Effects of gypsum addition on sedimentation characteristics ofTondano Lake clay, Indonesia
Meldi T.M. SINOLUNGAN1 , Fumiyoshi KONDO2 and Tatsuya KOUMOTO2

Abstract: A laboratory experiment was carried out to investigate the sedimentation characteristics of Tondano lakeclay and effect of gypsum addition, included the settling
patterns as the previous experiments. Firstly, the ﬂocculation and dispersion characteristics, which are evaluatedby turbidity, are substantially inﬂuenced by pH and salinity. Then, it was found that the addition of 5 % gypsum to this clay inﬂuenced the settling pattern and settling/consolidation rate. In the region in which consolidation settling does not occur (above 3000 % water content),
the effects of gypsum addition on the settling patterns are
signiﬁcant; whereas in cases that the consolidation settling
occurs (under 2000 % water content), the effects of gypsum addition are not so much signiﬁcant. The greater water content of the clay suspension could induce a higher
degree of ﬂocculation, thus explaining sedimentation characteristics induced by gypsum treatment.
Key Words : Tondano Lake, sediment, gypsum, ﬂocculation and dispersion, sedimentation

1.Introduction
Tondano Lake is one of the important and strategic natural resources for the people of North Sulawesi’s life. This
lake has multifunction, like source of fresh drinking water, washing and bathing, hydroelectric power, ﬁsh aquaculture, wet-rice ﬁeld/agriculture and tourism. It is situated 693 m above sea level with an area of 4,638 ha, andTondano River (41,100 m length) is its outlet, ﬂowing into
Manado Bay (Sinolungan et al., 2008). The Tondano attracts a lot of attention from a variety of societies, government and scientists because the condition of this lake tends
to be deteriorated. Some problems of Tondano Lake are: 1)
Shallowing the lake basin due to high erosion and sedimentation; 2) Decreasing the water ﬂow at the outlet (TondanoRiver); 3) Blooming the aquatic plants; 4) Decreasing thehabitats or species of ﬁshes. The shallowing phenomenon
may be seen from decreasing the water volume in dry mon1 The United Graduate School of Agricultural Sciences, Kagoshima University / Sam Ratulangi University, 1-21-24 Korimoto, Kagoshima 8900065, Japan / Kampus Unsrat-Bahu, Manado 95115, Indonesia. Corresponding author: Meldi T.M. SINOLUNGAN
2 Dept. of Environmental Sciences, Faculty of Agriculture, Saga University,1 Honjo-machi, Saga 840-8502, Japan
2008 年 5 月 22 日受稿 2008 年 12 月 8 日受理
土壌の物理性 111 号, 43–51 (2009)

soon, whereas in rainy monsoon the ﬂood comes over thelake and its environs (Kemur, 1998). The Tondano is beinga receiver of waste passing through the catchment’s basin.
For example, the Toliang Oki River carries an estimatedannual sediment load of 63.01 t ha−1 , comprised from avariety form of land uses (Natural Resources Management
Program, 2001).
Sedimentation phenomena have been the subjects ofmany decades of research, and while much insight has
been gained concerning the hydrodynamics of sedimentation processes, comparatively little is known about the roleof particle attractive forces in settling and sediment consolidation (Nasser and James, 2006). The ability of ﬁne particles to settle rapidly is made possible by the process ofparticle ﬂocculation (Kranck, 1973, 1980). Dispersion is
also inﬂuenced by clay type, and cation concentration andcomposition as reported by Agassi et al. (1980). Ahmad
and Karube (2005) found that kaolinite coagulated below
pH 6 and dispersed well above that for both Na- and Catype of clays after it was dialyzed. At pH 8, the criticalcoagulation concentration for kaolinite, which was higher
than for montmorillonite, was substantially inﬂuenced by
the small size of particles. Kondo and Torrance (2005)
demonstrated that, the presence of high-swelling smectite
to low activity-mineral-dominated Leda clay increased thesalinity and water content ranges above levels which consolidation settling and zone settling could occur. Thesesettling rate patterns change with time in a manner that isdependent on the settling mode, water content and salinity.
The settling behavior of clay suspension is determinedby interactions among particles as they are inﬂuenced byvarious aspects of the depositional environment. Understanding the depositional characteristics of soil suspension
has practical applications related to the settling behavior
during ﬂocculating treatment of waste water, land reclamation by using dredged mud (Kondo and Torrance, 2003),
and lake sediment formation. In this study, gypsum wasused as a representative additive material in analyzing theeffect of sedimentation characteristics of Tondano Lakeclay, due to the fact that it dissolves over time in water.
Gypsum is found most commonly in nature as a sparingly soluble salt (CaSO4 -2H2 O) (Nettlejohn et al., 1982;

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Table 1 Analytical characteristics of Tondano Lake clay.
KIND OF PROPERTIES
Physical properties:
Atterberg limits
Liquid limit, wL (%)
Plastic limit, wP (%)
Plasticity index, IP
Particle density (g/cm3 )Grain size distribution
Sand (> 0.075 mm) (%)
Silt (5μ m-0.075 mm) (%)
Clay (< 5μ m) (%)
Chemical properties:
pH (1:5, soil: water extract)
CEC (cmolc kg−1 )Organic matter (%)
C/N ratio (%)
Total heavy metal concentration (mg/kg):
Cd (United State-SQG = 1.2 ; Canada-SQG= 0.24)
Cr (United State-SQG = 81.0 ; Canada-SQG= 106.0 )Cu (United State-SQG = 34.0 ; Canada-SQG= 39.3 )Ni (United State-SQG = 20.9 ; Canada-SQG= 49.3 )Pb (United State-SQG = 46.7 ; Canada-SQG= 21.9 )Zn (United State-SQG = 150.0 ; Canada-SQG= 172.0 )Principal clay minerals

WEST

EAST

NORTH

SOUTH

197.0
131.0
66.0
2.32

253.0
135.2
117.8
2.18

304.5
180.8
123.8
2.16

141.2
79.5
61.7
2.44

3.0
36.5
60.5

3.0
28.5
68.5

10.5
37.5
52.0

23.0
34.5
42.5

6.3
29.8
12.8
8.8

6.4
27.1
14.3
8.5

6.6
30.8
14.7
9.1

6.5
25.0
8.0
8.2

2.25
11.32
54.92
24.75
6.72
89.74
Kaolinite

0.51
10.68
81.64
13.30
5.46
134.45
Kaolinite

0.63
11.85
26.23
28.67
7.44
22.04
Illite

0.52
15.36
30.07
22.06
0.74
38.43
Illite, smectite

ﬁelds was sufﬁcient to decrease the suspended solid (SS)concentration under 0.1 % in 12 hours, and basalt fertilizer
was also found to promote ﬂocculation to some extent anddid not hinder the ﬂocculation by gypsum.
The use of a number of amending materials including
traditional agricultural products such as natural gypsum
has been suggested for this purpose. The practical application of such a material would provide a better understanding of sedimentation characteristics in Tondano clay as prelude research. In the present work, the sedimentation characteristics of Tondano clay are experimentally examined inrelation to the inﬂuence of gypsum addition.

2. Materials and methods
2.1 Stock material and sampling sites

Sampling
Kind of activities :areas :→
Northern sewage disposals, clearing forest, paddy farming,
Western → ﬂoating net, tourism, mixture crops/paddy
Eastern → clove plantation, paddy ﬁeld, ﬂoating net
Southern → ﬁshing/duck husbandry, sand mine material, paddy
Fig. 1 Sampling sites of Tondano Lake clay, Indonesia (• =Sampling area).

Verhaye and Boyadgiev, 1997). The concentration of a saturated gypsum solution is about 15 mM (Stumm and Morgan, 1970). Akae (1994) has examined that, gypsum application rate of 30 kg / 10a in the puddled water of paddy

Tondano clay material was collected from the bottom of
Tondano Lake in September 2006. The sample was taken
up by an Eckman grab sampler and its messenger in four
different areas of the lake, as follows: 1) the northern outlet
area in Tondano sub-district, 2) the eastern area in Eris subdistrict, 3) the southern inlet area in Kakas sub-district, and4) the western area in Remboken sub-district, which are
shown in Fig.1. Each sampling area was determined by adistance of 50 meters, as counted from the margin line of
the lake. This ﬁgure also illustrates the various activities ineach area.
2.2 Physical and chemical properties of Tondano clay
Analytical characteristics of Tondano clay, which consist of some physical properties and chemical properties,論文： Effects of gypsum addition on sedimentation characteristics of Tondano Lake clay, Indonesia

are listed in Table 1. The physical properties consist of:plastic limit, liquid limit, plasticity index, particle density,and grain size distribution. Plasticity index was measured
by plastic limit and liquid limit values, where the highest
value (123.8) was at North clay. The highest particle density (2.4 g cm−3 ) was at South clay. Grain size distribution
was analyzed by a Hydrometer procedure and classiﬁed
with a particle size of ASTM. Tondano Lake sediment was
dominated by clay in all areas, and the eastern area had thehighest clay content (68.5 %).
The chemical properties consisted of: pH, CEC, organic
matter, C/N ratio, total heavy metal concentration, and clay
mineralogy. Compared with the Japanese Agriculture Water Standard (2008), the pH value of Tondano clay, which
has ranged from 6.3 to 6.6, was still suitable for all areas.
The highest contents of CEC, organic matter, and C/N ratio were observed in the North clay. The content of heavy
metals from Tondano clay has been compared with the
Sediment Quality Guidelines (SQG) of the United Statesof America (National Oceanic and Atmospheric Administration, 2000) and Canada (National Research Council
Canada, 1993). Some selected heavy metals, which contained in this clay, were: Cd, Cr, Cu, Ni, Pb, and Zn. Compared with the Japanese standard (Cu: 125 mg kg−1 , Zn:
140 mg kg−1 ), the contents of Cu and Zn in Tondano clay
were less (Ministry of the Environment of Japan, 1993).
Clay mineralogy was determined by X-ray analysis procedure (Wada, 1966). Some clay minerals of Tondano clay
were found. The mineralogy was dominated by illite (2:1type) at North and South, kaolinite (1:1-type) at West andEast, as well as a small amount of smectite (2:1-type) atSouth (Fig. 2). A clear peak can not be detected in thoseminerals due to the high organic matter content of the soils.
2.3 Experimental methods
2.3.1 Flocculation and dispersion measurement
The stock material (in diameter 425 μ m-sieved) has
been modiﬁed by the addition or removal of components,
as described below, in order to test the nature of inﬂuencesfrom various material factors on the sedimentation process.
In order to clarify the effects of pH and salinity on the characteristics of ﬂocculation and dispersion, all samples wereNa saturated. Na-saturation was achieved for each experimental sequence by ﬁrst washing the appropriate material
both with 1M (58.5 g L−1 ) sodium chloride (NaCl) solution and then twice with 30 g L−1 of the same sodium
solution. The sodium saturated Tondano clay served ascontrol material. The control material was treated into a
vacuumed-condition, and the test-tube experiment (internal diameter of test tube = 15 mm; water content = 6000%; volume of test tube = 20 mL) was done as shown below: (1) natural condition (with salinities from 30 to 2 g
L−1 and deionized water) and 0.1 M NaOH / 0.1 M HCl

45

solution (2, 4, 6, 8, 10 mL, respectively with salinity 30
g L−1 ). The salinity of this experiment was controlled byusing the NaCl solution; (2) alkaline and acid conditions
in experimental sequences by 0.1M NaOH / 0.1 M HCl;
0.01 M NaOH / 0.01 M HCl; and 0.001 M HCl (2, 4, 6, 8,
10 mL, respectively with deionized water). Values of pH,
salinity (converted from conductivity), and turbidity were
measured with a pH meter, electrical conductivity meter,
and turbidity meter.
According to the Japanese Industrial Water Standard(2008), the turbidity (suspended solid, SS) value is 15
g L−1 , whereas the Japanese Agriculture Water Standard(2008) is less than/equal 100 g L−1 . This ﬁgure has, therefore, shown that the ﬂocculation and dispersion patterns of
our results are divided into dispersion pattern (turbidity >100 g L−1 ); intermediate pattern (turbidity between 16–99
g L−1 ); and ﬂocculation pattern (turbidity < 15 g L−1 ).
2.3.2 Sedimentation experiment
The preparation of a sedimentation experiment was conducted using a 1 L plastic bottle for each test series withthe appropriate initial water contents (w0 ) which ranged
from 1000 to 8000 %. After these suspensions were left
for one day, the soil was removed into a 6 cm diameterand 26 cm high acrylic cylinder which was then ﬁlled to
20 cm with the appropriate suspensions. For each test condition, the sedimentation processes were monitored for 14
days. The experiments for each initial water contents began from each kind of Tondano clays (West, South, East,and North). This sedimentation experiment was done with
two kind of treatments, which were: 1) no added gypsum(as the control), and 2) with 5 % added gypsum. Throughout the experiments, sedimentation data were monitored
by measuring electrical conductivity, turbidity, and pH of
the suspensions after one day. All experiments were conducted in a laboratory at a room temperature of 27-29 ◦ C.
Sedimentation measurement (up to 48 h settlement) was
carried out until an equilibrium position of the sedimentsupernatant interface was attained. Subsequently the depthof interface was recorded with the settling time (Nasser andJames, 2006).
Natural gypsum powder was added to the stock material
to produce sub samples with the desired gypsum content
with a weight of 5 % from the total dry-soil weight. The
material was thoroughly homogenized using a household
mixer. The gypsum content in the suspensions are 5.0, 2.5,
1.3, and 0.63 g L−1 for initial water contents of 1000, 2000,
4000, and 8000 %, respectively. Subsequently, insoluble
gypsum powder remained in the suspensions below the water content of 2000 %, because the solubility is generally
known as 2.1 g L−1 at 25 ◦ C. Akae (1992) has demonstrated that the 0.25–1.0 g L−1 gypsum addition is very
effective for ﬂocculation of the puddled suspension. The 5

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Fig. 2 X-ray diffraction patterns of Tondano clays.

論文： Effects of gypsum addition on sedimentation characteristics of Tondano Lake clay, Indonesia

47◦

Fig. 3 Flocculation and dispersion patterns of Tondano clay. ( = dispersion (turbidity > 100 g L−1 ),  = intermediate (turbidity
16–99 g L−1 ),= ﬂocculation (turbidity < 15 g L−1 ), Solid lines distinguish the ﬂocculation and dispersion patterns)•% gypsum addition for dry soil assumed to be enough for
Ca adsorption to clay particles, and useful for ﬂocculationof Tondano clay. But the further research is still needed torealize the most appropriate gypsum content.
2.3.3 Sedimentation pattern determination
The sedimentation pattern of our results has been determined according to the settling pattern of clay suspensions
by Imai (1980), that, when the water content (or, particulate concentration) is selected as the physical conditionof the suspension and the salinity is selected as the chemical descriptor of sedimentation environment, these patterns
can be classiﬁed into four types: (1) dispersed free settling(DFS); (2) ﬂocculated free settling (FFS); (3) zone settling(ZS) and (4) consolidation settling (CS). In dispersed freesettling, no signiﬁcant interaction between particles occurs
and particles settle individually at rates according to theirrespective sizes; in ﬂocculated free settling, larger ﬂocs
settle faster than smaller ones and the boundary between
clear water and settling ﬂocs is not sharp; in zone settling,
ﬂocculation occurs and the ﬂocs settle in parallel creating a sharp interface above which the water is clear; andin consolidation settling, ﬂocs are not formed, but rather
the whole suspension gels into an interconnected ﬂocculation that extends throughout the total volume, it thencommences to consolidate (dewater) under its own weight.

These settling types are widely observed, not only withclay suspensions but generally with suspensions of ﬁne
particulate materials (Fitch, 1979).

3. Results and discussion
3.1 Flocculation and dispersion characteristics
of Tondano clay
Fig. 3 shows the ﬂocculation and dispersion patterns of
the Na-saturated material. Both ﬂocculation and dispersion
patterns were a part of the sedimentation characteristics.
These patterns were determined by their turbidity value.
Boundaries between the ﬂocculation and dispersion patterns for Tondano clay represent the result of interactions
between suspended particles under the speciﬁc conditions
imposed. The extent of both dispersion and ﬂocculation
patterns is inﬂuenced by the pH and salinity of clay suspension. Below some pH levels, at ﬂocculating salinities,
the particles are sufﬁciently close together that ﬂocculation
occurs very quickly and extends throughout the whole volume creating a gel.
Examination of the Fig. 3 for Na-saturated Tondano clayshows that, dispersion is the dominant pattern at < 10 g
L−1 salinity for West and South clays. However, the peaksalinity which the dispersion pattern can be seen for North
and East clays has been found at approximately pH 6–8. It

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Fig. 4 Time-sediment thickness curves for control material. (Initial water content:%).

has been concluded that the salinity at which a dispersion
pattern occurs as the pH of the suspension approximately
constant in all salinity ranges. At higher salinities, only
patterns that involve ﬂocculation and intermediate occur
for all clays. Flocculation is also observed at low pH in
spite of salinity range.
Because of the salinity and pH of each experimental
condition can be readily identiﬁed from the graphs, theresults will be identiﬁed by the alteration of experimental conditions from the standard (Na-saturated material)conditions. It is also possible to assess why the speciﬁcchange occurred. As related to the clay mineralogy, kaolinite (1:1-type) is a pH-dependent mineral, whereas illite(2:1-type) is not a pH-dependent mineral. According to
Brady and Weil (2002), kaolinite has mostly variable (pHdependent) negative charges and exhibits modest positive
charges at low pH values. Other factors which would affect the ﬂocculation and dispersion patterns are organic
matter and grain size distribution. Flocculation can occur
easily when organic matter is low and grain-size distribution is not dominated by clay, however, dispersing activityof added organic matter inhibited the processes of ﬂocculation and sediment accumulation (Kondo and Torrance,2003).

◦ = 1000 %,  = 2000 %,  = 4000 %, : = 8000

3.2 Sedimentation characteristics of Tondanoclay
3.2.1 Sedimentation patterns of Tondano clay
with no added gypsum
Settling (time-sediment thickness) curves for selectedwater content of the natural material at selected areas ofTondano clay are presented in Fig. 4. This ﬁgure represents the control response to which gypsum-addition experiments are compared. The ”sediment thickness” in all
ﬁgures represents the thickness between the bottom of sediment and the lower zone of water which is free, or almost
free, of particulate material, and the zone of high particulate concentration, where this interface can be deﬁnitively
identiﬁed. The ﬁrst point plotted for water content represents the earliest time that a sediment interface between
clear or almost clear water and settling ﬂocs or accumulating sediment can be identiﬁed. This does not always
represent the upper boundary of the accumulated sedimentbecause, in some cases, it is apparent that it represents theupper boundary of a large mass of settling ﬂocs.
In the previous part of this paper it was explained by
Imai (1980) that, there were various types of settling patterns such as: dispersed free settling (DFS), ﬂocculated
free settling (FFS), zone settling (ZS), and consolidation
settling (CS). All settling types were observed with the ex-論文： Effects of gypsum addition on sedimentation characteristics of Tondano Lake clay, Indonesia

Fig. 5 Time-sediment thickness curves for 5 % gypsum added material. (Initial water content: :%,  = 3000 %,  = 4000 %,  = 6000 %,  = 8000 %).

49◦ = 1000 %, :• = 1500 %,  = 2000

perimental materials, but all types were not represented in
all water contents/suspended particle concentrations. In
these control experiments: in North clay, CS occurred at1000 % water, ZS at 2000 % water, and FFS at 4000 %water, and DFS at 8000 % water; in South clay, CS did notoccur and the ZS boundary occurred at 1000 % water, FFS
at 2000 % and 4000 % water, and DFS at 8000 % water.
Only mixed CS and DFS occurred in West and East clays
for 1000-8000 % water. The differences in these patterns
were due to the difference in clay content among samplingsites. For example, West and East soils have more than 60% clay particles and their huge numbers per unit weightlead to a decrease in average inter-particle distance. Thisis probably a dominant factor and also explains the upward
shifting of boundaries between CS and ZS (at North andSouth) where the environment is strongly ﬂocculating, anddepends on water content. The differences between thesecases can be explained by an increased tendency towards
ﬂocculation in natural material.

lated to ﬂocculation treatment of lake sediments. At Westand East clays with low particle concentrations, the conditions are not favorable for ﬂocculation.

The interpretation placed on these results is that a settling pattern is determined by interplay among the particles
as inﬂuenced by the sedimentary characteristics of Tondano clay, such as the differences of physical and chemical properties (Table 1). Understanding depositional characteristics of soil suspension has practical applications re-

Table 2 has shown that as the salinity increased according to the gypsum addition, the turbidity decreased for
all clays (from 8000 to 2000 % water content). However, from 1500 to 1000 % water content, the turbidity increased. It was due to that their water content were low,
so the consolidation settling were occurred, consequently

3.2.2 Sedimentation patterns of Tondano clay
with gypsum addition
The settling patterns of Tondano clay supplemented with
5 % gypsum (Fig. 5) differ sharply from those of Tondano clay alone. The main differences are that DFS occurs
in both West and East clays (in control group), however,when gypsum is added to all the clays, a DFS pattern does
not occur. In this ﬁgure, the boundaries between the CS
and ZS, and between ZS and FFS shift to a higher watercontent in East, West and North clays. However the CS
does not occur in the South clay, but occurs directly on the
ZS boundary starting at 1000-3000 % water, FFS occurs at
4000-8000 % water. These trends for CS behavior to occur at a lower water contents, and for the CS–ZS boundary
and the ZS–FFS boundary to shift to higher water content
increase as the gypsum has added.

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Table 2 Experimental properties on turbidity of Tondano clay with 5 % gypsum addition and the control.
WEST
Clay + 5 % Gypsum
Items
w0 = 1000% 1500% 2000% 3000% 4000% 6000% 8000%
Gypsum concentration (g L−1 )
5.0
3.3
2.5
1.7
1.3
0.8
0.6
Turbidity(1 day, g L−1 )73.1 22.0 12.6 19.2 36.6 65.9
90.4
pH after 1 day
4.2
3.1
5.2
5.2
5.5
4.9
4.8
Electric conductivity (1day, mS cm−1 )
3.8
3.2
2.5
1.80 1.43 1.06
0.83
EAST
Clay + 5 % Gypsum
Items
w0 = 1000% 1500% 2000% 3000% 4000% 6000% 8000%
Gypsum concentration (g L−1 )
5.0
3.3
2.5
1.7
1.3
0.8
0.6
Turbidity(1 day, g L−1 )177.5 27.7 23.4 17.3 27.2
64.4 69.6
pH after 1 day
3.7
3.5
3.1
3.7
4.6
4.6
4.7
Electric conductivity (1day, mS cm−1 )
3.3
3.1
2.5
1.78 1.43 1.05
0.83
NORTH
Clay + 5 % Gypsum
Items
w0 = 1000% 1500% 2000% 3000% 4000% 6000% 8000%
Gypsum concentration (g L−1 )
5.0
3.3
2.5
1.7
1.3
0.8
0.6
Turbidity(1 day, g L−1 )36.5
6.2
8.9 69.4 89.1 114.8 150.7
pH after 1 day
6.4
6.3
6.2
6.2
6.1
6.0
6.1
Electric conductivity (1day, mS cm−1 )
3.3
3.0
2.5
1.86 1.47
1.09 0.87
SOUTH
Clay + 5 % Gypsum
Items
w0 = 1000% 1500% 2000% 3000% 4000% 6000% 8000%
Gypsum concentration (g L−1 )
5.0
3.3
2.5
1.7
1.3
0.8
0.6
Turbidity(1 day, g L−1 )22.6 17.8 12.6 19.2 36.6
65.9 26.7
pH after 1 day
6.2
6.0
5.2
5.2
5.5
4.9
5.8
Electric conductivity (1day, mS cm−1 )
2.9
2.7
2.2
1.64 1.31
0.94 0.76

the gypsum addition was not affected effectively. In the region in which consolidation settling does not occur (above
3000 % water content), the effects of gypsum addition on
the settling patterns are signiﬁcant; whereas, in cases thatthe consolidation settling occurs (under 2000 % water content),the effects of gypsum addition are not so much signiﬁcant. Gypsum is always used for calcium addition to
sodic soils in aggregation and improvement of permeability without changing pH value. However, the pH valuesof West and East clays were decreased down to approximately 3–4 with decreasing water content. The reason forthis may be attributed to the mineralogy of West and East
clays, which were kaolinitic. More experiments are neededto address this result because the Tondano clays mainlycontain organic matter, and also the interactions between
organic matter and gypsum are not fully clariﬁed at current stage.
Regarding the gypsum content, with clays and watercontent combinations where ZS and FFS settling occurred,
the maximum settling rate and the earliest timing for the
maximum rate occurred in the higher water content sample(Fig. 5). This is related to the process of ﬂoc formation in
the different environmental clays and a degree of interference with the ﬂocs in suspension. Once the settling process
has ended and only the consolidation process is active, therate of water loss decreases rapidly, and continues to decrease for the duration of the experiment. For the Tondanoclay control group (Fig. 4), the consolidation settling has
occurred at West, North, and East clays with 1000 % watercontent.

Control
1000% 2000% 4000% 8000%
0000631.2 92.8
155.3
332.4
6.0
5.7
5.0
5.4
0.48 0.175
0.112
1.14
Control
1000% 2000% 4000% 8000%
0
0
0
0
58.0
94.3
443.1
5.4
5.6
4.9
4.7
0.67 0.35
0.23
1.37
Control
1000% 2000% 4000% 8000%
000040.8 85.1
41.0
179.3
6.1
5.9
7.9
6.3
0.95 0.63
0.36
0.22
Control
1000% 2000% 4000% 8000%
0000271.2 27.1
45.2
342.8
6.2
5.8
7.3
5.8
0.54 0.31
0.25
0.109

The greater water content of the clay suspension could
induce a higher degree of ﬂocculation, thus explaining sedimentation characteristics induced by gypsum treatment.
Therefore, the effectiveness of a gypsum addition on thesedimentation characteristics of Tondano clay in improving the lake condition is well demonstrated by this paper
as the preliminary results have shown.

4. Conclusion
The experimental results demonstrate that the presenceof gypsum in relatively modest concentration has a substantial effect to the sedimentation characteristics of suspensions of ﬁne-grained soil material over a wide range ofwater contents.
Although the Tondano clay control exhibited fullvolume gelation and consolidation has occurred at North
clay, the presence of 5 % gypsum greatly extended the water content range over which this behavior was exhibited.
In the region in which consolidation settling does not occur (above 3000 % water content), the effects of gypsum
addition on the settling patterns are signiﬁcant; whereas,in cases that the consolidation settling occurs (under 2000
% water content), the effects of gypsum addition are notso much signiﬁcant. The greater water content in the clay
suspension could also induce a higher degree of ﬂocculation, thus explaining the greater sedimentation characteristics induced by gypsum treatment.
The results of these controlled experiments provide
guidance as to the effect of speciﬁc soil properties upon

論文： Effects of gypsum addition on sedimentation characteristics of Tondano Lake clay, Indonesia

51

the sedimentation characteristics of high particulate suspensions such as the dredging of ﬁne-grained lake sediments. They should prove useful in assessing what may
account for differences in the sedimentation characteristics of suspensions of natural materials. The applicabilityof this conclusion to Tondano Lake clays awaits further experimental results for those clays.

Kranck, K. (1973): Sediment deposition from ﬂocculated suspen-

Acknowledgements

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disturbed soft marine clay. Trans. Jpn. Soc. Irrigat. Drainage
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Kondo, F. and Torrance, J.K. (2005): Effects of smectite, salinity
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in the settling of ﬁne-grained sediment in still water. Can. J.

We wish to thank Professor Masami Ohtsubo of the Faculty of Agriculture at Kyushu University, Japan for his cooperation in analyzing some properties of Tondano Lakesoils.

Ministry of the Environment of Japan (1993): Japanese Envi-

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