DEL EMBARGO ADMINISTRATIVO

Foundation

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The construction of tower foundations shall be in accordance with IS 4091:1979 and Power Grid specifications.

3.3.1 Check Survey

This will be conducted to make a check on detailed survey and to locate the peg marks and the tower positions on ground conforming to the survey charts. In the process it is necessary to have the pit centres marked according to the excavation marking charts. The levels, up or down, of each pit centre with respect to the centre of the tower location shall be noted and recorded for determining the amount of benching or earthwork required to meet design requirements of the foundation.

If the levels of the pit centres be in sharp contrast with the level of the tower centre (say beyond a slope of 1:4), suitable "leg extension" may be deployed as required.

3.4

Preparation of Foundation Site

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3.4.1 After tower spotting is done during check survey, it is often the case that the area within the base of the tower is found uneven in level. Minor variation in level can be ignored but variations in level of more than 60 cms are to be dealt with benching the area to the reference level of centre peg. The foundation shall be placed at the design depth with reference to the centre peg level. The

area below the centre peg shall be back filled to get a leveled surface. If the level of back filling is considerably high, the filled area shall be enclosed by a revetment wall to prevent erosion.

3.4.2 In Hilly areas the difference of levels in the four legs is generally very high resulting in large quantity of benching and massive revetment walls, which will be very uneconomical. In these cases special hillside extensions are provided to place the four legs at different elevations.

3.5

Type of Foundation to be Adopted

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3.5.1 The following standard types of foundation are approved in Powergrid for adoption in transmission lines for different towers:-

i) Normal dry ii) Wet

iii) Partially submerged iv) Fully submerged v) Dry fissured rock vi) Wet fissured rock vii) Hard rock foundation. viii) Black cotton soil foundation

3.5.2 The above foundations are designed for the corresponding predominantly prevalent soils in the pits. However, there will be cases where a combination of soils in the pits may be observed and correct classification of the standard foundations is required to be done.

3.5.3 Type of foundations to be adopted with reference to the soils and sub-soils waters encountered in the pits are indicated in Table-1 to 3. Foundations may

be classified and adopted accordingly. In certain classifications of soils with sub-soil waters (items 6(b) and 7(b) of Table-1, items 1(b) and 2(c) of table 2 and items 1,2 & 3 of Table 3) standard foundations cannot straight away be adopted.

3.5.4 If the soil conditions differ with the four legs of the same tower necessitating adopting of different types of approved foundation for the different legs of the tower, classification of foundations may be done for each of the pits as required by the actual soil conditions in the pits.

3.5.5 If site feels that soil encountered at any of the locations do not tally with the description of soils given in the Annexure enclosed or if the soils are considered very bad/tracherous, foundations may not be decided at such locations by the site. Classification of the foundations of such locations may be done in consultation with Corporate Engineering department. If the locations fall in the back water of any river, tank or reservoir, the depth of the back water at those locations and the duration of the stagnation of water at those locations is to be ascertained and foundation to be decided after studying the detailed soil report.

Table (1) Adoption of Foundations in Different Soils

Sl. No. Types of Soil Encountered Type of Foundation to be Adopted

(1) (2) (3)

1. a) In good soil

b) Where black cotton soil does not extent beyond 30% of the depth from top with good soils thereafter

Normal dry

c) Silty sand mixed with clayey soil (In all the above cases water table is not encountered in the pit)

2. a) In paddy fields and sugar cane fields. b) Where black cotton soil exceeds 30 and

extends up to 45% from the Ground level and good soil thereafter.

c) Where sub-soil water is encountered in the pit beyond 1.5 m from ground level.

Wet d) Where silty sand mixed with clayey soil and

water encountered in the pit beyond 1.5 m from ground level.

3. a) Where black cotton soil extends upto 60% of the depth from G.L. and good soil thereafter. b) Where sub-soil water is encountered in the

pit between 0.75m depth and 1.5m depth from ground partially level.

Partially Submerged

c) Where silty sand mixed with clayey soil and water table in the pit is between 0.75m depth and 1.5m depth from ground level.

4. a) Where sub-soil water is encountered in the pit within 0.75m depth from ground level. b) Where silty sand mixed with clayey soil and

water table in the pit is within 0.75 m from G.L.

Fully Submerged

5. a) Where normal soil encountered with fissured rock at the bottom without the presence of water.

b) Where fissured rock is predominant in the pit without the presence of water

Dry fissured rock c) Where hard rock thickness at the bottom of

the pit is less than 2.0m

6. a) Where fissured rock is encountered with sub soil waster after 1.50m depth from ground level

Wet fissured rock

b) Where fissured rocks is encountered with sub-soil water within 1.5m depth from ground

Soil investigation is to be properly conducted at this location and the contractor has to develop new fdn. Design based on soil report and get

approved from Engg. Deptt. 7. a) Wherever hard rock is encountered and the

thickness of hard rock layer below the bottom of fdn. Is more than 2 meter

Hard rock

b) Wherever hard rock is encountered with sub- soil water

Soil investigation is to be conducted at this location & the contractor has to develop new fdn. Design based on soil report and get

approved from Engg. Deptt. 8. Wherever black cotton soil extends beyond 60% of

the depth from ground level.

Black cotton soil fdn.

Table (2) Black cotton soil in combination with rock

Sl. No. Types of Soil Encountered Type of Foundation to be Adopted

(1) (2) (3)

1. Black cotton soil with fissured rock combination

a) If fissured rock is encountered at the bottom of pit (with black cotton soil at top) for a depth of not less than 1000mm or the thickness of the bottom slab concrete of dry fissured rock foundation whichever is less.

Dry fissured rock foundation as per approved drg. may be adopted with wet chimneys because of black cotton soil encountered in the top portion of the pit.

b) If fissured rock in combination with water is encountered at the bottom of the pit with black cotton soil at the top.

Wet fissured rock foundation is to be adopted.

2. Black Cotton Soil with hard Rock Combination

a) If the hard rock is encountered at the bottom of the pit (with black cotton soil at top) and the thickness of the hard rock layer below foundation is more than 2m.

Hard rock fdn. Design to be adopted with wet chimneys because of the black cotton soil encountered in the top portion of the pit.

b) If hard rock encountered at the bottom of the pit with black cotton soil at the top and hard rock layer depth is less than 2m.

Fissured rock foundation design to be adopted with wet chimneys because of the black cotton soil at the top. c) If hard rock is encountered at the

bottom with water and black cotton soil at top and hard rock layer depth is less than 2m.

Wet fissured rock foundation as per approved drgs. are to be adopted.

Table (3) Sand in Combination with Sub-soil water

Sl. No. Types of Soil Encountered Type of Foundation to be Adopted

(1) (2) (3)

1. Predominantly sand mixed with clay or other soil (without sub-soil water)

Dry sandy soil foundation may be adopted.

2. Predominantly sand mixed with soils encountered with sub soils water in the pits

FS/Wet sandy soil foundation may be adopted depending on water table. 3. Pure sand encountered with sub-soil water

inside the pit

Same as above

Table (4) Clay Mixed with Silty Sand

Sl. No. Types of Soil Encountered Type of Foundation to be Adopted

(1) (2) (3)

1. Pure clay encountered without water Dry black cotton foundation may be adopted.

2. Predominantly clay mixed with silty sand encountered with sub-soil water in the pits.

Wet black cotton foundation may be adopted.

3. Predominantly silty sand mixed with clay encountered with sub-soil water.

Normal fdn. Based on the water table classification can be adopted.

Note : To facilitate foundation classification a check format has been developed and enclosed at Chapter-10 (Page Nos. 130, 131 & 132).