SOIL INVESTIGATION – SOME VIEWSDR K M SONI, CHIEF ENGINEER, CPWD, MUMBAI

FOUNDATIONS•FOUNDATION TRANSMITS WEIGHT

OF COMPLETE STRUCTURE TO THE SUBSOIL.

THUS SUBSOIL MUST BE ABLE TO BEAR THE LOAD WITHOUT FAILURE.

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.

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

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)

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)

CHOICE OF FOUNDATION•DEPENDS UPON

▫SOIL STRATA▫LOAD OF THE SUPERSTRUCTURE

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

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

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.

OUTLINE OF PROCEDURE•SITE PLAN OF THE SITE•PRELIMINARY DISTRIBUTION OF

THE BOREHOLES AND OTHER FIELD TESTS

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.

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

METHODS OF SOIL EXPLORATION•GEOPHYSICAL METHODS (INDIRECT

TESTS)•OPEN TRIAL PITS•BORINGS WITH SPTs•CORE DRILLING•SPECIALISED TESTS

SAMPLES•DISTURBED SAMPLES

▫TO DETERMINE PARTICLE SIZE DISTRIBUTION AND INDEX PROPERTIES

•UNDISTURBED SAMPLES▫TO DETERMINE DENSITY, STRENGTH

PARAMETERS AND CONSOILDATION PARAMETERS

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

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

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

• 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.

•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.

•PLEASE CORELATE N VALUES AND DENSITY OF SOIL PARTICULARLY IN SANDY SOILS

•ALSO CORELATE DENSITY VALUES WITH PHI VALUES PARTICULARLY IN SANDY SOILS

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.

• 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

•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

•

•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

•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

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.

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

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

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

•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.