The selection of the foundation and its depth, bearing capacity, and settlement analysis depend very much upon the several engineering properties of the foundation soils.

1. Soil Exploration
 The field and laboratory investigations needed to obtain the required
soil data for proper design and successful construction of structure at
the site are collectively known as the methods of soil exploration.
 The selection of the foundation and its depth, bearing capacity, and
settlement analysis depend very much upon the several engineering
properties of the foundation soils. With the help of different methods
of soil exploration, we can collect and analyze the soil data after
laboratory tests.

2. Why Soil Exploration?
Primary Purposes of Soil Exploration
 To determine the nature of the deposits of soil.
 To determine the depth and thickness of various soil strata and
their extent in the horizontal direction.
 To determine the location of the groundwater table(G.W.T).
 Collecting soil and rock samples from different strata and
determining the engineering properties of the soil and rock
strata that affect the structure’s performance.
 To determine the in-situ properties by conducting field tests.

3. REQUIRED DATA FOR ENGINEERING SOIL PROPERTIES:
1. Soil profile:
 Layer thickness and soil identification
2. Index Properties:
 Water content, Atterberg’s limit, etc.
3. Strength and compressibility characteristics
4. Others (e.g. water table depth, etc.)
5. Water Logged Areas
6. Unstable sites/slopes

4. Site Investigation Requirements
As per NBC 109: 1994, for load bearing masonry buildings and reinforced concrete
frame buildings with masonry infills, site exploration should be carried out by
digging pits, two as a minimum and more if the subsurface soil conditions show a
variation in soil types. Generally the depth for exploration for the building should
be minimum of two meters. In hilly areas, exploration up to the depth of bed rock,
if it lies shallower than two meter should suffice. No exploration shall be required if
the site is located on rock or fluvial terrace with boulder bed.
First Stage of Investigation
A site investigation is a process of collecting information, appraisal of data, and
assessing & reporting. Without soil exploration for construction, it is impossible to
know the risks in the ground beneath. The first stage of site investigation required
the following data, if available.
Aerial photographs
Topographical maps
Existing site investigation reports(for nearby sites)

5. Methods for soil exploration :
There are three main methods available for soil exploration, which are as follows.
Direct methods of soil exploration:
 Test pits, trial pits or trenches
Semi-direct methods of soil exploration:
 Boring
Soil exploration – indirect method:
 Soundings or penetration tests and geophysical methods
1. Probing
A steel bar of 25 to 40 mm is diameter hammered into the
soil until the hard sub-structure is met and the nature of soil
is obtained by examining the soil sticking to the sides of the bar.
Fig. Soil Testing Probe

6. 2. Means of Test Pit
Test pits are excavated holes which help the Geotechnical Engineer to ensure
ground conditions are suitable for the foundation of the proposed structure. The
average depth of test pits varies from 3-15 feet deep depending upon the soil
condition, allowing geotechnical engineers to assess soil composition before any
activity.
 Test pits or trenches are an open type or accessible soil exploratory system.
 Soils can be examined in their natural condition.
 Sampling procedures: can easily collect the underlying soil samples and be used
for finding strength and other engineering properties through relevant
laboratory tests.
 Test pits are considered fit only for small depths up to 3m; the cost of
investigation increases quickly with the pit’s depth.
 For greater depths, parallel supports or excavation bracing will be needed.
 Test pits are typically made only to improve other methods or minor structures.

7. 4. Boring
Making or drilling boreholes into the ground with an outlook
to obtaining soil or rock samples from particularised or known
depths is known as boring.
Different methods of boring are:
1. Auger boring
2. Wash boring
3. Rotatory Drilling
4. Shell and auger boring

8. Auger Boring
 Simplest method of exploration and sampling
 Power driven or hand operated
 Maximum depth upto 10 meters.
 Suitable in all types of soil above GWT but
only cohesive soil below GWT.
 Hollow stem augers used for sampling or
conducting Standard Penetration Tests.
Fig. Helical Auger

9. Wash Boring
Fig. : Wah Boring
• A casing is driven with a drop hammer. A
hollow drill rod with chopping bit is
inserted inside the casing.
• Soil is loosened and removed from the
bore hole using water or a drilling mud
jetted under pressure.
• The water is jetted in the hole through the
bottom of a wash pipe and leaves the hole
along with lose soil, from the annual space
between the hole and wash pipe.
• The water reaches the ground level where
the soil in suspension is allowed to settle
and mud is re-circulated.

10. The Spacing and depth of Borings:
Spacing of Boring:
Depending on the type of project, spacings of boring may be varied, which are
given below:

11. Depth of Borings:
The minimum Depth of Boring (according to ASCE, 1972) for a building project
with a breadth of 30.5 m (100 ft) will be as under.

12. Depth of Borings (according to IS 1892-1979) for different types of foundations
will be as under :
• For isolated spread footing or raft foundation, the boring depth should be
one and half times the width(B) of the foundation.
• Boring depth adjacent footings with precise spacing less than twice the width
should be one and half times the length (L) of footing.
• For pile foundation and well foundation to a depth of one and half times the
width of structure from the founding level(toe of pile or bottom of bearing
well).
• In the case of a road cut, the boring depth should be equal to the base width
of the cut.
• For road filling, it will be two meters below ground level or equal to the
height of the fill, whichever.

13. IMPROVING BEARING CAPACITY OF SOIL
Bearing capacity of soil:
In geotechnical engineering, Bearing capacity is the capacity of soil to
support the loads applied to the ground. The bearing capacity of soil
is the maximum average contact pressure between the foundation
and the soil which should not produce shear failure in the soil.

14. DIFFERENT METHODS OF IMPROVING BEARING
CAPACITY OF SOILS ARE:
 Increasing the depth of the foundation
 Draining the soil
 Compacting the soil
 Confining the soil
 Replacing the poor soil
 Using grouting material
 Stabilizing the soil with chemicals

15. INCREASING THE DEPTH OF THE FOUNDATION
 To enhance soil strength for building foundations, digging deeper is
an option, but it's expensive and suitable only for certain dry soils
like sand and gravel. Wet conditions can make this method
ineffective.

16. DRAINING THE SOIL
 When soil has higher water content, its bearing capacity decreases,
especially in sandy soil where it can drop by around 50%. To address
this, drainage methods are commonly used. Drains placed in the
foundation channel help remove excess water through pipes,
mitigating the negative impact on soil cohesion.

17. COMPACTING THE SOIL
 Soil compaction reduces gaps between
particles, making them more stable and
indirectly improving bearing capacity.
Methods include pressing glass, stones, or
sand into trench beds and using a suitable
roller at a defined speed on the soil.

18. CONFINING THE SOIL
 In this method, an enclosure is formed with the
help of sheet piles which help compact particles
of soil, resulting in improved bearing capacity of
the soil. Once the soil is confined, it is further
compacted to achieve even higher strength.
 Moreover, this method is particularly helpful for
shallow foundations.

19. REPLACING THE POOR SOIL
 Firstly, the poor-quality soil is removed by digging a trench of about
1.5 m in depth. After that, the trench is filled up with hard materials
like sand, gravel, stone etc.

20. USING GROUTING TECHNIQUE
 To address soil issues like pores or cracks beneath a foundation,
grouting is used. Cement grout is pumped into problematic soil
layers to harden them, filling any cracks that could compromise
bearing capacity. To ensure even distribution, the ground is bored,
and drain pipes are inserted to inject the grout.

21. STABILIZING THE SOIL WITH CHEMICALS
 For reinforcing soft soils at significant depths, chemical stabilization
involves injecting compounds like calcium chloride silicates. These
react with soil particles, forming a gel-like mass and improving soil
bearing capacity. Despite its effectiveness, this method is rarely used
due to its high cost and limited application.

22. INCREASING THE WIDTH OF THE FOUNDATION
 The load of the structure is evenly divided into all parts of the
foundation. If the area of the foundation is increased, the weight on
each part will then decrease. Thus, if the bearing capacity of the soil
is weak, this technique can be employed.
 However, this method has its limitations, as the foundation can not
be improved permanently.

23. LOAD TEST
 The two main types of load tests commonly used for soil
exploration are:
1. Plate Load Test
2. Penetration Test

24. 1. PLATE LOAD TEST
• The plate load test is a field test, which is performed to determine the ultimate
bearing capacity of the soil and the probable settlement under a given load. This
test is very popular for the selection and design of the shallow foundation.
• For performing this test, the plate is placed at the desired depth, then the load is
applied gradually and the settlement for each increment of the load is recorded.
At one point a settlement occurs at a rapid rate, the total load up to that point is
calculated and divided by the area of the plate to determine the ultimate bearing
capacity of soil at that depth. The ultimate bearing capacity is then divided by a
safety factor (typically 2.5~3) to determine the safe bearing capacity.

25. Figure: Test Setup for Plate Load Test

26. PLATE LOAD TEST PROCEDURE
 Excavate hole to depth> 4B (B=breath of hole).
 Plate is placed in the hole.
 Load is applied with the help of hydraulic jack in convenient increment.
 Settlements are recorded from the gauge.
Load increment =
1
5
bearing capacity of soil or
1
10
estimated failure load.
 Settlement is observed for each increment of load after a interval of 1, 4, 10, 20,40 and 60 minutes
and thereafter a hourly intervals until the rate of settlement becomes less than 0.02mm per hour.
 After this the load increment is applied.
 Test should continue until the maximum load = 1.5 × estimated load or 3 x proposed allowable
bearing pressure.

27. In summary, the Plate Load Test is a valuable tool in geotechnical engineering for
evaluating the load-bearing capacity of soil and predicting settlements under different
loads, contributing significantly to the design and construction of safe and stable
foundations.

28. 2. PENETRATION TEST
The Penetration Test is a widely used in-situ geotechnical test to assess the subsurface
soil conditions. This test provides valuable information for foundation design and
construction projects. Around the world, it is the most popular test of soil for
subsurface exploration. The measure of soil penetration resistance and collected soil
sample is used for foundation design.
This test involves the measurement of the resistance to penetration of a sampling
spoon a cone or other shaped tool under dynamic or static loadings. The resistance is
empirically correlated with some of the engineering properties of soil such as density
index, bearing capacity, etc. There are many types of penetration test, mostly used two
of them are:
 Standard penetration test.
 Dutch cone test.

29. SIGNIFICANCE OF PENETRATION TEST
 Angel of shearing resistance of cohesionless soils.
 The relative density of cohesionless soils.
 Unconfined compressive strength of cohesive soils.
By performing penetration test we can find out,

30. References
[1] Textbook of Building Technology
[2] https://www.constructioncivil.com/soil-exploration-for-
construction/?fbclid=IwAR0kkiKeWK2lhDNbkBoXtMV3Sj95y9rMG7
HIm e-nixLrMLrGYeHkRhRLC4o#gsc.tab=0
[3] NBC 109: 1994
[4] IS 1892-1979

31. THANK YOU !