Geotechnical Investigation (Soil Exploration), Purpose and Methods Disturbed and Undisturbed Sampling

1. Geotechnical Investigation
(Soil Exploration),
Purpose and Methods
Disturbed and Undisturbed Sampling
By:
Dr. Sajjad Ali Mangi
(Associate Professor)

2. 1. INTRODUCTION
Two very essential requirements for design and construction
of any civil engineering project:
I. SOIL INVESTIGATION
• Also called site investigation.
• Provides data regarding subsoil conditions(i.e.
underground conditions), groundwater position and its
fluctuations.
II. TOPOGRAPHIC SURVEY
• Furnishes information regarding ground surface features
of the site (i.e. surface conditions of the site).

3. 1. INTRODUCTION(Cont…….)
i. Soil Exploration Is Needed For:
• Planning, designing and constructing new projects,
• Investigating the causes of distress or failure and
developing remedial measures,

4. 2. PURPOSE AND OBJECTIVES
• To determine the stratigraphy of the site
 Primary Objectives:
• To assess sequence and thickness of strata.
• To assess quality of bedrock and depth of overburden soil.
• To determine position of groundwater table (GWT).
• To evaluate the characteristics of subsoil, rock,
and GWT.

5. 3. INVESTIGATION PHASES
In general can have Four Phases, as given below:
I. Feasibility or
reconnaissance
II. Preliminary exploration
III. Detailed exploration
IV.
pre-construction
phases
Construction/post construction stage exploration
• For small projects, all pre-construction phases are
combined into a single exploration operation and phase-4
may not be operated at all.

6. 3. INVESTIGATION PHASES
(Cont…..)
phase-I
I. Feasibility or Reconnaissance:
Generally following information is collected in this phase:
• Project details (type of structure to be constructed, structural
loads, intended use of the structure)
• Surface and sub-surface conditions of the site (obtained
through study of geological maps, aerial photographs, etc)
• Study of existing structures in the area (behavior of existing
structure adjacent to the site)

7. 3. INVESTIGATION PHASES (Cont.…)
phase II
II. Preliminary exploration:
During this phase
• Bedrock or hardpan is established
• Disturbed soil samples are recovered
• Position of GWT may also be
established

8. 3. INVESTIGATION PHASES (Cont.)
phase III
III. Detailed exploration:
This phase may include:
• Test pits excavation, boreholes, in-situ testing
• Collection of both disturbed and undisturbed samples
for detailed laboratory testing
• GWT may also be monitored by installing piezometers.

9. 3. INVESTIGATION PHASES (Cont..)
phase IV
IV. Construction/post construction stage exploration:
• Additional exploration may be required during
construction stage to cope with the unexpected subsoil
conditions.
• Monitoring of the movement of structure,
• Monitoring of groundwater fluctuations, both during
and after construction, may be required.

10. 4. EXPLORATION METHODS
I. Indirect methods
• Establish boundaries
between strata of
different composition,
• Determine the quality by
detecting changes in the
electrical resistivity or
wave velocity in the soil.
II. Direct methods
• Soil samples are taken
from the soil/rock strata
by making actual
excavations through
probing, borings, test
pits etc.
• Subsoil/GWT
characteristics are
determined by field
/laboratory test
performed on sample
recovered.
 The indirect methods do not provide direct information regarding the
characteristics of subsoil/rock and must be used in conjunction with
direct methods.

11. I. INDIRECT METHODS
• Also termed as GEO-PHYSICAL METHODS
• Used when soil exploration is to be carried out over large
area and where speed is of prime importance.
• These soil exploration methods are based on principle that
physical properties (like electrical conductivity, elasticity or
seismicity, magnetic susceptibility, density etc) vary for
different types of soils.

12. I. METHODS USED IN INDIRECT
METHODS(Cont…….)
• NOTE:
Here we will discuss only those methods which are being
widely used.
• Seismic refraction method.
Widely used
• Electrical resistivity method.
• Magnetic measurements method.
• Gravitational measurements method.
methods

13. I. METHODS USED IN INDIRECT
METHODS(Cont…….)
i. Seismic refraction method.
• This soil exploration method is based on the principle
that sound waves travel faster in rock than in soil.
• Shock waves (or sound waves of vibration) are created
into the soil at ground level or at a certain depth below
it, either by striking a plate on the soil with the hammer
or by exploding small charge in the soil.
• The shock waves so produced travel down in the sub-
soil strata and get refracted after striking a hard rock
surface below.

14. I. METHODS USED IN INDIRECT
METHODS(Cont…….)
i. Seismic refraction method.(Cont…….)
• The refracted or radiated shock waves are picked up by
the vibration detector (also known as geophone) where the
time of travel of the shock waves gets recorded.
• Knowing the time of travel of the primary and refracted
waves at various geophones, tune and distance graphs
are drawn based on which it is possible to evaluate the
depth of various strata in the sub-soil.

15. Fig. Seismic refraction method

16. I. METHODS USED IN INDIRECT
METHODS(Cont…….)
ii. Electrical resistivity method
• This soil exploration method is based on the principle that
each soil has different electrical resistivity, depending
upon the type of soil, its water content, compaction and
composition
• 4 electrodes are driven in the ground at equal distance
apart and in a straight line.
• A current is passed between the two outer electrodes
and the potential drop between the inner electrodes is
measured by use of potentiometer.
• Thus on knowing the values of change in resistivity of sub
soil strata at site, it is possible to establish the nature and
distribution of different type of soils in the formation.

17. Fig. Electrical resistivity method

18. II. Direct Methods
• A more detailed description of underground conditions at
specific locations are revealed with direct investigation
methods which obtain information by physically sampling or
testing soil, rock and groundwater.
• The sampling or testing devices are inserted into a
borehole advanced into the ground by drilling methods,
and either samples are returned to the surface for
evaluation and testing, or the properties of the
underground profile are measured by in-place testing
i. Methods used in DIRECT METHODS
a. Tests pits, trenches, shafts, tunnels
b. Probing or sounding
c. Test boreholes

19. i. METHODS USED IN DIRECT
MTHODS
a. Test pits, Trenches, Shafts, Tunnels:
• Also termed as open excavations.
• The most satisfactory method of inspecting soil
stratification through visual observations and recovering
both disturbed and undisturbed samples.
• Excavations may be made by hand by using manual labor
or by using earth moving equipments (i.e. backhoe,
trencher or dozer)

20. i. METHODS USED IN DIRECT
METHODS(Cont………)
MERITS
• Visual inspection of
stratification.
• Suitable for best quality
disturbed and
undisturbed sampling.
• Difficulties of excavating
can be assessed.
DEMERITS
• Limited to a depth of
about 3 m as the
cost beyond limit
increase rapidly.
• Suitable for exploration
only above GWT.
• Limited to shallow
exploration only.
a. Test pits, Trenches, Shafts, Tunnels (Cont……...)

21. i. METHODS USED IN DIRECT
METHODS (Cont.)
b. Probing or sounding
• Probing or sounding is made by driving a steel rod about
25 mm diameter into the ground.
• The soil type and its properties are related to the driving
resistance to the rod and from particles adhering to the rod
when it is pulled out.
• Sometime some small grooves are made along the rod to
collect some traces of soils through which it passes during
probing.

22. i. METHODS USED IN DIRECT
MTHODS(Cont………)
MERITS
• Suitable to locate
the thickness of
loose overburden
soil.
• Rapid method and
relative cost is very low.
• Very cheap and rapid
method of exploration.
DEMERITS
• A boulder may be
mistaken for bedrock.
• Method is of limited use
as soil sample for
identification are not
recovered.
• Suitable for depth about
10 to 20 m.
b. Probing or sounding (Cont………)

23. i. METHODS USED IN DIRECT
METHODS(Cont.)
C. Test Boreholes:
Boreholes in soil/rock may be drilled using:
 Auger Boring,
 Percussion rig,
 Wash boring rig and
 Rotary drilling rig.
 Auger Boring:
• Hand augers are used only for boring above GWT to a
maximum depth of about 10m.
• The range of diameter of holes is from 50mm(2inch) to
about 300mm(12 inch) drilled by different augers.

24. i. METHODS USED IN DIRECT
METHODS(Cont………)
 Auger Boring(Cont………)
• Fast and economical, using simple, light, instruments
for large to small holes.
• 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.

25. i. METHODS USED IN DIRECT
METHODS(Cont………)
Fig. Augers

26. i. METHODS USED IN DIRECT
METHODS(Cont………)
 Percussion rig:
• 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.

27. i. METHODS USED IN DIRECT
METHODS(Cont………)
Fig. Percussion Drilling
Lifting,
dropping,
cutting by
drilling bit.

28. i. METHODS USED IN DIRECT
METHODS(Cont………)
 Wash boring rig:
• It is a popular method due to the use of limited
equipments.
• Here first an open hole is formed on the ground so that the
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.

29. i. METHODS USED IN DIRECT
METHODS(Cont………)
Fig. Wash Boring

30. i. METHODS USED IN DIRECT
METHODS(Cont………)
 Rotary drilling rig:
• Rotary drilling method of boring is useful in case of highly
resistant strata.
• 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.

31. i. METHODS USED IN DIRECT
METHODS(Cont………)
Fig. Rotary Drilling
System

32. 5. SOIL SAMPLING
• A sample is said to be a representative sample when it truly
represents the characteristics of the stratum from which it is
recovered.
 In geotechnical engineering, generally following types of
samples are retrieved.
I. Disturbed Soil Samples (DS).
II. Undisturbed Soil Samples (UDS).

33. 5. SOIL SAMPLING(Cont………)
I. Disturbed Soil Samples:
• In disturbed soil samples the natural structures of soils
gets partly or fully modified or destroyed,
• Although with suitable precaution the natural water
content may be preserved.
• Disturbed sample can be obtained by direct excavations
by augers.
I. Undisturbed Soil Samples:
• In undisturbed soil samples the natural structure and properties remain
preserved.
• These samples are used to tests for shear, consolidation
and permeability.

34. 5. SOIL SAMPLING(Cont………)
 Causes Of Sample Disturbance:
• Volume disturbance.
• Side friction between the sampler and the soil.
• Moisture content changes.
• Loss of hydraulic pressure.
• Working environment.
• Improper shearing of the sample.
• Failure to shut-off the drilling fluid during sampling
operation.

35. 6. AMOUNT OF EXPLORATION
• By amount of exploration is meant to decide about the
extent of investigation i.e. to determine the number,
location, and depth of open excavation and boring
soundings.
i. The extent of exploration depends upon:
• Site conditions
• Nature and extent of the project.
• Availability of time, funds, equipment for exploration

36. 7. IN-SITU TESTS
 Following are the different tests performed in soil exploration:
• Standard Penetration Test (SPT).
• Static Cone Penetration Test (CPT).
• Field Vane Shear Test (FVT).
• Plate Load Test (PLT).
• Pressure meter Test (PMT).
• Dilatometer (DMT).

37. 8. PLANNING AN INVESTIGATION
The aim of planning an investigation programmed is to gather
adequate data regarding surface and subsurface conditions of
the site necessary for the design and construction of the new
facilities for evaluating the causes of distress and developing
remedies for the existing facilities.
i. Investigation program must include;
a. Amount and extent of exploration:
Spells out number of boring and test pits

38. 8. PLANNING AN INVESTIGATION
(Cont............)
b. Method of exploration:
Based on the information gathered from a critical review
of the background data of the site. It may be split into
following phase:
• Feasibility or reconnaissance
• Preliminary exploration
• Detailed exploration
• Construction/post construction stage exploration
c. Sampling method:
Types of method should be specified in the program. the
program should also specify the labeling, packing,
transporting procedures etc, of the samples from the site to
the laboratory.

39. 8. PLANNING AN INVESTIGATION
(Cont……..)
d. Laboratory testing program:
should include the number and types of tests required for the
investigation.
e. In-situ tests:
should specify the number and types of in-situ tests needed
for collecting information essential for the adequate design of the
works.
f.Recording of Field Data and Preparation of the
Geotechnical Report
It should specify the procedure of recording field data and
should include specimen boring logs or data recording sheets.
g. Time and cost of exploration
It should make estimates for the time and cost of investigation. it
must include the bill of quantities of the investigation.(BOQ)

40. 9. RECORDS AND REPORTS
I. Borehole/Test Pits Logs:
• should be prepared in the field providing all necessary
information about the site.
II. Data Sheets of In-situ Tests:
A geotechnical report should include:
• Introduction
• Brief project description
• Purpose and scope
• Techniques and geophysical survey record
• Boring logs and in-situ and laboratory test result
• Comments , Conclusion , and Recommendation

41. 10. CONCLUSION OF PRESENTAION
• For any construction the soil that it’s built on is of great
importance.
• If the foundation is not set on solid ground it can have
disastrous impact on the building.
• Necessary for the design and construction of the new facilities
for evaluating the causes of distress and developing remedies
for the existing facilities.