Model of Industrial Training Report for B.Tech

INDUSTRIAL TRAINING REPORT

Submitted in partial fulfilment of the

requirements for the award of the

Degree of Bachelor of Technology in Civil

Engineering

of the University of Kerala

Submitted By

SRIJITH BALAKRISHNAN

08403051

DEPARTMENT OF CIVIL ENGINEERING

THANGAL KUNJU MUSALIAR COLLEGE OF ENGINEERING

KOLLAM – 691005

2011

Industrial Training Report

DEPARTMENT OF CIVIL ENGINEERING

THANGAL KUNJU MUSALIAR COLLEGE OF ENGINEERING

KOLLAM

CERTIFICATE

This is to certify that, this report of the industrial training is a bonafied report of the work done by SRIJITH BALAKRISHNAN, in partial fulfilment of requirement for award of degree in Bachelor of Technology in Civil Engineering from University of Kerala.

Staff in Charge Head of Department

Mr. A Nizad Prof. ROSAMMA DANIEL

Assistant Professor Professor

Department of Civil Engineering Department of Civil Engineering

TKM College of Engineering TKM College of Engineering


Kollam Kollam

Table of Contents

Serial No. Contents Page No.

1 Abstract 12 Design Parameters 23 Stages of construction

Site Preparation (leveling) 4Site Marking 5Excavation 6Foundation Base leveling 7Bar bending, Placing and Tying 8Concreting of Mat Slab and Beams 10Concreting of First Lift of Columns 11Preparation of Templates 11Placing of Templates and Tower bolts & Shuttering 12

Concreting of Second Lift of Columns 12Removal of Shutter 13Curing 13Plastering 13Backfilling 13

4 Scheduling for Construction 145 Appendix

Excavation Plan 15Site Plan 16Foundation and Reinforcement Detailing & Drawing 17


Abstract

As a part of the academic requirements, an Industrial Training Programme was undergone from April 17 to April 27, 2011. During the training period, the different stages in the construction of the Foundation of a self-supporting Communication Tower (cell phone tower) were closely studied. The construction was undertaken by the firm, Lakshmi Enterprises, Chennai and the construction site was Edakkurissi in Palakkad District. The work was completed in a span of two-and-a-half weeks.

Due to the low bearing capacity of the soil, and since the tower height ranges from 40m to 75m, usually Reinforced Ribbed Mat Slab Foundations are used for such tall structures. Mat-slab foundations are used to distribute heavy column and wall loads across the entire building area, to lower the contact pressure compared to conventional spread footings. The mat footing constructed in the site was of dimension 7.7mx7.7mx3m. The first step in the construction was plot-leveling, followed by marking and excavation of soil according to the engineering drawings. PCC was laid to a thickness of 0.15m to level the base of the trench. Bar bending, placing and tying of rebar are also done. The foundation consists of two reinforcement mats. The raft is of 0.35m thick. The reinforcements are also made for the connecting beams (0.55m thick) and the four columns that are to be raised. Shuttering is also done. Concreting is carried out in three stages. On the first day of concreting, the bottom slab and the connecting beams are concreted. The next day concreting is done for the first 1.5m lift of the column. Since the tower is made of steel, the foundation is to be connected to the tower using tower bolts. So the tower bolts are fastened to their positions by using templates. On the next day the final stage of concreting is carried out. Shutters are removed and plastering is done. Continuous curing for seven days is compulsory for any concreting works. As a last step, backfilling of soil and


leveling are done which gives required strength for the foundation

to resist overturning due to seismic and wind loads.

Design Criteria

A Communication tower is a steel framed structure that is used to support Base Transceiver Stations that facilitates mobile phone communication. These are usually self supporting towers erected either on buildings tops or on the ground and have a height range of 50m to 200m. Since these are very tall structures, the foundation provided needs to satisfy several conditions other than the self weight of the structure. Therefore, a communication tower mounted over the ground is usually provided with Raft (Mat-slab) Foundation.

Mat-slab foundations are used to distribute heavy column and wall loads across the entire building area, to lower the contact pressure compared to conventional spread footings. Mat-slab foundations can be constructed near the ground surface, or at the bottom of basements. In high-rise buildings, raft foundations can be several meters thick, with extensive reinforcing to ensure relatively uniform load transfer.

Raft Foundations are stiffened by ribs or beams incorporated into the foundation. Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. They are often needed on


soft or loose soils with low bearing capacity as they can spread the loads over a larger area.

In the case of a communication tower, a steel framed structure is to be supported by the Mat slab foundation. Therefore, during the design of such a foundation, a number of parameters have to be taken into account. To meet the varying needs in respect of soil conditions and loading quantum, several tower foundations are used for communication towers. Design philosophy of tower foundation should be closely related to the principles adopted for the design of towers, which the foundation has to support. It should take care of all the loads such as dead load, live loads, wind loads, seismic loads, erection loads, etc. causing vertical thrust, uplift as well as horizontal reactions. For satisfactory performances, it should be stable and structurally adequate and be able to transmit these forces to the soil such that the limit soil bearing capacities are not exceeding.

The Design of the tower foundation has to be carried out as per the Indian and International Standards. Wind load calculations are done as per IS: 875:1964 PART III, material standards as per IS 2062 and IS 8500 and Design standards as per IS 802, 800 and 806. Wind loads are an important parameter in the design. Winds tend to create tensile uplift forces in the foundation. To calculate the wind load, the maximum wind fall in the last 50 years is collected from the meteorological department and double of that value is taken for the calculation. Design parameters like diameter, length and spacing are normally determined to ensure the overall stability of foundations in a given rock mass, allowable displacement and the risk of tendon corrosion.

Soil conditions is the other parameter that has to be given due consideration. The limit bearing capacity of soil, density of soil and the angle of frustum influence the design values. These soil properties are normally obtained either by conducting in-situ or laboratory tests on soil samples collected from the field during soil investigation or from available testing record of the area. Limit bearing capacity is vital from the point of view of establishing the stability of foundation against shear failure of soil and excessive settlement of foundation when foundation is subjected to total


downward loads and moments due to horizontal shears and/or eccentricities as applicable. The density of the soil is required to calculate the uplift resistance of the foundation. The angle of frustum is required for finding out the uplift resistance of the foundation.

Stages in Construction

Site Preparation (Leveling)

The first step in the actual construction is site preparation. Site preparation consists of clearing away the trees from construction site, and doing whatever rough grading that is necessary in the installation of the footings and foundation. After selecting a suitable site for the tower (probably an isolated, elevated area), clearing of the land is done. This involves the removal of trees, roots, and other vegetation from the site. This can be done using manual labor. The debris can be removed using a dozer.

After clearing the site, the undulations in the terrain have to be roughly leveled. Usually a dozer is used for carrying out the purpose. The cut and fill are balanced so that the leveling work is minimized.


A minor grade is induced deliberately for natural drainage of the surface. For small works, Manual labor and hand tools are used to roughly level the plot.

Site Marking

Before the excavation of soil is started, the layout of the structure has to be marked on the ground. This process is called Site marking. This is a very important step, because here we begin the construction part. The slackness made in layout causes extensively damage to the work and heavy loss to the owner. If a building is constructed with an inaccurate marking, it can even affect the stability and aesthetics of the building. The centre line, base line, etc. are fixed prior to the marking. Using pegs and ropes the layout for excavation is clearly marked to project the plan on the ground.


In the case of a communication tower, the foundation is symmetric, most often square in shape. Here, the dimension of the foundation is about 7.7mx7.7mx3m. A working clearance of 0.5m in each side has also to be taken into account. This is for the convenience of the laborers to work in different stages of construction like excavation, shuttering, concreting etc. The corners of the foundation have to be accurately checked for being right angle. For larger projects, this can be achieved by using a theodolite or total station. For smaller works, the diagonals are also measured to ensure the right angle.

Excavation

Excavation is the removal of soil from the surface to form a trench of required depth in which the foundation is built. Usually a backhoe or a digger (JCB) is used for this purpose. For smaller works manual labor is sufficient. The excavated soil can either be transported to some other place, or if backfilling is required, is dumped near to the trench or sideways. In the case of a raft foundation for a communication tower, backfilling is needed. Digging is started from a corner and is proceeded through an edge. This helps in dumping of the soil to the sides of the trench without causing difficulties for the backhoe to move. In case of any obstructions, digging is carried out manually. But the time required for manual excavation consumes much time.


At the site, A JCB digger was employed for excavation work. After the field layout is marked, a square trench of side 0.5m+7.7m+0.5m was made. By drawings and design, the actual size of the foundation required is a square mat of size 7.7mx7.7m. But an extra working clearance is allotted for the convenience of the workers to carry out the construction at different stages and also for placing and removal of shutters for concreting. The whole excavation was completed in a few hours.

Foundation Base Leveling

After excavation of soil, there still exist some minor undulations at the base of the trench. These may be formed due to the removal of large boulders, roots of trees, and due to the presence of burrows of rodents. Before the reinforcements are done for the mat slab, the base has to be leveled. So a Plain Concrete Mix is used to fill the crevices (pore-sealing) and undulations. It acts as bedding for the foundation and as a damp-proof.

The procedure for laying PCC concrete is as follows: first the excavation levels and dimensions are checked as per the drawings. The loose earth is then removed manually from the trench. Water is sprinkled over the base and is rammed my some means. Shuttering is done using planks and runners if the soil is very loose to avoid slope failures at the sides of the trench. Then the required amount of plain cement concrete is mixed in the required proportion and water cement ratio using a mechanical mixer. The concrete is laid over the base to the specified thickness evenly. Mild compaction is done by ramming and surface is smoothed using a trowel.


At the site, concrete was of grade M10 made using 40mm aggregate. The cement concrete was laid to a thickness of 0.15m.

Bar Bending, Placing and Tying

In a mat slab foundation, the load is distributed over the mat with the help of reinforcement bars. Reinforcement bars are otherwise known as rebars, and are made of steel and are of different grades such as Fe250, Fe415, and Fe500. It acts as a tensioning device in the mat slab holding the concrete in compression. It is often given with ridges or ribs for better mechanical anchoring into the concrete. The bar bending works are done at site according to a bar bending schedule or bar bending diagram. It is a comprehensive representation of cut and bend bars as per the design requirements. It helps in determining the appropriate material quantities, strength and cost estimation. Bar bending schedule is used as a guide in positioning various structural members. Some examples are bending of hooks, bending of stirrups and ties, meeting point of the top and bottom bars, bend for longitudinal position of bars. The bar bending schedule is used to communicate requirements for cut and bent shapes from the detailer to the fabricator. The precision in cutting and bending operations is crucial to ensure appropriate fit on site and to facilitate necessary anchorage lengths, lap length and cover.


Rebar cages are fabricated on or off the project site commonly with the help of hydraulic benders and shears, however for small or custom works a tool known as Hickey or hand rebar bender, is sufficient. The rebars are placed by concrete rod busters or concrete reinforcing ironworkers with bar supports separating the rebar from the concrete forms to establish concrete cover and ensure proper embedment is achieved. Cover blocks are provided at 1m c/c throughout the entire region of concreting before the reinforcement is placed. The rebars in the cages are connected by tying with steel wires.

In the case of a raft foundation of a communication tower, there are two mats of reinforcement, one at top and the other at bottom of the slab. To maintain the interval between the rebar mats, chairs are placed at 1m c/c throughout. The placing of rebars should be accurate, because it is the bench mark for the load capacity of the foundation. Even slight deviations from the design can cause serious consequences. Care is also taken to provide adequate space between bars for placing the concrete. The reinforcements for the four columns that are erected from the mat slab are also done. The compression bars are 24mm diameter with 16 in numbers in each column.


Concreting of Mat slab and Beams

The concreting of the mat slab and the connecting beams are done on the same day. On the previous day, shuttering is fixed for the slab and the connecting beams. Wooden or Metal shutters are used. The thickness of the slab after concreting should be 0.35m and that of connecting beams is 0.55m. M20 concrete is used for concreting works. Cement, sand and aggregate are in the proportion 1:1.5:3. The aggregate is of size 2omm. The concrete is mixed using a mixer that is placed near to the site. Admixtures are used in a specified amount. The concrete is mixed and are hand carried to the location. Concrete is placed in layers of 10cm and is instantly compacted by using needle vibrators. Special care is taken to avoid over compaction. The connecting beams help in keeping the columns intact and to act as a single unit. The concrete is dumped from a height not more than 30cm to ensure segregation does not take place. Over-compaction results in bleeding of concrete. Also it adversely affects the strength of the footing. Also the shuttering should stay in its position throughout the setting time.


Concreting of First Lift of Columns

The procedures for concreting the first lift of columns is same as that of the concreting of mat slab. But it is done in two phases. At first the first lift, i.e. 1.5m of the column is concreted. Compaction is done after placing the concrete. Before concreting the second lift, the tower setting bolts are to be fixed, hence we use templates.

Preparation of Templates

A template is a steel plate fabricated to fix the bolts in position before concreting is done. It is a 10mm thick steel plate with four bolt holes at corners and central circular holes. The tower bolts are fixed to the bolt holes and are fixed using nuts. The template on each column is connected to the adjacent ones by using a steel angle of accurate length. This helps the templates to be in position and acts as a single unit. Any alterations in position of any of the templates affect the whole configuration. The markings on the


template help us to appropriately fix the tower setting bolts to the specified position.

Placing of Templates and Tower bolts and Shuttering

After the templates are prepared, they are places over the column reinforcements. The templates are affixed to positions by using nails attached to some wooden beams The tower bolts are then inserted through the bolt holes. The tower bolts are of 32mm diameter and are of length 1.2m, out of which 15cm is thread. The bottom of tower bolts is bent to right angles for better anchorage into the concrete. Shuttering of the rest of the column lift is also done prior to concreting.


Concreting of second lift of Columns

The concreting of the second lift of columns is done on the next day itself. The concrete is poured only through the central hole of the templates provided. So care must be taken for the compaction of concrete and for not forming any voids inside. The columns should be concreted to a height of 0.5m above the ground level. It should be also noted that the tip of the tower bolts are on the same elevation and on the same line. If there is any deviation, the load coming on each of the column will be different which leads to differential settling and ultimately results in the overturning of the tower.

Removal of Shutter

The next day of concreting, the shutters are removed. This is done prior to concreting.

Curing

After the removal of shutters, on the same day, the curing of the foundation has to be started. Continuous watering is provided for the next 7 days. Curing is done by spraying of water. Pooling of water can be done over the mat slab. Jute bags are fixed over the columns and connecting beams and watering is done regularly. Curing helps in the strength attainment and prevention of temperature cracks.

Plastering


Plastering is essential for protecting the concrete from the natural agencies and to provide the structure with a better aesthetic appearance. Plastering is done using 53 grade cement mortar. A coat of 1cm mortar is applied over the entire structure and is smoothed at the surface using trowel.

Backfilling

Backfilling is an important step in the construction. The tower is always subjected to wind loads and hence chances for overturning are high. By backfilling, the wind loads are resisted by the self weight of the foundation. Also it reduces the quantity of concrete required for the construction.

Scheduling for Construction

The whole construction of the foundation is completed in about 17-18 days. The timeline of the stages of construction is as follows:

Day Work(s) Completed

1 Site Leveling, Marking and Excavation

2 Leveling of the trench using PCC

3, 4, 5 Bar Bending, Cutting, Placing, Binding and


Shuttering

6 Concreting of Raft and Connecting Beams

7 Concreting of First Lift of Columns (1.5m)

8 Bolt setting using templates and Column Shuttering

9 Concreting of Second Lift of Columns

10 Removal of shutter

10-17 Curing of Concrete by watering

18 Plastering

19 Backfilling of Trench


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