Geotechnical Engineering-I [Lec #28: Soil Exploration]

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Geotechnical Engineering–I [CE-221]
BSc Civil Engineering – 4th Semester
by
Dr. Muhammad Irfan
Assistant Professor
Civil Engg. Dept. – UET Lahore
Email: mirfan1@msn.com
Lecture Handouts: https://groups.google.com/d/forum/2016session-geotech-i
Lecture # 28
8-May-2018

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SITE INVESTIGATION
 Site investigation →
suitability of site for
proposed construction.
 Soil exploration (or
Geotechnical
Investigation) is a part of
site investigation.

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GEOTECHNICAL INVESTIGATIONS –
COST
Geotechnical investigations cost only ~0.1% to
4% of the total capital cost.

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WHAT ?
Attempt at understanding subsurface conditions:
 Soil and rock profile
 Geological features of the region
 Position and variation of ground water table
 Physical properties of soil and rock
 Contamination, if any
 General data of adjacent structures, hydrological
data, topography, soil maps, seismicity, etc.
SOIL EXPLORATION Nature of soil
Quality of soil

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WHY ?
 To predict possible geotechnical problems and devise
relevant solutions.
 To determine the type of foundation required for the
proposed project at the site, i.e. shallow foundation or deep
foundation.
 To make recommendations regarding the safe bearing
capacity or pile load capacity.
“Virtually every structure is supported by soil or rock.
Those that aren’t either fly, float, or fall over.”
Richard L. Handy (1995)
SOIL EXPLORATION

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LEANING TOWER OF
PISA AND SINKHOLES

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HOW ?
The three important aspect are planning, execution
and report writing.
Planning
 To minimize cost of explorations and yet give reliable
data.
 Decide on quantity and quality of geotechnical
investigations.
SOIL EXPLORATION

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Execution:
 Exploring sub-soil (i.e. drilling)
 Conducting in-situ tests and
obtaining soil properties in field
 Collection of disturbed and/or
undisturbed soil samples
 Laboratory testing on collected
samples
 Study of ground water
conditions
 Sample collection for chemical
analysis
 Geophysical exploration
SOIL EXPLORATION

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Report writing:
 Description of site conditions
 topographic features, hydraulic conditions, existing structures, etc.
supplemented by plans/drawings
 Description of proposed construction
 nature, type and loading arrangement, etc.
 Description of field activities
 Description of lab activities
 Analysis and discussion of data collected
 Preparation of charts, tables, graphs, etc.
 Calculations performed
 Conclusions & Recommendations
SOIL EXPLORATION

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GEOTECHNICAL DESIGN CYCLE
Construction Site
Geo-Laboratory
for testing
Design Office
for design & analysis
soil properties

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GEOTECHNICAL INVESTIGATIONS
Report Writing
 Description of site conditions
 topographic features, hydraulic conditions, existing structures, etc.
supplemented by plans/drawings
 Description of proposed construction
 nature, type and loading arrangement, etc.
 Description of field activities
 Description of lab activities
 Analysis and discussion of data
collected
 Preparation of charts, tables, graphs,
etc.
 Calculations performed
 Conclusions & Recommendations

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Deciding the
 Number of boreholes
 Depth of each borehole
 Frequency of sample collection
 Location and number of test pits, etc.
GEOTECHNICAL INVESTIGATIONS
Planning Phase
 Practically impossible to
explore the entire area
 Key/critical location
selected and explored

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BOREHOLES/TEST PITS
Test Pit
1-2m width
1-4m depth
~100-150 mm dia
~10-60m depth

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BOREHOLE SPACING
Thumb rule → One borehole every 10,000 ft2

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BOREHOLE DEPTH
Rough estimate for depth of boreholes (Sowers & Sowers, 1970).
Light steel or Narrow Concrete Buildings:
zb (m) = 3 S0.7
zb (ft) = 10 S0.7
Heavy Steel or Wide Concrete Buildings:
zb (m) = 6 S0.7
zb (ft) = 20 S0.7
where
zb = approximate depth of the boring
S = number of stories
~100-150 mm dia
~10-60m depth

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Test Pit
1-2m width
1-4m depth
~100-150 mm dia
~10-60m depth
SOIL EXPLORATION
Execution Phase

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 Permits visual inspection of
subsurface conditions in natural
state.
 Max. depth limited to 18-20 feet.
 Useful for gravelly soil where
boreholes may be difficult.
 Sampling/testing done on exposed
surfaces.
 Useful for pavement projects
where exploration of shallow
ground is required.
SOIL EXPLORATION
Test Pits

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REPORTING
Test Pit Wall Photograph
Western Wall Section
Test-pit Logs

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Advantages
 Cost effective
 Provide detailed information of stratigraphy
 Large quantities of disturbed soils are available for testing
 Large blocks of undisturbed samples can be carved out from the pits
 Field tests can be conducted at the bottom of the pit
Disadvantages
 Depth limited to about 6 m (20 ft) in stiff clays
 May require side supports in coarse-grained soils and soft clays
 Deep pits uneconomical
 Excavation below groundwater and into rock difficult and costly
 Too many pits may scar site and require backfill soils
 Time consuming
 Limited to depths above ground water level
SOIL EXPLORATION
Test Pits

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Test Pit
1-2m width
1-4m depth
~100-150 mm dia
~10-60m depth
SOIL EXPLORATION

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 Heavy structures transmit the
load to deep ground.
 Boreholes are used to explore
deep soil strata.
 Typical diameter → 60mm –
150mm
 Max. depth of borehole
depends upon soil strata/type
of loading/type of drilling.
SOIL EXPLORATION
BOREHOLES

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 Simplest method of exploration and
sampling.
 Mostly hand operated (Power driven also
available)
 Max. depth ~25 m
 Above GWT → suitable in all soils
 Below GWT → only in cohesive soils
BORING METHODS
Auger Borings
Hand Operated
Augers

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Hand Operated Augers Power Driven Augers

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Advantages
 Cost effective
 Not dependent on terrain
 Portable
 Low headroom required
 Used in uncased holes
 Groundwater location can be easily identified and measured
Disadvantages
 Depth limited to about 3 m (10 ft) to 25 m (80 ft)
 Labor intensive
 Undisturbed samples can be taken for soft clay deposit only
 Cannot be used in rock, stiff clays, dry sand, etc.
BORING METHODS
Hand Augers

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 Drilling operation by rotation of
a chopping bit attached to
drilling rig.
 Drilling bit lowered to the
bottom of hole by means of
drilling rods.
 Drilling mud (bentonite) jetted
under pressure to the bottom of
hole.
 Bentonite keeps temperature of
bit low, and
 Removes the loosened soil from
the bottom through its
circulation
BORING METHODS
Straight Rotary Drilling
VIDEO

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BORING METHODS
Drilling Bits
 Tricone bit is the most commonly
used type.

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 Main cutting force → power rotation of drilling bit
 Circulating fluid removes cuttings from hole.
Advantages
 Quick
 Undisturbed samples can be obtained quite easily
 Groundwater location can be identified
Disadvantages
 Cost of drilling increases with depth
 Site must be accessible to motorized vehicle
 Difficult to use in gravels or rocks
BORING METHODS
Straight Rotary Drilling

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 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.
Advantages
 Quick
 Undisturbed samples can be obtained quite easily
 Groundwater location can be identified
Disadvantages
 Site must be accessible to motorized vehicle
BORING METHODS
Percussion Drilling

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 Rate of progress, action of the rods, examination of
cuttings in the drilling fluid gives an idea of the
encountered strata.
 Bentonite, a montmorillonitic clay, is the typically used
drilling fluid.
 Bentonite stabilizes the borehole → avoids soil caving.
SOIL EXPLORATION

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 Borehole is washed to remove
bentonite coating.
 Any water in the borehole is
bailed out.
 Almost 24 hours given for
stabilization of groundwater
table (GWT).
 Depth of GWT determined.
 Whistle meter
SOIL EXPLORATION
Location of Groundwater Table

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CONCLUDED
REFERENCE MATERIAL
Principles of Geotechnical Engineering – (7th Edition)
Braja M. Das
Chapter #18
Foundation Design, Principles and Practices – (2nd Edition)
Donald P. Coduto
Chapter #4

Where should the borings be located
• Initial borings: to give general geological information
about the site.
• Initial borings should be located near heavily loaded
parts of the structures, special structures, suspected
dumpsites, old landslide areas, and ground
depression.
• Boreholes must be located within 5 m (15 ft) radius
from center of the load.
•37

Location and depth of boreholes •38
• Practically impossible to
explore entire site.
• Building/regulatory codes
provide guidelines on the
min. no of boreholes and
their depth.
• The footprint of a
structure should be
divided using a grid
approx. 20 to 40 m (60 to
120 ft) for large areas and
boreholes should be
located at note points on
the grid.
Preliminary plan of borehole
location at a site

Location and depth of boreholes
• In compressible soils such as clays, the borings should
penetrate at least 1 to 3 times the width of the proposed
foundation below the depth of embedment or untill the stress
increment due to the heaviest foundation load is less than
10%, which ever is greater.
• In very stiff clays and dense coarse-grained soils, borings
should penetrate 5 to 6 m (12 to 20 ft) to prove that the
thickness of the stratum is adequate.
• Borings should penetrate at least 3 m (10 ft) into rock.
• Borings must penetrate below any fills or very soft deposits
below the proposed structure.
• The minimum depth of boreholes should be 6 m (20 ft) unless
bedrock or very dense material is encountered.
•39

Location and depth of boreholes •40
Preliminary plan of borehole location at a site

Geotechnical Engineering-I [Lec #28: Soil Exploration]

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