Characterization of Clayey Diatomaceous Earth (1)

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Engineering geological characterization of clayey diatomaceous

earth deposits encountered in highway projects in the

Tengchong region, Yunnan, China

Yongshuang Zhang a,b,⁎, Changbao Guo a,b, Xin Yao a,b, Yongxin Qu c, Nengjuan Zhou a,b

a Key Laboratory of Neotectonic Movement and Geohazard, Ministry of Land and Resources, Beijing 100081, Chinab Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, Chinac Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

a b s t r a c ta r t i c l e i n f o

Article history:

Received 19 March 2013

Received in revised form 11 October 2013

Accepted 14 October 2013

Available online 22 October 2013

Keywords:

Diatomaceous earth

Clay mineral

Swelling

Mechanical property

Geohazard

A type of clayey diatomaceous earth of Pliocene Mangbang Formation (N2m) was encountered in highway pro-

jectsin the Tengchong region of YunnanProvince,Southwest China.The Tengchong clayey diatomaceous earth is

characterized using multiple test methods, such as granulometric analysis, chemical analysis, X-ray diffraction

(XRD) and scanning electron microscopy (SEM) analysis, shrinkage and swelling tests, unconned or uniaxial

compression test, triaxial test and direct shear tests. The chemical and mineralogical compositions, physical

and hydraulic properties and engineering properties of the clayey diatomaceous earth are presented in this

paper. It is revealed that the Tengchong clayey diatomaceous earth is an unusual soil or rock which has charac-

teristics of both typicaldiatomaceous earth andswelling clayeysoil or soft clay rock. Thediatoms in theearth en-

hance the connection between micro-structures, and improve the mechanical properties of diatomaceous earth.

However, due to the existence of a signicant amount of swelling clay minerals, the Tengchong clayey diatoma-

ceous earth becomes swelling soft rock, and is prone to engineering problems or geohazards. Based on the test

results, a number of issues regarding classication, discrimination and geohazard control relatedwith the clayey

diatomaceous earth are discussedto provide both some basic understandingand new insight into the character-

istics of the clayey diatomaceous earth deposits.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Diatomaceous earth deposits are located in marine and lacustrine

deposits of Miocene and Pliocene ageworldwide (Harben, 2002). Diato-

maceous earth is generally characterized as a chalk-like, soft, friable,

earthy, very ne-grained, siliceous sediment, usually light in color, i.e.,

white if pure, commonly buff to gray in situ, and rarely black ( Ilia

et al., 2009). It has been earlier studied in the eld of engineering geolog-

ical research (Iijima and Tada, 1981; Isaacs, 1982; Chaika and Dvorkin,

2000; Koizumi et al., 2009; Calvo et al., 2012), or as a material for indus-

trial applications in ltering, bleaching, lling, electrical insulating, and

building (Stoemer and Smoll, 2001; Fragoulis et al., 2005; Ilia et al.,

2009; Liu and Zhao, 2009; Van Garderen et al., 2011).

The engineering and mechanical properties of diatomaceous earth

have received continuous attention in the last decade. Day (1995) stud-

ied the diatomaceousll in south California, showing itshigh water con-

tent and low dry density. Tateishi (1997) studied diatomaceous earth in

Japan, indicating that it has not only very high water content, but also

very high strength and elastic modulus. Honget al. (2004a) investigated

changes of micro-structures of diatomaceous earth in Japan under dif-

ferent stress levels, and proposed the relationship between the micro-

pores in the diatomaceous earth and stress levels. The inuences of dia-

toms on engineering properties of diatomaceous earth have been ad-

dressed in many researches, which suggested that the presence of

diatoms can increase the plastic limit and the liquid limit of sediments

(Tanaka and Locat, 1999; Shiwakoti et al., 2002; Palomino et al., 2011),

the plasticity index, the shear strength and the internal friction angle

(Shiwakoti et al., 2002), the compressibility and hydraulic conductivity

(Rajasekaran, 2006), and decrease the shrinkage limit (Palomino et al.,

2011). The previous researches have revealed the contributions of dia-

toms in improving the mechanical properties of diatomaceous earth. For

example, the natural diatomaceous earth sample with diatom content

of about 75% from Linqu of Shangdong Province, China, remained inte-

grated without swelling–slaking when soaked in water (Hu and Wen,

2005). However, when the amount of clay minerals, especially the swell-

ing clay mineral of montmorillonite, increases to a certain level, the con-

trolling factors for the mechanical properties of diatomaceous earth may

be changed. This can be evidenced by frequent occurrence of slope

geohazards associated with the clayey diatomaceous earth, such as the

landslides of diatomaceous earth in Shengzhou of Zhejiang Province,

China (Gao et al., 2007). Thus, study of the engineering geological proper-

ties of clayey diatomaceous earth deposits, especially the inuence of

Engineering Geology 167 (2013) 95–104

⁎ Corresponding author at: Institute of Geomechanics, Chinese Academy of Geological

Sciences, Beijing 100081, China. Tel./fax: +86 10 68486765.

E-mail address: [email protected] (Y. Zhang).

0013-7952/$ – see front matter © 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.enggeo.2013.10.009

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swelling clay minerals, is also one of the interesting and essential subjects

in the engineering geology eld.

Although diatomaceous earth deposits were found in 18 provinces

of China (Cui, 2008), few studies have been conducted on their engi-

neering geological and mechanical properties. During engineering geo-

logical survey for highway projects in the Tengchong region, Yunnan

Province, Southwest China, in the past two years, a type of expansive

clayey diatomaceous earth of Pliocene Mangbang Formation (N2m)

was encountered (Zhang et al., 2012). Several slope failures including

surface debris avalanches and landslides occurred in the highwayslopes. In this paper, based on tests on typical diatomaceous earth de-

posits, the physical and engineering properties of the diatomaceous

earth are obtained and analyzed. The associated engineering geological

problems or geohazards, the control measures, as well as the classica-

tionconsidering theswellingpropertyof clayeydiatomaceous earth, are

discussed to provide both some basic understanding and new insight

into the characteristics of the diatomaceous earth deposits.

2. Geological settings

The Pliocene diatomaceous earth deposits in the Tengchong region

appear in lake facies sediments of the Mangbang Formation (N2m), es-

pecially in its third member(N2m3). Theformation is widely distributed

in basins along the Longchuan River, such as the Mangbang, Tengchong,

and Lianghebasins. It is 352–630m thick and consists of gravel-bearing

granitesand, gravel, clayeydiatomaceous earth, brown coaland interca-

lated basalt, volcanic breccia and tuff, forming several volcanic sedimen-tary cycles.

In the Mangbang basin, the Pliocene diatomaceous earth is mainly ex-

posed in such locations as Qushi, Yongan, Menglian, Sanjiajie, Tuantian,

Wuhe, Mangbang, and Shangyin (Fig. 1). According to Shang (2003)

and Zhang et al. (2012), the distribution of diatomaceous earth was obvi-

ously controlled by two factors. One is associated with Cenozoic basalt in

basins.Due to the warm climatic condition, chemical weathering of basalt

provided a lot of free SiO2 and Mg2+, which was favorable for the forma-

tion of montmorillonite or illite–montmorillonite mixed layer mineral

which is the main composition of Tengchong diatomaceous earth. An-

other is the Longchuan River fault zone (Fig. 1), which controlled the

distribution of such basins as Mangbang, Tuantian, and Puchuan where

diatomaceous earth was deposited.

The diatomaceous earth can coexist or be intercalated withbasalt or

weakly cemented clay rock or silty sandstone (Fig. 2a, b and c) or occur

as a single layer (Fig. 2d and e). In somesections, its thickness is evenup

to 40 m. The layered diatomaceous earth is nearly horizontal or gently

dipped. Along the Tengchong–Baoshan Highway and Tengchong–

Longling Highway under construction, it can be seen that undisturbed

diatomaceous earth is usually compact-massive and very light and has

well-developed lamination. It is mainly gray white, yellowish brown,

dark gray or black in color, depending on its existence environment.

After being air dried, it is usually cracked with shrinking ssures and

hence vulnerable to aking (Fig. 2a, e, f and h).

3. Materials and methods

Seven bulk samples of diatomaceous earth were collectedduringthe

geological survey. Two samples were taken from a highway slope southof Wuhe Town (G02-1, G02-2), another two from a highway slope in

Mannong (G05-1, G05-2), the others from a diatomaceous earth mine

slopein Manpa (G04-2), a borrowpit by the highway south of Mangbang

(G06), and a highway slope in Zhangjiacun (G07), respectively. Each of

them was extracted from subsurface without disturbance and natural

water content was maintained. The samples were ne-grained and ho-

mogenous, having three color tones: black or dark gray, gray white, and

yellow or yellowish brown respectively, which can reect the existence

of micro-environment.

Tests on the chemical and mineralogical compositions, physical and

hydraulic properties and engineering properties of the diatomaceous

earth, including compressive strength and shear strength, were con-

ducted. Unless specically indicated, all the test results were obtained

by a single test, and tests were carried out on bulk samples to ensuregood correlation between physicochemical and index properties.

The composition and structure of minerals were determined by X-ray

diffraction (XRD) and scanning electron microscopy (SEM) analyses. The

clay mineral tests by XRD were carried out, using the b2μ m fraction, on

untreated samples, glycol-saturated samples and samples heated to

550 °C, respectively. SEM was used to examine the microstructure of

the biogenic silica, mostly diatom frustules, in the bulk samples.

Fig. 1. Distribution of diatomaceous earth of the Pliocene Mangbang Formation in the

Tengchong region 1—Quaternary sediment; 2—Quaternary volcanic rock; 3—Pliocene

Mangbang Formation; 4—

Volcanic rock of Mangbang Formation; 5—

Miocene NanlinFormation;6—Carboniferous MenghongFormation; 7—Cambrian Shahechang Formation;

8—Cambrian Baoshan Formation; 9—Lower Paleozoic Gaoligongshan group; 10—Late

Yanshanian granite; 11—Early Yanshanian granite; 12—Regional tectonic zone; 13—Crater;

14—Sampling spot.

Fig. 2. Clayey diatomaceous earth exposed along the highways in Tengchong region. a —Thick-bedded clayey diatomaceous earth with air-dried ssures by the highway south of Wuhe

Town; b—Collapsed debris below the slope of thick-bedded clayey diatomaceous earth in Wuhe Town; c—Contact of diatomaceous earth with basalt on the highway slope in Mannong

Village; d—Diatomaceousearthlandslideof thehighway slope in Mannong Village; e—Slopeof thick-bedded clayey diatomaceous earthin ManpaVillage; f —Collapse debris on the high-

wayslopeof clayeydiatomaceous earthin Mannong Village; g—Clayeydiatomaceousearth with theweatheredupperparton thehighwayslopein Mannong Village; h—Weatheredthick-

bedded clayey diatomaceous earth in the highway borrow pit south of Mangbang Town.

96 Y. Zhang et al. / Engineering Geology 167 (2013) 95–104

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The analyses on the chemical, physical and hydraulic properties, as

well as the swelling property of the samples, were carried out following

the China Standard for Soil Test Method GB/T 50123-1999 (Ministry of

Water Resources, P.R. China, 1999), as well as the American Society for

Testing and Materials (ASTM D2487-06, 2006).

The unconned or uniaxial compression test, triaxial test, and direct

shear tests were conducted on 4 samples from G02-1, G04-2, G05-1 and

G06, respectively. The uniaxial compression tests were conducted on

specimens with a diameter of 39.1 mm and a height of 80 mm,

under a loading strain rate of 0.01–0.03/min. The triaxial tests were

conducted under consolidated–drained condition with a shearing rate

of 0.08 mm/min. The specimen size was also Φ39.1mm × 80mm. The

applied conning pressures were 100 kPa, 200 kPa, 250 kPa, and

400 kPa, respectively. The direct shear tests were carried out in three

ways: the quick shear test, the slow shear test, and the repeated shear

test. A strain-controlled direct shear apparatus with 4 shearing boxes

was used. The specimen size was Φ61.8 mm × 20 mm. The shearing

rate of quick shear test was 0.8 mm/min, while that of slow shear test

and repeated shear test was 0.02 mm/min. The vertical pressures were

50 kPa, 100 kPa, 150 kPa and 250kPa, respectively. The test data were

auto-collected by a data acquisition system.

4. Test results and analyses

4.1. Chemical and mineralogical compositions

The main chemical compositions of seven diatomaceous earth sam-

ples are listed in Table 1. It can be seen that the color of diatomaceous

earth is dependent on the organic content: the darker is the color, the

higher is the organic content. The organic content of gray white and

yellowish brown diatomaceous earth is generally much lower, b0.3%,

while that of dark gray and black diatomaceous earth is higher, up to

1.93–4.42%.

The undisturbed diatomaceous earth consists mainly of three types

of clay minerals, namely, illite–montmorillonite mixed layer mineral, il-

lite and kaolinite (Fig. 3). Illite–montmorillonite mixed layer minerals

are not only higher in the mixed layer ratio, up to 50–65%, but also

higher in content, up to 33–74% (Table 2). However, in the weathered

diatomaceous earth (samples G06 and G07), the content of illite–

montmorillonite mixed layer mineral is lower, instead, the content

of kaolinite is higher.

The granulometric analysis results for 7 samples using the pipette

method are also presented in Table 1. The particle fraction of d N 75 μ m

Table 1

Grain–size distribution, main chemical compositions and specic surface area of diatomaceous earth.

Sample no. Sampling location Color Grain–size distribution(μ m %) SiO2/% Organic content/% CaCO3/% Specic surface

area/m2 g−1250-75 75-10 10-5 b5 b2

G02-1 Highway slope south of Wuhe Town Gray white 0.05 35.07 9.96 54.92 37.08 56.45 0.15 0.21 291.97

G02-2 Black 0.01 42.15 11.64 46.20 32.40 56.55 4.42 0.51 397.14

G04-2 Slope in the diatomite mine of Manpa Dark gray 0.08 37.42 13.02 4 9.48 34.88 5 6.35 1.93 0.31 224.51

G05-1 Highway slope in Mannong Yellowish brown 0.34 45.46 4.04 50.16 46.16 50.00 0.16 0 242.68

G05-2 Gray white 0.29 35.15 13.00 51.56 31.72 54.69 0.23 0 291.48

G06 Borrow pit by highway south of Mangbang Yellowish brown 0.06 57.18 2.28 40.48 35.68 61.72 0.10 0.67 196.20

G07 Highway slope in Zhangjiacun Yellow 0.02 41.14 8.72 50.12 33.92 64.76 0.08 0.23 171.70

Fig. 3. X-ray diffraction analyses of the b

2 μ m fraction of the Tengchong diatomaceous earth① Natural sample,② glycol-saturated sample,③ Sample heated to 550 °C.




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is generallyb1% and particularly up to 0.34%; the fraction of d= 75–10μ m

ranges from35.07%to 57.18%,with the average being 41.94%; the fraction

ofd=10–5μ m is 2.28% to 13.02%,withan average of 8.95%; the fraction of

db5μ m is 40.48% to 54.92%, with an average of 48.99%;and thefraction of

db2μ m colloids is 31.72% to 46.16%,with an averageof 35.98%.Therefore,

the particle fraction of d= 75–10μ m (silt grain) forms the skeleton of the

diatomaceous earth. It canbe seen under SEMthat the minimum debris of

diatomaceous earth is as smallas 2–3μ m (Fig. 4). Since the allogeneic clay

mineral is less than 2 μ m in diameter, the colloids can be counted as clay

minerals.

Ascan beseen from Tables1 and 2, the Pliocene diatomaceous earth

in the Tengchong region contains 50–65% diatom relicsand 31–46% clay

minerals. To be more precise, it should be named as the clayey diatoma-

ceous earth (Cui, 2008). Furthermore, it is worth noting that the content

of diatom relics in the Tengchong region is lower than that found in

other places in China (Tian et al., 1989; Gu and Zhou, 1994).

4.2. Physical and hydraulic properties

The physical and hydraulic properties of the clayed diatomaceous

earth were measured as listedin Table 3. From Table 3, itcanbe seenthat:

(1) The natural density of 7 samples measured by the wax-sealing

method is 1.16–1.41g/cm3, close to that of pumice.

(2) Except the obviously air-dried sample G04-2, the water contents

of 6 samples range between 84.86%and 113.25%, with an average

of 95.73%. The water content is higher than that of any natural

rock or soil except seabed clay.

(3) The dry density of the clayey diatomaceous earth is very low,

generally 0.62–0.75 g/cm3, which is related to the low natural

density and extremely high water content.

(4) The porosity and void ratio of the Tengchong diatomaceous earth

are very high, ranging between 68.75%–73.55% and 2.20–2.78,

respectively. However, it canbe seen under SEMthat thepore di-

ameter is very small (Fig. 4). In addition, there are many closed

super-ne pores within diatom relics, which might be an impor-

tant reason for the low density.

(5) The specic surface area of 7 samples measured by the ethylene

glycol-ethyl ether adsorption method is in the range of

171.70 m2/g–397.14 m2/g. The specic surface is much higher

than that of the typical diatomaceous earth, suggesting that the

Tengchong clayey diatomaceous earth has very high physic-

chemical activity.

(6) The liquid limits of 7 samples are in the range of 66.13–108.18%,

and the plastic limits are in the range of 38.91–55.36%, and the

corresponding plasticity indexes vary between 27.22 and

52.12, with an average of 38.84. The high plasticity index of

the Tengchong diatomaceous earth is attributed to its high

Table 2

X-ray diffraction analysis results of the b2μ m fractions of the Tengchong diatomaceous earth.

Sample no. Sampling location Relative content of clay

mineral/%

Mixed layer ratio/% b2 μ m/% Content of clay mineral/%

S I/S I K C S I/S I K C

G02-1 South of Wuhe Town 74 3 23 65 31.04 22.97 0.93 7.14

G04-2 Slope in the diatomite mine of Manpa 53 15 32 60 34.88 18.49 5.23 11.16

G05-1 Highway slope in Mannong 28 18 54 45 46.16 12.92 8.31 24.93

G06 Borrow pit by highway south of Mangbang 33 23 44 50 35.68 11.77 8.21 15.70

Note: S—montmorillonite, I/S—illite–montmorillonite mixed layer mineral, I—illite, K—kaolinite, C—chlorite.

Fig. 4. SEM microstructures of clayey diatomaceous earth.




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clay content and especially to the existence of abundant

swelling clay minerals.

(7) The liquidity indexes of 5 samples with natural humidity were

measured to be 0.66–2.73, with an average of 1.31. According

to the consistency of clay, such diatomaceous earth should be

in a soft-plastic state or a ow-plastic state. However, the nat-

ural Tengchong clayey diatomaceous earth mostly appears as

a pure solid state. This is one of the characteristics different

from general clayey soils.

(8) The ltration coef cients of two typical samples G02-2 and

G04-2 are 2.77×10−8cm/s and 1.89× 10−7cm/s, respectively,

indicating a kind of relatively impermeable material in theory.

This might be closely related to abundant swelling clay min-

erals in the diatomaceous earth.

These properties indicate that the Tengchong clayey diatomaceous

earth is a kind of very light and soft rock with high swelling potential.

It hasproperties of both thetypical diatomaceous earth andthe swelling

clayey soil or soft rock. It is similar to the typical diatomaceous earth in

terms of high porosity, high percent sorption, and strong structural con-

nection, whereas similarto theswelling clayey soil or soft rock in terms

of high plasticity and signicant swelling–slaking property. Of course,

the organic content has some effect on the swelling–slaking property

of the clayey diatomaceous earth.

4.3. Shrinkage and swelling behavior

4.3.1. Slaking behavior

The typical diatomaceous earth usually does not exhibit swelling

and slakingbehaviors. For example, when soaked in the water, the nat-

ural diatomaceous earth clod (with diatom content of about 75%) from

Linqu, Shangdong Province remained integrated without swelling–

slaking (Hu and Wen, 2005). In the slaking tests of the Tengchong clayey

diatomaceous earth, however, it can be seen that, when soaked in the

water, all light colored dry samples such as the gray whiteand the yellow

brown split into thin sheets (Fig. 5). The earth after soaking feels soft like

mud, and a single slaking sheet is much thinner than 1 mm, and the

volume of sample is swelled to nearly twice the size of the original. How-

ever, the dark gray, gray black and black samples show no slaking and

argillation but cracking (Table 3). The slaking of the Tengchong clayed di-

atomaceous earth is obviously a main cause of failures of highway slopes

(Fig. 2b and f).

4.3.2. Shrinkage and swelling

The shrinkage and swelling tests of the Tengchong clayey diatoma-

ceous earth were conducted on samples with different water contents.

The specimen for the circle test was 2 cm thick and the working load

was 1.17–1.25kPa. The test results are listed in Table 4. From these re-

sults, the following relationships can be inferred:

(1) The volumetric shrinkage ratio is closely related to the water

content. Among the four samples under test, obvious shrinkage

occurs in three samples with high initial water content, as indi-

cated by the volumetric shrinkage ratio up to 11.85–20.14%.

(2) The swelling ratio is dependent on the weathering degree. After

being shrunk to a stable state and soaked in water, obvious volu-

metric swelling occurred in samples G05-1, G05-2 and G06. The

swelling ratio of unweathered samples G05-1 and G05-2 is 4.64 –

4.68%, whereas that of weathered sample G06 is up to 29.77%

(Table 4 and Fig. 6). It indicates that the drying and rewetting pro-

cess leads to structural softening of the clayey diatomaceous earth,as well as increased swelling deformation.

(3) Swelling is also related to the organic content. For instance, no

swelling occurred in sample G04-2 with an organic content of

1.93%, while samples G05-1, G05-2 and G06 with lower organic

contents swelled after test.

(4) In terms of the clay particle content, the effective montmorillonite

content and the specic surface area, the Tengchong clayey diato-

maceous earth is very close to the Neogene hard clay met in the

Middle Line of North-South Diversion Water Project in China

(Zhang et al., 2003). However, the swelling ratio of the former is

lower (Table 5). Of course, this might be related to a pronounced

inuence of diatoms as the cementation of siliceous material

on the engineering properties of diatomaceous earth (Palomino

Table 3

Physical and hydraulic properties of the Tengchong clayey diatomaceous earth.

Sample

no.

Water

content/%

Density/

g cm−3Dry density/

g cm−3Porosity Void

ratio

Plastic

limit/%

Liquid

limit/%

Plasticity

index

Liquidity

index

Behavior after

soaking

Saturated

percent

sorption/%

Specic surface

area/m2 g−1Filtration

coef cient/

cm s−1

G02-1 84.86 – – – – 35.96 86.26 50.30 – Thin-sheet crack, muddy 119.12 291.97 –

G02-2 88.29 1.16 0.62 72.81 2.68 50.56 108.18 57.62 0.66 Sheet, softened 109.43 397.14 2.77 × 10−8

G04-2 42.80⁎ 1.18⁎ 0.87⁎ – – 55.85 100.52 44.67 – Cat aclastic, not soft ened 70.82 224.51 1.89× 10−7

G05-1 107.43 1.37 0.66 72.61 2.65 39.92 98.08 52.16 1.29 Thin -sheet crack, muddy 87.70 242.68 –

G05-2 113.25 1.37 0.64 73.55 2.78 38.91 66.13 27.22 2.73 Thin-sheet crack, muddy 1 09.23 291.48 –G06 92.85 1.27 0.66 72.39 2.62 40.75 87.87 47.12 1.11 Thin-sheet crack, muddy 129.40 196.20 –

G07 87.68 1.41 0.75 68.75 2.20 55.36 98.43 43.07 0.75 Thin-sheet crack, muddy 111.11 171.70 –

⁎ Air-dried sample.

Fig. 5. Tengchong clayey diatomaceous earth before and after soaking (a) before soaking; (b) after soaking.




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determined, such guideline is expected to be helpful for a more elabo-

rate classication system of expansive soil or rock.

5.2. Swelling discrimination method

As noted above, it is dif cult to discriminate the swelling behavior of

clayey diatomaceous earth by the existing simple method. Qu et al.

(1988) suggested that the saturated water absorptivity of dry rock

block can be used as the engineering classication index for fast deter-

mination of the swelling potential of argillaceous rock. However, al-

though the porous pure diatomaceous earth has very high absorption

capacity (Rajasekaran, 2006; Palomino et al., 2011), no swelling occurs

when soaked in water. In this case, the index of saturated water absorp-tivity of dryrockblock is unsuitable. Another method is the plot of re-

lationship of plasticity index (PI) and content of d b 2 μ m fraction

developed by Van der Merwe (1964), which is widely used. Howev-

er, when this method is applied to the clayey diatomaceous earth, it

can make the determination result much higher than the nature of

clayey diatomaceous earth. Obviously, this method is not suitable for

the clayey diatomaceous earth too. In this study, it is suggested to pre-

liminarily determine the swelling behavior of the clayey diatomaceous

earth using the disintegration test and then accurately determine its

swelling ratio by the swelling ratio test under low load.

5.3. Geohazards related with clayey diatomaceous earth

It is well known that the existence of diatoms can pronouncedlyincrease the micro-structural connection (Tanaka and Locat, 1999;

Rajasekaran, 2006; Palomino et al., 2011), and accordingly improve

the mechanical properties of diatomaceous earth (Shiwakoti et al.,

2002). The diatomaceous earth usually has better engineering properties

than common clayey soils. However, due to its compositions, especially

with the signicant amount of swelling clay minerals, the Tengchong

clayey diatomaceous earth can be considered as swelling soft rock, and

its undesirable engineering geological properties may result in engineer-

ing problems or geohazards. With alternate dry and wet conditions, the

shrinkage and swelling deformation might inuence the stability of

highway slopes. For instance, after the clayey diatomaceous earth

slope was excavated for highways in the Tengchong region, intensive

dry-shrinking ssures were formed because of water loss. Meanwhile,

a large amount of irregular debris constantly piled up at the slope toe

(Fig. 2b, f).These problems maynot be serious, butcommonand frequent.

Therefore, they shouldnot be overlooked. Themost effective controlmea-

sure is to prevent rainwater inltration and seepage during construction

of highway slopes.

It is revealed by repeated shear tests that the internal structure of

Tengchong clayey diatomaceous earth can be easily destructed when

subjected to disturbance or vibration. As a result, the structural strength

decreasesrapidly.Therefore, fordesign and stability analysis of slopesin

the clayey diatomaceous earth, the residual strength index should be

taken into account, and the factors such as weathering, earthquake,

and external disturbances should also be considered. During construc-

tion, it is important to avoid or reduce disturbance to the cutting slope.

Besides the landslides which occurred in thick-layer clayey diatoma-ceous earth slopes, more landslidesoccurred in bedding slopes and slopes

in which the clayey diatomaceous earth is overlying on basalt layer

with ancient weathering crust (Fig. 2c), during construction of the

Tengchong–Baoshan Highway and the Tengchong–Longling High-

way in Yunnan Province. The slope conditions and landslides are

similar to those observed between the Hipparion Laterite and the

overlying loess in Northwest China (Qu et al., 1999). Thus, the analyses

on the engineering properties and existence environment of the clayey

diatomaceous earth deposits may help to understand the complex

mechanism of geohazards.

Table 5

Comparison of swelling ratios between the intact clayey diatomaceous earth and the Neogene hard clay.

Type of sample Region Volumetric

shrinkage

Swelling ratio of

natural state

Swelling ratio

of dry state

Conning pressure

Grayish-green hard clay Nanyang basin 9–23 (17)20 3–19 (10)17 42–72 (60)20 Unloaded

Grayish-green with brown belt hard clay Fangcheng–Baofeng region 6–28 (16)12 5–26 (15)12 48–83 (63)12 Unloaded

Grayish-green with brown belt hard clay Handan–Yongnian Region 11–20 (16)6 5–7 (6)6 50–73 (58)6 Unloaded

Clayey diatomaceous earth Tengchong region 2–20 (13)4 0–30 (10)4 Very low loads, 1.17–1.25 kPa

Notes: Data in parentheses are mean values and subscript number afterwards is the number of test samples.

Table 6

Mechanical properties of the Tengchong clayey diatomaceous earth.

Sample no. Uniaxial compressive

strength/MPa

Saturated uniaxial

compressive

strength/MPa

Softening

coef cient

Quick shear

strength

Slow shear

strength

Residual

strength

Triaxial

consolidated

drained strength

c /kPa φ /° c d/kPa φ d/° c r/kPa φ r/° c /kPa φ /°

G02-1 1.09 0.84 0.77 210.0 30.0 29.4 21.6 22.5 12.4

G04-2 2.11 95.0 23.5 58.9 23.4 5.8 12.3 155.2 26.1

G05-1 1.36 0.80 0.59 177.0 31.5 61.5 25.9 16.9 11.8 409.8 22.7

G06 0.40 0.34 0.84 69.0 29.5 79.8 21.6 45.9 17.9

Fig. 7. Relationshipbetweenporosity and uniaxial compressivestrengthof the Tengchong

clayey diatomaceous earth.




9/10

6. Conclusions

The Tengchong clayey diatomaceous earth is a type of regional

unusual soil or rock, which has the characteristics of both the typical

diatomaceous earth and the swelling clayey soil or soft clay rock. The

Tengchong clayey diatomaceous earth has high porosity, high plas-

ticity, high percent sorption, and relatively strong structure, and ex-

hibits signicant swelling–slaking behavior. In brief, the Tengchong

clayey diatomaceous earth is a kind of very light and soft rock with

high swelling potential.

Fig. 8. Triaxial consolidated drained test curves of the Tengchong clayey diatomaceous earth.

Fig. 9. Strength envelopes for repeated shear tests on the Tengchong clayey diatomaceous earth.


http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B9%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B8%80


10/10

The engineering geological properties are controlled by the diatoms

and the swelling clay minerals. The diatoms in the earth enhance the

micro-structural connection, and accordingly improve the mechanical

properties of diatomaceous earth. However, due to the existence of a

signicant amount of swelling clay minerals (illite–montmorillonite

mixed layer mineral and illite), the Tengchong clayey diatomaceous

earth becomes swelling soft rock, and its undesirable engineering

geological properties may result in geohazards.

Thetests on the swelling potential of theTengchong clayeydiatoma-

ceous earth indicate that the traditional methods for discrimination of

the swelling soil or rock are not suitable for the clayey diatomaceous

earth. It is dif cult to discriminate the swelling behavior of clayeydiato-

maceous earth by the existing methods. In this study, it is suggested to

preliminarily determine the swelling behavior of the clayey diatoma-

ceous earth using the disintegration test and then accurately determine

its swelling ratio by the swelling ratio test under low load.

Based on the test results, geohazards related with the clayey diato-

maceous earth for the highway slopes in the Tengchong region are

discussed. The most effective control measure is to prevent rainwater

inltration and seepage during construction of highway slopes. Mean-

while, repeated disturbances or vibrations shall be avoided if at allpossible.

Acknowledgment

This research was supported by the Project of Science and Technology

Development Plan of Railway Ministry of China (Grant No. 2008G027-B),

the Fundamental Research Fund of Institute of Geomechanics (Grant No.

DZLXJK201206), and the Project of the 12th Five-year National Sci-Tech

Support Plan of China (Grant No. 2011BAK12B09). We would like to

thank the three anonymous reviewers andthe editor for their helpfulsug-

gestions, which have improved the clarity of the paper.

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shear stress and vertical pressure.

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