Stone columns are constructed using down-hole vibratory probe methods similar to those used in vibro-compaction. The main difference is that instead of using coarsegrained soil to simply fill the void created by the vibro-compaction operation, stone or other clean, coarse grained materials are placed, and compacted, to form a narrow structural element (i.e. a column) which functions as one or more of the following:
1. enhance the average shear strength and bearing capacity of a weak soil mass,
2. transfer a surface load to deeper competent materials, or
3. provide easy drainage of temporarily high pore water pressures.
Stone columns are ideally suited for improving soft silts and clays and loose silty sands. Stone columns under suitable conditions will:
• increase a soil’s bearing capacity and shear resistance
• reduce settlements,
• increase the time-rate of consolidation,
• reduce liquefaction potential, and
• stabilize existing slopes affected by low shear strength soils .
Stone columns, in general, are most economically attractive for sites requiring column lengths less than 35 ft. deep and preferably about 20 ft. deep below the surface.
Unsuitable soil conditions for stone columns include soils having thick layers of very soft or sensitive clays and organic materials. If the thickness of the unsuitable soil layer is more than the diameter of the stone column, then stone columns may not be appropriate because the very soft soils will not provide adequate lateral support of the stone column. In addition, stone column construction can be hampered by the presence of a thick, dense overburden, or soils with boulders, cobbles or other obstructions that may require pre-drilling prior to installation of the stone column.
Stone columns are constructed using either a vibro-replacement or vibro-displacement installation with the stone aggregate placed using either top or bottom feed methods.
Vibro-Replacement :
Vibro-replacement involves a wet installation method that replaces deep, narrow pockets of the in-situ soil with stone aggregate columns. In this method a high-pressure water jet, located at the tip of the probe, is used to excavate a narrow, open (uncased) hole. Once the hole is progressed to the design depth, the hole is flushed out several times by raising and dropping the probe to remove any loose silt and sand at the bottom of the hole. The vibro-probe is retracted and a limited amount of stone is placed into the hole from the top. The probe’s vibration mode is turned on and it is inserted into the hole to compact the lift of stone. The probe is retracted again and the process repeated until the stone column is formed to the ground surface. During the entire operation, water is continually pumped into the hole to prevent collapse and to keep the aggregate clean. This method is best suited for sites with soft to firm soils with undrained shear strengths of 200 to 1,000 psf and a shallow groundwater table, and where drill wash and spoil
containment and disposal can be practically handled.
Vibro-Displacement :
When a cleaner or lesser environmental impact operation is preferred, stone columns should be constructed using the vibro-displacement method. The operation is a dry installation method where the stone aggregate can be placed into the hole from the top or from aggregate ports at the bottom of the probe. Although the probe’s dead weight and vibration, in lieu of water jetting, is used to excavate the hole, air jetting and/or pre-augering may be used to prevent clogging of the aggregate ports or to assist in advancing or extracting the probe. This method is best suited for
sites where collapse of the hole during the column’s installation is unlikely.
Vibro-compaction is a ground improvement method that uses a specialized vibrating probe for in-situ subsurface compaction of loose sandy or gravelly soils at depths beyond which surface compaction efforts are effective.
The vibrating probe densifies loose granular, cohesionless soils by using mechanical vibrations and, in some applications, water saturation to minimize the effective stresses between the soil grains which then allows the oil grains to rearrange under the action of gravity into a denser state.
Vibro compaction to densify loose, silty sands for an interim spent fuel cask storage pad in Braceville, Illinois.
Generally, vibro-compaction can be used to achieve the following enhanced soil performance or
properties:
• Increased soil bearing capacity
• Reduced foundation settlements
• Increased resistance to liquefaction
• Compaction to stabilize pile foundations driven through loose granular materials
• Densification for abutments, piers and approach embankment foundations
• Increased shear strength
• Reduced permeability
• Filling of voids in treated areas
Two rigs completing vibro compaction for liquefaction mitigation and settlement at a casino.
The vibrator is hung from a crane cable or, in some instances; it is mounted to leads in a similar fashion as foundation drilling equipment. The vibrator penetrates under its self weight (or crowd of the machine if mounted in leads) and, at times, with assistance from the action of water jets. The goal is that the vibration and water imparted to the soils ransforms the loose soils to a more dense state.
The Vibro Compaction Process
Advantages, Disadvantages and Limitations
1. Advantages
As an alternative to deep foundations, vibro-compaction is usually more economical and often results in significant time savings. Loads can be spread from the footing elevation, thus minimizing problems from lower, weak layers. Densifying the soils with vibro-compaction can considerably reduce the risk of seismically induced liquefaction. Vibro-compaction can also be cost-effective alternative to removal and replacement of poor load-bearing soils. The use of vibro-compaction allows the maximum improvement of granular soils to depths of up to 165 feet. The vibro-ompaction system is effective both above and below the natural water level.
2. Disadvantages and Limitations
Vibro-compaction is effective only in granular, cohesionless soils. The realignment of the sand grains and, therefore, proper densification generally cannot be achieved when the granular soil contains more than 12 to 15 percent silt or more than 2 percent clay. The maximum depth of treatment is typically limited to 165 feet, but there are very few construction projects that will require densification to a greater depth.
Like all ground improvement techniques, a thorough soils investigation program is required. Yet, a more detailed soils analysis may be required for vibro-compaction than for a deep foundation design because the vibro-compaction process utilizes the permeability and properties of the in-situ soil to the full depth of treatment to achieve the end result. A comprehensive understanding of the total soil profile is therefore necessary which typically requires continuous sampling or in-situ testing.
Equipment access over the site must also be considered. Since the operation requires use of a large crane, a relatively flat work bench with a width of at least 25 ft must be possible near all areas to be treated.
Wet vibro-compaction requires the use of water to jet the vibrator into the ground. The effluent from the jetting process requires at least temporary containment to allow any fine soil particles to settle out and be disposed. Further, this method of ground improvement may not be acceptable if the existing subsurface environment, either soil or water is contaminated. If contamination is present, use of water jetting may cause its dispersion and therefore other ground improvement methods should be considered.
What type of pavement is used for airports runway?
The materials used for airports is generally the same as what is used for roadways, however, the depths, or thicknesses are different, and the tolerances are much tighter at an airport. The material for runways usually needs to meet a much tighter spec.
A typical section for an airport can use asphalt or concrete. Below is a generic look at the structural section for either asphalt or concrete from an FAA Advisory Circular on Aiport Pavement Design and Evaluation.
You will notice that the materials in the middle are thicker and then taper to thinner. This is because the loads on the runway are primarily from the 2 landing wheels, which will be in the middle of the runway. The effective tire width is pictured below.
The surface must be smooth and well bonded, and resistant to the shear stresses of the airplane wheel loads. The non-skid surface must not cause undue wear on the airplane tires . The surface must be free of loose particles that could damage the airplane or people. In order to meet this requirement, there must be good control of the mix. This usually requires a central mixing plant be used for the hot mix asphalt.
The base course is integral to flexible pavement design such as asphalt. The loading in flexible pavements transfers downward and outward. For this reason, the base, subbase, if used, and subgrade contribute to the strength of the pavement section. For concrete pavement, the concrete provides the strength to the structural section.
The base course must be of sufficient quality that it won’t fail, or allow failure in the subgrade. It must be able to withstand the forces from the airplane wheel loading without consolidating which would cause the surface course to deform. The base course uses very select material with very hard and durable aggregate. The requirements for the base course are very strict.
Concrete slab moisture can cause problems with the adhesion of floor-covering material, such as tile, sheet
flooring, or carpet and bond-related failures of non-breathable floor coatings. Many adhesives used for installation of floor coverings are more water-sensitive than in the past, due to restrictions on the use of volatile organic compounds (VOCs).
To warranty their products, manufacturers require that the moisture emission from the hardened concrete slab be less than some threshold value prior to installing floor coverings or coatings. Fast-track construction schedules exacerbate the problem when floor-surfacing material is installed before the concrete slab has dried to an acceptable level.
WHAT are the Sources of Concrete Slab Moisture ?
a. Ground water sources and when the floor slab is in contact with saturated ground, or if drainage is poor. Moisture moves to the slab surface by capillary action or wicking. Factors affecting this include depth of the water table and fineness of soil below the slab. Fine grained soil promotes moisture movements from considerable depths compared to coarser subgrade material.
b. Water vapor from damp soil will diffuse and condense on a concrete slab surface that is cooler and at a lower relative humidity due to a vapor pressure gradient.
c . Wetting of the fill course/blotter layer, if any, between the vapor retarder and the slab prior to placing the slab will trap moisture with the only possible escape route being through the slab. A blotter layer is not recommended for interior slabs on grade (CIP 29).
d. Residual moisture in the slab from the original concrete mixing water will move towards the surface. It may take anywhere from six weeks to one year or longer for a concrete slab to dry to an acceptable level under normal conditions.
Factors that affect the drying rate include the original water content of the concrete, type of curing, and the
relative humidity and temperature of the ambient air during the drying period. This is the only source of moisture in elevated slabs. Any wetting of the slab after final curing will elevate moisture levels within the slab and lengthen the drying period.
HOW do You Avoid Problems?
Avoiding problems associated with high moisture content in concrete can be accomplished by the following means:
• Protect against ingress of water under hydrostatic pressure by ensuring that proper drainage away from the slab is part of the design.
• Use a 6 to 8 inch [150 to 200 mm] layer of coarse gravel or crushed stone as a capillary break in locations with fine-grained soil subgrades.
• Use a vapor retarder membrane under the slab to prevent water from entering the slab. Ensure that the vapor retarder is installed correctly and not damaged during construction. Current recommendation of ACI Committee 302 is to place the concrete directly on a vapor retarder for interior slabs on grade (CIP 29).
• Use a concrete mixture with a moderately low water-cementitious material (w/cm) ratio (about 0.50). This reduces the amount of residual moisture in the slab, will require a shorter drying period, and result in a lower permeability to vapor transmission. Water reducing admixtures can be used to obtain adequate workability and maintain a low water content. The water tightness of concrete can be improved by using fly ash or slag in the concrete mixture.
• Curing is an important step in achieving excellent hardened concrete properties. However, moist curing will increase drying time. As a compromise, curing the concrete under plastic sheeting for 3 days is recommended and moist curing times greater than 7 days must be avoided. Avoid using curing compounds on floors where coverings or coatings will be installed.
• Allow sufficient time for the moisture in the slab to dry naturally while the floor is under a roof and protected from the elements. Avoid maintenance and cleaning operations that will wet the concrete floor. Use heat and dehumidifiers to accelerate drying. Since moisture transmission is affected by temperature and humidity, maintain the actual service conditions for a long enough period prior to installing the floor covering.
• Test the slab moisture condition prior to installing the floor covering. When concrete slab moisture cannot be controlled, consider using decorative concrete, less moisture-sensitive floor coverings, breathable floor coatings, or install moisture vapor suppression systems (topical coatings).
HOW is Concrete Slab Moisture Measured?
Various qualitative and quantitative methods of measuring concrete slab moisture are described in ASTM E 1907.
Test the moisture condition of the slab in the same temperature and humidity conditions as it will be in service.
In general, test at three random sample locations for areas up to 1000 sq. ft. [100 m2] and perform one additional test for each additional 1,000 sq ft. Ensure that the surface is dry and clean. Record the relative humidity and temperature at the time of testing. Some of the common tests are:
Polyethylene Sheet Test (ASTM D 4263)
– is a simple qualitative test, where an 18 by 18 inch [450 by 450 mm] square plastic sheet is taped tightly to the concrete and left in place for a at least 16 hours. The presence of moisture under the plastic sheet is a positive indication that excess moisture is likely present in the slab. However, a negative indication is not an assurance that the
slab is acceptably dry below the surface.
Mat Test
– where the adhesive intended for use is applied to a 24 by 24 inch [600 by 600 mm] area and a sheet vinyl flooring product is placed face down on the adhesive and sealed at the edges. A visual inspection of the condition of the adhesive is made after a 72-hour period. This test is no longer favored since it can produce false negative results.
Test Strip
– in which a test strip of the proposed primer or adhesive is evaluated for 24 hours to predict its behavior on the floor. This procedure is not very reliable.
Moisture meters
– Measure electrical resistance or impedance to indicate slab moisture. Electronic meters can be useful survey tools that provide comparative readings across a floor but should not be used to accept or reject a floor because they do not provide an absolute measure of moisture conditions within the slab.
Gravimetric
– This is a direct and accurate method of determining moisture content by weight in the concrete slab. Pieces of concrete are removed by chiseling or stitch-drilling and dried in an oven to constant weight. The moisture content is then calculated as a percentage of the dry sample weight. This is rarely recommended by floor covering manufacturers.
Nuclear Density and Radio Frequency
– This nondestructive test instrument is relatively expensive and can take a long time to properly correlate correction factors for each individual project. The instrument has a radioactive source and therefore requires licensed operators.
Anhydrous Calcium Chloride Test (ASTM F 1869)
– is specified by most floor covering manufacturers for pre installation testing. A measured amount of anhydrous calcium chloride is placed in a cup sealed under a plastic dome on the slab surface and the amount of moisture
absorbed by the salt in 60 to 72 hours is measured to calculate the moisture vapor emission rate (MVER).
Maximum limits of vapor transmission generally specified are 3 to 5 pounds of moisture per 1000 square feet per 24 hours. This test is relatively inexpensive, and yields a quantitative result. However, it has some major shortcomings: it determines only a portion of the free moisture at a shallow depth of concrete near the surface of the slab. The test is sensitive to the temperature and humidity in the building. It provides only a “snapshot in time” of current moisture conditions and does not predict if the sub-slab conditions will cause a moisture problem later in the life of the floor.
Relative Humidity Probe (ASTM F 2170)
This procedure involves measuring the relative humidity of concrete at a specific depth from the slab surface inside a
drilled or cast hole in a concrete slab. The relative humidity is measured after allowing 72 hours to achieve moisture equilibrium within the hole. Typically a relative humidity of 75% to 80% is targeted for installation of floor coverings. Relative humidity probes can determine the moisture profile from top to bottom in a slab, conditions below the slab,
and can monitor the drying of a slab over time, leading to predictions of future moisture conditions. These instruments have been used for many years in Europe and are becoming more popular in the
Lean concrete and flowable concrete are terms used to describe low-grade concrete slurry that is used in a variety of construction projects. In some ways the two terms are interchangeable, both describing concrete made with lesser ingredients, but there are some differences in how the two are used. Lean concrete tends to be more long-lasting than flowable concrete, which is often temporary.
Lean Concrete
Lean concrete is made with low cementitious material content. This means that it does not have many of the heavy, high-density rock and sand elements that normal concrete has. Instead, it can use a mixture of standard concrete materials, reclaimed and crushed concrete, discarded sand and recycled ash. This makes lean concrete very cheap in nature and simple to make and use.
Uses
Main function of the lean concrete is to provide the uniform surface to the foundation concrete and to prevent the direct contact of foundation concrete from the soil.
Lean concrete is used under the foundations.
It is good for providing a flat bottom in uneven or dirt terrain.
Lean concrete has a lower level of cement in it, which is why it’s mostly used for fillings or under the foundations, to keep it protected from the soil.
Lean concrete is used to provide a level surface , where main foundation (raft, isolated or any other type) can be placed.
Another purpose is protection of main foundation from soil below, as moisture or other chemicals in soil like sulphates may attack concrete and can weaken it.
Flowable Concrete
Flowable fill concrete is a self-compacting cementitious slurry consisting of a mixture of fine aggregate or filler, water, and cementitious material which is used as a fill or backfill in lieu of compacted-soil backfill. This mixture is capable of filling all voids in irregular excavations and hard to reach places (such as under undercuts of existing slabs), is self-leveling, and hardens in a matter of a few hours without the need for compaction in layers.
Flowable fill is sometimes referred to as controlled density fill (CDF), controlled low strength material (CLSM), lean concrete slurry, and unshrinkable fill.
Flowable fill materials will be used as only as a structural fill replacement on VA projects. Unless otherwise noted, flowable fill installed as a substitution for structural earth fill, shall not be designed to be removed by the use of hand tools.
The materials and mix design for the flowable fill should be designed to produce a comparable compressive strength to the surrounding soil after hardening, making excavation at a later time possible to produce the compressive strength indicated for the placed location, as determined by the Engineer.
Purposes
Like lean concrete, flowable concrete or flowable fill is used for sub-bases and subfooting as well as abandoned wells and cavities. But flowable concrete is more associated with backfill projects where the concrete will be removed in several months when projects are completed. Because it will be taken away, it may be made of cheaper and less durable materials than lean concrete.
Concrete vibration – The why and how of consolidating concrete
What factor has a greater effect on concrete compressive strength than any other? Most engineers would say water-cement ratio … as water-cement ratio increases strength decreases. Duff Abrams showed this in 1919, and Abrams’ law is the principle behind most concreting proportioning methods used today. But Abrams ran his tests on fully consolidated concrete.
Unless concrete is properly consolidated, voids reduce strength regardless of the water-cement ratio. And, as shown in Figure1, the effect is significant.
Right after it’s placed, concrete contains as much as 20% entrapped air. The amount varies with mixtype and slump, form size and shape, the amount of reinforcing steel, and the concrete placement method. At a constant water-cement ratio, each percent of air decreases compressive strength by about 3% to 5%. Consolidating the concrete, usually by vibration, increases concrete strength by driving out entrapped air. It also improves bond strength and decreases concrete permeability.
Figure 1. Degree of consolidation can have as much effect on compressive strength as water cement ratio. Low-slump concrete may contain up to 20% entrapped air when placed.
Vibration is a two-part process
How does vibration consolidate concrete? Figure 2 shows it to be a two- part process. A vibrator creates pressure waves that separate aggregate particles, reducing friction between them. Piles of concrete flat-ten as the concrete flows around reinforcing steel and up to the form face. Large voids (honeycomb) disappear. But making the concrete flowable doesn’t finish the compaction proces s. Almost simultaneously, a second stage starts to occur as entrapped air bubbles rise to the surface.
This deaeration process continues after the concrete has flattened out. Until both vibration stages are complete, the concrete isn’t fully consolidated. If the vibrator is removed too soon, some of the smaller bubbles won’t have time to rise to the surface. Vibration must continue until most of the air entrapped during placement is removed. It’s usually not practical, though, to remove all the entrapped air with standard vibrating equipment.
Figure 2. A vibrator consolidates concrete in a two-part process. The first waves liquify the concrete so it flows better and the continuing waves knock out air bubbles.
Different vibrators for different jobs
The earliest form of equipment used as a vibrator was a rod stuck into the concrete, pushed down and pulled up. Rodding works for concretes with slumps greater than 3 inches, but it’s rarely used because of the costly labor required. Because rodding doesn’t put extra pressure on forms, howe ve r, it has helped more than one contractor complete a concrete pour when forms were bulging.
The most common vibrator used is the electric, flexible shaft type. Other types include electric motor-in- head, pneumatic, and hydraulic. Vibrator output, usually expressed as a frequency, is controlled in a different
way for each type of vibrator:
An electric vibrator uses voltage.
A pneumatic vibrator uses air pressure.
A hydraulic vibrator uses pressure and flow rate of hydraulic fluid.
On the jobsite the contractor can check the operating performance of his equipment by measuring frequency.
If it’s low he should check for voltage fluctuations, air pressure losses, or hydraulic pressure drops. The type of vibrator must match the requirements of the concrete and the jobsite (Figure 3). Frequency rates determine the amount of vibration time required to complete the two-stage consolidation process.
In the 1960s, vibration frequencies were much lower. To compact a 1⁄2-inch-slump concrete took 90 seconds at 4,000 vibrations per minute (vpm), 45 seconds at 5,000 vpm, and 25 seconds at 6,000 vpm. Today’s typical frequency of 15,000 vpm requires only 5 to 15 seconds of vibration
time.
Internal vibrators chosen for most jobs have a frequency of 12,000 to 17,000 vpm in air. The common flexible shaft-type vibrator reduces its frequency by about 20% when immersed in concrete. Motor-in- head types provide a constant frequency when in air or concrete.
Figure 3. The vibrator head must fit between the rebars and have a high enough frequency to quickly consolidate the concrete.
How to use an internal vibrator
Producing a dense concrete without segregation requires an experienced vibrator operator. Inexperienced operators tend to merely flatten the concrete because they don’t vibrate long enough to deaerate the concrete. Undervibration is more common than over vibration because of a worker’s effort to keep up with the concrete or to increase productivity. The operator can judge whether or not vibration is complete by watching the concrete surface.
When no more large air bubbles escape, consolidation is adequate. Skilled operators also listen to the pitch or tone of the vibrator motor. When an immersion vibrator is inserted in concrete, the frequency usually drops off, then increases, becoming constant when the concrete is free of entrapped air.
Never use a vibrator to move concrete laterally. Concrete should be carefully deposited in layers as close as possible to its final position in the form. As each layer is placed, insert the vibrator vertically. The distance between insertions should be about 11⁄2 times the radius of action (usually 12 to 24 inches). Radius of action is a distance from the vibrator head within which consolidation occurs. It varies with equipment and concrete mix.
Walls and columns
Special techniques are necessary to blend layers of concrete in walls and columns. Let the vibrator penetrate quickly to the bottom of the layer and at least 6 inches into the preceding layer. Then move the vibrator up and down, generally for 5 to 15 seconds, to blend the layers. Withdraw the vibrator gradually with a series of rapid up and down motions.
Elevated beams and slabs
Beams and joists placed monolithically with slabs should be vibrated separately before slab placement. Place the slab concrete after vibrating the beam, but before the beam concrete is set. Allow the vibrator to penetrate through the slab into the previously placed beam to blend the two structural elements.
Undervibration vs Overvibration
Undervibration is far more common than overvibration. Good quality normal- weight concrete is not readily susceptible to the problems caused by overvibration, so when in doubt, vibrate more.
The problems associated with undervibration include:
Honeycombing
Excessive entrapped air
Sand streaks
Cold joints
Subsidence cracking
The problems associated with overvibration include:
Segregation
Sand streaks
Loss of entrained air
Form deflection
Form damage or failure
Overvibrating, because it causes entrained air loss, might be expected to decrease freeze-thaw resistance. Research results don’t bear this out, however. In one study, overvibration of low- slump, air- entrained concrete had no effect on freeze-thaw resistance. Overvibration should not be a concern unless high- slump, improperly proportioned concrete is being placed.
Vibrating around congested reinforcement
To provide good concrete- to- steel bond, vibration is especially important in areas congested with rebar. Vibration alone doesn’t solve the problem. Other actions must be taken to help complete concrete consolidation, such as:
Using admixtures to increase flowability but limit segregation
Changing mix proportions or ingredients to increase flowability
Designing the reinforcing for ease of concrete placing
Figure 4. Vibration alone won’t consolidate concrete adequately when reinforcing is congested. To ensure adequate consolidation it may be necessary to use superplasticizers, reduce aggregate maximum size, or adjust rebar spacing.
To achieve proper consolidation by internal vibration in congested areas, the designer should provide obstruction-free vertical access of 4×6-inch minimum openings to insert the vibrator. Horizontal spacing of these openings should not exceed 24 inches or 11⁄2 times the vibrator’s radius of action. Engineers designing congested reinforcement should also design for proper consolidation, otherwise contractors can’t always guarantee adequate concrete to steel bond (Figure 4).
References
1. “Guide for Consolidation of Concrete,” ACI 309R-87, ACI Materials Journal, September-October 1987, American Concrete Institute, Box 19150, Detroit, Michigan 48219. 2. Whiting, D., G. W. Seegebrecht, and S. Tayabji, “Effect of Degree of Consolidation on Some Important Properties of Concrete,” SP-96, Consolidation of Concrete, American Concrete Institute. 3. Olsen, Mikael, “Energy Requirements for Consolidation of Concrete During Internal Vibration,” SP-96, Consolidation of Concrete, American Concrete Institute. 4. Troxell, Davis, & Kelly, Composition and Properties of Concrete, McGraw- Hill, New York, New York, 1968.
Road safety audit is the formal examination of existing roads, future roads or various sorts of traffic projects by any independent group of trained expertise. They examine the deficiencies in road safety. There are various stages of road safety audits. Number of stages depends on the number of stages in a road project before completion.
Feasibility study phase
Preliminary design phase
Detailed design phase
Pre-opening phase
In service phase
Feasibility Study
In feasibility study phase, trained specialist study and evaluate the results of following questions;
What is the scope of this project?
How many choices of routes available?
What will be the impacts on the existing transportation system?
Which design should be selected as a design standard for that road?
How long this route could be continued?
Which location will be the best location of interchanges?
Number of lanes required for managing maximum average daily traffic?
Where to provide the route terminals?
What will be the effects on the environment?
Control access
Technical team works on these questions step by step and at the end gives the most feasible solution possible.
Preliminary Design Stage:
Preliminary design is the second stage of road safety audits. In this stage designs of roads are carried out. As preliminary phase, therefore designing is not carried out in a very detailed. Following are road designs that are done in this phase;
Alignment of horizontal and vertical curves
Width of land and shoulder
Layout of intersection
Provision of super elevation and side slopes with pavements
Provision of overtaking lanes
Provision of separate way for pedestrians and cyclists.
Safety arrangements during construction on site
Provision of sign boards.
Design of Link roads
Space management
Detailed Design Stage
After the completion of 2nd stage, designs are carried out in detail. Following are the road designs that completes at the end of this stage;
Signals
Sign boards
Line marking
Lighting
Intersection details
Delineation
Provision of shoulders
Management of traffic during construction
Design of road drainage system
Provisions of way for road user groups. For example, pedestrians, cyclists, vans, trucks etc.
Provision of slopes.
Provision of road side objects
Pre-Opening Stage
In this phase technical audit team drive through the completed project. During drive, they observe the provision of safety level, quality, sign boards, road material and all other aspects that they took under consideration during the preliminary survey and detailed design phase. They came on the site during different weather conditions like during day time, night, etc… after the completion of survey they wrote a report on it and deliver it to the main authority.
In-service:
After the pre-opening examination, road is opened for public and during that still technical team remains active and they observe the working of safety features during heavy traffic.
Flexible pavement differ from rigid pavement in terms of load distribution. In flexible pavements load distribution is primarily based on layered system. While, in case of rigid pavements most of the load carries by slab itself and slight load goes to the underlying strata.
Structural capacity of flexible pavement depends on the characteristics of every single layer. While, the structural capacity of rigid pavements is only dependent on the characteristics of concrete slab. This is so, because of low bearing soil capacity of underlying soil.
In flexible pavements, load intensity decreases with the increase in depth. Because of the spreading of loading in each single layer. While, in case of rigid pavement maximum intensity of load carries by concrete slab itself, because of the weak underlying layer.
In flexible pavement deflection basin is very deep, because of its dependency on the underlying layers. While in case of rigid pavement, deflection basin is shallow, this is because of independency of rigid pavement on the underlying layers.
Flexible pavement has very low modulus of elasticity (less strength). Modulus of elasticity of rigid pavement is very high, because of high strength concrete and more load bearing capacity of the pavement itself. Than compared to flexible pavements.
In flexible pavements, underlying layers play very important role. Therefore, more role are playing only underlying layers. In case of rigid pavements, slight function of underlying layers. Maximum role is playing by the top layer (that is slab) by itself. Therefore, minute part is taking by sub layers.
Simulation of the Morandi bridge collapsed in Genoa, Italy, in 2018, performed with the Bullet Constraints Builder (BCB) structural simulation software for Blender. The model was built from plans true to scale. Reinforcement information was estimated in part from photos of the destroyed bridge. In order to narrow down the likely collapse scenario this video includes simulation results for different initial failure points. The characteristics of the debris heaps in comparison to reality often can provide an indication on what has probably happened.
Types of structural supports – Boundary Conditions
Types of supports
Defining the boundary conditions in a model is one of the most important part of preparing an analysis model, irrespective of the software that you use. Supports are an essential part of building your model to ensure accurate and expected results.
These are not to be ignored nor guessed as it can lead to your structure not behaving in the way you anticipated. To define supports you need to be aware about the support detailing in case of steel structures. For example, a support column in a steel structure can be pinned or fixed, depending upon the detailing adopted.
1. Fixed support:
This is the most rigid type of connection. It restrains the member in all translations and rotations, which means it can’t move or rotate in any direction. The best example of this is a column placed in concrete which can’t twist, rotate or displace. A fixed support in three dimensional model will have 6 degrees of freedom restrained, which are three translations and three rotations in three orthogonal directions, X, Y and Z.
These are beneficial when you can only use a single support. The fixed support provides all constrains necessary to ensure the structure is static. It’s the only support which is used for stable cantilevers.
The greatest advantage provided by this support can also lead to its downfall as sometimes the structure requires a little deflection or some play to protect the surrounding materials. For example, as concrete continues to gain strength, it also expands. Hence it’s crucial that the support is designed correctly else the expansion could lead to reduction in durability.
Fixed support reactions
Beam fixed on the wall as an example
2. Roller support:
This support can’t resist the horizontal support but can resist the vertical support. This connection is free to move in horizontal direction as there is nothing restraining it.
The most common use of this support is in a bridge. Typically a bridge consists of a roller support at one end to account for the vertical displacement and expansion from changes in temperature. It’s required to prevent the expansion causing damage to a pinned support.
The roller support doesn’t resist horizontal force which acts as its limit as the structure will require another support to resist the horizontal force.
For a structure to be stable roller support is used along with pin support.
Roller support reactions
Roller support on one end of a bridge
3. Pinned support:
A pinned support is a common type of support in civil engineering. Like hinge, this support allows rotation to occur but not translation which means that it resists the horizontal and vertical forces but not a moment.
Pinned supports are widely used in trusses. By joining multiple members by pinned connections, the members push against each other which will induce an axial force within the member. The advantage of this support is that the members won’t have internal moment forces, and can be designed only according to their axial force.
The pinned support can’t completely resist a structure on its own as you need at least two supports to resist the moment coming on the structure.
Hinge support reactions
Hinge support in sydney harbor bridge
4. Internal Hinge
Interior hinges are often used to join flexural members at points other than supports. In some cases, it is employed deliberately so that the excess load breaks the weak zone rather than damaging other structural elements.
