Transcript kuliah 14(permeabilitas)

PERMEABILITAS
Yulvi Zaika
SIKLUS HIDROLOGI
Muka Air Tanah
AQUIFER
Dari mana datangnya air tanah
Kemana air tanah mengalir?
Interaksi air tanah dan air permukaan
Aliran

Shallow, unconfined aquifers often follow the same flow
patterns as the watershed above them, as seen in the
illustration at right. Deeper, confined aquifers may be part of a
different flow system.
Bagaimana mengeluarkan air
tanah?
UKURAN BUTIRAN TANAH
Porositas
TANAH
BATUAN
Permeabilitas
Kemampuan air mengalir melewati tanah
KOEFISIEN PERMEABILITAS
Soil
Coarse gravel
Permeability
Coefficient, k
(cm/sec)
>10-1
Relative
Permeability
High
Sand, clean
10-1-10-3
Medium
Sand, dirty
10-3-10-5
Low
Silt
10-5-10-7
Very Low
Clay
<10-7
Impervious
Seepage Through Porous Media
W.T.
Impervious Soil
)h = hA - hB
W.T.
hA = total head
Impervious Soil
Datum
hB= total head
W.T.
Impervious Soil
)h = hA - hB
W.T.
hA
Water In
Impervious Soil
Datum
hB
q = v .A= k iA= k
Dh
A
L
Head Loss or
Head Difference or
Energy Loss
)h =hA - hB
i = Hydraulic Gradient
hA
(q)
Water
out
hB
ZA
ZB
Datum
To determine the rate of flow, two parameters are needed
* k = coefficient of permeability
* i = hydraulic gradient
k can be determined using
1Laboratory Testing
2Field Testing
3Empirical Equations
i can be determined
1from the head loss
2flow net
[constant head test & falling head test]
[pumping from wells]
Seepage Through Porous Media
Water In
L = Drainage Path
Head Loss or
Head Difference or
Energy Loss
i = Hydraulic Gradient
Dh =hA - hB
hA
Water
out
A
Porous
Stone
Soil
L
hB
Datum
B
Porous
Stone
Seepage Through Porous Media
Water In
L = Drainage Path
Head Loss or
Head Difference or
Energy Loss
i = Hydraulic Gradient
Dh =hA - hB
hA
Water
out
A
ZA
Porous
Stone
Soil
hB
B
Porous
Stone
L
Datum
ZB
No Seepage
Piezometer
In
Flow
3 ft
D
u = 6 x 62.4
C
u = 14 x 62.4
14 ft
Out
Flow
2 ft
4 ft
B
Ws
12 ft
W
Ws
s
Ws
Ws
Buoyancy
8 ft
A
3 ft
3 ft
Datum
In
Flow
Piezometer
3 ft
u = 6 x 62.4 + Du
Du
17 ft
u = 17 x 62.4
Upward Seepage
D
C
Out
Flow
2 ft
4 ft
B
12 ft
Ws
Ws
8 ft
Ws
Ws
Ws
Buoyancy + Seepage Force
A
3 ft
3 ft
Datum
Downward Seepage
Piezometer
3 ft
D
In
Flow
2 ft
u = 6 x 62.4 - Du
C
10 ft
u = 17 x 62.4
Out
Flow
4 ft
B
12 ft
Seepage Force
8 ft
Ws
W
Ws
s
Ws
A
3 ft
Ws
Buoyancy - Seepage Force
3 ft
Datum
No Seepage
1
g1 =110 pcf
2
1
2
W.T. 3 ft
4 ft
3
3
-
4
=
6 ft
4
12 ft
5Total Stress
Total Stress
s1 =
s2 =
s3 =
s4 =
s5 =
Pore Water Pressure
u1 =
u2 =
u3 =
u4 =
u5 =
5
Effective Stress
Pore Water
Pressure Effective Stress
s1 =
s2 =
s3 =
s4 =
s5 =
Ws
W
Ws
s
Ws
Ws
Buoyancy
No Seepage
W.T.
1
g1 =110 pcf
3 ft
4 ft
2
-
=
6 ft
3
12 ft
4
Total Stress
Pore Water
Pressure
Effective Stress
Ws
W
Ws
s
Ws
Ws
Buoyancy
No Seepage
W.T.
1
3 ft
3 ft
2 pcf
g1 =110
4 ft
3
-
=
6 ft
4
12 ft
5
Effective Stress
Total Stress Pore Water
Pressure
Ws
W
Ws
s
Ws
Ws
Buoyancy
Upward Seepage
1
2
5 ft
g1 =110 pcf
W.T. 3 ft
4 ft
3
-
=
6 ft
4
12 ft
54 Total Stress
4 Effective Stress
Pore Water
Pressure
Ws
Ws
Total Stress
Pore Water Pressure
Effective Stress
Ws
Ws
Ws
Buoyancy + Seepage Force
Downward Seepage
g1 =110 pcf
1
1
3 ft
2
W.T. 3 ft
4 ft
2
-
=
6 ft
3
3
12 ft
4
Total Stress
Total Stress
4 Effective Stress
Pore Water
Pressure
Pore Water Pressure
Seepage F
Effective Stress
Ws
W
Ws
s
Ws
Ws
Buoyancy - Seepage Force
g1 =110 W.T.
pcf 3 ft
W.T. 3 ft
4 ft
4 ft
6 ft
6 ft
12 ft
12 ft
PERCOBAAN LABORATORIUM

Figure 1 is a sketch of the
apparatus used to
determine the coefficient of
permeability in the
laboratory. The soil sample
should be as close to undisturbed as possible. While
performing the test, one has
to maintain a constant
water level at the top by
adding water at a rate of q
for some time interval of t.
Then

k

QL
HAt
and Q is the volume of
water collected in time t
from the outlet. The crosssectional area of the
specimen is indicated by A.
PERCOBAAN LAPANGAN
PERMEABILITY
Porositas mengacu pada kecendrungan material untuk
dilewati zat cair melewati pori-pori nya.
 Permeabilitas adlah parameter anah penting dalam
proyek dimana air mengalir melewati tanah atau batuan
seperti alairan yang melewati bawah dam , drainse pada
lapisan sabgrade dan timbunan, mengetahui kecepatan
sumur dapat teisi kembali dan dewatering untuk struktur
yang dekat dengan muka air tanah.


Ada beberapa faktor yang mempengaruhi permeabilitas
tanah adalah:
1. visikositas air ( dipengaruhi oleh suhu)
2. ukuran dan bentuk partikel
3. derajat kejenuhan
4. void ratio

In construction, if an excavation is to be done
below the water table, a dewatering plan needs
to be followed. A highly permeable soil will
require a pumping system with a comparatively
large capacity. A low permeable soil may not
require any pumping. In addition, fill material
that is highly permeable can usually be placed
and compacted immediately, but low permeable
fill may require days of drying before being
placed and compacted.
The fundamental description of permeability
is based on the equation q=vA which takes the
familiar form similar to river discharge. The
variable q is the discharge (Vol/Time), v is the
apparent velocity, and A is the area that is
related to the geometry of the situation. Now,
Darcy's Law describes the factors important in
determining the value of v, which is
 v=ki
 where k is a constant for the material and is
called the coefficient of permeability, and i
is the hydraulic gradient which is related to
the water pressure. The following table lists
some soil permeabilities.
