CE 152 Lecture 1

Geotechnical Engineering 2
ground

2
Learning Objectives:
Define foundation engineering

Describe the parts of an engineering structure

Enumerate the requirements in the design of

foundations

Discuss the principal kinds of failure for

foundation.

Enumerate the different types of foundation.

 Soil Formation

Soil derives from Latin word Solum

having same meanings as our modern
world.
From Geologist point of view, The
superficial unconsolidated mantle of
disintegrated and decomposed rock
material-The entire mantle or rock decay.
 Soil Formation

Soil is a complex of inorganic matters that

may or may not contain organic decomposed
organic residues and other substances, which
blanket the earths crust, which is formed by
the process of weathering (Disintegration and
decomposition) of rock and mineral.
The weathering agents include physical,
mechanical or chemical agents.
 The factors of weathering in the
process of soil formation may be
atmospheric such as pressure,
temperature, wind and water erosion
and transportation by the water
erosion and transportation by water
and glaciers, plant and animal life.
 Soil is a mixture of Water, Air and Solids. The
solids are mixture of mineral matters with
particle sizes differing in sizes, shapes and
structure and varying in chemical
compositions.
The top soil which supports vegetation is
called Top soil and the undisturbed strata
lying immediately below the natural top soil
is termed as sub soil.
 Types of Soils:
Gravels
Sands
Silts
Clay
Peats
 Types of Soils:
They are further classified into two
types:
Cohesive soils: Clay, shale and silts.
Non cohesive or Cohesion-less soils:
Sand and Gravels, which possesses no
plasticity and tend to lack cohesion
specially when in dry state.
Problems to be studied before execution of the
projects

How deep the soil exploration must be made?

What is the safe and allowed bearing capacity?
What is the load of structures to be applied at
the soil?
What is the intensity and stress distribution in
a soil induced by various kinds of loading?
Problems to be studied before execution
of the projects
How thick should be thickness of layer
of good soil over a poor one in order to
prevent the foundation from punching.
Does soil possesses properties ( friction
and cohesion) which will assure
satisfactory stability for foundation.
How much counter weight must be
placed as remedial measures against the
lateral motion of soil
Other considerations:
The settlement of soils under applied loads
and its rate and nature.
The depth of ground water and its variation
at various depths.
Depth of frost penetration and subsequent
depth of foundation and effect of freeze and
thaw on pavement and structures.
The suitability of soil for the construction of
structures like dams, roads and buildings.
The issues relating to water logging and
salinity in soils etc.
Identification of Soils
Soils are identified by visual examination
and by means of their index properties
(grain-size distribution, Atterberg limits,
water content, specific gravity, and void
ratio).

A description based on visual examination

should include color, odor when present,
size and shape of grains, gradation, and
density and consistency characteristics.
Identification of Soils
Coarse grained soils: soils have more than 50 percent
by weight retained on the No. 200 sieve and are
described primarily on the basis of grain size and
density
Fine-grained soils have more than 50 percent by
weight finer than the No. 200 sieve. Descriptions of
these soils should state the color, texture,
stratification, and odor, and whether the soils are
soft, firm, or stiff, intact or fissured.
The visual examination should be accompanied by
estimated or laboratory determined index properties.
Physical Properties Of Soils
Color: Depends on
Minerals of soil.
Organic contents
Amount of oxides
Color is natural state is noted.
Soil Structure: depends on
Size and shapes of soil particles.
Terzaghi grouped soil in three groups
Granular or single grained soil: Silt and Clay

Flocculent Structure: Clay

Dispersed Structure: Transportation process: Man fills.

Particle Shapes:
Angular

Sub Angular

Rounded

Elongated

Flaky
 Atterberg limits. The Atterberg limits indicate the
range of water content over which a cohesive soil
behaves plastically. The upper limit of this range is
known as the liquid limit (LL); the lower, as the plastic
limit (PL). The LL is the water content at which a soil
will just begin to flow when slightly jarred in a
prescribed manner. The PL is the water content at
which the soil will just begin to crumble when rolled
into threads 1/8 inch in diameter.

Density. The mass density of a soil material is its

weight per unit volume. The dry density of a soil is
defined as the weight of solids contained in the unit
volume of the soil and is usually expressed in pounds
per cubic foot.
 Specific gravity. The specific gravity of the
solid constituents of a soil is the ratio of the unit
weight of the solid constituents to the unit
weight of water. For routine analyses, the
specific gravity of sands and clayey soils may be
taken as 2. 65 and 2. 70, respectively.

Consistency. The consistency of an

undisturbed cohesive soil may be expressed
quantitatively by the unconfined compressive
strength qu.

17
Weight Volume relationships of soils

18
FOUNDATION ENGINEERING
Art and science of applying engineering judgment and
the principles of soil mechanics.
Two parts consisting an engineering structure:
1.Superstructure upper part
2.Substructure/Foundation lower part
- part of the structure which is usually placed
below the surface of the ground and which transmits the
load to the underlying soil or rock.
Two essential requirements in the design of
foundations:
1.Total settlement shall be limited to tolerable
small amount.
2.Differential settlement shall be eliminated as
nearly as possible.

To limit settlement, it is necessary:

1.To transmit the load of the structure to a soil
stratum of sufficient strength.
2.To spread the load over a sufficiently large area
of that stratum to minimize bearing pressure.
Two principal kinds of failure for
foundation:
1.Structural failure of foundation (bending,
shear, bearing).
2.Bearing capacity failure of the supporting soil
Three types of bearing capacity failures:
1.General shear failure
It occurs in soils that are relatively
incompressible and reasonably strong, in rock, and in
saturated, normally consolidated clays that are loaded
rapidly enough that the undrained condition prevails.
The failure surface is well defined and failure occurs
quite suddenly. Although bulges may appear on both
sides of the foundation, ultimate failure occurs on one
side only, and is often accompanied by rotations of the
foundation.
1.General shear failure
2.Local shear failure
The shear surfaces are well defined under the
foundation, and then become vague near the ground
surface. A small bulge may occur, but considerable
settlement, perhaps on the other half of the foundation
width, is necessary before a clear shear surface forms
near the ground. Even then, a sudden failure does not
occur, as happens in the general shear case. The
foundation just continues to sink deeper into the
ground.
2.Local shear failure
3.Punching shear failure
It occurs in very loose sands, in a thin crust of
strong soil underlain by a very weak soil, or in weak clays
loaded under slow, drained conditions. The high
compressibility of such soil profiles causes large
settlements and poorly defined vertical shear surfaces.
Little or no bulging occurs at the ground surface and
failure develops gradually.
3.Punching shear failure
FOUNDATION CLASSIFICATIONS:

1.Shallow Foundation
- Satisfactory soil directly underlies the structure.
- Spread footings, combined footings

2.Deep Foundation
- Adequate soil is not found immediately below the
structure.
- Caisson foundation, pile foundation
Typical Foundation Types
1.Foundation for buildings (controlled mainly by
vertical loads)
2.Foundations for smoke stacks, radio & television
towers, bridge, piers, etc. (lateral loads)
3.Foundations for ports & marine structures (lateral
loads & impact loads)
4.Foundations for machinery, turbines, generators,
etc. (vibration & weight)
5.Foundation elements to support open cuts or retain
earth masses or bridge abutments.
General Requirements of Foundations:
1.Depth must be adequate to avoid lateral expulsion of
material from beneath the foundation.

2.Depth must be below seasonal volume changes such as

the zone of active organic materials.

3.System must be safe against overturning, sliding or soil

rupture.

4.System must be safe against corrosion or deterioration

due to harmful materials present in the soil.
 5.System should be adequate to sustain some changes
in later site or construction geometry, or be easily
modified should later changes be major in scope.

6.The foundation should be economical in terms of

the method of installation.

7.Total earth movements & differential movements

should be tolerable for the foundation elements
and/or any superstructure elements.
HW # 1 Deadline: 9/27/17 (1pm)
1. What are the differences between Geotechnical
Engineer and Geologist?

2. Research at least 3 structures which have failures

because of its foundation.