.
Introduction:
An investigation of soil is essential for judging its
suitability for the proposed engineering works and for
preparing adequate and economic design.
In general, the purpose of soil investigation is to obtain
necessary information about the soil and to know the
engineering properties of soil which will be affected.
Earlier, the soil investigation of locations of
transmission line towers was not very popular and
general practice had been to adopt 4 to 5 types of
standard design foundations for different classes of
soils encountered.
.
Only special foundations in river beds necessitating
huge volumes of concrete were investigated for
properties of soils. Now the soil investigation of
normal foundations is also felt necessary in good
number of locations in the Transmission lines which
helps in better choice of standard foundation &
development of new designs to achieve overall cost,
economy and minimise chances of failure.
.
Purpose of soil investigation:
a) Technical Consideration
b) Economic Consideration
a) Technical Considerations : An inadequate design
or a conservative choice of standard foundation can
lead to a failure causing long outage of transmission
line. In modern practice, a large variety of
standardized foundations are being pre-designed
with different sets of properties attached to
foreseeable encountered soils. Large variety of soils
are encountered as length of transmission lines are
increasing with voltage levels going up.
.
To obtain optimal choice of pre-designed standard
foundations, it is very much necessary to have a proper
scientific knowledge of properties of soil against the
back-drop of increasing sizes of towers, foundations,
loads, thereby minimizing the risk of failures of
foundations
.
b) Economic Considerations : Among site erection
activities, the foundations form the major chunk of the
cost. The cost of foundations constitutes 50 to 70% of
the total cost of erection depending upon terrain
conditions. It forms 5 to 15% of the total cost of
transmission line. A considerable saving in the
foundation cost can be achieved by having detailed
knowledge of soil properties and making wide usage of
them in designing the foundations in sufficient types
and classification of the foundations in field to match
the most optimum size and type of foundation
Nature & Extent of Soil Invest.
Nature and extent of soil invest. depends on the Ultimate
use of the results of investigation
(A) For Structures which transmit heavy loads:
- Selection of proper type of foundation
- Determination of location of foundation
(B) In highway construction:
- Suitability of soil as const. material
(C) Dam:
- Hydraulic characteristics of soil
- Suitability of soil as const. material
Subsurface investigation
(A) Reconnaissance survey: No exploratory field
observation
Data is collected from
-Published journals, reports, geological maps,
soil survey map, local enquiry etc.
(B) Preliminary exploration:
-Few boreholes may be selectively sunk at site
to know soil profile and the depth of bed rock
-Used for relatively unimportant and light
structure
(C) Detailed exploration:
-Large number of boreholes are drilled
-Samples both disturbed and undisturbed sample
collected from different depth
-Used where important structure is proposed
Activities in soil investigation
- Drilling boreholes
- Sampling
- Field test
Operation in laboratory
Report
Common method used for drilling
boreholes for Soil investigation
Auger boring:
Used in soil having some cohesion.
Helical auger is used to advance the borehole
Well suited for depth upto 10m
In sandy soil bailer is used ( Bailer is a heavy cylindrical
metallic barrel with a flap valve at the bottom)
Wash boring: Water jet is used through drill pipe
along with casing for drilling upto higher depth
Rotary boring: For hard soil resistant to augering and
wash boring
Percussion drilling: Drilling through rocky strata
Depth of investigation
At least twice the width of anticipated largest size of
foundation
The borehole should be drilled to a depth beyond which
increase in stress due to foundation loading is not significant
(less than one tenth of load intensity at the contact surface of
foundation and soil)
SAMPLING
Samples are collected at regular intervals and at levels where
there is change in soil types for closer examination in the
laboratory
Sample types:
Disturbed sample
- Natural structural arrangement of
soil particles considerably disturbed
- Sampling is done using split spoon sampler
used in SPT
Undisturbed sample
- Retain the in-situ soil structure to a large extent
apart from being representative
- sampler used is thin walled sampler
(Area ratio less than 10% and inside clearance ratio bet. 1
to 3)
Area ratio= 100*(Do2- De2)/ De2 , Inside clearance ratio= (Di- De)/ De
Sample Size: Dia=150mm, Length=>5Do to 10Do in sand and =>10Do to 15Do in Clay
Sampling in rock
Done by use of rotary drilling
Dia of core drilled should be preferably not less than
54mm
Rock Quality designation
RQD= (Lc/La)X100 %
Lc-Total length of intact piece more than 100mm,
La-Total length of core advance
Rock classification based on RQD
RQD % Rock Description
<25 Very poor
25-50 Poor
50-75 Fair
75-90 Good
90-100 Excellent
Exercise : To find type of rock
METHOD OF INVESTIGATION
(A) FOR NORMAL LOCATIONS
(OPEN CAST FOUNDATION LOCATIONS)
Trial pit up to foundation depth for
suspension tower locations
(To ascertain the type of soil
and water table)
Detailed investigation at angle tower
locations up to 10m depth
METHOD OF INVESTIGATION
(B) FOR SPECIAL RIVER CROSSING
LOCATIONS
(BANK OR MIDSTREAM LOCATIONS)
Detailed investigation up to 40m/50 m depth
DETAILED SOIL INVESTIGATION
Normal locations
Minimum bore hole dia.150mm
SPT is carried out in the borehole at a
depth of 2.0m, 3.0m, 5.0m, 7.0m and 10.0m Disturbed
samples are collected at 0.5m interval starting from a
depth of 0.5m below GL and at every identifiable change
of strata
Undisturbed samples are collected at a depth of 1.0m,
4.0m, 6.0m and 8.0m and at every change of strata
DETAILED SOIL INVESTIGATION
Field & Laboratory test:
Required field & laboratory tests are conducted
to arrive at requisite soil parameters and
strength to facilitate in optimum foundation
design & selection of foundation classification.
Chemical tests on sub-soil water and soil
samples
DETAILED SOIL INVESTIGATION
For special foundation locations
Minimum borehole dia 150mm
SPT Carried out at 2.0m, 3.0m, 5.0m,
7.0m, 10.0m and thereafter at the rate of
3.0m intervals to 40.0m depth and at 40.0m.
Undisturbed soil sample collected at 1.0m,
4.0m, 6.0m, 8.0m, 11.0m and thereafter at
the rate of 3m intervals.
Disturbed soil sample collected at 0.5m
interval starting from 0.5m below GL
DETAILED SOIL INVESTIGATION
Required field & laboratory tests are
conducted to arrive at requisite soil
parameters and strength to facilitate in
optimum foundation design.
Chemical tests on sub-soil water and soil
samples
Highest flood level (H.F.L.), Maximum river
velocity and Maximum river discharge data
are collected.
PRESENTATION OF SOIL DATA SHEET
FOR NORMAL LOCATIONS
Date of boring: Project:
Surface elevation: Water table: Termination Bore hole no.:
depth:
Co-ordinates: Grain size Atterberg Densities (g/cc) Moistur Triaxial Remarks
Analysis (%) limit e Test/Direct
(%) content shear test
N-Value (corrected)
N-Value
Plastic
Silt
Sp. Gr.
Angle of Repose
Optimum
Clay
Dry Density
Depth
Sample nos. & type
Gravel
Confining pressure
Soil Classification &
Natural
Description
Plasticity Index
Bulk\Natural Density
Relative density
Sand
Proctor density
Cohesion
Liquid
Submerged Density
0 m to 10m
Following additional information may also be provided:
(i)Maximum expected yearly rise/fall of water table:
(ii)General topography of the location: Whether cultivated or barren, hilly/plain location, river bank location
etc
Note: Where ever undisturbed sampling is not possible, reasons must be clearly be indicated and all the tests
may be conducted on re-moulded samples
Prepared by Checked & Reviewed by Checked & Approved by
Soil investigator Line contractor Powergrid
representative
PRESENTATION OF SOIL DATA SHEET
FOR NORMAL LOCATIONS
Ultimate Bearing capacities considering appropriate correction for maximum rise of water
table:
Location Foundation Depth and size of Bearing Capacity*
no Classification footing
Based on settlement Based on shear failure
criteria criteria
(for 40mm total
settlement)
3.0m, 3mX3m
3.0m, 5mX5m
3.0m, 7mX7m
3.0m, 9mX9m
3.5m, 3mX3m
3.5m, 5mX5m
3.5m, 7mX7m
3.5m, 9mX9m
* Detailed calculations of all the bearing capacities should be enclosed with soil investigation
report
Prepared by Checked & Reviewed by Checked & Approved by
Soil investigator Line contractor Powergrid representative
PRESENTATION OF SOIL DATA SHEET
FOR SPECIAL LOCATIONS
(i) Silt factor:
(Supporting calc. to be annexed)
(ii)River Data:
(a)Maximum velocity :
(b)Maximum discharge :
(c)Highest flood level :
(d)Width of river channel :
(if available)
(iii)Max. Scour depth :
(Supporting calc. to be annexed)
Prepared by Checked & Reviewed by Checked & Approved by
Soil investigator Line contractor Powergrid representative
PRESENTATION OF SOIL DATA SHEET FOR SPECIAL
FOUNDATION LOCATIONS
SOIL DATA SHEET FOR PILE LOCATION
Date of boring: Project:
Surface elevation: Water table: Termination depth: Bore hole no.:
Co-ordinates: Grain size Atterberg limit Densities (g/cc) Moisture Triaxial Test/Direct
Analysis (%) (%) content shear test
& type
Clay
Index
N-Value
N-Value
Plastic
Sp. Gr.
Density
Density
pressure
Repose
Soil
Gravel
density
Optimum
Sand
Liquid
Plasticity
Dry Density
Natural
Bulk\Natural
Confining
Cohesion
(corrected)
Depth
Sample nos.
& Description
Silt
Submerged
Proctor
Classification
Angle of
0 m to 40m
(i)Maximum expected yearly rise/fall of water table:
(ii)General topography of the location: Whether cultivated or barren, hilly/plain location, river
bank location etc
Note: Where ever undisturbed sampling is not possible, reasons must be clearly be indicated
and all the tests may be conducted on re-moulded samples
Prepared by Checked & Reviewed by Checked & Approved by
Soil investigator Line contractor Powergrid representative
TESTS ON SOIL SAMPLES
Field Tests
Standard Penetration Test
Cone Penetration Test
Vane Shear Test
Plate load test
Laboratory tests
Grain Size Analysis (Sieve analysis)
Tri-axial test
Direct shear test
STANDARD PENETRATION TESTS (SPT)
Most commonly used in india
Test is carried out using split spoon sampler
Sampler is fitted at the bottom of drill rod
It is driven into the soil to a distance of 450mm
using a monkey wt. of 65kg falling freely from ht.
750mm
Number of blow required for last 300mm
penetration is entered as N-value at that particular depth
of borehole.
Refusal: If less than 300mm is penetrated in 50 blows
Test is discontinued if blow count is equal to 100 or the
penetration is less than25mm. SPT must be carried out at
termination depth also.
Static cone penetration test : At field SCPT is widely
used of recording variation in the in-situ penetration
resistance of soil in cases where in-situ density is
disturbed by boring method & SPT is unreliable below
water table. The test is very useful for soft clays, soft
silts, medium sands & fine sands.
Vane shear test: Used in soft and sensitive fine grained
soils where it is difficult to obtain an undisturbed soil
Sample.
Plate load test: It is a model test of prototype foundation
in which a rectangular or circular plate simulating the
foundation is subjected to series of increments of load
and its deformation behavior is observed.
LIST OF CODES FOR SOIL INVESTIGATION
Sl.No. Indian Standards Title
1. IS : 1498 Classification and identification of soils for General
Engineering purpose
2. IS : 1892 Code of practice for subsurface investigation for
foundation
3. IS : 1904 Code of practice for design and construction of
foundation in soils general requirements
4. IS : 2131 Method of Standard Penetration Test for soils
5. IS : 2132 Code of practice for Thin Walled Tube Sampling of
soils
6. IS : 2720 Methods of test for soil (relevant section)
7. IS : 2809 Glossary of Terms and Symbols relating to Soil
Engineering
8. IS : 3025 Methods of Sampling and Testing (Physical &
Chemical) for water used in Industry
9. IS : 4078 Code of practice for Indexing and storage of Drill
cores.
10. IS : 4434 Code of practice for In-situ Vane-shear test
11. IS :4453 Code of practice for exploration of pits, trenches,
drifts and shaft
LIST OF CODES FOR SOIL INVESTIGATION
Sl.No. Indian Standards Title
12. IS : 4464 Code of practice for presentation of drilling information and
core description in foundation investigation
13. IS : 4968 (Part-II) Method of subsrface sounding of soils, dynamic method
using cone and bentonite slurry
14. IS : 5313 Guide for core drilling observations
15. IS : 6403 Code of practice for determination of allowable bearing
pressure on shallow foundation
16. IS : 6926 Code of practice for Diamond core drilling for site
investigation for river valley projects
17. IS : 6935 Method of Determination of Water level in bore hole
18. IS : 8009 (Part I) Code of practice for calculation of settlements of foundations
(shallow foundations subject to symmetrical vertical loads)
19. IS : 8764 Method of determination of point load strength index of rocks
20. IS : 9143 Method of determination of unconfined compressive strength
of rock materials
21. IS : 9179 Method of preparation of rock specimen for lab. testing
LIST OF CODES FOR SOIL INVESTIGATION
Sl.No. Indian Standards Title
22. IS : 4091:1987 Code of practice for Design and Construction of Foundations
for Transmission line towers and Poles
23. IS : 9259-1992 Specification for Liquid Limit Apparatus
24. IS : 9640-1992 Specification for Split Spoon Sampler
25. IS : 10050-1991 Method of Determination of Slake Durability Index of Rocks
26. IS : 11315(Part-II)- Description of Discontinuities in Rock mass core recovery
1991 and rock quality
Field Investigation Equipments
FLOATING WORKING PLATFORM WITH SOIL TESTING EQUIPMENT
FOR RIVER CROSSING LOCATIONS
Field Investigation Equipments
BORE HOLE IN PROGRESS
IN RIVER RIVER BANK
Field Investigation Equipments
ROCK - DRILLING
THANKS
