General

Cement concrete footings shall be used for all types of towers, in conformity with the present day practice followed in the country, and the specifications laid herein. RCC foundation may be used for locations where cement concrete footings are not possible to be laid. All the four footings of the towers shall be similar.

Foundation Depth

The depth of the foundation may vary from 1.5 to 3.5 meters.

Classification of Foundation

• Depending on the type of soil, the sub-soil water table and the presence of surface water, four types of foundation will be used for each type of tower location. classified in the following manner-

Properties of Earth

The following general physical properties of earth under various conditions have been assumed for the design of foundation-

Table3-I

Testing of Soil

It is desirable to undertake testing of soil for all the tower locations and report should be obtained about the sub-soil water table, as prevalent in the month of September and October type of soil encountered, bearing capacity of soil, possibility of submergence and other soil properties required for the correct casting of casing of foundations. Testing should be carried out about soil resistively in dry season and its record should be kept properly along with route alignment map. After soil investigation along the line alignment, the final quantities of foundation types should be worked out based on the soil investigation carried out and such foundations should be casted and installed only after proper checking and approval.

Excavation

• Except as specifically otherwise provided, all excavation for footings shall be made to the lines and grades of the foundation. The excavation walls shall be vertical and the pit dimensions shall be such as to allow a clearance of not more so as to maintain a clean sub-grade, until the footing is placed, using timbering, shoring or casing, if necessary. Any sand, mud, silt or other undesirable materials which may have accumulated in the excavation pit shall be removed before placing concrete.
• The soil to be excavated for tower foundation shall be classified as under-
• Normal soil : Soil removable by means of ordinary pick axes, shovels & spades such as types of soil found in gigantic plains, black cotton soil etc.
• Wet Soil : Soil where the sub soil water table is encountered within the range of foundation depth, the soil below the water table and that at locations where pumping or bailing out of water is required due to presence of surface water, will be treated as wet soil.
• Rocky Soil :
• Soft rocks-This will mean decomposed rock, hard gravel, kankar, lime stone, laterite or any other soil of similar nature which can be easily excavated with pick axe or spade.
• Hard rocks- Hard rock will be that which requires chiselling or drilling and blasting.
• Where rock is encountered, the holes for tower footings, shall preferably be drilled, but where blasting is to be resorted to as an economy measure, it shall be done with the utmost care to minimise the use of concrete for filling up the blasted area. All necessary precautions for handling and use of blasting materials shall be taken. In case where drilling is done, the stubs may be shorten suitably with the approval of the Engineer.
• Shoring of pits with shuttering will be done when the soil condition is so bad that there is likelihood of accident due to the falling of surrounding earth. However, the necessity of shoring of pits with shuttering shall be decided by the Supervising Engineer, depending upon the site conditions.
• Depending on the condition of water available in the pits, following methods of dewatering will be adopted
• Manual : Where dewatering is done by men with the help of buckets etc.
• Mechanical : Where dewatering is done by hand pump.
• Power driven: When engines or electrical power driven pumps with power input not less than half H.P. are used for dewatering.

Setting up of Stubs

• The stub shall be set correctly in accordance with approved method at he exact location and alignments and alignment and in precisely correct levels. The stub setting templates shall be used for proper setting of stubs. Stubs shall be set in the presence of well conversant Junior Engineer/Assistant Engineer.
• The foundations are to be made as per designs and drawings approved by the Engineer. Extent of the work as defined by such drawings shall not be exceeded except in very exceptional cases where the prior approval of Supervising Engineer is to be obtained.
• Setting of stub at each location shall be approved by the Assistant Engineer/Executive Engineer.
• Details of Foundations
• The thickness have been designed such as to satisfy that conditions specified herein.
• The thickness o concrete in the chimney portion of the tower footing would be such that it provides minimum cover of not less than 100 mm from any part of the stub angle to the nearest outer surface of the concrete in respect. of all dry locations limiting the minimum section of chimney to 300 mm square. In respected of all wet locations chimney should have an allround clearance of 150 mm from any part of the tub angle limiting to 450mm square minimum.
• The chimney top or muffing must be at least 225 mm above ground level and also the coping shall be extended upto the lower most joint level between the bottom lattices and the main corner leg of the tower.
• The spread of concrete pyramid or slabs will be limited to 45 deg. with respect to the vertical.
• At least 50 mm thick pad of size equal to the bases of pyramid with its side vertical will be provided below the pyramid to account for the unevenness of soils and impurities likely to be mixed in concrete due to direct contact of wet concrete with earth and for allowing stone aggregate reaching upto corner edges. This pad will also be provided in cases where pyramids are provided over concrete slabs.
• In case of fully submerged type foundation one base slab, not less the 200 mm thick has been provided.
• The minimum distance between the lowest edge of the stubs angle and the bottom surface of concrete footings shall not be less than 100 mm or more than 150 mm in case of dry locations and not less than 150 mm or more than 200 mm in case of wet locations.
• The portion of the stub in the pyramid has been provided with the cleats.

Concrete

• All the properties of concrete regarding its strength under compressing , tension, shear, punching and bending etc. as well as well as workmanship will conform to IS :456 FO 1978. shall conform to specifications for coarse & for concrete as per IS: 383 of 1970.
• The sand used for the concrete shall be composed of hard siliceous material. It shall be clean and of a sharp angular grit type and free from earthy of organic matter and deleterious salts.
• The aggregate shall be of clean broken hard granite or other stone specified or approved by the Supervising Engineer. It shall be of hard, closed grained quality. It shall also be as far as possible cube like,  perfectly angular, but not flaky, perfectly clean and free from earth, organic or other deleterious matters, aggregate shall be of size as well pass through a mesh. 40 mm measured in the clear and 20 mm aggregate through 20 mm square mesh measured in clear.
• The water used for mixing concrete shall be fresh clean and free from oil, acid and alkali, organic material  or other deleterious substances. Saltish or blackish water should not be used. Potable water is generally satisfactory.
• The concrete shall be mixed in a mechanical mixer. Hand mixing shall not be permissible.
• Mixing shall be continued until there is uniform distribution of material and the mix is uniform in colour and consistency but in no case the mixing be done for less than two minutes. Normally mixing shall be done close to the foundation but in case it is not possible, the concrete may be mixed at the  nearest convenient place. deposit as rapidly as practicable by methods which shall prevent the segregation or loss of any ingredient. The concrete shall be placed and compacted before setting commences.
• The concrete should be mixed as stiff as the requirements of placing the concrete in the forms or moulds with ease and the degree to which the concrete resists segregation. Hence the quantity of water used should not be too much.
• Proper forms or moulds adequately braced to retain proper shape while concreting should be used for chimney or pyramid and slab portions. The mould should be made water-tight so that cement cream should not come out leaving only sand and jelly consequently forming of honey combing in the concrete. The form boxes shall be cleaned and oiled before these are used for concreting.
• The stub angle shall be free of rust and cleaned thoroughly and painted with cement paste, made of 1 part of cement and 3/4 part of  water (cement-slurry) to a thickness of 1.6 mm (1/16 inch) be fore the concrete is laid against the stub angles. The painting with cement slurry shall be done each time to such a height that concrete will be laid to that height before the cement wash becomes dry.
• The concrete shall be laid in 150 mm layer and consolidated well so that the cement cream works upto the top and no honey combing is left in the concrete. Concreting is to be done continuously so that the subsequent layers are laid before the initial setting of the bottom layer begins. If fresh concrete is to be laid on old concrete less than a week old, the surface of the old set concrete should be chipped and cleaned thoroughly with wire brush and washed with a layer of thick cement slurry before the new concrete is laid. If, However, the concrete is more than10 days old, the top layer of the set concrete should be chipped and cleaned thoroughly with wire brush and water, and layer of  cement mortar (1 : 1) 12 mm thick shall be laid evenly after giving a coat of cement slurry, as specified above to ensure proper bonding between old and new concrete.
• After concreting the chimney portion to the required height, the top surface should be finished smooth, with slight slope towards the outer edge to drain off the rain water falling on the coping.
• In wet locations, the site must be kept completely dewatered both during the placing of the concrete and for 24 hours after completion. There should be no disturbance of concrete by water during this period.
• The forms of moulds shall not be removed before a lapse of a bout 24 hours after the completion of concreting .After removal of the forms, the concerted surface, where required shall be repaired with a rich cement and sand mortar in the shortest possible time.
• In case pozolona cement is used, longer time at least 72 hours shall be permitted for proper setting of concrete. Before removing the form box, it should be ascertained by opining one plank of shutter whether strength of concrete has been achieved. If not, extra time shall be given till the concrete sets fully.

Back - Filling and Removal of Stub Template

• Following opening of Form-Box and removal of shoring and  shuttering, if any back filling shall be started after repairs, if any, to the foundation concrete. Back filling shall normally be done with the excavated soil, unless it consists of large boulders which shall be broken to a maximum size of 80 mm.
• The back fill materials should be clean and free from organic or other foreign materials. The earth shall be deposited in  maximum 200 mm layers, levelled, and wetted and damped properly before another layer is deposited. Care shall be taken that the backfilling is started from the foundation end of the pits, towards the outer ends. After the pits have been backfilled to full depth the stub template may be removed.
• The backfilling and grading shall be carried to an elevation of about 75 mm above the finished ground level to drain out water. After backfilling, 150 mm high earthen embankment (bandh) will be made along the sides of excavation pits and sufficient water will be poured in the backfilled earth for at least 24 hours.
• The stub setting template shall be opened only after the completion of backfilling. level of backfilled earth does not go below the surrounding ground level. However, level with the surrounding ground.

Curing

The concrete after it is 24/72 hours (as the case may be) old shall be cured by keeping the concrete wet continuously for a period of 10 days after laying. The pit may be backfilled with selected earth sprinkled with necessary amount of water and will consolidated in layers not exceeding 200 mm of consolidated thickness after a minimum period of 24/72 (as the case may be) hours and thereafter both the back filled earth and exposed chimney top shall be kept wet for the remainder of the prescribed time of 10 days. The uncovered concrete chimney above the backfilled earth shall be kept wept wet by providing empty cement bags dipped in water fully wrapped around the concrete chimney for curing and ensuring that the bags are kept wet by frequent pouring of water on them.

Earthing

• The footing resistance of all towers shall be measured in dry weather after their erection before the stringing of earthwire. In case the tower footing resistance exceeds 10 ohms, pipe type earthling/counterpoise earthling, wherever required shall be done in accordance with stipulations made in this specification.
• Pipe Earthing
  The grounding shall be effected by making about 300 mm dia 3750 mm deep pit at a distance of not less than 3650 mm away from the stubs and filling in the pit with finely broken coke having the granule size not more than 25 mm and salt in such a way that minimum cover of 125 mm thick salt mixed coke shall be maintained from the pipe on all sides and that the top edge of the pipe shall be atleast 600mm below the ground line. The G.S. strip shall be buried not less than 600 mm deep from the ground line.
• Counterpoise Earthing
  In place of high resistivity soil, special earthing arrangement shall be employed in the form of counterpoise earth to bring down the tower footing resistance to 10 ohms. The counterpoise earth shall be composed of 7/9 SWG Galvanised steel wire having suitable G.S. lugs soldered or compressed at its one end, complete with 16mm dia. bolts and nuts, required for connecting the earthing to the tower end. The counterpoise shall be buried radially away from the tower base at 600 mm below ground level. The lug should be preferable buried in the chimney portion of the foundation to avoid pilferage.

Steel Reinforcements

In case of R.C.C type foundations, mild steel red forcemeat bars conforming to IS:226 along with binding wires etc. shall be used.

Tower Footing Resistance & Soil Receptivity

• General
  The tower footing resistance is an extremely important parameter in the determination of lighting flashover rates. Unfortunately, it is a fluctuating statistical variable, the magnitude of which is governed not only by geography but also by non-linear conduction physics in the earth. It may swing over a two to one range or more owing to changes in stroke current, and even with constant current it will change with time.
  The resistively of the earth is far form uniform even within a local area. A general idea of the order of soil receptivity in any region may be had from the type of soil, temperature, seasonal variations etc. It is not possible to predict to a precise degree the resistively to be expected in given area of the electrode resistance at a given site. In every case it  is necessary to carry out an actual measurement to determine the earth resistively in a given case. The experiments establish that the four electrode method is the most suitable with large electrode spacing of about 50 metres. 
• Four Electrode Method
  The familiar Earth Receptivity Tester can be conveniently used for carrying out the four-electrode method of measuring resistivity. It has a generator producing DC at 500 or 1000 volts with the current coils connected in series. The potential coil is mounted on the same shaft as the current coil and has a definite inclination out to four independent terminals C1, C2, and P1, P2, Both the potential and current coils tend to rotate  in the field of a permanent magnet, when the generator is worked, the direction of the movement being opposite to each other .Therefore, when the instrument is connected to a test earth and generator is worked, the position taken by the combined current-potential coil well be proportional to the  ratio of V/I or the mutual resistance between the current and potential circuits.
  As indicated in Fig. 3-1, the Earth Resistively Tester has four terminals marked P1, P2, C1, C2, and four similar electrodes are driven into the ground at equal distances of 50 meters in the region, where the soil resistively is to be determined (should be driven about 1 metre depth). If these electrodes are designated as  A, B, C and the extreme electrodes A and D should be connected to C1, of the Earth Tester, The electrodes B and C should be connected to P1, and P2, By operating the Earth Tester Handle continuously at unifrom speed, we can read the electrode resistance 'R' on the Earth Tester scale.
  The Soil Resistivity can also be determined from the following formula-
  
  r = 2p ´ S ´ R      or          r = 31430  R
  where,   r = soil resistivity in ohms/cm3
  R = Earth Tester reading in ohms
  S = Distance between successive electrodes in metres which is normally kept
  50 metres.

Fig. 3-1

The Earth Tester is known to give fairly accurate results of earth resistivity. Where the average resistivity over a large area and depth are required, the electrode spacing has to be kept correspondingly high. While there should be no objection to keep such wide spacing with the Earth Tester, the skin effect of the ground current places a limitation on the spacing that can be used. The results with the Earth Tester reasonably reliable for electrode spacing upto about 45-60 metres. In all  measurements of soil resistivity, it is necessary to see that these are carried out in the driest part of the year, preferably during the months of April and May.