Foundation Engineering ECIV 4352
Chapter 2: Natural Soil Deposits and Subsoil Exploration
Foundation Engineering
ECIV 4352
Lecture No. (2)
Natural Soil Deposits Subsoil Exploration
The main topics that will be discussed in this lecture are:
1- Subsurface Exploration: Definition, Purpose and Process.
2- Soil Sampling.
3- Correlations from Standard Penetration Test (SPT).
4- Correlations from Cone Penetration Test (CPT).
1- Subsoil Exploration: Definition, Purpose and Process:
What is Subsurface Exploration?
It is the process of identifying the layers of deposits that underlie a proposed structure.
What are the Subsurface Purposes?





Evaluate the soil Bearing capacity and Select the suitable foundation type
for given structure.
Determine the depth of bedrock layer.
Determine the depth of water table.
Establish the expected settlement for the foundation.
Obtaining samples to prepare some tests.
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Foundation Engineering ECIV 4352
Chapter 2: Natural Soil Deposits and Subsoil Exploration
Subsurface Exploration Program:
Chart 1, shown below, illustrates the steps for subsurface exploration:
Subsurface Exploration
Collecting of Preliminary
information
Collect data related to
the type of structure to
be built
Reconnaissance
Make Visual inspection
to the site to obtain
information about:
1- Site Topography.
2- Ground Water Levels.
3- Type of vegetation at
the site.
etc …
Site Investigation
Planning, Making test
boreholes and collecting
Soil Samples at desired
intervals for laboratory
tests.
Chart 1: Subsurface Exploration Steps
The following section shows how to estimate the required depth of the boreholes.
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Foundation Engineering ECIV 4352
Chapter 2: Natural Soil Deposits and Subsoil Exploration
According to ASCE (1972) the depth of bore hole can be estimated as follow:
Figure 1 Distribution of stresses
The depth of bore hole is D, which is the minimum of depths D1 and D2.
 D1 is the depth of bore hole at which:
1
 @ D1  q
10
P
q
in case of rectangular foundation.
L B
P
 : Increase in stress   
B  D1 L  D1 
Note
For Circular Foundation of diameter (S):
P
q
 2
S
4
P
 

S  D1 2
4
Figure 2 Depth of bore hole D1
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Foundation Engineering ECIV 4352
Chapter 2: Natural Soil Deposits and Subsoil Exploration
 D2 is the depth of bore hole at which:
 @ D2  0.05 v'
 : Increase in stress   
P
B  D1 L  D1 
 v' : Effective vertical stress at depth D2.
Figure 3 Depth of bore hole D2
In this type there will be cubic equation which requires trial and error method to obtain
adequate roots.
Note:
‫ ولذلك بعد‬،‫ عند عمل حفر استكشاف يتم حساب عمق الحفر من سطح األرض الطبيعية و ليس من بطن القاعدة‬.1
Z ‫ نضيف لها عمق الحفر بين بطن القاعدة و سطح األرض الطبيعية‬،‫ لعمق حفرة االستكشاف‬D ‫حساب المسافة‬
.)Z+D( ‫فيكون عمق حفرة االستكشاف هو‬
Figure 4 Depth of bore hole for buried body
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Foundation Engineering ECIV 4352
Chapter 2: Natural Soil Deposits and Subsoil Exploration
‫ و‬،‫ فإن الحفر يتوقف عند هذه الطبقة‬،‫ عند وجود طبقة صخرية في طبقات التربة التي نجري عليها االستكشاف‬.2
،D1 = 80m, D2 = 112m and Rock at 60m ‫لذلك لو افترضنا أننا عند حساب عمق حفرة االستكشاف كانت‬
‫ أي أن عمق الحفرة‬، D1 , D2 ‫فبالتالي عمق حفرة االستكشاف سيكون األصغر من القيم الثالثة و ليس األصغر بين‬
.60m ‫سيكون‬
Practical data:
 Depth of bore hole (6 – 10) m is sufficient for non clayey soil.
 For small projects with uniform soil only 3 bores are sufficient.
 For buildings on uniform soil, a bore hole every 60m is adequate. For errant soil
it is preferred to use it every 15m.
 For highway on uniform soil, a bore hole every 300 m is adequate. For errant soil
it is preferred to use it every 30m.
In General we have to follow the requirements of the Client when preparing such holes.
The paragraph shown below is obtained from UNDP request for proposal in Gaza for
designing some housing units. The UNDP as Client requires the consultants to make
subsurface exploration by specified properties:
The Consultant have to conduct at least one bore hole for each building
(Buildings of residential units, schools, mosque, etc.). and three boreholes
for detached buildings. The depth of bore hole will be minimum 15m.
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Foundation Engineering ECIV 4352
Chapter 2: Natural Soil Deposits and Subsoil Exploration
Example 1)
Site investigation is to be made for a structure of 100m length and 70m width. The soil
profile is shown below, if the structure is subjected to 200 KN/m2 what is the
approximate depth of bore hole (Assume w=10KN/m3).
Figure 5: Example 1
Solution
The depth of bore hole is D which is the minimum of (D1 and D2).
1
 D1 is the depth at which:  @ D1  q
10
Figure 6: Example 1
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Foundation Engineering ECIV 4352
Chapter 2: Natural Soil Deposits and Subsoil Exploration
q
P
200  100  70

 200 KN / m 2 .
L B
100  70
  
P
B  D1 L  D1 

200  100  70
1.4  10 6

70  D1   100  D1  7000  170D1  D12
1
1.4  10 6
1
 @ D1  q 
  200  D12  170 D1  63000  0.00
2
10
7000  170 D1  D1 10
 170  170 2  4  1  63000
 180m.
2 1
 D2 is the depth of bore hole at which:
D1 
 @ D2  0.05 v'
 v'   '   w  D2   v'  18  10D2  8D2 .
200  100  70
1.4  10 6
  


B  D2 L  D2  70  D2   100  D2  7000  170 D2  D22
P
1.4  10 6
 0.05  8D2  D23  170 D22  7000 D2  3.5  10 6
7000  170 D2  D22
By trial and error; D2 = 101.4m.
As final result, D is minimum of {D1=180m, D2 = 101.4m, 130m}, so D=101.4m.
 @ D2  0.05 v' 
2- Soil Sampling:
There are two types of soil samples:
 Disturbed samples (‫)عينات مفككة‬: These types of samples may be used for some
types of soil tests as sieve analysis, water content and Attreberge limits.
The major equipment used to obtain disturbed samples is “Split spoon” which is a steel
tube has inner diameter of 34.93mm and outer diameter of 50.8mm
 Undisturbed samples (‫)عينات متماسكة‬: These types of samples may be used for
some types of soil tests as shear strength tests and permeability.
The major equipment used to obtain disturbed samples is “thin wall tube”
In general as we obtain soil samples, there will be degree of disturbance, if it was so high
we will be compelled (‫ )مضطر‬to take additional number of soil samples.
The degree of disturbance can be expressed from the following relation:
Do2  Di2
AR % 
 100
Di2
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Foundation Engineering ECIV 4352
Chapter 2: Natural Soil Deposits and Subsoil Exploration
AR % : Area ratio, if it was less than or equal 10% the sample can be assumed
undisturbed.
Do : Outer diameter of sampling tube.
Di : Inner diameter of sampling tube.
3- Correlations from Standard Penetration Test (SPT):
‫ وباإلضافة ألخذ العينات يمكن الحصول على‬،‫ألخذ عينات من التربة نقوم بعمل غرز فيها بواسطة معدات معينة‬
. SPT‫ ومن تجارب الغرز للتربة‬،‫معلومات تفيد في إيجاد بعض العالقات الهامة في خصائص التربة‬
،‫ ملم في التربة‬150 ‫ ملم نغرزه مسافة‬35 ‫ ملم و الداخلي‬50 ‫الفكرة تتمثل في وجود قضيب دائري قطره الخارجي‬
:‫ ملم األولى و عدد الضربات يسمى‬150 ‫ ملم بعد ال‬300 ‫بعد ذلك نحسب عدد الضربات التي تمكننا من اختراق‬
Standard penetration number (N)
Standard penetration number (N) is used for calculating some parameters such as:
C: Cohesion.
: Angle of friction.
OCR: Over Consolidation Ratio
 Find the undrained shear strength for clay (Cu):
Cu  KN
K : Cons tan t 3.5  6.5KN / m 2

Calculate the Over Consolidation Ratio (OCR):
0.689
N
OCR  0.193 ' 
  v' ( MPa)

 v
1 KN/m2 → 1  10 3 MPa
 Calculate the soil friction angle (:
  20 N cor  20
N cor  C N  N
1
 v'

  v' UStons / ft 2

CN 
9.78
1

'
v
 ( KN / m 2 )
2000
1UStons / ft 2 
 Ib / ft 2
2204.6
1Metric tons / ft 2 
 Ib / ft 2
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Foundation Engineering ECIV 4352
Chapter 2: Natural Soil Deposits and Subsoil Exploration
Example 2)
For the following soil profile find the OCR at point A
(Assume w=10KN/m3). N = 4
Figure 7: Soil profile Example 2
Solution
N
OCR  0.193 '
v



0.689
  v' ( MPa)
 v'  1.5  17  1.518.5  10  316.9  10  58.95KN / m 2  0.05895MPa.
4


OCR  0.193

 0.05895 
0.689
 3.66
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Foundation Engineering ECIV 4352
Chapter 2: Natural Soil Deposits and Subsoil Exploration
4- Correlations from Cone Penetration Test (CPT):
A cone of 60o vertex angle and 10 cm2 base area is pushed into the ground at 20 mm/sec
rate, and then we measure the cone resistance (qc) that is useful to find some soil
parameters such as:

q
  tan 1 0.1  0.38 log  c'
v

 q 
OCR  0.37 c ' v
 v






 v : Total vertical stress
 v' : Effective vertical stress
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Foundation engineering
Chapter 2: Subsoil Exploration
Home work
Student Name: -------------------------------------------Student Number: ----------------------------------------Q.1) a foundation investigation is to be made for a 40m diameter and 20 m height of
water tank, to be built on a thick layer. The soil profile is shown below; estimate the
depth of bore hole considering only the weight of water in the tank.
(Assume w = 10KN/m3).
 sat  17 KN / m3
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Q.2) Site investigation is to be made for 2500 KN load carried on (3.0 m x 2.0 m) footing.
The foundation will be built on layered soil as shown in the figure below, estimate the
depth of bore hole. (Assume w = 10KN/m3).
Dry Sand
 d  17 KN / m3
Sand  sat  18.5KN / m3
Clay  sat  16.9 KN / m3
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