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: zhys100@hotmail.com (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.
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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.
102 Y. Zhang et al. / Engineering Geology 167 (2013) 95–104
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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.
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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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