POWER GRID CORPORATION OF INDIA LIMITED. HYDERABAD SS. EXECUTION OF SOIL INVESTIGATION AND FOUNDATION. by M.KRISHNA MURTHY Sr.Engineer (TLM) Hyderabad SS. Soil Investigation.
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SOIL INVESTIGATION AND FOUNDATION
SHELL & AUGER UNIT CAN DRILL UPTO 40 M DEPTH THROUGH SOILS & MIXED WITH PEBBLES, GRAVELS, CHISELLING CAN BE DONE
1. Locate the exact location by Check Survey.
2. Taking the back reference fix the alignment in line with the route finalised during the Detailed Survey.
3. Once the direction is fixed mark the intermediate suspension tower’s center point peg.
4. Distance between the point of reference center point to the center point of the tower under reference should be exact distance as marked in the tower schedule.
5. Same process should be followed for all the intermediate towers till the next Angle Point.
6. Compare the physical situation of each tower location with the Elevation and Plan Section as detailed in the profile.
7. For each Location fix three pegs to give the line direction.
1. First center total station on center peg of the Angle Point
2. View the back reference / line direction peg ‘P’ by the Theodolite / Total Station.
3. Transit the telescope to the line direction peg ‘Q’.
4. Rotate telescope 30oL with reference to direction peg ‘Q’.
5. Mark the intermediate towers A1, A2, A3 ……
1. It is the most difficult task since the Transmission Line passes through different terrains and come across different soil strata and remoteness of the construction.
2. Various methods were adopted by different Organisations to classify the foundation. Generally the cost of foundation is 8% to 14% of the cost of Transmission Line.
3. Before we proceed further for classification of Foundations, we should know, what are types of forces acting on the foundation.
a) Uplift / the Tension.
b) Downward Thrust / the Compression.
c) Lateral Load.
d) Over Turning Moment.
The magnitudes of limit loads on foundation should be taken 10% higher than those adopted for the corresponding tower.
4. How forces act?
Total wind force on tower = F
Wind moment on foundation = T&L
Wind Shear = F
Compression on Foundation
( C =T )
Uplift on Foundation (T)
5. Soil parameters required to take care of the load as indicated above are as follows:
a) Limit bearing capacity of soil.
b) Density of soil.
c) Angle of Earth frustum.
6. For designing foundation the soil properties are generally obtained by the method as follows:
a) By conducting soil investigation.
b) From available data bank of soil strata of different states.
7. Limit Bearing Capacity: This parameter is vital to establish the stability of foundation against shear failure of soil and excessive / uneven settlement.
8. Density of soil: This parameter is required to calculate the uplift resistance of foundation.
9. Angle of Earth Frustum: This parameter is required for finding out the uplift resistance.
To determine the above check, various procedure are followed in the Transmission Line to find out the properties of the soil which are as follows:
1. To carry out the soil investigation throughout the Line at an interval of 350 to 400 m.
2. Present practice in POWERGRID is to carry out soil investigation at all the major Power Line crossing, Railway crossing, National Highway crossing, at an interval of 15 locations along the Route, at Angle Towers, where the soil strata changes and at Special Locations .
3. During execution stage trial pits upto a minimum depth of 3.0m shall be excavated at each and every location in order to classify the type of foundation after obtaining following details.
(i) Type of soil encountered.
(ii) Ground water table.
1. Where the soil investigations are carried out, the soil properties are determined and based on which classification can be done.
2. Where only trial pits or visual observation is done, then Engineer’s needs to apply the knowledge by their understanding of soil properties.
a) Non-cohesive soil:
1. Gravel, compact sand, gravel mixture and offering high resistance to penetration during excavation.
2. Coarse sand, compact and dry
3. Medium sand, compact and dry
4. Fine sand, Silt (dry lumps easily pulverized by the fingers)
5. Loose gravel or sand, gravel mixture loose coarse to medium sand, dry.
1. Soft shale hard or stiff in deep bed, dry.
2. Medium clay, readily indented with thumb nail.
3. Moist clay and sand clay mixture.
4. Soft clay and very soft clay.
5. Black cotton soil or other shrinking/expansive clay.
For further understanding of classification, the cohesive soil gets unified with the parent soil after back filling with the passage of time.
a) Normal soil having mixture of silt and clay (clay not exceeding 15%). This type of soil in wet condition can be rolled between the palms, only short threads can be made.
b) Clayee soils having high percentage of clay (more than 15% ) e.g., Black cotton soil (can be black or yellow). This type of soil in wet condition can be rolled within the palms a long thread can be made.
c) Marshy soil having sea mud (Marine soil) which are very sticky In nature.
a) Soft rock / Fissured rock: Include decomposed or fissured rock, hard gravel, kankar, lime stone, laterite or any other soil having similar nature. These types of rocks can be excavated using normal tools.
b) Hard rock: Include hard sand stone, quartzite, granite, basalt, hard marble etc. These types of rocks can be excavated through blasting, chiseling and drilling.
a) Classification of foundation mainly through visual observation and trial pits, depends on the factors as follows:
1. Type of Soil.
2. Water Table.
b) Based on this key factor the normal standardized design of foundation are as follows.
1. Normal dry Foundation: i) Soil cohesive and homogeneous up to full depth.
ii) Clay content (10% to 15%)
iii) Water below foundation level.
2. Wet Foundation. i) Soil condition as above.
ii) Sub soil water at depth 1.5m or more below GL.
iii) Standing water with penetration not exceeding 1m (Paddy field).
3. Partly Submerged. i) Soil condition as above.
ii) Sub-soil water at depth between 1.5m & 0.75m below GL.
4. Fully Submerged. i) Soil condition as above.
ii) Sub-soil water at a depth within 0.75m below GL.
5. Black cotton Foundation i) Soil cohesive, clay exceeding 15% having characteristic of high shrinkage and swelling (Black & Yellow).
ii) Soil of same nature exceeds 50% and extends upto full depth.
iii) Where top layer is normal / good soil up to 50% of the depth but below is WBC.
6. Soft Rock or Fissured i) Soil non-cohesive, decomposed or fissured rock, hard gravel, hard morrum, laterite or similar nature can be cut normal tools.
7. Submerged Fissured i) Soil is same as Fissured Rock.
ii) Sub-soil water within 0.75m or below 0.75m from GL.
8. Hard rock foundation i) Where hard rock is encountered at 1.5m or less below GL.
9. Partial black cotton i) Where top layer of black cotton soil exceed upto 50% of depth and there after good soil.
10. Hard rock foundation i) Where hard rock is from 1.5m to 2.5m below GL (top layer is good soil).
with chimney for normal
11. Hard rock foundation i) Where hard rock is 1.5m to 2.5m below GL (top layer is BC or fissured rock).
12. Normal dry with i) Where top layer upto 1.5m below GL is normal dry soil and thereafter hard soil / murrum.
13. Dry sandy soil i) Sandy soil with clay content not exceeding 10%.
14. Wet sandy soil i) Sandy soil with Water Table in pits.
with chimney for WBC
Any other combination of soil not covered above shall require development of special foundation design after detailed soil investigation.
1. Standardization of foundation to take care of normal variation of soil.
2. To implement speedy execution by quick decisions.
3. Simplifying the process of erection requiring less expenditure in execution.
Based on the above criteria of loading and soil parameters various types of foundations are designed for adoption in Line are generally as follows:
A) Chimney and Pyramid Type:
These foundations need form boxes. Commonly used in cohesive soils. Stepped foundations need less shuttering but adequate care should be taken for monolithic jointing of concrete in intermediate steps.
These foundations can be suitably designed for variety of soil conditions. These are commonly adopted type of foundations.
This type of foundation consists of a chimney and block concrete. This type is usually suitable where soft rock and hard rock are encountered.
1. Concrete poured should be in direct contact with inner surfaces of excavation pit. The thickness and the size of the block is decided based on the uplift and bearing required. The uplift resistance in this type of footing is provided by the bond between concrete and rock.
2. Preferable at least 1.5m below GL.
3. Though sometimes this type of foundations are used but under cut type for soft rock and rock anchor type for hard rock are preferred.
These foundations are suitable for hard murrum, fissured / soft rock, soil mixed kankar etc. This type of foundation has higher uplift resistance in comparison to the similar type of foundation without undercut.
1. The depth of embedment, diameter of bar & number of anchor bars will depend upon uplift force.
2. The diameter of bar shall not be less than 12mm.
3. The grouting hole shall normally be 20mm more than the diameter of bar.
4. Anchor strength can be increased using eye bolt, or threaded rods.
(A) For Suspension Tower:
1. Mark three pegs along with the line direction.
2 Mark two pegs at 90o on either side of line direction.
3. All marking shall done with the help of Theodolite.
X1 X2 X3= Line directional peg
X4 X5 = 90o peg with reference to the line direction
1. Mark five pegs to show the both line direction.
2. Bifurcate the angle and mark the direction peg of bifurcation.
3. Mark two pegs at 90o on either side of the angle bifurcation line.
X1 X2 X3 = Start line direction peg.
X4 X5 = Angle line direction peg.
X6 X7 = Angle bifurcation peg.
X8 X9 = 90o direction with ref. to bifurcation line
Pit marking shall be done as follows:
1. For Suspension Tower it should be with reference to line direction pegs and the peg at 90o direction of line peg.
2. For Tension tower it should be with reference to angle bifurcation peg and the peg at 90odirection of angle bifurcation peg.
3. Method of laying out / pit marking is same, after the consideration as above.
SIDE “X 1”
DIAGONAL OF PIT
DIAGONAL “X 2”
PIT MARK FOR FOUNDATION
DIRECTION OF TOWER LINE
1. Preferably avoid manual excavation.
2. Excavated earth should be stacked away from the pit specially in case of WBC, sand. Normal soil with high Water Table.
3. Preferably shoring and shuttering should be used in case of loose soft soil, WBC slushy soil.
4. Incase of WBC, predominantly sand, with water, the pit should not be kept open for long period.
5. Dewater pumped from the pit during excavation should be taken away from the location.
6. Before entering in pit check all the edges of excavated pit for ascertaining caving in condition.
7. Except Rocky Morrum, Soft Rock in all other conditions, follow slope cutting or box cutting, if Shoring and Shuttering is not used.
8. If blasting is resorted to proper license and trained people should be deployed for excavation. Blaster should have license as per Indian Explosives Act.
9. Storing and handling of blasting material should be done very carefully.
10. Check the length of the fuse wire during blasting.
GENERALLY FOR STABILITY OF THE PIT AT LEAST 1:4 SLOPE SHALL BE MAINTAINED. FOR WBC, LOOSE, SOFT OR SLUSHY SOILS, THE INCLINATION OF SLOPE SHALL BE INCREASED.
GENERALLY FOR EVERY 2 M DEPTH THE WIDTH OF THE STEP CUTTING SHALL BE 0.5M. FOR LOOSE WBC, SOFT OR SLUSHY SOILS WIDTH OF STEP SHALL BE SUITABLY INCREASED
Stub should be set in the manner so that distance between stubs, their alignment and slope are as per design and drawing. To achieve the following methods are generally followed:
1. A combined stub setting template.
2. Use of individual leg template for each stub.
3. Prop setting template.
90mm Ø WITH INSIDE THREADING
12mm TH. PLATE
ANCHOR BOLT 600mm LONG
90mm Ø WITH INSIDE THREADING
12mm TH. PLATE
1. Assemble the templates four sides as per drawing.
2. Place the four sides of the assembled template on the stub setting jacks.
3. Mark center point of the each side of the template.
4. Tie thread on the line center pegs and on pegs at 90o to line direction pegs in case suspension tower
5. In case of angle tower, tie thread on the angle bifurcation pegs and on the peg at 90o to angle bifurcation pegs.
6. Fix 4 Nos Plumb bobs (generally 0.9Kg) to the four center marks on four sides of the template.
7. Orient template to the alignment of the line and center it over center pegs of the location.
8. Fix up the stubs to the template corners with the help of Water level or Dumpy level, with reference to the point considered as reference point for excavation (generally the center peg).
9. Check both the diagonals of the template.
10. Ensure that all four sides are at the same level.
11. Check the alignment, centering and diagonals of template again.
1. The individual template comprises Steel Channel / Joist having length more than the size of the pit (around 2 to 3 meters).
2. A chamfered Cleat is welded at the center of the Channel/Joist to provide slope of the stub.
3. The stub is fixed to the cleat of the template with the help of bolts.
0m leg Extension
3m leg Extension
2m leg Extension
4m leg Extension
1. Position of template supporting jack should be selected properly .
2. Template supporting jack should be away from the edge of the excavated pit.
3. Supporting jack should be on firm ground. Avoid setting the jack on gunny bags sand bags etc.
4. Careful handling of template should be done while aligning with axis of the foundation.
5. Keep constant watch on collapsing soil of the pits or the arrangements made to resist collapsing.
6 use personal protective equipment while at work.
THE SUPPORTING JACK OF TEMPLATE SHOULD BE AWAY FROM THE PIT EDGE SPECIALLY IN CASE WBC, SAND PREDOMINANT, SOFT OR SLUSHY SOIL. BRIEFLY WHEREVER THE SOIL IS COLLAPSING THIS ARRANGEMENT
THE SUPPORTING JACK OF TEMPLATE SHOULD BE AWAY FROM THE PIT EDGE SPECIALLY IN CASE NORMAL SOILS, MOORUM, GRAVELL ETC.
1. Materials like coarse and fine aggregates shall be taken along the line route preferably for economy and expeditious execution.
2. Well in advance source should be identified, all the relevant tests shall be carried out at the approved third party lab.
3. Once, the satisfactory results of the source are available, necessary approval of source from client shall be obtained. For main foundation, 40mm size coarse aggregate, and for chimney 20mm is used.
4. Concreting mix can be prepared by:
a) Volume batching / Measurement.
b) Mix design (generally used in Pile foundations).
a) For lien concrete, Sub base or pad M10 (1:3:6)
b) For main foundation M15 (1:2:4), M20 (1:1½:3)
6. Prepare suitable measuring box of size 35x25x40cm (works out to 35 litres capacity which is equal to volume of 1 cement bag of 50 Kg).
7. Water Cement Ratio shall be between 0.5 to 0.6. Water cement Ratio depends on various factors like moisture content in aggregate, weather condition, time of the day etc.
8. Slump test shall be carried out for checking workability. Desirable slump limit is 25 to 50 mm.
9. Generally, the water shall be used from a nearest source of the foundation. pH value, and foreign matter in the water shall be checked.
10. Form box shall be fabricated as per design drawing, shall be sufficiently tight to avoid loss of cement slurry during concreting.
11. Inside the form box shall be cleaned properly. Inside surfaces shall be treated with approved, burnt mobile oil, shuttering oil etc., before use every time.
12. Concrete shall preferably mixed in mechanical mixture, but under certain unavoidable conditions hand mixing shall be permissible.
13. Mixing shall be continued until there is an uniform distribution of materials and the mass is uniform in colour and consistency.
14. In case of mechanical mixing, duration of mixing of each batch shall not be less than two minutes.
15. During concreting, following cares shall be taken:
a) No segregation between coarse & fine aggregate.
b) Compaction completed before Initial Setting.
c) Once compacted then it should not be disturbed.
16. Compaction shall be carried through mechanical vibrator or poking bar (16mm Ø bar pointed at the end). Excess compaction causes the liquid to flow causing segregation.
17. In case of RCC foundation, reinforcement shall be placed as per design and drawing, with minimum concrete cover of 50mm. Where double layer of reinforcement is provided in the foundation suitable chairs of reinforcement shall be provided to maintain the exact position of the 2nd layer.
18. Removing / opening form box shall be done in fair weather condition after 24 hours of concreting. In cloudy / rainy weather or at very cold temperature form box shall be removed after 48 hours.
19. After removing form the concrete surface is checked, then defects if any, be repaired with the application of cement and sand mortar of proportion same as that of concreting.
20. After removing the form box and shoring & shuttering (if any), back filling shall be started. Back filling shall be done with the excavated soil. In case of boulders the same shall be broken to a maximum size of 80mm.
21. The earth shall be put in layers of 150 to 300mm, levelled and compacted properly before laying next layer.
22. After all pits have been back filled to full depth the stub setting template may be removed.
23. The back filling shall be made upto 75mm above the GL. Also, built 50mm high bunds along the sides of the excavated pit and sufficient water shall be poured for at least 24 hours.
- Shall be as per shape, lines and dimensions shown in drawing
- Should be rigid & sufficiently tight
- Should be of light design, easily removable
- Shall be of steel, hardwood or framed plywood
- Inner surface to be smooth
- Shall be used d for casting of all type of foundations except at undercut interface
- Inner surface should be clean, coatedwith approved composition every time during use
- M-20 (1 cement :1.5 fine aggregates :3 coarse agg)
- 20 mm coarse aggregates for Chimney
- 40 mm coarse aggregates for Slab
- M-10(1 cement :3 fine aggregates :6 coarse agg)
- for pad (lean concrete) to avoid possibility of RF steel exposed to earth
- Design Mix
- mix be designed to achieve required work- ability & strength not less than appropriate value
- Nominal Mix
- proportion of materials prefixed
- used for M-20 or lower
- Used in normal foundation of TL
Proportion of Nominal Mix Concrete
Grade Fine + Coarse Agg Fine :Coarse Agg Water
(Max in Kg/bag cement)Max in liter/bag cement
M-10 480 1:2 34
M-15 330 1:2 32
M-20 250 1:2 30
- concrete to be brought to form work as fast as possible to prevent segregation of ingredients or ingress of foreign matter or water.
- special care to be taken during very hot or cold weather
- to be placed & compacted before start of setting
to avoid segregation
- care to take to avoid displacement of reinforcement or movement of formwork
- freefall not to be more than 1.5 meter
- to compact thoroughly & fully around reinforcement, embedded fixtures & into corners of formwork. 16 mm poking bar pointed at the end may be used.
- by mechanical vibrator. Manually in difficult terrain
- Over vibration & under vibration are harmful & to be avoided
- process of preventing loss of moisture from concrete
& maintaining a satisfactory temperature regime
- concrete to be kept continuously wet by ponding, covering with sacks, canvas, hessian or similar materials
- to be done for at least 10 days
- to remain sufficiently rigid during placing & compaction of concrete
- to prevent loss of slurry from concrete
- insides to be cleaned from all rubbish, i,e, chippings, shavings before concreting.
- insides to be treated with form release agent
- to be removed after at least 24 hrs of concreting
Pouring of concrete & vibration
- Slump Test
- to determine consistency of concrete/work ability
Apparatus for slump test
- A cone of metal with following
Bottom dia - 20 cm
Top dia - 10 cm
Height - 30 cm
- To be tested for 28 days compressive strength
- 3 samples of 15 cm cube shall be taken
- 28 days compressive strength for
a) for M-20 – 265 km/cm2
b) for M-15 – 210 km/cm2
- Average strength to be accepted provided strength
of each cube not < 70% or > 130% of above
- Shall be started after repair, if any, of foundation concrete after a
setting period of 24 hrs
- Generally done by excavated soil
- Clay type, black cotton soil & large
boulders (>80mm) not allowed
- Should be clean & free from organic
- Shall be deposited in max. 200 mm layers &
- Carried out about 75 mm above finished GL
- 50 mm earthen embankment along the sides of
excavation pits & to pour water for 24 hrs
- Stub template to remove only after completion
of back filling
- Measurement shall be made as per actual type of soil encountered so that total payment for excavation shall not exceed amount as payable for excavation considering the soil type same as that of foundation classification
-Unit rate includes all allied activities, i.e, dewatering, shuttering, form box etc
- Unit rate includes supply, cutting, bending as per design & placement
1. Construction material should be stacked at least 1.5m to 2.0m away from the edge of the pit.
2. Keep watch on caving in soil of pits, arrangement to resist the collapsing of soil of the excavated pit.
3. Suitable and stable strong ladder should be available in pit for going down and climbing up.
4. Keep constant watch on the supporting jacks which is required technically as well as for safety.
5. All the persons working at site should use PPE.
6. Concreting mixture shall be placed on firm ground securedly to avoid in toppling during the operation.
7. If any electrical connection is taken, check properly type of cable, connection etc., to avoid any electrocution.