2. Soil Exploration
• The process of collection soil data for the
assessment soil properties at a site
through series of laboratory and field
investigation is collectively called Sub-soil
Exploration
• Enables the engineers to draw soil profile
indicating the sequence of soil strata and
the properties of soil involved
3. Objective
• To access the general suitability of the site.
• To achieve safe and economical design of foundations and
temporary works.
• To know the nature of each stratum and engineering properties of
the soil and rock, which may affect the design and mode of
construction of proposed structure and foundation.
• To foresee and provide against difficulties that may arise during
construction due to ground and other local conditions.
• To find out the sources of construction material and selection of
sites for disposal of water or surplus material.
• To investigate the occurrence or causes of all natural and man
made changes in conditions and the results arising from such
changes. To ensure the safety of surrounding existing structures.
• To design for the failed structures or remedial measures for the
structures deemed to be unsafe. To locate the ground water level
and possible corrosive effect of soil and water on foundation
material.
4. General
• Attempt at understanding the subsurface conditions
such as:
– 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.
– Engineering properties of soil
5. General, Why soil Exploration
– 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.
– Ultimately, it is the subsoil that provides the
ultimate support for the structures
6. PURPOSE OF SOIL EXPLORATION
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. Determination of potential foundation problems (for example,
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 bulkheads, and braced cuts.
7. Establishment of construction methods for changing subsoil
conditions
7.
8. Planning For Subsurface Exploration
• Fact finding and geological survey
• Reconnaissance
• Preliminary Exploration
• Detailed Exploration
9. ASPECTS
• 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.
10. 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
12. 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
information
Preparation of charts, tables, graphs, etc.
Calculations performed
Recommendations
14. Methodology
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
15. Methodology
• 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.
16. Methods of site exploration
• The various types of site investigation are:
Open excavation
Boring
Subsurface Sounding
Geophysical Methods
• These available methods of exploration can be
broadly classified into two categories:
Direct methods
Indirect methods
17. Direct methods
• sinking a borehole at a predetermined
location to the required depth by a method
suitable for the site
• obtain fairly intact samples of soils from every
stratum encountered or at suitably selected
depths.
• information about the soil characteristics by
means of laboratory tests
19. Indirect methods
• sounding
In sounding methods, the variation in penetration
resistance of sample or cone is utilized to interpret
some of the physical properties of the strata
• geophysical methods..
In geophysical methods, the change in subsoil strata
are identified by measuring certain physical
characteristics, e.g. electrical conductance, wave
velocity of subsurface deposits.
• projectiles, probes, and aerial photographs are also
useful in interpreting the soil characteristics.
20. Classify the Site Investigation in
another way
Investigation of site for new works
Investigation of defects or failure of existing
works
Investigations as the safety or stability of
existing works.
Investigations relating to the suitability of
material for various constructional purposes.
21. Test/Trial pit
• Trial pits are the cheapest way of site exploration
• do not require any specialized equipment.
• A pit is manually excavated to get an indication
of the soil classification & obtain undisturbed &
disturbed samples. Generally Gravelly soil.
• Trial pits allow visual inspection of any change of
strata & facilitate in-situ testing.
• They are suitable for exploration of shallow
depth only(18-20 feet)
24. Boring
Drilling a hole into the soil strata upto specified
depth is known as boring
• 1. Displacement boring.
• 2. Wash boring.
• 3. Auger boring.
• 4. Rotary drilling.
• 5. Percussion drilling.
• 6. Continuous sampling.
25. 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
26. Displacement Boring
• 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.
27. Wash Boring
• Initially, the hole is advanced for a short depth by using
an auger.
• Then a casing pipe is pushed in and driven with a drop
weight. The driving may be with the aid of power.
• A hollow drill bit is screwed to a hollow drill rod
connected to a rope passing over a pulley and
supported by a tripod.
• Water jet under pressure is forced through the rod
and the bit into the hole.
• This loosens the soil at the lower end and forces the
soil-water suspension upwards along the annular
surface between the rod and the side of the hole
28. Wash Boring Contd
• This suspension is collected in a settling tank.
• Soil particles are allowed to settle down and water is
allowed to overflow into a sump which is then
recalculated
• Very disturbed sample is obtained. Hence cannot be
used for determining engineering properties.
• whenever a soil sample is required, the chopping bit is
to be replaced by a sampler.
• The change of the rate of progress and change of
colour of wash water indicate changes in soil strata.
• Below GWT. May not be used for soils mixed with
gravel and boulders
30. Auger Boring
• Drilling is made using a device called Soil Auger
• Power driven (upto 3 to 5m) and Hand operated
(Greater than 5m)
• Advancement is made by drilling the auger by
simultaneous rotating and pressing it into the soil
• Dry and unsupported bore holes
• When the auger gets filled with soil same, it is
taken out and the soil sample collected
31. Auger Boring
• It is very suitable for soft to stiff cohesive soils
and also can be used to determine ground water
table.
• Soil removed by this is disturbed but it is better
than wash boring, percussion or rotary drilling.
• It is not suitable for very hard or cemented soils,
very soft soils, as then the flow into the hole can
occur and also for fully saturated cohesionless
soil.
34. Rotary Drilling
• 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.
• Here, the bore holes are advanced in depth by rotary
percussion method which is similar to wash boring
technique.
• 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.
35. Rotary Drilling
• 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.
• 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
37. Percussion drilling
• In case of hard soils or soft rock, auger boring or wash
boring cannot be employed. For such strata, percussion
drilling is usually adopted.
• Here advancement of hole is done by alternatively lifting
and dropping a heavy drilling bit which is attached to the
lower end of the drilling bit which is attached to the cable.
• Addition of sand increases the cutting action of the drilling
bit in clays.
• When coarse cohesion less soil is encountered, clay might
have to be added to increase the carrying capacity of slurry.
• After the carrying capacity of the soil is reached, churn bit
is removed and the slurry is removed using bailers and sand
pumps.
38. Percussion Drilling
• Change in soil character is identified by the
composition of the outgoing slurry. The stroke of bit
varies according to the ground condition.
• Generally, it is 45-100cm in depth with rate of 35-60
drops/min. It is not economical for hole of diameter
less than 10cm. It can be used in most of the soils and
rocks and can drill any material.
• One main disadvantage of this process is that the
material at the bottom of the hole is disturbed by
heavy blows of the chisel and hence it is not possible
to get good quality undisturbed samples.
• It cannot detect thin strata as well.
40. Continuous sampling
• The sampling operation advances the borehole and the boring
is accomplished entirely by taking samples continuously.
• The casing is used to prevent the caving in soils. It provides
more reliable and detail information on soil condition than the
other methods. Therefore it is used extensively in detailed and
special foundation exploration for important structures.
• It is slower method and more expensive than intermittent
sampling.
• When modern rotary drilling rigs or power driven augers are
not available, continuous sampling may be used to advantage
for advancing larger diameter borings in stiff and tough strata
of clay and mixed soil.
