Chapter 2: Natural Soil Deposits and Subsoil Exploration
Foundation Engineering
ECIV 4352
Lecture No. (2)
Natural Soil DepositsSubsoil 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.
Subsurface Exploration Program:
Chart 1, shown below, illustrates the steps for subsurface exploration:
Chart 1: Subsurface Exploration Steps
The following section shows how to estimate the required depth of the boreholes.
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.
- D1is the depth of bore hole at which:
in case of rectangular foundation.
: Increase in stress
For Circular Foundation of diameter (S):
- D2 is the depth of bore hole at which:
: Increase in stress
: 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:
1. عند عمل حفر استكشاف يتم حساب عمق الحفر من سطح الأرض الطبيعية و ليس من بطن القاعدة، ولذلك بعد حساب المسافة D لعمق حفرة الاستكشاف، نضيف لها عمق الحفر بين بطن القاعدة و سطح الأرض الطبيعية Z فيكون عمق حفرة الاستكشاف هو (Z+D).
Figure 4 Depth of bore hole for buried body
2. عند وجود طبقة صخرية في طبقات التربة التي نجري عليها الاستكشاف، فإن الحفر يتوقف عند هذه الطبقة، و لذلك لو افترضنا أننا عند حساب عمق حفرة الاستكشاف كانت D1 = 80m, D2 = 112m and Rock at 60m، فبالتالي عمق حفرة الاستكشاف سيكون الأصغر من القيم الثلاثة و ليس الأصغر بين D1 , D2 ، أي أن عمق الحفرة سيكون 60m.
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.
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/m2what 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).
- D1 is the depth at which:
Figure 6: Example 1
- D2 is the depth of bore hole at which:
By trial and error; D2 = 101.4m.
As final result, D is minimum of {D1=180m, D2 = 101.4m, 130m}, so D=101.4m.
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 (مضطر) to take additional number of soil samples.
The degree of disturbance can be expressed from the following relation:
: Area ratio, if it was less than or equal 10% the sample can be assumed undisturbed.
: Outer diameter of sampling tube.
: Inner diameter of sampling tube.
3-Correlations from Standard Penetration Test (SPT):
لأخذ عينات من التربة نقوم بعمل غرز فيها بواسطة معدات معينة، وبالإضافة لأخذ العينات يمكن الحصول على معلومات تفيد في إيجاد بعض العلاقات الهامة في خصائص التربة، ومن تجارب الغرز للتربةSPT.
الفكرة تتمثل في وجود قضيب دائري قطره الخارجي 50 ملم و الداخلي 35 ملم نغرزه مسافة 150 ملم في التربة، بعد ذلك نحسب عدد الضربات التي تمكننا من اختراق 300 ملم بعد ال 150 ملم الأولى و عدد الضربات يسمى:
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):
- Calculate the Over Consolidation Ratio (OCR):
1 KN/m2→
- Calculate the soil friction angle (:
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
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:
: Total vertical stress
: 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).
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).
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