2018

Foundation Engineering
Course I
For
Fourth Year of Civil Eng.

2018
Prepared
By
Dr. Mahmood Rashid Mahmood

UNIVERSITY OF TECHNOLOGY CIVIL ENG. DEPT.

Foundation Engineering” Ch1 Site Investigations

C.E. 4244 Foundation Engineering (1) Theory: 2 hrs./ Week

Tutorial: 2 hr./ Week

Subject Time/Hrs
1- Soil investigation: -------------------------------- 4

Determination of spacing, No. of bore holes, depth of bore holes, type

and methods of drilling, sampling and samples, in situ tests, geophysical
exploration, report writing

2- Bearing capacity of Shallow foundation: --------------------- 10

Types of shear failure, Determination of ultimate bearing capacity of soil,

eccentrically loaded foundations, bearing capacity of footing on layered
soils, bearing capacity of footing on slopes, determination of bearing
capacity from field tests.

3- Settlements of shallow foundations: --------------------------- 6

Immediate or elastic settlements, consolidation settlements, secondary

settlements, prediction of settlement for cohesionless soils, elastic
settlements of eccentrically loaded foundations, allowable settlements.

4- Determination of Footing Dimensions: ------------------------- 2

Spared , Combined Rectangular and Trapezoidal and Raft foundations

5- Structural Design of Foundations: ------------------------------------ 6

Separated Footings, Combined Foundations, Rectangular Foundations,

Trapezoidal Foundations, Strap foundations and Raft Foundations.

6- Foundations on Difficult Soils: ------------------------------------ 2

Collapsing soils and expansive soils

Text Book :
Principles of Foundation Eng. By Braja M. Das 7th Ed.2011
References:

1- Foundation Analysis and Design 5th Edition By: Joseph E.Bowles

2- Basic of foundation Design By Bengt H. Fellenius 2006

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Foundation Engineering” Ch1 Site Investigations

Chapter One
Site Investigations
The aim of soil investigations:
The aim of soil investigations that take place before starting the
implementation of any structure is to achieve harmony between the
structure and nature of soil in which structure will be built on. Soil
investigation providing the required information for the nature of the soil
and engineering properties of soil, chemical properties of soil and find a
scientific engineering solutions for the design of foundations for various
structures and solving problems that may arise and hinder build of
structure and ensure include the following:

1. Safety of Structure:
Doing the soil investigations for any structure is to ensure safety
from the cracks, crevices, rapid or irregular settlement and sometimes
even from collapse. Soil investigations provide sufficient information for
the soil which helps on specify the types, depth of appropriate
foundations consistent with the purpose for which it created for.

2. The economy in the cost:

Knowing the properties of different soils offers a choice to
determine the most suitable types and depths of foundations for the
structure, as well as the easiest ways to implement these foundations
which can saving in construction materials then reducing the cost. The
lack of knowledge of soil properties causing that the designer engineer
making sometimes exaggerated design for foundation, to take precautions
in the design of footings which increase the cost of structure and the
actual needed for it.

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Foundation Engineering” Ch1 Site Investigations

3. Durability of Structure:
Soil investigation includes taking the necessary precautions to
avoid the influence of change the different soil properties with time on
the foundations set on such as high and low groundwater levels and
differences in the concentration of salts in the soil and its impact on
concrete foundations and other problems that affect the sustainability of
structure performance.
In general it is necessary to do the soil investigations for all
structures in order to ensure that the three factors above are achieved
whatever the structure was small or light.

Information from the soil investigations:

The soil investigation could give the following in formations:
Firstly: For new construction sites:
1. choosing the type and depth of the foundation to be used.
2. Estimate the appropriate soil for the project or the proposed works.
3. Determine the thickness and the extension of each soil layers within a
certain depth below the soil surface.
4. Getting the disturbed and undisturbed samples representsthe soil layers
to classify and identify them and use it in laboratory tests to determine the
engineering properties of the soil.
5. Calculate the bearing capacity of the soil under the foundation of
structure which carrying the applied loads.
6. Estimate the expected settlement of the foundations of structure.
7. Determine of groundwater level and seasonal change.
8. Determine the type of soil and soil’s problem on the foundations such
as gypsum soils, swelling soils and soft soils.
9. Determine the lateral earth pressure on retaining walls, shelters and
basements.

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Foundation Engineering” Ch1 Site Investigations

Secondly: For pre-built sites (Existing Sites):

1. Safety knowledge of the structure.
2. Knowing the expected settlement and the proportion of excessive
settlement.
3. Comparing the calculated settlement with the actual settlement.

Thirdly: For roads and airports:

1. Selection of the correct location of roads and airport runways.
2. Choose the best materials used as soil fill material.
3. Design and choose the best locations for crossings and drainage
channels.
4. Design the roads and choose the best quality of the pavement.
5. Find solutions for soil treatment and improve it in preparation for the
implementation of the roads

Planning of Exploration:
The soil investigations program generally can pass through the
following stages:

1. Reconnaissance Phase:
A. Getting the available information about the nature of the soil for the
site to be implemented, such as:
1. Topographic maps, geological and contour to know terrain type of soil
and geological formations in the region.
2. Information about the rivers and drainage of water.
3. Weather Information such as maximum and minimum temperature
degrees and humidity.
4. Soil investigation reports for sites built close to the region.
5. Information about the service lines in the area.

B. Site visit for the purpose of the following:

1. The topography of the region.
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Foundation Engineering” Ch1 Site Investigations

2. Get an idea about the nature of the soil surface.

3. Notice the buildings built near the project area.
4. Notice the soil layers appear from any previous excavation works.
5. Notice the highest level of water marks on the walls of the old
buildings and the piers of the bridges.
6. Filming of site location.
7. Getting general information such as services available for staff
working like accommodation, food, shops, water supply, electricity….etc.

C. Information about Construction design:

to determine the local soil investigation requirements we must know the
construction design accurately such as:
1. Buildings:
a) the size and height of buildings and the depth of shelters and
basements.
b) The distribution and arrangement of columns and walls.
c) Applied Loads on columns and walls.
d) The structure type of buildings (steel or concrete).
e) The purpose of the site construction.
f) Materials used in the construction of the exterior walls.

2. Roads and Bridges

a) Type and length of the bridge and bridge sectional.
b) Horizontal and vertical loads applied on piers in the water and on the
approach slabs of the bridge.

2. Preliminary investigation stage

Includes making number of boreholes to a specific depth to get
the following:
A. Depth and extent and composition of soil layers.
B. Groundwater level.
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Foundation Engineering” Ch1 Site Investigations

C. Disturbed and undisturbed samples to determine the engineering

properties of the soil.
D. initial determination for the type of foundations and preliminary cost
estimation.

3. Detailed investigation phase, which includes the following:

A. Determine the geological structure of the site.
B. Determine the groundwater situations.
C. Conduct field tests to determine the mechanical properties of the soil.

Determine Number, Spacing and Depth of Borings:

1. Determine the number of boreholes:

Table (1) can be adopted as a recommended guide to determine the
number of borings:

Table (1) Recommended Guide for number of Borings

Phase of Investigation Geological No. and Spacing Location of Boring

Structure of Boring in the Field
Preliminary Investigation Uniform 5 to 10 boring per Depends on
( to assess the suitability of km2 topography of the
site) Irregular or 10 to 30 borings site
Unknown per km2
General Investigation Uniform 300 x 300 m Regular square
(Selection of area of Most Irregular or 100 x 100 m network of borings
favorable ground) Unknown parallel to contour
Detailed Investigation (for Uniform At least 3 borings As regular as
individual building where (10 to 30 m apart) possible network
location has been fixed) Irregular or 3 to 5 borings for to suit individual
Unknown each building buildings taking
(10-30 m into consideration
diagonal) preliminary
investigation

2. Spacing between Boreholes:

The Spacing cannot be determined with absolute exactness, they depend
upon:
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Foundation Engineering” Ch1 Site Investigations

1. Nature and condition of soil.

a. If the soil conditions are of well-known stratification (simple
and thick layers) → widely spaced borings are sufficient.
b. If the soil conditions vary appreciably over site (thin layer)
then closely spaced borings are required.
c. If the soil conditions are uniform → moderate investigation
is justified.
2. The shape and extent of building (10-20m apart).
3. Importance of the project (cost of boring).

The Rules can be followed:

1. For individual buildings of less than 300 m2 plan area, 3 boreholes

are the minimum (not to be on a straight line).
2. For large sites or group of buildings, 5 boreholes are the minimum
(4 at corners and 1 at the middle).
3. As a guideline you may use Table (1).
4. For large site: probes are needed (penetration test, seismic method,
electrical resistivity method) to obtain information in areas
between boreholes.
5. In case of limestone rock (from geological information) use seismic
method between boreholes to check any cavities.
6. For some special structures
a. Retaining Walls:
Minimum spacing 120 m at centerline with some of these
B.H. located at both sides of the centerline.
b. Slope Stability Problems :
3 to 4 B.H. at critical zones and at least one B.H outside the
zone.

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Foundation Engineering” Ch1 Site Investigations

The following table can be used as a general guideline

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

3. Depth of Boreholes:
It can be developed following the general principles for the purpose
of determining the depths of boreholes.
a. The continuation of the drilling process in soft and burial layers of soil
until reaching the layer of acceptable strength.
b. Drilling can be stop when reaching the rock layers except in the case of
large loads for very large projects which drilling shall continue to a depth
of three meters to check the thickness of this layer unless there is
geological information that is constantly these rock layers.
c. the depth of drilling can be determined from the first borehole
especially in homogeneous soil regions.
d. In the dam reservoirs, drilling continues until reaching to the

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Foundation Engineering” Ch1 Site Investigations

impervious layer and the depth as much as twice the highest level of
water.
e. Drilling for roads and runways shall be at a depth of 5 meters if the
road was with the soil surface level and the depth will be twice the height
of the embankment on which the road set on, taking into consideration
that the drilling shall penetrate the clay layers, soft silt layers or soil
layers containing organic materials.
f. Drilling depth is twice the width of the footing for normal buildings i.e.
until the arrival of stress to this depth due to loads applied on these
footings.
g. Drilling depth is twice the height of the retaining wall or piles
supporting the walls.
h. A drilling depth is twice the width of the embankment base.

The Following points can be followed step by step to determine

borehole depths
1. Highway and airfields: minimum depth of borings is 3m but should
extend below organic soil, muck, artificial fill or compressible
layers such as soft clays.
2. Retaining Walls and slope stability problems:
a. Below organic soil, muck, artificial fill or compressible layers.
b. Deeper than possible surface of sliding.
c. Deeper than the width of the base of wall (increase of retaining
wall).
d. Equal to the width at bottom of cuts.
3. Structural Foundation: depends upon soil profile and the type of
feasible foundation
a. Below organic soil, muck, artificial fill or compressible layers.
b. Single separate narrow strip footings:
 Depth = 3 x width of footing >6m
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Foundation Engineering” Ch1 Site Investigations

c. Group of overlapping footings or raft.

 Depth = 1.5 x least width of the group or raft.
d. For heavy structures the depths of one of the boreholes should
extend to 2 x width of footing (Heavy loads >20 T/m2=200
kPa).
e. The depth of boreholes should extend to the point where the net
increase in stress due to the action of the load of the building is
less than 10% of the total surface load.
f. The depth of the borehole should extend to the point where the
net increase in stress due to building (Δqs) is less than 5% of the
overburden stress in soil.
g. For pile foundation, depth of boring should extend to the
bearing strata + (3 x pile diameter).

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Foundation Engineering” Ch1 Site Investigations

Example 1:

Four storey building (20mx30m) with a basement (depth= 4m below

ground surface) is proposed. The net pressure (Δ qs) of the building at the
basement level is 75 kPa. The soil is silty clay with a dry and submerged
unit weight equal to 16 kN/m3 and 9 kN/m3 respectively. The water table
was found at elevation 1 m below ground surface. Determine for a detailed
soil investigation the number, layout and depth of the boreholes.

Solution:

1. No. of B.H : Area = 30 x 20 = 600 m2 > 300 m2 → use 5

boreholes.
2. Layout : 4 at corners and one at the center.
3. Depth : Using the criteria in Page
a. No information.
b. Not Valid.
c. Depth (Z) = 1.5 x B = 1.5 x 20 = 30 m
d. One of the boreholes should extend to 40 m (= 2 x B) {Not
necessary}.
e. Assuming a 2:1 distribution
Q = Δ qs x B x L = q1 (L+Z)(B+Z)
For q1 = 0.1 Δ qs
Δ qs x 20 x 30 = 0.1 Δ qs (30 + Z ) (20 + Z)
Solving to get Z = 52.6 m
f. Effective stress at depth z (Po) = 1 x 16 + 9 x (Z+3) = 43 + 9Z
0.05 x Po' = q1 {where q1 is defined in item (e)}
0.05 x (43 + 9Z)=75 x 20 x 30 /((30 + Z) x (20 + Z))
Solving to get Z= 29.4 m which is the same as (c )
g. Not valid ( No Piles).

Therefore the final depth of boring = 30 m + 4 m = 34 m {choose

minimum of (e) and (f)}{4 m is the depth of basement}.

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Foundation Engineering” Ch1 Site Investigations

Example – 2

For the Building layout shown, find:

a. No. of Boring
b. Depth of boreholes

Given: Col Size 0.5x0.5m

Col. No. Load (kN)

C1 1500

C2 2500

Use Depth of Footing (1 m)

Note: Neglect weight of footing

Solution:

a) Area of Building = 8.5 * 8.5 = 72.25 m2 < 300 m2

Use 3 boreholes

b) Δ qs = PTotal/ Area = (5 * C1 + 4 * C2) / A = (5 * 1500 + 4 * 2500) /

(8.5 * 8.5) , Δ qs= 242.21 kN/m2 = 24.2 T/m2 > 20 T/m2
So, it is a heavy structure, then to calculate the depth of boreholes
below the footing base according the following:
1. Δ qs> 20 T/m2 → Depth = 2 * B = 2 * 8.5 = 17 m → Z = 17 m
2. (Δqs x B x L)/ (L+Z)(B+Z)= 0.1 Δqs
(8.5 * 8.5) / (8.5 + Z)(8.5 + Z) = 0.1 →Z = 18.4 m
3. ( Δ qs x B x L)/ (L+Z)(B+Z)= 0.05 (∑ ɣ’Z)
(242.21 * 8.5 * 8.5) / (8.5 + Z)(8.5 + Z) = 0.05 [1.5 * 17 + 1.5 * 19 + (Z –
2) * (19 –10)]
(242.21 * 8.5 * 8.5) / (8.5 + Z)(8.5 + Z) = 1.8 + 0.45 Z
Z = 27.26m
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Foundation Engineering” Ch1 Site Investigations

use min value of Z = 17.0m

Then the total depth of borehole is = 17.0m + 1m = 18.0m below N.G.L

Types and Method of Borings:

There is many methods for boring like:
Trial Pits:
 Simple excavation using ordinary tools (Shovels).
 Simple and reliable for stratification and types.
 Maximum depth 4-5 meter, suitable for exploration of
shallow only
1. Hand and Portable Augers:
 Used to depth around 6 m and very cheap.
 Not recommended for gravelly soils, very stiff soils and soils
which the borehole collapses when boring without casing
(such as sandy soils under water table).
 Types:
 Posthole or Iwan auger (Dia. up to 200mm).
 Small helical auger (Dia. 50mm).
2. Wash Boring:
 Water is pumped through a string of hollow boring rods and is
released under pressure through narrow holes in a chisel attached
to the lower end of the rods.
 The soil is loosened and broken up by the water jets and the up
and down movement of the chisel.
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Foundation Engineering” Ch1 Site Investigations

 Used for most types of soils (slow for gravel).

 Common method for advancing test holes.
3. Percussion Boring:
 A method of forming a hole using a “bailer”, which is lifted,
rotated slightly and dropped onto the bottom of the hole. Water
is circulated to bring the soil cutting to the ground surface.
Casing is required as well as a pump to circulate the water.
 Widely used in England and Iraq.
4. Rotary Drilling:
 Method of forming a hole by rotating an auger.
 If undisturbed samples are required then casing is used.
 Types:
a. Short flight augers
b. Continuous flight auger.
c. Continuous flight auger with hollow stem.
d. Bucket auger.

Helical Auger
Posthole or
Iwan Auger
Hand Tools
Augers

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Layout for Small-Scale Rotary Core

Drilling

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Foundation Engineering” Ch1 Site Investigations

Wash boring Rig

Drilling rig and hollow-stem auger

system

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Foundation Engineering” Ch1 Site Investigations

Sampling and Samplers:

A. Types of Samples:
1. Disturbed Samples
 Taken during boring in Plastic bags.
 Used mainly for classification purposes.
2. Undisturbed Samples:
 Special types of tubes are used such that the structure of the
grains is approximately the same as that in the site.
 Very difficult to obtain for various reasons which will be
discussed in article (C).
 Used to determine the mechanical properties of soil.
I. Shear Strength (Cu).
II. Consolidation characteristics (Cc, Cv)
III. Permeability (k).
IV. Stress strain relationship.
3. Remolded Samples:
 Disturbed samples compacted in special molds.
 For research purposes.
B. Types of Samplers

Many types of samplers were used to collect undisturbed soil

samples such as Shelby Tube of thin wall with a lower area ratio, its
diameter ranged between 50-450mm with a solid sharp adage for easy
penetrate the soil. This type used to collect soil samples of soft to medium
cohesive soils only.
The other type is a Pitcher Sampler which used in stiff and hard
cohesive soils and sand stone and soft rock.
Another type of thick wall samplers with a high area ratio which
cause high disturbance during sampling such as Split Spoon Sampler

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Foundation Engineering” Ch1 Site Investigations

which used in most soil types except the soil have big stones and
boulders. Another type of samplers is a Core Cutter Sampler which
used in rock layers and hard soils. With respect to the sandy soil special
sampler called Bishop Samplers which have one way valve used to
obtain undisturbed samples of cohesionless soils.
After extracting the sampler from the borehole, the soil removed up
to 50mm from the two ends and sealed with a paraffin wax and labeled
with information included the location name, borehole number and the
depth of boring, then transported to the soil laboratories by special
wooden boxes to reduce vibration effect.

Shelby Tube Sampler

Bucket Auger

Shelby Tube Sampler

Split Spoon Sampler

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Foundation Engineering” Ch1 Site Investigations

Requirements for obtaining non-confusing samples:

There are several types of mechanical disturbance occurs on the

samples during extraction, the most important of these types are:

1. Stress relief

The extraction of the sample from the mass of the surrounding soil
leads to a change in the stresses that were affected by the original, as the
total vertical stress and total horizontal stress on the sample originally
different amounts of each other and after the sample is extracted from the
mass of surrounding soil the total vertical and horizontal are vanished. This
is called stress relief, which affects the structure of the soil and causes some
disturbance of the sample.

2. Area Ratio (Ar) Area Ratio:

When soil sampling is carried out, part of the soil is removed to allow
the thickness of the pipe or the thickness of the cutting shoe to penetrate the
soil. This causes the soil to compress and cause some disturbance of the
sample. The smaller the area ratio of the removed soil, gives less
disturbance.

Area Ratio:

Ar % = [(Dw2 - De2) / De2] * 100

Where:

Ar: Area Ratio.

Dw: Outside Diameter of the Tube.

De: Inside Diameter of the Tube.

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Foundation Engineering” Ch1 Site Investigations

To obtain a sample with a low degree of disturbance, the area of Ar

should not exceed 20% for clay soil and 10% for weak clay soil. The area
ratio of Ar should not exceed 12% when using a tube of 50 mm diameter and
15% when using a tube of 75 mm diameter and 20% when using a tube of
100 mm diameter.

3. Inside Clearance Ratio

If the sampling tubes are long to their diameter, the friction or

adhesion forces between the sample and the internal walls of the tube may
reach a high amount that may lead to failure in the sample structure. This
causing a sample and to minimize friction or adhesion the inner diameter of
the tube makes some more of the inner diameter of the cutting shoe. This is
called inside clearance ( Cr) and is expressed as:

Cr % = [ ( Ds – De ) / De ] * 100

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Foundation Engineering” Ch1 Site Investigations

Where:

Cr : Inside Clearance Ratio.

Ds: Inside Dia. of the Tube.
De: Inside Dia of the cutting shoe.

The inside clearance reduces during the sampling process from

friction or adhesion along the internal cylindrical surface of the sample
and thus reduces disturbance. The increase in this ratio leads to an
undesirable adverse effect, which is to allow the sample to expand. The
percentage of inside clearance used in practical cases ranges from
0.3% to 4%, specifically as follows:

Cr > 0.5% ...... For Sand

Cr <3% ......... For Clay

4. Recovery Ratio

Lr = [Actual Length of Recovered Sample / Theoretical Length of

Sample (Tube length)] x 100

Lr = 1 → Very good sample

5. Disturbance During Sampling

1. Due to advancing the borehole (during boring).

a. Sample friction on the sides due to auger rotation.
b. Samples below water table may drain during covering
process.
c. Volume displacement of the tube.
2. Due to changes in prevailing condition
a. Loss of hydrostatic pressure may cause gas bubble voids to
form in the sample.
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Foundation Engineering” Ch1 Site Investigations

b. Changes in water and effective stresses during drilling.

c. Samples are always unloaded of the in-situ confining
pressure with some unknown resulting expansion.
d. Working environment (temperature).
e. Handling and transporting a sample from the site to the lab
and transferring the sample from sampler to the testing
machine.

Example: What is the area ratio of thin walled sample tube,if the
outer diam. Dw=50.8 and the inner dia. De=47.7?

Ar = [(Dw2 - De2) / De2] * 100

= [(50.82- 47.72 ) / 47.72 ] * 100 = 13.4%

Observation of Ground Water table

The presence of ground water table near the footings has a major
effect on soil bearing capacity and settlement. The groundwater level is
measured in the hole after a sufficient period of time for the completion of
the drilling process to allow the ground water to reach the equilibrium. This
time period depends on the soil permeability and is inversely proportional.
The time needed to reach the balance, in general must be at least 24 hours
before the completion of the drilling before measuring the level of ground
water. Measuring process occurred by special electrical measure tape
inserted within the borehole to investigate the water level.
If the permeability of the soil is very low this needed a long time to
investigate the true level of ground water table (many weeks). Observation
wells can be used and placement (Piezometer) or Hvorslev method can be
used.

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Foundation Engineering” Ch1 Site Investigations

Hvorslev Method to measure the Ground water level in the Soil

Hoverslev (1949) proposed a technique to determine the ground water

level see fig. This technique uses the following steps:

1- Bail out water in the borehole to a level below the estimated ground
water table.

2- Observe the water level in the borehole at times

t = 0 , t= t1 , t = t2 and t = t3

Note That

t1 – 0 = t1 – t2 = t2 – t3 = Δ t = 24 hrs.

Example:

Refer to Fig. Below, make the necessary calculation and locate the
ground water level. Given;

hW+ h0 = 9.5 m
Δ t = 24 hrs
Δ h1 = 0.9 m
Δ h2 = 0.7 m
Δ h3 = 0.54 m
Solution:

= = 4.05 m

= = 2.45 m

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Foundation Engineering” Ch1 Site Investigations

= = 1.82 m

It can be seen that hW = 9.5 – 4.05 = 5.45 m

In-Situ Testing:
In some types of soils such as loose granular soils and soft clayey
soils, it is not possible to obtain undisturbed samples, so it is basically
depends on insitu testing.

The Main Types of Insitue Tests:

1. Standard Penetration Test (S.P.T):

The principle of this test is based on measuring the soil resistance to
penetrate the cylinder of standard dimensions (Split Spoon Sampler).
 The test consists of driving a standard split spoon (50.8mm O.D
and 35mm I.D) into soil under the blows of a drop weight
(hummer) of 65 kg falling freely through 0.75m. The number of
blows required for 300mm of penetration of sampler in the soil is
designated as (N) values.
 The blows for the first 150mm are not used.
 Used as an estimate of the shear strength of soils.
 Good for cohesion less soils and gives rough results for cohesive
soils.
Correction for N- values:
1. Correction due presence of water table:

For Soils consisting very fine or silty sand below water table, a

correction is made when N > 15 because excess pore water

pressure set up during drilling the sampler cannot dissipate. You

may use one of the following:

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Foundation Engineering” Ch1 Site Investigations

Ncor = 15 + 0.5 * ( N – 15 ) ………..Terzaghi and Peck (1943).

Ncor= 0.6 * N ……………………………..Bazaraa (1967).

2. Correction due to Overburden Pressure:

N – Values for a depth corresponding to an effective overburden

pressure of 110 kPa( 1 T/ft2) is considered to be a standard. For

Po'>25 kPa (0.25 T/ft2) a correction factor (CN) should be used

CN = 0.77 log (2000/Po’)

Ncor = Nact * CN

Where Po’ is the effective overburden pressure at the depth of

sampling in kPa (Tsf).

Factors affecting N- values:

1. Variation of hummer height of fall.

2. Friction along the guides (rope, pulley)

3. Disturbed shoe of the spoon.

4. Inadequate cleaning of borehole.

5. Poor setting of spoon

6. Failure to maintain the hydrostatic pressure in the hole (i.e. quick

sand).

7. Careless of the crew.

8. Avoiding stones and obstruction.

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Foundation Engineering” Ch1 Site Investigations

Example:

The N-values for a test performed at a depth of 8 m below the

ground surface is 35, if the water table is at a depth of 2m and the dry unit
weight is 14 kN/m3 and the saturated unit weight is 18 kN/m3. Calculate
the corrected N if the soil is fine silty sand?

Sol.

1- Correction for water table:

Ncor = 15 + 0.5 * ( N – 15 ) = 15 + 0.5 * ( 35 – 15 ) = 25

2- Correction for overburden

Po’ = 14 * 2 + 6 * ( 18 – 10 ) = 76 kPa> 25 kPa

CN = 0.77 log (2000/ Po’) = 0.77 log (2000/ 76) = 1.093

Ncor = Nact * CN = 1.093 * 25 = 27.3 → Say 27 blows

Table below show correlations between SPT N-value and Consistency of

cohesive soils.
SPT N-Value Consistency of Clay Unconfined Compression Strength
qu (kN/m2)
0-2 Very Soft ‫ا‬ 0 - 25
3-5 Soft 25 – 50
6-9 Medium Stiff 50 - 100
10 - 16 Stiff 100 - 200
17 – 30 Very Stiff 200 - 400
> 30 Hard‫ا‬ >400

Table below show correlations between SPT N-value and Relative

Density & angle of Internal Friction Φ

SPT N-Value Approximate Relative Approximate angle of Internal

density Dr% Friction Φ
0-5 0-5 26 - 30
5 - 10 5 - 30 28 – 35
10 - 30 30 - 60 35 - 41
30 - 50 60 - 95 38 - 46

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Foundation Engineering” Ch1 Site Investigations

2- Cone Penetration Test (CPT):

A- Static Cone Penetration Test:

The testing tool consists of a hard cone at an angle of 600 and an

area of 10 cm2, it is connected to a solid steel bar covered with a sleeve
that allows the bars to pass freely. The cone is pressed by the hydraulic
jack at a speed of 1.5-2.0 cm/s to a distance of 80 mm in the soil and
recorded the pressure rate required by the pressure gauge. Then the cone
is established and the sleeve pushed to 80 mm. After that the cone and the
sleeve pushed to a distance of 120mm and recording the required force
for both. This process is repeated every 200 mm so that a cone resistance
and resistance Friction with depth can be determined.

1- Preferable for soft cohesive deposits (fine sands, silty fine sands
and clay).
2- Pushing hydraulically a steel cone (diameter = 35.7mm and apex
angle 60o) at a rate of 10 to 20mm / sec and recording the required
force and hence the stress (qc) can be calculated using the
following equation:
qc = (force required) / (base area = 1000 mm2)
3- The outer rod is pushed and the force required for pushing the
cone and sleeve is recorded and the stress (qt) can be calculated
4- Friction stress = qf = qt – qc
5- The data obtained are used for bearing capacity and settlement
analysis and static pile capacity.
6- For sand and coarse silt, you may use the following table:
qc (kPa) < 2000 - 4000 - 12000 12000 - >200000
2000 4000 200000
Rel. < 20 20 - 40 40 - 60 60 - 80 >80
Den.(Dr)
Φ (deg) 25 - 30 - 35 35 - 40 40 - 45 >45
30
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Advantages:
1- Fast and economical
2- Gives a continuous resistance of the strata.
3- Gives skin friction of soil (used for piles).
4- More reliable for sand below water table.
5- No boring is required.
Limitations:
1- Unsuitable for gravelly soils
2- Does not reveal the types of soil encountered.
3- No samples are taken.
4- Test depth 15 to 20m.

Relationship between N (SPT) and qc is shown in the following

table:
Soil Type qc / N (qc in Es (kPa)
kPa)
Silts, fine sand, slightly 150-300 (1.5-2) qc
cohesive soils
Fine to medium sand 300-450 (2-4)qc
Coarse sands 450-700 (1.5-3)qc
Sandy gravel, gravelly sand 700-2000
Stiff clay, sandy clay (5-7)qc

B- Dynamic Cone Penetration Test:

- Used for hard deposits.
- The cone is driven into the soil by drop of standard hammer falling a
standard distance.
- Record number of blows / 0.3 meter (NC).
- Friction on sides increases with depth; hence the diameter of the cone
must be greater than the outside diameter of the pipe.
- If the depth of investigation is more than 6m, use bentonite.
- Fast and economical (no borehole is required).

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Limitations:
1- No samples are taken.
2- Misleading results in gravel or boulder strata
3- NC values must be corrected for overburden pressure.
NC' = C1 * NC
Where C1 = 0.8 to 1.2 when bentonite is used.
= 1.5up to depth 3m and bentonite is not used.
= 1.75 for depth 3 to 6m and bentonite is not used.

Equipment for Cone Penetration Test (CPT)

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Foundation Engineering” Ch1 Site Investigations

3- Vane Shear Test

This test is used to measure the Undrrained Shear Strength (Cu) for
soft and medium clayey soils with a shear resistance of 5-75 kPa. In this
test, the vanes are lowered with the fixed bars to the bottom of the hole or
pushed in the soil directly to a depth of at least about 0.5 m below the
bottom of the hole, this distance to eliminate the disturbance of soil as a
result of the drilling. The torque part of the tool is then stabilized and the
torque scale is set to zero. The torque is set to rotate the blades at a speed
of 6-12 ° / min. until the amount of torque reaches the maximum that
causes the soil failure. The undrained shear resistance ( Cu) can be
calculated from the following equation:

Where:
T : Torque
d : Diameter of
h : Height of Blade

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Foundation Engineering” Ch1 Site Investigations

4- Plate Load Test:

The test involves loading a square horizontal plate with dimensions

0.3x0.3 or 0.45 x 0.4 m2 or round with a diameter of 0.3 or 0.45 m. This
test is used to estimate the bearing capacity of the foundations and the
settlement of the structures on the soil which cannot be examined in situ.
They are placed on the layer to be examined, at base level foundation.
The settlement is recorded as shown in the figure below, and the test is
performed by increasing the stress with a constant increments with time,
each increment represents 1/5 of the actual stress exerted by the structure
on soil based until the total stress reaches at least to a double of the stress
exerted from the footing or the loading continues adding more increments
until the soil under the plate reaches to the point of failure. The results
drawn the as load settlement relationship and ultimate bearing capacity
determined.

Limitations:

1- Suitable for sandy soil.

2- Usually of short duration, hence consolidation settlement does not
fully occur during this test.
3- Zone of stressed soil beneath the plate is much smaller than that
beneath the larger foundation so will be not unaffected by deeper
strata whose load bearing and settlement characteristic may affect the
behavior of the foundation.
4- To predict bearing capacity using one of the following:
a- Relation formulas
q (footing) = q (plate) ………………………………For Clay
q (footing) = q (plate) * ( BFooting/ BPlate)…………..For Sand

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Foundation Engineering” Ch1 Site Investigations

b- Using Housel Equation

V = A * q + P * s ………………………… ……For C-Φ soil
Where: V: Total load on a bearing area
A: Contact area of footing (or plate)
P: Perimeter of footing,
q: bearing pressure,
S: Perimeter shear

By conducting two plate load tests, we can solve the equation for q
and S and then re-use the equation for fully scale footing.

5- To predict Settlement
Sp = Sf * (Bp / Bf)a
Where:
a = 1/2 to 1/3 for sand and gravel
= 1/2 for saturated silt, = 1/2 to 2/3 for clay and dry silt
= 1 for compacted fill.
If ground water is at the level of the test plate, reduce the values by a
half (1/2).
Example:
Two Plate load tests were performed using plates (0.3 * 0.3m) and
(0.45 * 0.45m) for 12mm settlement, the loads were 37.5 and 75 kN
respectively. What size square footing is required to carry 80 kN column
load.
Solution:
V=A*q+P*s
[0.3 * 0.3m]………→ 37.5 = 0.09 * q + 1.2 * s
[0.45 * 0.45m]……→ 75 = 0.2025 * q + 1.8 * s
Solving the two equations to get ..q = 277.77 kPaand s = 10.41kN/m
[Full scale footing]…..→ 80 = 277.77 * B2 + 10.41 * 4 * B
Solving to get ….B = 0.47m say 0.5 m for 12mm settlement
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Foundation Engineering” Ch1 Site Investigations

Geophysical Exploration
They can be used in large engineering projects such as dams,
reservoirs and roads. These investigations can be carried out in large
areas, at a short time and at a relatively low cost, relative to the usual
investigations of the excavation. These Investigations can be used to
know the depth of the rock layers, the gaps, the depth of the groundwater
and the thickness of the layers.
The most important geophysical methods used in soil
investigations for civil engineering works are:
1. Seismic Method
This method depends on the speed of the shock wave transmission
in the different soil, the greater the density or the hardness of the soil, the
more rapid transmission of the shock wave. The shock wave transmission
depends on the water content of the soil, the percentage of the spaces, the
elastic constants; from these properties type of soil can be determined.
In this type of investigation, the shock wave is generated by an
explosion on the surface of the soil or by a heavy hammer hitting a metal
plate. The explosion generates three types of waves:

a. Compression Wave

b. Shear Wave

c. Surface Wave

In this type of investigation, the pressure wave is measured only

Which are on three types as follows?

a. Direct Wave: Take a straight path from the shock center to the surface
of the earth.

b. Reflected Wave: is commonly used in oil exploration investigations

c. Refracted Wave: This method is commonly used in civil engineering.

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Foundation Engineering” Ch1 Site Investigations

The latter are affected by the different physical properties of the soil in
which they pass.

To determine the velocity (V) of compression wave (P-waves) in

various layers and the thickness of those layers as the following:

a- Obtain the times of first arrival, t1, t2, t3,…..etc at various

distance x1, x2, x3,…etc from the field.
b- Plot a graph of time (t) vs distance (x).
c- Determine the slopes of the lines ab, bc, cd, ….etc
Slope of ab = 1 / V1,
Slope of bc = 1 / V2,
Slope of cd = 1 / V3
Where V1, V2, V3 are the P-wave velocities in layer 1,2,3,…
d- Determine the thickness of the top layer as:

The value of Xc is obtained from plot as shown in Fig. and determine the
thickness of the second layer Z2.

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Foundation Engineering” Ch1 Site Investigations

Where Ti2 is the time intercept of the line cd in Fig. extended backward.

Ray paths in a three layer soil

Graph of T vs x, ab=1/v1, bc=1/v2, cd=1/v3

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Foundation Engineering” Ch1 Site Investigations

Table below Shows Typical Value Ranges of P-Wave Velocity at

Shallow Depths

Material P-Waves velocity

m / sec ft / sec
Dry Sand 183-1220 600-4000
Clay and wet soils 610-1830 2000-6000
Loam ‫ﻣﺰﯾﺞ اﻟﻄﯿﻦ واﻟﺮﻣﻞ‬ 610-1525 2000-5000

Sandstone ‫اﻟﺤﺠﺮ اﻟﺮﻣﻠﻲ اﻟﺼﻠﺐ‬ 1525-4573 5000-15000

Shale ‫اﻟﻄﯿﻦ اﻟﺼﻔﺤﻲ اﻟﺼﻠﺐ‬ 1525-4268 5000-14000

1 m/sec = 3.28 ft/sec

Example:

The results of refraction survey at a site are given in the following table,
determine the P-wave velocity and the thickness of the material
encountered.

Distance from the source of Time of First arrival (sec x 10-3)

disturbance (m)
2.5 11.2
5 23.3
7 33.5
10 42.4
15 50.9
20 57.2
25 64.4
30 68.6
35 71.1
40 72.1
50 75.5

Solution:the time of first arrival are plotted against the distance from the
source of disturbance, the plot has 3-straight line segments. The velocity
of the top three layers can now be calculated as follows:

Slope of segmentoa = = =

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Foundation Engineering” Ch1 Site Investigations

Then = 228 m / sec (top layer).

Slope of segment ab = = =

Then = 814.8 m / sec (2nd layer).

Slope of segment bc = = =

Then = 4214 m / sec (3rd layer).

By comparing those velocities with those given in table, it appears that’s

3rd layer is rock layer

Thickness of layers:

From the above Fig. Xc = 10.5 m, So

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 Foundation Engineering” Ch1 Site Investigations

Where from Fig, Ti2 = 65 * 10 -3 sec, Then:

So the rock layer is located at a depth of (Z1 +Z2) = 3.94m + 12.66m =

16.6m

H.W:

Following are the results of a refraction survey (horizontal layering of

soil) showing the distance x and time of first arrival t, determine the P-
wave velocitiesof the soil layers and their thicknesses.

Distance from the source of Time of First arrival (sec x 10-3)

disturbance (m)
2.5 5.5
5 11.1
7.5 16.1
15 24
25 30.8
35 38.2
45 46.1
55 51.3
60 52.8

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Foundation Engineering” Ch1 Site Investigations

2- Electrical Resistivity Survey Method

The electrical resistivity method depends on the fact that the
different materials have different resistance to the flow of electricity and
that the resistance of the soil or rock to the passage of electricity depends
heavily on the concentration of dissolved salts and quality and the
following factors:

a. Moisture content

b. Percentage of blanks and particle size.

c. Stratification.

d. temperature.

Method of conducting the test:

The average resistivity of the earth can be obtained from the

equation below.

Where, R is the electrical resistivity.

The most common procedure for measuring electrical resistivity of

a soil profile makes use of four electrodes that are driven into the ground
and spaced equally a long a straight line. It is generally referred to as the
Wenner method. The two outside electrodes are used to send an electrical
current, I, (usually a Dc current is typically in the range of 50-100
milliamperes. The velocity drop, V, is measured between the two inside
electrodes, if the soil profile is homogenous, its electrical resistivity is :

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Foundation Engineering” Ch1 Site Investigations

To obtain the actual resistivity of various layers and their thickness,

an empirical method may be used. It involves conducting tests at various
electrode spacings (that is, d is changed). The sum of the apparent
resistivity, ∑ρ, is plotted against the spacing (d) as shown in Fig below.
The plot they obtained has relatively straight segments. The slopes of
these straight segments give the resistivity of individual layers. The
thickness of various layers can be estimated as shown in Fig.

The Range of resistivity generally encountered in various soils and rocks

is given in table below:

Material Resistivity (ohm.m)

Sand 500-1500
Clays, Saturated Silt 0-100
Clayey Sand 200-500
Gravel 1500-4000
Weathered Rock 1500-2500
Sound Rock >5000

Subsoil Investigation Report

After all the required information has been completed, a soil
exploration report is prepared for the use of the design office and for
reference during future construction work. Although the details and
sequence of information in the report may vary to some degrees
depending on the structure under consideration and the person compiling
the report, each report should include the following items:
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Foundation Engineering” Ch1 Site Investigations

1- The scope of the investigation.

2- A description of the proposed structure for which the subsoil
exploration has been conducted.
3- A description of the location of the site, including structure (s),
nearby, drainage conditions of the site, nature of vegetation on the
site and surrounding it, and any other features unique to the site.
1- Geological setting of the site.
2- Details of the field exploration. That’s, number of borings,
depths of borings, type of boring and so on.
3- General description of the subsoil conditions as determined from
soil specimens and from related Lab. Tests, standard penetration
resistance and cone penetration resistance, and so on.
4- Water table conditions.
5- Foundation recommendations, including the type of foundation
recommended, allowable bearing capacity, and any special
construction procedures that may be needed, alternative foundation
design procedures should also be discussed in this portion of the
report.
1- Conclusion and limitations of the investigations.

The following graphical presentation should be attached to the report:

1- Site location map.

2- A plan view of the location of the borings with respect to
the proposed structures and those existing nearby.
3- Boring log.
4- Laboratory test results.
5- Other special graphical presentations.

The exploration reports should be well planned and documented. They

will help in answering questions and solving foundation problems that
may arise later during design and construction.

Congratulations, you have finished

Chapter One

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