Detailed presentation on site investigation by static and cone penetration test.
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1. Foundation Engineering Site Investigations - Virncheepuram
Naveen
2. Standard Penetration Test Introduction: Most commonly used
In-situ test Especially for cohesion less soils, which cant be
easily sampled
3. Useful in finding out: Relative density of cohesion less
soils. Angle of shearing resistance of cohesion less soils.
Unconfined compressive strength of cohesive soils
4. INSTRUMENTS 1. Drilling equipment for boreholes 2.
Split-spoon sampler 3. Drive-weight assembly 4. Cathead 5. Hammer
a) Safety Hammer b) Donut Hammer c) Automatic Hammer
5. 1. Drilling equipment for boreholes: Any drilling equipment
is acceptable that provides a reasonably clean hole, which is at
least 5 mm larger than the sampler or sampling rods, and less than
170 mm diameter.
6. 2. Split spoon sampler It is a sampler for obtaining a
disturbed sample of soil and consists of-o Driving shoe : Made of
tool-steel, about 75 mm long o Steel Tube : 450 mm long, split
longitudinally in two halves o Coupling : 150 mm long, provided at
the top o Check Valve o 4 Venting Ports : 10 mm diameter
7. 3. Drive weight assembly Hammer of 63.5 kg A driving Head
(Anvil) A guide permitting a free fall of 0.76 m and over lift
capability of at least 100 mm.
8. 4. Cathead Operating at approximately 100 rpm Equipped with
suitable rope and overhead sheave for lifting drive-weight
9. 5. Hammer a) Safety Hammer Closed system Delivers
approximately 60% of the maximum free fall energy Highly variable
energy transfer
10. b) Donut Hammer Open system Delivers approximately 45% of
the maximum free fall energy Highly variable energy transfer
11. c) Automatic Hammer Safest system Delivers approximately 95
- 100% of the maximum free fall energy Consistent and effective
energy transfer Increased production
12. Procedure 1. Drilling of borehole 2. Driving the Casing 3.
Assembling equipment 4. Penetration testing 5. Handling sample
13. 1.Drilling of borehole Drill the borehole to the desired
sampling depth and clean out all disturbed material. The equipment
used shall provide a clean borehole, 100 to 150 mm in diameter, for
insertion of the sampler to ensure that the penetration test is
performed on undisturbed soil. Casing shall be used when drilling
in sand, soft clay or other soils in which the sides of borehole
are likely to cave in.
14. 2.Driving the Casing Where casing is used, it shall not be
driven below the level at which the test is made or soil sample is
taken. In the case of cohesion less soils which cannot stand
without casing, the advancement of the casing pipe should be such
that it does not disturb the soil to be tested or sampled; the
casing shall preferably be advanced by slowly turning the casing
rather than by driving, as the vibration caused by driving may
alter the density of such deposits immediately below the bottom of
the borehole.
15. 3.Assembling equipment Attach the split-spoon sampler to
the drill rod and lower into the hole until it is sitting on the
undisturbed material. Attach the drive weight assembly. Lift the
63.5 kg hammer approximately 0.76 m and allow it to fall on the
anvil delivering one seating blow. Mark the drill rod in 3
successive .15 m increments to observe penetration.
16. 4. Penetration testing Raise and drop the hammer 0.76 m
successively by means of the rope and cathead, using no more than
two and one forth wraps around the cathead. The hammer should be
operated between 40 and 60 blows per minute and should drop freely.
Record the number of blows for each .15 m of the penetration. The
first 0.15 m increment is the "seating" drive.
17. The sum of the blows for second and third increment of 0.15
m penetration is termed "penetration resistance or "N-value". If
the split spoon sampler is driven less than 45 cm (total), then the
penetration resistance shall be for the last 30 cm of penetration
(if less than 30 cm is penetrated, the logs should state the number
of blows and the depth penetrated). If the no. of blows for 15cm
drive exceeds 50, it is taken as a refusal and the test is
discontinued. Tests shall be made at every change in stratum or at
intervals of not more than l-5 m whichever is less. Tests may be
made at lesser intervals if specified or considered necessary.
18. The intervals be increased to 3 m if in between vane shear
test is performed.( as per IS:2131-1963) . The entire sampler may
sometimes sink under its own weight when very soft sub-soil stratum
is encountered. Under such conditions, it may not be necessary to
give any blow to the split spoon sampler and SPT value should be
indicated as zero.
19. 5. Handling sample Bring the sampler to the surface and
open it. Remove any obvious contamination from the ends or sides
and drain excess water. Carefully scrape or slice along one side to
expose fresh material and any stratification. Record the length,
composition, colour, stratification and condition of sample. Remove
sample and wrap it or seal in a plastic bag to retain moisture. If
the sample can be removed relatively intact, wrap it in several
layers of plastic and seal ends with tape.
20. Corrections: Dilatancy correction Overburden pressure
correction Gibbs and holtz correction (1957) Peck, hansen and
thornburns correction Peck and bazaraas correction
21. Dilatancy correction Silty fine sands and fine sand below
the water table develop pore pressure which is not easily
dissipated. Pore pressure increases the resistance of the soil
thus, Penetration Number (N) also increases This correction is
applied when observed value of N exceeds 15
22. Dilatancy correction Terzaghi and Peck (1967) recommended
the following correction-
23. Overburden pressure correction In granular soils,
overburden pressure affects the penetration resistance If two
soils, having same relative density but different confining
pressures are tested, the one with a higher confining pressure
gives a higher penetration number as the confining pressure in
cohesion less soils increases with the depth, the penetration
number for soils at shallow depths is underestimated and that at
greater depths is overestimated. For uniformity, the N- values
obtained from field tests under different effective overburden
pressures are corrected to a standard effective overburden
pressure.
24. Gibbs and holtz correction (1957)
25. Peck, Hansen and Thornburns correction
26. Peck and bazaraas correction One of the most commonly used
corrections According to them,
27. FACTORS COMMENTS Attitude of operators Blow counts for the
same soil using the same rig can vary, depending on who is
operating the rig, and perhaps the mood of operator and time of
drilling. Overdrive sampler Higher blow counts usually result from
an overdriven sampler. Sampler plugged by gravel Higher blow counts
result when gravel plugs the sampler, resistance of loose sand
could be highly overestimated. Plugged casing High N-values may be
recorded for loose sand when sampling below groundwater table.
Hydrostatic pressure can cause sand to rise within the casing.
28. FACTORS COMMENTS Overwashing ahead of casing Low blow count
may result for dense sand since overwashing loosens sand. Drilling
method Drilling technique (e.g., cased holes vs. mud stabilized
holes) may result in different N-values for the same soil. Free
fall of the drive weight is not attained Using more than 1-1/2
turns of rope around the drum and or using wire cable will restrict
the fall of the drive weight. Not using correct weight Driller
frequently supplies drive hammers with weights varying from the
standard by as much as 10 lbs.
29. FACTORS COMMENTS Weight does not strike the drive cap
concentrically Impact energy is reduced, increasing N-values. Not
using a guide rod Incorrect N-value obtained. Not using a good tip
on the sampling spoon If the tip is damaged and reduces the opening
or increases the end area the N-value can be increased. Use of
drill rods heavier than standard With heavier rods more energy is
absorbed by the rods causing an increase in the blow count.
30. Correlations between spt and soil properties - Relative
Density - Effective Stress Friction Angle - Unconfined Compressive
Strength *Some correlations require the raw N-values whereas others
use the corrected N-values.
31. Relative Density SPT N-Value Relative Density 0-4 Very
loose 25-32 4-10 Loose 27-35 10-30 Medium 30-40 30-50 Dense 35-45
Over 50 Very dense >45
32. Unconfined Compressive Strength Of Cohesive Soils
Consistency Very Soft Soft Medium Stiff Very Stiff Hard SPT N-value
30 qu 400
33. Advantages Relatively quick and simple to perform. Provides
a representative soil sample. Provides useful index of relative
strength and compressibility of the soil. Able to penetrate dense
layers, gravel, and fill. The SPT equipment is rugged, and the test
can be performed in a wide range of soil conditions.
34. Disadvantages The SPT does not typically provide continuous
data, therefore important data such as weak seams may be missed.
Limited applicability to cohesive soils, gravels, cobbles boulders.
Somewhat slower than other sample methods due to sample retrieval.
The greatest disadvantage to SPTs is the lack of reproducibility of
the test results
35. Precautions The drill rods should be of standard
specification and should not be in bent condition. The split spoon
sampler must be in good condition and the cutting shoe must be free
from wear and tear. The drop hammer must be of the right weight and
the fall should be free, frictionless and vertical. The height of
fall must be exactly 750 mm. Any change from this will seriously
affect the N value.
36. Static Cone Penetration Test
37. Static Cone Penetration Test Introduction: Cone Sleeve
Driving force Procedure Interpretation of results
38. Cone (dutch cone) Base area = 10 cm2 Apex angle = 600 To
measure tip resistance Related to un-drained shear strength
39. Cone specifications Diameter = 35.7 < d < 36 (mm)
Cone height = 31 < hc < 31.3 (mm) Cylindrical extension = 4.5
< he < 5.5 (mm)
40. CONE (10 cm2)
41. Cone (15 cm2)
42. Cylindrical sleeve specifications Area = 150 cm2 Height
=13.4 cm To measure frictional resistance
43. Procedure Cone pushed at 10 mm/sec (35 mm) Cone withdrawn
& sleeve pushed on to the cone and driven together.
44. Cone penetrometer
45. HAND OPERATED (30kn)
46. ENGINE OPERATED (200kN)
47. Advantages: Speed Economy Detailed & precise data
48. Disadvantages Soil sample not obtained Depth limited Most
useful in coarser, permeable soils ie: sands
49. Approximate relationship between point of resistance of
cone and penetration number i. Gravels qc=800N to 1000N ii. Sand
qc=500N to 600N iii. Silty sand qc=300N to 400N iv. Silts and
clayey silts qc=200N Where qc is point resistance of cone in
KN/m2
50. Dynamic cone test
51. Dynamic cone test Introduction This test is conducted by
driving the cone by blows of a hammer. The number of blows for
driving the cone through a specified distance is a measure of the
dynamic cone resistance.
52. Characteristics Dynamic cone test is performed by using a
50mm cone without bentotite slurry or by using a 65mm cone with
bentotite slurry. The driving energy is given by a 65kg-hammer
falling through a height of 75cm The number of blows for every 10cm
penetration is recorded. The number of blows for every 30cm
penetration is recorded as dynamic cone resistance.
53. Cross-section of dynamic cone: Holes of 3mm dia are
provided . Distributed for a height of 150mm Sleeve of dia 60mm
dia
54. Dimensional Details(Small scale DCP)
55. Correlation between standard penetration test N-value :
Ncbr=1.5N for upto 3m depth Ncbr=1.75N for depth between 3 to 6m
Ncbr=2.0N for depth grater than 6m Where Ncbr dynamic cone
resistance in KN/m2
56. Correlation between the dynamic cone resistance of 65mm
diameter cone without using bentonite slurry and the SPT number(N):
Ncbr=1.5N for upto 4m depth Ncbr=1.75N for depth between 4 to 9m
Ncbr=2.0N for depth grater than 9m Where Ncbr dynamic cone
resistance in KN/m2
57. Precaution If the skin friction is to be eliminated, the
test is conducted in a cased bore hole. When 65mm cone with
bentonite slurry is used , the set-up should have arrangements for
circulating slurry so that the friction on the driving rod is
eliminated.