geotechnical engineering

1. FOUNDATION
ENGINEERING
Krupa Sekhar Pattapu
Assistant
Professor
1

2. unit – I
Subsurface Investigations for
Foundations
2

3. Soil Exploration
 Collection of soil sample, field and laboratory investigations to obtain the
necessary data for the soils collectively called soil exploration.
 Soil Investigation consists of
1. Determination the soil profile of natural soil deposits at site,
2. Location of groundwater and fluctuations in GWT
3. Obtaining soil and rock samples from the various strata
4. Determination of Index Properties of the soils
5. Determination of Engineering properties of the soils
3

4. 1) To select the type and depth of foundation for a given structure
2) To determine the bearing capacity of soil
3) To estimate the probable maximum and differential settlement
4) To predict the lateral earth pressure against retaining walls
5) To select suitable construction techniques
6) To investigate the safety of existing structure and suggest the remedial
measures
7) To ascertain the suitability of soil as construction material
8) To establish the ground water level ad to determine the properties of
water
Purposes of Subsoil Exploration 4

5. Stages in sub-surface exploration
• Site visit
• Study of maps
• To decide method of exploration, type
of sampler..etc
Reconnaissance
• Depth and thickness of each strata
• Depth of bed rock and GWT
• Geophysical methods are used
Preliminary
explorations
• Engineering properties of soil
• Field test conducted for natural
properties
• For Complex projects
Detailed
explorations
5

6. Planning a Soil exploration Programme
 Depends upon the type and importance of the structure and the
nature of the soil strata.
 It include a site plan of the area, a layout plan of proposed
structures
 Resourceful and intelligent personnel
 Extent of soil Investigation depends upon the location of project
and variability of strata.
 maximum information that is useful in the design and construction
of project at a minimum cost
6

7. Planning a Soil exploration Programme
A detailed study of the geographical condition of the area
Preparation of a layout plan of the project.
Preparation of a borehole layout plan
Preparation of specifications and guidelines for the
field execution
Preparation of specifications and guide lines for
laboratory testing
7

8. Methods of Investigation
• Test pits, trial pits or trenches
Direct
methods
• Drillings / Borings
Semi-direct
methods
• Geophysical methods
• Soundings or penetration tests
Indirect
methods
8

9. Test pits or trenches
 Are open type or accessible exploratory
methods
 useful for conducting field tests such as
the plate-bearing test.
 suitable only for small depths—up to 3m
 cost increases rapidly with depth
 lateral supports or bracing of the excavations
will be necessary
9

10. Borings For Exploration
Auger
and
Shell
boring
Auger
boring
Type of soil
and
Purpose of
boring
Wash
boring
Rotary
drilling
Percussi
on
drilling
10

11. 1. Auger boring
 Used for relatively small depths (less than
3 to 6 m)
 Augers may be hand-operated or
power-driven
 Used in soils which can stay without
casing
 Useful for subsurface exploration for
highways, railways and air fields where
depth of exploration is small
 Soil Samples are highly disturbed
11

12. 2. Auger and Shell Boring
 If the sides of the hole cannot remain
unsupported, the soil is prevented from
falling in by means of a pipe known as
‘shell’ or ‘casing’
 An equipment called a ‘boring rig’ is
employed for power-driven augers,
which may be used up to 50 m depth
 Casings may be used for sands or stiff
clays
12

13. Wash boring
 used for exploration below ground water table for which the
auger method is unsuitable
 very disturbed sample and is not very useful for the evaluation of
the engineering properties
 The change of the rate of progress and change of colour of wash
water indicate changes in soil strata.
 Equipment is light and inexpensive
 Not suitable for rocks and soil containing boulders
13

14. 3. Wash boring 14

15. 4. Rotary drilling
 Can be used in clay, sand and rocks
 A drill bit, fixed to the lower end of a drill rod, is
rotated by power while being kept in firm contact
with the hole
 Drilling fluid or bentonite slurry is forced under
pressure through the drill rod and it comes up
bringing the cuttings to the surface.
 This method is not used in porous deposits as the
consumption of drilling fluid would be prohibitively
high.
15

16. 5. Percussion drilling
 Used for making holes in rocks, boulders and
other strata
 Heavy chisel is used to pulverize the material
 Water is added to pulvarised material and forms
slurry
 Sand pump or bailer is used to collect the slurry
 Disturbed samples
 More expensive
16

17. Depth of exploration holes
 Depends upon the degree of variation of the subsurface data in horizontal
and vertical direction
 Depth of exploration is depends upon Depth of influence zone
 Depth of influence zone depends upon type of structure, intensity of loading,
shape of loaded area, soil profile
 Significant depth : Depth upto which the stress increment due to super
imposed loads can produce significant settlement and shear stresses
 Significant depth is generally taken at which vertical stress is 10% of load
intensity
 Depth of exploration should be at least equal to Significant depth
17

18. Strip Footing
Plie foundation
For closely spacing Footing
18

19. Number/ Spacing of exploration holes
S.No. Nature of the project Spacing of borings
(metres
1 Highway (subgrade survey) 300 to 600
2 Earth dam 30 to 60
3 Borrow pits 30 to 120
4 Multi-storey buildings 15 to 30
5 Single story factories 30 to 90
NOTE: For uniform soil conditions, the above spacing are doubled, for
irregular conditions, these are halved.
19

20.  For a compact building site covering an area of about 0.4 hectare, one
bore hole or trial pit in each corner and one in the centre should be
adequate
 For smaller and less important buildings even one bore hole or trial
pit in the centre will suffice
20

21. Boring Log
21

22. Geophysical Methods
1. Seismic Refraction Methods
22

23.  Assumption made that the subsurface strata are such that the
velocity of shock waves increases as depth increases
𝑽 𝟑 > 𝑽 𝟐>𝑽 𝟏
 For Determination of thickness of different layers a distance – time
graph is plotted
 At a distance X1 (critical distance), both directed wave and
refracted wave reaches the geo phone simultaneously .
𝑋1
𝑉1
=
2𝐻1
𝑉1
+
𝑋1
𝑉2
𝐻1 =
𝑉2 − 𝑉1
𝑉2
.
𝑋1
2
𝑤ℎ𝑒𝑛 𝑤𝑎𝑣𝑒𝑠 𝑎𝑟𝑒 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑏𝑦 𝑠𝑖𝑛𝑢𝑠𝑜𝑖𝑑𝑎𝑙 𝑓𝑜𝑟𝑐𝑒𝑠
Thickness of layer 1 , 𝑯 𝟏 =
𝑽 𝟐−𝑽 𝟏
𝑽 𝟐+𝑽 𝟏
.
𝑿 𝟏
𝟐
23

24. 24Thickness of layer 2, 𝑯 𝟐 =
𝑽 𝟑−𝑽 𝟐
𝑽 𝟑+𝑽 𝟐
.
𝑿 𝟐
𝟐
+ 𝟎. 𝟖𝟓𝑯 𝟏
Find the velocities of
each layer from the plot
are nothing but slopes of
lines as shown in graph

25. Limitations of Seismic Method
1. Can’t be used if a hard layer with a greater seismic velocity overlies a
softer layer with a smaller seismic velocity
2. Can’t be used for the areas covered by concrete, asphalt pavement or
any other having high seismic velocity.
3. If the area contains some underground features (buried conduits,
irregularly dipping strata and irregular water table ) interpretation of
results are difficult
4. When the surface layer is frozen
5. Requires sophisticated and costly equipment
25

26. 2. Electrical Resistivity Method
a) Electrical Profiling Method
b) Electrical Sounding Method
Geophysical Methods
Type of
Rock/Soil
Sound
Rock
Weathered
Rock
Gravel Sand Clayey
Sand
Saturate
d Clay
and silt
Resistivity
(ohm-m)
>5000 1500 to
2500
1500
to
4500
500 t0
1500
200 to
500
2 to
100
26

27. a) Electrical Profiling Method
𝑚𝑒𝑎𝑛 𝑟𝑒𝑠𝑖𝑠𝑡𝑖𝑣𝑖𝑡𝑦, ρ =
2 𝜋𝑎𝑉
𝐼
27

28. b) Electrical Sounding Method 28

29. Limitations of Resistivity Method
1. Method is capable of detecting only the strata having different
electrical resistivity
2. Results are influenced by surface irregularities, wetness of strata
and electrolyte concentration of ground water
3. As the resistivity of different strata at the interface changes
gradually the interpretation is difficult
4. Services of an expert in the filed are needed
29

30. Types of
Samples
Disturbed
samples
Undisturbed
samples
30

31. Non-representative samples
1. Some mineral constituents have been lost or got mixed up
2. Soil samples obtained from auger borings and wash borings
3. Suitable only for providing qualitative information such as
major changes in subsurface strata.
Representative samples.
1. contain all the mineral constituents of the soil, but the
2. structure of the soil may be significantly disturbed
3. water content may also have changed
4. suitable for identification and for the determination of certain
physical properties such as Atterberg limits and grain specific
gravity.
Disturbed samples 31

32. Sample Disturbance
𝒂𝒓𝒆𝒂 𝒓𝒂𝒕𝒊𝒐 𝑨 𝒓 =
𝑴𝒂𝒙𝒊𝒎𝒖𝒎 𝒄.𝒔 𝒂𝒓𝒆𝒂 𝒐𝒇 𝒄𝒖𝒕𝒕𝒊𝒏𝒈 𝒆𝒅𝒈𝒆
𝑨𝒓𝒆𝒂 𝒐𝒇 𝒔𝒐𝒊𝒍 𝒔𝒂𝒎𝒑𝒍𝒆
=
𝐷2
2
− 𝐷1
2
𝐷1
2 × 100
= ≤ 10% 𝑓𝑜𝑟 𝑢𝑛𝑑𝑖𝑠𝑡𝑢𝑟𝑏𝑒𝑑 𝑠𝑎𝑚𝑝𝑙𝑒𝑠
𝑰𝒏𝒔𝒊𝒅𝒆 𝒄𝒍𝒆𝒂𝒓𝒂𝒏𝒄𝒆 𝑪𝒊 =
𝑫 𝟑 − 𝑫 𝟏
𝑫 𝟏
× 100
= 0.5 𝑎𝑛𝑑 3% 𝑓𝑜𝑟 𝑢𝑛𝑑𝑖𝑠𝑡𝑢𝑟𝑏𝑒𝑑 𝑠𝑎𝑚𝑝𝑙𝑒𝑠
𝑶𝒖𝒕𝒔𝒊𝒅𝒆 𝒄𝒍𝒆𝒂𝒓𝒂𝒏𝒄𝒆 𝑪 𝒐 =
𝑫 𝟐 − 𝑫 𝟒
𝑫 𝟒
× 100
= 0 𝑎𝑛𝑑 2 % 𝑓𝑜𝑟 𝑢𝑛𝑑𝑖𝑠𝑡𝑢𝑟𝑏𝑒𝑑 𝑠𝑎𝑚𝑝𝑙𝑒𝑠
32

33. Types of Samplers
1. Thick wall samplers (split spoon samplers)
2. Thin wall samplers (shelby tubes)
33

34. Split Spoon Sampler (Thick Wall) 34
 To collect the disturbed samples
 Not suitable for soils like fine sand
and below water level
 Thin metal or plastic tube liner is
provided in split steel tube to protect
the soil and hold it together.

35. Shelby tubes and Thin Wall Samplers 35
 To collect the undisturbed samples of
clay
 Length of the tube is 10-15 times of
diameter
 Thickness varies from 1.25 to 3.15 mm

36. Piston sampler 36
 To collect the undisturbed samples of soft
and sensitive clays
 Piston is attached to bottom of sampler
tube
 Piston provides protection against water
pressure on top of soil

37. Scraper Bucket sampler
 Used for sandy deposits which contain
pebbles and cohesion less soils below
water table
 Driving point attached to bottom and
vertical slit at top
 As the sampler is rotated, soil enters
into sampler through open slit
 Disturbed but representative soils
37

38. Hand curved sampler
 Used to collect the sample from test pits
 Hand curved samples are undisturbed
38

39. Methods of Investigation
• Test pits, trial pits or trenches
Direct
methods
• Drillings / Borings
Semi-direct
methods
• Geophysical methods
• Soundings or penetration tests
Indirect
methods
39

40. Sounding and Penetration Tests
(Field Tests)
1. Standard penetration test
2. Cone penetration test
a) Static Cone penetration Test
b) Dynamic Cone Penetration Test
3. In-situ Vane shear Test
40

41. Standard Penetration Test (SPT)
 suited for cohesion less soils
 Used to determine the relative density and angle of internal friction
 split-spoon sampler
 a bore hole 55 to 150 mm in diameter at the desired depth
 hammer of 640 N (65 kg) weight with a free fall of 750 mm
 The blow count is found for every 150 mm penetration
 The number of blows for a penetration of last 300 mm = SPT value (N)
 Usually SPT is conducted at every 2 m depth or at the change of stratum.
 If no of blows required for 150 mm drive exceeds 50, it is taken as refusal and
test is discontinued
41

42. 42

43. Over burden Pressure Correction :
Correct Penetration Number, 𝑵 𝑪 = 𝑵 𝑹 ×
𝟑𝟓𝟎
𝝈 𝒐+𝟕𝟎
Where, 𝜎 𝑜 = effective overburden pressure kN/m2 < 280 kN/m2
Dilatancy Correction:
when fine sands or silts below water table,
Correct Penetration Number, 𝑵 𝑪 = 𝟏𝟓 +
𝟏
𝟐
(𝑵 𝑹 − 𝟏𝟓) if 𝑵 𝑹 > 15
𝑵 𝑪 = 𝑵 𝑹 if 𝑵 𝑹 < 15
NOTE: The ratio of (
𝑵 𝑪
𝑵 𝑹
) should lies in between 0.5 and 2, or if it > 2, then it
is divided by 2
43

44. Condition N Dr φ
Very loose 0-4 0-15% <28
Loose 4-10 15-35% 28-30
Medium 10-30 35-65% 30-36
Dense 30-50 65-85% 36-42
Very Dense >50 >85% >42
Consistency N qu (kN/m2)
Very Soft 0-2 <25
Soft 2-4 25-50
Medium 4-8 50-100
Stiff 8-15 100-200
Very staff 15-30 200-400
Hard >30 >400
44

45. Static Cone Penetration Test (Dutch Cone Test)
 In-situ penetration resistance of soils
 standard penetration test unreliable especially under water
 The steel cone shall be of steel with tip hardened.
apex angle of 60° ± 15′ and
overall base diameter of 35.7 mm
cross-sectional area of 10 cm2.
 Cone is pushed down ward at a steady rate of
10mm/sec through a depth 35mm each time
 Refined Dutch Cone : Friction sleeve +Cone
45

46. Relation between Cone Penetration Resistance (qc) in kN/m2 and
SPT(N)
 Useful in determining the bearing capacity of the soil at various
depths below the ground level and skin friction values
Type of soil Cone Penetration Resistance
qc in kN/m2
Gravels 𝟖𝟎𝟎 𝒕𝒐 𝟏𝟎𝟎𝟎 𝑵
Sands 𝟓𝟎𝟎 𝒕𝒐 𝟔𝟎𝟎 𝑵
Silty Sands 𝟑𝟎𝟎 𝒕𝒐 𝟒𝟎𝟎 𝑵
Silts and Clayey silts 𝟐𝟎𝟎 𝑵
46

47. Dynamic Cone Penetration Test
 Driven by a 65 kg hammer with 750mm free
drop
 Find No of blows required for every 10 cm
penetration
Dynamic cone resistance 𝑵 𝒄𝒃𝒓 = No of blows
required for 30 cm penetration
 This test is performed either by using 50 mm
cone without bentonite slurry or 65 mm with
bentonite slurry
47

48. Dynamic Cone Penetration Test48

49.  If 50 mm cone is used,
𝑵 𝒄𝒃𝒓 = 1.5 N for depth upto 3m
𝑵 𝒄𝒃𝒓 = 1.75 N for depth upto 3m to 6m
𝑵 𝒄𝒃𝒓 = 2 N for depth > 6m
 If 65mm cone is used,
𝑵 𝒄𝒃𝒓 = 1.5 N for depth upto 4m
𝑵 𝒄𝒃𝒓 = 1.75 N for depth upto 4m to 9m
𝑵 𝒄𝒃𝒓 = 2 N for depth > 9m
Relation between Dynamic Cone Resistance(𝑵 𝒄𝒃𝒓 ) and SPT (N) 49

50. Pressure Meter Test
 Used for determining the stress
– deformation Characteristics
of soils H:pressure meter
test.mp4
 Helps to determine the modulus of
deformation, undrained shear strength,
angle of shearing resistance,
50

51. Recovery ratio (Rr) : Percentage ratio between the length of the core
recovered and the length of the core drilled on a given run
Recovery ratio Rr = L/H
where, L = length of the sample within the tube
H = depth of penetration of the sampling tube.
 Influenced by the drilling technique and the type and size of core barrel
used.
 Generally the use of a double tube barrel results in higher recovery
than can be obtained with single tube barrels
 value should be 96 to 98% for a satisfactory undisturbed sample
51

52. Rock Quality Designation (RQD) :
𝑳 𝒂
𝑳 𝒕
𝐿 𝑎 = Total length of intact hard and sound pieces of core of length
greater than 4 inch. Arranged in its proper position
𝐿 𝑡= Total length of drilling
RQD % 90-100 75-90 50-75 25-50 0-25
Rock
Quality
Excellent Good Fair Poor Very Poor
52

53. 53