characterization unsaturated zone with insitu test 2003 sa

1

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

2

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

3

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

4

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

5

.

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

6

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.