About Subsurface investigation, Depth of foundation, Significant depth, Types of investigation, Steps involved, Methods of boring, Types of samples and samplers, Core recovery and RQD.

1. By,
Ms. Shruthi Hiremath
M E (Geotechnical Engg.)
Assistant Professor
Sanjivani College of Engineering
Kopargaon

3. Investigation of the underground conditions at a site for
the economical design of the substructure elements.
Purpose of Exploration
❖ To determine the general suitability of the site.
❖ To find 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 find out the sources of construction material.
❖ To ensure the safety of surrounding existing structures
❖ To locate the ground water level and possible corrosive effect of soil
and water on foundation material.
❖ To predict the settlements
❖ Selection of suitable construction technique.
❖ Selection of type and depth of foundation.

4.  Foundations of Multi-storeyed Buildings (IS: 1892,1979)
 Earth and rockfill Dams (IS: 6955, 1973)
 Power House Sites (IS: 10060, 1981)
 Canals and Cross Drainage Works (IS: 11385, 1985)
 Ports and Harbours (IS: 4651 – Part 1, 1974)

5. Exploration program involves location and depth of
borings, test pits or other methods to be used, and methods
of sampling and tests to be carried out to determine the
stratification and engineering properties of the soils
underlying the site. The principal properties of interest will
be the shear strength, deformation, and hydraulic
characteristics of soil.
The program should be planned so that the maximum
amount of information can be obtained at minimum cost.
The actual planning of a subsurface exploration program
includes the following steps:
 Gather all available information
 Reconnaissance
 Preliminary exploration
 Detailed exploration

6. Assemble all information on dimensions, column
spacing, type and use of structure, basement
requirements, and any special architectural
considerations of the proposed building. For bridges the
soil engineer should have access to type and span lengths
as well as pier loadings. This information will indicate
any settlement limitations, and can be used to estimate
foundation loads.

7. This step includes visual inspection carried out at site
without drilling bore holes to reveal surface and
subsurface information. This includes
Collection of information about adjacent sites and
structures.
Type of vegetation.
Ground water levels that can be determined by
checking nearby wells.
The general topography of the site, the possible
existence of drainage ditches .
Soil stratification from deep cuts .

8. In this step a few borings are made to establish in a general
manner
 To know the stratification, types of soil to be expected, and
possibly the location of the groundwater table.
 If the initial borings indicate the upper soil is loose or
highly compressible, One or more borings should be taken
to rock, or hard strata,
 A feasibility exploration program should include collection
of enough site data and sample recovery to approximately
determine the properties of soil, foundation design and
identify the construction procedures.
 To find the thickness and composition of each soil layer.

9.  Here we make a detailed planning for soil exploration in the
form trial pits or borings, their spacing and depth.
Accordingly, the soil exploration is carried out.
 The details of the soils encountered, the type of field tests
adopted and the type of sampling done, presence of water
table if met with are recorded in the form of bore log.
 The soil samples are properly labeled and sent to laboratory
for evaluation of their physical and engineering properties.
 The report is prepared with clear description of the soils at
the site, methods of exploration, soil profile, test methods
and results, and the location of the groundwater. This should
include information and/or explanations of any unusual soil,
water bearing stratum, and soil and groundwater condition
that may be troublesome during construction.

10.  Generally soil exploration should be advanced to a depth
up to which the increase in pressure due to structural
loading will have no damaging effect (such as settlement
& shear failure) on the structure. In other words, the
depth at which soil does not contribute settlement of
foundation. This depth is termed as significant depth.
Significant depth

11.  Type of structure
 Weight of structure
 Dimension of structure
 Disposition of the loaded area
 Soil profile and layer properties

12.  It can be that depth where net increase in vertical
pressure becomes less than 10% of the initial
overburden pressure.
 The maximum depth reached by the pressure bulb or
isobar diagram drawn with an intensity of pressure
varying from 1/5th or 1/10th of the surface loading
intensity (i.e. 0.2Q to 0.1Q). (Where Q = Initial
loading intensity).
 It may be equal to one and half to two times the
width or smaller lateral dimension of the loaded area.

13. Sl. No. Type Of Foundation Depth Of Exploration
1 Isolated spread footing
or Raft
1.5 B
2 Adjacent footings with
clear spacing less than
2B
1.5 L
3 Pile foundation 10 to 30m OR 1.5B
4 Base of retaining wall
1.5 B(Base width)
1.5 H (Exposed height of wall face)
[whichever is Greater]
5 Floating basement Depth of construction
6 Dams
1. 1.5 times of bottom width of earth dams
2. 2 times of height from stream bed to crest for
concrete dams, for dams less than 30 m high
3. Upto bed rock, in all soft, unstable and
permeable strata.

14. Sl. No. Type Of Foundation Depth Of Exploration
7 Roads Cuts
1. 1.0 m where little cut or fill is
required
2. In cut sections, 1.0 m below
formation level
3. In deep cuts, equal to the bottom
width or depth of the cut
8 Road Fill
2.0 m below ground
level or equal to the height
of the fill whichever is
greater
 B = Width of the foundation
 L = Length of the foundation

15.  Open Excavations (Trial pits or Test pits)
 Boring Methods
 Geophysical Methods

16. Trial pits are applicable to all types of soils, which provides
visual inspection of soil in their natural condition in either
disturbed or undisturbed state. Here depth of investigation
is limited to 3 to 3.5m.
There are 2 ways
1. Pits and trenches
2. Drifts and Shafts

17. 1. Pits and Trenches
Pits: They are excavated at site for inspection of strata so
as t0 provide necessary working space. According to IS
4453 1967, a clear working space at the bottom of the
pit should be 1.2 m x 1.2 m. Shallow pits (upto 3m) do
not require lateral support. For depth greater than 3m
and GWT arises then lateral support in the form of
sheeting and bracing is required.

18. Trenches:
They can be defined as long shallow pits. It is continuous
over a considerable length and provides exposure along a
line. On slopes trenches are more suitable than pits.

20. Drifts:
They are the horizontal tunnels made in the hill sides to
determine the nature and structure of the geological strata.
According to IS 4453-1980 a drift should be 1.5m wide and
2m height in hard rock. In soft rock arched roof can be
provided.
Shafts:
Large sized vertical holes made in the geological formation
are called as shafts
For Circular Diameter = 2.4m (min)
For Rectangular Width = 2.4m
Usually done for depth greater than 4m.

22. Exploratory bore holes are excavated in relatively soft soil
close to ground. The location, spacing and depth depends
on type , size and weight of the structure. Bore holes are
generally located at
 The building corners
 The centre of the site
 The place at which heavily loaded columns are proposed
 At least one boring should be taken to a deeper stratum
When the depth of excavation is large, vertical boring
methods are adopted. Samples are extracted from bore
holes and tested in laboratory. GWT is located and Insitu
tests are carried using bore holes. Depending on type of
soil and purpose of boring the methods are classified as:

23. Sl. No. Type of project Spacing (m)
1 Multi-storey building 10-30
2 Industrial Plant 20-60
3 Highway 250-500
4 Residential Subdivision 250-500
5 Dams and Dikes 40-80

24. Auger Boring
Wash Boring
Percussion Boring
Core Boring OR Rotary Drilling

25. 1. Auger Boring
(Helical Augers)

26. Hand operated
Helical Auger
Mechanical Operated
Helical Auger
Hand operated
post hole
auger

27. 2. Wash Boring

29. 3. Percussion Boring

31. 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.
4. Core Boring or rotary
drilling

33.  Geo-physical methods are used when the depth of
exploration is very large, and also when the speed of
investigation is of primary importance. he major method of
geo-physical investigations are: gravitational methods,
magnetic methods, seismic refraction method, and
electrical resistivity method. Out of these, seismic
refraction method and electrical resistivity methods are the
most commonly used for Civil Engineering purposes. It is a
non-intrusive method of “seeing” into the ground.
Geophysical methods includes surface and down-hole
measurement techniques which provide details about
subsurface hydrogeologic and geologic conditions. These
methods have also been applied to detecting contaminant
plumes and locating buried waste materials. some methods
are quite site specific in their performance.

35. In this method, shock waves are created into the soil at their ground level or a
certain depth below it by exploding small charge in the soil or by striking a plate
on the soil with a hammer. The radiating shock waves are picked up by the
vibration detector (also called geophone or seismometer) where the time of
travel of the shock waves gets recorded. A number of geophones are arranged on
surface , The shock waves travels directly from the shock point along the ground
surface and are picked first by the geophone. The other waves which travel
through the soil get refracted at the interface of two soil strata. The refracted
rays are also picked up by the geophone. If the underlying layer is denser, the
refracted waves travel much faster. As the distance between the shock point and
the geophone increases, the refracted waves are able to reach the geophone
earlier than the direct waves. By knowing the time of travel primary and
refracted waves at various geophones, the depth of various strata can be
evaluated, by preparing distance-time graphs and using analytical methods.
Seismic refraction method is fast and reliable in establishing profiles of different
strata provided the deeper layer have increasingly greater density and thus
higher velocities and also increasingly greater thickness. Different kinds of
materlais such as gravel, clay hardpan, or rock have characteristic seismic
velocities and hence they may be identified by the distance-time graphs. The
exact type of material cannot, however, be recognised and the exploration
should be supplemented by boring or soundings and sampling.It is not suitable
for the ground with hard strata overlying soft strata.

37.  The electrical resistivity is resistance of the material to the
passage of electrical current.
Each soil has its own resistivity depending upon its water
content, compaction and composition; for example, it is low for
saturated silt and high for loose dry gravel or solid rock.
The test is conducted by driving four metal spikes to serve as
electrodes into the ground along a straight line at equal
distance. A direct voltage is imposed between the two outer
electrodes, and the potential drop is measured between the
inner electrodes. The mean resistivity Q (ohm-m) is computed
from the expression:
D = Distance beween the electrodes (m)
E = Current Flowing between outer electrodes (amps)
I = Potential drop between inner electrodes (Volts)
Ώ = 2П D E Ohm-m
I

39. Non-Representative samples
soil samples in which neither the in-situ soil structure,
moisture content nor the soil particles are preserved. They are
not representative. They cannot be used for any tests as the
soil particles either gets mixed up or some particles may be
lost. These Samples are obtained through wash boring or
percussion drilling.
Disturbed soil samples
Disturbed soil samples are those in which the in-situ soil
structure and moisture content are lost, but the soil particles
are intact. They are representative and can be used for grain
size analysis, liquid and plastic limit, specific gravity,
compaction tests, moisture content, organic content
determination and soil classification test performed in the
lab. These samples are obtained through cuttings while
auguring, grab, split spoon (SPT), etc.

40. Undisturbed soil samples
Undisturbed soil samples are those in which the in-situ soil
structure and moisture content are preserved. They are
representative and also intact. These are used for
consolidation, permeability or shear strengths test
(Engineering properties). In sand is very difficult to
obtain undisturbed sample. These samples are obtained by
using Shelby tube (thin wall), piston sampler, surface (box),
vacuum, freezing, etc.

42. Where,
D1 – Inner diameter of cutting edge
D2 – Outer diameter of cutting edge
D3 – Inner diameter of Sampling tube
D4 – Outer diameter of sampling tube
Causes of Soil Disturbances
 Friction between the soil and the sampling
tube
 The wall thickness of the sampling tube
 The sharpness of the cutting edge
 Care and handling during transportation
of the sample tube
To minimize friction
 The sampling tube should be pushed
instead of driven into the ground
 Sampling tube that are in common use have
been designed to minimize sampling
 disturbances.

43. 1. For obtaining good quality undisturbed samples, the
area ratio should be less than or equal to 10%.
2. It may be high as 110% for thick wall sampler like split
spoon sampler and may be as low as 6 to 9% for thin wall
samples like Shelby tube

44.  The inside clearance allows elastic expansion of the
sample when it enters the sampling tube.
 It helps in reducing the frictional drag on the
sample, and also helps to retain the core.
 For an undisturbed sample, the inside clearance
should be between 0.5 and 3%.

45.  Outside clearance facilitates the withdrawal of the
sample from the ground.
 For reducing the driving force, the outside
clearance should be as small as possible.
 Normally, it lies between zero and 2%.
 Co Should not be more than Ci.

46.  The friction on the inside wall of the sampling tube
causes disturbances of the sample.
 Therefore the inside surface of the sampler should
be as smooth as possible.
 It is usually smeared with oil before use to reduce
friction.
 The non – return value provided on the sampler
should be of proper design.
 It should have an orifice of large area to allow air,
water or slurry to escape quickly when the sampler
is driven.
 It should close when the sample is withdrawn

47.  The degree of disturbance depends upon the
method of applying force during sampling and
depends upon the rate of penetration of the sample.
 For obtaining undisturbed samples, the
sampler should be pushed and not driven
L = length of the sample within the tube and
H = Depth of penetration of the sampling tube
 Rr = 96 – 98 % for getting a satisfactory undisturbed
sample

48. ❖ Split spoon Sampler
❖ Scraper Bucket
Sampler
❖ Shelby tube or Thin
Walled Sampler
❖ Piston Sampler

49.  It has an inside diameter of 35mm and an outside
diameter of 50mm.
 Has a split tube which is held together using a screw-
on driving shoe at the bottom end and a cap at the
upper end.
 4 vent posts are provided to improve recovery of
sample.
 The thicker wall of the standard sampler permits
higher driving stresses than the Shelby tube but with
higher levels of soil disturbances. It is used in SPT test.
 Split spoon samples are highly disturbed.
 They are used for visual examination and for
classification tests.

50. Split Spoon Sampler

51.  Scraper bucket can be used in case of Sandy soil
containing pebbles (gravels) also below water table, it
is difficult to use split spoon sampler.
 Driving point is attached at the end.
 It has vertical slit in the upper portion of the sampler.
 As the sampler rotates the cutting of disturbed sample
is collected in slit.

52.  It is a thin-walled seamless steel tube of inside diameter 50 to
76.2mm, outer diameter upto 125mm and length of 600-900mm.
 The bottom end of the tube is sharpened. The tubes can be
attached to drilling rods.
 The drilling rod with the sampler attached is lowered to the bottom
of the borehole and the sampler is pushed into the soil, when the
required depth is reached it is twisted to 360’ twice.
 For sandy soil length of tube = 5xdia to 10xdia
 For Clayey soil length of tube = 10xdia to 15xdia
 The sheared soil sample inside the tube at the bottom is then
pulled out and the two ends of the sampler are sealed and sent to
the lab.
 The samples can be used for consolidation and shear tests as it is
undisturbed.

53. Shelby Tube Sampler

54.  When sampling very soft and sensitive clays to get high
quality undisturbed samples, they tend to fall out of the
sampler. Then piston samplers are used.
 They consist of a thin wall tube with a piston. Initially, the
piston closes the end of the thin wall tube. The sampler is
lowered to the bottom of the borehole and then the thin wall
tube is pushed into the soil hydraulically past the piston.
 Later the pressure is released through a hole in the piston
rod. To a large extent, the presence of the piston prevents
distortion in the sample by not letting the soil squeeze into
the sampling tube very fast and by not admitting excess soil.
 Consequently, samples obtained in this manner are less
disturbed than those obtained by Shelby tubes.

55. Piston Sampler

56.  It may be necessary to core rock if bedrock is
encountered at a certain depth during drilling. It is
always desirable that coring be done for at least 3 m. If
the bedrock is weathered or irregular, the coring may
need to be extended to a greater depth.
 For coring, a coring bit is attached to the core barrel
and core barrel is attached to the drilling rod. The
cutting element in the bit may be diamond, tungsten,
or carbide.
 The coring is done by rotary drilling. Water is
circulated and cuttings are washed out

57.  Rock cores obtained by such barrels can be fractured
because of torsion. To avoid this problem, one can use
double-tube core barrels.
On the basis of the length of the rock core obtained the
following quantities can be obtained for evaluation of
the quality of rock

58. RQD was developed in 1964 by D. U. Deere*. It is
determined by measuring the core recovery percentage
of core chunks that are greater than 100 mm in length.
Core that is not hard or sound should not be counted
even if they are 100 mm in length. RQD was introduced
for use with core diameters of 54.7 mm. It is a leading
indicator for low-quality rock zones. Today RQD is used
as a standard parameter in drill core logging and forms a
basic element value of the major mass classification
systems.