SOIL EXPLORATION

SERIN ISSAC, ASSISTANT PROFESSOR, NEW HORIZON COLLEGE OF ENGINEERING

1. SUBSURFACE
EXPLORATION
1

2. OVERVIEW
I.SITE INVESTIGATION
II.SOIL EXPLORATION
III.OBJECTIVES
IV.PHASES OF EXPLORATION
V.METHODS OF EXPLORATION
I.OPEN EXCAVATION
II.BORING
III.SUBSURFACE SOUNDINGS
IV.GEOPHYSICAL METHODS
VI.BOREHOLE LOGS
VII.SOIL EXPLORATION REPORT
2

3. SITE INVESTIGATION
Siteinvestigationisanengineeringprogrammeused
toassessthesuitabilityofasiteforaproposed
constructionwork.
Itisalsonecessaryinreportingthesafety&causes
offailuresofexistingworks.
3

4. Failures
4

5. Leaning Tower of Pisa
and Sinkholes
5

6. Steps in Site investigation:
1.Preliminary work
Collecting general information and already existing
data such as study of geologic , seismic maps, etc.
at or near site.
Study site history –if previously used as quarry,
agricultural land, industrial unit, etc.
2.Reconnaissance: Actual site inspection
To judge general suitability
Decide exploration techniques
3.Soil exploration
4.Report of investigations
6

7. SOIL EXPLORATION
Thefield&laboratorystudiescarriedouttoobtain
theknowledgeofsub-soilconditionsforasafe&
economicdesignofsubstructuresiscalledasSOIL
EXPLORATION.
7

8. OBJECTIVES OF SOIL
EXPLORATION
To know geological conditions of soil and rock formations.
To establish the ground water table & determine its properties.
To select type & depth of foundation of proposed structure.
To determine the bearing capacity of the site.
To estimate probable settlement.
To solve foundation problems.
To predict suitable construction techniques.
To select suitable construction material for foundation.
To investigate the safety of existing structures & suggest
remedial measures.
8

9. PHASES OF EXPLORATION
A.PHASE1-Collectionofavailableinformationsuchasasite
plan,type,size,andimportanceofthestructure,loading
conditions,previousgeotechnicalreports,topographicmaps,
geologicmaps,hydrologicalinformationandnewspaperclippings.
B.PHASE2-Preliminaryreconnaissance.Visualinspectionis
donetogatherinformationontopography,soilstratification,
vegetation,watermarks,groundwaterlevel,andtypeof
constructionnearby.
9

10. C.PHASE3-Detailedsoilexplorationintheformtrialpits
orboringsiscarriedout.Thedetailsofthesoils
encountered,typeoffieldtestsadopted,typeofsampling
done,presenceofwatertablearerecordedintheformof
borelog.Thesoilsamplesareproperlylabeledandsentto
laboratorytodetermineengineeringproperties.
D.PHASE4-Writeareportcontainingacleardescriptionof
thesoilsatthesite,methodsofexploration,soilprofile,test
methodsandresults,andthelocationofthegroundwater.
10

11. METHODS OF EXPLORATION
A.OPENEXCAVATION
B.BORING
C.SUBSURFACESOUNDINGS
D.GEOPHYSICAL METHODS
11

12. A. OPEN EXCAVATION
Test pits
Permits visual inspection of subsurface conditions in natural state.
Max. depth limited to 3m. Working space of 1.2m x 1.2m is
required at the bottom of the pit.
Used to obtain disturbed/ undisturbed samples.
Trenches
Used to provide a continuous exposure of soil strata.
12

13. Shafts
For greater depths, shafts are made with proper supports by
timber/ steel.
Boreholes
For depths greater than 6m & below WT.
Tunnels
Used to explore areas beneath steep slopes.
13

14. ADVANTAGES
Detailed information of
soil stratigraphy
Large quantities of
disturbed samples are
available for lab testing.
Blocks of undisturbed
samples can be carved out.
Bottom of pits can be used
for field tests.
DISADVANTAGES
Deep pits are uneconomical
Depth limited to 6m
Excavation below WT is
difficult.
14

15. B. BORING
Making & advancing of boreholes is known as
boring.
Methods of Boring are:
AUGER BORING
AUGER & SHELL BORING
WASH BORING
PERCUSSION BORING
ROTARY BORING
15

16. AUGER BORING:
Simplest method of exploration and sampling.
Method:
Auger is held vertically on ground surface & is
pressed out by rotating.
Turning action cuts the soil which fills the annular
space.
Auger is then withdrawn & cleaned. The process is
again repeated.
16

17. Types: Hand operated or
Power driven .
Hand operated augers
Post hole augers
Boring pits of sizes 7.5 to
30 cm in dia.
Used for depth upto 6m.
Casing pipes may be used
to prevent the collapse of
sides of boreholes.
If stones are encountered,
chisel bits are used to
break them.
Spiral/ Helical augers
CHISEL BITS
17

18. Power driven augers
Used for greater boring depths.
Useful in hard & stiff soil strata.
Continuous flight of augers are used.
Useful in all soil types.
18

19. Auger boring is suitable in loose, moderately cohesive soils,
partially saturated sand & silts.
Max. depth 10 m
Samples obtained are highly disturbed.
19

20. PERCUSSION BORING:
Percussion drilling
Grinding the soil by repeated lifting and
dropping of heavy chisels or drilling bits.
Water is added to form slurry of cuttings.
Slurry removed by bailers or pumps.
BAILER
20

21. Ingeneral,amachineusedtodrillholesiscalledadrill
rig(generallypowerdriven,butmaybehanddriven).
Awinchisprovidedtoraiseandlowerthedrillingtools
intothehole.
Sidesofholesarestabilizedbyusingcasingordrilling
mud(Bentoniteclaymixedwithwater).
Usefulinrocks,boulders&hardstrata.
Expensivemethod.
Highlydisturbedsamplesareobtained.
21

22. ROTARY DRILLING
Used to bore holes of few cm to 1m
dia& of greater depths.
Can be used in clay, sand & rocks.
Two types: Mud Rotary drilling &
Core drilling
Mud Rotary drilling
Hollow drill rods with a drill bit is
rotated into the soil by a chuck.
Drilling mud is continuously
pumped into the hole through drill
rods.
The bit grinds the soil and the
return flow brings the cuttings to
the surface.
No casing used.
22

23. Core drilling
Used for obtaining rock cores.
A core barrel is fitted with a
drill bit is attached to a string of
hollow drill rods and is rotated
into the hole.
As the bit advances, rock core
passes to the barrel& is
retained by core lifter.
Bit is kept cool by pumping
water through drill rods.
Examples: diamond bit or steel
bit.
23

24. TYPES OF SAMPLES
DISTURBED SAMPLES
REPRESENTATIVE –DISTURBED SAMPLES
NON REPRESENTATIVE –DISTURBED
SAMPLES
UNDISTURBED SAMPLES
24

25. DISTURBED SAMPLES
Samplesinwhichthenaturalsoilstructuregetsdestroyedor
modifiedduringthesamplingoperations.
REPRESENTATIVE SAMPLES:Disturbedsampleswhose
moisturecontent&proportionofmineralconstituentscanbe
preserved.
Usedtodeterminetheindexproperties(grainsize,plasticity
characteristics,specificgravity).
Usedforcompactiontests,stabilizationtestsetc.
NON-REPRESENTATIVE SAMPLES:Disturbedsamplesin
whichthemoisturecontent&proportionofmineralconstituents
cannotbepreserved.Theyarevirtuallyofnouse.
Obtainedduringaugerboring&percussiondrilling.
25

26. UNDISTURBED SAMPLES
Samplesinwhichthenaturalsoilstructureispreserved
&thepropertieshavenotchangedduringthesampling
operations.
Suitabletodetermineengineeringpropertiessuchas
compressibility,shearstrength&permeabilityand
indexpropertieslikeAtterberg’slimits.
Obtained using samplerslike Shelby tube, piston
sampleretc.
26

27. SAMPLE DISTURBANCE
Dependsuponthesamplerandsamplingmethod.
Itisgreatlyaffectedbythedimensionsofthe
cuttingedge&sampler.
D4
D3
D1
D2
D3
SAMPLING TUBE
CUTTING EDGE
27

28. Design features affecting the sample
disturbance:

28

29. Design features affecting the sample
disturbance:

29

30. Design features affecting the sample
disturbance:

30

31. Design features affecting the sample
disturbance:

31

32. Design features affecting the sample
disturbance:

32

33. SAMPLERS
1.OPEN DRIVE SAMPLERS
1.SPLIT SPOON SAMPLER
2.SHELBY TUBE SAMPLER
2.STATIONARY PISTON SAMPLERS
3.ROTARY SAMPLERS
4.BLOCK / CHUNK SAMPLES
33

34. 1.OPEN DRIVE SAMPLERS
Consists of a seamless –open ended steel tubewith a cutting
edge.
Tube is connected to a drill rod.
Sampler head has vents–permits water & air to escape
during sampling.
Check valve–retains the sample in the tube, while it is
withdrawn.
Two types:
1.SPLIT SPOON SAMPLER
2.SHELBY TUBE SAMPLER
34

35. SPLIT SPOON SAMPLER
Used for obtaining disturbed –
representative samples.
Consists of a solid tube, split
tubeor split tube with liner.
Used in Penetration tests.
Sample is collected by repeated
blows of a falling weight.
Outside diafrom 50 mm to 115
mm
Inside diafrom 38 mm to 100
mm.
35

36. 
36

37. SHELBY TUBE SAMPLER

37

38. Length of sampler is such that it can
be penetrated into:
Sandy soils from 5 to 10 times dia
Clays from 10 to 15 times dia
Sampler attached to the drill rod is
pushedinto the soil from the bottom
of borehole by a continuous rapid
motion.
38

39. 2.STATIONARY PISTON SAMPLER
Itisathinwallsamplerwithapiston
inside.
Pistonisattachedtothepistonrod.
Pistonkeepsthelowerendofthe
samplingtubeclosed,whensampleris
loweredtothebottomofhole.
Oncethesamplerislowered,pistonis
preventedfrommovingdownwards
usinglockingcone.
Samplerispushedpastthepistonto
obtainthesample.
Pistonremainsincontactwiththetopof
thesample–itpreventssample
disturbances.
39

40. Used in saturated sands & wet / soft clays to obtain
undisturbedsamples
Cannot be used in Hard & gravelly soils.
40

41. 3.ROTARY SAMPLER
Doublewalledtubesamplerwith
aninnerremovableliner.
Outertubehasremovablecutting
bitatitsbottom,whichrotates&
cutthesoil.
Cuttingsarewashedupfrom
boreholeusingwater/drillingmud,
whichiscontinuouslypumped
throughdrillrods&flowsbtwthe
tubes.
Innertubehasaspringcore
catchertoretainthesample.
Sampleisreceivedininnerliner.
41

42. Used in hard rocks, firm to hard
cohesive soils & slightly cohesive sands
Not suitable for gravelly soils, loose
sands & silts below WT & very soft
cohesive soils.
Quality of sample can be estimated
using a ratio –RQD (Rock Quality
Designation)
RQD = measure of amount of
fracturing/weathering in rocks
42
RQD ROCK
QUALITY
<25 Very poor
25 –50 Poor
50 –75 Fair
75 –90 Good
90 –100 Excellent

43. 4.BLOCK OR CHUNK
SAMPLES
Obtainedfromopenexcavations.
Possibleincohesivesoils.
Soilblockof40cmx40cmisleftundisturbedduring
excavation.
Fromthis,a30x30cmblockistrimmedout
undisturbed.
Openendedboxismadetoslideoverthetrimmed
block.
Spacebtwbox&sampleisfilledwithsand&topis
sealedwithparaffinwax.
Boxisthencutoutfrombaseusingaspade.
43

44. 44

45. DEPTH OF EXPLORATION
Should be done up to which the pressure due to
building loads causes undesirable settlement or shear
failure –Significant depth.
= 1.5 to 2 times width of loaded area.
Depends on :
Type of structure
Load intensity
Size, shape & disposition of loaded areas
Soil profile & properties.
45

46. ISOLATED SPREAD FOOTING / RAFT 1.5 B (WIDTH)
ADJACENT FOOTINGS WITH CLEAR SPACING < 2B 1.5 L
ADJACENT FOOTINGS WITH CLEAR SPACING ≥ 4B 1.5 B
PILE FOUNDATIONS 10 to30 m or 1.5 B
BASE OF RETAINING WALL 1.5 Bor1.5 H(greater)
DAMS 0.5 B or2 H
ROAD CUTS 1 m
ROAD FILLS 2 m below GL
or
Htof fill abvGL, (greater)
CONSIDERING WEATHERING 1.5 m , in general
3.5 m in blackcotton soil
MULTISTOREYED BUILDINGS
46

47. NUMBER & SPACING OF
BORINGS
Itshouldbesuchastorevealanymajorchangesin
thickness,depthorpropertiesofstrataaffectedby
constructionorimmediatesurroundings.
47
Multi storeyed building10 –30m
Industrial plant 20 –60 m
Highway 250 –500 m
Residential subdivision250 –500 m
Dams & dikes 40 –80 m
SPACING OF BOREHOLES

48. 48
Compactbuilding site 0.4
hectares area
1 in each corner& 1 at centre
Small & less important
building sites < 0.4 hectares
area
1at centre
Large area buildings > 0.4
hectares
Area divided into grids & 1 at every
100m
Dam sites 1 at every 50 m spacingalong top line
of upstream side & across abutments.
Road sites 1 at every 100 m spacing along centre
line & both side ditch lines.
1 at every 500 m,on uniform soils
1 at every 30 m, on non-uniform soils
NUMBER OF BOREHOLES

49. SUB SURFACE SOUNDINGS
Sub surface sounding is in-situ determination of variation
in penetration resistanceof a soil deposit along vertical
lines.
APPLICATIONS
To locate bedrock or hard strata underlying soft and loose
strata.
To explore an erratic soil profile
To determine boundaries of soft pockets.
To get information on the relative density of cohesionless
soils and consistency of cohesive soil.
To estimate the approximate bearing capacity of soil, length
and bearing capacity of piles and settlement of foundation.
49

50. TYPES OF SOUNDING OR
PENETRATION TEST
STANDARD PENETRATION TEST
CONE PENETRATION TEST
50

51. STANDARD PENETRATION
TEST (SPT)
Most extensively used method in India
Suitable in cohesionless soils
Carried out in a borehole, which is clean having 55 to 150 mm
diameter.
The borehole is advanced to required depth and cleaned.
Casing / Drilling mud is used to support sides.
Split spoon sampler diainner 35mm & outer 50.8mm is driven
for a distance of 450 mm with hammer of -
weight 65kg
Drop 750mm
Rate of penetration 30mm/minute
51

52. No of blows for every 150mm is recorded.
Blows for first 150mm penetration is discarded –seating drive.
The blows for last 2 penetrations are added together to give the
STANDARD PENETRATION NUMBER (N value).
Sampler is withdrawn and detached from drill rod.
Sample is extracted and sealed with paraffin wax and transported
for testing.
SPT is carried out at every 0.75 or 1m depth.
It is discontinued if :
Blows 50 –for 150mm
Blows 100-for 300mm
10 successive blows produce no advance.
Sample obtained-representative, disturbed.
52

53. SPT CORRECTIONS
OVERBURDEN PRESSURE CORRECTION
DILATANCY CORRECTION
53

54. Overburden pressure correction
Significant in granular soils
As depth increases, confining pressure also increases in granular
soils.
N value will be over estimated for greater depths
N value will be under estimated for shallow depths.
N values are corrected to a Standard effective overburden
pressure.
54

55. 
55

56. Dilatancy correction (fine sand & silt)

56

57. 57

58. 58

59. CONE PENETRATION TEST
(CPT)
Also called as Dutch cone test.
Useful in cohesionless soil & soft
sensitive silt & clay.
Soil sample cannot be obtained.
Types:
STATIC CONE
PENETRATION TEST
DYNAMIC CONE TEST
59

60. STATIC CONE PENETRATION TEST

60

61. 61

62. 
62

63. DYNAMIC CONE TEST

63

64. 
64

65. METHODS OF EXPLORATION
A.OPENEXCAVATION
B.BORING
C.SUBSURFACESOUNDINGS
D.GEOPHYSICAL METHODS
65

66. GEOPHYSICAL METHODS
Usedinpreliminaryinvestigationsofsubsoilstrata
tolocatedifferentsoillayers&forrapidevaluation
ofsoilcharacteristics.
Twomethods:
SEISMICREFRACTION METHOD
ELECTRICALRESISTIVITYMETHOD
66

67. SEISMIC REFRACTION METHOD
PRINCIPLE:seismicwaveshavedifferent
velocitiesindifferenttypesofsoil/rock.
Itgetsrefracted–whenitcrossesthe
boundaries–asdifferentsoiltypespossess
differentelasticproperties.
Usedtodeterminesoiltypes&approx.depthof
soilstrata.
67

68. 68

69. Procedure:
AnIMPACTorSHOCKisgenerated–
bystrikingaplateonsoilwithhammer
byexplodingasmallchargeat/neargroundsurface.
VibrationdetectorscalledasGEOPHONES ,placed
ongroundsurfaceatacertaindistancefromthe
SOURCE,picksuptheradiatingshockwaves.
Geophones/SEISMOMETER –recordthetimeof
travelofwaves.
69

70. 70
A B
V1
V2
Z1
Z2
TOP
SOIL
DENSER
SOIL
Refracted waves (V2)
Refracted waves (V3)
Direct waves (V1)
B
D1 D2
A

71. Direct/Primarywavestraveldirectlyfromsource
alongthegroundsurface&arepickedupbyfirst
geophone.
Ifsubsoilhasmorelayers–wavestravel
downwardstolowerlayer&getrefractedatthe
interfaceb/wlayers.
Iflowerlayerisdenser,-refractedwavestravel
faster–mergeagain-&reachgeophone.
Asdistanceb/wSource&Geophoneincreases–
refractedwaveswillreachfasterthandirectwaves.
71

72. P –source –shock waves are generated here.
A & B -Geophones –record the arrival of waves &
convert them to electrical impulses & transmit to
recording devices.
V1 –velocity of direct waves
V2 –velocity of refracted waves from top layer
V3 –velocity of refracted waves from denser layer.
V3 > V2 > V1
72

73. 73
DISTANCE –TIME
GRAPH

74. 
74

75. 
75

76. Limitations
Notapplicable,ifahardlayeroverliesasofterlayer
withsmallerseismicvelocity.
Notapplicableinareascoveredwithconcrete,
asphaltpavement,undergroundconduits,irregular
WT,frozensoillayer.
Costly
Requiressophisticatedequipment&expertpersons
forinterpretingtheresults.
76

77. ELECTRICAL RESISTIVITY
METHOD
77

78. 78

79. 79

80. Electrical Profiling / Resistivity
mapping
4 electrodes in the form of metal spikes are
driven into soil
Outer 2 –current electrodes
Inner 2 –potential electrodes
DC is applied to outer electrodes & Voltage
drop b/w inner electrodes are then measured.
To know horizontal changes in sub –soil, the
electrodes kept at a constant spacing are
moved as a group along the test line.
80

81. 81
Mean resistivity is given as,
d

82. Electrical Sounding / Resistivity
Sounding
Tostudyverticalchangesinsoilstrata,
electrodesystemisexpanded,byincreasing
spacingb/welectrodesuptoadistanceequal
todepthrequired.
82

83. Limitations
The methods are capable of detecting only the strata
having different electrical resistivity.
The results are considerably influenced by surface
irregularities, wetness of the strata and electrolyte
concentration of the ground water.
As the resistivity of different strata at the interface
changes gradually and not abruptly as assumed, then
the interpolation becomes difficult.
The services of an expertin the field are needed
83

84. STABILIZATION OF BOREHOLE
Itisimportanttopreventthecavinginofthesides
ofaborehole,whileitisdrilledinsuccessivestages.
METHODS:
1.SELFSUPPORTINGBOREHOLE
2.BYFILLINGWATERABOVEGWT
3.BYFILLINGDRILLINGMUDABOVEGWT
4.BYCASINGPIPE&FILLINGWITHWATER
ABOVEGWT
84

85. 1.SELFSUPPORTINGBOREHOLE
Inclays,boreholesareself-supportivebecause:
Dry–aboveWT–haveapparentcohesion
Wet–belowWT–haveundrainedshear
strength
Insilt,boreholesareself-supportivebecauseof
cohesionduetonegativeporepressure.
2.BY FILLING WATER ABOVE GWT
GWT-above the water in borehole –seepage
forces push the soil particles into the hole.
If water level in hole is raised -flow will be
reverse & the soil particles will remain in original
position.
Suitable in sandy soil
85

86. 3.BYFILLINGDRILLINGMUDABOVEGWT
Drilling mud/ Bentonite coating on sides of borehole –
have low permeability & high plasticity -prevents caving
in.
Suitable in fine sand.
4.BY CASING PIPE & FILLING WITH WATER ABOVE
GWT
Suitable in medium & coarse sand & soft clays
86

87. BORELOG
Afterthesoilinvestigationhasbeencompleted&
thelaboratoryresultsareavailable,theground
conditionsdiscoveredineachboreholeis
summarizedintheformofaBORELOG.
87

88. It consists of :
Name of Project & Client
Method of investigation & Details of equipment
used
Date of Start & Completion of project
Location, Ground level & diameter of borehole.
Soil profile & elevations of different strata
Ground water level
Depths at which tests are done & samples are taken.
Types of soil samples
Results of important Lab tests
N values obtained at various depths
88

89. 89

90. SOIL EXPLORATION
REPORT
90

91. It should contain the following data:
1.Introduction
Scope of investigation
Nature of project
2.Location & Description of the site & its geological conditions.
3.Details of soil exploration programme –Number of borings,
their location, depth etc.
4.Methods of exploration
5.Laboratory & Field tests conducted & their results, tables,
plots, test procedure, IS standards etc.
6.Depth of Ground water table & their changes
7.Borelogs& representation of data from a number of
boreholes as a surface profile.
91

92. 92

93. 8.Analysis of Data. It should include:
Reference to Borelog& subsurface profile
RL of GWT
Calculations for recommended type of foundation, size,
depth, allowable bearing pressure etc.
9.Recommendations
10.Conclusions
11.Limitations of investigation
12.References & relevant literature extracts.
93