EXCAVATION PIT MARKING:
After the location marking has been done, the marking of the pits for excavation for the foundation of the location is carried out. This is based on the specific foundation drawing of the type and make of the tower which is to be erected at that location.
The tower legs, footings and faces are designated as shown in the drawing below.
The pit marking shall be carried out according to the pit marking drawing which is prepared from the foundation drawing of the tower. The size of the pit, in case of open cut foundations, is worked out by adding 150 mm to the sides of the base pad on all the four sides for allowing working space. No margin is necessary in case of undercut foundations.
The excavation pit marking drawing indicates the distance of centres, sides and corners of the pits with reference to the centre point of the tower. These distances are measured and each pit boundary is marked in the field by means of spade or pick axe along the sides of the pit.
Construction Manual for Transmission Lines
A typical excavation pit marking drawing is given below.
From the dimensions shown in the drawing, the triangle ABC is first marked with the help of a measuring tape. The distance CD, equal to F (width of the pit) is marked on the ground. The triangle AB'C is then marked by shifting the point B and without changing the points A and C. The distance CD', equal to F, is then marked. The sides DE and D'E, both equal to F, are then marked. The procedure is repeated for marking the other three pits.
The dimension G shown in the drawing is the centre to centre distance between stubs of the tower at their lowest point. The dimension M is the diagonal distance between the ends of the stubs of the tower. The excavation pit marking drawing is prepared on the basis of these dimensions.
Sample calculation of the volume of excavation is shown in Appendix – C.
The depth of actual excavation at the pit centre is measured with reference to the ground level at the centre of the tower location. A typical example of determining the actual depth of excavation of the pits is shown below.
CLASSIFICATION OF SOIL:
The different types of soils are generally classified as under.
Normal Dry Soil:
Normal dry cohesive soils of any colour, viz., loose murram / sand, etc. which is readily removable with ordinary spades, pick axes and shovels.
Hard soil / Hard murram / Dense soil:
Generally any soil such as stiff clay, gravel, cobble stone and black cotton soil which requires the close application of pick axes or jumpers or scrifiers to loosen it.
Soft / disintegrated / fissured rock:
Soil consisting of decomposed or fissured rock, hard gravel, kankar, lime stone, laterite or any other soil of similar nature.
Soil in which chiselling, drilling or blasting is required for excavation.
Soil with negligible cohesion which is prone to collapsing when excavated.
CLASSIFICATION OF FOUNDATIONS:
The type of the foundation to be casted at any location depends upon the type of soil, sub – soil water level and the presence of surface water. While classifying foundations, the worst conditions are to be considered and not necessarily the conditions prevailing at the time of inspection. For example, there may be areas where sub – soil water rises when canal water is let out in the fields raising the sub – soil water level to a considerable degree. Similarly, the effect of monsoon or when nearby reservoirs are full is also to be considered and not the conditions prevailing in open season or summer when work is normally carried out. The classification of foundation is done as given hereunder.
Normal Dry Foundation:
This is used at locations where normal dry cohesive soils are met and...