Some views on soil investigation for buildings are presented.

1. SOIL INVESTIGATION – SOME
VIEWS
DR K M SONI, CHIEF ENGINEER, CPWD, MUMBAI

2. FOUNDATIONS
• FOUNDATION TRANSMITS WEIGHT OF
COMPLETE STRUCTURE TO THE
SUBSOIL.
THUS SUBSOIL MUST BE ABLE TO
BEAR THE LOAD WITHOUT FAILURE.

3. WORK IN FOUNDATION (CPWD
SPECIFICATIONS)
• WORK IN FOUNDATION AND PLINTH INCLUDES:
• FOR BUILDINGS: ALL WORKS UPTO 1.2 M ABOVE GL OR
UPTO FLOOR 1 LEVEL WHICHEVER IS LOWER.
• FOR ABUTMENTS: ALL WORKS UPTO 1.2 M ABOVE THE
BED LEVEL
• FOR RETAINING WALLS/WING WALLS/COMPOUND
WALLS/OHT/OTHER ELEVATED STRUCTURES: ALL
WORKS UPTO 1.2 M ABOVE GL.
• FOR BASEMENTS: ALL WORKS UPTO 1.2 M BOVE GL OR
UPTO FLOOR 1 LEVEL WHICHEVER IS LOWER.
▫ FLOOR 1 IS THE LOWEST FLOOR ABOVE THE GROUND
LEVEL IN A BUILDING UNLESS OTHERWISE SPECIFIED IN
A PARTICULAR CASE.

4. TYPES OF FOUNDATIONS
• SHALLOW FOUNDATION
▫ SPREAD/ISOLATED FOOTING
▫ COMBINED FOOTING
▫ RAFT
• DEEP FOUNDATION
▫ PILES (PRECAST, CAST IN SITU)
 END BEARING PILES
 FRICTION PILES
 COMPACTION PILES
▫ WELL

9. CLASSIFICATION OF SOILS
• COARSE GRAINED SOIL (G,S: G –
MEANS GRAVELS AND S MEANS SAND)
▫ W,P (W-WELL GRADED, P-POORLY
GRADED)
• FINE GRAINED SOIL (M,C,O: M –
MEANS SILT, C – CLAY AND O –
ORGANIC MATTERS)
▫ L,I,H (L-LOW COMPRESSIBILITY, I-
INTERMEDIATE, H-HIGH
COMPRESSIBILITY)

10. CLASSIFICATION OF SOILS AS PER
CPWD SPECIFICATIONS
• ALL KINDS OF SOILS
• ORDINARY ROCK (LIME STONE/SAND
STONE/HARD LATERITE/UNRFD CC)
• HARD ROCK
(QUARTZITE/GRANITE/BASALT/RCC)

11. CHOICE OF FOUNDATION
• DEPENDS UPON
▫ SOIL STRATA
▫ LOAD OF THE SUPERSTRUCTURE

12. GENERAL REQUIREMENTS
• SAFE AGAINST SHEAR FAILURE
• SHOULD NOT SETTLE SO AS TO
DAMAGE THE STRUCTURE
• SHOULD BE LOCATED WITH RESPECT
TO ANY FUTURE INFLUENCE
BEARING CAPACITY OF SOIL

13. BEARING CAPACITY
• MAXIMUM LOAD PER UNIT AREA WHICH
THE SOIL CAN SUSTAIN WITHOUT
RUPTURE
• TERMS USED ARE ULTIMATE BEARING
CAPACITY, NET BEARING CAPACITY, SAFE
BEARING CAPACITY AND ALLOWABLE
BEARING CAPACITY – EACH HAVING
DIFFERENT MEANINGS
DETERMINATION OF BEARING CAPACITY

15. GENERAL INFORMATION FOR SOIL
INVESTIGATION
• LOCAL KNOWLEDGE OF THE SITE
• RECORDS OF TRIAL PITS
• RECORDS OF BORE HOLES IN CLOSE
VICINITY
• BEHAVIOUR OF EXISTING STRUCTURES
• EARLEIR USES OF THE SITE
• SITE PARTICULARS LIKE STREAM/
NALLAH PASSING
• ABONDONED FIELDS/DEEP SEWER LINES
ETC.

16. OUTLINE OF PROCEDURE
• SITE PLAN OF THE SITE
• PRELIMINARY DISTRIBUTION OF THE
BOREHOLES AND OTHER FIELD TESTS

17. NUMBER AND DISPOSITION OF BORE
HOLES
• ABLE TO REVEAL SOIL CHARACTERISTICS
AND MAJOR CHANGES IN THE SITE
• NO. OF BORE HOLES DEPEND UPON THE
EXTENT OF THE SITE, IMPORTANCE AND
NATURE OF SITE.
• AS A GUIDELINE, FOR A COMPACT
BUILDING, FIVE BHs, ONE EACH AT
CORNER AND ONE AT CENTRE ARE
RECOMMENDED FOR A PLOT OF 4000
SQM.
• AT LEAST TWO BORE HOLES MAY BE
DONE.

19. DEPTH OF EXPLORATION
• NORMALLY DEPTH OF EXPLORATION
SHOULD BE BETWEEN ONE AND HALF
TIMES TO TWO TIMES OF WIDTH OF
FOOTING BELOW FOUNDATION LEVEL.
• IN CASE, GWT IS NEAR THE PROPOSED
DEPTH, DEPTH OF EXPLORATION MAY BE
INCREASED SUITABLY.
• ACTUALLY AT THE TIME OF SOIL
INVESTIGATION, WIDTH OF FOUNATION
IS NOT AVAILABLE, HENCE, SOIL
CONSULTANT HAS TO USE HIS
JUDGEMENT

20. METHODS OF SOIL EXPLORATION
• GEOPHYSICAL METHODS (INDIRECT
TESTS)
• OPEN TRIAL PITS
• BORINGS WITH SPTs
• CORE DRILLING
• SPECIALISED TESTS

21. SAMPLES
• DISTURBED SAMPLES
▫ TO DETERMINE PARTICLE SIZE
DISTRIBUTION AND INDEX PROPERTIES
• UNDISTURBED SAMPLES
▫ TO DETERMINE DENSITY, STRENGTH
PARAMETERS AND CONSOILDATION
PARAMETERS

22. NO. OF SAMPLES
• DISTURBED SAMPLES: 0.75M
INTERVAL INITIALLY SAY 6M DEPTH
AND THEREAFTER AT 1.5M INTERVAL
OR EVEN AT HIGHER INTERVAL
• UNDISTURBED SAMPLES: AT CHANGE
OF STRATA OR 1.5M INTERVAL WITHIN
CONTINUOUS STRATA

23. REMEMBER
• UDS PROVIDE MAIN PROPERTIES AND
THUS UDS ARE VERY IMPORTANT
• UDS SHOULD INCLUDE SAMPLES AS FAR
AS POSSIBLE FROM THE DEPTH AT
WHICH DEPTH OF FOOTING IS LIKELY TO
BE RECOMMENDED AND THE DEPTH
INFLUENCED BY THE LOAD OF THE
STRUCTURE
• UDS TAKEN FROM ABOVE THE
RECOMMENDED DEPTH OF FOOTING MAY
NOT BE VERY USEFUL

24. REMEMBER
• IN CASE OF CLAY/SILT CONSOLIDATION
PROPERTIES ARE IMPORTANT WHILE IN
SANDY SOILS, FRICTIONAL PROPERTIES
(PHI VALUE/ N VALUE).
• CONSOLIDATION TEST MUST BE DONE IN
CASE OF CLAYS/SILTS AND N VALUE IS
IMPORTANT IN SANDY SOILS.
CONSOLIDATION INDEX IN CASE OF
SANDY SOILS AND N VALUE IN CLAYS ARE
NOT SO IMPORTANT

25. • SOMETIMES SOIL CONSULTANTS
AVERAGE OUT SOIL PROPERTIES AND
THUS THEY ARE NOT
REPRESENTATIVE. THIS SHOULD NOT
BE DONE
• IN CASE ONLY THREE TESTS ARE
CONDUCTED FOR DETERMINATION OF
SOIL PROPERTIES, AVERAGING OUT
OR MOST FIT CURVE CAN PROVIDE
DIFFERENT VALUE OF PHI. SBC
CHANGES CONSIDERABLY WITH EVEN
SMALL CHAMGE IN PHI VALUE.

26. • PLEASE CORELATE C, PHI AND Cc
VALUES WITH THE SOIL
CLASSIFICATION FROM DISTURBED
SAMPLES. IN CASE OBSERVED
SAMPLES SHOW DIFFERENT TYPE OF
SOIL AND THEIR C/PHI VALUES
DERIVED SHOW DIFFERENT TYPE OF
SOILS, EITHER THE SAMPLING IS
INCORRECT OR TESTS ARE
INCORRECT.

27. • PLEASE CORELATE N VALUES AND
DENSITY OF SOIL PARTICULARLY IN
SANDY SOILS
• ALSO CORELATE DENSITY VALUES
WITH PHI VALUES PARTICULARLY IN
SANDY SOILS

28. BEARING CAPACITY OF SOILS
• MOSTLY SOIL INVESTIGATION IS
CARRIED OUT TO DETERMINE BEARING
CAPACITY OF SOILS.
• THERE ARE FEW TERMS USED LIKE
BEARING CAPACITY, UBC, SBC, SAFE
BEARING PRESSURE, ABC ETC.
• ALLOWABLE BEARING CAPACITY TERM IS
USED FOR LESSER OF SBC AND SAFE
BEARING PRESSURE.
• SBC IS BASED ON SHEAR FAILURE
CRITERION AND SAFE BEARING
PRESSURE ON SETTLEMENT CRITERION.

29. • SAFE BEARING PRESSURE IS IMPORTANT
IN CLAYEY SOILS WHILE SBC IN SANDY
SOILS THOUGH ALLOWABLE BC IS
MINIMUM OF TWO.
• FILLED UP SOIL CAN BE DETERMINED
FROM BORE LOG CHART WHEN N VALUES
ARE HIGHLY INCONSISTANT. AVOID
RECOMMENDING FOOTING ON FILLED UP
SOILS. IN CASE SMALL PORTION IS
FILLED UP, FILLED UP SOIL CAN BE
REMOVED AND LEAN CONCRETE CAN BE
FILLED UP. IN SUCH SOILS, ADOPT RAFT
FOOTING FOR HIGHER LOADS

30. • IN CASE OF LOOSE SOILS IN PART OF
THE PLOT, DEPTH OF FOOTING CAN BE
ADOPTED DIFFERENT FOR DIFFERENT
FOOTINGS
• IN CASE OF WATER TANKS, MINIMUM
TWO BORE HOLES SHOULD BE DONE
AND RAFT FOOTING RECOMMENDED
IF FEASIBLE
•

31. • PLEASE DO NOT RECOMMEND SBC/ABC
BASED ON INDIRECT TESTS
(GEOPHYSICAL). INDIRECT TESTS
MUST BE CONFIRMED FROM FIELD
TESTS BY DOING BORE HOLES
• INDIRECT TESTS SHOULD NOT BE
CARRIED OUT IF EXTERNAL
PARMETERS AFFECTING TESTS ARE
PREDOMINANT LIKE
ELECTRIC/TELEPHONE LINES, GI
WATER SUPPLY LINES ETC

32. • CONSIDER THE SEASON IN WHICH
SOIL INVESTIGATION IS BEING DONE
PARTICULARLY IF GWT IS IN CRITICAL
DEPTH OF AFFECTING DEPTH OF
INFLUENCE. ALWAYS CONSIDER THAT
DURING RAINY SEASON, GWT IS
LIKELY TO RISE AND THUS CALCULATE
SBC ACCORDINGLY

33. DEPTH OF FOUNDATION
• MINIMUM 50 CMS BELOW NATURAL GROUND
LEVEL
• RAFT FOUNDATION GENERALLY NOT LESS
THAN 1 M.
• IN ROCK, CAN BE DIRECTLY PLACED
PREVENTING SLIPPING/UNWANTED
MOVEMENTS BUT IT IS ADVISABLE TO
RECOMMEND FOOTING AT SOME DEPTH
MINIMUM 30 TO 50 CMS TO AVOID SLIPPING
• IN CASE OF RAFT/ROCK, UNIFORM LEVEL
CAN BE ACHIEVED BY FILLING LEAN
CONCRETE IN THE GAPS.

34. DESIGN CALCULATIONS
• SBC DEPENDS UPON THE FOLLOWING
FACTORS;
▫ DEPTH OF FOOTING
▫ DENSITY OF SOIL
▫ WIDTH OF FOOTING
▫ WATER TABLE
▫ SHEAR PARAMETERS i.e. c & Phi VALUES

35. SBC
• IS IT A FIXED TERM ?
• NO, IT DEPENDS UPON c, Phi, DENSITY,
WIDTH AND DEPTH OF FOOTING AND
GWT AND TYPE OF FOOTING.
• C, Phi, DENSITY ALSO MAY VARY WITH
DEPTH

36. REMEMBER
• BEARING CAPACITY IS NOT A FIXED TERM AT
ANY SITE
• BEARING CAPACITY DEPENDS ON MANY
FACTORS AND THUS CAN BE VARIED AS PER
DESIGNER’S REQUIREMENTS BY ADOPTING
DIFFERENT TYPE OF FOUNDATION AND
DEPTH/WIDTH OF FOOTING
• SO ASK SOIL CONSULTANT TO PROVIDE 3-4
COMBINATIONS OF DEPTH AND WIDTH OF
FOOTING IN MATRIX FORM (DEPTH AND
WIDTH)
• SOIL CONSULTANT SHOULD KNOW
APPROXIMATE REQUIREMENTS OF BUILDING
LOAD AND SUITABLY RECOMMEND THE SBC

37. • NORMALLY GEOTECHNICAL
CONSULTANT DOES NOT VISIT SITE
ONCE THE REPORT HAS BEEN GIVEN.
DURING ACTUAL EXCAVATION, SOME
DIFFERENT CONDITION MAY EXIST
AND THUS IT IS ADVISABLE TO VISIT
THE SITE AND IN CASE ABNORMALITY
IS OBSERVED, REPORT SHOULD BE
CONSIDERED FOR REVISION.