University of Duhok
College of Engineering
Water Resources Engineering Department
Report about:
Title
( )
By
Rasti muhammed ahmed
Supervised by Lecturer
Khalil Sadiq Ismae
Chapter 1 General
1.1 Soil mechanics,
the study of the physical properties and utilization of soils, especially used in planning
foundations for structures and subgrades for highways.
The first scientific study of soil mechanics was undertaken by French
physicist Charles-Augustin de Coulomb, who published a theory of earth pressure in
1773. Coulomb’s work and a theory of earth masses published by Scottish engineer
William Rankine in 1857 are still primary tools used to quantify earth stresses. These
theories have been amended in the 20th century to take into account the influence
of cohesion, a more recently discovered property of soils that causes them to behave
somewhat differently under stress than Rankine and Coulomb predicted.
Soil is a natural aggregate of mineral particles, sometimes including organic
constituents; it has solid, liquid, and gaseous phases. How the soil of a given site will
support the stresses put upon it by the weight of structures, or how it will respond to
movement in the course of construction, depends upon six properties—internal
friction (the resistance of a soil mass to sliding, inversely related to the amount of
moisture in the soil and thus greater in sands and gravel than clays) and cohesion
(molecular attraction between soil particles, much higher in clays than sands or silt),
both of which lessen the tendency of soils to shear, or slide along
planes; compressibility (the degree to which soil may be made denser by various
means including tamping and vibration, and thus able to support greater
loads); elasticity (the ability of soil to reexpand after being
compressed); permeability (the degree to which a soil will conduct a flow of water);
and capillarity (the degree to which water is drawn upward from the normal water
table).
The thoroughness of soil surveys at a given site depends on the size of the project to
be carried out. Visual examination of the surface may suffice in some cases. Soil
characteristics generally vary more rapidly vertically (with depth) than horizontally.
Subsurface examination techniques include trench-digging, boring (to test resistance
as well as to obtain samples), and pumping subsurface matter to the surface with
water. Seismic testing (measuring the speed with which shock waves generated by
explosives are transmitted through the ground) and measurement of the electrical
resistance of the soil also yield information helpful in the evaluation of soil. Grain
size and plastic properties of samples taken from the site are measured in a laboratory.
Occasionally data obtained from previous studies of soils near the site are useful
Foundations are designed to convey the weight of a structure to the ground
underneath and around it. Stress distribution that is not properly matched to the
characteristics of the soil may result in structural failure through shearing of the soil
or uneven settling. Spread foundations may be either of the spread footing (made with
wide bases placed directly beneath the load-bearing beams or walls), mat (consisting
of slabs, usually of reinforced concrete, which underlie the entire area of a building),
or floating types. A floating foundation consists of boxlike rigid structures set at such
a depth below ground that the weight of the soil removed to place it equals the weight
of the building; thus, once the building is completed, the soil under it will bear the
same weight it bore before excavation was begun. Deep foundations may be end-
bearing piles (which convey all the weight put on them end-to-end, from the building
above to the bedrock on which they are set), friction piles (which transfer some of the
pressure put on them to the soil around them, through friction or adhesion along the
surface where pile sides interface with soil), or caissons (extra-large piles cast in place
in an excavation, rather than prefabricated and sunk
Slopes stay in place because the downward pull of gravity is countered by forces of
cohesion and friction between particles. Various changes may upset the balance
between these forces, precipitating a slide; in particular, an increase in the amount of
water borne in the soil of a slope may drastically reduce cohesion and friction. The
stability of slopes is graded such that 1.0 indicates forces exactly balanced, 2.0
signifies that the forces of stability are twice as great as those tending toward
movement, etc. A slope with a reading of less than 1.0 is collapsing. The banks of
dams, highway cuts, and railway cuts are designed to certain standards of stability as
measured by this scale. Stability may be increased by draining, gradient leveling,
compacting, or reinforcing the slope with injections of cement. In dam construction
an impermeable core is used to prevent excess seepage of water from lowering
stability, while the slopes consist of permeable material that buffers the weight of
water along the dam.
Soil mechanics, by examination of the subgrade of roads and highways, helps to
determine which type of pavement (rigid or flexible) will last longer. The study of soil
characteristics is also used to decide the most suitable method for excavating
underground tunnels
Chapter 2 Objectives
2.1 For settlement
Before construction when the mathematical equation helps us know the depth of
landing of any building before construction and so that we can know how many years
l can build it begins to land, because any built landing can change the center of the
building because an example of the tower is the leaning tower of pisa in it
Leaning Tower of Pisa
in 2013
2.2 For shear strength of soil
The shear strength of soil is the resistance to deformation by continuous shear
displacement of soil particles upon the action of a shear principal engineering
property which controls the stability of a soil mass under loads. It governs the bearing
capacity of soils
Chapter 3 Introduction
3.1 Settlement of a foundation
Probably the most difficult of the problems that a soils engineer is asked to solve is
the accurate prediction of the settlement of a loaded foundation. The problem is in
two distinct parts: (1) the value of the total settlement that will occur, and (2) the rate
at which that value will be achieved. When a soil is subjected to an increase in
compressive stress due to a foundation load the resulting soil compression consists of
elastic compression, primary compression and secondary compression.
3.1.1 Components of the total settlement
1.Elastic compression
This compression is usually taken as occurring immediately after the application of
the foundation load. Its vertical component causes a vertical movement of the
foundation (immediate settlement) that in the case of a partially saturated soil is
mainly due to the expulsion of gases and to the elastic bending reorientation of the
soil particles. With saturated soils immediate settlement effects are assumed to be the
result of vertical soil compression before there is any change in volume.
2.Primary compression
The sudden application of a foundation load, besides causing elastic compression,
creates a state of excess hydrostatic pressure in saturated soil. These excess pore
water pressure values can only be dissipated by the gradual expulsion of water
through the voids of the soil which results in a volume change that is time dependent.
A soil experiencing such a volume change is said to be consolidating and the vertical
component of the change is called the consolidation settlement.
3.Secondary compression
Volume changes that are more or less independent of the excess pore water pressure
values cause secondary compression. The nature of these changes is not fully
understood but it is apparently due to a farm of plastic flow resulting in a
displacement of the soil particles. Secondary compression effects can continue over
long periods of time and, in the consolidation test, become apparent towards the end
of the primary compression stage: due to the thinness of the sample, the excess pore
water pressures are soon dissipated and it may appear that the main part of secondary
compression occurs after primary compression is completed. This effect is absent in
the case of an in situ clay layer because the large dimensions involved mean that a
considerable time is required before the excess pore pressures drain away. During this
time the effects of secondary compression are also taking place so that when primary
compression is complete, little, if any, secondary effect is noticeable. The terms
‘primary’ and ‘secondary’ are therefore seen to be rather arbitrary divisions of the
single, continuous consolidation process. The time relation-ships of these two factors
will be entirely different if they two test samples of different thicknesses.
3.1.2 Causes of total settlement
A-Direct causes
The direct couse of settlement is the weight of building incliding dead load and live
load
B-Indirect causes
1- failure of collapsible soil underground infiltration
2-yielding of excavation done adjacent to foundation
3- failure of underground tunnels and mines
4- collapse of cavities of limestones
5-undermining of foundation while flood
6- earthquake induced settlement
7- finally, due to extraction of ground water and oil
3.2 Shear strength of soil
One of the main characteristics of soils is that the shear deformations increase
progressively when the shear stresses increase, and that for sufficiently large shear
stresses the soil may eventually fail. In nature, or in engineering practice, dams,
dikes, or embankments for railroads or highways may fail by part of the soil mass
sliding over the soil below it. In this chapter the states of stresses causing such
failures of the soil are described. In later chapters the laboratory tests to determine the
shear strength of soils will be presented.
3.2.1 Coulomb
A slope in a soft soil may fail if the slope is too steep or the soil has insufficient
strength. A very small cause, such as a small load, or a small local disturbance, may
result in a large landslide. Other causes for such a landslide may be water waves
against the slope, or a rising groundwater table in the interior of a dam.
3.2.2 Mohr’s Circle
From the theory of stresses it is known that the stresses acting in a certain point on
different planes can be related by analytical formulas, based upon the equilibrium
equations. In these formulas the basic variable is the angle of rotation of the plane
with respect to the principal directions. These principal directions are the directions in
which the shear stress is zero, and in which the normal stresses are maximal or
Minimal
3.2.3 Mohr–Coulomb
Coulomb’s frictional law for finding the shear strength of soils requires that we know
the friction angle and the normal effective stress on the slip plane. Both of these are
not readily known because soils are usually subjected to a variety of stresses. that
Mohr’s circle can be used to determine the stress state within a soil mass. By
combining Mohr’s circle for finding stress states with Coulomb’s frictional law, we
can develop a generalized failure criterion
3.2.4 Factors affecting shear strength parameters
A-factors affecting internal friction angle
1-Normal stress
2-Grading
3-Density
4-Water content
5-Grain shape and size
B-Factors affecting cohesion
1-Void ratio
2-Water content
3-Stress history
4-Soil structure
Chapter 4 Theoretical concepts
4.1 For total settlement
If the stress change in the soil or in the currently build earth structure, caused by the
ground surface surcharge, is known, it is possible to determine the soil deformation.
The soil deformation is generally inclined and its vertical component is termed the
settlement. In general, the settlement is non-stationary dependent on time, which
means that it does not occur immediately after introducing the surcharge, but it rather
depends on the consolidation characteristics of a soil. Permeable, less compressible
soils (sand, gravel) deform fast, while saturated, low permeability clayey soils
experience gradual deformation called consolidation.
Time
dependent settlement of
soils
Applied load yields
settlement, which can be
subdivided based on
time- dependent
response into three
separate components:
1-Instantaneous settlement (initial),(Si)
2-Primary settlement (consolidation),(Sc)
2-Secondary settlement (creep),(Ssc)
ST=Si+Sc+Ssc
For clay
ST=Si(minimum)+Sc(major)+Ssc(small, but present to certain extent)
For sand
ST=Si(major)+Sc(present but mixed with Si)+Ssc(undefined)
4.2 For shear strength parameters
