Research on anti-seepage properties of geosynthetic clay lines in landfills

doi:10.4236/jbise.2009.24039

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J. Biomedical Science and Engineering, 2009, 2, 254-260

doi: 10.4236/jbise.2009.24039 Published Online August 2009 (http://www.SciRP.org/journal/jbise/

JBiSE

Published Online August 2009 in SciRes. http://www.scirp.org/journal/jbise

Research on anti-seepage properties of geosynthetic

clay lines in landfills

Xiao-Bo Xiong1, Guo-Qing Gui1, Shu-Zhi Ma2

1School of Engineering, Jinggangshan University, Ji’an, China; 2School of Engineering, China University of Geosciences, Wuhan, China

Email: thongtao2006@163.com; JHBMSZ@163.com

Received 23 February 2009; revised 20 March 2009; accepted 26 March 2009.

ABSTRACT

In recent years, geosynthetic Clay liners (GCLs)

are widely used in different kinds of anti-seep-

age projects and the anti-seepage availabilities

of GCLs are regarded as increasingly important

by engineers. Anti-seepage effectiveness of

GCLs involves at least two aspects, such as

Hydraulic conductivity of GCLs under engi-

neering practice conditions, and Absorption

ability of bentonite in GCLs in the course of liq-

uid permeation. In this paper, Hydraulic con-

ductivity tests are performed to obtain seepage

coefficient of GCLs, taking liquids such as dis-

tilled, deioned water and landfill leachate, and

solutions with single-species cation as the hy-

dration and permeation liquid. The results Show

that cation valence, cation concentration and

hydration ionic radius in hydration and permea-

tion liquids have influences on hydraulic con-

ductivity of GCLs.

Keywords: Landfills; GCLs; Lines System; Lmper-

vious Barrier; Experimental Analysis

1. INTRODUCTION

Along with the rapid economic development and ur-

banization, the generation of municipal solid waste

(MSW) increased drastically in China. Landfill is a pri-

mary method of ultimate disposition of municipal solid

waste in many countries. The impervious barrier of

cover system and lines system is the key to ensure that

the landfills should not be the source of secondary pollu-

tion to the surrounding. Applications of geosynthetics,

such as geotextile, geomembrance, GCLs, geonet (GN),

geocomposite (GC) and geofoam (GF), etc., they are

used to all aspects of landfill as liners and covers. With

regard to GCLs as a new type material of seepage barrier,

the authors present here some information on the com-

position of material, engineering characteristic, and

make suggestion on the future development of this

technology. It was investigated that the leakage preven-

tion structures of 56 landfill sites for domestic wastes in

China, and the results showed that leakage prevention

structures were found designed improperly and leakages

happened incidentally in landfills in China. Overseas

experts have already acquired certain research on prop-

erties of GCLs, but research on this product is rare in our

country. In this paper, we studied the properties of GCLs

by means of testing, then points out that making use of

GCLs is the trend of the applicable liner in future.

2. CURRENT STATUS AND PROSPECTS

OF GCLs

The protection liners system of clay layer was widely

used in USA before 1982. The new type of geosynthetics

named GCLs come out in 1987 at a Germany company.

In 1989, GCLs was widely applied in sanitary landfills

in USA. And since 1995, the national code for the new

type liner material was taken to draw up, which was fin-

ished in 1999 completely. Before 2000, only one docu-

ment was found during the literature survey that dis-

cusses the performance of GCLs when installed as a

single in-service liner (Lichtwardt and Comer, 1997).

Horace Moo-Young et al. (2004) reviews the state of

the science and practice on the infiltration rate through

compacted clay liner (CCL) for 149 sites and geosyn-

thetic clay liner (GCL) for 1 site. The field hydraulic

conductivities for natural clay liners range from

1×10-9cm s-1 to 1×10-4cm s-1, with an average of

6.5×10-8cm s-1. There was limited information on GCL.

For composite lined and geomembrane systems, the leak

detection system flow rates were utilized. The average

monthly flow rate for composite liners ranged from 0~32

l/hd for geomembrane and GCL systems. A GCL was

installed on the bottom of a salinity-gradient solar pond

in Texas in 1994. The pond has a surface area of ap-

proximately 1 acre (0.4 ha) and a water depth averaging

3.2 m. The GCL is a modified Gundseal, with a 30 mil

(0.07cm) flexible polypropylene geomembrane backing

with 0.5cm of bentonite clay bonded to one side. The

performance of the GCL is presented as the variation in

G. G. Adams et al. / J. Biomedical Science and Engineering 2 (2009) 254-260 255

hydraulic conductivity of the GCL over time for a 9-

week period. The hydraulic conductivity remains fairly

constant at 2×10−6 cm s−1. This represents an increase

over the initial conductivity, which was measured at

1×10−6 cm s−1. In general, a GCL is most effective when

placed under an effective overburden pressure where the

minimum permeability can be obtained. This data set

clearly indicates that additional research needs to be

conducted to quantify the field infiltration rate of GCLs

in landfills.

In China, Zhou Z. B. (2002) present the condition,

mechanism and process of ion exchange between the

ions of bentonite in the GCL and ions in leachate and

their effects on GCL's impermeability, and some resolu-

tion methods. Yao Q. (2003) introduced design of im-

pervious system of landfill and use of various impervi-

ous materials. Wang X. Q. (2004) analyzed the foreign

application of geomembrane liner system and it is useful

for landfill construction in China. Li Z. B. (2005) ad-

vanced a series of problems which needed to be deeply

researched based on the research achievements by home

and overseas scholars. Liu H. B. (2006) provided the

background and basis for developing standard “Geosyn-

thetic Clay Liner” and details product classification, test

items and methods and performance index. LU H. Y.

(2007) introduced the advantages of GCL serving as an

anti-seepage system of hazardous waste landfill sites and

demonstrates that the anti-seepage result of GCLs is

better than CCLs. The study shows that all the indexes

and benefits from this technique are much better than

those from the traditional anti-seepage technique (Ta-

ble1).

3. THE BASIC PROPERTIES OF GCLs

A GCL is a factory-manufactured, hydraulic barrier

typically consisting of bentonite clay, supported by geo-

textiles and/or geomembranes held together by needling,

stitching, or chemical adhesives. GCLs are typically

used in areas where clay is not readily available or where

conserving air space is an important factor. GCLs do not

have the level of long-term field performance data that

are available for GMs or CCLs because GCLs were de-

veloped recently (1986) and they are typically used with

a GM in a composite liner system.

In recent years, GCLs is widely used in different kinds

Table 1. Statistics of anti-seepage engineering for man-made

lake with GCLs in China.

ProvinceNumbers Area/m2 Province NumbersArea/m2

Beijing 9 167398 Shanghai 37 95907

Zhejiang28 138385 Jiangsu 22 77213

Tianjing1 7000Anhui 6 18395

Hubei 3 7280Shanxi 1 6000

Guangdong4 13240Yunnan 3 18220

Guangxi1 20000Jiangxi 2 7254

Gansu 1 10000Chongqing 1 15000

Neimenggu1 25000Hebei 1 50000

of anti-seepage projects, and the anti-seepage availabil-

ities of GCLs are regarded as increasingly important by

engineers. The four basic GCLs list as follows (see Fig-

ure 1), that is: two layer of GT/GM with bentonite and

bonding agent(binder), two layer of GT/GM with ben-

tonite and stitch fiber, two layer of GT/GM with bentonite

and mending fiber, one layer of GT/GM(beneath the ben-

tonite) with bentonite, etc.

Anti-seepage effectiveness of GCLs involves at least

three aspects: 1) Hydraulic conductivity of GCLs; 2)

Absorption ability of bentonite in GCLs in the course of

liquid permeation; 3) GCL internal shear strength while

used in anti-seepage system.

The above aspects are so important that they could

directly affect the factor of safety and effectiveness of

anti-seepage project. Section of composite liner system

landfill of MSW sanitary landfill (see Figure 2), Section

of the cap of MSW sanitary landfill (see Figure 3).

In this paper, GCLs for indoors tests was made from

the non-fabricated textile factory in Yixing city in ji-

angsu province, and two layer of GT/GM of bentonite

(made by the CETCO Company, USA) with stitching in

two geotextile fibers layer, it belongs to the type of

Bentomat (see in Figure 1, the second graph). The total

mass is 5.32kg/m2 per unit area, with the lower GT is a

kind of fabricated GT whose mass is 112g/m2 per unit

area, and with the upper GT is a kind of non-fabricated

GT whose mass is 221g/m2 per unit area.

Chemical components of Bentomat are: SiO2 57.23%;

Al2O3 18.45%; Fe2O2.57%; FeO 1.19%; MgO 2.22%;

CaO 1.53%; Na2O 2.55%; K2O 0.61%; H2O+4.30%;

H2O–7.9%; Cr2O3 0.0044%; ZnO 0.028%; NiO 0.0037%;

Li2O 0.003 8%; TiO2 0.24%; P2O5 0.034%; Mn0.016%;

Buring0.41%; Total 99.30%.

Figure 1. Sketch of four basic GCLs products.

er GT stitch fibe

er GT men

fibe

er GT

entonite

entonite+binde

lower GT

lower GT lower GT

lower GT

256 X. B. Xiong et al. / J. Biomedical Science and Engineering 2 (2009) 254-260

Figure 2. Section of composite liner. Figure 3. Section of the cap of landfill.

The swell stresses of a GCL, made by the CETCO

Company, USA, were measured directly using a custom-

made swell stress instrument under water-sorbed satura-

tion conditions. The results show that in the test process,

the variation of the swell stress curve with time can be

divided into three segments. To characterize the minera-

logical nature of the Nabentonite, six samples were

studied by X-ray diffraction and results show a relatively

homogeneous mineralogical composition. It consists of a

complex mixture of different smectites, those of sodium

nature clearly prevailing, with trace amounts of quartz,

calcite and muscovite. This is determined by the fact that

all the heated specimens collapse their main 001 spacing

to 10 A ˚ and expand that same spacing to 16.5~16.8 A˚

upon treatment.

By comparison of the seepage coefficient curve of the

two experimental conditions (see Figure 4), the seepage

coefficient are approximate same when the vertical

stress is less than 250kPa, otherwise, when the vertical

stress is more than 250kPa, the seepage coefficient by

Figure 4. Seepage coefficient test apparatus.

Table 2. The results of hydrated swelling experiments of GCLs

samples by main pressures. (Notes: MGCL is mass per unit

area of GCLs.)

σw/kPa MGCL/kg/m2H0/mm Hσ/mm Hw/mm w/%EB

3 5.39 6.84 6.72 9.67 131.33.38

15 5.37 6.87 6.52 8.61 112.62.91

50 5.48 6.85 6.27 7.70 102.12.44

150 5.33 6.87 5.86 6.52 92.41.98

300 5.45 6.85 5.46 5.85 70.11.72

400 5.44 6.87 5.37 5.69 63.21.65

experiments under conditions of hydrate firstly and

compress secondly (cw1) is lower than the seepage coef-

ficient by experiments under conditions of compress

firstly and hydrate secondly (wc2). That is, when the

vertical stress is more than 250kPa, and the experiments

under conditions of hydrate firstly and compress sec-

ondly, the thickness of GCLs is bigger, so that the seep-

age coefficient increase very slow.

The results of hydrated swelling experiments of GCLs

samples by main stress list in Table 2, through the data

of Table 2, firstly, the swelling deformation of GCLs

increase very fast because of the properties of water ab-

sorption of bentonite, till the thickness of GCLs without

change on the end. Under the series main stress such as

3kPa, 15kPa, 50kPa, 150kPa, 300kPa and 400kPa, the

hydrated time list sequences as 12.8d, 9.8d, 8.3d, 6.0d,

5.0d and 5.0d (see Figure 5). When at a low main

stress, the initial creep velocity is very high, and the

total hydrated thickness is big, with increasing of σw,

the thickness of GCLs decreasing, at the same time,

the water content(w) and porosity ratio decreasing

(show in Table 2).

From Table 3, we can find that the vertical pressure

take much effect on the seepage properties, the seep-

age coefficient increases along with the increasing of

the horizontal strain of GCLs, and when the horizontal

strain is more than 6.0%, the velocity of the seepage

coefficient increasing obviously. That is because,

when the GCLs was taken horizontal tense, not only

G. G. Adams et al. / J. Biomedical Science and Engineering 2 (2009) 254-260 257

100

150

200

250

300

350

400

050100 150200250 300 350400 450

Vertical pressure(kPa)

Seepage cofficent

(10

-11

m/s)

cw1

wc2



Figure 5. Seepage coefficient of GCLs under several conditions.

Table 3. Seepage coefficients by all grades Vertical pressure

of GCLs under the condition of compress firstly and hydrated

secondly or hydrate firstly and compress secondly.

seepage coefficient

(m/s)×10-10

seepage coefficient

(m/s)×10-10

Vertical

pressure

(kPa) Compress

firstly

hydrate

firstly

Vertical

pressure

(kPa) Compress

firstly

Hydrate

firstly

25 25.6 37.3 250 0.738 0.840

50 20.6 20.6 300 0.916 0.407

100 18.9 13.1 350 1.40 0.463

200 2.32 3.78 400 1.43 0.581

the upper and the low GT cause horizontal extension,

and the bentonite stitched in the middle could be scat-

tered by horizontal tension, at the same time, the po-

rosity of the bentonite increases after water hydrated,

then the seepage coefficient increases. When the hori-

zontal strain is big (usually more than 6.0%), even

when the horizontal strain attain to 16.0%, the ben-

tonite grains decreasing, the thickness of some parts of

the GCLs is very low, the seepage properties is very

high, and the anti-seepage properties of GCLs vanish

wholly. From the above statement, in practical anti-

seepage engineering projects, the non-uniform settle-

ment of should be calculated before construction, and

the horizontal strain of GCLs is extremely controlled

to 6.0%, thus, the anti-seepage properties of GCLs

could be run ordinary.

In a newly built landfill facility, the lower most

leachate barrier is composed of GCL strips; adjoining

strips are not seamed and, instead, they partially overlap

and are ‘‘sealed’’ by a layer of sodium bentonite powder.

The performance of this Na-bentonite is the main object

of investigation of the present study. In laboratory re-

constructions, the mechanical and geotechnical behavior

of the Na-bentonite placed in between adjoining GCL

strips was evaluated, as it is in this situation that the

Na-bentonite is less constrained physically and thus

more prone to deformation and rupture. X-ray diffrac-

tometry was used to identify the minerals making up the

clay powder; several laboratory tests were performed to

geotechnically characterize the clay; and cakes with dif-

ferent water contents of sodium bentonite were submit-

ted to pure shear deformation using an automated pure

shear rig to evaluate its behavior when subject to load,

and wetting and drying cycles. The results prompted the

search for field evidence of clay rupture around the

landfill facility by means of geophysical and geochemi-

cal investigations, together with a detailed structural

study of the fracture network of the granite. The swelling

behavior of the Na bentonite is highly dependent on the

type of encapsulation between the cover and the carrier

geotextiles and on the chemical composition of the fluids

involved.

From the above, a reliable, easily handled and cheap

waterproof roll liner material was designed, in which the

Bentonite, a kind of clay soil with unusual self-sealing

ability, was used as the protective barrier for contain-

ment of hazardous waste in landfills. On the basis of the

materials’ choice, the permeability properties of this

new-style synthetic liner under various conditions were

tested. The permeability coefficient of Na+-Bentonite

was less than that of Ca2+-Bentonite. The suitable adhe-

sives can enhance the waterproof of Na+-Bentonite. Af-

ter Na+-Bentonite and the new liner were pretreated by

water and well maintained, they hardly changed their

permeability when they met such leachates as inorganic

acids/bases and organic solvent . The new liner presents

good “welding” property even if it was damaged. The

experimental results indicate that this kind of bentonite

waterproof roll liner is very suitable for landfill.

When MSWL was taken as the hydrated liquid and

seepage liquid, to analyze the chemical component of

the liquid seeping from GCLs, and attain all kinds of

single-species cation iron and Chemical Content of

BOD5, COD and NH4-N of Leachate from landfills,

from the data of Table 4, we can find that GCLs took

effective absorption with all kinds of single-species

cation iron and Chemical Content of BOD5, COD and

NH4-N. The results show that GCLs has large absorption

ability on the permeation liquids. But the ability is de-

creasing with increase of permeation volume, and the

types of hydration liquids exercise great influence on the

variation of GCLs absorption, and the content some sin-

gle-species cation iron of the permeation liquids has no

change, it shows that the absorption of GCls to cation

iron is very low.

Table 4. Chemical Content of BOD5, COD and NH4-N of

Leachate from landfills.

Flow of Porosity VolumeBOD5/mg/l COD/mg/l NH4-N/mg/l

0.50 800 6200 1059

1.25 900 6800 1538

2.09 1600 15800 1585

Original liquid 2500 16000 3426

258 X. B. Xiong et al. / J. Biomedical Science and Engineering 2 (2009) 254-260

A series of confined swell tests were conducted on a

needle-punched GCLs with tap water as the hydration

medium. The effects of the static confining stress on the

swelling characteristics of GCLs and the hydration time

under different confining stresses were explored. In-

creasing the static confining stress led to: shorter hydra-

tion time; smaller final GCL height; less final GCL bulk

void ratio; smaller final bentonite moisture content.

4. DESIGN CALCULATOR-LEAKAGE

RATE THROUGH GCLs

4.1. Problem Statement

This calculator computes the rate of leakage through

defects in a composite liner, i.e. GCLs (see Figure 6).

The thickness of a hydrated GCLs depends on the com-

pressive stress applied during hydration. Typical values

are between 5 and 10 mm. Field evaluation, sponsered

by USEPA, of leakage rate for double-lined landfills

indicates that GM/GCL composite liners outperform

GM/ CCL liners (Othman et al., 1998.)

The rate of leakage through a GCL due to GCL per-

meability is negligible compared to the rate of leakage

through defects in the GCL Hence, only leakage through

defects will be considered. If there is a defect in the GCL,

the liquid first passes through the defect, then it flows

laterally some distance between the GCL and the

low-permeability soil, and, finally it infiltrates in the low

permeability soil.

Flow between GCL and low-permeability soil is

called interface flow, and is highly dependent upon the

quality of contact between the two components Contact

conditions are defined as follows:

1) Good contact conditions correspond to a GCL in-

stalled, with as few wrinkles as possible, on top of a

low-permeability soil layer that has been adequately

compacted and has a smooth surface. Contact quality

factor (Cqo) (circular, square, rectangular) is 0.21, Con-

tact quality factor (Cq∞) (infinite length) is 0.52.

* Space exaggerated to show interface flow

Figure 6. Schematic diagram of GM/GCL calculation section.

Table 5. Representative installation defect densities.

Installation qualityDefect density

(number per acre) Frequency

(percent)

Excellent Up to 1 10

Good 1 to 4 40

Fair 4 to 10 40

Poor 10 to 20* 10

2) Poor contact conditions correspond to a GCL that

has been installed with a certain number of wrinkles,

and/or placed on a low-permeability soil that has not

been well compacted and does not appear smooth. Con-

tact quality factor (Cqo) (circular, square, rectangular) is

1.15, Contact quality factor (Cq∞) (infinite length) is

1.22.

*Higher defect densities have been reported for older

landfills with poor installation operations and materials;

however, these high densities are not characteristic of

modern practice.

The Help model provides guidance for estimating the

defect densities. Some useful information on the Help

model is given in the Technical Note on Using HELP

Model (ver 3.07). There are mainly two types of defects,

manufacturing defects and installation defects. Typical

geomembranes may have about 0.5 to 1 (1 to 2 per hec-

tare) pinholes per acre from manufacturing defects

(Pinholes are defects with a diameter equal or smaller

than the geomembrane thickness. The density of instal-

lation defects is a function of the quality of installation,

testing, materials, surface preparation, equipment, and

QA/QC program. Representative installation defect den-

sities as a function of the quality of installation are given

in Table 5 for landfills being built today with the state of

the art in materials, equipment and QA/QC.

Studies by Giroud and Bonaparte have shown that for

geomembrane liners installed, with strict construction

quality assurance, could have one to two defects per acre

(4000m2) with a typical defect diameter of 2mm (i.e., a

defect area of 3.14×10-6 m

2). Typical for liner perform-

ance evaluation one defect per acre (4000m2) is consid-

ered with a defect area of 0.1 cm2 (equivalent to defect

diameter of 3.5 mm), for a conservative design a defect

area of 1 cm2 (equivalent defect diameter of 11 mm) can

be considered.

4.2. Problem Solution

Different geomembrane defect shapes will be consid-

ered: Circular defect with diameter of d

74.09.02.095.0 ])/(1.01[976.0 Ssqo khdthCn

Q

Rectangular defect with width of b and length of B:

87.045.01.095.0

74.09.02.095.0

)(])/(2.01[

])/(1.01[

Ssq

Ssqo

khbBthCn

khbthCn







G. G. Adams et al. / J. Biomedical Science and Engineering 2 (2009) 254-260 259

Q-Leakage rate through the considered geomembrane

defect (m3/s);

Q*-Leakage rate per unit length of geomembrane de-

fect (m3/s.m);

A-Considered geomembrane area (m2);

N-Number of defects per considered geomembrane

area (A);

Co or Cq ∞-Contact quality factor (see Table 6);

H-Hydraulic head on top of the geomembrane (m);

ts-Thickness of the low-permeability soil component

of the composite liner (m);

D-Diameter of circular defect (m);

b-Width of defect (m);

B-Length of rectangular defect (m).

Limitation of the equations presented:

1) If the effect is circular, the defect diameter should

be no less than 0.5 mm and not greater than 25 mm. In

the case of the defects that are not circular, it is proposed

to use these limitations for the defect width.

2) The liquid head on top of the geomembrane should

be equal to or less than 3 m.

4.3. Input Values

Case 1: Geometry of circular defect

① Considered geomembrane area(A) is 4000m2; ②

Hydraulic head on top of the geomembrane(m) is 0.3m;

③ Thickness of the low-permeability soil(m) is 2m; ④

Permeability of the low-permeability soil(m/s) is

1.0×10-7m/s.

Properties of circular defect

① Contact (good or poor) Good; ② Number of de-

fects (n) is 1; ③ Diameter of defect (d) is 0.0002m.

Case 2: Geometry of Rectangular Defect

Properties of Rectangular Defect

Width of defect (b) is 0.002m; Length of defect (B) is

0.01m, the else conditions like Case 1.

5. CONCLUSIONS

In this paper, a series of research work including about

the laboratory tests of GCLs is conducted and several

valuable conclusions are obtained:

Table 6. Comparison between theoretical

Case

number Theoretical

structure Practical anti- seep-

age structure

Maximum

flow, m3/

(m2.d)

Case1

One layer GM,

eneath high

seepage layer

GM+ foundation with

high permeability in-

dex or geocomposite

(such as geotextile)

3.14×10-3

Case2

One layer GM,

eneath low

seepage sub-

stituted layer

GM+geocomposite

with low permeability

index (such as GCL)

7.97×10-8

1) A geosynthetic hydraulic conductivity device is de-

signed to measure seepage coefficients of GCLs under

constant normal stress conditions. Seepage coefficients

of GCLs is measured for different hydrated and perme-

ated liquids using the apparatus. Free swelling tests and

hydration swelling tests of GCLs and bentonnite per-

formed in order to research the swelling characteristics

of GCLs and its influencing factors, including normal

stress, loading-hydration sequence and hydration liquid.

The results show that these factors have influences on

GCLs swelling characteristics. Similar influencing laws

can be obtained in the two tests.

2) Seepage coefficients tests are performed to obtain

hydraulic conductivity of GCLs, taking liquids such as

distilled, deioned water and landfill leachate, and solu-

tions with single-species cation as the hydration and

permeation liquid, the results show that cation valence,

cation concentration and hydration ionic radius in hydra-

tion and permeation liquids have influences on hydraulic

conductivity of GCLs. The influences of stress condi-

tions and loading-hydration sequence on GCLs hydraulic

conductivity are researched, the results Show that nor-

mal stress and horizontal strain, as well as load-

ing-hydration sequence, all have influences on the vari-

ety of hydraulic conductivity.

3) Absorption ability of GCLs in the course of liquid

permeation is studied, and its influencing factors are also

discussed, including hydration liquid and permeation

time. The results show that GCLs has large absorption

ability on the permeation liquids. But the ability is de-

creasing with increase of permeation volume, and the

types of hydration liquids exercise great influence on the

variation of GCLs absorption.

4) Analysis are made to combine the conclusion ob-

tained in this study with practical engineering projects,

and proposals of building method of GCLs are ad-

vanced.

6. ACKNOWLEDGEMENTS

Some research results described in this paper were de-

veloped by the author in his graduate study between

1997 and 2000 in Nanjing University. The author grate-

fully acknowledges his teacher (Academician of Chinese

Academy of Sciences Prof. Jun SUN) for his helpful

direction and suggestion to the author during his study-

ing in Tongji University. The authors would like to ex-

press their thanks to many teachers and staffs at the uni-

versity of tongji university, nanjing university, and china

university of geosciences, and jinggangshan university

for the help in presenting some related experimental

data.

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