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.
More than half of construction firms now using drones to capture data
The use of digital/reality capture information from drone technology continues to increase in the UK and Irish construction industries, with 52% of respondents to a new survey now using the technology compared with 33% in 2017.
This increase has been revealed in a poll by aerial mapping, inspection and surveying specialist ProDroneWorx. However, more advanced digital/reality capture outputs continue to be underutilised, the survey found.
ProDroneWorx asked senior figures within the construction, infrastructure and asset inspection markets about their perception, usage and understanding of the digital/reality capture outputs from drone technology. A total of 150 respondents took part across the UK and Ireland.
Construction company Kier said: “The latest ProDroneWorx survey demonstrates how far drone technology has progressed. Kier is working closely with ProDroneWorx on some key projects to realise the benefits from drone technology, including progress capture, 360 photography and photogrammetry.”
Steven Hedley, vice president technical at the Chartered Institute of Architectural Technologists, said: “As regulation and licencing laws surrounding drone usage tighten, it is imperative that specialist drone operators continue to facilitate the development of drone technology and its integration with Building Information Modelling within our industry to maximise benefits and minimise misuse.”
According to the survey, the top three reasons for adopting the technology are improved data quality (56%), time saving (54%) and the reduction of risk (42%). Interestingly, fewer companies than last year are planning on utilising drone technology in-house, reflecting, perhaps the level of knowledge and expertise needed to deploy them.
However, the findings also demonstrate that drone technology is currently being underutilised. While 74% of respondents are using drone technology for photography and video, fewer than 30% of respondents are using the technology for value-added services such as aerial LiDAR, 3D point clouds, 3D modelling, digital surface/terrain models, orthophotos and thermal imaging.
Ian Tansey, managing director at ProDroneWorx, said: “In a world of very tight margins of about 2% in construction, and an increasingly competitive landscape, the use of digital/reality capture data gives firms a significant competitive advantage over their peers through improved data quality, reduced costs, increased productivity gains and the mitigation of risk”.
Tansey says that digital/reality capture data outputs created from drone technology using photogrammetry and LiDAR are starting to transform traditional business models, helping to reshape the construction, infrastructure and asset inspection markets.
This is happening through the improved management of assets digitally, deeper data insights, better collaboration on projects, improved data deliverables to clients, cost reductions and the reduction of risk.
The survey also found that of the 52% that are currently using drone technology, the majority (45%) have been using it for less than a year, and only 14% of this sub-group have been using the technology for the last three to five years, making them very early adopters
The three main reasons firms are using the technology are:
Improved data quality (56%)
Time saving (54%)
Reduced risk (42%)
Other findings:
74% of those not using the technology plan on using it in future, so usage should continue to increase.
Only a small proportion (15%) of firms have no plans to use the technology in the future.
Construction firms have two options when it comes to deciding how to incorporate drone technology into their business models and workflow: creating an internal drone unit/function; or using a third party specialist. The vast majority of firms plan on outsourcing the business to third party companies; only 23% of companies plan on using the technology themselves (in-house), down from 28% in 2017.
The majority of respondents (78%) were from England, followed by the Republic of Ireland at 11%; a smaller number were from Scotland (7%) and Wales (3%).
26% of respondents were from Construction, 17% Architecture, 17% Civil Engineering, 9% Building Surveying, 7% Surveying and 23% in other sectors.
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.
Permeability of Soil: Definition, Darcy’s Law and Tests
Definition of Permeability:
It is defined as the property of a porous material which permits the passage or seepage of water (or other fluids) through its interconnecting voids.
A material having continuous voids is called permeable. Gravels are highly permeable while stiff clay is the least permeable, and hence such a clay may be termed impermeable for all practical purpose.
The study of seepage of water through soil is important for the following engineering problems:
1. Determination of rate of settlement of a saturated compressible soil layer.
2. Calculation of seepage through the body of earth dams and stability of slopes for highways.
3. Calculation of uplift pressure under hydraulic structure and their safety against piping.
4. Groundwater flow towards well and drainage of soil.
Darcy’s Law (1856) of Permeability:
For laminar flow conditions in a saturated soil, the rate of the discharge per unit time is proportional to the hydraulic gradient.
q = kiA
v = q/A = Ki … (7.1)
Where q = discharge per unit time
A = total cross-sectional area of soil mass, perpendicular to the direction of flow
i = hydraulic gradient
k = Darcy’s coefficient of permeability
v = velocity of flow or average discharge velocity
If a soil sample of length L and cross-sectional area A, is subjected to differential head of water h1 – h2, the hydraulic gradient i will be equal to [(h1 – h2)/L] and we have q = k. [(h1 – h2)/L].A.
When hydraulic gradient is unity, k is equal to V. Thus, the coefficient of permeability, or simply permeability is defined as the average velocity of flow that will occur through the total cross-sectional area of soil under unit hydraulic gradient. Dimensions are same as of velocity, cm/sec.
The coefficient of permeability depends on the particle size and various other factors. Some typical values of coefficient of permeability of different soils are given in Table 7.1.
Discharge Velocity and Seepage Velocity:
The total cross-sectional area of the soil mass is composed of sectional area of solids and voids, and since flow cannot occur through the sectional areas of solids, the velocity of flow is merely an imaginary or superficial velocity.
The true and actual velocity with which water percolates through a soil is called the velocity of percolation or seepage velocity. It is the rate of discharge of percolating water per unit of net sectional area of voids perpendicular to the direction of flow.
Validity of Darcy’s Law:
In accordance with the Darcy’s Law, the velocity of flow through soil mass is directly proportion to the hydraulic gradient for laminar flow condition only. It is expected that the flow to be always laminar in case of fine-grained soil deposits because of low permeability and hence low velocity of flow.
However, in case of sands and gravels flow will be laminar upto a certain value of velocity for each deposit and investigations have been carried out to find a limit for application of Darcy’s law.
According to researchers, flow through sands will be laminar and Darcy’s law is valid so long as Reynolds number expressed in the form is less than or equal to unity as shown below –
Where v = velocity of flow in cm/sec
Da = size of particles (average) in cm.
It is found that the limiting value of Reynolds number taken as 1 is very approximate as its actual value can have wide variation depending partly on the characteristic size of particles used in the equation.
Factors affecting permeability are:
1. Grain size
2. Properties of pore fluid
3. Void ratio of the soil
4. Structural arrangement of the soil particle
5. Entrapped air and foreign matter
6. Adsorbed water in clayey soil
4. Effect of degree of saturation and other foreign matter
k will decrease if air is entrapped in the voids thus reducing its degree of saturation. Percolating water in the field may have some gas content, it may appear more realistic to use the actual field water for testing in the laboratory.
Organic foreign matter also has tendency to move towards critical flow channels and choke them up, thus decreasing permeability.
5. Effect of adsorbed water – The adsorbed water surrounding the fine soil particle is not free to move, and reduces the effective pore space available for the passage of water.
Capillarity-Permeability Test:
The set-up for the test essentially consists of a transparent tube about 40 mm in diameter and 0.35 m to 0.5 m long in which dry soil sample is placed at desired density and water is allowed to flow from one end under a constant head, and the other end is exposed to atmosphere through air vent.
At any time interval t, after the commencement of the test, Let the capillary water travel through a distance x, from point P to Q. At point P, there is a pressure deficiency (i.e., a negative head) equal to hc of water.
If the coefficient of permeability is designated as ku at a partial saturation S, the above expression can be rewritten as –
In order to find the two unknowns k and hc in the above equation, the first set of observations are taken under a head h1. As the capillary saturation progresses the values of x are recorded at different time intervals t.
The values of x2 are plotted against corresponding time intervals t to obtain a straight line whose slope, say m, gives the value of [(x22 – x21)/(t2 – t1)] . The second set of observation are taken under an increased head h2 and values of x2 plotted against corresponding values of t to obtain another straight line, whose slope m2 will give the value [(x22 – x21)/(t2 – t1)].
By substitution in Eqn. 7.1, we obtain the following two equations, which are solved simultaneously to get k and hc.
The porosity n required in the above equation is computed from the known dry weight of soil, its volume and specific gravity of soil particles.
Permeability of Stratified Soil Deposits:
In general, natural soil deposits are stratified. Each layer may be homogeneous and isotropic. When we consider flow through the entire deposit the average permeability of deposit will vary with the direction of flow relative to the bedding plane. The average permeability for flow in horizontal and vertical directions can be readily computed.
Average Permeability Parallel to Bedding Plane:
Figure 7.9 shows several layers of soil with horizontal stratification. Let Z1, Z2, ….Zn be the thickness of layers with permeabilities k1, k2, … kn.
For flow parallel to bedding plane the hydraulic gradient i will be same for all layers. The total discharge through the deposit will be the sum of discharges through individual layers.
Average Permeability Perpendicular to Bedding Plane:
For flow in the vertical direction for the soil layers shown in Fig. 7.10.
In this case the velocity of flow, v will be same for all layers the total head loss will be sum of head losses in individual layers.
h = h1 + h2 + h3 + … + hn (i)
If kz denotes average permeability perpendicular to bedding plane, applying Darcy’s law, we have –
Compaction test is conducted in the laboratory to determine the relation between the dry density and the water content of the given soil compacted with standard compaction energy and to determine the OMC corresponding to the MDD.
The OMC obtained from the laboratory compaction test will help in deciding the amount of water to be used for compaction in the field. The MDD obtained from the laboratory compaction test helps in knowing the dry density achievable in the field compaction and also as a check for quality control.
Based on the compacted energy used in compacting the soil in the laboratory test, the laboratory compaction tests are of two types:
1. Standard Proctor test.
2. Modified Proctor test.
1. Standard Proctor Test:
In this test, the soil is compacted in three layers, each layer subjected to blows of a rammer with falling weight of 5.5 pounds (2.6 kgf) falling through a height of 12 in. in a cylindrical mold of internal diameter of 4 in. and effective height of 4.6 in.
The compaction parameters in a standard Proctor test are – W is the weight of rammer blow = 5.5 pounds, h is the height of fall = 12 in. = 1 ft, n is the number of blows per layer = 25, l is the number of layers = 3, and Vm is the volume of the mold = 1/30 cubic ft. The total compaction energy imparted on the soil per unit volume in this test is –
IS – 2720 (Part 7) – 1980 recommends the Indian Standard light compaction method based on standard Proctor test in metric system. The standard Proctor test or IS light compaction can be used as a criteria for the compaction of subgrades on highways and earth dams, where light rollers are used.
2.Modified Proctor Test:
Advances in construction technology resulted in the development of heavier rollers, which impart higher compaction energy during field compaction. To provide a laboratory control criterion for the higher compaction energy, the modified Proctor test was developed and standardized by the American Association of State Highway (and Transportation) Officials (AASHO or AASHTO) and by US Army Corps of Engineers for airfield construction.
In modified Proctor test, the soil is compacted in the same mold as in standard Proctor test, which has internal diameter of 4 in. and affective height of 4.6 in. giving a total internal volume of 57.805 cubic in. or 1/30 cubic ft. The rammer is bigger with a hammer of weight 10 pounds (4.5 kgf) falling through a height of 18 in. The soil is compacted in five layers, each layer being given 25 blows.
The compaction parameters in modified Proctor test are as follows – W is the weight of hammer blow = 10 pounds, h is the height of fall = 18 in. = 1.5 ft, n is the number of blows per layer = 25,l is the number of layers = 5, and Vm is the volume of the mold = 1/30 cubic ft.
The compaction energy imparted to the soil, per unit volume, in the modified Proctor test is:
Thus, the compaction energy in the modified Proctor test is (56250/12375) 4.55 times that in standard Proctor test.
If the soil fraction retained on 20 mm sieve is more than 5%, a bigger mold of 5.9 in. (15 cm) internal diameter and 5 in. (12.73 cm) internal height giving a total volume of 137.04 cubic in. or 1/12.611 cubic ft (2250 cm3) is used. When the bigger mold is used, the soil is compacted with 56 blows for each layer.
IS – 2720 (Part VII) – 1980 and Part VIII-1983 have recommended procedures corresponding to these two tests as follows:
1. IS light compaction test.
2. IS heavy compaction test.
1. IS Light Compaction Test:
The objective of the IS light compaction test is to determine the relation between the water content and the dry density of compacted soil and to determine the MDD and OMC from this test. The compaction energy used to compact the soil corresponds to that of standard Proctor test.
The compaction parameters in IS light compaction test are as follows – W is the weight of hammer blow = 2.6 kgf, h is the height of fall = 31 cm, n is the number of blows per layer = 25, and I is the number of layers = 3, and Vm is the volume of the mold = 1000 cubic centimeter (cc). The total compaction energy imparted on the soil in this test is –
E = Whnl = 2.6 × 31 × 25 × 3 = 6045 kgf cm
The total compaction energy imparted on the soil per unit volume in this test is –
E = 6045/1000 = 6.045 kgf cm/cm3
2. IS Heavy Compaction Test:
The IS heavy compaction test is similar to IS light compaction text except for the following differences:
1. A heavy rammer of 4.9 kgf falling weight that falls through a height of 45 cm is used for compacting the soil in IS heavy compaction.
2. The soil is compacted in five layers of equal thickness in the compaction mold.
3. The initial water content to be used in the first trial is 3%-5% for sandy and gravelly soils and 12%-16% below plastic limit for cohesive soils.
To increase the accuracy of the test results, it is desirable to reduce the increment of water in the region of OMC.
Shifting and tilting problems occurs generally during sinking process of well foundations. If proper care is not taken, they will cause serious problems and weaken the stability of foundations. Precautions to avoid shifting and tilting, limitations and rectifying methods are discussed below.
Shifting and Tilting of Well Foundations
When the well is moved away horizontally from the desired position, then it is called shifting of well foundation.
When the well is sloped against vertical alignment,it is called tilting of well foundation.
Precautions to Prevent Shifting and Tilting
It is safer and economical to avoid tilting and shifting of wells by adopting the following preventive measures:
The outer surface of the well curb and steining should be level, straight, and smooth.
The radius of the well curb should be kept 2-4 cm more than the outer radius of the well steining.
The cutting edge should be sharp and of uniform thickness.
The steining should be built in lifts and the entire steining height should be built in one straight line from bottom to top at right angles to the plane of the curb.
Dredging should be uniform on all sides of the well. For a twin well, dredging should be uniform in both the wells.
The well should be constructed in stages of small height increments.
The magnitude and direction of sinking of wells should be properly and carefully monitored on a continuous basis to identify any tilt or shift and adopt appropriate corrective measures immediately to rectify the same.
If the well shows a tendency for tilting, dredging should be done on the higher side. If this does not bring required improvement, sinking should be stopped and should be resumed only after the tilting is corrected.
Dredged material should not be deposited unevenly around the well.
When a kentledge is used to provide additional sinking effort, it should be placed evenly on the loading platform.
Limitations
The maximum tilt allowed in case of well foundation is 1 in 60.
The shift in well foundation should not be more than 1 % of depth of sunk.
Beyond the above limits, well foundation is considered as dangerous and in such a case, remedial measures to rectify shifting and tilting should be followed.
Rectifying Methods
Rectifying methods for Rectification of shifting and tilting problems in well foundations are as follows:
Eccentric loading
Excavation on higher side
Water jetting
Pulling the well
Using hydraulic jacks
Using struts
Excavation under cutting edge
Wood sleeper under cutting edge
1. Eccentric loading
The well tilt can be rectified by placing eccentric loading on the higher side. Higher side is nothing but the opposite side of tilt or lower side.
A loading platform is constructed on the higher side and load is placed on it.
This eccentric load will increase downward pressure on higher side and correct the tilt.
The amount of load and eccentricity is decided based on the depth of sinking.
Greater is the depth of sinking of well, larger will be the eccentricity and load.
Fig 1: Eccentric Loading on Well Foundation
2. Excavation on Higher Side
When well is tilted to one side, excavation should be increased on the other side which is opposite to tilted side.
This technique is useful only in the initial stages of well sinking.
Fig 2: Excavation on Higher Side
3. Water Jetting
Water jetting on external surface of well on the higher side is another remedial measure for rectifying tilt.
When water jet is forced towards surface of well, the friction between soil and well surface gets reduced and the higher side of well becomes lowered to make well vertical.
Fig 3: Water Jetting on Higher Side
4. Pulling the Well
The well can be pulled towards higher side using steel ropes.
One or more steel ropes are wound around the well with wooden sleepers packed in between well and ropes to prevent damage to the well steining by distributing load over to larger area of steining.
Pull should be carefully done otherwise,shifting of well foundation may occur.
Fig 4: Pulling the Well Foundation
5. Pushing using Jacks
Another method to rectify tilting and shifting of well foundation is using hydraulic jacks or mechanical jacks, the tilted well can be pushed from lower side to higher side.
Neighbor vertical well foundations or suitable arrangements made will give support to the jack system.
Care should be taken while pushing the well otherwise the well may shifts.
Fig 5: Pushing using Jacks
6. Using Struts
By providing struts as supports on the lower side or tilted side of well, further tilting can be prevented.
Wooden sleepers are provided between struts and well steining to prevent damage to well steining and to distribute pressure to larger area.
Struts are rested on firm base having driven piles.
Fig 6: Strutting the Well From Lower Side
7. Excavation under Cutting Edge
This technique is used for hard strata soils. In this method, the well is de-watered first and open excavation is carried out exactly under the cutting edge on the higher side.
If de-watering is not possible, soil strata is loosened using suitable equipment with the help of professional divers.
8. Wood Sleeper under Cutting Edge
If tilting towards lower side is increasing,then wooden sleepers are placed under cutting edge on lower side to control the tilting temporarily.
When well is corrected to vertical level, this sleepers can be removed.
