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Compass surveying can be defined as the method of surveying in which the directions of the survey lines is determined by means of a compass and also by using tape or a chain. This measurement is directly done on the surface of the earth. The use of compass as a navigator is a practice followed for centuries. This was the only means through which directions and the horizontal measurements are made, before the invention of a theodolite or sextant.

What is Compass Surveying?
What is Compass Surveying? 


Even after different findings in the field of surveying, this method of compass surveying is still used for preliminary measurements in engineering surveying. Most of the land surveying is conducted by compass surveying by the civil engineers and surveyors which helps the foresters, geologists etc.
Initially, the theodolites where equipped with a compass which made the same heavy. New theodolite does not have a compass in it.

Mainly the compass surveying method make use of a compass to carry out the angular measurements. The compass surveying procedure is done through traversing.
Magnetic Compass in Compass Surveying

The magnetic meridian is the basis on which the magnetic compass measures the directions. This will require the magnetized needle, a line of sight and a graduated circle. The important types of compass used in compass surveying are the:

  • Prismatic Compass
  • Surveyor’s Compass
  • Transit compass

Principle of Magnetic Compass: The basic principle of working of a magnetic compass is that a magnetized steel or an iron that is suspended on a pivot at its center which lets it to oscillate freely about the vertical axis, will take the direction of the magnetic meridian of with respect to that place.
The basic parts of any magnetic compass used in surveying are the magnetic needle, Graduated ring, sighting vanes, reading system and a tripod to support the compass.

Principle of Compass Surveying

The main principle of compass surveying is traversing. Traverse, a series of connected lines is measures by a prismatic compass. Compass surveying is mainly employed in areas that is free from local attraction. This can be used for surveying the land area with undulations which is crowded and large.

Every bearing at each of station can be observed by conducting the compass survey by letting the needle to float. This approach is known as loose-needle or free surveying. But the same can be carried out with a compass with a Vernier fitted that is connected with the line of sight and moves over a fixed graduated circle. This method is called as fixed-needle or fast – needle surveying. The use of compass in compass surveying is found easy and portable which makes it suitable for exploratory surveys and reconnaissance. This bring a great application for the preliminary route surveys of lakes, streams, survey of rivers and for topographical works.

Traversing in Compass Survey 

One of the main applications of compass surveying is traversing. Traverse can be defined as a series of connected straight lines each will join at two points on the ground. The length and directions of these lines are determined from the field measurements. Compass surveying is one of the widely practiced method to determine the relative locations of the points.

The traverse can be of two types:

  • Open Traverse
  • Closed Traverse

Open Traverse: A traverse that never ends or returns to the starting point or the starting station or neither ends at a known station is called as an open traverse. The figure-2 below shows the pathway of an open traverse.

Fig.2. Open Traverse
Fig.2. Open Traverse


Route surveys mainly employ open traverse. This method does not have error checking methods, so measurements are repeated to avoid mistakes.

Closed Traverse: This type of terrace will end the work at the starting point itself hence forming a polygon. This is closed both geometrically and mathematically. Ending the survey at a point that is known before will also make it a closed traverse. The figure -3 below shows examples of closed traverse.

Fig.3. (a) A closed traverse that is geometrically and mathematically closed (b) Closed traverse that is geometrically open but closed mathematically.
Fig.3. (a) A closed traverse that is geometrically and mathematically closed (b) Closed traverse that is geometrically open but closed mathematically.

The closed traverse will provide a check on the angles and the distances. This method will help in the property surveys, construction surveys, topographic surveys and control surveys. 
The traverse surveys will determine the interior angles, the deflection angles, the angles to the right, bearing and azimuths.

Compass Traversing – Fieldwork

The two fieldworks employed in compass surveying are:
  1. Measurement of the Bearing of all the Survey Lines
  2. Measurement of length of all the lines using a tape or a chain
The prismatic compass is mainly employed for compass surveying in order to measure the angles. This will measure the forward and the backward bearings. This measurement will help in eliminating the errors that is caused due to local attraction. 

While conducting survey, the traverse stations have to be selected carefully in order to satisfy the usual conditions. A field notebook is kept in order to take the observations.

Fore Bearing and Back Bearing in Compass Surveying

Every line in a traverse is defined by two bearings. They are the Fore bearing and the back bearing. The bearing measured is represented in the Whole Bearing System (WCB) and these differ by 180 degrees. 

Fore or forward bearing (F.B) is the bearing of the respective line in the direction of progress of the survey. The bearing that is measured in the opposite direction of survey is called as the back bearing (B.B).

Fig.4.The fore Bearing and Back bearing conducted on a line AB.
Fig.4.The fore Bearing and Back bearing conducted on a line AB.


As shown in fig-4 above, the bearing taken in the direction of AB is called as Fore bearing and the measurement taken in the direction of BA forms the back bearing as shown in the figure-4. 

Temporary Adjustments in Compass Survey -  Using a Prismatic Compass


The figure-5 below shows the schematic diagram of a prismatic compass. The adjustment of the prismatic compass in tripod will involve the following procedure:
  • Centering
  • Levelling
  • Focusing
Centering: The procedure of centering will involve the placing of a compass over the station where the bearing has to be determined. Dropping a pebble from the bottom side of the compass will help to know the correct position. If the pebble falls correctly over the station then the centering is correct. If not correct, the adjustment of the tripod will help in correcting the position.

Fig.5.Prismatic Compass
Fig.5.Prismatic Compass

Levelling: Free swing of the graduated circular ring in the prismatic compass is achieved by the leveling procedure. The tripod possesses a ball and socket arrangement that helps to achieve a proper level of the compass.

Focusing: Until we see the aluminum ring clearly, the prism is moved up or down. The vision of the observer is controlled by the position of the prism. 





The word plum means large stones which are termed as boulders or coarse aggregates if technically speaking. The plum concrete is defined as:-


A mixture of coarse aggregate and cement that are placed below the foundation or footing of a structure so as to achieve a level surface for equal distribution of load. 


The plum concrete is actually an economical variation of mass concrete. The use of plum concrete is preferred if the required thickness of PCC is excessive or large.

This is mainly done below the foundations where due to steep slope of the strata, the quantity of leveling course could be excessive.

What is Plum Concrete?
What is Plum Concrete? 


Application of Plum Concrete 


  1. In one example the plum concrete can be used below footings of residential buildings having small portion where the slope of ground below single footing is 1:10 to 1:50 so to save the cost of concrete plum concrete is usually preferred. This will result in minimization of the construction cost of building as a whole.
  2. Plum Concrete is also used in areas that require massive concrete placements like concrete dams or bridge piers. In such cases pieces of rock about 150 mm in size are used as coarse aggregates to mix a plum concrete.
  3. Similarly in another example; suppose the foundation pit for a machinery bed is proposed to have a certain depth but the hard stratum is available at 3 m below the ground level.  Whereas in design It was proposed that the depth of foundation for bed would be 1.5 m. Since the foundation has to be rested on the hard strata of soil, so plum concrete of lean ratio with 80mm -100mm size stones would be poured which will be rammed upto 1.5 m and thereafter regular designed raft, footings may be laid. 


Mix Design of Plum Concrete 

I was actually working on a building project a few years back, when I joined the project the excavation work was nearly done and my senior engineer told me that they are getting prepared for placement of plum concrete. And when I heard the word “plum concrete” I was stunned as I haven’t heard that thing in the past as I was fresh obviously :D.

But the reason of explaining above fact was that I had actually worked on the project where we placed plum concrete.  So for that particular project we used 40% boulders and 60% concrete which means M15 was used.

Methodology or Construction Process for Plum Concrete

Step 1 – Transportation of Plum Material
The boulders are large Stone size which can be lifted by laborers easily, not too big or small.
Step 2 – Leveling and Cleaning – Surface Preparation
The first step that is done initially is leveling and cleaning of the surface by removal of soft soil that can cause low bearing capacity. After clearing and grubbing water is sprayed to the surface to make it wet before placement of plum concrete. The reason of sprinkling water is to ensure proper bond of plum concrete with the ground surface. After sprinkling of water anti-termite chemical is sprayed which is essential now a days for the flawless foundation of a structure.
Step 3 – Pacing and Spreading of Plum Concrete
Boulder's are spread over the ground with little gapping layer by layer and concrete is spread using pump all over the boulder's in each layer which slowly permeate inside the gapping between boulder’s. This helps to bind it properly, after pouring concrete, boulders are thrown into the concrete again and the process repeats till required level surface is reached.
Step 4 – Curing of Plum Concrete
Curing should be done for next 7 days minimum. Jute bags are spread over the whole area of plum concrete to retain moisture for a long time after curing.

Boulder's should be inspected before spreading and if any dirt or Clay is found, then it should be washed properly.

Boulder's should be strong and should not be falky. since, whole building load is going to act upon it.
Fact is that nobody in construction site, measures that how much boulder's and concrete is used in doing plum concrete. Sometimes, more boulder's are needed when there is dearth of concrete or for economical purpose etc.

Specifications for plum Concrete


Plum shall comprise of black trap basalt of crushing strength of minimum 100 Kg /Sqcm.
The concrete shall be of nominal mix 1:4:8 using aggregate of maximum size 25mm.
The volume of the plum in the concrete shall not exceed 50% of total volume.
The crevices shall not be less than 150 mm.
Layer shall not exceed 900mm.

The  nominal maximum size of coarse aggregate should be as large as possible  within the limits  specified  but in  no  case  greater  than  one-fourth  of  the  minimum thickness  of  the  member,  provided  that the  concrete can be placed  without  difficulty  so as to surround all reinforcement   thoroughly  and  fill  the  comers  of  the form. 

As for the requirement of the aggregate size that can be used in the plum concrete there are different recommendations by the IS code; the code says that for most of the common works the size of 20 mm aggregate is suitable and best. If there are no restrictions for flow of concrete into section 40 mm or larger can also be used. But where there are reinforcement or the cover is small with the formwork 10 mm nominal maximum size of plums should be used in plum concrete.

Plums  above  160 mm  and up to any  reasonable  size may be used in plain concrete  work up to a maximum limit   of  20  percent   by  volume   of  concrete   when specifically  permitted  by the engineer-in-charge. 

However for all the cases, the plums shall be distributed evenly and shall not be closer than 150 mm from the surface.

Difference between Plum Concrete and Mass Concrete


Sometimes the people do have misunderstanding about the difference between plum concrete and the mass concrete. The both are actually one and the other thing; mass concrete is a term used for concrete that is placed in large size and huge quantity that can either be a plain concrete or reinforced according to the requirements of the structure or project. The term mass concrete is just used to define the massiveness in quantity.

But as for the plum concrete it is by all means plain having large size of aggregates, as explained above called plums and they can be used with different grades of concrete.

What’s further…???

So in this post you have completely learnt about what is plum concrete, what is the use and advantages of plum concrete, what is the mix design and what are the different specifications or guidelines for the plum concrete.

If you think I have missed anything that you know must be added than please do comment your thoughts below. 

Raft foundation are among the most common types of foundations used these days keeping in view the structural safety, requirements and the ground bearing capacity.  

What is Raft Foundation?
What is Raft Foundation? 

So you might also be the one that are told by their contractors or construction experts that your house is going to be built on the raft foundation and you are searching for; 

  1. what is raft foundation, 
  2. when the raft foundation is a must to be used, 
  3. what are the different types of raft foundations
  4. what are the advantages or disadvantages of using a raft foundation. 


So hang-on guys we would be answering your all of such questions relevant to raft foundation

Introduction to Raft Foundation


In Civil Engineering and in structures, foundation is the supporting base of a structure which forms the interface across which loads are transmitted to the underlying soil or rock.

The foundation can be of plain concrete or reinforced concrete, sometimes foundation is of earth or rock like in roads, embankment and dams. 

What is Raft Foundation?
What is Raft Foundation? 

Related



Raft foundation as type of shallow foundation

Shallow Foundations are sometimes called spread foundations and include isolated pads, strip footings and raft. The most usual definition of a shallow foundation refers to the founding depth being less than the breadth. For the most part in the case of pad foundation and strip footings this is acceptable, but for wide rafts it is clearly unacceptable. It is sensible, therefore to limit the term ‘shallow’ to mean less than 3 m or less than the breadth of the footing. 

What is Raft Foundation? 

Raft foundation, sometimes also referred as mat foundation is a combined footing covering the entire area underneath the structure and thus providing supports to all the walls and the columns above. The raft foundation can either be placed directly on the soil or rock, but can also be placed directly on the piles if the underneath soil is that much weak. 


Situations in which Raft Foundation must be Used 

Although, it is quite impossible to lay down hard and fast rule to define situations where a raft foundation is required. It is actually the decision of the geotechnical engineer and designer based on his judgment, keeping in view the site situation, cost, safety and ease of construction.  There can always be differences of opinion about the solution therefore it is the experience and the testing that can lead you to a favorable, optimum and economical solution. 

The following situations could help you in making decision of requirement for a raft foundation :-

1. If the underneath soil has low bearing pressure or the loads superimposed by the building or structure are very heavy such that the individual area needed for allowable pressure is larger sometimes more than 50 % of the total area than in such cases raft foundation. 
a. Using raft foundation will increase the width and thus the ultimate bearing capacity will increase, because increasing width will bring deeper soil layers in action. 
b. Increasing the depth of action of the foundation would decrease the settlement. 
c. As raft foundation bridges over cavities in the underneath soil, it diminishes the chances of differential settlement. A structure is always designed in a way to bear a safe differential settlement but above that critical dangerous stresses would develop that would not be bearable for the structure. Thus raft foundation ensures equal settlement and thus increases the bearable settlement than that if the foundation is build on individual footings. 

It must be mentioned here that you can not always expect the deeper layer of soils to be of great bearing quality and capacity, reverse can also occur. In such situations it will be advantageous to provide individual footings so that the zone of influence of the footings remains within the top stronger layer. 

2. Sometimes it is decided by the designers that if more than 50% of the area of the structure will be occupied by individual footings than in such cases it is necessary to provide an overall raft. But this is not always true, the quantity of reinforcement steel and concrete required to avoid excessive deflections and cracking of raft carrying unequal column loads, may be larger and may make raft foundation uneconomical. In such situations it is more economical to excavate the entire site to a level formation, construct individual closed space footings (sometimes touching each other) and then backfill around them. In these cases, however, one must weigh formwork costs against the extra footing material required by use of mat foundation. 

3. Situations exist in practice when a soil stratum contains compressible lenses or the soil have a formation where individual layers of soil are neither parallel nor can be reasonably stratified into different layers of known properties to enable calculations of settlement to a reasonable accuracy. In such situations, individual footing, if provided, would undergo widely varying settlements which cannot be tolerated by the structure. 

4. Raft foundation is unavoidable in cases where structures and equipments to be supported are very sensitive to differential settlement. Where structures naturally lend themselves for the use of raft foundation such as silos, chimneys, water towers etc.  

5. You must have heard about floating foundations, apparently you may have got picture of a foundation floating on the surface of water or air; however, this is not the floating foundation. Sometimes we may encounter a soil that is very compressible and soft as well, such that buildings cannot be founded on them.  In such cases a solution exists, how if we remove the equal quantity of soil, in terms of weight, to that of the load to be supported on the soil and thus there would be no change in the stress. Such a foundation is called floating foundation. 

However, in practice it is rarely possible to balance the loading so that no additional pressure comes on the soil. However, in such cases still, it is only a part of the total load which comes on the bottom soil and, thus it is possible to construct a building inducing a much larger load than the soil would have otherwise supported. 

After said that, we know that soil if was under excess loading is suddenly released than it swell and come up thus in such cases reconsolidation of the soil must be considered a necessary step to be taken to prevent detrimental effects. 

6. Basements located below ground water table should use a mat as their base to provide water tight construction. The alternative of having individual columns footings connected by thin slabs has not proved to be successful in most of the cases;  presents difficulties in water proofing, causes concentration of stresses at the junction of the thin slabs and footings and also at the junction of the basement walls and raft causing cracks to develop. This arrangement, therefore, should not be resorted to unless the economy is of such a magnitude as to outweigh all other considerations. 

Even in cases where sub-soil water Level is low and basement does not extend below ground water table, long-term built-up of surface water accumulating against basement walls and bottom should be allowed for. This is particularly so in case of impermeable soils or of large surface areas draining towards the building like areas on sloping ground near hillrocks. The basement walls should be also normally be designed as self-supporting cantilever retaining walls even though they may eventually be strutted by floor construction. It is convenient and often impossible to provide temporary raking struts to support a basement retaining wall until time as strutting given by ground floor or intermediate floor is completed. 

7. Situations also arise when isolated footings are subjected to very large eccentric loadings, and one is faced with the possibility of excessive footing rotation, excessive differential settlement or possibility of exceeding the allowable bearing capacity of the soil at some location. This can happen when the building consists of shear walls and columns, shear walls sharing most of the horizontal load subjecting its footings to large settlements and rotation, decreasing the effectiveness of the shear walls and also creating difficulties by way of large differential settlements raft, if provided, will even out these deformations.  

Raft foundation construction process


Step 1 – Excavation and Ground Preparation


As per the design depth of the raft foundation from the plinth level of the building along with the depth of the raft foundation slab the ground is excavated with excavator or manual labor depending on the quantity of excavation and requirement of the project.  After excavation and removal of the vegetative top soil the subgrade is compacted and compressed while sometimes hardcore broken rocks or stone is spread to raise the ground floor level and for a stable footing. 

Step 2 – Placement of Blinding Concrete


After achieving a compacted surface, a blinding or leveling concrete is poured so as to get a level bed for reinforcement to be fixed and concrete to be poured. 

Step 3 – Fixing of Formwork and Final Survey


After the blinding has been allowed to cure for some days, then the temporary formwork (wood) is then erected over it all around that will serve as the support to for the reinforced steel bars and concrete work. 
Mould oil can be used on the surfaces and sides of the formwork. This makes the finished concrete work to have a clean look or surface by preventing the concrete from sticking to the wood used.

Step 4 – Fixing of Reinforcement and Pouring of Concrete


The reinforced steel bars will then be placed into the formwork following the bending schedule the structural engineer has provided. After this is done the concrete which is mixed to a given ratio is then poured in and spread all over to cover the reinforcement. 

Step 5 – Curing and Application of Damp Proof Membrane


When the concrete has been allowed to cure for some days. A layer of damp proof membrane is spread around the entire area of the foundation and then over it reinforced mesh wire is laid, which will receive the concrete for the main floor slab (German floor). Before the concrete is poured all necessary plumbing piping is done. After the concrete has been cured the main walling for the building can begin.

Curing concrete prevents the concrete from drying too quickly. This is done by pouring water over it, using formwork or a plastic shield or cover. This process stops the concrete from cracking or becoming too weak when it dries too fast.

Types of raft foundation

As we know that in civil engineering projects the professionals have to understand the unique and challenging underground soil conditions that vary from project to project so for different geotechnical and structural requirements there exists many types of raft foundations to mitigate uncertainties.  
The classification and types of raft foundation are depending on two things :-

1. Different Supports and Restraints Conditions
2. Different Structural and Framing System

For the first classification type there are three different types of raft foundation :- 

1. Raft on Soil / Ground
2. Pile supported raft
3. Raft with Buoyancy Effects 

Depending on structural system they are also classified in three categories:

a. Rafts having uniform thickness of slab, sometimes they may have pedestal
b. Raft with beams & slab system; 
c. Framed raft or cellular raft having foundation slab, columns, walls rendering essential rigidity to structure.
Based on these different structural systems there are different famous named types of raft foundation which we will discuss now in the coming paragraphs:- 
1. Flat Plate Raft Foundation
2. Plate thickened under Columns Raft Foundation
3. Two-way beam and slab raft Foundation
4. Raft foundation with Pedestals
5. Rigid Frame Raft Foundation
6. Raft supported by Pile Foundation
7. Wide Toe Raft
8. Slip Plane Raft
9. Blanket Raft
10. Slab Beam Raft
11. Cellular Type Raft

Raft foundation advantages

Due to the construction of raft foundation we can achieve a bigger bearing area for load distribution that can ensure a stable column foundation without differential settlement which will avoid walls deformation. 
Raft foundations tend to be cheaper and quicker to use than traditional footings. There are a number of reasons why this is the case:
The foundation and floor slab is combined, which saves time and materials
Less excavation is required
Other reasons that make raft foundations preferable to footings are due to their engineering benefits. They are ideal for poor ground condition where normal footings would not cope well as they cannot spread the load as effectively.
Related to this is that raft foundations can reduce differential settlement, where settlement occurs at different rates across the ground surface of the building, which reduces cracking and other more serious problems.



The construction industry, if given name as deadliest industry would not be wrong. It is inherently a hazardous industry with special thanks to the heavy equipments and giant mechanized robots contributing significantly in the occurrence of fatal accidents at construction sites. 

Construction Accidents : Risks and Types
Construction Accidents : Risks and Types 

The fatalities caused by road accidents, agriculture is parallel to the fatalities caused by construction industry. Every 4th person working in the construction industry die at site while working out of the 10,000 workers.  These accidents and fatalities have monetary consequences on the project as well as to the client involved. A major share of the construction cost is spent for repairing structural failures and due to delays caused by the accidents at construction site. 

Due to these construction accidents the construction works and managers deal with danger every days, whether they are working on a small scale projects like single room remodeling project or construction of a 300 stories mega projects. 

Classification of Construction Accidents 

1. According to the cause of occurance
2. According to the nature of injury sustained
3. Temporary disablement
  • Partial disablement
  • Total disablement
  • Total disablement
  • Death


3.     According to the severity of injury
  • Minor accident
  • Major accident
  • Accident hazard

Types of Construction Accidents

Unsafe working conditions

It is a condition in which the physical layout of the workplace or work locations, the status of tools, equipment, and / or material are in violation of contemporary safety standards

Unsafe working conditions
Unsafe working conditions


Falling from heights

Falling accidents typically occur when a worker steps to or backwards an open-sided floor or slab and focuses on the work. Slips, trips and low falls are the most common forms, and usually cause minor injuries (e.g. musculoskeletal disorders, puncture wounds, etc.). by contrast, fatal accidents can be caused by falls from height, and in most cases workers suffer from head or shoulder wounds. Lack of opening protections, inadequate edge protections, may lead to potential falling hazards. 

Falling from heights
Falling from heights

Falling objects

In the site when an object is dropped onto you from above without any warning or the ability on your part to move out of the way causing an injury to you which makes it impossible for you to continue working. Injuries could vary from minor cuts and bruises to more serious crush injuries and head and brain injuries. 

Falling objects
Falling objects

Electrocutions and power tool accidents

Electrocutions can be caused by body contact with power cables, cable strikes on underground utilities, or defective of ground-fault protection. Power tool accidents, such as nail gun injuries, are usually caused by inappropriate use or inadequate eye protection. Furthermore, improper guards in place on power tool management may lead to more severe injuries. 


Excessive noise / vibrating tool hazards

Noise and vibrating tools are also a cause for concern whilst working in the construction industry as excessive noise from power tools can cause hearing loss or problems such as tinnitus (ringing in the ears). The vibration from power tools can cause nerve and tendon damage to the hands, arms and wrists which if experienced can be extremely debilitating with reduction and sometimes total loss of grip strength. 

Scaffolding accidents

Scaffolding accidents cause large numbers of deaths and injuries every year. Most accidents are caused by scaffold equipment failure, inadequate scaffolding safety training, lack of personal fall protective system, or improper scaffolding equipment operation. In addition, materials and tools falling off scaffolding may also cause injuries. 

Scaffolding accidents
Scaffolding accidents


Stepladder misuse

The majority of falls in construction accidents are due to stepladder misuse or failures. Ladder injuries include loss of balance, improperly ladder setting up, loss of balance, and exceeding the ladder weight capacity etc. 

Construction vehicle accidents

Large construction equipment (e.g. cranes, forklifts, dump trucks, road graders and concrete mixtures, etc.) may become potential dangers on a construction site. Due to their weight and size, workers and drivers can be seriously injured or killed in an accident. In some cases, drivers may not have competent driving skills, or their visibility may be limited due to site constraints. 


In a research by R.A. Haslam et al. on contributing factors causing construction accidents revealed that most of the construction accidents were due to problems arising from workers or the work team (about 70%); while others include workplace issues, shortcomings with equipment (including PPE); problems with suitability and conditions of materials and deficiencies with risk management. 


Among the most recent construction accidents that have been reported on New York times includes the partial collapse of a construction crane in Miami. The collapse was caused because of Category 5 hurricane that caused crane collapse at 2 different spots in the city. In another incident a 59 year old worker named Wilfredo Enrique died after falling from scaffolding at the site of the residential development in Brooklyn which was reported as the 11tst construction death this year. 
  

Key take-a-ways??? 



Even with all the safety education, you just have a monetary lapse while trying to fix something. And it just takes a few seconds of not slowing down and following procedure. 

Rescue and recovery of the victims is always of primary importance, and often requires engineering knowledge for stabilization and extraction. For this reason the rescue team always requires the services of a structural engineer. After the victims have been cared for, the accident area is evaluated, and unsafe conditions, such as unstable debris or dangerous materials, are mitigated so that investigation and clearing of the site can be proceed. 

Piling rigs are a common place feature on building sites in cities and towns today. The continuous introduction of new, more powerful, and self-erecting machines for installing piled foundations has transformed the economics of this form of construction in ground conditions where, in the past, first consideration would have been given to conventional spread or raft foundations, with piling being adopted only as a last resort in difficult ground. 

The increased adoption of piling is not only due to the availability of more efficient mechanical equipment. Developments in analytical methods of calculating bearing capacity and dynamic methods for load and integrity testing have resulted in greater assurance of sound long-term performance. Further economics in foundation superstructure design are now possible because of the increased ability to predict movements of piles under load, thus allowing engineers to adopt with confidence the concept of redistribution of load between piles with consequent savings in overall pile lengths and cross-sectional dimensions, as described in this new edition.

Pile Design and Construction Practice Book PDF by Michael Tomlinson & John Woodward
Pile Design and Construction Practice Book PDF by Michael Tomlinson & John Woodward 

Title of the Book

Pile Design and Construction Practice

Author of the Book

Michael Tomlinson John Woodward

Content of the Book


1. General Principles and practices
2. Types of Pile
3. Piling Equipment and Methods
4. Calculating the resistance of piles to compressive loads
5. Pile groups under compressive loading
6. The design of piled foundations to resist uplift and lateral loading
7. Some aspects of the structural design of piles and pile group
8. Piling for Marine Structure
9. Miscellanous piling problems
10. The durability of piled foundation
11. Ground investigations, piling contracts, pile testing
12. Properties of Materials

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We have already defined the pile foundation, its type and importance in case of low bearing ground conditions. But this post will mainly deal about the procedures and steps that are undertaken for the construction of pile foundation. Pile foundation construction procedure is not simple as it involves heavy machinery at times and requires fair assessments of the underground conditions prior to design and preliminary survey. 



Pile foundation act as a steady support for structures making use of the skin friction with the ground as well as the increased bearing capacity of the soil at greater depths. We would not deal here the design of the pile foundation but would only talk about its construction procedure at site. 

Construction Procedure for Bored Piling 


Bore piles are commonly used to support heavily loaded structures such as high-rise buildings and bridges in view of the low noise, vibration and flexibility in size to suit different loading conditions and subsoil conditions. These vibrations are mostly attached to other conventional piling system like driven piles.  The construction sequence / procedure of the bored cast in-situ piles is as follows:-

  1. Setting Out
  2. Excavation
  3. Placement of Reinforcement
  4. Concreting 
  5. Stipping Pile Heads and Bonding
  6. Piling Records

Setting Out

The first and foremost step and is no doubt the most important step for carrying out piling is setting out. Generally a licensed Surveyor hired by the contractor will set up the positions of the piles which is shown in the pile layout plans of the detailed designs showing the Northing and Easting of the center of each piles along with diameters etc. The positions set out by the Surveyor are secured and preserved by pegs. 

Setting out (pic credit indiaMART)


The positions of the pile are produced as-built drawings showing the position of all the piles constructed or installed.  The surveyor will be checking the correct position and verticality of the pile foundation.  Mostly the verticality of the finished pile from the vertical at any level is given tolerance of 1 in 150. 


Contractual Submittings by the Contractor

The contractor before carrying out any physical work must submit the list of persons in charge of the supervision work for pile foundation for the approval of the Engineer. In Addition to this all the technical details manufacture comments of the piling equipment and accessories must be made along with sequence for construction of the piling work. 

Excavation

The excavation of boring of the pile is carried out by auger method of boring or conventional percussion boring. In Auger cast piling procedure, a continuous flight auger drill is used to excavate a hole by screwing a hollow shaft auger into a depth of at least equal to the length of the pile. 
Continuous Flight Auger (CFA) piling method is the quietest from of piling and is a fast and very economical technique. It is a cast in-situ process, very suited to soft ground where deep casings or use of drilling support fluids might otherwise be needed. 

Auger Piling (Pic Credit: manavinfra)
Auger Piling (Pic Credit: manavinfra) 

The word auger boring can be defined as a method of boring beneath the earth’s surface by means of a jack and bore. It works by simultaneously jacking the casing pipe while the rotating augers remove the excavated soil. An auger is a drilling device, or a drill bit, that usually includes a rotating helical screw blade called a “flighting” to act as a screw conveyor to remove the drilled out material. 
Percussion drilling, on the other hand, is a manual drilling technique in which a heavy cutting or hammering bit is attached to a cable which is lowered in the open hole or inside a temporary casing.

Percussion Boring (Pic Credit : geositesurveys) 


Reinforcement lowering


Most of cast in-situ concrete piles are reinforced throughout its length with a cage of reinforcement fixed at ground and then lowered with the help of crane before concreting. The reinforcement usually used in pile foundation includes vertical main bars of #11 diameters with the spiral of lower dia reinforcement for providing shear and avoiding the buckling failure in the pile foundation.

The reinforcement bending schedules are first properly  prepared by the contractor which is submitted to the Engineer for approval. After approval the bar bending schedule is used for cutting and fixing of the rebar at site. 

Rebar Cage Lowering
Rebar Cage Lowering

Concreting

The tremie method of concrete placement is used for the concrete pouring of the pile foundation. This method uses a vertical pipe, through which concrete is placed by gravity feed blow water level. The lower end of the tremie pipe is kept immersed in fresh concrete so that concrete rising from the bottom displaces the under ground water. The dia of the tremie pipe varies from 20 to 30 cm. 

Tremie Method of Concrete piling
Tremie Method of Concrete piling

Deep foundation especially pile foundation is best suited and mostly used for a low bearing and bad textured soil or for the heavy load structures. Bearing capacity is the deciding factor for the geotechnical design of foundation. Bearing capacity of soil is defined as load bearing capacity of the soil without having different settlement as well as shear failure in the undernetath soil of the foundation.

Difference between End Bearing Piles and Skin Friction Piles.
Difference between End Bearing Piles and Skin Friction Piles. 


Generally the bearing capacity of the soil is directly related to the depth from the top soil upto a certain extent after which it again reduces. Terzaghi’s and Meyerhof’s theory of Bearing Capacity are mostly used for the geotechnical design of foundations.

A pile in civil engineering & construction is basically a long cylinder of a strong material either concrete or steel that may be casted at site or driven or pushed into the ground to act as a steady support for the structures built on top of it.

Cases to Use Pile Foundation

Following are the cases in which pile foundation is recommended :-
  1. When there is a layer of weak soil at the surface. This layer cannot support the weight of the building, so the loads of the building have to bypass this layer and be transferred to the layer of stronger soil or rock that is below the weak layer. 
  2. When a building has very heavy concentrated loads, such as in a high rise structures, bridges or water tanks. 
  3. Subsoil water level is high so that pumping of water from the open trenches for the shallow foundation is difficult or uneconomical. 
  4. Large fluctuations in subsoil water level that may cause sinking foundation in case of shallow foundation. 
  5. Structure is situated near river bed, where there is a danger of scouring action of water. 
  6. The top soil is of expansive nature. 


End bearing piles and friction piles are basic two types of pile foundation mostly used throughout the worlds. 

End Bearing Piles 

These piles are used to transfer the load through water or soft soil ground to a suitable hard bearing stratum.

End bearing piles are used to transfer load through water or soft soil to a suitable bearing stratum. Such piles are used to carry heavy loads safely to hard strata. Multi-storeyed buildings are invariably founded on end bearing piles, so that the settlements are minimized.

End bearing piles are typically driven through soft soil, such as a loose silt-bearing stratum underlain by compressible strata. Remember this factor when determining the load the pile can support safely.

Friction Piles

These piles are used to transfer to a depth of a friction load carrying material by mean of a skin friction along the length of piles.  Friction piles are used to transfer loads to a depth of a friction-load-carrying materials by means of skin friction along the length of the pile. Such piles are generally used in granular soil where the depth of hard stratum is very great.

Some piles transfer the super-imposed load both through side friction as well as end bearing. Such piles are more common, especially when the end bearing piles pass through granular soils.

Friction piles are used in the soil of fairly uniform consistency and the tip is not seated in a hard layer, the load carrying capacity of the pile is developed by skin friction.  The load is transferred downward and laterally to the soil.




Key takeaways…?


So after reading the difference between End Bearing Piles and Friction Piles you have understood the basics of pile foundation, when to use pile foundation, when to use friction piles, when to use end bearing piles.
We normally use frictional pile when we discover by geotechnical investigation that the hard strata (e.g. Rock etc) lies at unreachable depth (let’s say more than 60m) and it would be difficult to execute with available machinery of such capacity. So what we do , we provide frictional pile in that case whose purpose is to transfer large structural imposed loads to soil instead of transferring to end tip of pile.
Similarly when hard strata is found at reachable depth (let’s say less than 40m) we transfer superimposed load to end tip, this type of pile is end bearing.

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