THEORETICAL SUBJECTS for the graduating examination - Civil Engineering specialization (ICE) Discipline: SOIL MECHANICS 1. Soils components – solid phase, chemical and mineralogical composition. Answer 1: From physical point of view, soils are dispersed, three-phase mediums, generally composed from the following phases: solid phase (solid particles composing the mineral skeleton of soil); liquid phase (water in the spaces between the solid particles, named voids); gaseous phase (air or gases in voids unoccupied by water). Fig. 1. Soils components: 1 – solid particle; 2 - water; 3 – air The mineral skeleton of soils was formed by physical weathering and chemical weathering of the minerals contained in the pre-existing rocks (primary minerals resulted from physical weathering and secondary minerals resulted from chemical weathering of the primary minerals, resulting new minerals). Most frequent primary minerals parts of sandy and silty soils are: quartz, feldspar, calcite, mica, etc. Characteristic to clays are secondary minerals as: montmorillonite, kaolinite, etc.
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THEORETICAL SUBJECTS
for the graduating examination - Civil Engineering specialization (ICE)
Discipline: SOIL MECHANICS
1. Soils components – solid phase, chemical and mineralogical composition.
Answer 1:
From physical point of view, soils are
dispersed, three-phase mediums, generally
composed from the following phases: solid phase
(solid particles composing the mineral skeleton of
soil); liquid phase (water in the spaces between the
solid particles, named voids); gaseous phase (air or
gases in voids unoccupied by water).
Fig. 1. Soils components:
1 – solid particle; 2 - water; 3 – air
The mineral skeleton of soils was formed by physical weathering and chemical
weathering of the minerals contained in the pre-existing rocks (primary minerals resulted
from physical weathering and secondary minerals resulted from chemical weathering of the
primary minerals, resulting new minerals). Most frequent primary minerals parts of sandy and
silty soils are: quartz, feldspar, calcite, mica, etc. Characteristic to clays are secondary
minerals as: montmorillonite, kaolinite, etc.
2. Physical characteristics of the soils – solid particles density and the density of the soils
(ρs, γs, ρ, γ).
Answer 2:
Solid particles density represents the ratio between the solid particles mass Ms from a
soil sample and their volume Vs; is expressed by the relationship:
s =
s
s
V
M [g/cm
3]
The solid particles density varies between relatively restricted limits (2,6-2,8 g/cm3).
In laboratory the solid particles density is determined using the picnometer.
Unit weight of the solid particles is the ratio between the weight of the solid
particles, Gs from a soil sample and their volume, Vs. The calculus relationship is: s =
gV
gM
V
Gs
s
s
s
s
[kN/m3] in which g is the gravitational acceleration.
Bulk density represents the ratio between the mass of a soil sample M and its total
volume V, including the volume of the voids (spaces between the solid particles). It’s
expressed by the relationship: = V
M [g/cm
3]
Bulk density varies widely, for the same soil, with the same porosity (voids volume),
function of the water content.
Bulk unit weight is the ratio between the weight of a soil sample G and its volume V:
= gV
gM
V
G
[kN/m
3]
3. Moisture content and degree of saturation of the soils (w, Sr).
Answer 3:
Moisture content of a soil represents the ratio between the water mass Mw from the
voids of a soil quantity and the mass of the solid particles Ms from the same soil quantity. It’s
quoted with w and is expressed by the relationships:
w
s
w
M
M or: in percent w 100
M
M
s
w [%]
In laboratory, moisture content is determined by soil samples drying (in the oven at a
temperature of 105°C) till a constant mass.
Degree of saturation is defined as the ratio between the volume of water from a soil
sample and the voids total volume of the same soil sample:
p
wr
V
VS
Function of the degree of saturation value the soils are classified into:
- Dry soil, Sr ≤ 0,40;
- Wet soil, 0,40 < Sr ≤ 0,80;
- Very wet soil, 0,80 < Sr ≤ 0,90;
- Practically saturated soil, 0,90 < Sr ≤ 1,00.
4. Void ratio, porosity and the density index of cohesionless soils (e, n%, ID).
Answer 4:
Void ratio (e) represents the ratio between the voids volume Vp and the volume of the
solid particles for the considered soil sample: e = Vs
Vp
Porosity (n) expresses the ratio between the voids volume and the total volume of the
considered soil quantity:
n =V
Vp or in percent: n = 100V
Vp [ % ] where: Vp – volume of the voids
from the considered soil sample; V – total volume of the considered soil sample.
Density index, ID is defined by the following relationship: ID =
minmax
max
ee
ee
where: emax – void ratio corresponding to the loose state of the soil; emin – void ratio
corresponding to the compacted state of the soil; e – void ratio corresponding to the natural
state of the soil.
5. Plasticity limits, plasticity index and consistency index (wL, wP, IP, IC).
Answer 5:
Moisture contents that define the plastic cohesive soils behaviour are named plasticity
limits.
Plastic limit wp, represents the minimum moisture content from which a clayey soil
behaves as a plastic body, the soil passing from a hard (semisolid) state into a plastic state.
Liquid limit wL represents the maximum moisture content till which a clayey soil has
a plastic behaviour, the soil passing from a plastic state into a yielding state. For moisture
contents greater than wL the soil yields under self-weight.
Property of the cohesive soils to behave, in a certain moisture content limits, as a
plastic body, is named plasticity. Quantitatively, plasticity is expressed by plasticity index Ip,
that represents the moisture content interval in which cohesive soils are in plastic state, being
defined by the relationship: Ip = wL - wp.
Consistency index Ic expresses quantitatively the consistency state of the cohesive
soils, being defined by the following relationship: Ic =
p
L
pL
L
I
ww
ww
ww
.
6. Study of the soils compressibility in the laboratory. Oedometric test.
Answer 6:
In laboratory, for the compressibility study is used an apparatus named oedometer
(fig. 1). During this test, upon the soil sample is applied, through a piston, a vertical
compression load in steps. For water drainage from soil sample voids, the soil sample is
placed between two porous plates.
Fig. 1. Oedometer Fig. 2. Load-settlement curve
The main characteristic of the compressibility test consists is the fact that the lateral
deformation of the soil sample is completely hindered.
On the basis of the compressibility test can be drawn the load-settlement curve (fig. 2).
From the load-settlement curve is determined: specific deformation:
0
ii
h
h100
[%] and
oedometric modulus of deformation:
ppptgM
12
12
M value is computed for the pressures: p1 = 200 kPa and p2 = 300 kPa; this value is
quoted M2-3.
7. Shear strength of soils, definition, Coulomb’s Law.
Answer 7:
The application of an exterior load upon a soil mass (fig. 1) and its own weight
develops in its mass normal and tangential stresses. Normal stresses produce reduction in
voids volume and tangential stresses tend to displace soil particles laterally one towards the
other. The shear strength of soils f opposes to displacements produced by tangential stresses,
being generated by the bound forces between its particles.