Soil Penetration Tests to assess the strength parameters of soil

UNIT I
SITE INVESTIGATION AND SELECTION OF FOUNDATION
Scope and objectives – Methods of exploration –
Auguring and boring – Wash boring and rotary
drilling – Depth and spacing of bore holes – Soil
samples – Representative and undisturbed –
Sampling methods – Split spoon sampler, Thin wall
sampler, Stationary piston sampler – Penetration
tests (SPT and SCPT) – Data interpretation - Strength
parameters and Evaluation of Liquefaction potential -
Selection of foundation based on soil condition- Bore
log report.
1
Jeevanantham V (AP/Civil- RIT RJPM)

Penetrationtests
Penetration
tests
Standard
Penetration Test
Cone
Penetration Test
Jeevanantham V (AP/Civil- RIT RJPM) 2

StandardPenetrationTest
❖ The standard penetration test is carried out in a borehole
❖ All the Penetration tests measure the resistance of the soil
strata to penetration.
❖ Useful empirical correlations are available between
penetration resistance and soil properties for use in
foundation design.
❖ This is the most extensively used penetrometer test and
employs a split-spoon sampler, which consists of a driving
shoe, a split-barrel of circular cross-section which is
longitudinally split into two parts and a coupling.
❖ IS: 2131-1981 gives the standard for carrying out the test.
3

Objective :
❖Commonly used for cohesion less & cohesive soil
❖Used to determine relative density, angle of shearing
resistance and unconfined compressive strength
Procedure:
➢Bore hole is drilled to a certain depth, the drilling tools are
removed and the spilt spoon sampler is attached and lower
to the bottom of hole
➢Sampler is driven by drop hammer of 63.5kg falling from
height of 750mm.
➢The sampler is driven for every 150mm depth and number
of blows is recorded
➢The number of blows for the last two 150mm is noted as
Standard Penetration Number (N)

• The SPT is carried out at every 0.75 m vertical intervals in a
borehole.
• This can be increased to 1.50 m if the depth of borehole is
large.
• Due to the presence of boulders or rocks, it may not be
possible to drive the sampler to a distance of 450 mm.
• In such a case, the N value can be recorded for the first 300
mm penetration.
• The test is stopped (Refusal) if,
1. 50 blows for any 150mm penetration or
2. Total 100 blows for 450mm penetration or
3. 10 successive blows produce no advance.

Split Spoon Sampler

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. The
bottom of the borehole must be properly cleaned before
the test is carried out.

• When a casing is used in borehole, it should be ensured
that the casing is driven just short*** of the level at
which the SPT is to be carried out.
• When the test is carried out in a sandy soil below the
water table, it must be ensured that the water level in
the borehole is always maintained slightly above the
ground water level.
• If the water level in the borehole is lower than the
ground water level, ‘quick' condition may develop in the
soil and very low N values may be recorded.

Correction
• SPT values obtained in the field for sand have to be
corrected before they are used in empirical correlations
and design charts.
• IS: 2131-1981 recommends that the field value of N be
corrected for two effects, namely,
(a) effect of overburden pressure, and
(b) effect of dilatancy.

(a) Correction for overburden pressure
• N value in a granular soil is influenced by the overburden
pressure.
• Since the confining pressure (which is directly
proportional to the overburden pressure) increases with
depth, the N values at shallow depths are
underestimated and the N values at larger depths are
overestimated.
• To allow for this, N values recorded from field tests at
different effective overburden pressures are corrected to
a standard effective overburden pressure of 100kpa

(b) Dilatancy correction
• Applied after overburden correction, exceeds 15 in
saturated fine sands and silts.
• IS: 2131-1981 incorporates the Terzaghi and Peck
recommended dilatancy correction (when > 15)
• >15 is an indication of a dense sand. In such a soil, the
fast rate of application of shear through the blows of a
drop hammer, is likely to induce negative pore water
pressure in a saturated fine sand.
• Consequently, a transient increase in shear resistance will
occur, leading to a SPT value higher than the actual one.

( )
15
2
1
15
15
NR
−
+
=

R
c N
N
if
14

‘N’ Value Correlation to Engineering Properties

Relation Between
angle of internal
friction & N value
16
‘N’ Value Correlation to
Engineering Properties

‘N’ Value Correlation to Engineering Properties

A SPT is conducted in fine sand below water table
and a value of 25 is obtained for N. what is the
corrected value of N?
NR=25,
( )
15
2
1
15
15
NR
−
+
=

R
c N
N
if
( ) 20
15
25
2
1
15 =
−
+
=
c
N
18

Staticconepenetrationtest

Static cone penetration test
• At field SCPT is widely used of recording variation in the
in-situ penetration resistance of soil in cases where in-
situ density is disturbed by boring method & SPT is
unreliable below water table.
• The test is very useful for soft clays, soft silts, medium
sands & fine sands.

Cone and Friction assembly
21
Dutch cone
Apex angle of 600,
Diameter 35.7mm and
Area of 10cm2

Procedure
• After installing the equipment as per IS-4968, part III the
sounding rod is pushed into the soil and the driving
(Gradual push) is operated at the steady rate of 10 to 20
mm/sec approximately so as to advance the cone only by
external loading to the soil strata.
• After recording the cone resistance value, the sleeve is
pushed on to the cone and both are driven together into
the soil and combined resistance is noted.
• The resistance of sleeve is find out by subtracting cone
resistance from the combined resistance.

Typical Test Set up for SCPT

Correction in SCPT test
Other than the external Downward pressure applied self
weight of Cone & Sounding rod exerts additional
pressure on Cone.
CN = (m+n.m1) x 10 kpa
Where,
m = mass of cone (Usually 1.1 Kg)
m1= mass of each sounding roads (Usually 1.5 Kg)
n = No. of rods used

Friction ratio
fr = qs / qc
Where,
fr = Friction ratio
qs =measured site/slip friction
qc = tip resistance/point resistance

Limitations
1. This test is unsuitable for gravelly soil & soil for having
SPT N value greater than 50.
2. Also in dense sand anchorage becomes to cumbersome
& expensive and for such cases Dynamic SPT can be
used.
3. This test is also unsuitable for field operation since
erroneous value obtained due to presence of brick bats,
loose stones etc.

Type of Soil qc (kN/m2)
Gravel 800N to 1000N
Sands 500N to 600N
Silty sands 300N to 400N
Silts and clayey silts 200N
30
‘N’ Value relationship to the qc value

Dynamicconepenetrationtest(DCPT)

Dynamicconepenetrationtest
Dynamic cone penetration test (DCPT)
• The aim is to determine the effort required to force a
point through the soil and so obtain resistance value
which corresponds to the mechanical properties of the
soil.
• The preliminary use is in cohesionless soils when static
penetration test is difficult to perform or dynamic
properties of the soil are of special interest.

DynamicConePenetrationTest
Procedure:
• Test is performed by using a
➢50mm cone without bentonite slurry (or)
➢by using a 65 mm cone with bentonite slurry.
• For the circulation of the bentonite slurry the pumping unit of
the bentonite slurry is properly connected to the guide rod
through flexible tube.
• The standard 600 cone is connected to the drilling rod. The
driving head with the guide rod is connected and properly
fixed on the top of the drilled rods.
• This complete assembly is kept in position with the cone
resting vertically on the ground where the test is to be carried
out.

Procedure:
• The cone is driven by a 65kg hammer falling through a
height of 75 cm.
• The number of blows for every 10cm penetration is
recorded
• The number of blows for 30cm penetration is taken as
dynamic cone resistance
• When a 65mm cone is used, sufficient circulation of the
bentonite slurry is necessary for elimination of the
friction on the rods.
• Sometimes the bentonite slurry is not used when the
investigation is required up to a depth of 6m only.

Correlation with SPT 'N' Value
The resistance NC is correlated quantitatively to the
standard penetration test value (N) by C.B.R.I. Roorkee as

Advantages :
• The test does not need a borehole.
• It can be done quickly to cover a large area economically.
• The test helps to identify variability of subsoil profile and
to locate soft pockets such as filled up ponds.
• When DCPT is carried out close to a few boreholes,
suitable corrections may be obtained for a particular site
• and the number of bore holes can be reduced.

Disadvantages :
• The test is normally not suitable for cohesive soils or very
loose cohesionless soils.
• It is normally not possible to evaluate the mechanical
properties of the soil at great depths when the friction
• along the extension rod is significant.

BoreLog

Atypicalboringlogreport
41

A soil exploration report generally consists of the following
1. Introduction
2. Description of the proposed structure
3. Location and geological condition of the site
4. Methods of exploration
5. Number of borings, their depth and location
6. General description of sub-soil condition as obtained
from the SPT and cone test
7. Details and results of the laboratory test conducted
8. Depth of ground water table and its fluctuations
9. Discussions of the results
10. Recommendation about allowable bearing pressure,
depth and type of foundation
11. Conclusions and limitations of the investigations
43

Thankyou
44

Soil Penetration Tests to assess the strength parameters of soil

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