Soil Exploration
Techniques
Introduction
•Site investigation, in general deals with
determining the suitability of the site for
the proposed construction.
•Soil exploration is a part of site
investigation.
Why Do It?
Leaning Tower of Pisa
and Sinkholes
Introduction (Cont’d)…
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 depending on type, size and
importance of project and whether investigation is
preliminary or detailed.
Introduction (Cont’d)…
• Execution:
• Collection of disturbed and/or
undisturbed samples of
subsurface strata from field.
• Conducting in-situ tests of
subsurface material and
obtaining properties directly or
indirectly.
• Study of ground water conditions
and collection of sample for
chemical analysis.
• Geophysical exploration, if
necessary.
• Laboratory testing on samples
Introduction (Cont’d)…
• Report writing:
• Description of site conditions – topographic features,
hydraulic conditions, existing structures, etc. supplemented
by plans/drawings.
• Description of nature, type and importance of proposed
construction
• Description of field and lab tests carried out.
• Analysis and discussion of data collected
• Preparation of charts, tables, graphs, etc.
• Calculations performed
• Recommendations
Introduction (Cont’d)…
A complete site investigation will consist of:
• 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.
• Site Reconnaissance: Actual site inspection.
• To judge general suitability
• Decide exploration techniques
Introduction (Cont’d)…
• Exploration
• Preliminary Investigations: Exploratory borings or shallow test pits, representative
sampling, geophysical investigations, etc
• Detailed Investigations: Deep boreholes, extensive sampling, in-situ testing, lab
testing, etc.
• Depth and spacing: In general, depth of investigation should be such that any/all
strata that are likely to experience settlement or failure due to loading. Spacing
depends upon degree of variation of surface topography and subsurface strata in
horizontal direction. Refer to Alam Singh.
Some Common Objectives
□
Identify & describe pertinent surface conditions
□
Determine location and thickness of soil and rock strata (subsurface soil
profile)
□
Determine location of groundwater table
□
Recover samples for laboratory testing
□
Conduct lab and/or field testing
□
Identify special problems and concerns
Site Exploration Overview
• Review of Background Information
• Field Reconnaissance
• Field Exploration (Drilling, Sampling, In-situ Testing)
• Laboratory Testing
• Geotechnical Interpretations, Analysis
• Report of Exploration
Field Reconnaissance
□ Observation of Surface Conditions
■
■
■
■
■
■
■
Accessibility
Traffic Control
Surface Drainage
Geologic Features (Rock outcrops)
Vegetation
Slopes
Drainage Patterns
Geophysical Methods
□
□
Electrical Resistivity Surveys
Geophysical Logging
Example
Non-Intrusive Exploration
Example
Non-Intrusive Exploration
Field Exploration; Intrusive
□ Geotechnical drilling
■ Auger drilling (solid stem auger; hollow stem auger)
■ Rotary drilling (rotary wash boring)
■ Rock coring
□ Soil (and rock) sampling
□ In-situ testing (SPT, CPT and other)
How Many Borings & How Deep?
“No hard-and-fast rule exists for determining the number of
borings or the depth to which borings are to be advanced.”
How Many Borings?
• Conventional Wisdom
• The number (density) of borings will increase:
• As soil variability increases
• As the loads increase
• For more critical/significant structures
• Rules of Thumb:
• Soft soils, critical structures – 50'
• Soft Soils - Space 100' to 200'
• As soils become harder, spacing may be increased up to 500’
How Many Borings?
Structure or
Project
Highway
Subgrade
Multistory
Building
Subsurface
Variability
Spacing of Borings (ft)
Irregular
100-1000 (200, typical)
Average
200-2000 (500, typical)
Uniform
400-4000 (1000, typical)
Irregular
25-75
Average
50-150
Uniform
100-300
Source: Sowers 1979
How Many Borings?
How Deep?
How Deep (Bridges)?
• Boring depth is governed by various factors, including:
• Foundation type
• Foundation load
• Lowering of grade line at underpass?
• Channel relocation, widening, dredging?
• Scour?
• Rules of Thumb
• Generally speaking, 50’- 80’ is reasonable
• Local experience is helpful
• Look at nearby structures if available
• If no experience or other info available, plan for long first hole, then adjust.
Spacing of boring
• Depth of boring is dependent on the uniformity of soil deposit to determine
the spacing of borings
• It is related to type, size and weight of the proposed structure, variation of
strata and availability of funds
• Spacing is decreased if additional data are required and increased if the
strata is uniform
Fig. 11 Details regarding boring spacing
Depth of boring
• Depth of exploration at particular site depends upon degree of variation
of subsurface data in horizontal and vertical direction.
• For square footing 1.5B=D
• For strip footing 3B=D
• For pile foundation D=1.5 times the width of the pile group
• In case of friction pile D=1.5 times the width of the pile group measured
from lower 3rd point
• In case of multistory building D=C(S)0.7
Here D= depth of exploration, C=constant equal to 3 for light steel
buildings and narrow concrete buildings. It is equal to 6 for heavy steel
buildings and wide concrete building, S=number of stories
• In case of road fill minimum Depth of boring= 2m below ground surface
or= to the height of the fill, whichever is more
• In case of gravity dam minimum depth of boring is twice the height of
the dam
Methods of Investigation
The methods to determine the sequence, thickness and lateral extent of soil strata and appropriate
level of bed rock
The common methods include
• Test pits
• Boring or drilling
Test pits- This is the simple and reliable method. In which the depth is limited to 4-5m only.
In-situ conditions are examined visually. It is easy to obtain disturbed and undisturbed samples.
Fig. 12 Test Pit
Boring or Drilling
Boring refers to the advancement of hole into ground
The boring methods are used for exploration at greater depths where direct methods fail. These
provide both disturbed as well as undisturbed samples depending upon the method of boring. In
selecting the boring method for a particular job, consideration should be made for the following:
• The materials to be encountered and the relative efficiency of the various boring methods in such
materials
• The available facility and accuracy with which changes in the soil and ground water conditions can
be determined
• Possible disturbance of the material to be sampled
The different types of boring methods are:
• Displacement boring
• Wash boring
• Auger boring
• Rotary drilling
• Percussion drilling
• Continuous sampling
Displacement boring
It is combined method of sampling & boring operation. Closed bottom sampler, slit cup, or piston type
is forced in to the ground up to the desired depth. Then the sampler is detached from soil below it, by
rotating the piston, & finally the piston is released or withdrawn. The sampler is then again forced
further down & sample is taken. After withdrawal of sampler & removal of sample from sampler, the
sampler is kept in closed condition & again used for another depth.
• Features
• Simple and economic method if excessive caving does not occur. Therefore not suitable for loose
sand.
• Major changes of soil character can be detected by means of penetration resistance.
• These are 25mm to 75mm holes.
• It requires fairly continuous sampling in stiff and dense soil, either to protect the sampler from
damage or to avoid objectionably heavy construction pit.
Auger Boring
• It is the simplest method of boring used for small projects in soft cohesive soils. This can be done
using mechanical and hand auger.
• For hard soil and soil containing gravels boring with hand auger becomes difficult
• Hand augered holes can be made upto about 20m depth, although depth greater than 8-10m is
usually not practical.
• The length of the auger blade varies from 0.3-0.5m.
• The auger is rotated until it is full of soil, then it is withdrawn to remove the soil and the soil type
present at various depth is noted
• Repeated with drawl of auger for soil removal makes boring difficult below 8-10m depth
• The soil samples collected in this manner are disturbed samples and can be used for classification
test. Auger boring may not be possible in very soft clay or coarse sand because the hole tends to
collapse when auger is removed
Methods of Boring
• Auger Borings:
• Simplest method of exploration and sampling.
• Power driven or hand operated.
• Max. depth 10 m
• Suitable in all soils above GWT but only in cohesive soil
below GWT
• Hollow stem augers used for sampling or conducting
Standard Penetration Tests.
Hand operated
augers
Power driven augers
•Mechanical Auger means power operated augers. The power required to rotate the auger depends on the
type and size of auger and the type of soil.
•Downwards pressure can be applied hydraulically, mechanically or by dead weight.
•The diameter of the flight auger usually is between 75 to 300mm, although diameters up to 1m and bucket
augers up to 2m are available.
•Borehole depths up to 50m are possible with continuous-flight augers.
•The most common method is to use continuous flight augers. Continuous flight augers can be solid stem or
hollow stem with internal diameter of 75-150mm.
•Hollow stem augers are used when undisturbed samples are required. Plug is withdrawn and sampler is
lowered down and driven in to the soil below the auger.
•If bed rock is reached drilling can also take place through the hollow stem.
•The presence of cobbles and boulders create problems with small-sized augers.
•There is a possibility that different soil types may become mixed as they rise to the surface and it may be
difficult to determine the depths of changes of strata. Experienced driller can however detect the change of
strata by the change of speed and the sound of drilling.
Hand Augar
a)
b)
Plugged while advancing the auger
Plug removed and sampler attached
a
Continuous Flight Auger
Truck mounted auger boring machine
Drilling:
Continuous Flight Auger
Hollow Stem Auger
■ Casing with outer spiral
■ Inner rod with plug/or pilot assembly
■ For sampling, remove pilot assembly and
insert sampler
■ Typically 5ft sections, keyed, box & pin
connections
■ Maximum depth 60-150ft
Hollow Stem Auger
Wash boring
It is a popular method due to the use of limited equipments.
The main advantage is the use of inexpensive and easily portable handling and drilling equipments.
Methodology• An open hole is formed on the ground for soil sampling or rock drilling operation can be done
below the hole.
• The hole is advanced by chopping and twisting action of the light bit.
• Cutting is done by forced water and water jet under pressure through the rods operated inside the
hole.
• In this, a pipe of 5cm diameter is held vertically and filled with water using horizontal lever
arrangement and by the process of suction and application of pressure, soil slurry comes out of the
tube and pipe goes down.
• This can be done upto a depth of 8m –10m (excluding the depth of hole already formed
beforehand)
• Just by noting the change of colour of soil coming out with the change of soil character can be
identified by any experienced person. It gives completely disturbed sample and is not suitable for
very soft soil, fine to medium grained cohesionless soil and in cemented soil.
Fig. 13 Wash boring arrangement
Schematic for wash boring
Rotary boring
Rotary drilling method of boring is useful in case of highly resistant strata. It is related to finding out
the rock strata and also to access the quality of rocks from cracks, fissures and joints. It can
conveniently be used in sands and silts also.
Methodology• A heavy string of the drill rod is used for choking action. The broken rock or soil fragments are
removed by circulating water or drilling mud pumped through the drill rods and bit up through the
bore hole from which it is collected in a settling tank for recirculation.
• If the depth is small and the soil stable, water alone can be used. However, drilling fluids are useful
as they serve to stabilize the bore hole. Drilling mud is slurry of bentonite in water.
• The drilling fluid causes stabilizing effect to the bore hole partly due to higher specific gravity as
compared with water and partly due to formation of mud cake on the sides of the hole. As the
stabilizing effect is imparted by these drilling fluids no casing is required if drilling fluid is used.
Various types of diamond drill
bits for rotary drilling
Rotary Drilling
• This method is suitable for boring holes of diameter 10cm, or more preferably 15 to20cm in most
of the rocks. It is uneconomical for holes less than 10cm diameter. The depth of various strata can
be detected by inspection of cuttings.
Fig. 14 Rotary drilling arrangement
Percussion boring
• Percussion drilling is an alternative method of
advancing a borehole, particularly through hard soil
and rock.
• The boring rig consists of a derrick, a power unit and
a winch carrying a light steel cable which passes
through a pulley on top of the derrick.
• In this technique, the borehole is advanced by the
percussive action of the tool which is alternately
raised and dropped (usually over a distance of 1–2m)
by means of the winch unit.
• Borehole diameters can range from 150 to 300mm.
The maximum borehole depth is generally between
50 and 60m.
Fig 17 Percussion Drilling Setup
Rotary Drilling
□
□
□
□
Bit at the end of drill rod rotated and
advanced
Soil/rock cuttings removed by circulating
drilling fluid
Common drilling fluid; bentonite in water
with slurry density 68-72pcf
Air may be used as drilling fluid
Drilling vs. Sampling
• Drilling – “Just” a hole… no sample
• Disturbed Sampling
“…Estimating the nature of the formation from the cuttings is like
identifying the cow from the hamburgers.” G.F. Sowers
• Undisturbed Sampling
• Retrieve a continuous core
• Applicable to both soil and rock
Split Spoon (Barrel) Sampler
• Suitable for stiff soil, sand gravel
• Thick-wall Steel Tubes
• 1.5” ID, 2.0” OD, 18”-30” long
Shelby Tube Sampler
• Suitable for Soft Soil
• Thin-wall Steel Tubes
• 3.0" OD, 2.875" ID, 30.0" long, 7.2 lbs
Rock Coring
■ Double-tube core
barrel is
typical
■ Diamond or tungsten-carbide
tooth bit
■ Size of core samples varies (NX,
NQ, HQ, etc.)
Rock Core Quality
■
■
Core recovery percentage
Rock Quality Designation (RQD)
□
□
Defines the fraction of solid core recovered greater
than 4 inches in length
Calculated as the ratio of the sum of length of core
fragments greater than 4 inches to the total drilled
footage per run, expressed as a percentage
Groundwater Monitoring
▪ Groundwater level must be determined
during geotechnical exploration
▪ Measure at time of drilling and later (24
hrs, 1 week, etc.)
▪ Can be accomplished by leaving
selected soil borings open
▪ Or, install a piezometer
Ground Water
□ Piezometers
□ Monitor Wells & Sampling
□ Permeability Tests
SPT
79
SPT (Standard Penetration Test)
SPT (Standard Penetration Test)
The result of the Standard penetration test (SPT) is a number of blows N (penetration resistance),
needed to penetrate a sampling device to the soil or rock by a so-called Interval of penetration
depth 0,3 m (1 ft).
The number of blows N measured during the SPT test is correlated for the reason of various testing
devices and for the influence of the weight of overburden in sand. Corrected (correlated) value N60
is used in calculations.
Typical Plot For SPT
81
Typical Plot For SPT
103
Correlation Of N Value
Penetration
resistance N
4
10
30
50
-
Approxima Density
te angle
index (%)
(degree)
25-35
27-32
30-35
35-40
38-43
0
15
35
65
85
100
Description
Approximate
unit rate
kN/m3
Very loose
Loose
Medium
Dense
Very dense
1.12- 1.6
1.44-1.84
1.76-2.08
1.76-2.24
2.08-2.40
104
Penetration Resistance And Empirical Correlation For
Cohesive Soil –
Penetration
Resistance
(Blows)
Unconfined
compressive
strength (t/m2)
Saturated
density
(t/m2)
Consistency
0
0
--
Very soft
2
2.5
1.6 – 1.92
Soft
4
5
8
10
1.76 – 2.08
Stiff
16
20
1.92 – 2.24
Very stiff
32
40
Medium
Hard
105
CONE PENETRATION TEST
• The CPT is performed with a cylindrical penetrometer with a conical
tip (cone) penetrating into the ground at a constant rate of
penetration.
• During the penetration, the forces on the cone and the friction sleeve
are measured.
• The measurements are carried out using electronic transfer and data
logging, with a measurement frequency that can secure detailed
information about the soil conditions.
• A CONE WITH APEX ANGLE 600 AND BASE AREA 10 cm2.
107
Cone penetrometer
108
CONE PENETRATION TEST
• The CPT is performed with a cylindrical penetrometer with a conical
tip (cone) penetrating into the ground at a constant rate of
penetration. During the penetration, the forces on the cone and the
friction sleeve are measured. The measurements are carried out using
electronic transfer and data logging, with a measurement frequency
that can secure detailed information about the soil conditions.
109
• The results from a cone penetration test can in principle be used to
evaluate:
• stratification
• soil type
• soil density and in situ stress conditions
• shear strength parameters
• The results from cone penetration tests may also be used, directly, for
design of piled foundations in sand and gravel. Indirectly it can be
used (shear strength) for piles in clay.
110
• The results from a cone penetration test can in principle be used to
evaluate:
• stratification
• soil type
• soil density and in situ stress conditions
• shear strength parameters
• The results from cone penetration tests may also be used, directly, for
design of piled foundations in sand and gravel. Indirectly it can be
used (shear strength) for piles in clay.
111
Cone penetrometer
113
DCPT
• Cone having apex angle 600
• Dia of cone 50 mm or 65 mm
• Hammer weight 65 kg
• Free falling height 750 mm
• Blow count for every 100 mm penetration of the cone
• Continuous recording of blow count
• No. of blows required for 300 penetration is noted as dynamic cone
resistance (Ncd)
• No sample is collected
• The correlation can be used to to obtain N values from Ncd values.
DCPT
From 50 mm diameter cone
• Ncd=1.5 N upto 3 m depth
• Ncd=1.75 N for depths from 3 to 6 m
• Ncd=2.0 N for depths greater than 6 m
Vane Shear Test
Field Vane Shear Test
Assignment questions
• 1. Discuss about the importance of sub soil exploration?
• 2. List and explain the methods of explorations
• 3. Explain the advantages and disadvantages of non distractive explorations
• 4. Define the following-undisturbed sample, disturbed sample and
• representative sample
• 5. Explain split spoon sampler.
• 6. What are the parameters to be considered in soil exploration report?
• 7. On what parameters the depth and number of boring are fixed?
• 8. List and explain different types of sampler used in soil sampling.
• 9. Explain different types of boring.
• 10. Difference between hand and mechanical augers.
Quiz questions
1.The methods of site investigation are dependent upon
a) Climatic condition
b) Nature of engineering project
c) Local topography
d) All of the mentioned
2.The general exploration gives information about which of the following features?
a) Depth of rock
b) Composition of soil strata
c) Ground water level
d) All of the mentioned
3.What are the methods used for general exploration?
a) Subsurface penetration
b) Ground water exploration
c) Rock Cuttings
d) All of the mentioned
4.Exploratory borings in general exploration is carried out by using
a) Auger
b) Bore equipment
c) Well curb
d) All of the mentioned
5. Hand auger can be used for depths up to ________
a) 7 m
b) 6 m
c) 2 m
d) 10 m
6. Auger boring is used in __________ type of soil.
a) Cohesion less soil
b) Cohesive soil
c) Coarse-grained soil
d) Pervious soil
7. Rotary boring can also be called as ___________
a) Percussion boring
b) Wash boring
c) Core boring
d) Pit boring
8. Wash boring cannot be used for _________ type of soil strata.
a) Cohesive soil
b) Cohesion less soil
c) Boulder
d) All of the mentioned
9. The type of boring method that can be used for both rock and soils are
a) Shell boring
b) Wash boring
c) Auger boring
d) Rotary boring
10. The most commonly used sampler for obtaining disturbed sample of soil
a) Split spoon sampler
c) Piston sampler
b) Open drive sampler
d) Thin wall shell by tube sampler
IS Codes
• For Depth of exploration – IS: 1892-1979
• For SPT – IS: 2131 -1981
THANK YOU
Samplers
P. Bala Ramudu
Sampler
• The basic tool used for the collection of
undisturbed soil samples from test pits or
boreholes at required depth is known as a soil
sampler.
• The fundamental requirement of a sampling
tool is that when it is forced into the ground, it
should cause as little displacement,
remolding, and disturbance to the soil as
possible.
Type of soil samples
• Disturbed Samples
• Representative Samples
• Non-representative samples
Undisturbed samples
Type of samples
Disturbed Sample
Disturbed Sample
• Disturbed sampling refers to methods of
retrieving samples that incidentally cause the
material to be remolded or at least partially
altered.
• It should be understood that disturbed
samples generally are not suitable for
specialized tests requiring undisturbed soil
specimens.
Undisturbed Sample
Undisturbed Sample
The disturbed vs undisturbed sample
• Disturbed
– Change in stress condition
– Change in water content and void ratio
– Chemical change
– Mixing and segregation of soils
• Undisturbed samples
– No change in soil structure
– No Change in water content and void ratio
Types of samplers
• Disturbed
– Continuous augur
– Bulk sampler
– Split barrel sampler
• Undisturbed samples
– Thin wall Shelby sampler
– Piston sampler
The design features of the sampling
tool control the degree of disturbance
• 1. Cutting edge.
• 2. Inside-wall friction.
• 3. Non-return valve.
• 4. Recovery ratio.
1. Cutting Edge
• Cutting edge is the beveled and sharp edge at
the bottom of the soil sampler. It may be an
integral part of the soil sampler or a separate
cutting bit may be screwed to the bottom of
the sampler. It mainly facilitates driving of the
soil sampler through the soil.ratio.
Design features
• i. Inside Clearance
• ii. Outside Clearance
• iii. Area Ratio
2. Inside-Wall Friction
• The inside-wall friction should be minimized
for minimizing the disturbance of the sample
by the following actions:
• i. Provide suitable inside clearance.
• ii. Provide a smooth finish to the sampling
tube.
• iii. Oil the inside surface of the sampling tube
properly.
3. Non-Return Valve
• The valve should have a large orifice to allow
air and water to escape quickly and easily
when driving the sampler.
4. Recovery Ratio
• It is defined as the ratio of length of the sample within
the sampling tube to the depth of penetration of the
sampler during sampling. Thus, recovery ratio (Rr) is
expressed by –
• Rr = L/H
• where L is the length of the sample within the tube and
H is the depth of penetration of the sampling tube.
• For satisfactory undisturbed sampling, when excess soil
is prevented from entering the tube, the recovery ratio
should be between 96% and 98%.
Types of Soil Samplers
• On the basis of area ratio, soil samplers are
classified into the following types:
– i. Thin-wall samplers.
– ii. Thick-wall samplers.
– Thin-wall samplers are the samplers in which the wall
thickness of the sampling tube is less than 2.5% of the
diameter. In other words, thin-wall samplers are those
for which the area ratio is less than or equal to 10%.
Samplers for which the area ratio is more than 10%
are known as thick-wall samplers.
Types of Soil Samplers
• Later studies have modified the definition of
thin-wall samplers to include the effect of
cutting edge on sample disturbance.
• Accordingly, thin-wall samplers may be
defined as those with an area ratio less than
20% when the sampler has a suitably designed
cutting edge. Thick-wall samplers are those
with an area ratio more than 20%.
Based on the sampler design and use,
soil samplers are classified into the
following types
• 1. Open-tube sampler.
• 2. Standard split-spoon sampler.
• 3. Stationary piston sampler.
• 4. Rotary sampler.
• 5. Scraper bucket sampler.
1. Open-Tube Sampler
• The open-tube samplers are the simplest type of
samplers for collection of undisturbed samples.
• They are thin-wall tube samplers made of
seamless steel and are also known as thin-wall
Shelby tube samplers.
• The bottom of the tube is sharpened and
beveled, which acts as a cutting edge. The upper
portion of the sampler has threads on the inside
surface to enable the sampler to be attached to
the bottom of the drill rod.
Method of Sampling
• i. The sampler is attached to the bottom of the drill rod and is lowered to
the bottom of the bore, where the undisturbed sample is to be collected.
• ii. It is then driven into the soil, with fast and smooth strokes of careful
hammering. Erratic pressure, if applied during hammering, becomes a
source of sample disturbance.
• iii. After driving the sampler up to the required length (equal to the
sampler length minus provision for waxing), the sampler is rotated twice
completely to shear off the sample from its intact bottom and is then
withdrawn.
• iv. The tube is removed from the sampler head and its ends are sealed
with molten wax before transportation.
• v. The sampler head is provided with vents at top to permit water to
escape when sampling under water.
• vi. It is also provided with a check valve to help retain the sample when
withdrawing the sampler.
Thin-Wald Sampler
• Thin-walled models can be used to obtain
undisturbed specimens in soft clay and plastic
silt (IS: 11594, 1985).
• However, No separate cutting shoe is attached to
the lower end, but the bottom of the specimen
itself is machined to act as a cutting edge (Fig).
• High-quality undisturbed samples are possible if
the soil is not disturbed during Ar <10% and
boring operation. This model can be used more
conveniently in experimental pits and shallow
boreholes.
Thin-Wald Sampler
• High-quality undisturbed samples are possible
if the soil is not disturbed during Ar <10% and
boring operation. This model can be used
more conveniently in experimental pits and
shallow boreholes.
Suitability
• Shelby tube samplers are commonly used for
collection of samples in fine-grained soft soils.
The presence of obstructions to the sampler
during driving, in the form of gravel or stones,
cramps the beveled bottom edge and causes
disturbance to the sample collected. The
Shelby tube samplers are, therefore,
unsuitable for hard or dense gravelly soils.
Thin-Wald Sampler
• Thin-walled models can be used to obtain
undisturbed specimens in soft clay and plastic
silt (IS: 11594, 1985).
• However, No separate cutting shoe is attached
to the lower end, but the bottom of the
specimen itself is machined to act as a cutting
edge (Fig.).
Shelby tube and Thin Wall Samplers
Shelby tube Samplers
Thin Wall Shelby Sampler
• To obtain relatively undisturbed samples of
cohesive soils for strength and
consolidation testing.
• Commonly, it has a 76 mm (3.071in) outside
diameter & a 73 mm (2.875 in) insidediameter,
• Resulting in an area ratio of 9 percent
• Vary in outside diameter between 51 mm (2.0
in) and 76 mm (3.0 in)
• typically come in lengths from 700 mm (27.56
in) to 900 mm (35.43 in),
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Larger diameter sampler tubes used when higher quality samples are required andsampling
disturbance must be reduced.
• The thin-walled tubes are manufactured using carbon steel, galvanized-coatedcarbon steel,
stainless steel,and brass.
• Carbon steel tubes
• the lowest cost tubes but are
• unsuitable if the samples are to be stored in the tubes for more than a fewdays or if the inside of
the tubes become rusty,
• significantly increasing the friction between the tube and the soil sample.
• Galvanized steel tubes
• preferred in stiff soils
• carbon steel is stronger,
• less expensive
• galvanizing provides additional resistance to corrosion.
• Stainless Steel tubes
• preferred for offshore bridge borings,
• salt-water conditions, or
• long storage times
Both ends of the tube should then be sealed with at least a 25 mm (1 in) thick layer
ofmicrocrystalline (non-shrinking) wax after placing a plastic disk to protect the ends ofthe sample.
igure (e) Shelby Tube Sealing Methods
(a) Microcrystalline wax (b) O-ring
pack
Open-Drive Sampler
• It consists of a steel tube with a screw thread at each
end. The lower end is usually fitted with a cutting shoe
but sometimes with an extension piece.
• The top end is fitted with a sample head, which
includes a non-reversible valve.
• Non-return The valve allows air and water to escape
when the specimen enters and closes the specimen to
the surface, thus retaining the specimen inside the
tube (Figure)
• This is the simplest and most common type of model.
Split-Spoon Sampler
• This includes providing a longitudinally split
tube or barrel with a shoe and sampler head
for air release (IS: 9640, 1980).
• Therefore, The splitting element of the
specimen allows the sample to be opened for
testing and for further dispersion in sample
containers (Fig. 18.11).
Standard Split-Spoon or Split-Barrel
Sampler
• It is the most commonly used sampler for
obtaining undisturbed soil samples.
• It is also known as split-barrel and split-tube
sampler.
• A split-spoon sampler is also used to conduct SPT
in the borehole.
• When the SPT is conducted, the soil sample
simultaneously enters the sampler by the end of
the test, which is then withdrawn and taken to
the laboratory.
Split Spoon Sampler
Features of the Sampler
• i. A driving shoe made of 7.5-cm-long tool steel at the
bottom.
• ii. A 45-cm-long steel tube, split into two halves
longitudinally, as shown in Fig. 14.12(b).
• iii. A coupling or sampler head at the top of the tube, about
15 cm long.
• The inside diameter of the steel tube is 3.8 cm (1.5 in.) and
the outside diameter is 5.08 cm (2 in.). The coupling head is
provided with a check valve and four venting ports of 1-cm
diameter to improve sample recovery. In some split-spoon
samplers, a separate sampling tube of 3.8-cm internal
diameter is provided.
Split Spoon Sampler
Split Spoon Sampler
The procedure for the collection of
soil samples
•
•
•
•
•
•
•
•
i. After the borehole has been made, the sampler is attached to the bottom of the
drill rod and lowered to the bottom of the hole.
ii. The sampler is forced into the soil by repeated blows of a drop hammer.
iii. The sampler is driven to the required length, taking care not to overstress the
sample, and is then withdrawn.
iv. The split tube is separated after removing the shoe and the coupling, and the
sampling tube with sample is taken out.
v. The sampling tube containing sample is then placed in a container, sealed, and
transported to the laboratory.
vi. In the samplers with separate a 3.8-cm-diameter tube inside the split barrel, the
tube is taken out, waxed on both ends, and transported to the laboratory.
vii. If the soil encountered in the borehole is fine sand below the water table, the
sample recovery becomes difficult. For such soils, a spring core catcher device is
used to aid recovery. As the sampler is lifted, the springs close and form a dome to
retain the sample.
viii. While taking samples, the water level in the borehole should be always
maintained higher than the GWT to prevent quick conditions.
• Samples obtained using this model are rated
as representative. This model is suitable for
sands and is only used in the standard
penetration test (SPT).
• Moreover, Split-quick models can be provided
with a liner, thin metal, or plastic tube
embedded in a split spoon.
Split barrel sampler
•
•
•
•
•
•
•
•
•
•
Split Barrel Sampler
• Used to obtain disturbed samples in all types of soils.
• Typically used in conjunction with the Standard Penetration Test(SPT),
• The sampler is driven with a 63.5-kg (140-lb) hammer dropping
from a height of 760mm (30 in). (AASHTO T206 and ASTM D1586),
• Available in standard lengths of
– 457 mm (18 in) and 610 mm (24in)
– Inside diameters ranging from 38.1 mm (1.5 in) to 114.3 mm (4.5 in) in
12.7mm (0.5 in) increments.
– The 38.1 mm (1.5 in) inside diameter sampler is popular
because correlations
• High area ratio disturbs the natural characteristics of the soil being
sampled, thus disturbed samples are obtained.
• This corresponds to a relatively thick-walled sampler with an area rati
Split Barrel Sampler
• when the shoe and the sleeve of this type of sampler
are unscrewed from the splitbarrel, the two halves of the barrel
may be separated and the sample may be extracted easily.
• The soil sample is removed from the split-barrel sampler it is either
placed and sealed in a glass jar, sealed in a plastic bag, or sealed in a
brass liner.
• Separate
containers
should
be
used
if
the sample contains different soil types.
• Alternatively, liners may be placed inside the sampler with the
same inside diameter as the cutting shoe.
• This allows samples to remain intact during transport to
the laboratory.
• In both cases, samples obtained with split barrels are disturbed
and therefore are only suitable for soil identification and general
classification tests
Figure (a) Split-Barrel Samplers: Lengths of 457 mm (18 in) and 610 mm (24 in)
Figure (b) Inside diameters from 38.1 mm (1.5 in) to 89 mm (3.5 in
Piston Sampler
• However, The purpose of the liner is to protect the specimen during
handling, shipping, and storage. Piston Sampler. For very soft alloys
and clays, piston models are quite useful.
• Moreover, These include a thin-walled tube, which includes a piston
device, which helps push the thin-walled tube from the bottom of
the boring to unstoppable soil (IS: 10108, 1982).
• However, The piston is locked in the bottom position and the
sample is lowered to the bottom of the borehole. The piston is
provided with a seal that prevents water and debris from entering.
• When it is unlocked, the tube is driven into the soil for the entire
journey of the piston.
• Since, After locking the piston on the top of the tube, the entire
assembly is lifted back to the surface (Fig.).
• The specimen is separated from the head and piston. However, It is
then sealed at both ends.
Stationary Piston Sampler
• A piston sampler consists of two parts – (a) a
sampler cylinder and (b) a piston system. The
piston rod is 30 cm (12 in.) in diameter at the
bottom end and fits easily inside the hollow
drill rod.
Piston Sampler
Piston Sampler
Method of Sampling
• Figure illustrates the operation of stationary
position sampler. For obtaining a sample, the
bottom of the piston is brought flush with the
cutting edge of the sampler and the sampler is
lowered into the borehole. When the sampler
reaches the bottom of the hole, the piston rod is
held fixed relative to the ground surface and the
thin-wall tube is pushed into the soil by hydraulic
pressure or mechanical jacking. The sampler is
never driven. The sampling tube is pushed to the
required depth and then the drill rod along with
the whole sampler is withdrawn.
Suitability
• The stationary piston sampler is used for
sampling soft-to-stiff cohesive soils. The
quality of samples obtained is excellent and
the probability of obtaining a satisfactory
undisturbed sample is high. The use of piston
enables a high recovery ratio for the soil
sample, but at the same time, prevents
recovery ratios higher than 100%, preventing
entry of excess soil into the sampler.
Scraper Bucket Sampler
• Scraper Bucket Samplers are used to collect
the samples in case of sand mixed with
pebbles
Scraper Bucket Sampler
Scraper bucket sampler
• Sampling with a standard split-spoon sampler becomes difficult if
the soil contains pebbles. Even if the sampler is fitted with a spring
core catcher, the pebbles interfere with the springs and obstruct
their closure. A scraper bucket sampler may be helpful for the
collection of undisturbed samples in such soil deposits. However, it
is possible to collect only disturbed samples using a scraper bucket
sampler. The sampler is also useful to obtain samples of
cohesionless soil below GWT.
• The scraper bucket sampler contains a vertical slit at its upper
portion and a driving point at its bottom. As the sampler is rotated,
the scrapings of the soil enter into the sampler cylinder through the
vertical slit. When the sampler is filled with the scrapings, it is lifted
up and collected into a separate container. Although the sample is
thoroughly disturbed, it still represents the soil at exact depth from
where it is collected.
Rotary Sampler
• The rotary samplers are double-tube samplers with a
removable thin-wall tube, known as liner, inside an outer
tube provided with a cutting bit. The rotary sampler has an
outside diameter of 6.35-19.7 cm (3.5-7.75 in.) and a length
of 61 cm (24 in. or 2 ft). The outer tube with the cutting bit
is rotated and pushed down into the soil for the required
length and the sample enters the liner. The inner tube, that
is, the liner, provided with a smooth cutting shoe, remains
stationary and the sample cut by the rotating outer tube
slides into the liner. The sample is thus received in the liner.
• Rotary samples can be used for collection of undisturbed
samples in stiff-to-hard clays, silts, and sands with some
cementation and also in soft rock. The sampler is, however,
unsuitable for gravelly soils and loose cohesionless soils.
THANK YOU
Department of Civil Engineering
IIT (BHU)
Chapter 2:
Bearing Capacity of Shallow
Foundations
Bala Ramudu, P.
Associate Professor
Shallow Foundations
Bearing Capacity
• The problems of soil mechanics can
be divided into two principal groups stability problems and elasticity
problems
- Karl Terzaghi, 1943
CE 321: Geotechnical Engineering - II
□
□
The shallow foundation shown in Figure 1. has a width B and a length L. The
depth of embedment below the ground surface is equal to Df. Theoretically, when
B/L is equal to zero (i.e., L = ∞), a plane strain case will exist in the soil mass supporting the
foundation.
For
most
practical
cases,
when
B/L
≤
1/5
to
1/6,
the
plane
strain theories will yield fairly good results. Terzaghi1 defind a shallow foundation
as one in which the depth Df is less than or equal to the width B (Df/B ≤1).
□
CE 321: Geotechnical Engineering - II
(a) Total Overburden
Pressure q0
□ qo is the intensity of total overburden
pressure due to the weight of both
soil and water at the base
level of the foundation.
CE 321: Geotechnical Engineering - II
Total and effective overburden
pressures
CE 321: Geotechnical Engineering - II
(b) Effective Overburden
Pressure q'0
CE 321: Geotechnical Engineering - II
(c) The Ultimate Bearing
Capacity of Soil, qu
□ qu is the maximum bearing capacity
of soil at which the soil fails by shear.
CE 321: Geotechnical Engineering - II
(d) The Net Ultimate
Bearing Capacity, qnu
CE 321: Geotechnical Engineering - II
(e) Gross Allowable Bearing
Pressure,qa
CE 321: Geotechnical Engineering - II
(f) Net Allowable Bearing
Pressure,qna
CE 321: Geotechnical Engineering - II
(g) Safe Bearing Pressure,
qs
□ qs is defined as the net safe bearing
pressure which produces a settlement
of the foundation which
does not exceed a permissible limit.
CE 321: Geotechnical Engineering - II
SUMMARY of Terminology
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
Common Types of Footing
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
Location and depth of
Foundation
▪
The following considerations are necessary for deciding the location and depth of
foundation
As per IS:1904-1986, minimum depth of foundation shall be 0.50 m.
Foundation shall be placed below the zone of
The frost heave
Excessive volume change due to moisture variation (usually exists
within 1.5 to 3.5 m depth of soil from the top surface)
Topsoil or organic material
Peat and Muck
Unconsolidated material such as waste dump
Foundations adjacent to flowing water (flood water, rivers, etc.) shall be
protected against scouring. The following steps to be taken for design
in such conditions
Determine foundation type
Estimate probable depth of scour, effects, etc.
Estimate cost of foundation for normal and various scour conditions
Determine the scour versus risk, and revise the design accordingly
CE 321: Geotechnical Engineering - II
Location and depth of
foundation
CE 321: Geotechnical Engineering - II
Location and depth of
foundation
CE 321: Geotechnical Engineering - II
Location and depth of
foundation
CE 321: Geotechnical Engineering - II
Bearing Capacity Failure
CE 321: Geotechnical Engineering - II
Transcosna Grain Elevator
Canada (Oct. 18, 1913)
CE 321: Geotechnical Engineering - II
West side of foundation sank 24-ft
Stability Problem
Bearing Capacity Failure
• Chapter 6. Bearing Capacity Analysis
• How do we estimate the maximum
bearing pressure that the soil can
withstand before failure occurs?
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
Bearing Capacity Failures
Types/Modes of Failure
□ general shear failure
□ local shear failure
□ punching shear failure
CE 321: Geotechnical Engineering - II
General Shear Failure
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
Local Shear Failure
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
Punching Shear Failure
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
Model Tests by Vesic (1973)
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
General Guidelines
□ Footings in clays - general shear
□ Footings in Dense sands ( Dr > 67%)
-general shear
□ Footings in Loose to Medium dense
(30%< Dr < 67%) - Local Shear
□ Footings in Very Loose Sand (Dr < 30%)punching shear
CE 321: Geotechnical Engineering - II
Bearing Capacity Formulas
CE 321: Geotechnical Engineering - II
Failure zones
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
Terzaghi Bearing Capacity
CE 321: Geotechnical Engineering - II
Terzaghi Bearing Capacity
Formulas
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
Terzaghi Bearing Capacity
CE 321: Geotechnical Engineering - II
Terzaghi Bearing Capacity
CE 321: Geotechnical Engineering - II
RelationShip foR PPq (ϕ ≠ 0,
γ = 0, q ≠ 0, c = 0)
CE 321: Geotechnical Engineering - II
RelationShip foR P
Pc (ϕ ≠ 0, γ = 0, q = 0, c ≠ 0)
CE 321: Geotechnical Engineering - II
RelationShip foR P
Pγ (ϕ ≠ 0, γ ≠ 0, q = 0, c = 0)
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II
Terzaghi Bearing Capacity Formulas
For Continuous foundations:
For Square foundations:
For Circular foundations:
CE 321: Geotechnical Engineering - II
Terzaghi Bearing Capacity Factors
CE 321: Geotechnical Engineering - II
Bearing Capacity Factors
CE 321: Geotechnical Engineering - II
Terzaghi Bearing Capacity Formulas
□ D≤B
□ No sliding between footing and soil
□ soil: a homogeneous semi-infinite
mass
□ general shear failure
□ footing is very rigid compared to soil
CE 321: Geotechnical Engineering - II
Further Developments
□
□
□
□
□
□
□
Skempton (1951)
Meyerhof (1953)
Brinch Hanson (1961)
De Beer and Ladanyi (1961)
Meyerhof (1963)
Brinch Hanson (1970) See Extra Handout
Vesic′ (1973, 1975)
CE 321: Geotechnical Engineering - II
Vesic′ (1973, 1975) Formulas
Shape factors….…
Eq. 6.14, 6.15 and 6.16
Depth Factors …….
Eq. 6.17, 6.18 and 6.19
Load Inclination Factors …. Eq. 6.20, 6.21 and 6.22
Base Inclinations factors ..
Eq. 6.25 and 6.26
Ground Inclination Factors….Eq. 6.27 and 6.28
Bearing Capacity Factors …. Eq. 6.29, 6.30 and 6.31
CE 321: Geotechnical Engineering - II
Vesic′ Formula Shape Factors
CE 321: Geotechnical Engineering - II
Vesic′ Formula Depth Factors
CE 321: Geotechnical Engineering - II
Bearing Capacity of
Shallow Foundations
□
□
□
□
□
□
6.3 Groundwater Effects
6.4 Allowable Bearing Capacity
6.5 Selection of Soil Strength Parameters
6.6 Local & Punching Shear Cases
6.7 Bearing Capacity on Layered Soils
6.8 Accuracy of Bearing Capacity
Analyses
□ 6.9 Bearing Capacity Spreadsheet
CE 321: Geotechnical Engineering - II
Groundwater Table Effect
CE 321: Geotechnical Engineering - II
Groundwater Table Effect;
Case I
1. Modify σ′zD
2. Calculate γ′ as follows:
CE 321: Geotechnical Engineering - II
Groundwater Table Effect;
Case II
1. No change in σ′zD
2. Calculate γ′ as follows:
CE 321: Geotechnical Engineering - II
Groundwater Table Effect;
Case III
1. No change in σ′zD
2. No change in γ′
CE 321: Geotechnical Engineering - II
Groundwater Table Effect
CE 321: Geotechnical Engineering - II
Allowable Bearing Capacity
■
■
….. Allowable Bearing Capacity
F …. Factor of safety
CE 321: Geotechnical Engineering - II
Factor of Safety
Depends on:
▪ Type of soil
▪ Level of Uncertainty in Soil Strength
▪ Importance of structure and
consequences of failure
▪ Likelihood of design load occurrence
CE 321: Geotechnical Engineering - II
Minimum Factor of Safety
CE 321: Geotechnical Engineering - II
Selection of Soil Strength
Parameters
▪ Use Saturated Strength Parameters
▪ Use Undrained Strength in clays (Su)
▪ Use Drained Strength in sands,
▪ Intermediate soils that where partially
drained conditions exist, engineers
have varying opinions; Undrained
Strength can be used but it will be
conservative!
CE 321: Geotechnical Engineering - II
Accuracy of Bearing Capacity
Analysis
▪ In Clays …..Within 10% of true value
(Bishop and Bjerrum, 1960)
▪ Smaller footings in Sands…. Bearing
capacity calculated were too conservative –
but conservatism did not affect construction
cost much
▪ Large footings in Sands … Bearing capacity
estimates were reasonable but design was
controlled by settlement
CE 321: Geotechnical Engineering - II
Skempton’s Bearing
Capacity Analysis for
cohesive Soils
CE 321: Geotechnical Engineering - II
Example 1
□ A square foundation is 1.5 m × 1.5 m
in plan. The soil supporting the
foundation has a friction angle of ϕ =
20° and c = 15.2 kN/m2. The unit
weight of soil γ is17.8 kN/m3.
Determine the ultimate gross load the
foundation can carry. Assume the
depth of the foundation (Df) to be
one meter, and general shear failure
occurs in soil.
CE 321: Geotechnical Engineering - II
CE 321: Geotechnical Engineering - II