2. 2
Definition
The process of determining the layers of
natural soil deposits that will underlie a
proposed structure and their physical
properties is generally referred to as
subsurface exploration/Sub Soil
Exploration/Investigation
3. 3
The purpose of the soil – exploration
progrm
To obtain information that will help the
geotechnical engineer in the following:
1.Selection of the type and the depth of
foundation suitable for a given structure
2.Evaluation of the load bearing capacity of the
foundation
3.Estimation of the probable settlement of a
structure
4. 4
4. Determination of potential foundation
problems (expansive soil, collapsible soil,
sanitary landfill, and so on)
5. Establishment of ground water table
6. Prediction of lateral earth pressure for
structures like retaining walls, sheet pile,
bulk heads, and braced cuts
7. Establishment of construction methods for
changing subsoil conditions
5. 5
Scope of Soil Investigation
The scope of a soil investigation depends on the
• the type, size, and importance of the structure
• the client and the engineer’s familiarity with
the soils at the site, and
• local building codes.
6. 6
Phase I. Collection of available information
phaseII. Reconnaissance survey of a
proposed site.
Phase III. Preliminary soil exploration.
Phase IV. Detailed soil exploration
Phase V. Writing a report
Phases of a Soil Investigation
A site investigation must be developed in phases.
7. 7
Phase I: Collection of available information such as
site plan,
type, size, and importance of the structure,
loading conditions,
previous geotechnical reports,
topographic maps,
airphotogrphs,
geological maps,
hydrological information
Existing Infrastructure and
Highway department manuals
11. 11
Existing Data Sources
Various online sources like Google Earth
Geological survey of Pakistan
National/Local Geological Survey maps, reports,
and publications.
Flood zone maps prepared by National or local
Departments (Department of Irrigation, National
Disaster Management Authority of Pakistan)
12. 12
Site Plans showing locations of ditches,
driveways, culverts, utilities, and pipelines.
Maps of streams, rivers and other water bodies
to be crossed by bridges, culverts, etc.,
Earthquake data, seismic hazards maps, fault
maps, and related information
13. 13
Phase II : Preliminary reconnaissance or a site
visit to provide a general picture of the
geotechnical topography and geology of the
site.
Information collected during this phase:
Design and construction plans
General site conditions
Access restrictions for equipment
14. 14
Traffic control requirements during field
investigations
Location of underground and overhead
utilities
Type and condition of existing facilities (i.e.
pavements, bridges, etc.)
Adjacent land use (schools, churches,
research facilities, etc.)
Restrictions on working hours
Right-of-way constraints
15. 15
Environmental issues
Erosion features, and surface settlement
Flood levels
Water traffic and access to water boring sites
Benchmarks and other reference points to aid
in the location of boreholes
Equipment storage areas/security
16. 16
Phase III: Detailed soil exploration.
The objectives of a detailed soil exploration are:
1.To determine the geological structure, which
should include the thickness, sequence, and
extent of the soil strata.
2.To determine the ground water conditions.
3.To obtain disturbed and undisturbed samples
for laboratory tests.
4.To conduct in situ tests.
17. 17
• Phase IV : Writing a Report
The report must contain a clear description of
the soils at the site, methods of exploration, soil
profile, test methods and results, and the
location of the ground water.
18. 18
• Phase IV Preliminary site investigation
In addition to desk study and
reconnaissance, a few confirmatory
bore holes are sunk or probing is
done.
19. 19
Site Exploration Plan
Once the site has been selected, a detailed
investigation has to be conducted.
Since the cost of such an investigation is
enormous, it is important to prepare a
definitive plan for the investigation, especially
in terms of the
bore-hole layout and spacing between bores
the depth of boring at each location.
20. 20
Depth of Bore holes or Test pit
Approximate required minimum depth of bore hole shall be
predetermined
Depth can be changed during drilling depending upon
subsurface conditions
There is no hard and fast rule….
21. 21
Approximate method to determine
depth of boring holes or test pit
Net Increase in stress
due to structure
Vertical effective stress
23. 23
(a)Min Depth of boring is at which the
net increase of stress = 1/10 estimated net stress (q)
on the foundation
(b) Min Depth of boring is at which the
05.0'
0
=
∆
σ
σ
Approximate Min Depth shall be minimum of (a) and (b)
26. 26
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.
27. 27
Boring depth is governed by various factors,
including:
– Wall type (Fill vs. Cut)
– Lowering of grade line at wall?
– Scour?
• Rules of Thumb:
– Fill Walls: Depth = Wall Height +/-
– Soil Nailed Walls: Depth = Through Nailed Area,
plus 10’
– Drilled Shaft Walls: Depth = Exposed Wall Height plus
150% of Wall Height
How Deep (Retaining Walls)?
28. 28
A BB
L P B
D =1 ½ B, when A P4B
L
(L>B)
B B BA A
D =1 1/2 L, when A <2B
(A)
ISOLATED SPREAD OR MAT FOOTINGS
(B)
ADJACENT FOOTINGS
29. 29
W
AB B BA
BBA
L=W
D = 4 ½ B, when A P 2B
= 3B, when A > 2B
= 1 ½ B, when A > 4B
(c)
ADJACENT ROWS OF FOOTINGS
30. 30
B
H
D = 1 ½ B or 1 ½ H
which ever is greater
(D)
RETAINING WALLS
D = 25′ to 100′,
confirm
competent
strata
Pile
(E)
PILES
31. 31
Water Surface
B
H
D = 10′ minimum *
= B when B ≤ H
= H when B > H
d
(A) DEEP CUT AND FILL SECTIONS ON SIDE HILLS
32. 32
D = 10′ minimum *
(B)
NORMAL CANAL SECTIONS
34. 34
Points to note
The above approximate methods are not useful for
bedrocks
If the foundation load be transferred to bedrock
Min depth of boring in bed rock is 3 m
For irregular or weathered rock , it may be deeper
Further…when deep excavations are required, the min
depth of broing be 1.5 times the depth of excavations,
37. 37
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.”
But guidelines exist in –
• Textbooks
• Design manuals
38. 38
Spacing of Bore holes or Test pit
The spacing can be decreased or increased
depending on sub soil conditions
If subsoil is uniform and predictable less
number of bores may be needed
If subsoil is non uniform and un predictable
more number of bores are needed
40. 40
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?
41. 41
How Many Borings?
Source: Sowers 1979
Structure or
Project
Subsurface
Variability
Spacing of Borings (ft)
Highway
Subgrade
Irregular 100-1000 (200, typical)
Average 200-2000 (500, typical)
Uniform 400-4000 (1000, typical)
Multistory
Building
Irregular 25-75
Average 50-150
Uniform 100-300
44. 44
A soil exploration program usually involves test
pits and /or soil borings.
A detailed soil exploration consists of:
1.Preliminary location of each bore hole and /or
test pits.
2.Numbering of the bore holes or test pits.
3.Planned depth of each bore hole or test pit.
4.Methods and procedures for advancing the bore
holes.
5.Number of samplings and their frequency.
6.Requirements for ground water observations.
46. 46
Methods of Subsurface
Exploration
The following are the methods of subsurface
exploration to determine the stratification and
engineering characteristics of sub-surface soils.
Trial pits or trenches,
Hand Auger Borings (post hole, helical or
spiral , dutch auger, gravel auger; barrel
auger)
Mechanical Auger Borings
48. 48
Trial Pits and Trenches
Size - 1.5m * 1.5m
Depth – normal 3.0m,
Backfilled with proper compaction.
Cheapest method for shallow depth
Can be excavated either by labors or
mechanical excavator.
Any weak lenses or pockets can be
seen.
Expensive when depth is above 6m
or below water table specially when
the subsoil consists of sandy soil
49. 49
the excavated material should be placed
on surface
separate stacks to for the materials
obtained from different depths
Various measurements should be
recorded such as…
orientation,
depth of the pit,
depths and the thickness of each stratum
51. 51
Screw Auger( spiral auger)
Used in very cohesive, soft or
hard soils.
cannot be used in very dry or
sandy soils since these soil
types will not adhere to the bit.
Good for boring holes quickly,
but more difficult to remove
from the hole.
52. 52
Bucket Auger
They are made of a cylinder or
barrel to hold the soil, which is
forced into the barrel by the cutting
lips.
Bucket augers work well in most soil
conditions. Therefore, it is
considered the most universal
auger.
These augers are available with
different tips designed for specific
soil types, such as mud and sand.
53. 53
1. The sand auger tips are formed to touch in order
to hold the very dry and sandy soils.
2. Tips of the mud auger are spaced further apart
than the regular soil bucket to allow for easier
removal of heavy, wet soil and clay.
These also have an opening in the cylinder for
removal of the cohesive soils.
Sand Auger Mud Auger
54. 54
Dutch Auger: (Edelman auger)
designed for wet, clay, high fibrous, heavily
rooted swampy areas, and extremely wet
boggy soil.
A sand version is available where the blades
are much wider and closer together to help
capture the loose material, but a bucket
auger can retain this material with greater
ease
Bucket augers bore more slowly than the screw
and Dutch but are easier to remove from the hole
and can provide a semi undisturbed sample.
55. 55
Planer Auger
Similar to the bucket auger with its
cylinder shape,
but designed to flatten and clean out
the bottom of the predrilled hole in
preparation for core sampler to obtain
a quality undisturbed sample.
60. 60
Continuous augers can be classed as:
(i)solid stem continuous-flight augers
or
(ii)hollow stem continuous-flight augers
May be operated by hand or power
Cheap method to determine the type
of soil.
62. 62
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
63. 63
This method may be used in all types of soil
including sandy soils below the water table
but is not suitable if the soil is mixed with
gravel, cobbles etc.
A hollow stem is sometimes preferred since
standard penetration tests or sampling may be
done through the stem without lifting the auger
from its position in the hole.
64. 64
Besides, the flight of augers serves the purpose
of casing the hole.
In case of hollow stem, the hollow stem can be
plugged while advancing the bore and the plug
can be removed while taking samples or
conducting standard penetration tests (to be
described later on).
Due to plugging, hollow stem functions like solid
stem
65. 65
The drilling rig can be mounted on a truck or a
tractor.
Holes may be drilled by this method rapidly to
depths of 60 m or more.
66. 66
Advantages and Disadvantages of
CFA
soil moving up from the base of the hole is
free to mix with the soil at higher levels on the
edge of the borehole
Hollow-stem auger drilling is often fast and
reliable method of boring.
67. 67
Advantages and Disadvantages of
CFA
in coarse gravels the continuous-flight auger is
unusable because it must be removed each
time a sample or in-situ test is to be carried
out. At this stage the hole will collapse.
68. 68
Cleaning of Auger Flights
• In case of boring in very cohesive soils, like
clay, the cleaning of auger flights is
cumbersome.
Advantages and Disadvantages of
CFA
71. 71
Hollow Stem Continuous
Flight Auger Drilling Systems:
(a) Comparison with solid
stem auger;
(b) Typical drilling
configuration;
(c) Sizes of hollow stem
auger flights;
(d) Stepwise center bit;
(e) Outer bits;
(f) Outer and inner
assembly.
72. 72
Boring bits/dia 7.5 cm to 30 cm.
Applicable to where soil can stand without
casing or bore hole stands
unlined/unsupported due to cohesion.
Not suitable in a deposits containing large
cobbles or boulders.
77. 77
BUCKET AUGER
Bucket auger consists of an open-topped cylinder
which has a base plate with one or two slots
reinforced with cutting teeth, which break up the
soil and allow it to enter the bucket as it is
rotated.
78. 78
BUCKET AUGER
The top of the bucket is connected to a rod which
transmits the torque and downward pressure
from the rig at ground level to the base of the
hole: this rod is termed a ‘Kelly’.
They are used for subsurface exploration in the
USA, but are rarely used in the other parts of the
world.
This is probably because they require a rotary
table rig, or crane-mounted auger piling rig for
operation, and this is usually expensive to run.
80. 80
Shell and Auger Borings/Percussion
boring/percussion drilling
The auger consists of a cylinder with bit at
the bottom.
Shell is also similar as a cylinder with
cutting edge and hinged flap at the
bottom.
Used for drilling a deeper bore hole.
83. 83
Shell and Auger Borings
Shell & auger rigs are simple and cheap to
operate, they are excellent for drilling sand &
gravel and soft clays and chalk.
84. 84
Percussion Drilling
The drilling rig (Fig) consists of a collapsible ‘A’
frame, with a pulley at its top,
a diesel engine connected via a hand-
operated friction clutch (based on a brake
drum system) to a winch drum which provides
pulling power to the rig rope and can be held
still with a friction brake which is foot-
operated used to raise and lower a series of
weighted tools on to the soil being drilled.
87. 87
The rig is very light and can be
readily towed with a four-wheel
drive vehicle. It is also very easy
to erect, and on a level site can
be ready to drill in about 15
minutes.
In clays, progress is made by
dropping a steel tube known as a
‘clay cutter’ into the soil (see
Fig.).
88. 88
This is slowly pulled out of the borehole and is
then generally found to have soil trapped
inside it
When the claycutter is withdrawn from the
top of the hole, the soil is removed with a
metal bar which is driven into it through the
open slot in the claycutter side
89. 89
In granular materials, such as sands or
gravels, a shell is used.
At least 2 m of water is put in the bottom of
the borehole, and the shell is then surged,
moving about 300mm up and down every
second or so.
Surging the shell upwards causes water to be
drawn into the bottom of the hole, and this
water loosens the soil at the base of the hole
and forces it to go into suspension
90. 90
As the shell is dropped on the bottom of the
hole the mixture of soil and water passes up
the tube of the shell, past the simple non-
return valve (sometimes called a ‘clack’). As the
shell is raised, the clack closes and retains the
soil
By repeatedly surging the shell up and down at
the base of the hole, soil can be collected and
removed from the hole. By this method the
boulder or rock formation are pulverized .
Highly disturbed samples are collected.
91. 91
WASH BORING
Wash boring is a relatively old method of boring
small-diameter exploratory holes in fine-
grained soil.
Soil exploration below the ground water table is
usually very difficult to perform by means of
pits or auger-holes. Wash boring in such cases is
a very convenient method provided the soil is
either sand, silt or clay.
The method is not suitable if the soil is mixed
with gravel or boulders.
92. 92
The purpose of wash boring is to drill holes only
and not to make use of the disturbed washed
materials for analysis.
Whenever an undisturbed sample is required at a
particular depth, the boring is stopped, and the
chopping bit is replaced by a sampler. The
sampler is pushed into the soil at the bottom of
the hole and the sample is withdrawn.
93. 93
Figure shows the assembly for a wash boring.
To start with, the hole is advanced a short
depth by auger and then a casing pipe is
pushed to prevent the sides from caving in.
The hole is then continued by the use of a
chopping bit fixed at the end of a string of hollow
drill rods.
A stream of water under pressure is forced
through the rod and the bit into the hole, which
loosens the soil as the water flows up around the
pipe. The loosened soil in suspension in water is
discharged into a tub.
95. 95
The soil in suspension settles down in the tub
and the clean water flows into a sump which
is reused for circulation.
The motive power for a wash boring is either
mechanical or man power. The bit which is
hollow is screwed to a string of hollow drill
rods supported on a tripod by a rope or steel
cable passing over a pulley and operated by a
winch fixed on one of the legs of the tripod.
99. 99
Coring Bits
Three basic categories of bits are in use: diamond,
carbide insert, and saw tooth.
Diamond coring bits may be of the surface set or
diamond impregnated type.
• Diamond coring bits are the most versatile of all the
coring bits since they produce high quality cores in rock
materials ranging from soft to extremely hard.
100. 100
Carbide insert bits
Carbide insert bits use tungsten carbide
in lieu of diamonds.
Bits of this type are used to core soft to
medium hard rock.
They are less expensive than diamond
bits
but the rate of drilling is slower than
with diamond bits.
101. 101
Saw-tooth bits,
• the cutting edge comprises a series of teeth.
• The teeth are faced and tipped with a hard metal
alloy such as tungsten carbide to provide wear
resistance and thereby increase the life of the bit.
• These bits are less expensive but normally used to
core overburden soil and very soft rocks only.
105. 105
Open-holing
The formation of a hole in the subsoil
without taking intact samples is known
as ‘open-holing’.
carried out in a number of ways, but in
site investigation a commonly used tool
is the ‘tricone rock roller bit’ (or roller
core bit) (Fig.).
In site investigation such methods are
usually used to drill through soft
deposits, which have been previously
sampled by light percussion or auger
rigs.
106. 106
Sampling during open-holing is usually limited
to collecting the material abraded away at the
bottom of the borehole, termed ‘cuttings’, as
it emerges mixed with ‘flush fluid’ at the top
of the hole
107. 107
Coring
Double-tube swivel type core barrel
with face discharge bit
Double tube core barrel is that the inner tube is
mounted on anti-friction bearings and remains
virtually stationary while the outer barrel and
bit rotate. There is less tendency to grind core
with swivel type barrels.
The swivel type - double tube core barrel is the
generally accepted tool for obtaining core in
fractured, broken formations. It gives high
recovery when used properly.
109. 109
(a) Core barrels to which (b) coring bits
are attached to obtained rock cores
110. 110
Coring
Rotary coring in ground investigations is used to
obtain intact samples of the rock being drilled, at the
same time as advancing the borehole
In the rotary drilling method a cutter bit or a core
barrel with a coring bit attached to the end of a
string of drill rods is rotated by a power rig.
The rotation of the cutting bit shears or chips the
material penetrated and the material is washed out
of the hole by a stream of water just as in the case
of a wash boring
112. 112
ROTARY DRILLING
Rotary drilling uses a rotary action combined
with downward force to grind away the
material in which a hole is being made
may be applied to soil or rock, but are
generally easier to use in strong intact rock
than in the weak weathered rocks and soil
113. 113
Rotary drilling consists of a number of elements
(Fig.): ‘rotary rig’ , at the ground surface, which
delivers torque and thrust;
a flush pump, which pumps flush fluid down the hole,
in order to cool the mechanical parts and lift the
‘cuttings’ of rock to the ground surface as drilling
proceeds;
a ‘string’ of hollow drill rods, which transmit the
torque and thrust from the rig, and the flush fluid
from the flush pump to the bottom of the hole; and
a core barrel, which grinds away the rock, and in
addition may be designed to take a sample.
114. 114
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
Rotary Drilling
117. 117
Figure : Split Barrel Sampler: (a) Open sampler with soil sample and cutting
shoe; (b) Sample jar, split-spoon, Shelby tube, and storage box for transport
of jar samples.
119. 119
A ball check valve incorporated
in the sampler head facilitates
the recovery of cohesion less
materials.
This valve seats when the
sampler is being withdrawn
from the borehole, thereby
preventing water pressure on
the top of the sample from
pushing it out.
If the sample tends to slide out
because of its weight, vacuum
will develop at the top of the
sample to retain it.
120. 120
Split spoon sampler
Driven in the soil by hammering action
When the Area Ratio is 10% or less, the sample is generally
considered to be undisturbed
Samples are generally taken at intervals of about 1.5 m
Spring core catcher is added when samples from sand are
required
124. 124
Scraper Bucket
When soil deposit is mixed with sand and pebbles
Pebbles prevent spring core catcher from closing
Scraper bucket driven down in the soil and rotated
Scrapping from side fall into bucket
Highly disturbed sample
125. 125
Thin Wall Tube (Shelby Tubes)
The system is used to
sample soils that are
particularly sensitive to
sampling disturbance as it
has a very low wall
thickness-to-sample area
ratio.
This sampler is usually
only suitable for cohesive
soils up to a firm-to-stiff
consistency and free from
large particles.
It is not suitable for
granular non-cohesive soils.
131. 131
Common Rotary Sampling methods
Sample by rotary core drilling by
rotating drill bits and cylindrical
core barrel i.e single, double, triplesingle, double, triplesingle, double, triplesingle, double, triple
core barrel.core barrel.core barrel.core barrel.
Drilling fluid pump down
through drill rod to lubricate drill
bits and flushes cuttings/drill
debris up the borehole.
Drill hole normally supported
by casing to avoid collapse.
Flushing media : water, air, foamwater, air, foamwater, air, foamwater, air, foam
or drilling mudor drilling mudor drilling mudor drilling mud
132. 132
Rotary Core Samples
Single Tube Core Barrel
Rotary coring by portable
electric drill,
Hand portable single
tube
core barrel, without
protection between sample &
flushing medium useful for
checking mass concrete /
masonry retaining wall
thickness, geometry and
filling materials
140. 140
Handling samples
Proper Storage
Ensure sample test data
representative
Avoid storing samples in heated up
area
Preserve original moisture contents
Store up in sealed container
Use wax as sealer
147. 147
Block Sample
Sample Packing
Sample collected
by hand dug from
Trial Pit or Trial
Trench,
Record the
position, depth
and orientation of
the sample
collected。
148. 148
Field Samples Storage / Delivery
Field samples kept
in core boxes for
delivery and storage.
Disturbed jar
samples
Rock cores - H
sizes
154. 154
Observation of water levels
Presence of water effects both bearing capacity and
settlement
Water level can change seasonally
Establishing highest and lowest water level during the
life of project may become necessary
In permeable soils, the level of water will stabilize within
24 hours, after completion of bore hole, the depth of water
can then be recorded by lowering chain or tape
In impermeable soils water level may not stabilize for
several weeks. In such cases piezometer can be used
156. 156
Collecting ground water
sample, 2 liters per
sample
Check any
contamination from
ground surface or
flushing medium,
Take water sample once
hit groundwater,
Sample may change
,deliver to Laboratory.
158. 158
Bore logs
Detailed information obtained from each bore
hole is presented in a graphical form called boring
log
The driller generally record the onsite bore
information as borehole advance
The information should never be left to memory,
but shall be properly recorded on site
160. 160
After completion of laboratory test results,
Geotechnical Engineer prepare a finished
log…which contains
Driller’s filed log
Results of test conducted in laboratory
Bore logs
165. 165
Material Descriptions - Examples
Soft to firm yellowish brown slightly silty
CLAY with subangular gravels Dense
reddish brown spotted clayey silt SAND
Moderate Strong light pinkish grey
slightly to moderately weathered fine to
medium grained GRANITE with smooth
planer closely-spaced joints
166. 166
Soil Exploration Report
After all the required information has been
obtained a report has to be generated for soil
exploration results
The soil exploration report will be used by
Design Engineers/Contractors for construction
work and design
The format of soil exploration report may vary
project to project or Engineer to Engineer or client
to client depending upon the requirements
175. 175
Design Consideration for Sampling
Methods (Sample Quality Classes)
Determine which class of sample to be
collected, consider ground conditions and test
requirements
Design which sampling method to use,
Classification of Sample Quality (Class 1 to Class5(Class 1 to Class5(Class 1 to Class5(Class 1 to Class5)
Undisturbed samples: Class 1Class 1Class 1Class 1 –––– Class 2Class 2Class 2Class 2,,,,
Disturbed samples: Class 3Class 3Class 3Class 3 –––– Class 5Class 5Class 5Class 5
179. 179
Field Samples Handling & Storage
CoreCoreCoreCore----boxboxboxbox
Small jar samplesSmall jar samplesSmall jar samplesSmall jar samples
Rock coresRock coresRock coresRock cores
180. 180
Method of logging
Logging description/procedures follow
GEOGUIDE 3: Soil and Rock
Soil Description
Rock Description (i.e.Granite/Volcanic﹚﹚﹚﹚
Material Decomposition Grades (I, II,III, IV, V
VI)
204. 204
Depth of borings
• ASCE Rule:
1.Determine the net increase in the effective
stress, , under a foundation with depth as
shown in Figure.
2.Estimate the variation of the vertical effective
stress, σo ′, with depth.
3.Determine the depth, D = D1, at which the
effective stress increase , ∆σ′ is equal (1/10) q (q
= estimated net stress on the foundation).
205. 205
4 . Determine the depth, D = D2, at which
∆σ′/σo′ = 0.05.
5. Choose the smaller of the two depths, D1 and
D2, just determined as the approximate
minimum depth of boring required, unless
bedrock is encountered.
Based on the above rules the depths of
boring for a building with a width of 30 m will
be approximately the following:
207. 207
When deep excavation is anticipated, the
depth of boring should be at least 1.5 times
the depth of excavation.
In case rock bed is encountered at shallow
depth during the boring, the minimum 3 m
deep should be bored.
210. 210
"M" Design core barrels and bits are
used when drilling soft, friable, or
broken formations. They are available
with conventional waterways or
bottom discharge design.
Bottom discharge bits have internal
water ports that permit the water to
bypass the core and prevent erosion
of soft materials.
212. 212
Rotary drilling is used primarily for
penetrating the overburden between the
levels of which samples are required.
Coring bits, on the other hand, cut an annular
hole around an intact core which enters the
barrel and is retrieved. Thus the core barrel is
used primarily in rocky strata to get rock
samples. As the rods with the attached bit or
barrel are rotated, a downward pressure is
applied to the drill string to obtain
penetration,
213. 213
and drilling fluid under pressure is introduced
into the bottom of the hole through the
hollow drill rods and the passages in the bit or
barrel. The drilling fluid serves the dual
function of cooling the bit as it enters the hole
and removing the cuttings from the bottom of
the hole as it returns to the surface in the
annular space between the drill rods and the
walls of the hole.
In an uncased hole, the drilling fluid also
serves to support the walls of the hole.
214. 214
When boring in soil, the drilling bit is removed
and replaced by a sampler when sampling is
required, but in rocky strata the coring bit is
used to obtain continuous rock samples.
216. 216
Soil Samples
Soil samples obtained for engineering testing and
analysis, in general, are of two main categories:
Disturbed (but representative)
Undisturbed
Disturbed samples are those obtained using
equipment that destroy the macro structure of the
soil but do not alter its mineralogical composition.
samples can be used for determining the general
lithology of soil deposits, for identification of soil
components and general classification purposes, for
determining grain size, Atterberg limits,and
compaction characteristics of soils.
217. 217
Disturbed samples can be obtained with a number of
different methods as summarized in Table 3-4.
Undisturbed Samples
Undisturbed samples are obtained in clay soil strata
for use in laboratory testing to determine the
engineering properties of those soils.
Undisturbed samples of granular soils can be obtained,
but often specialized procedures are required such as
freezing or resin impregnation and block or core type
sampling.
It should be noted that the term “undisturbed” soil
sample refers to the relative degree of disturbance to
the soil’s in-situ properties.
218. 218
As shown in Figure 3-8a, when the shoe and the sleeve
of this type of sampler are unscrewed from the split
barrel, 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 (Figure)
Fig. SPLIT BARREL SAMPLER :
(a) Lengths of 457 mm (18 in) and 610 mm (24 in);
(b) Inside diameters from 38.1 mm (1.5 in) to 89 mm (3.5 in).
219. 219
Trial Pits and Trenches
Small hand-dug pits or wide trenches
excavated mechanically.
Examine soil in place
Obtain large disturbed samples &
undisturbed block sample.
Always install shoring D > 1.2m
Allow no accumulation of water
Backfilled with proper compaction,
222. 222
Exploration pits and trenches permit detailed
examination of the soil and rock conditions at
shallow depths and relatively low cost.
Important where significant variations in soil
conditions occur (vertically and horizontally),
large soil and/or non-soil materials exist
(boulders, cobbles, debris) that cannot be
sampled with conventional methods, or buried
features must be identified and/or measured.
225. 225
Common Rotary Sampling methods
Sample by rotary core drilling by
rotating drill bits and cylindrical
core barrel i.e single, double, triplesingle, double, triplesingle, double, triplesingle, double, triple
core barrel.core barrel.core barrel.core barrel.
Drilling fluid pump down
through drill rod to lubricate drill
bits and flushes cuttings/drill
debris up the borehole.
Drill hole normally supported
by casing to avoid collapse.
Flushing media : water, air, foamwater, air, foamwater, air, foamwater, air, foam
or drilling mudor drilling mudor drilling mudor drilling mud
226. 226
Piston Sampling
Piston sampler Ø= 75mm or
100mm (± 1mm), or up to 250mm ,
L = 1000mm
Single barrel thin wall
Area ratio 10%, tapered 10°
Min core recovery 90%
Piston remains stationary when
barrel driven down by static thrust.
Suitable for sampling in very soft to
firm soils. Highest quality
227. 227
Rotary Sampler - Triples tubes
(Non-R)
Slightly/ moderately
weathered Rock
Good for soil and
rock weak layers
228. 228
Triple-tube core-barrel
Outer and Inner Tubes , L ~
1.1m, PVC Liner ~ 1m,
Area ratio 10 ~ 15%, inside
clearance 1.5 ~ 3.5%
Cutting shoe protect sample
from flushing medium,
Samples require proper
storage and protection from
damages/disturbance during
delivery
233. 233
Steel or plastic sample retainers are often required to keep
samples of clean granular soils in the split-barrel sampler.
Figure shows a basket shoe retainer, a spring retainer and a
trap
valve retainer.
They are inserted inside the sampler between the shoe and the
sample barrel to help retain loose or flowing materials.
These retainers permit the soil to enter the sampler during
driving but upon withdrawal they close and thereby retain the
sample.
Figure 3-9: Split Barrel Sampler. (a) Stainless steel and brass
retainer rings (b) Sample catchers.