water in soil and rock mass
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Agua en masas de suelo y rocasTRANSCRIPT
UNIVERSIDADE DE BRASÍLIA. DEPARTAMENTO DE ENGENHARIA CIVIL E AMBIENTAL
PROGRAMA DE PÓS-GRADUAÇÃO EM GEOTECNIA
WATER IN SOIL AND ROCK MASS
Prof.: Hernán Eduardo Martínez -Carvajal, PhD
Design based on empirical rules
1930 (oral tradition)
Note: The theoretical basis for a rational
analysis were already known.
1856 Henri Darcy
GENERAL
STATEMENT
1880 Forcheimer
Hydraulic Head function
To satisfy the continuity
equation
Explain the flux
in porous
media
Continuity
equation
Basis for developing the graphic method
known as FLUX NETWORKS
Laplace
GENERAL
STATEMENT
1937 A. Casagrande
(“Seepage Through Dams” _ Contributions to
Soil Mechanics 1925 – 1940. Boston Society of
Civil Engineers 1940).
Graphic solution to LAPLACE equation
Ξ FLOW NETWORKS-
GENERAL
STATEMENT
¿ Why is important to solve problems related to
flow in porous media?R/ To obtain information about:
1. INFILTRATION THROUGH REGIONS SUBMETED TO FLOW – Earth
dams (Quantification of the flow).
2. INFLUENCE OF THE FLOW ON THE GLOBASL STABILITY OF THE
SOIL MASS – Slope stability.
a. Direction of the flow (hydrodynamic pressure)
b. t =( - u) tg
3. INTERNAL EROSION – Piping
(Earth dams, embankments, slopes, retaining walls)
GENERAL
STATEMENT
CONCEPT of
PRESSURE (STRESS)
P=F/S
Hydraulic Jack Volume compatibility : S1 . L1 = S2 . L2
Work compatibility : F1 . L1 = F2 . L2
1
2
1 2F2
F1
L1
L2
BASIC CONCEPTS
s2
s1
The Hydrostatic Paradox
Before the principles of hydrostatics were understood, the behavior of liquids was
often very puzzling. For example, Figure 1 shows a vessel with two interconnected
chambers which are open at the top and have bottom openings with the same
cross-sectional areas. If you pour water into either chamber, it flows up into the
other one until the water levels in both are identical. Is this not a paradox? Surely
the chamber containing the larger volume of water must have a greater force per
unit area at its base, and shouldn't this make the water in the smaller chamber rise
to a higher level?
Blaise Pascal asked this question nearly 300 years ago. He even built an
apparatus, now known as 'Pascal's vases', to demonstrate the paradox.
This apparent paradox can be easily resolved by the application of some elementary
mechanics. The pressure at a point in a static liquid is due entirely to the weight of
liquid (plus the atmosphere) directly above it.
An even more striking paradox is that associated with the horizontal
pressures on a dam.
The explanation follows the same reasoning as above: the horizontal pressure at a point must be equal to the vertical pressure, but the vertical pressure depends only on the depth of the water, not on its horizontal extent.
Hydrostatic pressure
Pe
A
h
BASIC CONCEPTS
ATMOSPHERIC PRESSURE
Mercury (Hg)
Hg = 13.6 g/cm3 = 13600 kg/m3
To sea level, h = 76cm
Units: * International System Patm = N/m2
Patm = 760 mmHg = 1 standard atmosphere= 1.033 kgf/cm2
1 technical atmosphere= 1 kgf/cm2
Patm
A
h
BASIC CONCEPTS
A B
PA= .g.hA PB= .g.hB
PA> PBbecause h2> h1
FLUIDS MOVEMENT OCCURS BECAUSE OF THE ENERGY
DIFERENCES
hA
Experience: Opening the valve movement toward 1
Experience: PRESSURE (Expressed as a height)
BASIC CONCEPTS
hB
A B
What happens in point C at equilibrium?
PC= Patm. And PA > PC nevertheless
there is no movement! Why?
HAhA
BASIC CONCEPTS
(by unit weight)
HAhA
0
BASIC CONCEPTS
g
gHE A
potC
g
VE cin
C
2
21
APLICATIONS: OPEN TUBE (CASAGRANDE)
hBZA
ZB
A
B
N.R. (Z=0)
0
BASIC CONCEPTS
YZA
ZB
A
B
N.R. (Z=0)
Inside the pipe there is no loss of head, so the piezometric heights of A and B are
the same.
Pressure in point B :
“The height of the water column indicated by an open piezometer in any point of a
terrain is equal to the static pressure of water on the point divided by the
volumetric weight of the water”.
BASIC CONCEPTS
PÉRDIDAS DE CARGA
Situación ideal en régimen estacionario
V1V1
A B N.R.
BASIC CONCEPTS
PÉRDIDAS DE CARGA
Situación real en régimen estacionario
hf
BASIC CONCEPTS
PÉRDIDAS DE CARGA
Situación real en régimen estacionario.
Cambio de sección= cambio de velocidad
Head lost (without
friction)
+ Head lost due to
friction
AB
BASIC CONCEPTS
PIEZOMETRIC HEAD IN SOIL MASS
DIFFERENCE IN ENERGY CAUSES
MOVEMENT OF FLUIDS
V is very low compared with
other components so
consider negligible
POSITIONPRESSURE VELOCITY
Water moves form high piezometric heads to low piezometric
heads.
BASIC CONCEPTS
GRADIENT
Gradient = = i
L
A
B
h
N.R.
Lost of head (h = hA – hB)
In a lenght L
h
L
BASIC CONCEPTS
QUALITATIVE INTERPRETATION OF THE GRADIENT
L
hkAQ L
h2
h2 h
h
QQA BC
BASIC CONCEPTS
Tres situaciones
adicionales de
gradientes variables:
•Implicaciones en la
permeabilidad.
•Tipos de suelo
•Suelos estratificados
•Suelos con
permeabilidad variable
INTERPRETACIÓN CUALITATIVA DEL GRADIENTE
BASIC CONCEPTS
FREE WATER
FREE WATER
FREE WATER
How to produce this situations?
DARCY´S LAW
1. INCREASING (Q)
ALSO INCREASES hL
2. INCREASING L
DECREASE Q
3. INCREASING A
ALSO INCREASES Q
Sectional
area, A
N.R.
BASIC CONCEPTS
1 3 42
k : hydraulic
conductivity
BASIC CONCEPTS
L
hkAQ L
L
hA L
Q
UNITS
L
hAkQ
**
hA
LQk
*
*
T
LQ
LA
LL
Lh
3
2
T
L
LL
LT
L
*
*
2
3
BASIC CONCEPTS
FACTORIZATION OF THE DEPENDENCY
Cases:
•Different terrains
•Water? Other fluid?
(change the
permeability)
•Temperature
•Salt content
m = viscosidad dinámica
del fluido
DARCY:
By definition, a material of 1
darcy permits a flow of 1 cm3/s
of a fluid with viscosity 1 cP (1
mPa.s. Water at 20°C ) under a
pressure gradient of 1 atm/cm
across an area of 1 cm2.
m
m
gkkk
** 00
Depends on
the material
Depends on
the fluid
BASIC CONCEPTS
Typical values (cm/s)
Tomado de Juarez-Badillo & Rico Rodriguez. “Fundamentos de la Mecánica de Suelos”
BASIC CONCEPTS
VELOCITIES: DISCHARGE, FILTRATION AND FLOW
l
Av
As
kiv
A
Qv
vAkiAQ
e
ev
A
A
A
Q
A
Qv
vv
f
1*
e
e
V
V
LA
AL
vv
1
(Discharge velocity)
Filtration velocity
v
See that:
BASIC CONCEPTS
Attention! Pores and pipes are not rectilinear
but highly curvilinear.
l
lvv R
fR(Real velocity or flow
velocity)
“sinuosity” factor = f(lR)
lR : real length of flow
UNKNOWN!!!
BASIC CONCEPTS
representative values, Castany (1963)
Tomado de Vélez Otálvaro, M.V. “Hidráulica de Aguas Subterráneas”
BASIC CONCEPTS
APLICABILIDAD DE LA LEY DE DARCY
EXISTE UNA RELACIÓN
LINEAL ENTRE EL
GRADIENTE HIDRÁULICO
Y LA VELOCIDAD DE
DESCARGA DEL FLUJO A
TRAVÉS DEL MEDIO
POROSO.
D
TfvC ,
1T: temperatura
D: Diámetro de la conducción en [cm]
Turbulento
Laminar
vc
i
6.5vc v
BASIC CONCEPTS
m
vDR
V : Velocidad de drenaje [cm/s]
D: Diámetro prom. partículas [cm]
: Densidad del fluido [gr/cm3]
m: Coef. viscosidad [(gr*s)/cm2]
En medios porosos: Reynolds (1883)
El valor limite del numero de Reynolds para que un fluido
cambie de laminar a turbulento oscila entre 1 y 12
R (agua) ≤1 con v= 0.25 cm/s y D <0.4mm (arena gruesa)
Flujo laminar válida la Ley de Darcy
BASIC CONCEPTS
HYDROGEOLOGY
Where is Earth's water located?
Picture of Earth showing if all Earth's
water (liquid, ice, freshwater, saline) was
put into a sphere it would be about 860
miles (about 1,385 kilometers) in
diameter.
An aquifer is a geological formation capable of yielding useful
groundwater supplies to wells and springs. All aquifers have two
fundamental Characteristics: a capacity for groundwater storage and a
capacity for groundwater flow. But different geological formations vary
widely in the degree to which they exhibit these properties and their
areal extent can vary with geological structure from a few km2 to many
thousands of km2.
HYDROGEOLOGY
The most significant elements of hydrogeological diversity are:
● major variation of aquifer unit storage capacity (storativity), between
unconsolidated granular sediments and highly-consolidated fractured rocks
● wide variation in aquifer saturated thickness between different
depositional types, resulting in a wide range of groundwater flow potential
(transmissivity).
The vast storage of many groundwater systems (much larger than that of the
Biggest man-made reservoirs) is their most distinctive characteristic. In
consequence most groundwater is in continuous slow movement from areas of
natural aquifer recharge (from rainfall excess to plant requirements) to areas of
aquifer discharge (as springs and seepages to watercourses, wetlands and coastal
zones).
What is the relationship between groundwater and surface water?
• Streams and rivers on which an aquifer is dependent as a significant source of its overall recharge
• Rivers that in turn depend significantly on aquifer discharge to sustain their dry-weather flow.
Why is the estimation of aquifer replenishment important?
● Contemporary aquifer recharge rates are a fundamental consideration in the sustainability of groundwater resource development. Furthermore, understanding aquifer recharge mechanisms and their linkages with land-use is essential for integrated water resources management.● The quantification of natural recharge, however, is subject to significant methodological difficulties, data deficiencies and resultant uncertainties because of:
● wide spatial and temporal variability of rainfall and runoff events● widespread lateral variation in soil profiles and hydrogeological conditions.
How can the ‘safe yield’ of an aquifer be defined?
All groundwater flow must be discharging somewhere, and
abstraction will reduce these discharges. But the source of
groundwater pumped can be complex. So-called ‘safe
yield’ is clearly bounded by the current long-term average
rate of aquifer recharge
Conceptual effects of abstraction on the groundwater resource balance
● discharge to freshwater systems required to sustain downstream water-supply or river ecosystems
● discharge via natural vegetation, including that sustaining ecologically and/or economically valuable freshwater wetlands
● discharge to saline areas, including coastal waters, salt lakes and pans and make allowances for those parts of these discharges which need to be conserved.
When can an aquifer said to be ‘overexploited’?
The term ‘aquifer overexploitation’ is an emotive expression not capable
of rigorous scientific definition. But it is a term which water resource
managers would be wise not to abandon completely, since it has clear
register at public and political level. Some regard an aquifer as being
overexploited when its groundwater levels show evidence of ‘continuous
long-term’ decline.
In practice, when speaking of aquifer overexploitation we
are invariably much more concerned about the
consequences of intensive groundwater abstraction
than in its absolute level.
Thus the most appropriate definition is probably an
economic one: that the ‘overall cost of the negative
impacts of groundwater exploitation exceed the net
benefits of groundwater use’, but of course these
impacts can be equally difficult to predict and to cost.
Water yiels and subsidence induced settlements in Mexico City
22 /4,3/2
1,55,6
cmkgcmkgmv
1cm2/kg=0,01m2/kN
1600 water wells
80 m3/s
Piezometric depletion 10 m
in 15 years (down town)
Qual é a situação atual dos projetos de aproveitamento racional
do aquífero Guaraní?
Rio
K=1x10-6 m/s
Mina
1. Piezometric head in points A, B and C in natural conditions (before the mine).
Explain each calculation
2. If the mine is instantly opened. Is there any flow of water into the excavation?
Explain your answer.
3. In steady state regime (time after the excavation) calculate piezometric head
in points A, B and C. Explain.
4. Considering vertical flow only how much water enters into the excavation?
Justify your answer.
Imagine at least three different possible
conditions for the groundwater condition
for this geological situation, in terms of
the piezometric and phreatic heads.
CONCEITOS BÁSICOS DE FLUXO DE
AGUA EM SOLOS
PRESSÃO HIDROSTÁTICA
GRADIENTE
LEI DE DARCY
PERMEABILIDADE
EQUAÇÃO DE BERNOULLI
VELOCIDADE DE DESCARGA, DE
INFILTRAÇÃO E REAL.
SEGUE:
SOLUÇÃO DA EQUAÇÃO DE
LAPLACE
ANÁLISE BASEADO EM MÉTODOS
GRÁFICOS
EFEITO DA AGUA EM ESCAVAÇÕES E
TALUDES
TENSÕES TOTAIS, EFETIVAS E
NEUTRAS
TUDO ENTRA NA PROVA!!!!
RESUMO