not used for tunneling operations.) Laser guidance stations and targets mounted on equipment blades can work in any outdoor situation, with or without overhead blockage from trees and tall buildings. GPS, however has a much greater effective operating range, limited only by the availability of open sky. Lasers systems are usually limited to about 1,500 feet. GPS can allow for the construction of complicated surface models as well as flat, sloping surfaces. Lasers, being line of site instruments, are usually limited to operations on long, flat, or sloping surfaces. (Though they, too, can build surfaces as directed by a 3D design model represented by a surface AutoCAD file.)
Grade Control System
Productivity can be measured in many ways: time savings, labor costs, material costs, fuel costs, quality bonuses, and finish bonuses. It begins as early as the job layout.
With Machine Control and GPS, you don’t have to wait for someone to stakeout the project or weather that permits someone to stakeout. This allows you to get started sooner. Any design change will also benefit as productivity increase due to not having to wait for restaking.
As the operator starts moving material you will see great value in being able to move the correct amount of material, to the correct location, the first time. This, along with only using the exact amount of material, will translate into a productivity savings.
You can also use GPS by having job layouts of the site so the machines or supervisors’ tablets can have the precise storage locations of materials, job trailers, or site boundaries. This can help having the right things in the correct location or, better yet, not in the way, therefore reducing excessive handling of material.
Drilling Wells Methods – Advantages and Disadvantages
There are several different types of drilling methods. Choosing a method depends on many factors including soil type, underground water level and common practice in every country. Cities using groundwater usually depend upon deep wells. These wells have the advantage of tapping deep and extensive aquifers. All deep wells are drilled wells. The successful digging of drilled wells requires special training, experience, tools and equipment.
The main methods of drilling wells are :
1- Rotary Drilling
The principle of rotary drilling is based upon a rotating drill stem made of lengths of drill pipe about 15 feet long. A bit is attached to a heavy stabilizer or drill collar at the end of the column of drill pipe. The extra weight and larger outside diameter of the stabilizer just above the bit helps to maintain a straight drill hole. The drill stem is hollow and has a drilling fluid of either mud or air circulating down the drill stem out through the nozzles in the bit and up along the outside of the drill stem. The rotating action of the bit breaks up the material and the drilling fluid carries the cuttings to the surface where they settle out in a mud tank.
Advantages:
Speed of Drilling: 5 to 7 times faster than cable tool, capable of several hundred feet per day (dependent on geologic material).
Options of Well Design: Screen can be telescoped or attached, separate screens can be installed, filter packing to enhance formation production, down holle casing hammer method.
Grouting: Oversized borehole requires grouting of annular space surrounding casing, most adaptable to various grout placement methods, practical for grout placement thru casing.
Disadvantages:
Cost of Equipment: 5 times as costly as a cable tool or jetting rig, bit cost and tooling more
Maintenance & Support: Much more extensive and costly than cable tool, higher fuel consumption, water truck needed.
2- Cable Tool Drilling
Cable tool drilling, also known as percussion drilling or spudding, is a widely used well drilling method.
Although it is a slower drilling method, the cable tool is less costly and simpler to operate than a rotary drill rig and is suitable for most geologic conditions.
The cable tool operates by raising and dropping a heavy drill string in the drillhole. The drill string, with bit on the bottom and rope socket (or swivel socket) on top, is suspended in the hole with a cable. The cable is threaded over the crown sheave located at the top of the mast, down to the walking beam, and onto the cable drum where it is stored.
The up-and-down drilling action imparted to the drill stem and cable by the walking beam. The walking beam is pivoted at one end, has a cable sheave at the other end and is connected to the crank gear with a pitman. Rotation of the crank gear causes the walking beam to move up and down. Additional cables called sand lines or casing lines are used to raise and lower casing, bailers, plungers, or other tools.
Advantages:
Inexpensive Equipment: 1/5 cost of rotary rig, less grouting equipment needed, large water truck unnecessary, lower fuel consumption, lower operating cost.
Limited Tooling Required: Bits can be resurfaced, less expensive tooling, used items readily available.
Less Material Removed During Drilling: Generally, no oversized borehole, material removed from casing inside diameter, lighter soils can be bailed from casing.
Repair Work: Cable tool rigs ideal for casing reaming, screen replacement, and
Disadvantages:
Slow Drilling Speed: Bedrock drilling – 1/7 as fast as rotary drilling, Glacial drift drilling – 1/5 as fast as rotary drilling
Depth Limitations with Single Casing String: Driving generally difficult in caving formations, ability to drive casing is limited by tool weight and ground
Outer Casing Needed for Gravel Packing or Full Length Grouting: 3 to 4 inch larger casing needed to maintain annulus and must be extracated during grouting.
Steel Casing Material Only: PVC casing can not be used unlesss installed in an oversized borehole without driving.
3- Auger Drilling
The auger method utilizes spiral augers, usually in 5 foot lengths. The auger stem is turned by a hydraulically-controlled rotary drive head. After drilling the length of an auger, the auger joint is broken and another 5 foot section is added.
Cuttings spiral their way up to the surface where they appear around the borehole, making formation identification relatively simple. If enough clay is present in the formation, the drillhole will remain open when augers are removed.
Dry sands and other caving formations may be a problem for the auger driller and will occasionally result in the loss of long flights of augers. When the auger encounters saturated sand (the water bearing formation to be screened), drilling generally can be continued for a short distance but the hole will not remain open in the saturated formation when the augers are removed.
The auger flight is then broken down and removed from the drillhole after drilling the depth of the well or when changing to another type of drilling operation.
Advantages:
Speed of Drilling: Fast for shallow holes without cobbles or gravel and with low water table, auger/cable tool or jetting combination rigs are common
Limited Equipment: Less expensive than rotary, minimal amount of equipment
Disadvantages:
Limited Depth: Poor results in caving formations, gravel, or high water table, less than 100 feet.
4- Hand Driving
Driven wells are common in many areas, especially around lakes where groundwater may be close to the surface.
Simple installation methods and the low cost of materials make them attractive to homeowners or cottage owners who wish to install their own water supplies.
However, since the well point and casing are driven into the ground, soil conditions are a major factor in suitability of the site. The site must be generally sandy and free of boulders or bedrock to be suitable for a driven well. Hard clay, silt, and very fine sand are generally difficult to drive through.
The installation of a driven well often begins by augering a hole with a hand auger or posthole digger as far as possible. A drive point, consisting of a reinforced well screen with a steel point on the end, is coupled to a 5 foot length of galvanized casing. The most common casing size for driven wells is 1-1/4 inch inside diameter.
A drive cap is placed an the top of the casing and a heavy weight is used to strike the top of the drive cap, driving the point into the ground, When the drive cap is driven close to the ground and driving cannot be continued, another length of casing is added and driving is resumed.
Special drive couplings are used to join sections of casing, Hand driving is usually accomplished by using a weighted driver consisting of a 3 or 4 foot piece of 3 inch diameter pipe capped on the top end. Extra weight is placed in the top portion of the driver. The driver fits over the casing and is guided by it. Another type of driver has a steel rod on the bottom that slides into the casing through a hole in the drive cap.
Raising and dropping the driver is done with the use of handles welded on the sides of the driver. The weighted driver may also be suspended from a tripod and tackle arrangement. The use of a sledge hammer for driving is not recommended since it may result in bent or broken casing from glancing blows.
5- Jetting
Jetting is a drilling method suited for the sandy areas. Jetting remains a popular method for drilling small diameter wells due to its simplicity and inexpensive cost of equipment. Many of the portable, do-it-yourself drilling machines advertised in magazines utilize the jetting method.
Jetting and hollow-rod equipment are quite similar except that drilling water is pumped with the jetting method and the direction of water flow is opposite.
The jetting method involves using a high velocity stream of water to break up the formation material and wash the cuttings away. A chisel-shaped bit with holes to serve as nozzles is attached to the end of a string of hollow drill pipe. Water pressure is provided to the nozzles by using a high-pressure pump.
Water exits from the nozzles and loosens the material being drilled while keeping the bit clean. The bit is raised and lowered and rotated slightly to maintain a round hole. The cuttings are washed to the surface on the outside of the drill pipe and flow into a settling pit or tank.
Cutting samples are easily obtained at this point. A 55 gallon drum is often used for this purpose. After cuttings are allowed to settle, the water is recirculated through the pump, swivel, drill pipe and down to the bit. Jetting can also be done without recirculation of the drilling water; however, a continuous supply of water must be available at the site.
6- Hollow-Rod
Drilling Hollow-rod is referred to as the hydraulic-percussion drilling method.
The hollow-rod is an old drilling method that can be time consuming in some situations, but remains popular due to its simplicity and relatively low cost of equipment.
Most hollow-rod wells are 2 inch diameter, but 4 inch casings are installed occasionally. This method is well suited for sand and soft clay formations with relatively few boulders. It can also be used for drilling rock wells, but progress is slowed considerably. Wells several hundred feet in depth have been completed by the hollow-rod method.
The drill string used in hollow-rod drilling is similar to that used in jetting, except that the chisel bit has a ball check valve in it. Water or a clay-water mixture is kept in the annular space between the drill rods and well casing to help prevent the uncased portion of the hole from collapsing.
The water is supplied to the annulus by gravity intake from a small mud tank. A 55 gallon drum is often used as a settling tank. Drilling is done by lifting and dropping the drill stem and bit. The drill pipe used has triple wall thickness to add weight to the drill string.
The drill string is also rotated slightly by hand during each stroke to maintain a round drill hole. As the bit drops, the ball check opens and mud and cuttings enter the hollow drill rods. On the upstroke, the check valve closes and keeps the cuttings in the drill rods.
Advantages:
Inexpensive Equipment: 1/5 cost of rotary rig, less grouting equipment needed, large water truck unnecessary, lower fuel consumption, lower operating cost.
Limited Tooling Required: Bits can be resurfaced, less expensive tooling, used items readily available, many tools
Less Material Removed During Drilling: Generally no oversized borehole, material removed from casing inside diameter.
Repair Work: Jetting rigs ideal screen replacement and development.
Disadvantages:
Slow Drilling Speed: Bedrock drilling – uncommon, requires heavy drill bar, 1/7 as fast as rotary drilling, Glacial drift drilling – 1/5 as fast as rotary drilling, limited use in gravel formations.
Depth Limitations with Single Casing String: Driving generally difficult in caving formations, ability to drive casing is limited by tool weight and ground
Outer Casing Needed for Gravel Packing or Full Length Grouting: 3 to 4 inch larger casing needed to maintain annulus and must be extracated during grouting.
Steel Casing Material Only: PVC casing can not be used unlesss installed in an oversized borehole without driving.
Since wide scale coal firing for power generation began in the1920s, many millions of tons of ash and related by-products have been generated. The current annual production of coal ash world-wide is estimated around 600 million tones, with fly ash constituting about 500 million tones at 75–80% of the total ash produced.
Thus, the amount of coal waste (fly ash), released by factories and thermal power plants has been increasing throughout the world, and the disposal of the large amount of fly ash has become a serious environmental problem. The present day utilization of ash on worldwide basis varied widely from a minimum of 3% to a maximum of 57%, yet the world average only amounts to 16% of the total ash.
Fly ash is generally grey in color, abrasive, mostly alkaline, and refractory in nature. Pozzolans, which are siliceous or siliceous and aluminous materials that together with water and calcium hydroxide form cementitious products at ambient temperatures,are also admixtures.
Fly ash from pulverized coal combustion is categorized as such a pozzolan. Fly ash also contains different essential elements, including both macro nutrients P, K, Ca, Mg and micro nutrients Zn, Fe, Cu, Mn, B, and Mo for plant growth. The geotechnical properties of fly ash (e.g., specific gravity, permeability,internal angular friction, and consolidation characteristics) make it suitable for use in construction of roads and embankments, structural fill etc.
The pozzolanic properties of the ash, including its lime binding capacity makes it useful for the manufacture of cement, building materials concrete and concrete-admixed products.
Fly Ash Types :
There are two common types of fly ash: Class F and Class C.
Class F fly ash contain particles covered in a kind of melted glass. This greatly reduces the risk of expansion due to sulfate attack, which may occur in fertilized soils or near coastal areas. Class F is generally low-calcium and has a carbon content less than 5 percent but sometimes as high as 10 percent.
Class C fly ash is also resistant to expansion from chemical attack. It has a higher percentage of calcium oxide than Class F and is more commonly used for structural concrete. Class C fly ash is typically composed of high-calcium fly ashes with a carbon content of less than 2 percent.
Currently, more than 50 percent of the concrete placed in the U.S. contains fly ash.2 Dosage rates vary depending on the type of fly ash and its reactivity level. Typically, Class F fly ash is used at dosages of 15 to 25 percent by mass of cementitious material, while Class C fly ash is used at dosages of 15 to 40 percent.3
Fly Ash Applications :
Utilization of fly ash appears to be technically feasible in the cement industry. There are essentially three applications for fly ash in cement
(1) replacement of cement in Portland cement concrete
(2) pozzolanic material in the production of pozzolanic cements
(3) set retardant ingredient with cement as a replacement of gypsum
Cement is the most cost and energy intensive component of concrete. The unit cost of concrete is reduced by partial replacement of cement with fly ash.
The utilization of fly ash is partly based on economic grounds as pozzolana for partial replacement of cement, and partly because of its beneficial effects, such as, lower water demand for similar workability, reduced bleeding, and lower evolution of heat.
It has been used particularly in mass concrete applications and large volume placement to control expansion due to heat of hydration and also helps in reducing cracking at early ages.
The major drawback of fibre reinforced concrete is its low workability. To overcome this shortcoming, a material is needed, which can improve the workability without comprising strength.
The use of fly ash in concrete enhances the workability of concrete and being widely recommended as partial replacement of cement. This also reduces the cost of construction. Fly ash concrete provides much strong and stable protective cover to the steel against natural weathering action.
Stone column method for ground improvement is a vibro-replacement technique, where the weak soil is displaced using a cylindrical vibrating probe (i.e. vibroflot), thus creating a column that is then filled and compacted with good-quality stone aggregates.
With the inclusion of stone aggregates to the in situ soil, its stiffness and load-carrying capacity increases. It also helps to reduce the static as well as differential settlement of the soils.
Bulging action of the stone columns imparts lateral confinement to the surrounding soils and it also acts as a drainage path accelerating the consolidation of cohesive soils.
These stone columns are generally used for soils that are much more compressible but not weak enough to necessitate a pile foundation. Moreover, for the construction of low-to-medium rise buildings on soft soils, pile foundation sometimes becomes expensive. In such cases, stone columns are preferred.
Stone columns are very useful for the improvement of cohesive soils, marine/alluvialclays, and liquefiable soils. Stone columns have been used successfully for a widerange of applications from the construction of high-rise buildings to oil tank foundation, and for embankment and slope stabilization.
Stone column installation methods
For the installation of stone columns a vibrating poker device is used that can penetrate to the required treatment depth under the action of its own weight, vibrations, and actuated air, assisted by the pull-down winch facility of the rig.
This process displaces the soil particles and the voids created are compensated with backfilling of stone aggregates. The vibroflot penetrates the filled stone aggregates to compact it and thusforces it radially into the surrounding soils.
This process is repeated till the full depth of the stone column is completed. The lift height is generally taken as 0.6–1.2 m for the filling and compaction of the stone aggregates.
Depending upon the feeding of stone aggregates into the columns there are basically two methods for the installation of stone columns:
1- Top-feed method
In the top-feed method, the stone aggregates are fed into the top of the hole. The probe is inserted into the ground and is penetrated to the target depth under its own weight and compressed air jetting. However, jetting of water is also done especially when the soil is unstable. This also helps to increase the diameter of the stone columns and to washout the fine materials fromthe holes.
The top-feed method is suitable when water is readily available and there is enough working space to allow for water drainage. Moreover, the soil types should be such that it would not create messy surface conditions due to mud in water.
The top-feed method is preferable when a deeper groundwater level is encountered.
Stone Columns installation Top-feed Method
2- Bottom-feed method
The bottom-feed method involves the feeding of stone aggregates via a tremie pipe along the vibroflot and with the aid of pressurized air. The bottom-feed method is preferable when the soil is highly collapsible and unstable. However, the stability of holes will also depend upon the depth, boundary conditions, and the groundwater conditions. In areas, where the availability of water and space and the handling of mudin process water are limiting factors, the bottom-feed method can be implemented.
Due to limited space in the feeding system, a smaller size of aggregates is used inthe bottom-feed method compared with that used in the top-feed method. On the otherhand, the flow of stones to the column is mechanically controlled and automatically recorded in the bottom-feed method.
The purpose of the tie is to cushion and transmit the load of the train to the ballast section as well as to maintain gage.
Wood and even steel ties provide resiliency and absorption of some impact through the tie itself. Concrete ties require pads between the rail base and tie to provide a cushioning effect.
Ties are typically made of one of four materials:
Timber
Concrete
Steel
Alternative materials
1. Timber Ties
It is recommended that all timber ties be pressure-treated with preservatives to protect from insect and fungal attack. Hardwood ties are the predominate favorites for track and switch ties.
Bridge ties are often sawn from the softwood species. Hardwood ties are designated as either track or switch ties.
Factors of first importance in the design and use of ties include durability and resistance to crushing and abrasion. These depend, in turn, upon the type of wood, adequate seasoning, treatment with chemical preservatives, and protection against mechanical damage. Hardwood ties provide longer life and are less susceptible to mechanical damage.
Hardwood Track Ties
2. Concrete Ties
Concrete ties are rapidly gaining acceptance for heavy haul mainline use, (both track and turnouts), as well as for curvature greater than 2°. They can be supplied as crossties (i.e. track ties) or as switch ties. They are made of pre-stressed concrete containing reinforcing steel wires. The concrete crosstie weighs about 600 lbs. vs. the 200 lb. timber track tie.
The concrete tie utilizes a specialized pad between the base of the rail and the plate to cushion and absorb the load, as well as to better fasten the rail to the tie. Failure to use this pad will cause the impact load to be transmitted directly to
the ballast section, which may cause rail and track surface defects to develop quickly.
An insulator is installed between the edge of the rail base and the shoulder of the plate to isolate the tie (electrically). An insulator clip is also placed between the contact point of the elastic fastener used to secure the rail to the tie and the contact point on the base of the rail.
Concrete Track Ties
3. Steel Ties
Steel ties are often relegated to specialized plant locations or areas not favorable to the use of either timber or concrete, such as tunnels with limited headway clearance. They have also been utilized in heavy curvature prone to gage widening. However, they have not gained wide acceptance due to problems associated with shunting of signal current flow to ground.
Some lighter models have also experienced problems with fatigue cracking.
Steel Track Ties
4. Alternative Material Ties
Significant research has been done on a number of alternative materials used for ties. These include ties with constituent components including ground up rubber tires, glued reconstituted ties and plastic milk cartons.
Appropriate polymers are added to these materials to produce a tie meeting the required criteria. To date, there have been only test demonstrations of these materials or installations in light tonnage transit properties. It remains to be seen whether any of these materials will provide a viable alternative to the present forms of ties that have gained popularity in use.
The list of potential causes of concrete failures is a long one. A few examples include chemical reactions, shrinkage, weathering, and erosion. Many other potential causes exist, and we will explore them individually. Understanding the causes of concrete structure dam-age is an important element in the business of rehab and repair work.
1. UNINTENTIONAL LOADS
Unintentional loads are not common, which is why they are accidental. When an earth-quake occurs and affects concrete structures, that action is considered to be an accidental loading. This type of damage is generally short in duration and few and far between in occurrences.
Visual inspection will likely find spalling or cracking when accidental loadings occur. How is this type of damage stopped? Generally speaking, the damage cannot be prevented, because the causes are unexpected and difficult to anticipate. For example, an engineer is not expecting a ship to hit a piling for a bridge, but it happens. The only defense is to build with as much caution and anticipation as possible.
2. CHEMICAL REACTIONS
Concrete damage can occur when chemical reactions are present. It is surprising how little it takes for a chemical attack on concrete to do serious structural damage.
Examples of chemical reactions and how they affect concrete:
a- Acidic Reactions
Most people know that acid can have serious reactions with a number of materials, and concrete is no exception. When acid attacks concrete, it concentrates on its products of hydration. For example, calcium silicate hydrate can be adversely affected by exposure to acid.
Sulfuric acid works to weaken concrete and if it is able to reach the steel reinforcing members, the steel can be compromised. All of this contributes to a failing concrete structure.
Visual inspections may reveal a loss of cement paste and aggregate from the matrix. Cracking, spalling, and discoloration can be expected when acid deteriorates steel reinforcements, and laboratory analysis may be needed to identify the type of chemical causing the damage.
b- Aggressive Water
Aggressive water is water with a low concentration of dissolved minerals. Soft water is considered aggressive water and it will leach calcium from cement paste or aggregates. This is not common in the United States. When this type of attack occurs, however, it is a slow process. The danger is greater in flowing waters, because a fresh supply of aggressive water continually comes into contact with the concrete.
If you conduct a visual inspection and find rough concrete where the paste has been leached away, it could be an aggressive-water defect. Water can be tested to determine if water quality is such that it may be responsible for damage. When testing indicates that water may create problems prior to construction, a non-Portland-cement-based coating can be applied to the exposed concrete structures.
c- Alkali-Carbonate Rock Reaction
Alkali-carbonate rock reaction can result in damage to concrete, but it can also be beneficial. Our focus is on the destructive side of this action, which occurs when impure dolomitic aggregates exist. When this type of damage occurs, there is usually map or pattern cracking and the concrete can appear to be swelling.
Alkali-carbonate rock reaction differs from alkali-silica reaction because there is a lack of silica gel exudations at cracks. Petrographic examination can be used to confirm the presence of alkali-carbonate rock reaction. To prevent this type of problem, contractors should avoid using aggregates that are, or are suspected to be, reactive.
d- Alkali-Silica Reaction
An alkali-silica reaction can occur when aggregates containing silica that is soluble in highly alkaline solutions may react to form a solid, non expansive, calcium-alkali-silica complex or an alkali-silica complex that can absorb considerable amounts of water and expand. This can be disruptive to concrete.
Alkali-silica reaction in concrete
Concrete that shows map or pattern cracking and a general appearance of swelling could be a result of an alkali-silica reaction. This can be avoided by using concrete that contains less than 0.60% alkali.
e- Various Chemical Attacks
Concrete is fairly resistant to chemical attack. For a substantial chemical attack to have degrading effects of a measurable nature, a high concentration of chemical is required. Solid dry chemicals are rarely a risk to concrete. Chemicals that are circulated in contact with concrete do the most damage.
When concrete is subjected to aggressive solutions under positive differential pressure, the concrete is particularly vulnerable. The pressure can force aggressive solutions into the matrix. Any concentration of salt can create problems for concrete structures. Temperature plays a role in concrete destruction with some chemical attacks. Dense concrete that has a low water–cement ratio provides the greatest resistance.
The application of an approved coating is another potential option for avoiding various chemical attacks.
f- Sulfate Situations
A sulfate attack on concrete can occur from naturally occurring sulfates of sodium, potassium, calcium, or magnesium. These elements can be found in soil or in ground water. Sulfate ions in solution will attack concrete. Free calcium hydroxide reacts with sulfate to form calcium sulfate, also known as gypsum. When gypsum combines with hydrated calcium aluminate it forms calcium sulfoaluminate.
Either reaction can result in an increase in volume. Additionally, a purely physical phenomenon occurs where a growth of crystals of sulfate salts disrupts the concrete. Map and pattern cracking are signs of a sulfate attack. General disintegration of concrete is also a signal of the occurrence.Sulfate attacks can be prevented with the use of a dense, high-quality concrete that has a low water–cement ratio. A Type V or Type II cement is a good choice.
If pozzolan is used, a laboratory evaluation should be done to establish the expected improvement in performance.
g- Poor Workmanship
Poor workmanship accounts for a number of concrete issues. It is simple enough to follow proper procedures, but there are always times when good practices are not employed. The solution to poor workmanship is to prevent it. This is much easier said than done. All sorts of problems can occur when quality workmanship is not assured and some of the key causes for these problems are as noted below:
Adding too much water to concrete mixtures
Poor alignment of formwork
Improper consolidation
Improper curing
Improper location and installation of reinforcing steel members
Movement of formwork
Premature removal of shores or reshores
Settling of concrete
Settling of subgrade
Vibration of freshly placed concrete
Adding water to the surface of fresh concrete
Miscalculating the timing for finishing concrete
Adding a layer of concrete to an existing surface
Use of a tamper
Jointing
3. CORROSION
Corrosion of steel reinforcing members is a common cause of damage to concrete. Rust staining will often be present during a visual inspection if corrosion is at work. Cracks in concrete can tell a story. If they are running in straight lines, as parallel lines at uniform intervals that correspond with the spacing of steel reinforcement materials, corrosion is prob-ably at the root of the problem. In time, spalling will occur. Eventually, the reinforcing mate-rial will become exposed to a visual inspection.
Techniques for stopping, or controlling, corrosion include the use of concrete with low permeability. In addition, good workmanship is needed. Some tips to follow include:
Use as low a concrete slump as
Cure the concrete
Provide adequate concrete cover over reinforcing
Provide suitable
Limit chlorides in the concrete
Pay special attention to any protrusions, such as bolts and
4.FREEZING AND THAWING
Freezing and thawing during the curing of concrete is a serious concern. Each time the concrete freezes, it expands. Hydraulic structures are especially vulnerable to this type of damage. Fluctuating water levels and under-spraying conditions increase the risk. Using deicing chemicals can accelerate damage to concrete with resultant pitting and scaling. Core samples will probably be needed to assess damage.
Prevention is the best cure. Provide adequate drainage, where possible, and work with low water–cement ratio concrete. Use adequate entrained air to provide suitable air-void systems in the concrete. Select aggregates best suited for the application, and make sure that concrete cures properly.
5. SETTLEMENT AND MOVEMENT
Settlement and movement can be the result of differential movement or subsidence. Concrete is rigid and cannot stand much differential movement. When it occurs, stress cracks and spall are likely to occur. Subsidence causes entire structures or single elements of entire structures to move. If subsidence is occurring, the concern is not cracking or spalling; the big risk is stability against overturning or sliding.
A failure via subsidence is generally related to a faulty foundation. Long-term consolidations, new loading conditions, and related faults are contributors to subsidence. Geotechnical investigations are often needed when subsidence is evident.
Cracking, spalling, misaligned members, and water leakage are all evidence of structure movement. Specialists are normally needed for these types of investigations.
6. SHRINKAGE
Shrinkage is caused when concrete has a deficient moisture content. It can occur while the concrete is setting or after it is set. When this condition happens during setting, it is called plastic shrinkage; drying shrinkage happens after concrete has set.
Plastic shrinkage is associated with bleeding, which is the appearance of moisture on the surface of concrete. This is usually caused by the settling of heavier components in a mixture. Bleed water typically evaporates slowly from the surface of concrete.
Concrete Shrinkage
When evaporation occurs faster than water is supplied to the surface by bleeding, high-tensile stresses can develop. This stress can lead to cracks on the concrete surface.Cracks caused by plastic shrinkage usually occur within a few hours of concrete placement.
These cracks are normally isolated and tend to be wide and shallow. Pattern cracks are not generally caused by plastic shrinkage.
Weather conditions contribute to plastic shrinkage. If the conditions are expected to be conducive to plastic shrinkage, protect the pour site with windbreaks, tarps, and similar arrangements to prevent excessive evaporation. In the event that early cracks are discovered, revibration and refinishing can solve the immediate problem. Drying shrinkage is a long-term change in volume of concrete caused by the loss of moisture.
A combination of this shrinkage and restraints will cause tensile stresses and lead to cracking. The cracks will be fine and the absence of any indication of movement will exist. The cracks are typically shallow and only a few inches apart. Look for a blocky pattern to the cracks.
They can be confused with thermally induced deep cracking, which occurs when dimensional change is restrained in newly placed concrete by rigid foundations or by old lifts of concrete.
To reduce drying shrinkage, try the following precautions:
Use less water in
Use larger aggregate to minimize paste
Use a low temperature to cure concrete
Dampen the subgrade and the concrete
Dampen aggregate if it is dry and
Provide adequate
Provide adequate contraction joints
7. FLUCTUATIONS IN TEMPERATURE
Fluctuations in temperature can affect shrinkage. The heat of hydration of cement in large placements can present problems. Climatic conditions involving heat also affect concrete; for example, fire damage, while rare, can also contribute to problems associated with excessive heat.
There are several types of TBMs. The best TBM for a project is based on the geological conditions of the site and the project’s features.The general classification of the different types of TBMs for both hardrock and soft ground are presented here.
Fig 1. TBM’s Classification
1. Gripper Tunnel Boring Machine
A gripper TBM is a rock tunnel boring machine which generally utilizes roller disc cutters as excavation tools and which moves forward by reacting (i.e., exerting shove forces) against the tunnel walls through a hydraulic gripper reaction system.
It is is suitable for driving in hard rock conditionswhen there is no need for a final lining. The rock supports (rock anchors, wire-mesh, shotcrete, and/or steel arches) can be installed directly behindthe cutter head shield and enable controlled relief of stress and deformations.
The existence of mobile partial shields enables gripper TBMs to be flexible even in high-pressure rock. This is useful when excavating in expanding rock to prevent the machine from jamming.
Fig 2. Typical diagram of an open gripper main beam TBM.Courtesy of TheRobbins Company
2. Double-Shield Tunnel Boring Machine
A double-shield TBM is generally considered to be the fastest machine for hard rock tunnels under favorable geological conditions with installation of the segment lining. It is possible to drive 100 m in 1 day. This type of TBM consists of a rotating cutter head and double shields (Fig. 3), a telescopic shield (an inner shield that slides within the large router shield), and a gripper shield together with a shield tail.
While boring, gripper shoes radially press against the surrounding rock to hold the machine in place and take some of the load from the thrust cylinders. For the motion of the front shield the gripper shoes are loosened, before the front shield is pushed forward by thrust cylinders protected by the extension of the telescopic shield.
Because regripping is a fast process, double-shield TBMs can almost continuously drill. As for the shield tail, it is used to provide protection for workers while erecting,installing the segment lining, and pea grouting.
Fig 3. Typical diagram of a double-shield TBM.Courtesy of The Robbins Company
3. Single-Shield Tunnel Boring Machine
Single-shield TBMs (Fig. 4) are used in soils that do not bear ground-water and where rock conditions are less favorable than for double shields, such as in weak fault zones. The shield is usually short so that a small radius of curvature can be achieved.
Fig 4. Typical diagram of single-shield TBM.Courtesy of The Robbins Company
4. Earth Pressure Balance Machines
Earth pressure balance (EPB) technology (Fig. 5) is suitable for digging tunnels in unstable ground such as clay, silt, sand, or gravel. An earth paste face formed by the excavated soil and other additives supports the tunnel face. Injections containing additives improve the soil consistency, reduce soil stick, and thus its workability.To ensure support pressure transmission to the soil, the earth paste is pressurized through the thrust force transfer into the bulkhead.
The TBM advance rate (in flow of excavated soil) and the soil outflow from the screw conveyor regulate the support pressure at the tunnel face. This is monitored at the bulkhead by the readings of pressure sensors.
Fig 5. Scheme of an EPB machine
5. Slurry Tunnel Boring Machine
Slurry TBMs are used for highly unstable and sandy soil and when the tunnel passes beneath structures that are sensitive to ground disturbances. Pressurized slurry (mostly bentonite) supports the tunnel face. The support pressure is regulated by the suspension inflow and outflow. The slurry’s rheology must be chosen in accordance with the soil parameters and should be carefully and regu-larly monitored.
6. Mixshield Technology
Mixshield technology (Fig. 6) is a variant of conventional slurry technology for heterogeneous geologies and high water pressure. In mixshield technology an automatically controlled air cushion controls the support pressure, with a submerged wall that divides the excavation chamber.This wall seals off the machine against the excess pressure from the tunnel’s face. As air is compressible in nature, the mixshield is more sensitive in pressure control and thus will provide more accurate control of ground settlement.
Fig 6. Overview of a mixshield TBM.Courtesy of Herrenknecht AG
7. Pipe Jacking
Pipe jacking (also called microtunneling) is a micro- to small-scale tunneling method for installing underground pipelines with minimum surface disruption. It is used for sewage and drainage construction, sewer replace-ment and lining, gas and water mains, oil pipelines, electricity and telecommunications cables, and culverts .
A fully automated mechanized tunneling shield is usually jacked forward from a launch shaft toward a reception shaft. Jacking pipes are then progressively inserted into the working shaft. Another significant difference between the pipe jacking method and shield method is that the lining of the pipe jacking is made of tubes and the lining of the shield method is made up of segments. In order to significantly reduce the resistance of the pipes, a thixotro-pic slurry is injected into the outside perimeter of the pipes. The thixo-tropic slurries can also reduce disturbance to the ground while pipe jacking slurry thickness. The thickness should be six to seven times thevoid between the machine and pipes.
Fig 7. Typical Components of a Pipe Jacking Operation
8. Partial-Face Excavation Machine
Partial-face excavation machines (Fig. 8) have an open-face shield andcan sometimes be more economical in homogeneous and semistable ground with little or no groundwater. In boulders layer, the open-face can deal with boulders much easier than closed shield machines. In cavity ground, the open-face can avoid the risk of falling down into the bottom of the cavity. Thanks to their simple design and that the operator workplace is close to the open tunnel face,these machines can easily be adapted to changing geological conditions. Good excavation monitoring can also be carried out.
Fig 8. Two kinds of partial-face excavation machines.Courtesy ofHerrenknecht AG
