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Tunnel Boring Machine Types

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

Soil Nail – Helping Combat Climate Change with Extraordinary Geotechnical Techniques

Pollution caused global warming which threatens our climate, our Earth. There have been significant changes in the behavior of Earth’s top layer and climate.

The risks of climate change require swift and deep reductions in emissions of heat-trapping gases and investments to prepare for now unavoidable impacts. Geo engineering is just one measure to combat the daunting challenge of keeping the rise in global temperatures in check.

Geo engineering or Climate engineering is the intentional large-scale intervention in the Earth’s climate system to counter climate change. It includes techniques like removing CO2 from the atmosphere, and steps to rapidly cool the Earth by reflecting solar energy back to space.

Geotechnical Engineering is a part of geoengineering that involves the application of soil and rock mechanics as well as engineering geology to solve engineering problems. These are design of foundations, slopes, excavations, dams, tunnels and other Civil, Mining and Environmental engineering projects relating to the mechanical response of the ground, and the water within it. It deals with many types of infrastructure – tunnels, bridges, dams, buildings, roads, railways, ports and landfills – that are built on the ground.

Soil nailing and its advantages

Soil nailing implies using grouted, tension-resisting steel elements (nails) to reinforce in situ soils and creating a gravity retaining wall for permanent or temporary excavation support.

Common uses

Stabilize slopes and landslides
Support excavations
Repair existing retaining walls

Advantages

Equipment is small enough to use in areas with restricted access
Often a more cost effective and faster solution for excavation support
Can be installed from crane-suspended working platforms for existing steep slopes, such as bluffs or existing retaining walls
Allows excavation to start at the same time as the shoring system is being installed

What are the main types Of Grout Used In Construction

What Is Grout?

Grout is a thick paste that is used to fill gaps, voids, or joints. It is mainly a composite material used for grouting, repairing concrete cracks, filling gaps and sealing the joints between tiles, waterproofing, soil stabilization, etc.

In this article, I will discuss different types of grout materials used in construction, tile installation, and other works. Differing from mortar, grout has a low viscosity and lack of lime, which makes it thinner and easier to work with.

Types Of Grout:

1. Cementitious Grout:

Cementitious grouts are the traditional grouting material, used both in residential as well as some commercial applications. It is also known as slurry grouting or hydraulic cement grouting. Materials used in cementitious grout are :

Portland cement,
Filler particles of different sizes,
Water-retentive additive,
Colored pigments.

Cementitious grouts are available in a variety of colors that let you match or contrast with the tile. Water is mixed with the grout and then applied using a trowel. The water retentive ingredient in cementitious grouts slows down the drying time, allowing the cement to cure slowly achieving maximum hardness.

Cementitious grouts are further classified into three types:

Sanded grout
Unsanded /Non-sanded grout
Latex modified grout.

a. Sanded Grout:

Sanded grout is composed of portland cement, sand, and other additives. The sand used is relatively larger in size. It is typically recommended for tile floors where joints are 1/8 inch to 3/8 inch wide. Sand provides extra strength to the grout joints, as it is one of the best building materials.

Sanded grout is absorbent and easily attracts dirt, therefore it is always better to seal the joint when grouting is done. As sand easily makes scratches, this type of grout should not be used on easily scratched tile like marble.

Sanded tile grout is less costly than unsanded grout because sand is a cheaper filler than polymers. It also provides a tight lock and a neat & clean finish.

Sanded grouts are ideal for use in Kitchen floor Bathroom floor Entryways Shower pan etc.

b. Unsanded Grout:

Unsanded grout is made of portland cement and color powder pigments. No sand is used here hence also known as non-sanded grout.

This type of grout is suitable for joint thickness between 1/8 to 1/16 inches. Unlike sanded grout, it is useful in scratchable surfaces such as metal, glass, marble, and natural stone tile.

Unsanded grout provides a much smoother texture since the mineral particles present in it are very fine powders without having grit. However, these grouts easily develop cracks due to lack of binding power.

Unsanded grout is costly than sanded and you may have limited color options. Unsanded grouts are suitable for use in Rectified tiles, Polished stones. Bathroom walls, Shower walls etc.

c. Latex Modified Grout:

Sanded grouts may be composed of a latex polymer additive which increases the strength and water-proofing properties of grout. The additives can be mixed in both dry and wet conditions.

2. Chemical Grouts:

Chemical grouts consist of polymers such as epoxy, acrylic, polyurethane, sodium silicate, or any other suitable polymer. It requires injection of chemical grouts into finer cracks that are not groutable by cementitious grouts. Some useful types of chemical grouts are discussed below.

a. Epoxy Grout:

Epoxy grout is consists of epoxy resin, silica fillers, pigments, and a hardener. It neither uses Portland cement nor uses water during the mixing process.

Epoxy grout is very strong and durable. Additionally, it is highly resistant to stains, cracks, chemicals attack, harsh weather conditions, and climatic changes.

Epoxy grouts are considerably less porous than cementitious grouts and set up quickly. It is also much costlier than any other type of grout. With light maintenance, epoxy grout can last lifelong if applied correctly.

The strength and other properties make it perfect for any tile work, indoors or outdoors. It is suitable for use in High traffic areas like entryways, hallways, and foyers.

Any type of flooring exposed to harsh conditions like grease and acid. Kitchen counters and backsplash, bathrooms, etc. Keep in mind, porous and unglazed surfaces, such as limestone or quarry tiles need to be sealed before applying epoxy grout, otherwise, it can stain the tile surfaces badly.

Another major disadvantage that it is much more difficult to shape and slope. If not done correctly, it will look plastic.

b. Furan Grout:

Furan grout is similar to epoxy but composed of polymers of fortified alcohols. There is no water used in this type of grout installation. Furan is basically a combination of furan resin and a filler powder with an acid catalyst.

The acid catalyst helps the resin to cure making it a thermosetting resin that has unsurpassed chemical, physical, and thermal resistance.

The tile surfaces may be smooth, abrasive or non-skid hence it should be sealed with wax coating right after furan grout installation to protect from staining. Furan grout is the strongest and most expensive grout material available in the market.

Because of the difficulty of installation, it requires proper precautions and skilled labours. This type of grout is suitable for use in Brick pavers, Quarry tiles. Industrial projects, such as laboratories, factories, dairies, and meat-packing plants. Areas highly exposed to chemicals and/or grease.

c. Acrylic Grout:

Acrylic grout is an acrylic latex admixture composed for use as a substitute for water when grouting ceramic tile. It is specifically produced for use with AccuColor Portland cement grouts. Acrylic grout helps to make joints less susceptible to water penetration.

It is very much essential when grouting wet areas. The additive further helps the grout retaining its color and resisting stains. It gives good stability in freezes and thaws. It also enhances grout flexibility. It has greater adhesion properties. Additionally, you don’t need to cover the entire work surface.

You can just apply it in between tile joints. Another advantage of using acrylic latex grout is that it can be used in small spaces. It is mostly suitable for joint thickness less than 1/8 inch. This type of grout is especially suitable for use in Outdoors such as deck or garage projects. Fast-food restaurant floors. Marble work etc.

Standard Culvert and MEL Culvert Definition

Introduction :

A culvert is a covered channel of relatively short length designed to pass water through an embankment (e.g. highway, railroad and dam).

It is a hydraulic structure and it may carry flood waters, drainage flows, natural streams below earthfill and rockfill structures. From a hydraulic aspect, a dominant feature of a culvert is whether it runs full or not.

The design can vary from a simple geometry (i.e. box culvert) to a hydraulically smooth shape (i.e. minimum energy loss (MEL) culvert)

Culvert Parts :

A culvert consists of three parts: the intake (also called inlet or fan), the barrel (or throat) and the diffuser (also called outlet or expansion fan) (Fig.1-a).

The cross-sectional shape of the barrel may be circular (i.e. pipe), rectangular (i.e box culvert) or multi-cell (e.g. multi-cell box culvert) (Fig.1-b).

The bottom of the barrel is called the invert while the barrel roof is called the soffit or obvert. The training walls of the inlet and outlet are called wing walls.

Fig 1. Sketch of a culvert: (a) box culvert

Fig 1. (b) MEL culvert

Standard Culverts :

A standard culvert is designed to pass waters at a minimum cost without much consideration of the head loss. The culvert construction must be simple: e.g. circular pipes and precast concrete boxes.

MEL Culverts :

An MEL culvert is a structure designed with the concept of minimum head loss. The flow in the approach channel is contracted through a streamlined inlet into the barrel where the channel width is minimum, and then it is expanded in a streamlined outlet before being finally released into the downstream natural channel. Both the inlet and outlet must be streamlined to avoid significant form losses

What are Rivet Connections?

Rivets are non threaded fasteners that are usually manufactured from steel or aluminium. They consist of a preformed head and shank, which is inserted into the material tobe joined and the second head that enables the rivet to function as a fastener is formedon the free end by a variety of means known as setting.

A conventional rivet before and after setting is illustrated in Fig. 1.

Fig.1 Conventional rivet before and after setting

Rivets are widely used to join components in aircraft (e.g. see Fig.2) boilers, ships and boxes and other enclosures. Rivets tend to be much cheaper to install than bolts and the process can be readily automated with single riveting machines capable of installing thousands of rivets an hour.

Fig 2. Two historical examples of the use of rivets on the Lockheed Electra and RB211engine nacelle.

Rivets can be made from any ductile material such as carbon steel, aluminium and brass. A variety of coatings are available to improve corrosion resistance. Care needs to be taken in the selection of material and coating to avoid the possibility of corrosion by galvanic action.

In general a given size rivet will be not as strong as the equivalent threaded fastener.

The two main types of rivet are tubular and blind and each type are available in amultitude of varieties. The advantage of blind rivets (Fig.3) is that they require access to only one side of the joint.

Fig 3. An example of the application of a closed end blind rivet

A further type of rivet with potentially many over-all advantages, from the production perspective, is the self-piercing rivet that does not require a predrilled hole. The rivet is driven into the target materials with high force, piercing the top sheets and spreading outwards into the bottom sheet of material under the influence of an upsetting die to form the joint.

Factors in the design and specification of rivets include the size, type and material for the rivet, the type of joint, and the spacing between rivets.

There are two main types of riveted joint: lap-joints and butt-joints(Fig.4).

In lap joints the components to be joined overlap each other, while for butt joints an additional piece of material is used to bridge the two components to be joined which are butted up against each other.

Rivets can fail by shearing through one cross-section known as single shear, shearing through two cross-sections known as double shear, and crushing. Riveted plates can fail by shearing, tearing and crushing.

What are Truss Structures?

A truss is a special type of structure renowned for its high strength- to- weight and stiffness- to- weight ratios.

This structural form has been employed for centuries by designers in a myriad of applications ranging from bridges and race car frames to the International Space Station.Trusses are easy to recognize: lots of straight slender struts joined end- to- end to form a lattice of triangles, such as the bridge in Fig.1.

Fig.1 Truss bridge in Interlaken, Switzerland

In large structures, the joints are often created by riveting the strut ends to a gusset plate as shown in Fig.2.

A structure will behave like a truss only in those regions where the structure is fully triangulated; locations where the struts form other polygonal shapes (e.g., a rectangle) may be subject to a loss of stiffness and strength.

Fig.2 Joint formed by riveting a gusset plate to converging members

The special properties of a truss can be explained in terms of the loads being applied to the individual struts. Consider the three general types of end loadings shown in Fig.3 tension, compression, and bending.

If you were holding the ends of a long thin steel rod in your hands and wanted to break it or at least visibly deform it, bending would be the way to go. Thus, if we could eliminate bending of the struts as a potential failure mode, the overall strength and stiffness of the truss would be enhanced.

Fig.3 Different end loading possibilities. The dashed represents the deformed shape produced by the applied forces

This is precisely the effect of the truss geometry on the structure, as the stiff triangular lattice serves to keep any bending induced in the struts to a minimum.

What is Warm Mix Asphalt (WMA) ?

The increase of scheduled commercial flights at busy civil airports have made it imperative that airfield pavement rehabilitation and asphalt overlay be performed without disrupting airport operations.

For this purpose, the off-peak period (nighttime) construction has become one practical solution for airport authorities. Using this approach, the airfield facilities are closed at night for a few hours when the flight volume is at the lowest, and then quickly opened to air traffic in the next morning.

During this closed period, aircraft will use other runway facilities, if parallel runways are available, or airport operation will be postponed. Time is the essence of the construction during the off-peaktime.

The typical unoccupied time of airfield pavement rehabilitation is as short as 6–8 h per night. It is a period from 23:00 to 6:00 that was specified for runway overlay in Fukuoka airport. The similar night time construction period can also be found in these following airport projects: San Diego International airport in1980 (8 h), Frankfurt airport, Germany, in 2005 (8 h)and Hong Kong airport in 2006 (8 h).

However, with the increase of 24-hour airport operation, the period for night time construction has become limited. The decrease was observed in the largest Australian airports, where the available night time construction was generally reduced from eight hours in 2005 to five hours in 2015.

Rapid construction is expected to reduce the disruption due to the airport closure and allow more time for contractors to produce the maximum volume of asphalt each night to achieve satisfactorily constructed pavement.

One of the approaches for rapid night time construction is to shorten the cooling time of freshly paved asphalt overlay. In this case, with its advantage of lower production and compaction temperature, warm mix asphalt (WMA) gives an advantage of a lower cooling time of asphalt; thus, the pavement can be quickly opened to traffic.

In the situation where the closure of the runway is substantially critical, the use of WMA is expected to shorten the runway closure time each night. In addition, in the case that the closure hours are fixed for each night, the use of WMA would enable more volume of asphalt to be laid each night, increase the target length of pavement to be done each night, thus, shortening the overall project time, compared to HMA.

The use of WMA technology for airport pavements has been few until now. The technology has more popularly been adopted for road pavement projects than airfield pavements. However, extensive research has been carried out in the last few years on the use of WMA for airside applications.

Recent evidence suggests the suitability of using WMA for airfield pavement. Although considerable researches have been done, there has been no detailed investigation into the advantages of the use of WMA on shortening the construction time of pavement.

What is High Performance concrete – HPC?

1. Definition Of High Performance Concrete:

High-performance concrete may be defined as concrete with strength and durability significantly beyond those obtained by normal means. The required properties for concrete to be classiffied as high performance therefore depend on the properties of normal concrete achievable at a particular time and location.

At the present time, high-performance concrete in developed countries usually refers to concrete with 28-day compressive strength beyond 70±80 MPa, durability factor (defined as the percentage of original modulus retained after 300 freeze/thaw cycles) above 80%, and w/c below 0.35.

It is made with good quality aggregates, high cement content (450±550 kg mw-3), and a high dosage of both silica fume (5±15 wt.% of cement) and super plasticizer (5±15 l mw-3). Sometimes other pozzo-lanic materials are also used.

The high performance is achieved with the use of low w/c (0.20±0.35) as well as pozzolans to produce a dense microstructure that is high in strength and low in permeability.

Superplasticizer is added to keep the mix workable.With high cement content, the use of super-pasticizers and silica fume and the need for more stringent quality control the unit cost of high-performance concrete can exceed that of normal concrete by 30±100%.

2. When High Performance Concrete is Used:

Despite the higher material cost, the use of high-performance concrete is found to be economical for columns of tall buildings, as the amount of steel reinforcement can be reduced.

In bridges, the reduction in deck size and weight effectively increases the allowable unsupported span. For a continuous bridge, the number of piers can be reduced. In many infrastructure projects, high-performance concrete is chosen for its durability against various types of chemical (e.g., sulfate or chloride) and physical attack (e.g., abrasion).

High-performance concrete can also be produced with lightweight aggregates. However, the aggregate needs to be very carefully chosen to make sure it is sufficiently strong. As long as the light weight aggregateis strong enough, its use can indeed be advantageous.

By saturating their pores with water before mixing, these aggregates can act as internal reservoirs that supply water to ensure continued cement hydration and prevent auto geneous shrinkage due to self-desiccation. This aspect is of particular relevance to concrete with a very low w/c, in which the early development of high density and low permeability makes it difficult for water to penetrate uniformly forthe hydration process to continue.

Besides the production of high-performance concrete, superplasticizers are also commonly used in the production of high-workability concrete. With aslump value of 180±230 mm, high-workability concrete can be pumped rapidly over long distances, easily compacted in structures with highly congested re-inforcement, and can even be self-compacting (i.e.,requiring no external compaction effort).

With super-plasticizers, it is also possible to reduce the cement content while retaining the same workability. The possibility of thermal cracking in massive structures can therefore be reduced.

In the continual quest for improving concrete performance, it was soon realized that the size of aggregates is an important factor. By using very fine aggregates, superplasticizers, and a high dose of silica fume (about 20±30% of the cement content) concrete strength beyond 200 MPa can now be achieved by conventional techniques. One example is DSP–densified system with fine particles.

Using strongaggregates of small size (e.g., calcined bauxite withmaximum size of 4 mm), DSP with compressive strength over 250 MPa can be produced.

Reaction powder concrete (RPC) is another example. With the maximum particle size limited to 0.4 mm, the compressive strength reaches 170 MPa by 28 days under room temperature curing. Curing at 80±90∞C will further increase the strength to 230 MPa. If pressure is applied before and during setting, and curing is carriedout at 400∞C, strength as high as 680 MPa can be attained. With very high strength, both DSP and RP Care extremely brittle. Fiber reinforcement is therefore essential to prevent catastrophic failure at ultimateload.

Precast Prestressed Spun Concrete Piles

Precast prestressed spun concrete piles are closed-ended tubular sections of 400 mm to 600 mm diameter with maximum allowable axial loads up to about 3 000 kN.

Pile sections are normally 12 m long and are usually welded together using steel end plates. Pile sections up to 20 m can also be specially made.

Precast prestressed spun concrete piles require high-strength concrete and careful control during manufacture.

Casting is usually carried out in a factory where the curing conditions can be strictly regulated.

Special manufacturing processes such as compaction by spinning or autoclave curing can be adopted to produce high strength concrete up to about 75 MPa. Such piles may be handled more easily than precast reinforced concrete piles without damage.

This type of piles is generally less permeable than reinforced concrete piles and may be expected to exhibit superior performance in a marine environment. However, they may not be suitable for ground with significant boulder contents. In such cases, preboring may be required to penetrate the underground obstructions.

Spalling, cracking and breaking can occur if careful control is not undertaken and good
driving practice is not followed

Precast Reinforced Concrete Piles

Precast reinforced concrete piles are not common nowadays.

These piles are commonly in square sections ranging from about 250 mm to about 450 mm with a maximum section length of up to about 20 m. Other pile sections may include hexagonal, circular, triangular and H shapes. Maximum allowable axial loads can be up to about 1 000 57kN.

The lengths of pile sections are often dictated by the practical considerations including
transportability, handling problems in sites of restricted area and facilities of the casting yard.
These piles can be lengthened by coupling together on site.

Splicing methods include welding of steel end plates or the use of epoxy mortar with dowels.

This type of pile is not suitable for driving into ground that contains a significant amount of boulders or corestones.