Site Investigation and Types of Shallow Foundation

1. SITE INVESTIGATION AND TYPES OF
FOUNDATION
Prepared by
Dr. V. VIGNESH
Assistant Professor
Sanjivani College of Engineering, Kopargaon

2. Content
• Types of Shallow Foundations
• Soil Investigation or Exploration
• Types of Borings
• Spacing of Boring
• Types of Soil Samples
• Types of sampler
• Geophysical Exploration

3. DEFINITION
 Foundation is the lower portion of the building, usually located below the
ground level, which transmits the load of the super-structure to the sub- soil.
 Types of foundations:-
 Shallow foundation
 Deep foundation
SHALLOW FOUNDATION
Shallow foundations are further classified into the following types:
 Spread footing
 Combined footing
 Strap footing
 Grillage foundations
 Raft foundations

4. SPREAD FOOTING
 The footing whose base is extended or spread to distribute the load of the
structure over a large area of sub-soil is called spread footing.
 Types:-
 Single footing:- Suitable for light loaded column.
 Stepped footing:- For Heavily loaded column if single footing is provided, the
footing may fail or crack in the cantilever portion hence to avoid this
It is used in load
stepped footing is provided.
bearing structures.
 Slopped footing

5. SPREAD FOOTING

6. COMBINED FOOTING
 Sometimes two columns are located very near to each other in a structure. If
separate footing under these columns are provided, they may interfere with
other. Therefore, providing a combined footing, is essential.
 Types:-
 Rectangular:- when columns carry equal load
 Trapezoidal:- when columns carry unequal load

7. STRAP FOOTING
 If the independent footing of two columns are connected by a beam, is called a
strap footing.
 It may be used where the distance between the columns is so great.
 Each column is provided with its independent footings & a beam is used to
connect the two footing.

8. GRILLAGE FOUNDATION
 It is a special type of isolated footing generally provided for these locations
where bearing capacity of soil is poor.
 The depth of such a foundation is limited to 1 to 1.5 m.
 The load of the column is distributed or spread to a very large area by means
of two or more layers of rolled steel joists, each layer being laid at right angle
to the layer bellow it.
 Both the tiers of the joists are then embedded in cement concrete to keep the
joists in position and to prevent their corrosion.

9. TYPES
 Depending upon the material used in construction, grillage foundations are
further classified into two types.
 Steel Grillage Foundation
 Timber Grillage Foundation
 Steel grillage foundations are useful for structures like columns, piers, stanchions
subjected to heavy concentrated loads and hence are employed for foundations
of the buildings such as theatres, factories, town, halls etc.
 Timber grillage foundations re usually provided for timber columns subjected to
heavy concentrated loads.
 Timber grillage foundation can also be safely used for light buildings where the
soil encountered is soft and is permanently water-logged.

10. RAFT OR MAT FOUNDATION
 The foundation consisting of a thick R.C.C slab covering the whole area of a
mat is known as raft foundation.

11. SUITABILITY
 This type of foundation is useful for public buildings, office buildings, school
buildings, residential quarters etc, where the ground conditions are very poor and
bearing power of the soil is so low that individual spread footing cannot be
provided.

12. Isolated Footing
Combined Footing
Raft or Mat Foundation

13. Factors affecting the selection of type of foundations
There are certain factors that should be considered during the selecting
the foundation.
1.Type of structure
2.Type of loading pattern
3.Location of the building (region of building)
4.Soil condition
5.Water table level
6.Types of material that will used in construction
7.Lifespan of structure

14. Choosing the right foundation type based on soil condition is a
critical step in the construction process and can significantly impact
the safety and longevity of the structure. It's essential to rely on the
expertise of geotechnical and structural engineers to make informed
decisions throughout this process.

15. STEPS IN CHOOSING TYPES OF FOUNDATION BASED ON
SOIL CONDITION:
Choosing the right type of foundation for a building based on the soil
condition is crucial for ensuring the structural integrity and stability of
the structure. Here are the steps to follow:
1. **Soil Investigation:**
- Start by conducting a thorough soil investigation. This typically
involves hiring a geotechnical engineer or soil expert to assess the
soil's composition, bearing capacity, and other relevant properties.
- Soil tests may include boring, sampling, and laboratory testing to
determine soil types, moisture content, compaction, and load-bearing
capacity.
2. **Soil Classification:**
- Based on the results of the soil investigation, classify the soil into
categories such as clay, silt, sand, gravel, or a combination thereof.
- Determine the soil's properties, such as its cohesion, angle of
internal friction, and bearing capacity.

16. 3. **Load Analysis:**
- Calculate the loads that the foundation will need to
support, including the weight of the building, live loads, and
any additional loads such as equipment or snow loads.
- Consider both vertical and horizontal loads.
4. **Foundation Types:**
- Select potential foundation types that are suitable for
the soil condition. Common types include:
• Shallow Foundations: Suitable for soils with good bearing
capacity.
• Spread Footings: Used for evenly distributed loads.
• Mat Foundations: Suitable for soft soils or heavy loads.
• Deep Foundations: Used when the soil near the surface is
not suitable for bearing the loads.
• Piles: Driven into the ground to transfer loads to deeper,
more stable soil or bedrock.
• Caissons: Large-diameter, drilled shafts used in similar
situations as piles.

17. 5. **Bearing Capacity Calculation:**
- Calculate the ultimate bearing capacity of the selected foundation
types to ensure they can support the expected loads.
- Ensure that the calculated bearing capacity exceeds the design
loads with an appropriate factor of safety.
6. **Consider Settlement:**
- Evaluate potential settlement issues based on the soil's
compressibility. Soils with high compressibility may require
special foundation designs or ground improvement
techniques.
7. **Consider Environmental Factors:**
- Consider any environmental factors that may affect the
foundation, such as groundwater levels, soil expansion or
shrinkage due to moisture changes, and seismic activity.
8. **Consultation with Structural Engineer:**
- Work closely with a structural engineer who can help you
make informed decisions based on the soil investigation, load
analysis, and other relevant data.

18. 9. **Foundation Design and Detailing:**
- Once you've chosen the appropriate foundation type, work with
an engineer to design the foundation with the necessary dimensions,
reinforcement, and construction details.
10. **Construction and Inspection:**
- Ensure that the foundation is constructed according to the design
specifications.
- Regularly inspect the construction to ensure quality and
compliance with the design.
11. **Monitoring and Maintenance:**
- After construction, monitor the foundation's performance and
address any settlement or stability issues promptly.
- Implement a maintenance plan to ensure the foundation's long-
term integrity.
12. **Documentation:**
- Keep comprehensive records of the soil investigation, design
calculations, construction process, and any modifications made
during the construction phase.

19. SITE INVESTIGATION OR SITE
EXPLORATION
• Site Investigation is the process of collecting information, assessment of the data
and reporting potential hazards beneath a site which are unknown.
• Site investigation can be broadly classified into four stages:
• Reconnaissance
• Data and map study
• Detailed investigation and
• Laboratory testing.

20. Purpose of Site Investigation
 1. To select the type of foundation and depth of foundation.
 2. To determine the bearing capacity of the soil.
 3. To locate the ground water table.
 4. To select suitable construction techniques.
 5. To estimate the probable maximum and differential settlement.
 6. To investigate the safety of existing strutures and to suggest the remedial
measures.
 7. To ascertain the suitability of base of soil as a construction material.

21. Methods of Exploration
 1. Open Excavation
 a) Pits and Trenches (Long shallow pits)
 b) Drifts and Shafts
Horrizontal tunnels Large vertical holes
2. Borings
a) Auger boring
b) Wash boring
c) Rotary drilling
d) Percussion drilling
e) Core boring

22. Auger boring

23. Wash Boring

24. Rotary drilling

25. Percussion Drilling
Percussion drilling is a drilling method which involves lifting and dropping heavy
tools to break rock, and uses steel casing tubes to stop the borehole from
collapsing. Percussion drilling is carried out by breaking up the formation by
repeated blows of a heavy bit or a chisel inside a casing pipe.

27. • Core drilling is the process of drilling below the earth's surface to obtain a
core of soil or rock sample in order to determine its properties.
• The core boring operation is the insertion of this cutting circle into the
material that is being drilled with a power drill or other means of pushing
this cutting bit into the material.
Core boring

28. SPACING OF BORING

29. 1. For a compact building site covering an area of about 0.4 hectare (i.e. 4000
m2), one bore hole in each corner and one in the centre (i.e. 5 boreholes in all)
should be adequate.
2. For smaller areas and less important buildings even one bore hole in the
centre should be sufficient.
3. For very large areas covering industrial and residential colonies, the
geological nature of the terrain will help in deciding the number of bore holes.
Dynamic or static cone penetration tests may be performed at every 100
metre by dividing the area in a grid pattern and numbers of boreholes are
decided by examining the variation in the penetration curves.
4. For Highways-Along the centerline-150 to 300m spacing
5. In case of Gravity dam – 40 to 80 m.

30. TYPES OF SOIL SAMPLE
Basically, in civil engineering, there are two main types of soil sample that is
collected for the study of the properties of soils:
• Disturbed Soil Samples
• Undisturbed Soil Samples
Disturbed Soil Samples
During the sampling process of the soil sample If the natural structure of the soil
gets disturbed, then this type of soil sample is called a Disturbed Soil Sample.
The Disturbed Soil Samples can be used for the determination of the grain size,
plasticity characteristics, and specific gravity of the soil.
The collection of disturbed soil samples is done by different methods such as
Auger Boring, Wash Boring, Rotary Drilling, and Percussion Drilling, even if the
disturbed soil sample is collected by hand excavating of soil with picks and
shovels.

31.  Undisturbed soil sample:
 During the sampling process of the soil sample, if the natural structure of
the soil and water content does not disturb, that means the soil retained its
natural structure and water content, then these types of soil samples are called
undisturbed soil samples.
 The undisturbed soil samples are used for the determination of engineering
properties of soils such as shear strength, permeability, and compressibility.

32. SOIL SAMPLER
 Types of sampler
1. Split spoon sampler
2. Thin-walled tube sampler
3. Stationary piston sampler

33. Design Factors Affecting Sample Disturbance
The disturbance of soil depends mainly depends upon the following
design features:
1.Area Ratio
2.Inside Clearance
3.Outside Clearance
Area Ratio(Ar):
• It is ratio of the volume of the soil displaced by the sampler to
the ratio of the sample volume.
• The larger the value, the larger is the degree of disturbance.
• For obtaining a good quality undisturbed soil sample, the area
ratio should be 10% or less.

34.  Inside Clearance (Ci):
• It is to reduce the friction between the soil sample and
the sampler, when the soil enters the tube by allowing
for elastic expansion of the sample.
• For an undisturbed sample the inside clearance should
be between 1 to 3%.
 Outside Clearance (Co):
• This will help in reducing the friction while the
sampler is driven and when it is being withdrawn after
the collection of sample.
• For an undisturbed soil sample the outside clearance
should be lies between 0 and 2%. Co should be less
than Ci.

36. Examples:
1. A sampler has the following dimensions, internal dia of sampler
tube is 70mm and outer dia is 72mm , the internal dia of cutting
edge is 69 mm and the outer dia is 73mm. Compute its inside
clearance, outside clearance and area ratio. Also comment on the
sample collected from the sampler.
2. The cutting edge of the sampling tube has outer dia of 75mm
and wall thickness of 1.7mm. Find out the area ratio.
3. Compute the area ratio of thin walled tube sampler having an
external dia of 6cm and a wall thickness of 2.25 mm. Do you
recommend the sampler for undisturbed soil sample and why?
4. One sampler has the area ratio of 8% while another has 16%
which of these samplers do you prefer to extract undisturbed
soil sample and why?

37. BORE LOG REPORT
• Information on subsurface conditions obtained from the boring
operation is typically presented in the form of a boring record
commonly known as ‘boring log’.
• It is also known as sub-soil investigation report which should
contain the data obtained from boreholes, site recommendations
about the suitable type of foundation, soil pressure and expected
settlements.
• It is essential to give a complete and accurate record of data
collected. All relevant data for the bore bole is recorded in a
boring log.
• A boring log gives the description or classification of various
strata encountered at different depths.

38.  A soil exploration report generally consists of the following:
1. Introduction, which gives the scope of the investigation.
2. Description of the proposed structure, the location and the geological
conditions at the site.
3. Details of the field exploration programme, indicating the number of
borings, their location and depths.
4. Details of the method of exploration.
5. General description of the sub-soil conditions as obtained from in-
sites tests, such as standard penetration Test, cone test.
6. Details of the laboratory test conducted on the soil samples obtained
and the results obtained.
7. Date and weather condition during investigation.
8. Depth of ground water table and the change in water levels.
9. Discussion of the results.
10. 10.Recommendation about the allowable bearing pressure, the type
of foundation or structure.
11. 11.Conclusion:The main findings of the bore hole investigations
should be clearly stated.

40. GEOPHYSICAL EXPLORATION
• Geophysical exploration may be used with advantage to locate
boundaries between different elements of the subsoil as these
procedures are based on the fact that the gravitational, magnetic,
electrical, radioactive or elastic properties of the different elements of
the subsoil may be different.
 Different methods of geophysical explorations
1. Electrical resistivity method
• Electrical Profiling method
• Electrical Sounding method
2. Seismic Method

41. Electrical Resistivity Method
 Electrical resistivity method is based on the difference in the
electrical conductivity or the electrical resistivity of different
soils. Resistivity is defined as resistance in ohms between the
opposite phases of a unit cube of a material.
ρ is resistivity in ohm-cm,
R is resistance in ohms,
A is the cross sectional area (cm2),
L is length of the conductor (cm).

42.  The electrical resistivity methods are of the following two types:
1. Electrical Profiling Method.
2. Electrical Sounding method.
 Electrical Profiling Method.


43. • The method is also known as the resistivity mapping method.
Four electrodes are used at a constant spacing ‘a’. To conduct the
test, four electrodes, which are usually in the form of metal
spikes, are driven into the ground.
• The two outer electrodes are known as current electrodes.
• The two inner electrodes are called potential electrodes.
• The mean resistivity of the strata is determined by applying a
D.C. current to the outer electrodes and by measuring the voltage
drop between the inner electrodes. A current of 50 to 100
milliamp is usually supplied.
• The mean resistivity (ρ) is given by the formula, ρ = 2πaV/I
Where I= Current supplied, a=Spacing of electrodes, V=Voltage
drop

44. 2. Electrical Sounding method

45. • In this method, the electrode system, consisting of four
electrodes, is expanded about a fixed location, say P. The spacing
in the first setting is a1, which is increased to a2 in the second
setting and to a3 in the third setting.
• The spacing is thus gradually increased to a distance equal to the
depth of exploration.
• As the depth of the current penetration is equal to the electrode
spacing, the changes in the mean resistivity is correlated to the
changes in strata at that location.

46. Limitations of Electrical resistivity method:
(1) The methods are capable of detecting only the strata having different
electrical resistivity.
(2) The results are considerably influenced by surface irregularities,
wetness of the strata and electrolyte concentration of the ground
water.
(3) As the resistivity of different strata at the interface changes gradually
and not abruptly as assumed, the interpretation becomes difficult.
(4) The services of an expert in the field are needed.

47. Seismic Method:

48. • The seismic methods are based on the principle that the elastic
shock waves have different velocities in different materials. At the
interface of two different materials, the waves get partly reflected
and partly refracted. Seismic methods of subsurface explorations
generally utilise the refracted waves.
• The shock wave is created by a hammer blow or by a small
explosive charge at a point P.
• The shock wave travels through the top layer of the soil (or rock)
with a velocity V₁, depending upon the type of material in layer-I.
• The observation of the first arrival of the waves is recorded by
geophones located at various points, such as A, B, C.
• The geophones convert the ground vibration into electrical
impulses and transmit them to a recording apparatus

49. Limitation of the seismic methods
(1) The methods cannot be used if a hard layer with a greater
seismic velocity overlies a softer layer with a smaller seismic
velocity.
(2) The methods cannot be used for the areas covered by concrete,
asphalt pavements or any other artificial hard crust, having a
high seismic velocity.
(3) If the area contains some underground features, such as buried
conduits, irregularly dipping strata, and irregular water table,
the interpretation of the results becomes very difficult.
(4) If the surface layer is frozen, the method cannot be
successfully used, as it corresponds to a case of harder layer
overlying a softer layer.
(5) The methods require sophisticated and costly equipment.
(6) For proper interpretations of the seismic survey results, the
services of an expert are required.

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