characterization unsaturated zone with insitu test 2003 sa
TRANSCRIPT
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1 INTRODUCTION
Indonesian territory has a specific characteristic geology, it is lay on tropical zone and influence by tectonic or volcanic forces. This condition shapes the soil characteristics, and categorized the soils as tropical volcanic residual soil.
Residual soils is a soil exist from weathering process of rock, as a result of exposure to weather and still lay above it’s base rock. Tropical soil is a soil located between Tropic of Cancer (23.5
0 N) in North
dan Tropic of Capricorn (23.50S) in South. Volcanic
soils is a soils derived from volcanic activity, directed from volcanic ash or as a weathering product of volcanic base rock (magma). Tropical volcanic residual soil of West Java in this research, has a weathering process consist of high temperature, high rain fall and good drainage condition.
Landslide in West Java region is numerous, more than 100 cases each year. Almost every landslide cases, induced by rainfall (rainfall induced lanslide).
The water table lay deep below surface, this means that soil is under unsaturated condition. Besides of the weight of water, landslide occurs as the result of decreasing the shear strength of soils when soils become saturated from unsaturated condition. As an initial effort for define and anticipated the landslide, it is important to first define the characteristic of the unsaturated zone.
Weathering process from base rock to residual soils is the independent variable to define another residual soils characteristics. Therefore as a first step, it is necessary to define the mineralogy of the residual soils. Weathering process than become the main influence factor to define physical properties characteristics, shear strenght, compressibility, in-situ stress, and slope hidrology characteristics of residual soils.
In dry condition, a slope of tropical volcanic residual soils has a high shear strength characteristics. The slope commonly stand up with steep slope, the angle of slope is higher than it’s shear angle. This is commonly the effect of two factors, the soil structure
Charaterization of Unsaturated Zone Susceptible to Landslides in
Tropical Volcanic Residual Soil with In-Situ Tests
P. P. Rahardjo Catholic University of Parahyangan Bandung, Indonesia
R. Karlinasari Islamic University of Sultan Agung Semarang, Indonesia
ABSTRACT: Tropical residual soil has unique characteristics due to the influence of the weather. The intensity of the rainfall is much higher than it’s soil permeability, and landslide become a serious threat. The number of landslides in West Java is numerous, more than 100 events yearly. Landslide is produced if the unsaturated zone on the upper side of a slope becomes saturated, hence it is necessary to characterize the unsaturated zone phrone to landslide on a slope. A research of weathering zone on residual soil were conducted with Scan Electron Microscopy (SEM) and X-Ray Diffraction (XRD) methods to gain the physical and geo-chemical characteristic of slopes, then a suction measurement of Filter Paper Method were conducted. This resulted in a conclusion that the weathering zone is a major reason to define the depth of unsaturated zone. A zone called zone 4 (the term from Little,1969) is defined as a zone where weathering process change between disintegration of base rock to lateritation process. The profile above the zone 4 is a zone where unsaturated zone highly influenced by weather and potential to slide. A series of in-situ tests (CPT-u and Dilatometer Test) and drilling for undisturbed samples and laboratory tests were conducted, to confirm the characteristic of this zone 4. The in-situ test were proved to be effective to define and characterize this zone, and consequently proved to be successful to define the unsaturated zone for landslide analysis. Keywords : Unsaturated Zone, Tropical residual soil, weathering zone, SEM, XRD, Filter Paper, CPT-u,
Dilatometer
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and the contribution of matrix suction to shear strength (Fredlund & Rahardjo, 1993; Melinda, 2004). How this two factors work on slope is a problem that need more study.
Tuncer & Lohnes (1977) published a profile of mineralogy, physical properties and shear strength parameters of Hawaiian and Puerto Rico tropical volcanic residual soil. This profile is very interesting, it is show a different condition over weathering zone. Using the same method of profiling, we intent to study further, in purpose to have a depth knowledge and define the mechanism of the two factors, the soil structure and the matrix suction controlling the shear strength of slope.
2 MINERALOGY OF TROPICAL VOLCANIC
RESIDUAL SOILS Based on mineralogy of tropical volcanic residual soils from Older Volcanic Products Formation (Qob) it is known, that the weathering process occur in stages as follow (Fig 2-1) :
a. The Beginning of Weathering Process. In this stage a disintegration of base rock is occuring. In the layer of soils a feldspar minerals were found in a significant number. Feldspar mineral were found as a common mineral in base rock. In microscopic (from SEM), it is observed that the grain of the soil is still in some chrystal shapes and in a non pararel structure. Feldspar mineral which found in example is Oligoclase, Na0.8Ca0.2Al1.2Si2.8O8, and Orthoclase KAlSi3O8. Feldspar mineral than broke by rainfall’s water and composed a carbonat and chlorit mineral.
b. The Washing of Carbonat and Chlorit Process. In this stage a wash of carbonat and chlorit mineral is occuring. This washing process made the soil behave dispersively. The soil
become lighter in colour, it is white or yellowish.
c. Oxidation Process.
In this stage, an oxidation process is occuring.
Mineral from oxide and hydroxide group,
which produced by an oxidation process of
iron were found, such as Goethite (FeO(OH))
and Hematite (Fe2O3). The soil become
reddish in present of iron mineral. In the
influence of drying-wetting weather condition,
the soil become flocculate, the mineral grain
clump together into floc, with a pararel
structure.
Based on these three specific process the profile of
tropical volcanic residual soils, then can be divided
into three different zone. The three specific zone are
The Beginning of Weathering Zone, The Washing
of Carbonat and Chlorit Zone, and Oxidation Zone.
Based on Tuncer & Lohness,1977 profile, this three
zones categorized into a four zones as follow:
a. Zone 2, as the beginning of weathering zone
b. Zona 3, as the washing of carbonat and
chlorit zone
c. Zona 4, as the beginning of oxidation zone
d. Zona 5, as the end of oxidation zone
3 PHYSICAL PROPERTIES CHARACTERISTIC OF TROPICAL VOLCANIC RESIDUAL SOILS
Based on Tuncer & Lohnes diagram, the
physical properties of tropical volcani residual
soils were evolve as follow : Stage 1 and stage 2
were categorized as the beginning of weathering
zone (where the base rock start to break), stage 3
was categorized as the washing of alkaline
mineral zone where clay percentage start to
increase, and stage 4 -
Figure 3-1. Weathering zones of tropical volcanic residual soils
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5 categorized as the oxidation zone, where the
sesquioxides (Fe203 dan Al2O3) were present.
In the stage 3 the broken mineral, the alkalin, is a
dispersive agent, it makes the soil grain become
finer. As result the void ratio reduce and soil’s
permeability decrease.
Figure 3-2. The variation of physical properties of Basalt rock
in weathering to laterite (Tuncer and Lohnes, 1977)
In the stage 4, the increase on sesquioxides (Fe2O3
dan Al2O3), means the increase on specific gravity
because the iron mineral have a relatively high
specific gravity. The clay particle cemented in
flocculation process of sesquioxides. The void ratio
increase because higher specific gravity means
decrease on solid volume (Vs). In stage 5 a decrease
on degree of saturation (Sr) occured, the soil become
unsaturated. In unsaturated soil the index relation
become as follow :
(1)
On equation 1, if an increasing of density and
specific gravity occured, in decrease of degree of
saturation will result on decrease of void ratio.
Specific gravity and void ratio become a main
parameters in controlling another soil parameters. In
conclusion, this two parameters profile are describe
as follow:
Specific Gravity. A difference on type of minerals in
each zone results on a difference of specific gravity,
as an example the minerals resulted from oxidation
process as Geothite and Hematite has a higher
specific gravity of 3.3 – 3.5 and 4.9 - 5.3. So the
profile of specific gravity has a unique shape as
shown in figure 2-2.
Void ratio. The difference of minerals behaviour as
dispersive in zone 3 and flocculative in zone 4 and 5
resulted on the difference on void ratio (Figure 2-2).
Dispersive behaviour resulted on a decreasing of
void ratio, meanwhile flocculative behaviour
resulted on increasing of macro void ratio and
decreasing on micro void ratio. The void ratio
increase on zone 2, decreasing on zone 3 and
increasing back on zone 4, it’s achieve a maximum
value at the border of zone 4-5. Void ratio decrease
again on zone 5 because of the relatively high
decreasing of degree of saturation as described in
equation 1.
Figure 3-3.(a) Specific gravity of minerals and l Gs profile,
(b) Void diameter and void ratio profile
The high magnitude of void ratio in the zone 4
towards the border of zone 4-5 consequence on a
higher permeability, this zone become a collecting
place of water (water trap zone). The degree of
saturation and water content in this zone become
higher.
3.1 Matrix Suction
To define the matrix suction profile an
instrumentation on Cijengkol Slope, a slope of
tropical volcanic residual soil, was conducted with
the instalation of 1 set Jetfill Tensiometer for
monitoring matrix suction and 1 set ADR
Thetaprobe for monitoring volumetric water content.
The depth of instalation are 0.6 m, 1.2 m and 2.1 m.
Meanwhile a Filter Paper method of measuring
matrix suction was conducted at laboratorium.
Figure 3-3 below show all the measuring matrix
suction profile. In conclusion, the profile of matrix
suction was describe as follow : suctions on soils
consist of two type of suction, matrix suction and
osmotic suction. Osmotic suction is a suction
related to a specific mineral on soils, as an example
a soil with high carbonat and chlorite has a high
value of osmotic suction. Matrix suction is a suction
related only to soil structure, the value of void ratio
between soil grain, micro or macro void. The
magnitude of macro and micro soils are dependent
to the type of mineral in soils, so the matrix suction
also influence by the type of mineral in soils
indirectly. In this research the matrix suction profile
is defined as follow : on zone 3 matrix suction is low
because the grain size of soil categorized as silty or
sandy clay. On zone 5 matrix suction relatively
higher because the grain size of soil categorized as
we
Ses
G
1
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clay. The condition of matriz suction on site is as
follow, matrix suction on zone 4 is the lowest
because the water trap condition, on zone 5 matrix
suction varied over degree of saturation, in general
because of the high rainfall intensity, matrix suction
fell below the soil’s air entry value .
Figure 3-4. Matrix suction profile of BH02 Cijengkol Slope
4 IN-SITU CHARACTERISTICS OF
TROPICAL VOLCANIC RESIDUAL SOILS
4.1 CPT-u Test
CPT-u test was conducted at residual soil of Kalijati,
West Java. This residual soils is a weathered result
from a base rock of Qos formation, a sediment
formation of Older Volcanic Product (Qob). This
area is located at a lower elevation than Cijengkol
slope, so the weathered zone is thicker. The soil
profile is homogen from silty clay to clay, without
any other insertion of different type of soil (sand).
A Sta 109+500 (Figure 4-1) zone 4 located at 10 to 16 m, it is marked by a value of FR from 2 to 4. Besides the value of FR, the value of u2 and the value of Bq could also be as an indicator of zone 4. The value of u2 at zone 4 is between 0.2 to 0.6 MPa, while the value of Bq is around 0.2 to 0.6 %. The same range of values were obtain from other bore holes as shown on table 4-1 below.
Table 4-1. Values of FR , u2 and Bq from CPTu test at Kalijati
residual slope
Sta Zone 4 (depth) FR u2 (MPa) Bq (%)
Sta 109+500 10 - 16 m 2 – 4 0.2 - 0.6 0.2 – 0.6
Sta 113+650 13 – 18.5 m 2 – 6 0.2 - 0.4 0.2 – 0.4
Sta 116+900 6.5 - 9.5 m 2- 6 0.0 - 0.4 0.0 - 0.4
Inconclusion, zone 4 is a position where positive
water pressure present (u2 present).
Schneider,Peuchen & Mayne, 2001 in their paper of
Piezocone Profiling of Residual Soils, concluded
that positive water pressure u2 profile indicate
clearly the different in stratigraphy of residuals soils
that related to weathering profile. As an example in
Schneider profile of CPT-u test result on Singapore
residual soils as follow (Figure 4-2), zone 4 is
indicate at 12-13 m depth, because u2 is at 0.2-0.4
MPa and FR is around 2 to 4 %.
This same conclusion were produced from this
research, moreover this conclusion also confirm by
the results of characteristics profile of mineral and
physical properties.
With knowledge to define zone 4 location, then it
will be possible to predict the unsaturated behaviour
of the soil profile, because the residual soil undergo
the same weathering process.
Figure 4-1. Parameters profile from CPT-u test
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.
Figure 4-2. CPTu profile of residual soil (weathered product
from clay stone and breksi ) below marine clay layer of
Singapore, from Schneider, et all
4.2 Dilatometer Test
Insitu test with dilatometer were conducted at
four location at Neglajaya slope and one location at
Cijengkol slope. One of the results was shown in
Figure 4-3 below. Based on corrected first reading
(po) and coreccted second reading (p1), an
intermediate parameters of ID (material index), KD
(horisontal stress index) and ED (dilatometer
modulus) were obtained. From this intermediate
parameters then derived by empirical formula
another parameters : Ko, OCR and MDMT (Vertical
Drained Constrained Modulus). Different from
sediment soils, OCR value obtained from this
dilatometer test was not correlated with the thick of
fill ever happen in past, but more correlated to the
present or not a structures in soils. Because of those
different perception Fonseca,et all, 2008, proposed a
change of term to vOCR or virtual OCR.
From the research, in general p1 profile is almost
the same as po profile with a maximum value as
follow :
Table 4-2 The values of maximum po and p1 from dilatometer
tests
Lokasi Kedalaman
(m)
po
max
p1 max
(kPa)
DMT01 BH-02 Neg 12 1100 1600
DMT02 BH-03 Neg 10 1000 1600
DMT03 BH-04 Neg 13 1200 1500
DMT04 BH-05 Neg 12 600 1000
DMT01 BH-03 Cij 10.6 460 1600
KD, Ko and vOCR profile in general is almost in the
same shapes. The dilatometer parameters in each
zone were discussed in order as follow.
4.2.1 Dilatometer parameters of zone 4
The value of KD at this zone shown a relatively the
same value between 2 to 4. Marchetti states KD
equal 2, as a Normally Consolidated (NC) condition
on sediment soil, so for residual soils, based on
characteristics of mineral and physical properties, it
is concluded that KD < 4 as an indicator of lost of
structure soil’s zone.
Table 4-3 Parameters value from Dilatometer Test at zone 4
Location po
(kPa) p1
(kPa) ID KD
D01-2 Neg 100-500 700-1000 1-3.75 (sand) 2-4 D02-3 Neg 400-500 700-1000 0.6-1.6(silt) 2-4
D03-4 Neg 200-500 500-1000 0.6-1.8(silt) 2-4 DMT04 BH-05 200-400 400-1000 0.6-1.0(silt) 1-2 DMT01B-03 Cj 200-400 500-900 1-4(sand) 2-4
Location ED
(MPa) Ko vOCR MDMT
(MPa)
D01-2 Neg 10-20 0.5 1-2 10-25 D02-3 Neg 10-20 0.5 2 18-22 D03-4 Neg 10-22 0.5-0.8 1-2.5 10-30 DMT04 BH-05 5-13 0.5 1 2-13
DMT01 B-03 Cj 10-30 0.5-0.8 1-2.5 10-40
Figure 4-3. Parameters profiles from dilatometer tests
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The value of Ko is around 0.5, a common value for
NC soil. The valur of vOCR is between 1 to 2,
indicated the soil as an NC to lightly Over
Consolidated (OC). The ID parameter shown
uncorelated to the result of index properties test at
laboratory. ID parameter is a function of po, p1 and u0
as folow :
(2)
It is shown that the ID value is higher than the ID
value of the index properties type of soils, which is
clay and silt. This is possible because of the
unsaturated condition. If the water pressure become
negative (in unsaturated soil), therefore the ID will
be lower and in suitable range of value, of clay and
silt type of soils.
4.2.2 Dilatometer parameters of zone 5
The parameters from dilatometer test of zone 5 is
shown in the table 4-4 below.
Table 4-4 Parameters from Dilatometer tests of zone 5
Location po(kPa) p1(kPa) ID KD ED
(MPa)
DMT01
BH-02 Neg
0-100 0-300 0.6-2.8
(Silt to
sand)
2-8 0-10
DMT02
BH-03 Neg
200-
800
500-
1200
0.3-4.0
(Clay to
sand)
4-
>20
5-25
DMT03
BH-04 Neg
200-
400
250-
750
0.5-1.8
(Silt)
2-10 10-22
DMT04 BH-05 Neg
100-350
400-650
0.7-3.0 (Silt to
sand)
1.5-2.5
5-14
DMT01
BH-03 Cij
100-
200
250-
500
0.8-4.0
(Silt to
sand)
2 -
>8
7-15
Location Ko vOCR MDMT(MPa)
DMT01 BH-02 Neg 0.5-1.6 2 - >8 0-10
DMT02 BH-03 Neg 1-3 4 - >20 10-80
DMT03 BH-04 Neg 0.5-2 1 - >10 8-30
DMT04 BH-05 Neg 0.5-0.7 1 – 1.5 2.5-15
DMT01 BH-03 Cij 0.5- > 2 1 - >10 8-30
4.2.3 Dilatometer parameters of zone 3
The parameters from dilatometer test of zone 3 is
shown in the table 4-5 below.
Table 4-5 Parameters from Dilatometer tests for zone 3
Location po(kPa) p1(kPa) ID KD ED
(MPa)
DMT01
BH-02 Neg
500-
1000
600-
1500
0.3-1.0
(Clay to
silt)
3-6 10-30
DMT02 300- 650- 0.6-0.8 2-6 12-22
BH-03 Neg 1000 1600 (Silt)
DMT03
BH-04 Neg
400-
1200
500-
1550
0.2-0.8
(Clay to
silt)
2-6 8-20
DMT01
BH-03 Cij
100-
600
450-
1600
1.8-7.0
(Sand)
1.5-
3
10-50
Location Ko vOC
R
MDMT(MPa)
DMT01 BH-02 Neg 0.75-
1.25
2 - 6 10-50
DMT02 BH-03 Neg 0.5-1.30 1-4.5 5-25
DMT03 BH-04 Neg 0.5-1.25 1-4.5 5-25
DMT01 BH-03 Cij 0.4-0.75 1-2 25-70
5 CONCLUSION
Zone 4, where water trap condition occured,
becomes an indicator zone to devide the tropical
volcanic residual soil into weathering zones. With
knowledge to define the zone 4 location, then it will
be possible to predict the unsaturated behaviour of
the soil profile, because the residual soil undergo the
same weathering process.
This research proved the effectiveness of insitu
tests to define the zone 4 location. The Dilatometer
and CPT-u test are among the best insitu tets to
define the weathering zone. The insitu parameters
from each different zone of weathering, have its
specific range of values. The Index Material
parameter ID from Dilatometer test is required to
redefine because it is not yet included the effect of
matrix suction on soils.
REFERENCES Blight, G.E., 1988, Keynote Paper : Construction in Tropical
Soils, Proceedings Geomechanics in Tropical Soils Semi-
nar, NTU-ISSMFE-SEAGS, Singapore. Fonseca, A.V, Coutinho, R.Q. , 2008, Characterization of re-
sidual soils, Geotechnical and Geophysical Site Characteri-
zation – Huang & Mayne (eds), Taylor & Francis Group,
London.
Fredlund, D.G. and Rahardjo, H. ,1993, Soil Mechanics for
Unsaturated Soils, New York: John Wiley & Sons, Inc.
Karlinasari, R., 2009, Study of Characteristics of Tropical
Volcanic Residual Soil From Older Volcanic Products
Formation (Qob), West Java, PhD Dissertation at Catholic
University of Parahyangan, Bandung. Marchetti, Monaco, DMT Course Notes (2001), 77 p, Insitu
2001, Bali. Melinda, F., Rahardjo, H.,Han, K.K., and Leong,E.C.,2004,
Shear Strength of Compacted Soil under Infiltration Condi-
tion, Journal of Geotechnical and Geoenvironmental Engi-
neering, Vol. 130, No. 8, August 1, 2004,ASCE.
Schneider, J.A., Peuchen, J., Mayne, J., and McGillivray, A.V.
(2001) Piezocone profiling of residual soils, Proceedings,
International Conference on In Situ Measurements of Soil
Properties and Case Histories, Bali, Indonesia, May 21-
24,2001, pp. 593 – 598,GEC, Bandung.