10th Asian Regional Conference of IAEG (2015)

Analysis of the engineering properties of Xigeda stratum in Lamaxi Gully,
Sichuan Province, China
Xiyong WU(1), Yongliang PENG(2), Qixiang YANG(3), Xin LIAO(4),Yong REN(5), Baolong ZHU(6)(1) Department of Geological Engineering, Southwest Jiaotong University, China
E-mail:wuxiyong@126.com(2)Department of Geological Engineering, Southwest Jiaotong University, China(3)Department of Geological Engineering, Southwest Jiaotong University, China(4)Department of Geological Engineering, Southwest Jiaotong University, China(5)Department of Geological Engineering, Southwest Jiaotong University, China(6) School of Civil Engineering and Architecture, Southwest University of Science and Technology,
China
Abstract
Attribution to the poor engineering properties of Xigeda strata, landslides easily occur
in the areas where it is distributed widely. Taking the Xigeda strata in Lamaxi Gully, Sichuan
Province, China as object, the characteristics of its particles, minerals and soluble
constituents were studied to analyze the weathering process and the landslide features. It is
indicated that the soft lithology of Xigeda strata is significantly important to induce thegeological disasters.
Keywords: Xigeda strata, Engineering property, Landslide
1. Introduction

for the engineering properties of Xigeda strata.
Under the consideration of the geological factors
which could affect the property of the Xigeda
strata, the grain size, chemical composition andsoluble salt were experimental studied to discuss
their influences on the rock’s engineering
properties.

The late Cenozoic Xigeda strata are
distributed widely in southwestern China, suchas in the valleys of Jinsha River and AnlingRiver, Dadu River and Yabi River and so on.
Xigeda strata mainly consist of gray-black,gray-green and gray-yellow claystone, silty
claystone and siltstone.
The study area Lamaxi gully is in the Xigeda
strata district, where mainly distributes the
Tertiary Pliocene (N2) lacustrine depositionalsemi-solid soft rock with poor geologicalengineering properties. The pale and light yellow
and silty sandstone and shale has significant
differences with the general rock soil mass inengineering properties. Under the influence of
the Jiuxiang fault zone, there are ubiquitouscrusher rocks and erosion gully caused by thelong term slope surface erosion. The gully iswith large longitudinal gradient and incision
depth of 30~40 m. The sidewall soils are relative
loose so that it is prone to result in instability inthe process of the gully bed down-cutting
erosion. The expand trend to the gully head andsidewalls year by year would induce theretrogressive erosion and slope instability.
Therefore, it is necessary to conduct the research

2.

Analysis of the geological factors of the
Xigeda strata

For the bearing capacity of rock mass, theXigeda strata are relatively good bearing layer offoundation. However, it is worth noted that there
are also some harmful geological factors, which
can be classified into the following aspects:
1. Though Xigeda strata have experienced
early diagenesis, their strength is still relatively
lower than that of the other rocks in the previous
geological time. Especially under the action ofwater, the bearing capacity and shear strength
will be reduced significantly.
2. Attribution to the well-developed
beddings in shale and mudstone of the Xigeda
strata, the rocks are ease to collapse
under wetting and drying cycles.
3. Because the (silty) sandstone and1

10th Asian Regional Conference of IAEG (2015)

mudstone are interbedded in Xigeda strata, there
are some weak sedimentary structure planes.
These structure planes and the contact interfaces
between the Xigeda strata and overlying andunderlying
strataarethemain
ill-structured surface which could trigger
landslide and collapse.
4. As the strong neotectonic movement in
Panxi district, there are many fracture planes
mainly consisted of fault and structural fissures.
This will seriously affect the slope stability in
Xigeda district.
3.

settling velocity was calculated by the Stokes’
equation.
Chemical analyses were conductedwith an X-ray fluorescence spectrometer (ARL
Advant’ XP). The soluble salts were determinedby the hydrochemical analysis of the leachingsolutions. 50~100 g soil samples were weighed
to place in a jar. The weight depends on saltcontent and test project. Then the water at theratio of 5:1 was added and stirring for about 5
min. When the salt content is as high as dozens
of grams, the ratio of water to soil can be higher,
but not over 20:1. The leaching solution was
obtained by filtration after 12 hours. The
following indexes for the leaching solutions aretested: alkalinity, carbonate, bicarbonate,calcium, magnesium, potassium, sodium
chloride and sulfate ions.

Methods

In the study area, Xigeda strata are mainlycomposed of three sections. The lower stratum
are lacustrine sediments, which mainly consist ofsets of sedimentary rhythm containing
yellow-grey and livid silt clay or fleshy redinterbedded clay; the middle stratum are mainly
black muddy-silt and sandy clay with clear
horizontal bedding; the top stratum are mainly
ochre thick sand layer with mica sheet.
According to the field investigation, three
Xigeda soil samples with different colors(Lmx-1, pale yellow; livid, Lmx-2; fleshy red,Lmx-3) were collected to be analyzed.
The grain size analysis was conducted by thehydrometer method and diffusion method. The

4.

Test results and discussion

4.1 Grain size
The results of grain size are shown in Table 1
and Fig. 1. It shows the grain sizes of three soil
samples are mainly distributed in the scope of
0.075-0.005 mm, whose proportion is about
66%~85%; followed is the scope of 2-0.075 mm,
whose proportion is about 10%~20%. The
remaining grain sizes are all less than 0.005 mm.

Table 1 Result of the grain size
sample number

2-0.075（mm） 0.075-0.005（mm） <0.005（mm）

Lmx-1

10.7

84.1

5.2

Lmx-2

19.6

66.3

14.1

Lmx-3

12.1

84.5

3.4

Average

14.1

78.3

7.6

90
80
70
60
50

sand

40

silt

30

clay

20
10
0
Lmx-1

Lmx-2

Lmx-3

Fig. 1 The contrast of the grain size content

2

10th Asian Regional Conference of IAEG (2015)

4.1.1 Granularity and corrosion resistance
It is reported that the resistance will increase
with the grain size decreasing. As the grain size
is homogeneous, the stability of the soil will beworse, vice versa. For the soils with more than
30% ~ 35% clay content, the cohesion isrelatively higher and it could form stable soil
crumb structure. This kind of crumb structure
occupied higher resistance to raindrops hit andsplash erosion (Kirkby and Morgan, 1987).
The average content of the sand of the
Xigeda soil in the Lamaxi Gully is about 14.1%,
while that of the silt is about 78.3% and that of
the clay is about 7.6%. It is indicated the soil
composition is mainly silt and relatively
homogeneous. Thus, the loose structure will
result in the weak anti-erosion capability.

interparticle cohesion. Correspondingly, thestability and anti-erosion capability of the silty
particles in the Xigeda soils are relatively weakin water.
4.2 Chemical component
Table 2 lists the analytical results ofchemical composition. It shows the contents ofeach element oxides are similar in different
samples. The content of SiO2 is in the range of
52.94%～62.98%, Al2O3 is 12.16%～15.22%,
Fe2O3 is 4.78% ～ 6.24%, MgO is 1.46% ～2.87%. In Xigeda strata, the sum of instablechemical component contains Na2O, K2O andCaO occupied 3.64～10.82% of the total. Due tothe active chemical properties of the soil, somesoluble chemical compositions can migrate tocause local mineral enrichment or be taken awaywith water under a special hydrological andclimatic conditions, such as Ca2+, Na+, K+ andso on. It will gradually enlarge the void amongrock mineral and aggravate the intensity ofchemical weathering, eventually lead to thedestruction of the rock mass structure. The
content of instable chemical compositions isrelatively lower in Lmx-3 sample, presumably
due to the result of the eluviation loss of the soil
samples near the surface.

4.1.2 Mechanical properties of the grain
The grain size has influence on the soil
strength. Generally, the soil strength can be
represented by the coulomb's law:general soil: τ = c+σ•tgφsandy soil : τ = σ•tgφ
From the above equations, it shows the soil
strength is not only relevant to the internal
friction angle φ, but also to the cohesive force C
at the condition of constant stress. The intensity
curve is a line with intercept C on vertical axis
(Fig. 2). When the grains become coarser, thecohesion value decreased gradually and theintensity curve (b) moves down along thevertical axis trending to the curve (a). When thecohesion value decreases to zero, curve (b) andcurve (a) are overlap and their strength become
equal. Meanwhile, the soil turns to be loose drysand with low strength. It will be eroded anddisintegrated immediately once being saturated
with water.

4.2.1 Chemical weathering degree
The chemical weathering degree iscontrolled by many factors, such as geographic
and geomorphic conditions, lithology, climatefeatures (rainfall), etc. Generally, it is identified
by the CIA value and Na/K index of acidnon-soluble substance. The bigger the CIA valueis, the higher lever rock chemical weathering
occupied, which suggests that feldspar corrosion
is being gradually altered into clay minerals.
CIA value is expressed as (Nesbitt and Young,1982):

CIA  [ Al2O3 /( Al2O3  CaO  K 2O  Na2O)]  100
Formula above is the mole ratio of theoxides. CaO represents the content in silicate
minerals, but not include the content incarbonate and phosphate. It shows the average
CIA value of Xigeda strata is 63.4, which iswithin the range of the quaternary glacial clay
(Table 2). This value is slightly lower than thatof shale (70-75), and far lower than that ofresidual soil (85-100). It indicates that theoverall weathering resistance degree of Xigeda
strata is relatively weak. But compared to thequaternary loess and paleosol, there are noFig. 2 Soil strength curve
In consequence, as the grain size is coarser itwill cause smaller natural porosity and lower

310th Asian Regional Conference of IAEG (2015)

significant differences among them, whichreflecting that the Xigeda soil is still in theprocess of strong chemical weathering.
Na/K ratio is an index to evaluate theplagioclase weathering degree. It can also be
used to represent chemical weathering degree of
sediment. Because weathering rate of

Lithology

plagioclase is faster than potassium feldspar,
Na/K ratio is inversely proportional toweathering degree and is on contrary of thechanging characteristics of the CIA parameters.
The Na/K value is close to loess and paleosol
indicating the strong chemical weathering of
Xigeda soil as well.

Table 2 Chemical composition comparisons among the Xigeda soil,
Loess and Paleosoil (Chen et al, 2001)
Xigeda strata
Loess

Paleosoil

NUM

Lmx-1

Lmx-2

Lmx-3

Ave.

scope

Ave.

scope

Ave.

SiO2

59.38

52.94

62.98

58.43

63.83～69.47

66.4

63.20～67.97

65.18

Al2O3

12.16

13.63515.21513.67

13.14～15.04

14.2

13.96～15.51

14.79

Fe2O3

4.775

5.98

6.24

5.67

4.18～5.50

4.81

4.66～5.54

5.12

CaO

6.7

6.905

0.68

4.76

0.87～1.22

1.02

0.57～0.95

0.83

MgO

2.195

2.87

1.46

2.18

1.99～2.60

2.29

1.86～2.60

2.21

K 2O

2.255

2.58

2.565

2.47

2.62～3.23

3.01

2.56～3.35

3.15

Na2O

1.51

1.33

0.395

1.08

1.32～2.03

1.66

1.03～1.79

1.41

TiO2

0.665

0.765

0.9

0.78

0.67～0.79

0.73

0.72～0.78

0.75

MnO

0.0855

0.12

0.11

0.11

0.05～0.09

0.07

0.06～0.09

0.08

P2O5

0.165

0.21

0.04

0.14

0.12～0.18

0.15

0.09～0.14

0.11

CIA

53.75

55.77

80.69

63.40

62.93～68.56

65.46

65.99～69.71

67.99

Na/K

66.96

51.55

15.40

44.64

50.38～62.85

55.15

40.23～53.43

44.76

SiO2/R2O3

1.95

1.54

2.28

1.89

2.26～2.56

2.38

2.21～2.48

2.29

4.2.2 The stability of anti-erosion
SiO2/R2O3 index is commonly used toindicate the anti-erosion stability of the soil
(Wang et al, 2001). R2O3 represents the oxides
except SiO2. The bigger the value of SiO2/R2O3
is, the weaker the stability of anti-erosion is.
From the result in Table 2, the value of Xigeda
soil is 1.54~2.28; the loess is 2.26~2.56; thepaleosol is 2.21~2.48. It is suggested that theanti-erosion stability of the Xigeda soil is similar
to that of the loess and the paleosol. Thus, we
can apply some scientific research achievementsof loess erosion to the study of Xigeda soil.

occurs.
(1) Analysis of the rock mass stress
Collapse is one of the main gravitational
erosion type. Gravitational erosion occurs whendifferent weathering degree leads to different
erosion rate, and the overlying strata is hanging.
For the gravitational erosion of sandstone, there
are two different directional forces on the rockmass before collapsing: one is the verticallygravity and the other is shearing force. Under
this circumstance, τ= C + σ·tanφ, σ=0. It
indicates there is no stress upon the free face. So,
T=C. When C is greater than the gravity of rockmass, collapse won’t happen. On the contrary,collapse occurs. Therefore, based on the force
analysis, the gravitational erosion is related tovalue of the rock’s cohesion.
(2) The relationship between the chemicalcomposition and cohesion
The data from the factor correlation analysisof the chemical composition and the cohesionsuggested that cohesion increases with theincrease of Fe2O3 .MgO, MnO ,Na2O and FeO in
the rock mass, and decreases with the increase of
K2O, CaO and SiO2 (Ye et al, 2006). The

4.2.3 The relationship between the chemicalcomposition and the shear strength
The ability of the rock to bear the shearing
force is called the shear strength. The shear
strength of the rock consists of two parts, thecohesion (C) and the internal friction resistance(σ·tanφ), which are related to the strong
connection of rock. When the gravity or its
component imposed to the rock mass in a slope
is greater than the residual shear strength, thegravitational erosion phenomenon like collapse

410th Asian Regional Conference of IAEG (2015)

characteristics of correlation analysis show thatdifferent chemical composition has different
effect degree on the cohesion. The correlation
has significant difference in linear correlation or
non-linear correlation, which shows thatdifferent chemical composition has independent
effect on cohesion. Although the test result only
denotes the total content of certain chemicalcompositions, the result reflects the relationshipbetween the cohesion and the chemicalcomposition of the rock to a certain extent. The
content of Fe2O3, MgO, MnO, Na2O of Xigeda
soil comprised 8.02%~10.3% of the total amountof chemical constituents while the content of
K2O, CaO and SiO2 comprised 62.43～68.34,

which indicates the cohesion of Xigeda soil iscomparatively low.
4.3 Composition of soluble salt
Soluble salt refers to the salt easily dissolvedin water, including all chloridum, potassium
sodium sulfate and potassium sodium carbonate.
This test is applicable to various types of soil
whose grains are less than 2 mm. The soil withgrains greater than 2 mm must be sieved out
before, and parallel determination should be
adopted. The analytical results of soluble salt
test are shown in Table 3:

Table 3 Analytical results of the soluble salt
total

total

hardnesssalinity

0.20

54.79

203.00

0.40

0.19

86.39

148.59

0.22

0.16

42.14

196.81

Samples

pH

CO2

HCO3-

CO32-

Cl-

SO42-

Ca2+

Mg2+

Na++K+

Lmx-1

8.90

0.21

1.54

0.00

0.06

0.86

0.42

0.13

Lmx-2

8.63

0.10

2.23

0.00

0.09

0.25

0.46

Lmx-3

8.56

0.21

1.34

0.00

0.06

0.90

0.20

Note: The unit of ion concentration is mmol/L; the unit of hardness and salinity are mg/L.
In general, leaching deformation is closely
related to the long-term immersion of soluble
salt in soil. The results in Table 3 show that thecontents of soluble salt are low, with a value ofabout 4.01 mmol/L. The positive ion content isabout 0.79 mg/100g, of which Ca2+ and Mg2+
occupy the major content which indicates themthe main loss of ions in Xigeda soil. Referring toprevious study (Kong and Li, 2001), the leaching
deformation is small when the content of soluble
salt is less than 90 mg/100g. As the content ofsoluble salt is greater than 90 mg/100g, theleaching deformation will increase significantly.
Based on the test data, Xigeda soil has leaching
deformation in a certain extent.

Samples

Table 4 Substituted quantity computation sheet of the cations
Mg2+
Na++K+
Cation exchange capacity
Ca2+

Lmx-1

0.042

0.013

0.020

0.075

27

Lmx-2

0.046

0.040

0.019

0.105

18

Lmx-3

0.020

0.022

0.016

0.058

28

ESP（%）

Note: the unit is cmol/kg. ESP is potassium and sodium percentage of the exchanged cations.
The soil with an ESP value ≥ 15% is calledhigh dispersal soil, which leads to erosion easily.
Encountering with water, this kind of soil iseasily to be dispersed and taken away. The
exchanged amount of the cations can becalculated from unit conversion in the dilute
solution. The Table 4 shows the ESP value of
Xigeda soil is 18%～28%, which belongs tohigh dispersal soil. The poor physicochemical
property further illustrates the weak erosion
resistance.
In brief, the content of soluble salt has
significant effects on soil loss. The Xigeda soil
in Lamaxi valley contains a certain amount ofsoluble salt, which is easily to be dissolved anddecrease the cohesion under the influence ofwater. Thus, the amount of soluble salt is asignificant factor in soil erosion and soil loss on
slope surface.
5. Conclusion
The serious headward erosion is closely
associated with the geotechnical characteristicsof the Xigeda strata in our study area. The
shearing resistance is affected by themineralogical and chemical compositions as well5

10th Asian Regional Conference of IAEG (2015)

as the rock structures in the erosion process. The
grain size and chemical compositions of therocks are the internal factors impact on the
rock’s strength. Furthermore, the soluble salt andclay mineral are the primary cause of strength
reduction.

Kong, H., Li, G. (2001): Mechanism and factorof loess soil deformation by seepage lixiviation,
Geotechnical Investigation & Surveying, No. 5,
pp. 29-30.
Nesbitt, H., Young, G. (1982): Early Proterozoic
climates and plate motions inferred from major
element chemistry of lutites, Nature, Vol. 299,
pp. 715-717.
Wang, Y., Zhan, Q., et al. (2001): Structures andCharacters of Debris flow, Changsha: Hunan
Science and Technology Press.
Ye, H., Shi, J., Wang, G., et al. (2006): Effect ofchemical compositions of Pisha sandstone on
the gravity erosion, Hydrogeology &Engineering Geology, No. 6, pp. 5-8.

References

Chen, J, An, Z., Liu L.,et al. (2001): Recently
2.5 Ma change of chemical composition on
the loess plateau and the Asian inland
chemical weathering, Science in China(Series D), Vol. 31, No. 2, pp. 136-145.
Kirkby, M.J, Morgan, R.P.C. (1987): Soil
Erosion, Beijing. Water and Hydropower Press

6