SOIL INVESTIGATIONS

Topics:
1. General

2. Purpose of Soil Investigation/Subsurface Exploration

3. Subsurface Exploration Program

4. Depth and Spacing of Soil Investigation

5. Method of Boring/Drilling of Bore Holes

CE 333

6. Soil Sampling

7. Standard Penetration Test (SPT)

8. Cone Penetration Test (CPT)

General
The process of identifying the layers of deposits that underlie a proposed
structure and their physical characteristics is generally referred to as soil
investigation/subsurface exploration.

Purpose of soil Investigation/Subsurface Exploration: The purpose

of subsurface exploration is to obtain information that will aid the
geotechnical engineer in –
1. Selecting the type and depth of foundation suitable for a given
structure.
2. Evaluating the load-bearing capacity of the foundation.
3. Estimating the probable settlement of foundation.
4. Determining potential foundation problems (e.g., expansive soil,
collapsible soil, sanitary landfill, and so on).
5. Determining the location of the water table.
6. Predicting the lateral earth pressure on earth retaining structures.
7. Establishing construction methods for changing subsoil conditions.
Subsurface exploration may also be necessary when additions and
alterations to existing structures are contemplated.

Subsurface Exploration Program

Subsurface exploration comprises several steps, including –

1. The collection of preliminary information

2. Reconnaissance Survey
3. Site investigation
• Planning
• Making test boreholes
• Collecting soil samples at desired intervals for subsequent
observation
4. Finally laboratory tests
How deep would be the boring?
The approximate required minimum depth of the borings should be
predetermined. The depth can be changed during the drilling operation,
depending on the subsoil encountered. To determine the approximate
minimum depth of boring, engineers may use the rules established by the
American Society of Civil Engineers (1972):

For shallow foundation:

1. Determine the net increase in the effective stress, Δ 𝜎 ′ , under a
foundation with depth as shown in Figure.
2. Estimate the variation of the vertical effective stress, 𝜎0′ with depth.
3. Determine the depth, D = D1 at which the effective stress increase,
1
Δ𝜎 ′ = q (q = estimated net stress on the foundation).
10
4. Determine the depth, D = D2, at which Δ𝜎 ′ /𝜎0′ = 0.05
5. Choose the smaller of the two depths, D1and D2 just determined as the
approximate minimum depth of boring required, unless bedrock is
encountered.
 Figure: Determination of the minimum depth of boring
If the preceding rules are used, the depths of boring for a building with a
width of 30 m will be approximately the following, according to Sowers
and Sowers (1970):
To determine the boring depth for hospitals and office buildings, Sowers
and Sowers (1970) also used the following rules.

1. For light steel or narrow concrete buildings,

Db
0.7 =a . . . . .(i)
S

Where,
Db = depth of boring
S = number of stories
a = 3 if Db is in meter and 10 if Db in feet

2. For heavy steel or wide concrete buildings,

Db
0.7 = b . . . . .(ii)
S

Where,
b = 6 if Db is in meter and 20 if Db is in feet
According to IS: 1892-1972:

For two adjacent footing, each size B ×L, spaced at clear spacing A –

a. The minimum depth of boring should be 1.5B when A ≥ 4B

b. The minimum depth of boring should be 1.5L when A< 2B

For adjacent rows of such footings –

a. The minimum depth of boring should be 4.5B when A< 2B

b. The minimum depth of boring should be 3.5B when A> 2B
c. The minimum depth of boring should be 1.5B when A ≥ 4B

For deep foundation:

• In case of pile foundation, the depth of bore hole below the tip of
bearing piles kept at least 1.5B.

• In case of friction piles the depth of bore hole is taken 1.5B measured
from lower third point.
• Sometimes, subsoil conditions
require that the foundation load
be transmitted to bedrock. The
minimum depth of core boring
into the bedrock is about 3 m.
If the bedrock is irregular or
weathered, the core borings
may have to be deeper.

• When deep excavations are

anticipated, the depth of boring
should be at least 1.5 times the
depth of excavation.

• In case of road cuts, it is taken at least to the width of the cut and in case
of road fill, the minimum depth of boring is 2m below the ground
surface or equal to the height of the fill, whichever is greater.

• In case of gravity dams, the minimum depth of boring is twice the height
of the dam.
What would be the boring spacing?
There are no hard-and-fast rules for borehole spacing. Table below gives
some general guidelines. Spacing can be increased or decreased, depending
on the condition of the subsoil.
If various soil strata are more or less uniform and predictable, fewer
boreholes are needed than in nonhomogeneous soil strata.

Table: Approximate spacing of boreholes

Type of project Spacing (m)
Multistory building 10–30
One-story industrial plants 20–60
Highways 250–500
Residential subdivision 250–500
Dams and dikes 40–80
General guideline for boring layout:
This particular layout assumes that the shape of the building is rectangle,
but basic pattern could be improvises for non-rectangular buildings by
merely adding rid blocks of M and N dimensions, as required by the shape
of the building.
Boundary Line
Method of Boring/Drilling of Bore Holes
Depending upon the type of soil and the purpose of boring, the following
methods are used for drilling the bore holes.
1. Auger boring
• Auger boring is the simplest method of making exploratory boreholes.
• Hand augers cannot be used for advancing holes to depths exceeding 3 to
5 m.
• Can be used for soil exploration work on some highways and small
structures.
• When power is available, continuous-flight augers are probably the most
common method used for advancing a borehole.
• Boreholes up to about 60 to 70 m can easily be made by this method.
• The soil samples obtained from Auger borings are highly disturbed.
2. Wash boring
• In this method, a casing about 2 to 3 m long is driven into the ground.
• The soil inside the casing is then removed by means of a chopping bit
attached to a drilling rod.
• Water is forced through the drilling rod and exits at a very high velocity
through the holes at the bottom of the chopping bit.
• The water and the chopped soil particles rise in the drill hole and
overflow at the top of the casing.
3. Rotary drilling
• Rotary drilling is a procedure by which rapidly rotating drilling bits
attached to the bottom of drilling rods cut and grind the soil and advance
the borehole.
• Rotary drilling can be used in sand, clay, and rocks.
• Water or drilling mud is forced down the drilling rods to the bits, and the
return flow forces the cuttings to the surface.
• Boreholes with diameters of 50 to 203 mm (2 – 8 inch) can easily be
made by this technique.
• The drilling mud is a slurry of water and bentonite.
• Generally, it is used when the soil that is encountered is likely to cave in.
• When soil samples are needed, the drilling rod is raised and the drilling
bit is replaced by a sampler.
4. Core Boring
• The core drilling method are used for drilling holes and obtaining rock
cores.
• In this method, a core barrel fitted with a drilled bit is fixed to a hollow
drilling road. As the drilling rod is rotted, the bit advances and cutes an
annular hole around and intact core.
• The core is then removed from its bottom and is retained by a core lifter
and brought to the ground surface.
• The core drilling may be done by using a diamond studded bit.

5. Percussion drilling
• Percussion drilling is an alternative method of advancing a borehole,
particularly through hard soil and rock.
• A heavy drilling bit is raised and lowered to chop the hard soil.
• The chopped soil particles are brought up by the circulation of water.
Types of Soils Samples
Soils samples are obtained during subsurface exploration are generally
classified into two categories:
1. Disturbed samples
These are the samples in which the natural structure of the soil gets
disturbed during sampling. However, these samples represent the
composition and mineral content of the soil. Disturbed samples can be used
to determine the index properties of the soil, such as grain size, plasticity
characteristics, specific gravity etc.
The degree of disturbance for a soil sample is usually expressed as –
𝐷02 − 𝐷𝑖2
𝐴𝑅 % = 2 × 100
𝐷𝑖
Where, 𝐴𝑅 = area ratio (ratio of disturbed area to total area of soil)
D0 = Outside diameter of the sampling tube
Di = Inside diameter of the sampling tube

When the area ratio is 10% or less, the sample generally is considered to be
undisturbed.
2. Undisturbed Samples:
These are the samples in which the natural structure of the soil and the
water content are retained.
It should be mention that it is impossible to get truly undisturbed sample.
Some disturbance is inevitable during sampling, even when the outmost
care is taken.
Even the removal of the sample from the ground produces a change in the
stresses and causes disturbances.
Undisturbed samples can be used to determine the engineering properties of
soils, such as compressibility, shear strength, and permeability. Some index
properties such as shrinkage limit can also be determined.

“The engineer should also take into account the ultimate cost of the
structure when making decisions regarding the extent of field exploration.
The exploration cost generally should be 0.1 to 0.5% of the cost of the
structure.”
Standard Penetration Test (SPT)
The standard penetration test is widely used and economical means to
obtain subsurface information. The method has been standardized as ASTM
D1586 since 1958 with periodic revisions to date. The test consists of the
following –
• Driving the standard split-spoon sampler of dimensions shown in Figure
(next slide) a distance of 18 inches into the soil at the bottom of the
boring using a 63.5 kg (140 lb) hammer free falling height of 30 inch
(760 mm).

• Counting the number of blows to drive the sampler the last two 6 inch
distances (total = 12 inch) to obtain the N number.

• SPT blow count (N) = blows for 2nd 6 inch penetration + blows for 3rd 6
inch penetration.

• If 3rd increment cannot be completed, the sum of first two 6 inch

penetrations is recorded as N.

• SPT N value/samples generally are taken at intervals of about 1.5 m.

 Figure: Standard split-spoon sampler.

The boring log shows refusal and the test is halted if –

• 50 blows are required for any 6” (150 mm) increment.

• 100 blows are obtained (to drive the required 12” (300 mm)).

• 10 successive blows produce no advance.

SPT Energy and Hammer Efficiency
The input driving energy and its dissipation around the sampler into the
surrounding soil as the principal factors for the wide range in SPT N values.
It should be marked that the blow count would be directly related to the
driving energy, which is theoretically computed as follows:

Ein = Wh

Where, Ein = input energy, W= weight of hammer and h = height of free fall

The SPT hammer energy efficiency can be expressed as –

actual hammer energy to the sampler

Er or 𝜇𝐻 = × 100
Ein

In the field, the magnitude of Er or 𝜇𝐻 can vary from 30 to 90%. The

standard practice now in the U.S. is to express the N-value to an average
energy ratio of 60 %.
The variation of Er or 𝜇𝐻 given in table below

Conversion of field SPT N to N60(energy efficiency correction)

It is often necessary to convert field SPT N value to an equivalent
standardize N value such as N60. This can be done by following correlation:

Er or 𝜇𝐻
N60 = N ×
60

Where,
N = SPT blow count in the field
N60 = equivalent SPT blow count, if efficiency of SPT device is 60%.
Er or 𝜇𝐻 = Hammer energy efficiency of the field SPT.
SPT Correction Factors (all correction): Following correction should
be made on field SPT N value –
N CN μH μB μS μR
N1(60) =
60
Where,
N = field SPT N value.
μH = hammer efficiency (%)
μB = correction for borehole diameter
μS = sampler correction
μR = correction for rod length
CN = overburden correction, and can be computed as –

2000 0.5
𝐶𝑁 = (𝑝𝑠𝑓) ------ (i)
𝜎𝑧′

95.76 0.5
or, 𝐶𝑁 = (𝑘𝑃𝑎) ------ (ii)
𝜎𝑧′

20
or, 𝐶𝑁 = 0.77𝑙𝑜𝑔10 (𝑡𝑠𝑓) [valid for 𝜎𝑧′ ≥ 0.25 𝑡𝑠𝑓] ------ (iii)
𝜎𝑧′
Variations of μH, μB, μS and μR based on recommendations by Seed et al.
(1985) and Skempton (1986), are summarized in Table below –
Example-1:
Correct the SPT values shown in figure for energy ratio of 60% using a high
efficient Japan type donut hammer in a 2.5 inch diameter boring.
SPT Correlations: The SPT has been used in correlations for –

• Unit weight (γ)

• Relative density (Dr) and Consistency
• Angle of internal friction (φ)
• Undrained compressive strength (Su)
• It has also been used to estimate the bearing capacity of foundations.

“ Relation between the SPT-N values and the internal friction (φ) for
granular soil."

SPT N - value Angle of internal friction (φ)

<4 < 30
4 -10 30 - 35
10 - 30 35 - 40
30 - 50 40 - 45
> 50 > 45
 Relation between the SPT-N values and the unit weight (γ).

SPT N - value Unit weight, γ (pcf)

0-4 70 - 100
4 - 10 90 - 115
10 - 30 110 - 130
30 - 50 110 - 140
>50 130 - 150

Relation between the Corrected Values N1(60) and the Relative

Density of Sands

Corrected Values N1(60) Relative Density (Dr) %

0–5 0–5
5 – 10 5 – 30
10 – 30 30 – 60
30 – 50 60 – 95
Correlation between SPT N values and the undrained compressive
strength (Su).

SPT N- Value Estimated Consistency Su (kPa)

0-2 Very Soft 0 - 25
2-4 Soft 25 - 50
4-8 Medium 50 - 100
8 - 15 Stiff 100 - 200
15 - 30 Very Stiff 200 - 400
>30 Hard >400
Cone Penetration Test (CPT)

• The cone penetration test (CPT), originally known as the Dutch cone
penetration test (developed in Netherlands in 1930s), is a versatile
sounding method that can be used to determine the soil stratigraphy and
estimate their properties.
• The test is also called the static penetration test, and no boreholes are
necessary to perform it. This test has been standardized by ASTM as
D3441.
• Particularly used for soft clays, soft silts, and in fine to medium sand
deposits. The test is not well adapted to gravel deposits or to stiff/hard
cohesive deposits.
• The test consists in pushing a standard 600cone with a base area of 10
cm2 into the ground at a rate of 10 to 20 mm/s and recording the
resistance.
• Data usually recorded are the cone resistance (qc), frictional resistance
(fc) with depth. In addition to that, pore pressures (u), vertical alignment,
temperature and shear wave velocity may also be taken if allowed by the
equipment configuration.
• The cone resistance (qc) is equal to the vertical force applied to the cone,
divided by its horizontally projected area.
• The frictional resistance (fc) is equal to the vertical force applied to the
sleeve, divided by its surface area.
• There are at least five cone types in use –
1. Mechanical cone
2. Electric friction cone
3. Electric piezocone
4. Electric piezocone/friction cone
5. Seismic cone.
 f
e
d
c
b

a
Figure: Various Cone/Penetrometers (bottom to top):
a. Miniature 4 cm2 Electric Cone
b. 10 cm2 Type 2 Piezocone
c. Type 1 (midface) piezocones
d. Type 2 Seismic Cone
e. Hogentogler Dual Type1 & 2 Seismic
f. 15 cm2 Fugro Triple-Element Cone.
Conventional CPT rig
CPT Sounding/Data
Cone Penetration Test (CPT) Correlations
Soil classification chart

frictional resistance (fc)

Friction ratio, Rf = × 100
cone resistance (qc)
Correlation between Dr and qc for normally consolidated sand –
Correlation between Nk and Ip for clay to determine Su –
Correlation between ∅' and qc for un-cemented quartz sand –

Cone tip resistance qc (MPa)

Example:
Classify the soil at the 10 -12 m depth. Also estimate the undrained shear
strength (su) if the average γ = 19.65 kN/m3 for the entire depth of the CPT.
It is known that the profile is entirely in cohesive soil. Also given the cone
resistance, qc= 11 MPa, frictional resistance, fc= 450 kPa and Ip = 10% from
recovered sample. (note: 1 MPa = 10.197 kg/cm2)
Solution:
fc 450
The friction ratio, Rf = × 100 = × 100 = 4 %
qc 11×103
From qc versus Rf chart this is a very stiff fine grained soil.

At the mid depth, (i.e.11m), σ′o = average γ × height = 19.65× 11 = 216 kPa

From Ipversus Nk chart, for Ip = 10 % the value of Nk = 18

qc ;σ′o 11 ×103 ;216

Therefore, the undrained shear strength, su = = = 600
Nk 18
kPa.
CPT versus SPT

CPT advantages over SPT –

• Provides much better resolution, reliability.
• Versatility, can also measure pore pressure and dynamic soil
properties.
• Both theoretical and empirical basis for interpretation of soil
properties, while SPT correlations are empirical.
• Provides continuous test data along the depth, which may be
important for stratified sols.
• Considerable data can be obtained in short time.

CPT disadvantages –
• Not possible to obtain soil sample with conventional CPT
• Will not work in gravelly soil and hard cohesive soil.
• Need to mobilize a special rig.
Bearing Capacity From SPT
Allowable bearing capacity of soils may be calculated from the following
equations proposed by Meyerhof (1956, 1974):

𝑁 𝐷
𝑞𝑎 = × 1 + 0.33 𝑤𝑕𝑒𝑛 [𝐵 ≤ 𝐹4 ] . . . . . . . . .(1)
𝐹1 𝐵
𝑁 𝐵 + 𝐹3 2 𝐷
𝑞𝑎 = × × 1 + 0.33 𝑤𝑕𝑒𝑛 𝐵 > 𝐹4 . . . . . . . . . . (2)
𝐹2 𝐵 𝐵
Where,
𝑞𝑎 = Allowable bearing capacity in kPa, for ∆𝐻= 25mm settlement
𝐷
and 1 + 0.33 ≤ 1.33
𝐵

The F factors are:

′
N55 𝐍𝟕𝟎
F1 0.05 F1 0.04
F2 0.08 F2 0.06
F3 0.3 F3 0.3
F4 1.2 F4 1.2
Meyerhof Method for determining qp and qf in Sand for deep foundation:

D
Theoretical unit tip-bearing capacity for driven piles in sand, when > 10
B
qp = 4Nc (ton/ft ) 2

Theoretical unit tip-bearing capacity for drilled piles in sand:

qp = 1.2Nc (ton/ft 2 )
Theoretical unit friction-bearing capacity for driven piles in sand:
N
q𝑓 = (ton/ft 2 )
50
Theoretical unit friction-bearing capacity for drilled piles in sand:
N
qf = (ton/ft 2 )
100
Where:
D = pile embedment depth, ft
B = pile diameter, ft
N = N-Value from SPT test
20
Nc = 0.77 log ′
σz
σ′z = effective overburden stress at pile embedment depth, (tons/ft2)
Parry (1977) proposed computing the allowable bearing capacity for
cohesionless soils as-
𝑞𝑎 = 30𝑁55 . . . . . . . . . . . . (3)

[Where, N55 is the average SPT value at a depth about 0.75B below the
proposed base of the footing]

and, the allowable bearing pressure is computed for settlement (∆𝐻 =

20mm) checking as –
𝑁55
𝑞𝑎 = 𝑘𝑃𝑎 . . . . . . . . . . . . (4)
15𝐵 (𝑖𝑛 𝑚)

[Note: use the smaller of the computed values from 3 & 4 for design]
Example:
Compute the allowable beading capacity of a footing. Use the following
information given below:
Footing width, B = 3.0 m
Footing depth, D = 1.0 m
′
Normalized SPT 𝑁70 = 24
Use Meyerhof’s equations.

Solution:
From F factors –
F2 = 0.06
F3 = 0.3

As B > F4, from equation (2);

𝑁 𝐵 + 𝐹3 2 𝐷
𝑞𝑎 = × × 1 + 0.33
𝐹2 𝐵 𝐵
24 3:0.3 2 0.33×1
= × × (1 + )
0.06 3 3

= 537 kPa
Bearing Capacity From CPT
The bearing capacity factors for use in the Terzaghi bearing-capacity
equation can be estimated as –

0.8 Nq = 0.8 Nγ ≅ qc

Where,
qc is averaged over the depth interval from about B/2 above to 1.1B below
the footing base. This approximation should be applicable for D/B < 1.5.

For cohesionless soils one may use –

Strip: qult = 28 - 0.0052(300 - qc)1.5 (kg/cm2)
Square: qult = 48 - 0.009(300 - qc)1.5 (kg/cm2)

For clay one may use –

Strip: qult = 2 + 0.28qc (kg/cm2)
Square: qult = 5 + 0.34qc (kg/cm2)